# Rebel 4X, no, 6X! Feel the power!



## Oznog (Jun 14, 2008)

Well, here it is in all its glory. 4x Rebel Red-Orange 50's on a Cree mount!

They float together when you reflow. You can't space them even if you wanted to. 200lumens @350mA, and putting it side-by-side with a red Lamnia BL-2000, this is slightly brighter at a fraction of the power.
4W, not really asking too much of the mounting especially considering the device-to-Star interface is solder.

Reflow with a heat gun to the bottom.

I had one of the 4x in series blow up... I was having trouble with the test lead though, I was having trouble adjusting the power supply and I think I had it set too high and it gave a pulse from the supply's output cap before the current limit kicked in. Or maybe the reflow didn't work under the pad, because that first time I was using the toaster oven and maybe there was a problem, I'm not sure. The remaining devices seem ok.

I rearranged it so there are two 2x series strings in parallel and turning up to 700mA. They don't turn on at the same time, and at _low_ power one side shines much brighter. Still, once I turn the power up they look pretty equal. In any case, the devices are each rated up to 700mA so even they don't share well it's still within the limits.

Actually when I still had them in series, I tried it at 350mA and 700mA with the BL2000 on a fixed current beside it for reference. Honestly, not a lot of intensity difference when doubling the current, in this configuration apparently the increased die temp is hurting nearly as much as the extra current is helping.


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## Oznog (Jun 15, 2008)

*Re: Rebel 4X! Feel the power!*






Behold, the Rebel 6x Spidereye!

I had another look. The heat spreader, a copper fill area that the thermal pad is made of, has wings off to either side. They're just covered by soldermask. By removing about 1mm from either side, there's actually enough space here to mount *6* Rebels, as shown above.

Well, 6 watts is kind of a lot of power, but it fits right in with my ideal requirements. My ideal voltage match would have series strings of 3x devices. I don't have enough room for a second Star, not unless I cut it up, and it could only carry 2 additional devices on it and still meet my voltage limitation. 

Oh yeah it's bright. Like unbelievably bright here. It tasered my retinas.


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## TigerhawkT3 (Jun 15, 2008)

Cool stuff! 

You might want to current-balance the two strings to avoid thermal runaway.


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## Oznog (Jun 15, 2008)

Well, I think I may not have thermal runaway issues. The thermal pads are joined by the copper foil heat spreader on top of the Star, due to package geometry the thermal pads are nearly adjacent. And I'm providing a regulated 700mA, trying to get 350mA per string to optimize the light output but 700mA is electrically and thermally permissible.

It does _look_ like it's sharing properly once the power's kicked up.

Thing is, I was trying to keep this a 2-wire device and there's really not room for resistors, esp ones which generate a lot of heat. Well, I'm looking at the issue. Dropping 1V @ 350mA would be plenty of ballast but that's 350mW of heat and the way I'd do this is heat shrink it inline before it joins up and that's not tolerant of high temps.

I gotta say, it's pretty awesome. The BL2000 was 13.6W at full rated power, but even with a pretty low thermal resistance that brought the die temp up and it degraded the light output pretty heavy. This is giving out way more light in less than half the power, it's not as flat but the diameter's not too different.

It really sucks that Future raised its low-quantity Rebel prices, that's gonna hurt big time. I need a bunch but still not 1000x.


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## frenzee (Jun 15, 2008)

Great work Oznog. I had the same imbalance issues at lower currents even when everything was in series. I did four white rebel 100s a couple months ago and the the light bounce off the wall 20 ft away with no optics at all was like that of a welding arc. In fact at 1A, if you held a sheet of Kleenex in front of it, poof!


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## Oznog (Jun 15, 2008)

frenzee said:


> In fact at 1A, if you held a sheet of Kleenex in front of it, poof!



"Poof"? What, the kleenex caught fire or the LEDs smoked?

What type of board is that one you've pictured there?


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## jeffosborne (Jun 16, 2008)

I like using a few re-orange Rebels with several neutral whites to warm up the light for photography. I drive them at 850ma. They do have a very respectable output. So Oznog, what are you lighting up with so many red-orange lights? Cheers, Jeff


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## Oznog (Jun 16, 2008)

jeffosborne said:


> I like using a few re-orange Rebels with several neutral whites to warm up the light for photography. I drive them at 850ma. They do have a very respectable output. So Oznog, what are you lighting up with so many red-orange lights? Cheers, Jeff



Rebel was only rated for 700mA.
Have you checked the graphs? The problem is that Rebel red-orange generated about 3.3x the heat at 850mA, and the thermal resistance of the package is on the high side. The efficiency of the die goes down quite a bit, you get only a 65% light gain when doubling the current even if the heatsink maintains 25C. That penalty's basically instantaneous when turning the device on, the heatsink will be hotter unless it's enormous and that will produce additional losses over a few minutes as it heats up- which is kinda unfortunate for photography because your color balance will change over that time.

I crunched the numbers and 6 devices at 350mA was _much_ better than 3 devices at 700mA. The additional forward voltage at 700mA resulted in an additional 24% of heat, AND the 3 devices would only go up by 65% even if I could maintain the same sink temp whereas doubling the number of devices to 6x at at 350mA is double the output.

So it really does come down to increasing the number of devices is more beneficial than increasing the current past the suggested 350mA. Thus the reason for uber-packing the Star instead of jacking up the current. The technical justification is sound.


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## frenzee (Jun 16, 2008)

Oznog said:


> "Poof"? What, the kleenex caught fire or the LEDs smoked?
> 
> What type of board is that one you've pictured there?



LOL, the Kleenex, of course. Therefore, I suggest you don't use your fingertip to see if how hot these emitters get

The board is a 21mm dia. Bergquist copper core disk. Here's the trace layer if you're interested, and few other crazy stuff, in case you decide to put up to 18 of these little guys on a single star:


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## Jarl (Jun 16, 2008)

How are they connected to each other?


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## rizky_p (Jun 16, 2008)

Jarl said:


> How are they connected to each other?



they are basicaly 2 series and 3 paralel right in 6x setup? (imagining Cree board)


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## Oznog (Jun 16, 2008)

The Cree board has a long trace on either side for the power, each one has a solder pad on each side. Each trace was cut in 3 places and the Rebel's power terminals straddle the cut. So on one side 3 devices are in series without doing any wacky soldered micro-wiring hidden in there. Which is genius, if I do say so myself.

So we've got 4 wires available, we can make the 2 strings into series (18V @ 350mA) or 2 in parallel (9V @ 700mA). Like I say, the current sharing isn't all that bad with the thermal pads connected to each other by copper, and the packages are rated to 700mA anyways so there's no real risk of destructive runaway.

It is essential to use solder paste and reflow it. Hand work is impossible because there's no exposed pad area to apply a soldering iron to!


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## Oznog (Jun 22, 2008)

frenzee said:


> LOL, the Kleenex, of course. Therefore, I suggest you don't use your fingertip to see if how hot these emitters get
> 
> The board is a 21mm dia. Bergquist copper core disk. Here's the trace layer if you're interested, and few other crazy stuff, in case you decide to put up to 18 of these little guys on a single star:



frenzee, do these things exist or are they hypothetical designs? It is obtainable?

On the Cree Star, with 6 devices running @ 1W each, I used a tiny thermocouple to measure on a tiny blob of solder off to the side of the thermal pad. I get about 15C above ambient after about 30 sec, and frankly the Star's mounted like crap, I only have 1 screw I can tighten down onto the sink so a some of that rise is simply Star-to-sink thermal resistance.


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## frenzee (Jun 24, 2008)

Oznog said:


> frenzee, do these things exist or are they hypothetical designs? It is obtainable?...



I've actually made One and Two and they exist and I've posted pictures before. Three and Four would be very straightforward to build. Five is, well, unobtainium at the moment. Although the pad topology would allow 18 Rebels to coexist, designing the trace layer (for a series connection) would be quite a challenge, not to mention the driver.


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## Gomer (Jun 26, 2008)




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## mrsinbad (Jul 2, 2008)

Gomer, I like that... tell me more!


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## Gomer (Jul 3, 2008)

http://www.asiansignals.com/RebelBoards.htm

I have ZERO affiliation with them nor can I vouch for their products in any way shape or form.


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## Oznog (Jul 3, 2008)

Gomer said:


> http://www.asiansignals.com/RebelBoards.htm
> 
> I have ZERO affiliation with them nor can I vouch for their products in any way shape or form.



I have a sample of that specific 6x board from that company.

It is NOT an aluminum-core thermal board. It's a fiberglass core double-sided PCB. It's got plated throughholes and all to try to conduct the heat to the bottom, but the thermal impedance looks very poor to me.


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## Gomer (Jul 9, 2008)

Oznog said:


> I have a sample of that specific 6x board from that company.
> 
> It is NOT an aluminum-core thermal board. It's a fiberglass core double-sided PCB. It's got plated throughholes and all to try to conduct the heat to the bottom, but the thermal impedance looks very poor to me.


Any chance you have measurements on the thermal conductance?


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## Oznog (Jul 9, 2008)

No, but the board's 0.042" total, foil and glass. The foil appears quite thin, the difference in depth between the white paint and the gold plated foil areas is 0.002". Since I don't know the paint thickness I can't say for sure what the foil thickness really is. There is a tiny rim where the foil on back ends before the pcb edge, no paint... it's real hard to read, and I may be reading edge imperfections from the cutting process, but I get a foil thickness of ~3.7 mils. Probably 2 oz copper before plating then.

Aside from the 3 large mounting screw PTHs, each of the three "pairs" has one Rebel mounting area surrounded by 4 small PTHs and one mounting area with 11 small PTHs. In addition, there are 2 *extremely* small PTHs directly under the Rebel's thermal pad. There are also 2 holes between the mounts making up a pair, but they removed the copper under them on the backside, thus these have no thermal conduction value.

I know Luxeon Application Brief AB32 shows a number of designs relying on the PTH copper plating to bring the heat to the back, achieving a thermal resistance of 7C/W or even 3C/W to the back (note: _not_ to ambient). This board does not have a similar density of PTHs. So, assuming the conduction is all about PTH (which AB32 says a lot about) and the 7C/W board with 31 large PTHs can be compared directly with the 5 PTH of this board... *I get an estimate of over 40C/W on that 6x circle board.* If that estimate's correct this is a failed, ineffective thermal design and inappropriate for a powerful Rebel application.

I don't know what they were thinking. Maybe you are supposed to use aluminum screws and they'll do the lion's share of the conduction to a sink? However, I did some calculations awhile back, the thermal resistance of copper foil when used to conduct heat laterally is limited, which is probably why Luxeon did not recommend adding more thermal vias outside of about 1mm-2mm from the perimeter; the thermal resistance from pad to the outer vias would be so great that it would yield little additional benefit. So even the screw would not get a great thermal resistance because it's only on one of the 4 sides of the pad.


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## Gomer (Jul 10, 2008)

I just re-dug up this info. It was posted by the designer on a reef forum



> The board is technically copper clad which means it is a basically a sandwich of copper and fiberglass. In this case we reduced the thickness of the fiberglass a bit and increased the thickness of the copper to 3oz on both sides (typical boards are 1/2 oz to 1oz copper). Then we add a number of plated vias through the board to bond the top and bottom plates. It are these vias that really set this board apart from the aluminum ones. You do essentially have a path of solid copper from the top to the bottom of the board.
> 
> It's not a good as a solid piece of copper but it is likely to be better then any aluminum based insulated board.
> 
> Those aluminum boards are designed for LEDs that require electrical insulation. That means that there is some dielectric material that is bonded between the metal in the LED and the aluminum base. There is no metal to metal contact. You are up against limitations when you can't make a metal to metal bond.


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## Oznog (Jul 11, 2008)

Hahaha.... well, I'm sure you accurately quoted his claim and thank you for passing it along, but I'm not buying it. Thermal vias are not as effective as he is making it out to be, the cross section is an incredibly thin tube. Copper's a great thermal conductor, almost twice that of aluminum, but still it's not a thermal _super_conductor.

And he's definitely wrong on the aluminum boards being poor thermal conductors because of the insulation. Look at the P7 board... that's an insulator yet it's got a pretty nifty lack of thermal resistance. Well, my crude measurement of the insulating Cree board suggested like 2C/W thermal impedance. I wish it wasn't insulating and they'd somehow plated solderable copper straight on the aluminum and got me sub-1C/W performance. But, oh well. You work with what you have.

The Luxeon recommended design using PTH thermal vias in the Rebel spec sheet bragged about 7C/W, but that's all about keeping cost down so I'm sure that's dandy for some apps. But the Luxeon design is probably "best possible" and featured far more thermal vias. 

Using 1/6th the number of vias as the 7C/W Luxeon design, using smaller vias, with apparently random (virtually inexplicable) placement but still beating a 2C/W-range aluminum-core board is technically absurd. So... the AsianSignals 6x claim looks like a load of crap.


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## KeithInAsia (Aug 24, 2008)

Oznog said:


> Hahaha.... well, I'm sure you accurately quoted his claim and thank you for passing it along, but I'm not buying it. Thermal vias are not as effective as he is making it out to be, the cross section is an incredibly thin tube. Copper's a great thermal conductor, almost twice that of aluminum, but still it's not a thermal _super_conductor.
> 
> And he's definitely wrong on the aluminum boards being poor thermal conductors because of the insulation. Look at the P7 board... that's an insulator yet it's got a pretty nifty lack of thermal resistance. Well, my crude measurement of the insulating Cree board suggested like 2C/W thermal impedance. I wish it wasn't insulating and they'd somehow plated solderable copper straight on the aluminum and got me sub-1C/W performance. But, oh well. You work with what you have.
> 
> ...


 

You bad mouth that product quite a bit but you don't show any testing. The bottom line is -- the product works and works well.

It is true that Luxeon created a design that included more vias with a different placement -- did they product that design and actually use it? My guess would be no because they never published a real picture of it. It's quite possible they didn't expect the LED to be mounted down to any heat sink as there are no mounting elements in that design -- notice that? The had no plan for anchoring the circuit board down. Why not? That would be a factor wouldn't it?

The AsianSignals design is in actual use and it works very well.

Please be a bit more responsible and actually test the design before drawing conclusions that are likely to be weak.

For your information -- that thickness on the top and bottom laywer of the board is 3 ounces of copper all of which is plated in gold. The vias in combination with the mounting holes are completely effective in transferring away heat from the LEDs. 

In fact, based on thermal measurements, it is difficult to see how the design would become much more effective than it already is. The heat transfers down to the heak sink just that quickly.

Test it for yourself and post the results here for other to see.


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## VanIsleDSM (Aug 24, 2008)

KeithInAsia said:


> You bad mouth that product quite a bit but you don't show any testing. The bottom line is -- the product works and works well.
> 
> It is true that Luxeon created a design that included more vias with a different placement -- did they product that design and actually use it? My guess would be no because they never published a real picture of it. It's quite possible they didn't expect the LED to be mounted down to any heat sink as there are no mounting elements in that design -- notice that? The had no plan for anchoring the circuit board down. Why not? That would be a factor wouldn't it?
> 
> ...



Appears to me you have some connection with the manufacturer of these boards, which is fine, but if so, why don't you test your boards and publish the actual performance of them yourself? Doing this would quell any bad speculation of your product in the first place.

By looking at the boards, I don't believe that the thermal transfer can be that good.. and there's no way that fewer, larger vias will perform better than more, smaller vias. But I dunno? Lets see some testing


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## Oznog (Aug 24, 2008)

Well it is what it is. If the copper's actually supposed to be 3oz, I'll buy that, my measurement attempt was very shaky.

But fiberglass is of course useless as a thermal conductor, and there are apparently only 4.5 large vias and 2 small vias per Rebel and they are not plug vias. http://www.lumileds.com/pdfs/AB32.pdf gives some idea of the thermal conductivity value of these and it's not exceptional. It can of course "run" and thus "work" but people here are very aware of how critical it is to get a low thermal resistance. People here generally want to overdrive for flashlights. I cited the reasons right there and unless this was some magic type of fiberglass or there are many more plugged vias that are invisible under the plating, then there's no evidence of it having an especially effective thermal design compared to MCPCBs.

The fact that people use them means little. A high resistance board won't keep it from lighting up. In fact let's see... Rebel spec sheet shows that 25C ambient running 1W on a 40C/W sink is STILL about 90% for White. AlInGaP red/orange/amber take a far more significant hit. But there's also a very significant lumen maintenance issue here with elevated temps. Few customers would even be able to notice a 10% or 20% drop in lumens, but it's a problem nonetheless. The AlInGaP colors are especially sensitive and this sort of C/W may have significant consequences. And this is not an exceptional sink for driving at 700mA/1000mA or whatever.

Alright, upon review I would like to retract my calling this a "failed" thermal design. That was uncalled for and I apologize. Plenty of guys here are able to calculate thermal design concerns, and the recommendations of AB32 are right there, and the details of the AS board implementation are there. You guys can pretty well figure out what the C/W will be and decide if it's adequate for your application.

So will it "work"? Yeah, it'll light up. It should be well under the absolute max junction temp when used at nominal current and mounted on an aluminum bar. 
Does it compete with the thermal resistance of an MCPCB as claimed? Seems _highly_ doubtful. 
Can it be driven to 700mA or 1A drive currents? Seems like the thermal resistance here is gonna make that a real bad idea. 
Is it good for RGB? Well, it sure looks like that AlInGaP red is gonna take a significant hit because of its efficiency loss at high temps, which sucks because isn't red at 50lumens @350mA @25C already going to be the limiting factor in how much you can turn it up and still get something "white"? This does not seem like a particularly top-grade solution but it will of course work, and given that Rebels ARE difficult to find thermal mounting solutions for then it's something an engineer _could_ consider.

Has Asian Signals ever actually claimed a specific C/W figure?


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## Oznog (Aug 25, 2008)

BTW, yes the Rebel thermal-via design WAS produced, I do recall seeing live pictures of it and some vendor is selling them.

They are designed for a rear heatsink of course. That's in the tech documents. None of the math deals with a thermal resistance to ambient.

But buying them kinda defeats the purpose. MCPCB is superior and they have 3x Rebel Stars at least so if you're buying someone else's stuff you probably want to buy MCPCB. No, the point here is the designer can mount Rebels in his _own_ design, either integrating with the power supply or making up their own array, without involving a prohibitively expensive custom MCPCB run. That's why Luxeon went into painful detail about how YOU can design your own thermal via boards rather than simply selling a thermal via board.


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## KeithInAsia (Aug 25, 2008)

Having tried both the aluminum metal sandwich board and these copper clad boards, I don't see the advantage of the aluminum boards. 

They have some liabilities. First, most manufactures of those aluminum board don't offer enclosed mounting holes on the boards. That make it difficult right there to mount them. Next, because there is aluminum under the soldering pads, that interferes with solding wires to them. I for one am a little but sckeptical that this exotic bonding between metal substate and insulation layer is going to last. 

You are correct, fiberglass is not a very good conductor of heat. But copper is such a champion of it -- it's doesn't take much to beat the aluminum board. And I have no doubts that a soldered bond is going to last and last.

I'll grant you that I have not run any scientific thermal tests other than looking at the LED and heat sink with an inferred themometer. But by inspection, the results look very compelling.

Now -- (small confession here), I am involved with the product directly but I don't want my comments here become a commerial endorcement and spoil the basic opportunity to add value to the discussion.

What I was about to say is -- we are about to produce board that could not possibily be made without layers. The Rebel Spot board will have 36 LEDs on it in a very tight pattern. You can't route it correctly without a number of copper layers. Now we'll see if copper boards can really compete.

It's funny how there is such a division on the aluminum vs copper issue. People are very motivated to be on one side of the fense or the other.

Me, I just want a board that works -- one that isn't expensive to use and one that I can solder to without any trouble. And as a bonus the gold plated copper gives me a highly reflective surface for free.

I'm sure there metal sandwich board has have a place. I presonally don't like to work with LEDs with an electrically conductive thermal plate. There is risk there. So, that's my position.

This is just a forum, and it's not a real big deal -- but don't knock the product until you've tried it. I mean -- hell, you tore the board open and measured the thicknesses of the layers but you didn't try to acutally use it? That is kind of hurts your credibility - doesn't it. You were looking for faults with a closed mind instead of really seeing if it had any value.

What am I allow to post here in the way of tests before I'm banded for going commercial? Can someone explain this to me?


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## VanIsleDSM (Aug 25, 2008)

As I'm not a staff member of this board, I can't officially answer your last question, all I can say is that for myself, I find it very rewarding when manufacturers will join in on the conversation and offer up some actual testing and information. It's not like you are advertising your product or starting up a thread about it... your product was being discussed, and you took the time to register so you could make a comment.. I think that's great. Even if this is only a forum.. and I'm sure the bread and butter comes from commercial orders of parts for actual products, yet.. I see much development on this very forum. I believe there have been many more than 1 commercial product produced by CPFers, which often seem to always be of the highest quality. I can't help but think that's because the person who finally goes off to create the product is so in touch with what is desired, usually someone who has benefited from the huge amount of collected wisdom on this forum. We may just be a bunch of LED or flashlight nuts here.. but together we have an astonishing collection of information and knowledge.

Rather than much more speculation though, a C/W figure backed up by testing would be the way to go.


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## Oznog (Aug 26, 2008)

KeithInAsia said:


> First, most manufactures of those aluminum board don't offer enclosed mounting holes on the boards. That make it difficult right there to mount them. Next, because there is aluminum under the soldering pads, that interferes with solding wires to them. I for one am a little but sckeptical that this exotic bonding between metal substate and insulation layer is going to last.



Every Star/round MCPCB design I've seen has mounting screws at the perimeter- this requires the board to be thicker to get the pressure without distortion. But they are thick enough to do that so there was never a problem unless you dislike the board thickness.

No one has ever reported a failure of an MCPCB AFAIK. There is a lot of technological drive to make MCPCBs. 



KeithInAsia said:


> But copper is such a champion of it -- it's doesn't take much to beat the aluminum board. And I have no doubts that a soldered bond is going to last and last.
> 
> I'll grant you that I have not run any scientific thermal tests other than looking at the LED and heat sink with an inferred themometer. But by inspection, the results look very compelling.



Well, I think we found the problem right there! It's impossible to get a useful reading off an IR thermometer!

For one, copper foil is a very high lateral thermal resistance- yes, even 3oz copper. The ability to conduct away heat is weak outside of about 2mm and insignificant past about 4mm. An IR therm cannot focus on the hot spot at all.

But the thing is copper, aluminum, and gold reflect IR from "significantly" to "extremely well", so you're not even measuring the thermal IR from the target, you're reading the reflected ambient temp.

The only way to do this is a thermocouple, and a tiny one at that, soldered right next to the pad. It'll still be off because the again the lateral thermal resistance of copper foil is significant. But really you can already do these calcs without ****ing around with funky measurement tricks. Thermal conductivity of copper is known, thermal conductivity of copper vias both plugged and unplugged has been well studied!

You need something to go off of. I mean most of your descriptions are based on what you feel and that's not really something you take to the bank. You need either thermal calculations or actual measurements. 

I mean, what are you even estimating the thermal resistance of this arrangement is? That's a good starting point. Under 5C/W? 10C/W? Under 20C/W? 40... 50... 100?



KeithInAsia said:


> we are about to produce board that could not possibily be made without layers. The Rebel Spot board will have 36 LEDs on it in a very tight pattern. You can't route it correctly without a number of copper layers. Now we'll see if copper boards can really compete.


 
The copper via construction is SEVERELY impacted by thicker FR4. Even with an optimum via layout it's going to have a cost.



KeithInAsia said:


> And as a bonus the gold plated copper gives me a highly reflective surface for free.



The reflectivity of the gold is not as high as you think. Clean white and polished silver can be in the high 90%'s but gold is lower. I have an AS board right here and it's a standard gold plating as expected for a PCB... it's not a super reflection, like 20%-30%. I mean the white silkscreen is brighter right here. It's not gonna matter much either way LEDs throw forward not back, but you're better off with white silkscreen in this regard.


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## Oznog (Aug 26, 2008)

KeithInAsia said:


> It's funny how there is such a division on the aluminum vs copper issue. People are very motivated to be on one side of the fense or the other.
> 
> Me, I just want a board that works -- one that isn't expensive to use and one that I can solder to without any trouble.



Nobody's motivated to bash copper. I bash pseudoengineering claims based on "feel", it's annoying when real engineering is easily available. In fact it's not that you're associate with the seller so much that you're pushing poor design, we'd respond badly to someone talking about how "GREAT our flashlight with DIRECT DRIVE to a 18650 is that doesn't waste power like other drivers!" Sure it works, sort of, but it's poor design and nothing to brag about.

*The obvious thing here is that this board COULD have an effective thermal design- for as much or less than it costs to make them now.* Now if this board actually does have hidden filled-and-capped vias I will eat my words; that would be a nice board- but I don't see any evidence of it. So anyways here comes the Free Advice Section.

*Copper PCBs using thermal vias effectively can be under 5C/W!* The thing is the AS boards clearly don't have a good thermal via design, and the thing is AsianSignals type stuff COULD be that effective at essentially the same cost! *You do not need the exotic "plugged and capped" vias to do this, but the same PTH vias on the board now just a lot more of them.* The exotic plugged-and-capped is needed only if you want those devices to be very close to each other, less than 2mm apart.

Well lemme just summarize the engineering situation here:

First thing you need to understand- *the top layer has such poor thermal resistance it's not even worth mentioning. All the thermal conductivity comes from thermal vias* or the screws. And unless they're aluminum screws then it's pretty poor there too. Anyhow, low via count/low size=high thermal resistance=hot junction=poor LED performance and low life and basically the design is a failure on the thermal front. We DO use large copper areas on PCBs to mount power components, but the situation is different- no heatsink is available so thermal vias only give you access to the backside. That yields >100C/W on small devices like SOT89 when given very larger copper areas (more than a sq in!), and SOT89 looks pretty close to the thermal pad area of the Rebel so really it's a dead-on comparison. 100C/W is helpful for an SOT89 but it's virtually useless for a Rebel unless you're gonna keep it at say 50mA. Now ok, the situation is a bit different in that there is some thermal conduction through FR4 to a heatsink in back, a normal PCB doesn't have that, but FR4 is a good thermal insulator and the cooling through the board is LOW. That's where I came to that point- *the copper area itself is fairly useless for dissipating power LED heat, regardless of copper thickness.*

*All the thermal conduction comes from properly sized vias placed close to the thermal pad.* *33x 0.5mm vias with a 0.9mm pitch all within 2mm of the part, , and 2x 0.35mm microvias right at the pad, on a 1oz copper 0.8mm thick FR4 can yield 7K/W!* Multiple devices cannot be closer than 2mm because their thermal zones will overlap. That's for open vias, the same as are there on the board right now (just not enough of them and the arrangement is poor) and they probably won't cost more from the PCB mfg. Many mfgs don't even care how many holes you put in there, the real time expenditure's in the tool changes. Smaller or larger vias will generally be worse and 0.5mm (20mil) vias is accepted by any PCB mfg at no additional charge. Note smaller vias have lower thermal conductivity yet can't be bunched much closer together because of mfg's Design Rules. For larger vias, going twice the diameter doubles the circumference so that doubles the thermal conductivity of the vias (yay)- BUT you can only fit 1/4 as many vias per sq in of board (boo) so it's a net loss.

*Thermal vias are only effective when placed close to the device due to the high lateral thermal resistance of the copper foil. On 1oz copper, vias outside 2mm have little additional effect. Thicker copper allows you to place more vias further out.* If you _don't_ plan to place vias past 2mm or 3mm, >1oz won't help much and is probably a waste of money. *Thicker copper will do virtually nothing to reduce thermal resistance without thermal vias.* Thicker copper will allow you to place multiple devices marginally closer together because it expands the area where it is still effective to place thermal vias out past 3mm. So you can still find room for 33x vias per device if you still have some board space within range of the device to place more vias. There is a limiting factor- each Rebel needs at least 25mm squared of area for its own footprint and the 33x thermal vias, not including the area for the power connections. Thicker copper does nothing to change this problem: 33x vias around the device or the thermal resistance will go up.

Now look what thicker multi-layer board will do. It doubles the thermal resistance of each via outright, which is bad. We know changing via size is ineffective. We know we can only place 33x within the effective heat spreading area with 1 oz copper. But... with 3 oz copper, the effective heat spreading area is more than 3x larger! Actually closer to 5x! So plan on 66x vias/device to get the 7C/W figure again. This will require you to raise the area per Rebel to at least 40 mm^2 per device, not counting wire connection area, simply because a Rebel with 66x 0.5mm vias cannot be placed in a 25mm^2 space anymore.

*MCPCB is around 5K/W to 10K/W.* That's the figure to beat.

*FR4 has its own reliability problems.* There are thermal stresses here to worry about that can break solder or vias because the copper and FR4 have very different thermal expansion coefficients and change temps at different rates. This is a fairly rare problem situation to encounter though. Luxeon says they tested the design for 1000 cycles without failure (which actually isn't a very impressive number IMHO but they didn't say it failed at 1000 they said they did 1000 cycles without failure). Anyways there are millions of SMD power transistors going through thermal cycling on FR4 every day and they're not failing all that often.

Keep in mind this is of course only a MOUNTING solution, it is not a HEATSINKING solution. But once you've got the mounting and heat spreading basically down, then heatsinking is high school stuff. Don't forget to take temps with an actual thermocouple in the metal this time.

BTW AsianSignals board seems to caliper at 0.9mm. This is thicker than Luxeon's calcs so increase the thermal resistance estimate by 12.5% on that factor alone.


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## Oznog (Aug 26, 2008)

Actually though I gotta say that going through the thermal data's details has been enlightening BTW, I didn't do it just because I can't lose an argument on the net. 

I was impressed to see just how low this design gets the thermal resistance, it really does compete with a MCPCB at a fraction of the price. Plus bare MCPCB board for custom designs is just about unobtanium anyways. 

In fact I can already see a couple of solid opportunities to lower the thermal resistance BELOW what Luxeon used in their designs and beat out the MCPCB for real. This could be kick-***...


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## KeithInAsia (Aug 26, 2008)

*Re: Rebel 4X no, 6X, NO 54X SEE THE COOLING! (Little humor here guys)*

The proof is in the pudding. What do you think guys?.....





























48 degrees C was the hot as this piece would get under normal ambiant temperature conditions and the fan is a nice Delta unit that spins at 8k (which is pretty high speed), but the fan was backed off a few inches for a reasonable test.

The amperage over each LED was 466 mA. You're looking at 6 sets of 3 parallel circuits of 3 LEDs in a series. Each circuit powered at 1.4 amps. The input voltage was 12 volts to nearly match voltage drop (plus a volt or two) so the drivers would run nominally.

The visa on these boards are normal and open. No capping nor pouring.

You know people -- I have been to engineering school. I'm been though all the math, and physics and crap -- but I think the first think you have to do is look at the big picture and see if something looks reasonable. Then you can dive into the details and do the math, etc.

I don't have the equipment here to correctly cacluate the thermal efficiency. I would like to get that someday (along with an accurage lumen meter). So, I can't claim any numbers here. But, I can assue you that it works fairly well -- even under an extreme test like this. And I think 54 LED is a pretty decent test.

It think the top plate and the stainless steel screws do most of the work.

Copper is a winner in many ways. If the fiberglass could reduced, then this could be even better. Why doesn't someone produce a copper sandwiched board with a few fiberglass layers to run the power?


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## Illum (Aug 26, 2008)

*Re: Rebel 4X no, 6X, NO 54X SEE THE COOLING! (Little humor here guys)*



KeithInAsia said:


> The proof is in the pudding. What do you think guys?.....



I think you've went off the deep end my friend:naughty:


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## VanIsleDSM (Aug 26, 2008)

the faults of using IR thermometers were already discussed.

You have to measure nearest to the LED junction as possible with a thermocouple, or use the voltage droop method. Proving that the heatsink is relatively cool doesn't prove anything about your boards. In fact showing that the heatsink isn't very hot, only proves that they may not be doing a very good job of thermal transfer into the heatsink.

As far as thermal vias and FR-4 go, yes, they can be much better than expensive MCPCB.. I'm really not sure why there aren't more FR-4 mounting solutions. But FR-4 thickness is key, and so are lots of small vias.

Check this out:

http://www.molalla.net/~leeper/mcpcb1.htm

"Using vias as thermal transfer points can be quite effective. You could make the board 39 mils thick (1mm), and do nothing more than put only 9 vias of 0.5mm in diameter, and still end up with a thermal resistance of 9 C/W. If you reduce the board thickness to one of the standard pre-preg board materials, like the 3 mil thick material (0.0762 mm), you end up with a thermal resistance of only 0.69 C/W."


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## Oznog (Aug 26, 2008)

Ya I just tried this out myself... putting a bar of aluminum stock on the stove and the thermocouple shows it at 250F. The IR therm thinks it's 115F! Ya right. There's a bit of blue paint overspray from the end markings on the bar and the IR therm jumps to 150F when you get into that area just because the emissivity of paint is compatible with the IR therm's functionality.

BTW, one other side note- stainless is a _really_ poor thermal conductor. 1/17th the thermal conductivity of aluminum. That really limits their effect.

You're not measuing pad temp. The only indication of pad temp is really really close (<1mm) from the edge of the Rebel. Even then, it's quite different. Note that heatsink temp doesn't even change with junction-to-sink thermal resistance anyways. A 1W device on a 5C/W sink at 25C ambient gives a heatsink surface temp of 30C regardless of whether the mounting is 5C/W or 50C/W. But in the second case the junction is 45C hotter!

Darn it, I wrote up a detailed analysis of what that square board is but the board dumped the post!!! DAMNIT!!! Anyhow, I went to a lot to show that board comes to about 43.75C/W- and that was being generous on several fronts! I'm really pissed the board dumped the post. Under 10C/W is "normal". Ya this board provides only a 0.4mm boundary around each device which is far less than the 2mm minimum, has an inexplicably variable number of vias per pad- about 8x really, whereas the optimum is 33x. And it's a 1mm FR4 not 0.8mm. Luxeon already had shown that dropping the outer vias and outer copper pad area puts a design ">30C/W". Increasing board thickness by 25% and halving the thermal vias brought me to 43.75C/W.

Running a board at 700mA on a 5C/W heatsink will exceed the absolute max junction temp (165C at 25C ambient) and normally we don't want to be anywhere near that. In fact, unlike simple poor heatsinking, you can't even briefly go that high because of the low thermal mass on the junction side of the thermal resistance. The junction temp will probably shoot up above absolute max in a second or two.

At 350mA, your junction temp is a permissible 84C but that's assuming there's no enclosure, the ambient is maintained at 25C, and... no way is your heatsink giving 5C/W on 54x devices. You'd need 0.09C/W total to give that to each device if they're all in use. Your sink is a fine choice but still far far above that. And I also didn't try to estimate board-to-sink resistance which is a good question since the offset screw may actually reduce board-to-sink pressure as it's tightened due to board distortion.

Basically that extra 35C die temp you're adding at even 350mA (1W) with this design over MCPCB is losing about 25% of the light output of AlInGaP reds. And it's really bad for lumen maintenance and lifespan. And it's actually quite an expensive solution, I mean saving 50 cents on boards but losing 25% of the light output on nearly $10 of RGB LEDs?

FR4 board CAN perform well. Very well! Just get 0.8mm FR4, 1oz copper, 33x thermal vias in a 25mm^2 area around the Rebel. NO THICK MULTI-LAYER BOARDS without many more vias and thicker copper. That's literally all you need to do to go from this really poor solution to an excellent one. And the gold plating is a waste of money frankly, if you do it do it for marketing reasons if you think it we be well received because it looks "cool". In truth it has no electrical or thermal benefit and doesn't have the optical advantage over silkscreening you talked about.

At 7C/W, it's very competitive to MCPCB. The insulated MCPCB, like the P7, gave 6-something C/W IIRC. It's not outstandingly BETTER but it's just fine. I mean once you've fixed this >40C/W problem 7C/W vs 6C/W is a foolish argument.


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## R33E8 (Aug 26, 2008)

Oznog said:


> Ya I just tried this out myself... putting a bar of aluminum stock on the stove and the thermocouple shows it at 250F. The IR therm thinks it's 115F! Ya right. There's a bit of blue paint overspray from the end markings on the bar and the IR therm jumps to 150F when you get into that area just because the emissivity of paint is compatible with the IR therm's functionality.
> 
> You're not measuing pad temp. The only indication of pad temp is really really close (<1mm) from the edge of the Rebel. Even then, it's quite different. Note that heatsink temp doesn't even change with junction-to-sink thermal resistance anyways. A 1W device on a 5C/W sink at 25C ambient gives a heatsink surface temp of 30C regardless of whether the mounting is 5C/W or 50C/W. But in the second case the junction is 45C hotter!
> 
> ...



um...

Well there is a fan blowing onto his heat sink... No one here except for him knows how much air its pushing... But I do agree that the aluminum may be giving wrong readings to the IR temp sensor... I would suggest painting the heat sink with a thin coating but complete of flat black paint and remeasure.. Or use a thermocouple.. But we would also need to figure out how well the heatsink performs and how much air is being blow out of it... The only way I see us figuring that out is with some finite analysis programs like Ansys or something..

and if I remember correctly, the thermal resistance lowers at higher temperature differences.. While trying to find info to back myself up, I found this:


> LRC researchers found:
> 
> 
> Thermal resistance increases when the power or the ambient temperature increases.
> ...


http://www.lrc.rpi.edu/programs/solidstate/cr_thermalresistance.asp

I have no idea how you figured out a thermal resistance of 43.75C/W, without some finite analysis and an accurate model of the board..

I believe the only way to truly end this argument is through testing the die temperatures on the cold/hot plate with an oscilloscope.. Of course you would need like 3 or more each PCB and take an average to get the final result..


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## Oznog (Aug 26, 2008)

Sry my big post was killed. The way I got to 43.74C/W on the 3x square board:

AB32 lays the groundwork:
0.8mm FR4 31x 0.5mm thermal vias on a pad within 2mm around the device with 2x 0.35mm vias= 7C/W
Removing the 14 outer vias, leaving 17x 0.5mm with a 2mm copper boundary= 8C/W
Omitting the outer vias and the extra copper around the pad: >30C/W

Square board:
1mm FR4. ~8x 0.5mm vias and 2x 0.35mm vias per board. Rebels are 0.8 mm from one edge to the other so it's a 0.4mm boundary owned by each device. The width of the common thermal strip is ~3.3mm. 

So the situation is it did indeed omit the outer vias and copper boundary. Well, there IS a 0.4mm boundary left, so I gave it 20C/W which may be overly generous.
There as only half as many inner vias left. But at this point I think the conduction through the FR4 is the dominant factor anyways, I gave it 35C/W for that. On second thought I might have overestimated the effect of the loss of half the vias but anyways that's what I used.
The 25% thicker board increases the via resistance AND the FR4 resistance by 25% flat-out. That brought it to *43.75 C/W*.

Is is exact? No... it's a a fairly rough guess but it shows the problem. There is no magic going on here. Even if I was grossly off no way is it anywhere near 10C/W.

It is EXTREMELY difficult to measure pad temp accurately when you can't access the pad. I mean putting a greased thermocouple on the copper next to the pad already has a major lateral conductivity factor through the copper foil and a major dissipation factor on the probe. That is, you've got a probe with 0.2% of its surface area touching the area to be measured and 98.8% of it being cooled by ambient air. Fortunately the thermal conductivity of the air is relatively poor, but these fine-point readings are of pretty questionable accuracy.

Heatsink readings, as previously noted, are completely irrelevant for determining mounting thermal resistance_ even if _you measured them accurately with a thermocouple not an IR shooter.


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## Oznog (Aug 26, 2008)

Also if I read your description and trace the picture correctly 3 of the 3x series strings were put in parallel without any ballast resistance. That's very prone to LED destruction due to the negate temp coefficient causing runaway, in fact it's nearly inevitable.

You may think that placing the paralleled strings on different parts of the sink would get around that. In fact, it intensifies the problem. See you can put 2 LEDs straight in parallel IF there's a very tight thermal path between them. Lamina BL2000's did that, so do the SSC P7's. Having paralleled Rebels soldered near each other onto the same shared piece of copper foil should do it. But placing them on separate boards and on separate parts of the heatsink will NOT be conducive to paralleling. 

Actually just the fact that they're on separate boards already kills the idea of paralleling. I'm just saying that if you did have a theory that separating them on the sink fixes paralleling problems- which seems like it could be the case because the wiring is a bit more complicated when done this way so I think that's why it was done- then no, it won't.


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## Oznog (Aug 26, 2008)

BTW- actually you certainly CAN make an RGB board with large numbers of devices on a 2-layer board.

The problem you surely encountered is that 1) the soldermask over the traces is not sufficient to insulate against mechanical contact with the heatsink, and 2) the soldermask spaces the exposed copper thermal bottom off the heatsink.

The solution is obvious: size the board so that it overhangs past the heatsink. Leave no soldermask in the area covered by the sink. Do all your routing that needs the bottom layer in that outer zone with no heatsink beneath it. There is no reason you can't route out there.

The problem of course is that >2 layer boards become thicker. Thermal resistance is simply the reciprocal of the thickness for the same via and copper pattern. So you want to avoid this at all costs.


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## Oznog (Aug 26, 2008)

Well like I said this got me interesting in thermal calcs.

Given that copper has a thermal conductivity of 401 W(m*K), and a plating thickness of 0.035mm inside the via:

0.5mm via is 49C/W on a 1mm board
0.35mm via is 72C/W on a 1mm board

The lateral thermal conductance down a 1mm wide trace is 70C/W per mm of distance on 1oz copper, or 23.3 C/W on 3oz.

I am skeptical of these claims of 3oz copper. I think I've got a fairly good measure here of the square board showing the thickness of both sides' copper is 0.006" combined, or 4.3oz total. Could it be 3oz on top and 1oz on bottom??

So... well, that's not all that easy to calculate from unfortunately due to the complexity of integrating the lateral path. 

Well, I get... let's see, the 0.5mm vias are 0.25mm apart in a row. Due to the row configuration there's no integration problem. I'm kinda seeing the path distance here as ~0.15mm, which would rate the thermal resistance to each via at 14C/W. 

Huh, well I dunno then. That would place the thermal resistance per via at 63 C/W and 8 would bring it down to 7.875C/W. Actually by rating the 2x 0.35mm centers as zero lateral resistance I get 6.278C/W. 

I have reason to doubt that calculation. By Luxeon's own thermal modeling they got 8C/W on the 1oz 0.8mm board (thus 10C/W on a 1mm board) with the 17 close-in 0.5mm vias and the 2x 0.35mm. So taking the 2x 72C/W 0.35 vias that leads us getting only 250C/W for each of the 17 other vias. If the via itself gave 49C/W then the path to the via would be 201C/W which is too much of a difference to account for with just the thinner copper- that would only be 42C/W. I'm probably skipping over something significant.


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## Oznog (Aug 27, 2008)

I may have used a high number for the copper. Elsewhere I saw 355 W/(m*K) in the PCB context. The condition and temp range of the copper does matter and maybe I didn't have the most accurate figure. 

But I wanted to figure FR4 conduction too. So the figure I have for that is 0.25 W/(m*K) from a TI app note. I've seen .36 and .4 mentioned elsewhere but anyhow I'm looking at TI's number.

So I get... 4000C/W per sq mm on 1mm board. Well 40 C/W per sq cm. Ya hardly good conduction but it does have an effect.


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## KeithInAsia (Aug 28, 2008)

Oznog, your logic is weak on a number of points. I'm not going to have enough time right now to hit them all...

I'll try to hit the a few of them.....

You say the Gold plating is a waste of money. I disagree. Coper must be coated to prevent oxidation. Gold is a great conductor also, it's better than tin. It provides a very smooth surface for good contact (Tin does not). It's nicely reflective which will help inside some light fixtures. This idea that gold plating is throwing off my thermal reading is highly dubious. What I read thermally is what I feel by touch as well. The measurements make sense. Looking into the front of the unit, you will get an average of the sensor area. That is to be expected. By inspection, the unit is not much hotter up front than it is in back or on the sides. That is an effective solution. You just can't knock a common sense observation like that. I don't care how many numbers you calculate. 

Let consider a very simple idea..... Look at that bottom of a Luxeon Rebel. Just how big is the surface areas of that thermal pad? It's very small. It looks to be about 2mm x 1.5mm. Just how much heat is going to travel through that point? All I have to do is match and slightly exceed that thermal area. That's it. I don't need a dozen vias to effectively match that thermal path on the LED.

Coppy is very effective -- it just is. 

For my circle product (a sample seen earlier in the thread) where the screw holes bond to the top and bottom plates -- I'll bet you that I don't even need visas on that board for it to work well. 

Fan speed on this test below means almost nothing related to the argument. If the heat sinking solution is good, that heat sink is going to get hot. 54 Rebels at 466mA pushes some heat -- it most certainly does.... The whole heat sinking design does get work the whole unit would over-heat. In this case whole unit warms up very evently -- that suggests that the boards are effective.

You said you had a sample -- correct? You tore one open right? Do you still have a good one to use in a real test?

If not, I'll send you a small handful if you promise to post the results here. How do I contact you for an address?

Will you run a real test? 

Throw your thermal couple on there as you like. Let's see the real results and not a long laundry list of conjecture.


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## Gomer (Aug 28, 2008)

KeithInAsia said:


> If not, I'll send you a small handful if you promise to post the results here. How do I contact you for an address?
> 
> Will you run a real test?
> 
> Throw your thermal couple on there as you like. Let's see the real results and not a long laundry list of conjecture.




If only Tullio read this and took a hint lol (OT from another LED+reef light situaiton)


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## Oznog (Aug 28, 2008)

Copper does not need to be protected against oxidation where it's soldered onto of course. The rest can be protected by mask or silkscreening or tinning. Actually mask IS much better than gold plating for protecting against the environment, but that's mainly gonna come into play if it's exposed to moisture which really shouldn't happen anyways.



KeithInAsia said:


> Look at that bottom of a Luxeon Rebel. Just how big is the surface areas of that thermal pad? It's very small. It looks to be about 2mm x 1.5mm. Just how much heat is going to travel through that point? All I have to do is match and slightly exceed that thermal area. That's it. I don't need a dozen vias to effectively match that thermal path on the LED.



Who told you this? Seriously, who?? The board is an insulator. The Rebel makes a fair amount of heat yet cannot be allowed to reach high temps. We really really need low thermal resistance.

The mfg never intended those Rebels to be mounted on a single-sided board without vias for example, not at full power. That wasn't their idea. Of couse even a single-sided board with a wide copper land is vastly improved than running the Rebel bare (which would burn it up in a second or two)- but no one would ever run a Rebel bare that's not relevant. Doesn't mean it's adequate. I mean doubling the area of the pad isn't twice the minimum cooling it needs. It's still many times less than the absolute minimum to run at even 350mA.

Spreading the heat only dissipates it through the board or off the top surface itself- the top surface's thermal resistance to air is way too high and it's totally ineffective if it's got a cover over it.

If the screws are stainless then the problem is that the stainless is a terrible thermal conductor. Like bad. 20x-30x less than copper. And unless there's thermal grease between the screw and board there's a significant boundary layer. I guess you could put grease in there but... well, stainless is just not an effective thermal conductor on this scale.

So if you discarded the vias you're limited to board conduction- which is 36C/W per sq cm on 0.9mm FR4. The round doesn't have anything like that. It's got under 1/2 a sq cm per device. So over 72C/W conduction through the circle board if it didn't have vias and we assumed the copper was so thick that the spreading is "perfect". That's terrible.

Again, we need 10C/W to be "good". That is, if you want to run it at full power, maybe a bit more, with a good heatsink behind it.

As we talked about- it is nearly impossible to actually measure the pad temp with any accuracy. I tried the delta-Vf method once but was somewhat disappointed, it's very small and hard to read. With photometric equipment, yeah, you could see the degradation in output as die temp increases, that'll be fairly clear on AlInGaP types. Perhaps you could engage someone here with that equipment?

It's not the board cost, but packing 6x Rebels on there is a fairly significant cost. And this is kinda silly because you're talking "common sense" based on feel- which doesn't mean much- and bogus ideas of measurement like heatsink temps and IR thermometers. Of COURSE gold plating reflects IR, plus the area is far too large. The measurement method is nonsense.

Vias are everything. Add vias. Many, many vias. For the most part the thicker you can place them the lower the thermal resistance, but eventually it may mechanically undermine the board's strength or the mfg's Design Rules Check will kick out your submission for not following the rules.

So what are you claiming these are? If a copper board is 5C/W or 10C/W, then are you saying it's less? What power level are you claiming they can sustain by design? 1A? 750mA? 350mA? Because I feel the way this is going to go is simply going to be you revising it to "well, obviously they work fine at >100C die temp because they're in use that way".


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## KeithInAsia (Aug 29, 2008)

"Copper does not need to be protected against oxidation where it's soldered onto of course. The rest can be protected by mask or silkscreening or tinning."

I disagree -- The entire bottom of the board is touching the heat sink. You don't want a layer of Tin down there. Tin is a poorer conductor of heat when compare to copper and gold. Without the gold you will get oxidation on the copper -- that is negative. You also need a very smooth surface on the bottom. Tinning does not render that.

Quote:
Originally Posted by *KeithInAsia* 
_Look at that bottom of a Luxeon Rebel. Just how big is the surface areas of that thermal pad? It's very small. It looks to be about 2mm x 1.5mm. Just how much heat is going to travel through that point? All I have to do is match and slightly exceed that thermal area. That's it. I don't need a dozen vias to effectively match that thermal path on the LED.

_"Who told you this? Seriously, who?? The board is an insulator. The Rebel makes a fair amount of heat yet cannot be allowed to reach high temps. We really really need low thermal resistance."

Are you following me at all? I said -- there is a very small thermal pad on the bottom of the LED that is mainly responsible for removing the heat from the die. All I have to do is match that thermal pathway. 
As small as it is -- the challenge is not that great. That is what I'm saying..... You disagree in principle?

"If the screws are stainless then the problem is that the stainless is a terrible thermal conductor."

The screw hold the gold plated copper to the heat sink. This screw is the correct choice (although titanium might be a better choice). I'm extremely dissapointed with those who product the "Star" for providing such a terrible mounting method. How do those stars sit flat with nice even pressure when the screw holes are so hard like up with the product? I have never been able to get it just right. 

"So what are you claiming these are? If a copper board is 5C/W or 10C/W, then are you saying it's less? What power level are you claiming they can sustain by design? 1A? 750mA? 350mA? Because I feel the way this is going to go is simply going to be you revising it to "well, obviously they work fine at >100C die temp because they're in use that way". "

Are you stuck on numbers? Thermal resistance has not been caclulated. I make the claim that the board works. I don't know the actual numbers. 
But, if you're the expert on doing these kind of measurments - I'll let you give it shot.

Tell us you're more than just a "armchair quarterback" type. Tell us that you'll take the challenge and I'll send you enough samples of the board to run a fair and complete test. Then all this guess work will be over....

Or -- are you only firm and convinced on what you "think" is correct? 

Come on now -- I'm putting the challenge in front of you. Do you have the time and resources to find out how well the "rubber meets the road" on this little board?


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## TigerhawkT3 (Aug 30, 2008)

KeithInAsia said:


> ...[snip]
> ...there is a very small thermal pad on the bottom of the LED that is mainly responsible for removing the heat from the die. All I have to do is match that thermal pathway.
> [snip]...


It's true that you can't do much to increase the contact between the die and the heatsink, but the heatsink does need to have some combination of 1) enough mass to sink the heat from the die and 2) a method (fins, fan, heatpipes, Peltier, whatever) to remove that heat from the heatsink fast enough to keep the whole thing from getting too hot for the LED(s). I'm sure that you knew all that and meant something different, but I probably misunderstood it. Could you clarify what you meant?



KeithInAsia said:


> ...[snip]
> Are you stuck on numbers?
> [snip]...


I should hope so. I'd much prefer some (accurate) figures over "seems to work okay" or "seems not to work okay."

Oh, BTW, does your IR thermometer have adjustable emissivity? If you know the emissivity of your boards' surface coating, you could get pretty good readings with it. If you let the device run until the temperature stabilizes, you could get a reasonable estimate (just a tad low) of the temperature of the LEDs, AFAIK.


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## KeithInAsia (Aug 31, 2008)

"It's true that you can't do much to increase the contact between the die and the heatsink, but the heatsink does need to have some combination of 1) enough mass to sink the heat from the die and 2) a method (fins, fan, heatpipes, Peltier, whatever) to remove that heat from the heatsink fast enough to keep the whole thing from getting too hot for the LED(s). I'm sure that you knew all that and meant something different, but I probably misunderstood it. Could you clarify what you meant?"

This LED board is not designed to be any kind of full heat sink. That task is solidly placed on the heat sink for which it is mounted on. So, the primary objective is to act as a conduit to pass heat from the LED to the heat sink below. Just how much mass and density would you say it takes to make that thermal conduit?


I am not stuck on numbers..... It would nice to have some of those numbers, but even numbers are just a arbritray measurments. Once you have those "numbers", then you have to work them over for stastical accuracy and experimental error based on calibration of your measuring equipment and the inherriant affects of the measurement equipment has on the system that it is measuring, etc. Getting numbers that everyone really respects is fairly difficult.

Remember all the old claims of Hi-fi equipment sellers back in the 70's who boasted about how many watts they had at the speakers and how low the distortion was in db's? Who was really right and who was just fudging "the numbers".

I'm telling you -- you first look for a common sense observation to point you in the right direction.... and my guess that this board is looking very good ise being suppported by the fact that this board is not hot to the touch anywhere on it. And I'm touching as close as I can to the emitter.

Now, I had a few lenses fall off - and then unknowingly I touched the emitter -- that burned me. That was painful -- so certainly that area is above 100C. But no were else did I feel any discomfort.

My inferred thermal measuring device reads perfectly when I aim it as my skin or at boiling water ... there is no mysterious "reflection" error going on there.....

In fact, if you look at the picture below, any reflection would only artificially increase the reading -- would it not? That 48 degree reading is influented directly by what is being seen right inside the die - is it not? (and just for the record I moved my sensor around to achieve the highest reading possible)

So 48 degrees is a pretty good number generally.

We all have been heavily influence and brainwashed to think that metallic sandwich boards are superior because they were first on the scene for LEDs. That was directly influence by LEDs which all had an electically active base. 

Many if not most of the newer LEDs are coming out with electrically isolated bases, so why not modify our thinking about the type of board that it should sit on?

Here is one last point.... Will my design be affected by the heat sink that is under it? If I mount on aluminum (as seen in the previous pictures) and then I mount a group of the same LEDs on a copper heat sink -- which group will function with great cooling? All thing being equal - a copper heat sink will make my board work better..... So now what? Oh -- the board is ok now? No -- the board just the same as it was.

What if I use a more superior heat sinking compound between this board and the heat sink? Will the board be better? It's still the same board.

So, I think we need to look at the entire picture.

Having a guideline C/w measurement for this board would be interesting, but it not the only factor in the over-all LED heat sinking system. So, tell me again why we can't consider anything else without those C/w numbers?


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## VanIsleDSM (Aug 31, 2008)

Well.. a die temperature over 100C, and a heatsink temperature of 48C. What does that tell you about well your boards are conducting heat?

I agree MCPCB aren't the best, that's why I built these boards:







Solid copper for the heat path. If you reduce the thickness of your FR-4 and add many more tiny vias.. then you'd had a really nice product I think.

But as it is.. without using numbers, and just my common sense as you like to do.. I think the board, as it is, will not conduct heat near well enough for the kind of performance I am looking for.


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## Oznog (Aug 31, 2008)

But what would I be testing? Suppose this shows up at 20C/W. You're going to say it "feels cool to me" and "the LEDs are blinding bright" and "it's in use and works fine so whatever number it is it's GREAT cooling!" These claims are nonspecific. Do you want to claim it's superior to MCPCB at least? Better than the 5C/W-10C/W range? How much better do you think it is? What power level do you claim these can be run at continuously or peak?

I did think about it... if I score the top and bottom copper into 6 pie slices, I can isolate the thermal performance of each device pad. That would accurately show the thermal performance without loading the board up with 6 devices. I don't have 6 to spare.

The IR therm wouldn't work even if you could adjust the emissivity unless you also had one which could focus to a fine point. The temp at the pad is the only relevant temp and an IR therm 1" away has a measuring zone at least 100x too large to give any information on pad temp or thermal resistance. It's reading temp across the board and LED face which doesn't tell you anything (and the emissivity question wasn't resolved anyways).

Yes C/W is everything as far as thermal performance goes. Cheap and easy to mount and high component densities can of course be good features, but thermal performance is all about C/W to the heatsink.

Actually the HiFi sellers are a good parallel- sure people can give false numbers but anyways guys here won't buy it. The alternative was numberless descriptions of the results of how wonderful $1000 8-ga silver-plated oxygen-free Litz-wire speaker cables "sound" when electrical and acoustical analysis shows no difference whatsoever. And you're kind of in that class unfortunately, going on what you "feel" and see as "common sense" theories about thermal conduction.



> I said -- there is a very small thermal pad on the bottom of the LED that is mainly responsible for removing the heat from the die. All I have to do is match that thermal pathway.
> As small as it is -- the challenge is not that great. That is what I'm saying..... You disagree in principle?


Yes I disagree. The pad on the Rebel is already a restriction, not too bad though. However, any additional restrictions will increase die temp and reduce the power level that can be sustained as well as the efficiency and life. For example, the junction-to-pad resistance on the Rebel InGaN is 10C/W. The MCPCB is maybe 8C/W, a moderately large sink 5C/W. That's a total of 23C/W. My point is that even though the MCPCB and heatsink were a lower resistance than the pad, they contribute heavily to the bottom line total for thermal performance. Thermal transfer performance is a cumulative number, the performance is not simply equal to the weakest link in the chain.



> I'm extremely dissapointed with those who product the "Star" for providing such a terrible mounting method. How do those stars sit flat with nice even pressure when the screw holes are so hard like up with the product? I have never been able to get it just right.


I don't understand. The Star mounts just fine! It has holes, the AS board has holes, and either one needs alignment of PCB holes over the heatsink holes. It actually had 6 and doesn't necessarily need all of them you can just screw down 4 in most cases- or 2 or 3 probably. Could you describe this problem?


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## TigerhawkT3 (Sep 1, 2008)

Well, I typed up a huge reply (epic, really), but the board ate it. "You have logged in since loading this page," or something. Never seen that one before.

I'll try to make a condensed version.

...

Okay, I read the preview of the condensed post, and it came off really argumentative. I'll keep this short and sweet. Here's draft 3:



People do care about measurements (just look at any datasheet, product manual, etc.). We also care about the methodology used to obtain those measurements. "Fudging the numbers" is not entirely dependent on there being numbers attached. Someone could say, "these speakers sound great" and be lying. The point is that we hope that people don't lie.

A finger is not a very good thermometer.

A more reflective surface (lower emissivity) makes for a low readout in a fixed-emissivity IR thermometer. The material will be hotter than the readout shows, as Oznog's experiment with the aluminum and the stove demonstrates.

If an emitter is hot to the touch, so should be the board and possibly the heatsink (depending on distance to the emitter, size of the heatsink, etc.). If an emitter burns you but the surrounding area feels fine, that means the heat is staying in the emitter.

Different heatsink compound, different heatsink material, etc. may help the performance of the system, but they don't affect the performance of your board.

C/W seems like one of the few relevant quantifiable properties of such a product as these mounting boards. I still can't understand why you don't care about it.


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## frenzee (Sep 1, 2008)

Here's my 2 cents:

Your finger is definitely not a good heat sensor. A rebel or Cree running at 1 amp will easily burn your finger if you put it right on the dome, no matter what the jT. There is almost 1W of energy coming out of a area the size of a match head.

An LED is probably not the best candidate for testing a board's C/W. Why not use a metal film resistor where you can accurately measure how many watts of heat is being generated? Of course it'll have to be properly thermal epoxied or soldered to one side of the board and insulated from the surroundings air to mitigate the effects of emissivity and convective cooling, and you'll need two thermocouples, one on each side of the board.

Infrared thermometers typically have a 15- or so degree viewing angle and basically take an average reading of whatever comes in within that angle. So unless you are looking at a surface with a uniform temperature, you can't really tell what the peaks and valleys are. The only way to get an accurate reading, in the context of an LED, would be to use a digital thermograph and those things are pretty expensive.


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## Oznog (Sep 1, 2008)

Here's an example of thermal vias for a Cree, that's a 1W nominal device just like the Rebel:


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## Frobe22 (Sep 1, 2008)

Bare metal acts like a mirror when you use IR thermal measurement. (You may even measure your own skin temperature if you have 90° angle).

When I use a thermal camera (superiour to a simple IR thermometer) I have to add coating or a thermal pad to metal surfaces, otherwise they just show up at room temperature or reflect any hot surroundings. The pad/tape/paint shows up in the display as a bright beacon compared to the dark bare metall. I know it is possible to adjust for emissivity, but most metals don't have a specific number, so the easiest solution is to "patch it" to get correct temperature.

Glass is a barrier to IR Thermal measurements, and will show up at room temperature. Cree LED lenses are glass, but I think Rebel use some type of silicone?

Optimal junction temperatures are below +80°C, and maximum are about +150°C.


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## KeithInAsia (Sep 2, 2008)

Frobe22 said:


> Bare metal acts like a mirror when you use IR thermal measurement. (You may even measure your own skin temperature if you have 90° angle).
> 
> When I use a thermal camera (superiour to a simple IR thermometer) I have to add coating or a thermal pad to metal surfaces, otherwise they just show up at room temperature or reflect any hot surroundings. The pad/tape/paint shows up in the display as a bright beacon compared to the dark bare metall. I know it is possible to adjust for emissivity, but most metals don't have a specific number, so the easiest solution is to "patch it" to get correct temperature.
> 
> ...


 
Bare with me people, I have a few posts to make....

I hear your point, but I also had my hands on it. The reading made some sense. Could they be a few degrees off? Yes, but you have to look at the general results and factor in the experiemental error.

The results are that the front of the unit close to the LEDs were not that much hotter than the back of the unit.... Conclusion -- heat sink and mounting PCB are working in a satisfactory way.

There is no trick photography here.

And granted, I do not have a method to measure the junction tempertures. Not many people do.


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## KeithInAsia (Sep 2, 2008)

Oznog said:


> Here's an example of thermal vias for a Cree, that's a 1W nominal device just like the Rebel:


 
This generally is a decent design, but I would have done it differently.

I think putting the screws far away from the die is a mistake. 

Also, the thickness on this copper looks rather thin. You can see thick copper by the change in light reflection next to the traces. LED Dynamics favored a more standard copper thickness and compensated with a larger number of vias.

Here is a big mistake. They should have never interrupted the flow of heat away from the LED by putting traces around it. That boxed in their design. Traces should have exited the design left and right and heat plane should have extened out to the screw holes. They were more concerned with having + and - outlets on both sides of the board over having a better heat sink.

I also would not have insulated the thermal plane with a solder mask. I favor gold plating so that heat has a chance to radiate from that area.

You may disagree with my points. These are just the things that I see.

I would reduced the number of visas and increased the copper. In fact, I in about 3 weeks I will have a 22mm square board that will mount this LED. And I will have 22mm designs to accept XP and MC-E units as well. Each one with the same approach with heavy copper, mounting holes closer to the die, minumum solder masking, and fewer vias.


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## KeithInAsia (Sep 2, 2008)

frenzee said:


> Here's my 2 cents:
> 
> Your finger is definitely not a good heat sensor. A rebel or Cree running at 1 amp will easily burn your finger if you put it right on the dome, no matter what the jT. There is almost 1W of energy coming out of a area the size of a match head.
> 
> ...


 


Well, granted my elbow would be a better heat sensor... (you guys know this don't you? This is the reason you see guys squirting baby milk on their elbows in old movies. We have better thermal nerve endings down there.)

I think my fingers do just fine for a raw inspection....

And NO - I don't feel any over-heating when I touch a lens dome of a Rebel. Not at all. There had been a couple of domes that I knocked off and then touched the actual die. That was painful. I don't then any mounting technic can prevent that.

The idea of testing with a metal film resistor may be interesting. What is the determination of watts? Do you measure the voltage drop and assume all waste energy is being turn into heat?

I agree with the digital thermograph. I'd love to have one those because you could see the full spread of heat dissipation. I agree fully.


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## KeithInAsia (Sep 2, 2008)

Oznog said:


> I don't understand. The Star mounts just fine! It has holes, the AS board has holes, and either one needs alignment of PCB holes over the heatsink holes. It actually had 6 and doesn't necessarily need all of them you can just screw down 4 in most cases- or 2 or 3 probably. Could you describe this problem?


 
I never seem to get my mounting holes in the right place and the screw can easily be off center and hanging off the edge of the plate. I use a drill press and my holes are staight, but they are never on center and these open holes don't natrually allow you to center up your screw to any degree.

If there was a full encapsulated hole there, a hole drilled slightlyl off-center just a bit would kind of self correct.

So, that is just a gripe. I like fully encapsulated holes and further -- I would want a drilling template to nail those holes with some precision.

Correct mount of screw is a significant contribution of good surface contact and good cooling.... 

I provide drilling templates with my products that accept a 1mm hole so anyone can get the hole drilled correctly. They tap a start with a 1mm bit and then follow up with a 2.7mm bit to make the final holes. All screws are on center and perfect. That is thinking ahead and giving the customer the best chance to do a great job.

I fault the companies who make the "star" boards with lack of quality in this regard. The are all designing like zommies following each other down a bad path. Are the bean counters making them optimize the surface area out of these designs?


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## KeithInAsia (Sep 2, 2008)

Oznog said:


> Thermal transfer performance is a cumulative number, the performance is not simply equal to the weakest link in the chain.


 
Agreed, every link in the chain has an effect, but the weakest link in the chain is the bottle neck to effective heat sinking.

I don't need a hunderd visas to keep up with the smaller pads on these Rebel parts. I believe there is a dimishing return after a certain point.....


People -- since none of you have actually seen this product in action, you are somewhat disadvantaged in your perspective.

Further, I see no takers to my offer of a few free boards to make your own real tests.

I think this is rather foolish and short sighted to tear a board part and comment about it's strengths and weakness when in fact you never put it through a real test.

Thats like pulling the wheels of a Ferarri, slicing a cross section of a tire and saying that it can't possibly accelerate from 0 to 60 in 4.3 second because the tires shouldn't work under those conditions.

You alll sound very close minded.

Here is a little reality check about the supposed unbeatable metal PCB.

At some point there will be a delamination of the layers. Its going to happen. Obvoiusly at this point it won't matter how good the C/w thermal conductance it.

What I think is interesting is that the diaelectric layer looks very much like fiberglass in nature. Hum....

I suppose this could be my fault since I soldered a fairly large wire onto the pads. This weight could have stressed the top layer. Of course there could have also been over factors of wind. Or moisture worked its way in between the layers, etc.


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## KeithInAsia (Sep 2, 2008)

You built this for the MC-E right?

One the surface, a slab of copper of course is unbeatable for heat sinking. You will have some trouble heating this this enough to solder the LED down to it. That is going to be a bit challenging....The solder will be over-temp for long periods of time driving the flux out of the joint.

But I think you have another issue lurking here....

Pan head screws are going to short out your traces when you screw this unit down --- Or am I missing something? Is there a particular reason that you placed the traces right on the holes?


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## snarfer (Sep 2, 2008)

It seems to me that an easy (and cheap) way to test out boards would be to use the relative forward voltage of the LEDs. Heat the entire assembly to various temperatures and measure forward voltage at a known current. You would have to pulse the current for very short time in order to avoid allowing the LED to heat beyond the measured temperature. 

Then when you have various datapoints at different temperatures you can test forward voltage at actual operating conditions and have an accurate idea of what the die temperature is. A digital thermometer will give you heat sink temperature and the math to extrapolate to temp rise per watt is simple. Since temp rise per watt is not necessarily linear it would be useful to measure under various drive wattages and ambient temperatures.

Procedure is further explained here.

Keith I don't think anybody is really interested in sacrificing still hard to obtain (in small quantities) Luxeon Rebels for your test. Nevertheless I agree with your sentiment that the proof is in the pudding. I don't know if I'd go so far as to compare it with pulling the tires on a Ferrarri in order to calculate maximum acceleration of the vehicle. I would say that your product is more similar to the tires if anything, and people would really like to know whether they are racing slicks or retreaded economy tires before they get in the driver's seat.

As a manufacturer I think this is really some testing you should do in an accurate and repeatable way. If someone is going to incorporate your product into a luminaire of any sort they will need the information in order to determine the parameters of the thermal design. This is especially true for natural convection applications. At least if there is a fan you could turn it up or get a more powerful one, but if you are using natural convection you'd have to redesign the whole heatsink in case of excessive die temperatures. 

If you want I can refer you to an engineer at an independent testing laboratory in Bangkok. He does exactly this sort of testing, with thermocouples, for major international OEMs, on a daily basis. Also his rates (for side jobs) are quite reasonable.

Finally I have to point out that the feedback you're getting here is really very valuable. Ignoring for the moment the merits of the thermal issues raised, what you can extrapolate from this thread is important information about what a certain number of your potential customers would purchase. If I were in your shoes I'd be thinking along the lines of redesigning my product to fit what the customers want, rather than spending too much energy trying to persuade them that the existing product is already adequate. If there is an unmet market need for a commercial product that will allow you to mount your LEDs to a heatsink through a piece of solid copper, well then you better believe someone is going to fill that need sooner or later.


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## KeithInAsia (Sep 2, 2008)

snarfer said:


> It seems to me that an easy (and cheap) way to test out boards would be to use the relative forward voltage of the LEDs. Heat the entire assembly to various temperatures and measure forward voltage at a known current. You would have to pulse the current for very short time in order to avoid allowing the LED to heat beyond the measured temperature.
> 
> Then when you have various datapoints at different temperatures you can test forward voltage at actual operating conditions and have an accurate idea of what the die temperature is. A digital thermometer will give you heat sink temperature and the math to extrapolate to temp rise per watt is simple. Since temp rise per watt is not necessarily linear it would be useful to measure under various drive wattages and ambient temperatures.
> 
> Procedure is further explained here.


 
Interesting idea. I would need a heat generator that is steady and a chamber that is extremely well insulated.



snarfer said:


> As a manufacturer I think this is really some testing you should do in an accurate and repeatable way. If someone is going to incorporate your product into a luminaire of any sort they will need the information in order to determine the parameters of the thermal design. This is especially true for natural convection applications. At least if there is a fan you could turn it up or get a more powerful one, but if you are using natural convection you'd have to redesign the whole heatsink in case of excessive die temperatures.
> 
> If you want I can refer you to an engineer at an independent testing laboratory in Bangkok. He does exactly this sort of testing, with thermocouples, for major international OEMs, on a daily basis. Also his rates (for side jobs) are quite reasonable.


 
Yes, I need to go through formal testing at some point. Sure, I'd be happy to have that information. Thanks.




snarfer said:


> Finally I have to point out that the feedback you're getting here is really very valuable. Ignoring for the moment the merits of the thermal issues raised, what you can extrapolate from this thread is important information about what a certain number of your potential customers would purchase. If I were in your shoes I'd be thinking along the lines of redesigning my product to fit what the customers want, rather than spending too much energy trying to persuade them that the existing product is already adequate. If there is an unmet market need for a commercial product that will allow you to mount your LEDs to a heatsink through a piece of solid copper, well then you better believe someone is going to fill that need sooner or later.


 
I don't know that I totally agree with you. China right now is trying to give everyone the screw-in-the-socket LED light bulb. I won't be a part of recommending something that I don't belive in. 

ColorKinetics wants everyone to adopt a technology that rides control signals on top of electric feeds. That concept is ideal for selling mass quantities of LED electronics to pre-existing wired building -- but I don't believe in that approach because it fundementally is a weak design over out-of-circuit control (network control NOT on normal electrical lines). 

Just going along with the flow because it looks right to the general public to me is a bit irresponsible towards the customer.

This is just my personal preference. I may also be a bit more sensitive to this because I live in Asia where everyone is very keen on making copies of everything and very few dare to think outside the box.

Life to too short to not venture out and use some logic and push the envelope. This is how we got to the moon.

Incidentally, I'm bold about my claims because these boards are working and working well. I have sold about 5,000 of them (Rebel Squares) and about 500 of the Circles. Feedback has been good. That is why I'm shaking my head over here in disbelief because I'm seeing comments about why this type of board doesn't work by guys who admit they have never used it.

I keep thinking about the basic logical observation: I have a the thermal pad (Rebel) soldered to a 3oz copper plate and by inspection that copper plate stays cool to the touch over long periods of time. Further, the heat sink below the board come up to meet that temperture -- not 100% but reasonibly close to it. I know the design is effective. I just can't put a number on it yet. If the LED had only thermal greese below it, then I might doubt that I'm feeling the right temperature, but that is soldered - so I know I'm feeling the temperature of that thermal pad (darn close to it). 

Anyhow, yes, thanks for the information on that testing facility. I would be interested in that.


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## Oznog (Sep 2, 2008)

I tried to do Vf testing once. The problem is the thermal coefficient of forward voltage is fairly small and highly variable. Also die temp rises fast, and high current MUST be applied because the effect of the dynamic resistance of forward voltage is more significant than the thermal coeff of forward voltage. So you need to apply an accurately regulated current with short-duty PWM pulses (giving it no time to heat up) and measure the value of the forward voltage accurately during the pulse. My scope can't, unfortunately. And like *snarfer* said it looks like the temp coefficient of Vf needs to be determined experimentally for a device on one known setup (soldered to a copper strip) then transferred to the test heatsink. The reflow process might change the temp coefficient of Vf, I don't know.



> I keep thinking about the basic logical observation: I have a the thermal pad (Rebel) soldered to a 3oz copper plate and by inspection that copper plate stays cool to the touch over long periods of time. Further, the heat sink below the board come up to meet that temperture -- not 100% but reasonibly close to it.


Remember, heatsink temp tells you nothing whatsoever about the pad-to-sink thermal resistance. Nothing. The sink will be the same temp regardless of die temperature. 
And you never measured the temp near the LEDs at all. The IR therm reading is far, far too wide to measure LED temps and cannot read on a shiny metal surface at all. Shiny metal surfaces do not radiate significant amounts of IR.



> I also would not have insulated the thermal plane with a solder mask. I favor gold plating so that heat has a chance to radiate from that area.


This may seem "weird" to you but shiny metal surfaces actually CANNOT radiate infrared effectively. It is a near perfect IR insulator (cooling by conduction is another matter, if cooling airflow is available). The emissivity of gold is 0.03, bare aluminum like 0.07 or so. The emissivity of white epoxy paint is ~0.9, about 30x higher! Black paint is 0.98. In fact all nonmetallic surfaces are ~0.9-1.0 or so. This is why aluminum heatsinks are anodized black and never left in a shiny state. It dramatically improves the IR radiation, which granted is only a minor part of the total cooling since convection is the primary method of cooling but it's still quite significant.

This is also why gold (or any shiny metal) will not read on an IR thermometer. The emissivity is so low that increasing the temp of a gold surface by 300C will only increase the reading on an IR therm by about 10C. Since emissivity varies so much for the exact condition of a shiny metal surface, we cannot simply look up the textbook emissivity for the surface type. We could always experimentally measure the surface temp with a thermocouple then adjust the emissivity reading on the IR gun but if you already have a thermocouple reading then you should be using the thermocouple reading.


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## R33E8 (Sep 2, 2008)

KeithInAsia said:


> Interesting idea. I would need a heat generator that is steady and a chamber that is extremely well insulated.



All you need is a cold/hot plate...

When I did the measurements I used this:

http://www.tetech.com/Cold-Plate-Coolers/CP-200HT.html
http://www.tetech.com/Temperature-Controllers/TC-24-25.html
(they do have cheaper setups..)

Then you need a good oscilloscope, a switch box to switch from high power to low, a driver, and a multimeter for checking...

If you do choose to go with this route, I would recommend building a protective box around the cooler to protect it from air movement...

I must say that this would be quite expensive but very accurate...


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## R33E8 (Sep 2, 2008)

Oznog said:


> I tried to do Vf testing once. The problem is the thermal coefficient of forward voltage is fairly small and highly variable. Also die temp rises fast, and high current MUST be applied because the effect of the dynamic resistance of forward voltage is more significant than the thermal coeff of forward voltage. So you need to apply an accurately regulated current with short-duty PWM pulses (giving it no time to heat up) and measure the value of the forward voltage accurately during the pulse. My scope can't, unfortunately. And like *snarfer* said it looks like the temp coefficient of Vf needs to be determined experimentally for a device on one known setup (soldered to a copper strip) then transferred to the test heatsink. The reflow process might change the temp coefficient of Vf, I don't know.



First you have to run the device at the current you intend to use it at (700mA?) and wait for the voltage to stabilize.. Then you press some button on the oscilloscope (like freeze or some sort of trigger, I have not seen the oscilloscope or performed this process in months sorry ..) and drop the current down to like 50mA with a switch. On the oscilloscope, it should show a dip in the Vf and then the Vf stabilizing over time.. The depth of the dip is like 50-200mV (depending on the temperature) and does not last long at all.. Using short duty PWM pulses will not let the die stabilize and mess up the final reading... The testing is also not the variable... With repeat testing, my results are usually exactly the same or within 2mV of the first test.. They would vary a lot if you did not let the die's Vf stabilize before dropping the current.


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## Oznog (Sep 2, 2008)

R33E8 said:


> First you have to run the device at the current you intend to use it at (700mA?) and wait for the voltage to stabilize.. Then you press some button on the oscilloscope (like freeze or some sort of trigger, I have not seen the oscilloscope or performed this process in months sorry ..) and drop the current down to like 50mA with a switch. On the oscilloscope, it should show a dip in the Vf and then the Vf stabilizing over time.. The depth of the dip is like 50-200mV (depending on the temperature) and does not last long at all.. Using short duty PWM pulses will not let the die stabilize and mess up the final reading... The testing is also not the variable... With repeat testing, my results are usually exactly the same or within 2mV of the first test.. They would vary a lot if you did not let the die's Vf stabilize before dropping the current.



I don't see how that would work. The delta-Vf between I=700mA and I=50mA is due to both the dynamic resistance which causes voltage changes nearly instantaneously when current changes and the temp coefficient which takes quite a few milliseconds for the temp to adjust to a change in current. The short PWM pulse allows you to observe delta-Vf due to dynamic resistance (I=700mA die remains at ambient) and then running the die at 700mA @ 100% duty allows you to see the additional result of the temp coefficient since the die will be hot. However, the exact temp coefficient remains unknown (Rebel spec sheet gives a huge variability) so knowing delta-Vf due to temp will not alone give you an accurate die temp.

Or, well, I guess you could do a one-shot of a long pulse, see the initial [email protected] then see the Vf drop due to temp coefficient. The difficulty there is that the initial Vf figure is a very small instant compared to the time span needed for the die to heat up. My scope couldn't read anything meaningful this way, I did try. It's digital but not a very advanced scope. Anyhow that still doesn't get you an accurate die temp from delta-Vf.

How long did it take temp to rise & Vf to drop in your tests?


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## snarfer (Sep 2, 2008)

Yes I don't understand this procedure either. What Oznog said makes sense. You measure Vf with target I forward at different ambient temperatures, with the die also at ambient temperature. That gives you your temperature coefficients. Then you measure Vf in the actual working conditions, at same I forward, and extrapolate die temperature. 

With the procedure described by RE388 I don't see where you ever have a known die temperature to extrapolate from. :thinking:

Clearly this was not the complete description of the procedure, because there is no explanation of when and how the cold and/or hot plate is employed...


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## R33E8 (Sep 2, 2008)

Ok, I think I should have read the link posted on the testing the die temperature before posting... I assumed it was the same method as mine..

So yeah, the method I used different.. Sorry for the confusion...:sigh:

The method I used finds the die temperature and uses the temperature from the cold plate to find thermal resistance..

I'll keep the process I used simple:

Put the led on a cold plate at a set temperature
Wait for the Vf to stabilize at a high current
Switch from high current(700mA) to low current(50mA) (instantly using a switch or something)
Use a trigger function on a Oscilloscope capture the dip in Vf
Find the difference in Vf from the bottom of the dip to where the Vf stabilizes at low current (in mV)
Do your calculations..


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## Oznog (Sep 2, 2008)

I'm not sure what purpose the cold plate serves?
The LED needs to be cooled well enough to keep the die temp low but also we need to know the exact pad temp. Really the only good way to do this is a strip of solid copper, the important part is to achieve a homogeneous temp near the pad that we can the get pad temp off of. It doesn't matter so much what the copper temp actually is as long as the LED's not overheating and we know the pad temp.

Ah, I get it, with the cooling plate cooling the copper strip we can run the LED continuously at 700mA and measure Vf at pad=80C and pad=25C or whatever by adjusting the cooling plate. Yeah, that will be a nice way to do it. But since a copper block responds quickly and homogeneously any form of cooling (including sticking an ice cube on the copper strip) could yield a decent measurement too.


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## snarfer (Sep 3, 2008)

It took me a while to get my head around this too. 

Expanding on R33E8 instructions:

First you mount the LEDs on the target board to be measured. The target board is attached to the cold plate. Have to have a very good junction between cold plate and board because we don't want that interface to get into our calculations. So then with cold plate set at specific temperature we run 700mA through the LEDs until they reach their equilibrium. Now we know typical V forward at 700 mA, also what temperature the cold plate was set at. 

Switch the current to 50mA and measure V forward again. V forward then drifts up to equilibrium.

You need also to calculate the correlation between temp and V forward. But it's not hard because you can just go through various cold plate temperatures with LED at 50mA generating negligible heat.

With that correlation and delta V forward at 50mA you can calculate what temperature the die was at immediately prior to being switched. Since you know how many watts it was running, no problem calculating degrees C/watt. 

So you don't actually need to solder the LEDs twice. And you might even be able to get away without a scope if you had a volt meter that can detect minimum and maximum volts. 

I like this method a lot. Much simpler than having to mess around with pulsing the current.


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## pspec (Sep 3, 2008)

KeithInAsia said:


> Further, I see no takers to my offer of a few free boards to make your own real tests.
> 
> I think this is rather foolish and short sighted to tear a board part and comment about it's strengths and weakness when in fact you never put it through a real test.


 
I'll take some boards. I want to build a RGB swimming pool light. You think they will hold up underwater in the can? How can I transfer heat from the LED boards to the can housing/water? You tell me what to buy and I'll put it together and take pics. I want to control it with either preprogrammed fading or dmx.


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## pspec (Sep 3, 2008)

I checked out your website. Lots of cool stuff. Will that titan style light engine put out enough light to light up a 25,000 gallon swimming pool? What about 1 on each end of the pool? RGB controlled not just cool white.


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## Oznog (Sep 3, 2008)

AsianSignals does have an acrylic cover with a genuinely waterproof seal. That fits any type of LED board out there. It's got an o-ring and the 2 screw holes are outside the o-ring to no leaking in through the threads. Screw that into aluminum and it's great.

The covers do lack any kind of diffusing features and can be pretty intense since you're viewing the LED directly. There's not a lot of clearance inside to add diffusing optics or anything either. So it makes more sense for indirect lighting or signal lighting which is normally viewed from a ways off.

You will of course need to ensure that water cannot enter through the holes in back that the wires come in through.


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## pspec (Sep 3, 2008)

i think thats just a cover for 1 star. I want to build an array. I was thinking about using 12 luxeon rebel rgb stars. I think they are called endor starts or something. But after seeing some of the stuff from asian signals, I might be better off getting a denser array instead of using 12 smaller boards with optics on each one. 

Would that titan style flood work good for a pool light?


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## Oznog (Sep 4, 2008)

OK... welll...

I took my one and only unused Rebel white to try to get a reading on the AS 6x board. For that, I took an engraver and pie-sliced the copper foil on top and bottom to give the device 1/6th the board. Right between one of the LED pairs and on the other side through the screwhole. And continued the cut through the copper on the inside of the screwhole. Sry my camera's battery door is busted. So anyhow the device only has a thermal connection to its 1/6th share of the board so mounting a single device gives it a fair reading.

I used Arctic Silver thermal compound on a CPU heatsink and secured it with a single screw in the middle. It seems to seat flat and I'm not inclined to drill and tap more.

I have a Type K thermistor here with a fairly thin gauge going to the junction. I made sure the wires are spread so they only make the junction at the tip so it's a fine-point probe. I dabbed a little Arctic Silver thermal compound on that too. I took readings very very close to the side of the device after about a minute of running, and did a thorough search for the hottest spots. Hottest reading was right next to the cut edge where it would have butted up against the other Rebel, so there's all the lateral conduction to this point but very little dissipation area so that should pretty much be reading pad temp if this is 3oz copper. The temperature was fairly stable at this point, the board's rising at 0.1C every 30 sec or so and the heatsink's rising at the same rate. So I passed the effect of the thermal mass of the board.

Throwing in a 0.8 estimate to account for power lost through light emission:

Well, here's what I got:
I=700mA Vf=3.4V P=1.904W @ 20% eff
Tsink=43.0C
Tpad=56.2C
deltaT=13.2C

Which yeah this _does_ shock the hell outta me. Because this is only *6.93C/W per device*, which should be on par with the MCPCBs mounting only one device! 

I tried the same procedure with the 1-Rebel Star by "Opulent", scraping a bit of the black soldermask off the copper spreader up top to allow for a temp measurement. I think the single screw seemed to make this board a bit tilted but well it's got 6x the bottom surface anyways. And I get:
I=700mA Vf=3.5V P=1.96W @ 20% eff
Tsink=41.1C
Tpad=52.5C
deltaT=11.4C
*Rth=5.82C/W*

So yeah, it's about 20% more resistance, but that's still almost unbelievable esp considering there's 6x on the device here. I'm looking over what I did to see if I screwed anything up significantly and can't find it. The only additional factor is the steel screwhead can transport the head to parts of the board intended to other parts of the board but I doubt that's very significant. I guess I could set it back up with some tape under the screwhead?

Now I also see the Rebel Endor Moon has a datasheet here, for single and triples.
That claims an Rth of *3C/W for singles* and *8.9C/W per device for the 3x configuration*. Had to do some math since they quote as Rth-junction-sink not Rth-pad-sink and the 3x was per watt to all devices not to one. I assume they're giving a figure just for junction heat not total power into the device so the 20% power lost as light is not a factor.


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## KeithInAsia (Sep 6, 2008)

Oznog, thanks taking a shot at testing a portion of that Rebel Circle board. I appreciate it.


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## pspec (Sep 6, 2008)

I'm getting ready to purchase 12 RGB endor stars and 12 optics, aswell as the luxdrive RGB controller and a power supply. Can you offer something similar or better before I buy them.


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## spencer (Sep 6, 2008)

What is the luxdrive RGB controller? Can you please give me a link?


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## KeithInAsia (Sep 6, 2008)

pspec said:


> I'm getting ready to purchase 12 RGB endor stars and 12 optics, aswell as the luxdrive RGB controller and a power supply. Can you offer something similar or better before I buy them.


 
I do not have any RGB products for optics. I will have some single LED boards for optics in about 3 weeks.

And my controller/driver systems are much more sophisticated than the Luxdrive Quad pack unit. In fact, we are putting a hold on sales to new customers right now pending the completion of software and a few hardware issues.


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## Oznog (Sep 6, 2008)

KeithInAsia said:


> Oznog, thanks taking a shot at testing a portion of that Rebel Circle board. I appreciate it.



Well I would have done it sooner if it were easier.

Yeah I'm sorry I spoke so harshly. Really the thermal performance appears to be "fair" per device and "really good" for a high density app (that is, it's already better than an Endor 3x and this is a 6x).

The board COULD have been significantly better, just by using more vias in a better layout. The room's there to do it. In fact, I think I've got a hang of the estimation process now and the numbers I'm using are ballpark agreeing with my measurement on the AS board. I do see that you can actually get equivalent or lower C/W that this even with common 1 oz copper and 1.575mm board. Or could have gotten it crazy low resistance and/or supported crazy high number of LEDs per board if better via placement was used on this 1mm thick 3oz board.

Part of it comes down to what the PCB mfg will let you do. The vias are already far closer than the _electrical_ design rules would ever allow (touching land area). That doesn't concern the mfg though. What does concern them is if the close holes will cause a physical mfg problem, but I don't know what's allowed or not I'm talking to a mfg about this.


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## pspec (Sep 7, 2008)

http://www.leddynamics.com/LuxDrive/datasheets/4016-QuadPuck.pdf

quad puck costs $160.00 and can run 12 endor stars at 350mA for RGB control and still run 4 white stars at 350mA. What do you have thats comparible


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## KeithInAsia (Sep 7, 2008)

pspec said:


> http://www.leddynamics.com/LuxDrive/datasheets/4016-QuadPuck.pdf
> 
> quad puck costs $160.00 and can run 12 endor stars at 350mA for RGB control and still run 4 white stars at 350mA. What do you have thats comparible


 
Without turning this into a sales post, I can give you a basic idea of how our bottom line equipment compares.

Our smallest system is 16 channels; 4 dedicated PWM processors with 4 channels of output (RGBA). We have a 5th processor dedicated to DMX/Serial control and power supply control (PC Power supplies).

Includes a set of 3.5 amp drivers (one for each output channel) and an interrconnect board. IC board supplies 4 PWM output, 12 volts out to the light fixtures, and one wire feedback control. The one feedback wire can be PWM'ed to throtle down output (typically used for thermal control feedback from the light fixture).

All of that is mounted on a back board design to fit inside a PC case. PS are NOT included. Case not included. All connectors are included. Price is approximately $650.

Also will include free PC software that gives you the ability to load up to 60 unique LED lighting sequences that you design. Then you can play them back at will. (we are soon to add battery backed up RTC to help trigger squences by time and date). We do have a switch input to call up sequences by hand. Or they can be called by serial port and the PC (RS485 network).

There are a few more features, but that is the general scope. This unit is much larger than the LEDDynamics product.

The drivers are not potted and can be repaired easily. If you have an Oops, nothing to worry about. Drivers are servicable.

We have 2 years of development in on this and we are about 85% complete. We have to spin the controller board one more time and finish out the software and the docs. We should have most of the documents and the PC software up on the web in about a month, and they won't be entirely finished but we'll begin to expose the design the tools for pre-sales inspection.

One last thing that some of you will appreciate. The output timing for the PWM processor will be highly deterministic. I believe you will be able to program timed bursts of light with a high degress of accuracy.


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## Oznog (Sep 8, 2008)

BTW, the settling time of the AsianSignals 6x board is about 6 seconds. That is, when you turn it on it will reach constant temp in about 6 seconds. That makes the time constant about 2 sec I guess, I'd have to do some rigging to get the thermistor probe into the scope to see it. 

This does come up when doing flashes in excess of the constant thermal rating of the setup. If you give it a 2amp flash for example (700mA being the max "normal" rating), the die will heat up in just a few milliseconds across the 10/12 C/W die-to-pad resistance but the pad attached to the top of the board won't heat up much if it's under half a second or so. Of course the heatsink won't heat up for minutes.


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