# Need help designing custom copper MCPCB for an XML



## archer6817j (May 2, 2011)

Hi folks,

I've been talking to some MCPCB manufacturers about making custom 2mm thick copper core boards to use in my custom lights. 

I just saw the "thermal path" thread and realized I should ask the experts here to get some advice and to make sure I'm getting my money's worth from the manufacturer. 

First of all, my application has some limitations and I won't be able to create the "ideal" thermal situation but I want to make it as good as possible with what I've got to work with. 

I've also been updating my testing methods to the ANSI standard for output. The good news is that @ 3 mintues the output is stable. The bad news is, I'm loosing about 100 lumens compared to the "@ turn on" output. I'd like to get this loss down as much as possible. Here are the constraints: 

1) I'm using a solid head (no pill) and the LED is inserted from the front of the head. The surface immediately behind where the LED is mounted is 3/8" of aluminum...but keep in mind the head is solid. 







2) I'm using thermal tape with no screws, but following the manufacturer's recommendation for pressure and time. I know this isn't as good as arctic silver and screws but the main reason I'm doing this is that the screws interfere with the optics I'm using (and most optics I've tried). So, I decided the best way to increase the thermal efficiency is to go with a copper MCPCB vs. an aluminum one. 
3) the MCPCB thickness will be 2mm. This is the thickness of my current aluminum MCPCBs and I can't go any thicker without altering the dimensions of the head. However, it seems like 2mm should be a "decent" thickness considering the copper stars from KD (or wherever) are around .8mm 
4) the star size is 20mm

So, given the above, do you have any thoughts or advice on the physical design of the board with respect to trace layout, circuit layer thickness, dielectric layer thickness, etc? 

Is this something people would be interested in purchasing as a component? The cost is quite high at a volume of 100 pcs...say in the neighborhood of $15 just for the MCPCB (not including the LED). 

Let the games begin!


----------



## CKOD (May 2, 2011)

for 2), Use arctic silver thermal adhesive or arctic alumina thermal adhesive if possible, this makes rework at a later point difficult. Heat should degrade the epoxy, so you may be able to bake the head/MCPCB, degrade the epoxy and use a screwdriver or the like to sheer the PCB from the base. But I'd call that a permanent solution either way. If you want to be able to change it out, find the best tape you can. 

For your MCPCB, you want to use the best electrical insulation/thermally conductive layer you can get, I dont know any by name, others may, but this is the bottleneck, and where MCPCBs go good or bad. As far as layout, get the heaviest copper you can get for the top copper, IIRC 8oz + copper is available, no 1 oz sissy stuff here, kthx  And you want to flood as much of the board as possible to allow for as much radial heat spreading you can get before it has to go though the insulator layer. You dont need the thermal pad to be isolated from both electrical pads on a single LED, so the only one lead needs to be isolated from the rest of the copper flood on top, and just use soldermask to define the pads. For the solder pads for soldering your wire, try and add some thermal relief, MCPCBs are hard enough to solder too. 
http://en.wikipedia.org/wiki/File:PCB_copper_pour_thermal_pads.png
similar to how all the pads that are connected to the pour in that image are. 


If youre trying to sell the stars, obviously screw slots will be needed for other people, if not, then just go with whatever fits your application.

edit:
scratch all that about the flood/insulator etc...

http://www.metalcorepcb.com/en/pcb-capabilities/mcpcb/design-construction/stack-up-drawing-for-mcpcb

if you get PCBs made with a direct thermal connection to the substrate, like in picture 7 on the link above 
( http://www.metalcorepcb.com/images/Stack%20Up%20Drawing%206.png ) I know you could sell some MCPCBs on here. Thats what all the junkies that love to drive the heck out of LEDs want. If they turned out to be in the $15/per range, and they were the usual star shape for screwholes, I might consider buying ~36-40 or so. Obviously lower is better.


----------



## bshanahan14rulz (May 2, 2011)

Never used arctic silver epoxy, but couldn't you use the epoxy and screws, but just remove the screws after the epoxy has had time to set? If that's the only reason you are avoiding 

Directly attaching the heat pad on the LED to the metal material of the MCPCB would be best, but if that isn't possible, try to spread the trace that attaches to the heat pad on the LED to the largest area you can, while still allowing enough room for sufficient traces for the anode and cathode traces. 

You probably already figured these things, but it might as well be said for whoever digs up this thread in the future.


----------



## srfreddy (May 2, 2011)

Thermal tape is really poor for conduction...couldn't you just clamp the star down and epoxy it? Or even just use the reflector to push the star down....


----------



## onetrickpony (May 2, 2011)

Here's a thought: Counter sink the screws! Have the stars machined with counter-sunk holes, or just have holes and counter sink them yourself, then just use some thermal compound (paste) and screw the suckers down. Best of all possible solutions, and it won't interfere with your optic.

Metal to metal contact with just enough thermal gunk to fill in the voids seems to be the accepted ideal method of heat transfer in most situations. Just make sure you use a counter sink that matches screws you have available.


----------



## Al Combs (May 3, 2011)

Have you seen this thread? I guess it's not very practical for production purposes. But it is an interesting method.


----------



## MikeAusC (May 4, 2011)

If you're going to the effort of making a super-conductive star, it seems such a waste to then put in a thermal-tape heat barrier in front of the torch body.

The best thermal compound has conductivity at least ten times worse than aluminium - it should only be used to fill in the air gaps resulting from irregularities in two smooth metal surfaces.


----------



## onetrickpony (May 4, 2011)

MikeAusC said:


> If you're going to the effort of making a super-conductive star, it seems such a waste to then put in a thermal-tape heat barrier in front of the torch body.
> 
> The best thermal compound has conductivity at least ten times worse than aluminium - it should only be used to fill in the air gaps resulting from irregularities in two smooth metal surfaces.


 
Eggggsactly.


----------



## archer6817j (May 4, 2011)

I agree that thermal tape may not be the "ideal" solution. However, could you agree that a copper MCPCB would be at least incrementally better than using an aluminum one? Without actual data I find it hard to say to what extent one solution is better than another. I think "heat barrier" might be a little over the top when describing thermal tape 

My hope: by using a copper MCPCB w/ thermal tape I can get thermal performance that is at least on par with an aluminum MCPCB and screwed down with AS. My understanding is that moving head away from the LED heat pad is the primary objective. I wish I could see a temperature gradient map with both the alu and copper MCPCBs. My hunch is that the copper moves the heat away so quickly that the ability to dissipate the heat into the flashlight head is less important than if you had an aluminum MCPCB. I'll keep thinking about a better "screw down" solution. I find it confusing that MANY optics and holders cannot accommodate screw heads or even stars with wires soldered to them...or perhaps that people that design stars don't think about attaching optics! 

In the end I suppose I'm chasing incremental improvement, not the end-all solution for LED cooling  If I can figure out how to get some sample copper boards it would be great to do some applied science and actually measure relative outputs in my sphere, with different MCPCB and thermal compound configurations. I supposes given the crowd here that might have been done already. Anyone know?


----------



## Oznog (May 5, 2011)

archer6817j said:


> I agree that thermal tape may not be the "ideal" solution. However, could you agree that a copper MCPCB would be at least incrementally better than using an aluminum one? Without actual data I find it hard to say to what extent one solution is better than another. I think "heat barrier" might be a little over the top when describing thermal tape


I would say no, actually.

First off, modern MCPCBs are QUITE good. The C/W of the package and heatsink are probably much more significant than the MCPCB. 

Second, I'm pretty sure almost, well, NONE of the thermal resistance comes from the aluminum MCPCB body itself. It's in the electrical isolator, with a thermal resistance orders of magnitude greater than the aluminum, but nonetheless, not very significant in the big picture. It IS important that the top copper be over as much area as possible and is as thick as practical to minimize the lateral thermal resistance. See, if there's not lateral spread on top and the heat path is only of the area of the slug, the thermal resistance IS high. If the heat spreads out 1cm laterally from the slug and transfers heat through a wider area of the insulator, it's much lower.

Aluminum XML MCPCBs should be under 2C/W to begin with. Copper will only be SLIGHTLY less, because the insulator is still there.

DX sells or used to sell a copper-MCPCB XML for a few bucks more than the regular. That's your shortcut right there. I don't have the SKU though.


----------



## Nos (May 5, 2011)

Oznog said:


> DX sells or used to sell a copper-MCPCB XML for a few bucks more than the regular. That's your shortcut right there. I don't have the SKU though.


 
ma sha1 did some testing with those DX copper stars, but both tests failed afer soldering (see post#59), probably due to bad leads on in the star 

I still think a star without a insulator under the termal pad of the emitter would a great idea. And of course this star has to be made of copper since its hard to solder on aluminium.


----------



## srfreddy (May 5, 2011)

I think that the thermal tape should not be there-your star doesn't do anything if the heat is transfering very slowly to the heatsink.


----------



## SemiMan (May 5, 2011)

I got a bit "lambasted" on another thread for suggesting that proper calculations are needed, but in this case, that is exactly what is needed.

1) Are you using a recognized metal core board supplier, or just buying from DX or similar where you can't get proper specifications? If you are using a proper metal core board supplier, what is the thermal resistance (or conductivity) of the insulator and how thick is the surface copper? At first pass I expect that you have a so-so board but would like to be wrong.

2) How thick is the metal core board?

3) What are the characteristics of the thermal tape you are using in terms of thermal resistance/conductance?

Once you have these parameters you can approximate your thermal resistance at each point and determine where the biggest gains are and by how much. If I had to hazard a guess on the limited information:

a) Heat sink is too small. If you are losing 100 lumens and it is stabilizing after 3 minutes then odds are your heat sink is probably on the small size.
b) Poor metal core board in that the thermal interface material is no good and copper on surface is too thin and/or poor contact between the LED and board. Could be wrong here, but have seen it often enough to expect it.
c) Thermal tape: Thermal tape is not great, but on the other hand you have a lot more surface area so even though resistance/area is higher, there is a lot of area to transfer heat.

My expectation is that copper over aluminum is going to solve little until you solve the basics first. Of course your copper board should be quite good.

You said you lost 100 lumens, but where did you start. If you started with 1000, then losing 100 is bad, but again, you have a small heatsink so perhaps your expectations are high. How hot does the heat sink get?

Your bare board costs are so high because your volume is low. Setup charges on metal core boards are high so you end up paying a lot of money unless you run volumes of material.

Semiman


----------



## archer6817j (May 5, 2011)

Anyone here use solidworks/cosmos? I have a full seat but don't really know how to use cosmos. I imagine they have some sort of thermal simulation tool? 

Semiman:

1) I buy my leds pre-mounted from cutter. I don't have any specs for those boards
2) The MCE board is 2mm thick and the XML board is 1.6mm
3) Thermal tape is Bond-Ply 100 (5mil) http://www.bergquistcompany.com/thermal_materials/bond_ply/bond-ply-100_5.htm

The entire head of the light is the heat sink, so making the heat sink bigger means a bigger light. The head gets quite hot if you let it run in air. I think part of the limitation is putting a lot of power into a 1" light. This is basically pushing the envelope and I'm trying to get more power and better lumen maintenance without increasing the size...and no, I'm not going to start making solid copper heads...unless anyone wants to buy them? I also want to keep in mind the thermal sag is only an issue if you are running full throttle. 

As for the lost lumens with the MCE:

Turn on: 575
30 sec: 553
3 min: 445

The XMLs strange results: 

Turn on: 610
30 sec: 575
3 min: 453

So, the XML starts out (a bit) brighter but settles to exactly the same point. Keep in mind the XML is on a 1.6mm star and the MCE is on a 2mm star. 

I have been looking at some of the lists of lumen readings from Bigchelis (he helped me calibrate my own sphere) and it seems like I'm in the top 1/3 for performance on 2.8A regulated single-MCE lights. There are many that are a lot lower. Obviously this doesn't compete with direct drive. However, I was surprised that many production lights have as much (if not more) drop in lumen output than my lights. From that I infer that I'm doing "something" right but I'd like to move closer to the top of that list  

I'm off to the shop to do some science!


----------



## archer6817j (May 5, 2011)

srfreddy said:


> I think that the thermal tape should not be there-your star doesn't do anything if the heat is transfering very slowly to the heatsink.


 
Hey srfreddy,

That's true but I don't know that the heat is transferring "very slowly." Since there is no data available there is no way to quantify "very." I will concede that it's transferring "more slowly" than if I used AS-5. However, we still don't know what the target rate of heat transfer is. Of course more heat transfer is better, but at some point there is a diminishing return. For example if the LED is a luxeon rebel driven at 750ma...using indium to solder it to a pound of copper probably isn't going to get a lot more output than if it was just soldered to a low quality aluminum star. So, if AS-5 and screws can get me 50 more lumens, I'm on board. If the difference is 10 lumens...then the hassle of drilling, tapping, coutnersinking, dropping tiny screws on the floor, applying AS, getting it all over, etc...isn't worth it.  

Same goes for the copper boards, if it's 50 lumens it's worth the expense. If it's less...diminishing return. It's all relative, for me at lest. Does that mean you aren't going to buy a light till I get rid of that thermal tape?


----------



## MikeAusC (May 5, 2011)

The majority opinion here seems to be against using Thermal tape because there are much easier ways of dramatically reducing the temperature drop.

All other options involve metal to metal contact - except for the air pockets cause by surface irregularities.

Thermal tape totally prevents ANY metal-to-metal contact.

No-one here has quoted practical measurements of temperature drop across Thermal Tape - because no-one here uses it.

When people complain about poor thermal performance using grease or epoxy, it's when they don't understand the significance or metal-to-metal contact and they don't apply pressure to ensure that compound only fills in surface irregularities - not using compounds as a barrier layer.


----------



## Oznog (May 6, 2011)

You don't need Cosmos flow to calculate heat here.

A 20mm star is about 0.5sq in. The thermal tape would be 1.12C/W IF high pressure (>10PSI) is used to seat it. You should be using thermal epoxy (Arctic Silver).

A Star is "under" 2C/W total, insulator and metal. Thermal resistance if junction-to-pad inside the XML itself is 2.5C/W.

Aluminum is 250 k-W/(m*K). So a 20mm Star is about 0.00034 m^2 and about 0.00175m thick. If I calc'ed that right, I get 0.0206 C/W in the aluminum itself. 
Which means the thermal resistance of the aluminum is already so close to zero, copper won't improve anything. ALL the thermal resistance is coming from the insulator layer and the aluminum clearly doesn't matter at all.

Well, the calc is flawed because the Star's utilization of the aluminum depends on how much area the top copper trace spreads it out over. But it's beside the point- if it were 10x higher than that, it STILL would hardly be significant!

If you're seeing high thermal resistance, it's probably coming from the thermal tape. No serious LED modder will use that, it's too high for these small areas, and if you have no screws, it probably won't work reliably. That's your problem. Like I say, Arctic Silver.

Heat's always an issue, but a lot of people obsess over it far beyond its true significance. The XML loses 10% of its output if the die temp increases 50C. So, OK, if you're running REALLY intense at 10W, the thermal board's 2C/W is causing a temp rise responsible for a 4% share of degradation of output. That's... not much. Sure, it'll also degrade a bit faster due to the extra 20C die temp, although in a flashlight it'll probably become obsolete before the die degrades. In any case, the heatsink is probably gonna be >2C/W so the Star is hardly the problematic factor.


----------



## Al Combs (May 6, 2011)

archer6817j said:


> I agree that thermal tape may not be the "ideal" solution. However, could you agree that a copper MCPCB would be at least incrementally better than using an aluminum one?



I think you are on the right track with the copper star idea. I believe it represents more than just an incremental improvement. The other thread I pointed to Mick made the case of 20+ watts of an SST-90 (really more like 35 watts) going through a thermal vias that only has a surface area of 5.1mm x 9mm. The XM-L's thermal pad is only 2.8mm x 4.8mm. The surface area of the XM-L thermal pad vs the amount of heat it has to dissipate is in similar ratio to that of the SST-90.

I have two links from Lux-RC here and here. He's the guy that makes the 20mm triple XP-G R5 stars. The first shows a picture of raised contact pads on his new style gold plated copper stars. Look towards the bottom of the page for a thumbnail link to a larger picture. Later the insulating layer and the traces for electrical contact are added making the overall surface flush. When he re-flows an LED, it's being solder directly to gold plated copper. The second is a discussion thread where he mentions copper has ≈ 100 times the thermal conductivity of the best insulating materials available on common stars.

As a quantitative example of that concept, on page 13 of the Luminus SST-90-W data sheet, the thermal resistance from the junction to the ceramic substrate is 0.64°C/W. Once it's mounted on an MCPCB star, the thermal resistance increases to 2.02°C/W. It's not that aluminum is a crappy material to make stars from. It's the insulator used under the contact pads that becomes the limiting factor. At the 35 watt power level, the SST-90 junction is 22.4°C hotter than the back surface of the ceramic substrate but 70.7°C warmer than the back of a MCPCB star! Another way of looking at that might be to say when the head of a flashlight with a star mounted LED is 29.3°C / 84.74°F or barely warm to the touch, the LED junction is already hot enough to boil water.

Referring again to page 13 of the SST-90 data sheet, the eGraf 1205 TIM Luminus used in their test only increased thermal resistance another 0.13°C/W. Thermal tape is fine as long as it has a large enough surface area to dissipate the heat through. Incidentally, I downloaded the pdf file on eGraf 1205 that Luminus mentioned just to see and that particular TIM has more than double the thermal conductivity of what Lux-RC refers to as, "thermally-enhanced insulator material". See the section labeled, "Improved Heat Conductivity". Thermal conductivity of the enhanced insulator on the MCPCB star is only 5.0 W/mK while the eGraf 1200 TIM is 12W/mK. The 1205 is the thinnest in the series @ 5 mills thick.

So here's a hopefully not too provocative question for the group. If using a TIM for thermal coupling is by consensus a bad idea, how can considering the use of an MCPCB star in the first place be a good one?


----------



## bshanahan14rulz (May 6, 2011)

MCPCBs are about compromise, and I think that is what the OP is going for, something that is easy to assemble but still performs better than the mass-produced silicone-the-star china lights.

BTW, that DS333 blank looks beautiful!


----------



## MikeAusC (May 6, 2011)

Al Combs said:


> . . . . So here's a hopefully not too provocative question for the group. If using a TIM for thermal coupling is by consensus a bad idea, how can considering the use of an MCPCB star in the first place be a good one?


 
The unchangeable problem isn't the star - it's the tiny surface area at the back of the chip that is such a small area to conduct the heat out of the LED - while also providing the electrical connection.

It doesn't matter whether you mount the LED+ceramic onto a block of copper or an aluminium star - all the heat has to spread from that small area. 

That's why 99% of high-power LEDs are mounted on stars - it works almost as well as the next best thing. If you're trying to get the last possible Lumen out of the LED you do something else - but it's a lot more work for a little extra benefit.


----------



## Harold_B (May 7, 2011)

Would it be possible to mount your MCPCB and optic on a separate heat sink and retest the output over time test? Most LEDs have an initial output drop as they warm up. Phosphor selection by the manufacturer will effect the drop over temperature (For example most YAGs will drop less than Silicates). That would give you some data for comparison between an optimum heat sink and the system you are building. Our pre-release XM-L's were mounted on 2mm copper stars by our vendor. I can try to get one in the integrating sphere next week but our lab is pretty busy right now so if you can do the test it would happen sooner. What voltage are you running at just in case?


----------



## Al Combs (May 7, 2011)

MikeAusC said:


> It doesn't matter whether you mount the LED+ceramic onto a block of copper or an aluminium star - all the heat has to spread from that small area.


How can the choice of materials not matter? Yeah I get how the thermal pad on the back of the LED is a very small and an inherent bottleneck. All the more reason to make sure it's mounted to something as thermally conductive as possible. If the thermal pad on the back of the LED was as you suggest the limiting factor, wouldn't you expect the SST-90 junction to ceramic substrate spec to be almost the same as junction to MCPCB spec? Why then did the data sheet give a spec of more than triple the thermal resistance *after* passing through the star? Doesn't that indicate the heat backed up in the star and not the LED's thermal vias?

Did you actually look at the Lux-RC links, the first one in particular? I understand why he didn't give too much detail. He is after all trying to sell something. But he mentions after mounting XP-G LED's directly to copper stars, he was able to drive them to a stable 50% power, if not actual lumen output increase. There is also this thread mentioned by archer6817j in the first post. At least I think that is the thread he was taking about. Heat pipes look even more promising than solid copper. Insulated aluminum wasn't even in the running. Manufacturers don't use stars because of their effectiveness. They are simply cheap and easy to work with until something better comes along.


----------



## Harold_B (May 7, 2011)

You are right that the shape of the star has little to do with thermal performance although symmetry is never a bad thing when dumping heat. The real drivers behind the star shape as I understand it is that is (1) an unoffical industry standard for mounting and (2) it makes a very efficient use of the material for MCPCB Arrays (less waste).


----------



## SemiMan (May 7, 2011)

Wow, lots of hand waving on here with little thought to the information available and what it means.

Looking at your results, you have a relatively small loss after 30 seconds, i.e. 3-6% depending on what which LED. That tells me that you have tolerable transfer of heat into the star. Not fantastic for the XML, but ok.

I would contact Cutter and ask them specifically what is the thermal prepreg material used for the star (not just the brand, but the actual material) and what is the finished copper thickness at the surface. The finished copper thickness at the surface will determine how much area on the star is used to transfer heat.

I am still surprised you stabilize after 3 minutes and that tells me that your head is woefully inadequate for what it is being asked to do. I understand you have size limitations. As opposed to making it larger, consider fins or other methods to increase the surface area. if the surface is flat now, you could increase the surface area 2-4X by changing the surface structure and hence greatly increasing the dissipation. Also, is it anodized black or is it silver? Black has higher emissivity and will enhance the radiation of excess heat.

For all the complaints about tape on here, I have seen no reference to actual values so I will post them here:

Initial Assembly Pressure (psi for 5 seconds) 10 25 50 100 200
TO-220 Thermal Performance (°C/W) 0.005" 5.17 4.87 4.49 4.18 4.10
TO-220 Thermal Performance (°C/W) 0.008" 5.40 5.35 5.28 5.22 5.20
TO-220 Thermal Performance (°C/W) 0.011" 6.39 6.51 6.51 6.50 6.40
Thermal Impedance (°C-in2/W) 0.005" (1) 0.56 0.84 0.52 0.50 0.50
Thermal Impedance (°C-in2/W) 0.008" (1) 0.82 0.80 0.78 0.77 0.75
Thermal Impedance (°C-in2/W) 0.011" (1) 1.03 1.02 1.01 1.00 0.99

You need some initial pressure when assembling (true to glue too) and hence I hope you have a jig to maximize pressure and contact. The biggest killer tends to be how flat the contact is. Let's say that for a 20mm2 star you get effectively 1/4 square inch of contact .. that puts you at 4C/watt using some of the worst case figures from above with the thickest material. You may have been "fooled" into using the thin material but this can behave worst if you have surface irregularities that make contact difficult. The thicker material will be better in this case. I expect from experience, you are likely in the 2-3C watt range with the tape. 

A few comments on here talk about the thermal resistance "through" the aluminum star. However, you also need to spread heat sideways and down to increase the effective thermal path. As you are currently using tape, a copper star will give you a larger effective thermal path. It is also more malleable and under pressure may adapt to the aluminum heat sink surface better.

Real easy test though: Use your star and tape and attach it to a very large aluminum heat sink. See the results you get. If they are similar to what you have now, then you know its the tape and/or board. If it is much better, then you have to work on your head. If you determine it is the tape/board, then use arctic silver, clamp while drying, then repeat. If the results are still not great (unlikely), then you need a better board.

Semiman




archer6817j said:


> Anyone here use solidworks/cosmos? I have a full seat but don't really know how to use cosmos. I imagine they have some sort of thermal simulation tool?
> 
> Semiman:
> 
> ...


----------



## Harold_B (May 7, 2011)

The drop between 30 seconds and 3 minutes is around 20% isn't it? If I stop waving my hands long enough to count my fingers that is....:thinking:


----------



## Oznog (May 8, 2011)

Al Combs said:


> I think you are on the right track with the copper star idea. I believe it represents more than just an incremental improvement. The other thread I pointed to Mick made the case of 20+ watts of an SST-90 (really more like 35 watts) going through a thermal vias that only has a surface area of 5.1mm x 9mm. The XM-L's thermal pad is only 2.8mm x 4.8mm. The surface area of the XM-L thermal pad vs the amount of heat it has to dissipate is in similar ratio to that of the SST-90.
> 
> I have two links from Lux-RC here and here. He's the guy that makes the 20mm triple XP-G R5 stars. The first shows a picture of raised contact pads on his new style gold plated copper stars. Look towards the bottom of the page for a thumbnail link to a larger picture. Later the insulating layer and the traces for electrical contact are added making the overall surface flush. When he re-flows an LED, it's being solder directly to gold plated copper. The second is a discussion thread where he mentions copper has ≈ 100 times the thermal conductivity of the best insulating materials available on common stars.
> 
> ...


 
I don't think the size of the thermal pad is that big of an issue, that is, with properly applied silver grease or epoxy, the pad-to-star thermal resistance should be negligible.
The original question was in the context of an XML, so he's talking ~8W at most. Not 35W. At 8W, the Star contributes 16C and according to spec sheet (which doesn't always hold to reality, I know) will lose 3.2% of its output due to the Star. Which is kind of significant, but 8W is already on rather an "extreme" end of usage.

But again, in any case, IF it's an MCPCB, the ONLY issue is the MCPCB insulator. Going from aluminum to copper or diamond or nanocarbon is irrelevant, because the metal contributed virtually nothing to thermal resistance to "fix".

If you want to mill a metal slug to solder the XML to directly, yeah, that WILL be better than an MCPCB. This isn't too hard, you'd just mill a circle out with enough metal removed on the sides to allow a PCB frame around it, that would be my choice. No major setup expense there. The soldering stands a minor chance of damaging the LED because of the time it takes to heat up and cool down a piece of metal of that size. Even though these devices are usually designed for soldering now (and seem to be soldered to the MCPCB Stars, IIRC), this is somewhat longer than those. Probably not a problem as long as you don't screw around.

Keep in mind, it's only gonna save most of the ~2C/W, which is good for performance attempts. For running at the rated power, it's just not necessary. The XML is 2.5C/W anyways, and that won't change. If it's a flashlight, the case as a heatsink is more C/W than either of those.

Aluminum should be similar performance to copper, honestly, and it's far cheaper and easier to mill. Brass and copper alloys (like per-1983 pennies) are not as conductive as copper, closer to aluminum actually, or worse. Common yellow brass is not even HALF the conductivity of aluminum. Aluminum CAN be soldered now, with organic acid flux. It's a bit tricky to use, but it DOES work just fine. You'd just run one pass to successfully "tin" the surface first then return to do the LED itself on top of it. One thought was to minimize the heat exposure of the die by heating the slug first, then adding the LED, but the thermal shock of going from room temp to soldering temp may be worse than letting it experience the extended time spent heating the slug up. But IMHO this flux is probably reactive enough that you'd want to remove it thoroughly before applying the LED anyways to ensure it doesn't react with the LED in the long run. Once the aluminum is successfully tinned with solder, you do not need the special aluminum flux to solder the LED on in a separate operation.

Gold plating does not improve thermal performance. Bare copper can lose solderability rapidly with exposure to air and contaminants like fingerprints. So the industry preserves solderability through gold plating, plating with tin, tinned with solder as Hot Air Solder Level (HASL), or with Organic Solderability Preservative (OSP)- a clear film leaving the pink color of clean copper.

So, gold plating is ONE way to do it, but it's primarily for show. I don't see how it'll add to the thermal performance. Just "pretty" coming out of the parts bin.


----------



## bshanahan14rulz (May 9, 2011)

archer6817j said:


> 3) Thermal tape is Bond-Ply 100 (5mil) http://www.bergquistcompany.com/thermal_materials/bond_ply/bond-ply-100_5.htm


 
Bond-Ply 100
-5mil
-needs 100psi to get thermal impedance of:
-0.86°C-in2/W
-conductivity: 0.8 W/m-K

9882
-2mil
-needs 50psi to get impedance of:
-0.35°C-in2/W
-conductivity: 0.60W/m-k


----------



## Al Combs (May 9, 2011)

Before committing yourself to having 100 copper stars made, I was just wondering if you already tried a proof of concept light? You mentioned $15 a piece so they want $1,500 for a run. There must be a profit margin to allow for shipping and your time. But still a lot of money to risk on something that may not be what you're hoping for. You could reflow the LED directly onto a copper disk the right diameter and thickness. Do the electrical contacts and all. Then with a carbide drill you could remove the 4 electrical vias connections from above. Perfect job for your fancy new CNC mill. Or a mini carbide burr in the Dremel if you've got a good loupe. It might be easier than milling out slots underneath, or maybe not. A milled rectangle the size of the LED's thermal pad would guarantee it would be perfectly centered after the reflow. Otherwise you need some kind of a jig. Scrape off the insulation and solder on the wires from the top. File a notch in the reflector if you need it for wire clearance. Hey it's only a test, it doesn't have to look pretty. And compare the initial turn on with the three minute lumen value ratios. Best way to see if there is a difference between bare copper and insulated aluminum stars.


----------



## Harold_B (May 10, 2011)

More data for your consideration. Our pre-release XM-L's came on copper stars, one of which I mounted on a heat sink capable of dissapaiting 7W with a dab of Artic Silver compound. I did not calibrate our sphere but the reading are reference only anyway. I allowed the the sphere to vent and the heat sink and LED to reach room temp between tests. time in seconds, reading in lumens:

@1.0A:
0, 371
30, 371
60, 372
90, 373
120, 374
150, 375
180, 375

@1.5A:
0, 542
30, 540
60, 540
90, 539
120, 539
150, 539
180, 538

@2.0A:
0, 699
30, 696
60, 695
90, 695
120, 694
150, 694
180, 693

@3.0A:
0, 969
30, 965
60, 963
90, 961
120, 960
150, 959
180, 957

My two cents would be that the heat sink of the light being developed is not up to the task as the LED is stable when the heat sink is large enough.


----------



## archer6817j (May 10, 2011)

Okay I'm starting to get confused with my other thread. At this point they should probably be combined. 

My question for the day: Is it possible that thermal paste will "wet out" over time and become more efficient? 

I noticed Bergquist claims this on there site regarding thermal tapes. The reason I ask is I've been collecting more data and it's beginning to look like my lumen readings are going up over the course of 3 test runs. It's just a little bit, and I need more data, but it seems like there is a short "break in" period...certainly for bondply and maybe for AS-5. Everything else I've been keeping the same, including the ambient temperature. 

Also, right now I'm seeing about 36 lumen advantage with the MCE and AS-5 between 30-180 seconds and only a 21 lumen advantage at turn-on. The chart below is slightly revised from the one on my other thread about ANSI FL1 lumen reporting. It's an average of 3 test runs. I plan to do 2-3 more. 







It is becoming clear (as some have suggested) that the total thermal mass might be more of a limiting factor than the MCPCB material or the thermal interface material. Basically I'm seeing large drop between 0 and 30 seconds, and then it gets relatively stable, regardless of ply vs. paste. To me it seems like if I had more thermal mass in the head I could avoid that large drop between 0-30 seconds. This also seems to indicate you can't put much more power in a 1" light without loosing a lot of it. 

I'm talking to a couple of members about getting a copper MCPCB for testing so hopefully I'll be able to generate some more data for you guys. 

*Please note, all of the readings in the chart were taken with a bare LED mounted in a head (no optic, no window)*


----------



## Al Combs (May 10, 2011)

Wow, it's interesting the difference between a TIM and AS5 only took 1 second to become apparent.


----------



## archer6817j (May 10, 2011)

Oops! that axis label is totally wrong. Each mark is 30 seconds! I'll fix that soon.


----------



## Al Combs (May 10, 2011)

Ahh, thanks for the clarification.


----------



## bshanahan14rulz (May 11, 2011)

I don't remember where I saw this, so it's almost pointless to mention, but I think I remember Arctic Silver 5 claiming that part of the reason why their paste works so well is that over time, heat causes the specially-shaped silver particles to fit together better over time.

Edit: heh, I guess this is where I saw it 
http://www.arcticsilver.com/as5.htm


----------



## Oznog (May 13, 2011)

Thermal mass only _delays_ the problem slightly, and makes for an unwieldy flashlight (assuming this is a flashlight).
A flashlight needs a way to dissipate heat. Even a single 18650 light has a practical limit of about 3W before the case temp becomes way too hot in constant use. Takes about 20 min to reach an equilibrium temp, won't get hotter over time. You could add a 2oz thermal slug in there and it'll only delay the equilibrium by a few minutes and won't change the long-term running temp one bit.

Note this- the thermal mass on the front of the MCPCB is quite low. A gram of LED metal and top copper. Whatever the thermal resistance of the MCPCB insulator, the FRONT will warm up to a stable temp over the back temp within a few seconds! You have a 2C/W MCPCB, metal backing at 25C, turn it on at 10W, in well under 30 sec the die temp will be 45C and demonstrating the performance loss of a 45C die temp. Over the next minute, the thermal mass of the case will warm up quite a bit and increase the temp will increase further.

ANY longer-term effect _cannot_ be due to thermal resistance of the MCPCB. That's either localized heating due to poor thermal resistance to the case, or just overall case-to-ambient heating.


----------



## archer6817j (May 14, 2011)

Oznog, I think I get what you are saying...so, any suggestions related to design?


----------



## Al Combs (May 14, 2011)

Ultimately all the heat the flashlight produces has to be dissipated into the air. Fins are always a good idea if you have enough thickness to allow for them. They increase the surface area of the air to metal boundary. Passive heat sinks require thicker fins than ones made for use with fans. That 3/8" thick area directly behind where your star is mounted could easily have 2 or maybe 3 fins of decent depth that wouldn't compromise the strength of your light.

And I still think it couldn't hurt to at least try a copper heat spreader. The experts aren't always right.:devil:


----------



## archer6817j (May 15, 2011)

Al Combs said:


> And I still think it couldn't hurt to at least try a copper heat spreader. The experts aren't always right.:devil:



Hehe I know that's true for my expertise  It's also why I like applied research. 

So funny you should mention fins. I finned out one of my heads a couple days ago just to "see what would happen." I was shocked to find that it makes a significant difference even without any (appreciable) air moving over the fins. I reduced a 55 lumen drop to a 35 lumen drop at 30 seconds. 20 lumens isn't going to be visible to the eye...but it is a measurable difference. In fact, it's about the same magnitude as the difference between using bondPly and AS-5. 

I also went back over my data and found some results that didn't "fit." I went back over my testers and discovered a driver that is only putting out 2.3A...so I tossed those results and put my chart back together. Here it is: 






So, this means that a finned head with bondPly should almost be on par with a solid head and AS-5. This also tells me that the increases in efficiency and output are going to be VERY incremental. I'm going to have to add up several changes to make a (barely) visible difference in output. The only other option to turn that curve into a line...is a much bigger head. Maybe a lego turbohead with a lot of thermal mass and a cavernous reflector? :devil: I'd still like to bust the 800 lumen mark and that's not going to happen in a light this size. I have a pretty good TIR I mounted up with and XML and the best I can do is about 625 lumens. 

This also makes me wonder, in terms of overall flashlight design...I figure a solid aluminum head like mine should be fairly comparable to a star mounted to a small copper pill and placed into a separate head...a P60 drop-in for example. I don't actually own a lot of lights so I'm interested to know what the commercial companies are doing in terms of head construction and thermal management: surefire, Fenix, JetBeam, etc. It doesn't seem like their high power 800-1000 lumen XML based lights could really be putting out that many lumens at 30 seconds and beyond. Granted, the reflector I'm using is quite small and floody so it's not going to pump out the lumens like a much larger light. But it seems like in my "weight class" I'm pretty close to the top in terms of output. 

With that in mind, what else do I need to do in order to impress you guys?  I figure even with a copper MCPCB, AS-5 and screws, and a finned head I might get an additional 50 lumens max. Is all that worth it? Don't get me wrong, I'm a tuner. If it was just MY light I'd want to eek every last lumen out of it and [email protected] the expense. However, as consumers, do you guys care about that last 10%? I still like the idea of an uber MCPCB just cause it's cool! :naughty:


----------



## MikeAusC (May 16, 2011)

archer6817j said:


> . . . . . With that in mind, what else do I need to do in order to impress you guys?  I figure even with a copper MCPCB, AS-5 and screws, and a finned head I might get an additional 50 lumens max. Is all that worth it? Don't get me wrong, I'm a tuner. If it was just MY light I'd want to eek every last lumen out of it and [email protected] the expense. However, as consumers, do you guys care about that last 10%? I still like the idea of an uber MCPCB just cause it's cool! :naughty:



Lumen output isn't the only benefit of improved heatsinking - lower Junction temperature also improves LED lifetime and reliability.


----------



## HarryN (May 16, 2011)

I know that a finned head can help, especially if there is poor conduction from heat to body, but that isn't the case your light.

My prior (very crude) testing indicated that if the body and head are fully integrated like your design, and the metal sections are not too thin, then your body will move the first 10 watts rather than the fins. I guess that means that you could run a test comparing having your hand in the normal position on the light vs not, and see if it matters. If nothing else, more science. (or at least data for discussion )


----------



## Oznog (May 16, 2011)

First off, get rid of the thermal tape if you haven't already.
Well, you could machine a direct-to-aluminum slug yourself, and use aluminum flux to achieve tinning on aluminum, and direct-solder. It will yield SOME gains, but nothing mind-blowingly excellent. The basic limitation is these flashlight bodies add a substantial star-to-air thermal resistance and there's no way to improve that without getting extra wacky. There's also the question of how you're gonna supply a lot of power for this. Most 18650 li-ion batts doesn't like over 1C discharge rates, it will "work" at first but drastically reduce their lifetimes.

Imagine what you'd say to the person who says "I need to build a perpetual motion machine. If magnets won't work, what do you suggest?" Well, that's a problem, the path is not the issue- the goal is not possible, and thus the goals must be revised into something possible.

You can resolve that this will be super-powerful but only for intermittent use, <3-10min to avoid the body overheating. That's possible. 
You can add fins and/or fan for cooling, or some wacky water sponge. These are possible, and innovative, but they do impair their ability to operate as a convenient flashlight.

Perhaps you could clarify- do you want to build a STUNT flashlight that produces a ridiculous amount of light but not for extended periods, or actually a "practical" light you'd use for camping, car repair, power outage, caving, etc?


----------



## Mick (May 16, 2011)

For those of you that aren't afraid of "heavy lifting" there is a finite element analysis program with a thermal modeling component at a price that is way below the "pro" programs. It is from Canada and called LISA. It is NOT user friendly but great for the price. ($50 CDN) Just don't come to me for help using it. LOL

http://www.lisa-fet.com/


----------



## archer6817j (May 17, 2011)

Oznog said:


> Perhaps you could clarify- do you want to build a STUNT flashlight that produces a ridiculous amount of light but not for extended periods, or actually a "practical" light you'd use for camping, car repair, power outage, caving, etc?



Since you are using the future tense I thought I'd point out I'm already making lights. I'm just trying to figure out if I can (reasonably) make them any better. My current design allows for unlimited run time on high as long as you are holding it in your hand and can dissipate some of the heat. Since it has 3 modes you can also decide how much light/heat you can tolerate for your current situation. "With great power comes great responsibility."  

I do like the water sponge idea. I'll try that and post some photos! (good natured joking around here) It actually reminds me of water cooled sound suppressors...so maybe it's not that crazy of an idea. Uh oh, now you have me thinking.


----------



## archer6817j (May 17, 2011)

HarryN said:


> I know that a finned head can help, especially if there is poor conduction from heat to body, but that isn't the case your light.
> 
> My prior (very crude) testing indicated that if the body and head are fully integrated like your design, and the metal sections are not too thin, then your body will move the first 10 watts rather than the fins. I guess that means that you could run a test comparing having your hand in the normal position on the light vs not, and see if it matters. If nothing else, more science. (or at least data for discussion )


 
Hey Harry,

That's a good point. I've been letting the lights sit "in air" between tests...mainly to get consistent results. So, in many ways my readings are also "worst case" since you would normally be holding the light and cooling it. I would love to get around to some comparative data on "in air" vs "hand hold" at some point. I imagine if shallow fins alone makes a difference, hand holding would have a similar effect...maybe good for another 10 or 20 lumens.


----------



## Frobe22 (May 18, 2011)

There are three ways a flashlight body can get rid of the heat:
- Conduction through the users hand or an external (solid or liquid) surface.
- Convection through air. Larger surface increase heat loss.
- Emission. Body coating increase heat loss, polishing decrease heat loss.

To see how the surface treatment helps heat emmision check this artice from Newbie: http://www.molalla.net/members/leeper/coatbar.htm


----------



## bshanahan14rulz (May 18, 2011)

1: Machine some pockets into empty space in the metal
2: install valve to release gas only
3: fill pockets with rubbing alcohol
4: watch phase change cool your light. 
5: watch house burn down if the isopropyl ignites


----------



## archer6817j (May 18, 2011)

archer6817j said:


> My current design allows for unlimited run time on high as long as you are holding it in your hand and can dissipate some of the heat.



*Oznog! I modify my above comment.* I just did a "desk run" with the XML version and after about 5 minutes the light gets "maybe I should turn it off" hot. 3 minutes is warm but fine. My previous comment as based on actual use while I was on a recent camping trip. The ambient temperature was quite cold, as were my hands. So, unless it's really cold you can't run high continuously. I still attest that the "practical" mode is the meager 180 lumens on medium  

Just to confuse things. I rounded up the MCE version and after 10 minutes (fresh battery) it's hot but totally fine to hold in your hand. This is a bit of a mystery. I thought the XML was supposed to be more efficient?


----------



## archer6817j (May 19, 2011)

I had an inspiration last night and thought I figured out why the XML was getting so much hotter than the MCE. I concluded it was because the XML does not sit quite as high in the reflector, some of the light was leaking out (also accounting for the "low" output of 500 lumens), and heating up the head through direct radiation. I further posited that a "glow ring" version of the light with a GITD o-ring to fill the gap between the LED and the reflector would prevent this light from spilling out and heating up the head. I tried two things: 

1) I took several reflectors and progressively cut the base back so the XML would sit deeper in the reflector (3 different depths). I tested all of these in the sphere and, at most, saw an improvement of 13 lumens. Hmmm. This would seem to indicate that the extra lumens are NOT leaking under the reflector and heating the light. 

2) 10 minute run test with a glow ring installed. My last test was without. To my hand there wasn't really any appreciable improvement in the coolness of the light. It still got pretty hot. However, the MCE runs distinctly cooler. 

So the question is...why? Anyone? Thoughts?

Oh, and sorry to everyone for hijacking my own thread. I hope to get back on topic


----------



## MikeAusC (May 19, 2011)

archer6817j said:


> I had an inspiration last night and thought I figured out why the XML was getting so much hotter than the MCE. I concluded it was because the XML does not sit quite as high in the reflector, some of the light was leaking out (also accounting for the "low" output of 500 lumens), and heating up the head through direct radiation. . . . . .


 
An LED radiates very little heat as it's operating at < 100 degC - A light bulb operates at 5000 degC which is why it radiates a lot of heat.

That's why LEDs need serious heatsinking - the waste heat isn't being radiated out.


----------



## Frobe22 (May 20, 2011)

If I have done the calculations right a MiniMag 2AA anodized body at 65°C will radiate about 1W if the ambient temperature is 25°C.
A 45°C body will only radiate 0.5W, while a 85°C body will radiate 1.7W.
If you remove the anodizing and polish the body it will only radiate 0.1W.

Radiation is very important at high temperatures and less at low as MikeAusC says, but if the light is too hot to hold it may have potential to loose at least 1W with different surface treatment (anodizing/paint).


----------



## SemiMan (May 20, 2011)

You show the current into the LED, but not the current FROM the battery, at least not that I could find. I am guessing that there is something funky with the driver and the XM-L and that extra heat is from the driver. Just a thought.

Semiman


----------



## archer6817j (May 20, 2011)

SemiMan said:


> You show the current into the LED, but not the current FROM the battery, at least not that I could find. I am guessing that there is something funky with the driver and the XM-L and that extra heat is from the driver. Just a thought.
> 
> Semiman



Hey SemiMan. So the driver is rated @ 2.8A. When I measured current that is actually at the battery...not sure what that means for the actual drive current to the LED. I take the head off the body, sit the battery positive on top of the driver, one probe on the battery negative and the other probe on the head. I consistently get around 2.80-2.84A with a fresh battery. I "think" that's independent of the type of LED (MCE or XML). 

I also ran for 10 minutes, pulled the head off and stuck my finger on the driver...could barely tell that is was warm. I assume you meant the driver may be getting hotter with the XML but it doesn't seem to. Any more ideas?


----------



## SemiMan (May 21, 2011)

Is the "hot" uniform ... i.e. all surfaces on both the XML and MCE version? I.e. is there some funky heat transfer issue with the XML implementation?

Semiman


----------



## archer6817j (May 21, 2011)

It does seem uniform. I'm going to test multiple lights side-by-side before I dig my own hole any deeper.  maybe it's just that one sample. Fingers crossed.


----------



## archer6817j (May 24, 2011)

Hey guys, I just re-read this entire thread from the beginning and I'm hoping to bring it back on topic. Let's forget about the design of my particular light for the moment and just focus on the star. If you can't forget about the practical application, lets say the star is screwed down to a gigantic heat sink with AS-5  

I'd still like help withe an actual design for the ultimate copper MCPCB. Some have suggested that (given identical designs) that copper will not significantly outperform aluminum. I can live with that...but I still want to use copper...just because it's pretty. I reserve the right to change my mind later  

Part of the reason to make these, is to have stars for the lights I'm currently making. I also want to use them on future lights that will have much better heat sinking. I'd also like to sell stars to folks here on the forums for whatever crazy applications they come up with. In all cases I'd like these to be the "best" MCPCB's possible. And have better/versatile mounting options. Possibly two countersunk screw holes + typical cut-outs. 

I've been talking with several manufacturers and emailing with some folks on the forum about MCPCB design. One option, similar to what lux-rc is doing is a direct thermal pad connection: 






Apparently this works great but is really expensive. One manufacturer suggested thermal vias as an equally effective solution at much lower cost: 






This seems to make sense to me and gets around the issue of the dielectric pre-preg being a thermal barrier. 

At this point I'm leaning towards using vias. I just got some awesome info from a member here, which is the Phillips design guide. This white paper suggests that an FR4 based board with thermal vias can *outperform* an MCPCB. This is pretty heavy reading, very interesting, and maybe good information for Version 2. 

At this time I'm still going with metal core. So, let's spec out a MCPCB shall we? 

1) Use of thermal vias
2) 2oz circuit layer
3) 2mm total thickness
4) Fancy bergquist thermal pre-preg dielectric of some sort (3 mil) 
5) Actual layout (this is where I need a lot of help)
6) What else is there? 

Okay experts, have at it!


----------



## MikeAusC (May 24, 2011)

I think it's worthwhile questioning the fundamentals that drive current MCPCB design.

Why put an insulator between the LED and the star ? In most cases, one side of the LED can be electrically connected to the heatsink. 

So for lowest possible thermal resistance in this high heat-density area, solder the LED directly to the copper star, with just one terminal insulated. There are two options - 
- a. Machine a slot in the star around one contact pad on the underside of the LED.
- b. Modify the LED to electrically isolate one contact pad on the underside of the LED.

If you really want both terminals of the LED electrically isolated, then do the above for both electrical contact pads.


----------



## HarryN (May 24, 2011)

I probably just missed it, but what is the diameter of the board you are trying to make ? The reason I ask, is that that it might be possible to machine an existing star to the size you need. Digikey sells some pre-made stars for various LEDs.

edit - I see that it is for your 18650 size light, so I am guessing around 15mm.

In any event, if you go with the Bergquist material, the vias are no longer of real value, especially if they are more than a few mm from the LED thermal pad. Their stuff is not perfect, but it is pretty dang decent. Watch your minimum solder thickness spec though or you can get delamination of the LED from a MCB. Thin solder sounds great for heat transfer, but it isn't necessarily ideal for part life, as the solder also acts as a mechanical buffer to the substantial difference in CTE.


----------



## SemiMan (May 25, 2011)

> At this point I'm leaning towards using vias. I just got some awesome info from a member here, which is the Phillips design guide. This white paper suggests that an FR4 based board with thermal vias can outperform an MCPCB. This is pretty heavy reading, very interesting, and maybe good information for Version 2.



This is almost true with Luxeon Rebels as the heat pad allows easy thermal connection to vias. This is not true for Cree XPG where you have limited ability to use thermals or heat transfer.

If you want the ultimate, there was a company doing traces right on heat sinks and I am sure they could do it on copper. There was very little thermal resistance.

Semiman


----------



## archer6817j (May 25, 2011)

SemiMan said:


> This is almost true with Luxeon Rebels as the heat pad allows easy thermal connection to vias. This is not true for Cree XPG where you have limited ability to use thermals or heat transfer.
> 
> If you want the ultimate, there was a company doing traces right on heat sinks and I am sure they could do it on copper. There was very little thermal resistance.
> 
> Semiman



Do you have any more info on this? 

Also, soldering direct to the copper and isolating one pad in an interesting idea. Do you have any thoughts on exactly what that would look like? I'm a mechanical person, not so much electronics. If you can show me how to do it then I can get it done. 

Also, I'm looking at 20mm stars at this point. 

Side note: my crazy hot XML. I think there is a problem with the particular light I was testing. I don't have any other finished XML lights on hand to compare to but...I checked the tailcap current and it's at 3.5A (with a 2.8A unmodified driver) and the lumen output is also way lower than it should be even for 2.8A. Is it possible that the LED itself is somehow bad?


----------



## MikeAusC (May 25, 2011)

archer6817j said:


> . . . . soldering direct to the copper and isolating one pad in an interesting idea. Do you have any thoughts on exactly what that would look like? I'm a mechanical person, not so much electronics. If you can show me how to do it then I can get it done. . . .


 
All you need is a slot below one of the contact pads on the underside of the LED - the big thermal pad and the other contact pad get soldered to the copper.


----------



## sn0wBLiND (May 26, 2011)

imo the cheapest way to deal with this problem would be to use a separate fr4 pcb for the electrical contacts and then place it on top of a machined copper plate to get rid of the heat.







The picture is only a quick sketch I drew with mspaint but you should get the idea =)


----------



## Harold_B (May 26, 2011)

We have built prototypes this way and it is more expensive than you might think. The cost drivers are turned copper slugs and controlling tolerances. FR4 and copper will also expand at different rates as the LED heats up so thre will be a stress on the LED substrate to deal with. We were able to use the parts as concept prototypes because we were building chip on board and the connection was via wire bonds. Copper MCPCB is prety cheap in modest volumes.


----------



## Oznog (May 26, 2011)

Frobe22 said:


> If I have done the calculations right a MiniMag 2AA anodized body at 65°C will radiate about 1W if the ambient temperature is 25°C.
> A 45°C body will only radiate 0.5W, while a 85°C body will radiate 1.7W.
> If you remove the anodizing and polish the body it will only radiate 0.1W.
> 
> Radiation is very important at high temperatures and less at low as MikeAusC says, but if the light is too hot to hold it may have potential to loose at least 1W with different surface treatment (anodizing/paint).


Very true. Your aluminum polish is pretty but detrimental to radiation heatsinking, which is very important here.

You can anodize at home. I know you may not like the idea, but there is a physical, functional need to do so.


----------



## Oznog (May 26, 2011)

Harold_B said:


> We have built prototypes this way and it is more expensive than you might think. The cost drivers are turned copper slugs and controlling tolerances. FR4 and copper will also expand at different rates as the LED heats up so thre will be a stress on the LED substrate to deal with. We were able to use the parts as concept prototypes because we were building chip on board and the connection was via wire bonds. Copper MCPCB is prety cheap in modest volumes.


 
I would not glue the FR4 to the slug, just cut the slug slightly deeper than the board thickness, and make it donut-shaped as shown above and let it sit captive, but floating.

If you have a CNC mill, cutting slugs and board is accurate and no problem at all. Very low cost.


----------



## Oznog (May 27, 2011)

Aluminum is vastly cheaper than copper, and while I don't have a finite point analysis for an exact answer, it seems to me that the thermal resistance of solid aluminum is already insignificant so extraordinary measures (pure copper) to "reduce" it are uncalled for. And you can make the ENTIRE part here, like the pill and threads in both directions- out of a signal piece of aluminum rod, so there's no need for an interface between copper slug and aluminum. 

All you need is an aluminum flux and tin the mating surface first. Let it cool and wash off the flux and then reheat.  Drop the "O" board, apply the LED over it, let it cool, solder the pads with an iron.

How much resistance does aluminum have?

Let's see. Aluminum near room temp is 250 k - W/(m.K). The way we state thermal resistance is the reciprocal there- two opposite faces of a 1M cube are 0.004 C/W.

Let's say that after a depth of 1mm, the heat will probably spread out so much that thermal resistance is dramatically less. The critical part is the 2.8mmx4.8mm (13.44 sq mm) area under the XML pad, to a depth of say 1mm. The board will be thicker than 1mm, but the heat will spread laterally and greatly reduce the thermal resistance per mm by then. Say it's a 14mm dia circle, the area is - once it spreads laterally and uses the entire area, the resistance per mm of a 14mm round column is 11.44x lower that a column of the die area.

So even assuming there's a 2.8mm x 4.8mm sheer-walled column here 1mm deep, with no side spreading, the resistance to 1mm below it is 0.2976C/W. That's nothing- at 10W, it's only 3C increase. And that was already kinda conservative, if you elongate the "O" so a line of aluminum passes under the thermal pad, there will be a great deal of lateral spread and the numbers will be lower. So we don't NEED to get a finite point analysis tool in here to figure this out- aluminum estimates are already so low they cannot be a significant contribution to die heat at the 10W power level. Copper is at least 10x more expensive and only 60% more conductive.

Well the minimag CASE was projected at 40C/W there. I think that's too high- my single 18650 case eventually gets unbearably hot at 3W if it's not winter. I'm guesstimating 130F in an 80F room, comes to 9.26C/W. 

Basically shows that if you're after practical runtimes- more than the 5-10 min it takes to heat up the body of the case- the case-to-air thermal resistance IS the limiting factor. Getting rid of a 2C/W MCPCB thermal resistance is "nice" but will only increase capabilities by maybe 15%-20%. There is no significant difference between aluminum and copper slugs though.

Only if you are after a "stunt light", which can project much higher lumens but limited to <<5min runtime so the case doesn't heat up, does losing the MCPCB become a VERY attractive option, because the operating principle is such that we ignore the case-to-air thermal resistance and focus only on moving heat out of the die to the case at far higher rates.


----------



## HarryN (May 27, 2011)

Oznog - Those are good points. Using either copper, brass, or Al as a line or slug soldered to the bottom of the LED and through the FR4 is not a bad approach. I am not sure how I managed to forget, but that is how I built the 2xCR2 light in my profile.

In that case, I silver epoxied small copper discs to the bottom of the Lux V thermal slug and poked it through the FR4. 

The only concern I would have with soldering to multiple material types is that LED domes are pretty delicate, and especially if you use Pb free solder, the difference between reflow temperature and LED damage isn't all that large, and flux on an LED dome can ruin a lot of nice thermal path work. Brass and copper - pretty close, but throw Al into the mix, and that is one more thing that can go wrong. (at least for me)


----------



## CKOD (May 27, 2011)

Oznog said:


> Aluminum is vastly cheaper than copper, and while I don't have a finite point analysis for an exact answer, it seems to me that the thermal resistance of solid aluminum is already insignificant so extraordinary measures (pure copper) to "reduce" it are uncalled for. And you can make the ENTIRE part here, like the pill and threads in both directions- out of a signal piece of aluminum rod, so there's no need for an interface between copper slug and aluminum.
> 
> *All you need is an aluminum flux and tin the mating surface first. Let it cool and wash off the flux and then reheat*. Drop the "O" board, apply the LED over it, let it cool, solder the pads with an iron.
> ...


 
Nope, not so lucky. Yes you can solder to aluminum with the right flux, but reworking it isnt so easy, it doesnt stay tinned like copper does, its wierd. Ive soldered on aluminum shielded rigid coax that comes coated with a flux so you can solder it with plain electrical solder. Putting on RF connectors, and it may go on ok the first time around, doesnt turn out as nice as copper, or silver plated stuff, but if you ever try to re-work it, it goes to crap. It wont take the solder and most the time you have to cut it back and do it on a fresh area, or just trash the whole cable assembly you cant cut it back. If you want to play around with plating, you can "zincate" the aluminum part, then plate it with copper according to caswell plating, but by time you mill an aluminum slug, put on a little FR-4 electrical contact, mess with plating, and soldering, and given that the thread starter wanted to get boards made, you might as well go with what I linked in the first reply, 
http://www.metalcorepcb.com/en/pcb-capabilities/mcpcb/design-construction/stack-up-drawing-for-mcpcb

Where they just take a plain copper plate, lay on the dielectric (doesnt even need to be thermally conductive, just a plain dielectric) with a hole in it and plate in the center to make a direct connection to the substrate and you can make it with normal soldering practices. Once youre on the star, the watts/mm^2 of the heat coming out has dropped compared to what it was at the back of the die, and standard methods of mounting a star should suffice. Once you get a *thin* proper bondline and a lower power density, your thermal compound matters less and less. For testing, Ive jb-welded stars down to heatsinks and had it work fine, because I put it in a 2-ton press and hung some weight on the arm while it cured, to get an absolute minimal bondline. 


It would be nice to make the thermal path a solid piece but it brings up a few DFM issues 

Machining: you have to have a boss/pillar at the bottom of the light, if your cavity is deeper, this could require a cutter which is long and skinny, causing chatter issues, or slow cycle times if they have to baby it. Also, if your light was previously made on only a lathe, that adds a milling setup that might not have been there before. 

Plating: with the one peice body, youre certainly going to want the body anozide or plated, that means youre going to need to get the pillar masked off, which adds to the price, and if youre going to plate it with copper for solderability, then selectve plating of 2 different materials is even more difficult, ($$$) 

Solderability: as stated, reflowing aluminum without the aluminum flux can be problematic, the fluxes can be more corrosive to attack the aluminum oxide, But your anodizing is made from aluminum oxide, and maybe even the LED base too, so you better clean good. Also, 63/37 might not be the ideal solder, temp profiles could be different, have to check compatiblity with the LEDs temp profile. 

mechanical stability of the FR-4 electrical board: Assuming you have a slightly snug fit around the post, it should be ok, but if its not, the solder pads and the LED itself are the only thing preventing it from moving, be mindful when soldering, etc, as applying force to the board (via wires or accidental touching etc..) could transfer into board, and to the LED's ceramic base. 


It is certainly doable for a few units, but the cost increase and added difficulty of a few 'unusual' processes would make commercial lights problematic. Plus if you want to change emitters in the same body, youre S.O.L. vs a 20mm star where you just use the same housing with a new LED/driver/optics and keep trucking. 


But you are right on dissipation vs capacity, dissipation capability is more importanty then heat capacity, and with enough dissipation even plain aluminum/thermal dielectric stars with thermal grease/glue (or even tape if youre feeling lazy) will work fine.


----------



## Oznog (May 27, 2011)

You'd think, being an alloy of 65% copper, it would be a good heat conductor, but yellow brass (common type) is only HALF the thermal conductivity of pure aluminum. The alloy is important- well, ALL the copper and aluminum alloys take big hits:

http://www.engineersedge.com/properties_of_metals.htm

Hmm, my calcs are low on the aluminum thermal resistance there- you'll surely be working with an aluminum alloy, not pure aluminum, so those calcs should be about 50% higher.

Silver epoxy is much worse thermal resistance that silver grease, and this has significant consequences when trying to connect through a small cross-section of a Lux V thermal slug. I understand you needed to have it physically hold it down, but the bottom line is, this isn't a performance thermal solution at all.

LED domes are now made of optical silicone so they WILL take reflow temps. The dome shouldn't be exposed anyways, not if the slug's heated to the melting point of solder and the LED dropped onto it.

Nor should the dome be exposed to the flux. And I did recommend cleaning the aluminum flux off- that stuff has got to be funky, and I can't guarantee what would happen if left on the part. Once solder has wetted across the aluminum, that flux is not needed anymore.


----------



## HarryN (May 27, 2011)

The correct silver filled epoxies, properly cured, to a copper slug are very comparable to the results of similarly available greases available - at that time frame.

In addition, thermal grease provides no mechanical stability at all, while an LED, properly prepared, using cured silver filled epoxy can take a heck of a beating. I personally did this testing and remember throwing mounted LEDs at concrete walls to see if they would delaminate. 

Compare the thermal slug size of a Lux V (5 watts of power) compared to the size of the thermal path of a Rebel or XR-E - it was not so small, although it had other limitations. Compared to other available technologies at that time, it was a pretty reasonable solution.

Coming back to the more modern point, your idea of soldering the LED thermal path to a metal (pick your favorite) part and embedding this path into a circuit board so the heat goes through to the larger, underlying heat spreader is reasonable. I do similar things as well. Usually it works, but the yield is not 100% either, so it is hard to get commercial board houses to go along with it.

I usually don't rework parts (except proto work for my own use) so reduced yield is a cost issue, but that is just life.


----------



## Al Combs (May 30, 2011)

archer6817j said:


> Hey guys, I just re-read this entire thread from the beginning and I'm hoping to bring it back on topic. Let's forget about the design of my particular light for the moment and just focus on the star. If you can't forget about the practical application, lets say the star is screwed down to a gigantic heat sink with AS-5
> 
> I'd still like help withe an actual design for the ultimate copper MCPCB. Some have suggested that (given identical designs) that copper will not significantly outperform aluminum. I can live with that...but I still want to use copper...just because it's pretty. I reserve the right to change my mind later
> 
> ...



Is the pre-preg applied before or after the star is made? I really have no idea what the process is. Just going back to the Lux-RC thread for a moment, it was a visitor that said the copper star was machined. Looking at the picture it seemed more likely it was stamped out like a coin. Is it possible to have them stamped in one shop and coated with pre-preg by another? Is one of those $200 Harbor Freight 20 ton presses enough force to cut 2 mm thick copper with the proper set of dies? Just a thought...


----------



## Harold_B (Jun 3, 2011)

I know this thread has gone cold but I've been messing with an XM-L on a copper star and have been able to get a couple images from our thermal camera of a bare die. I have removed the silicone dome over several days by soaking the LED in IPA and then picking the chunks out under a scope. Hope the links work and I realize this is more of an FYI or curiosity at this point in the thread.

http://i1083.photobucket.com/albums/j390/Harold_LF/XM-LDie.jpg
http://i1083.photobucket.com/albums/j390/Harold_LF/XM-LTestset-up.jpg
http://i1083.photobucket.com/albums/j390/Harold_LF/XM-LTemp.jpg


----------



## archer6817j (Jun 3, 2011)

Harold_B said:


> I know this thread has gone cold but I've been messing with an XM-L on a copper star and have been able to get a couple images from our thermal camera of a bare die. I have removed the silicone dome over several days by soaking the LED in IPA and then picking the chunks out under a scope. Hope the links work and I realize this is more of an FYI or curiosity at this point in the thread.


 
Pretty cool images! Any chance you have shots of an MCE to go with that? I'm still trying to figure out why the XML is getting way hotter than the MCE when everything else is identical. I've tested a couple of samples of both and I get the same result. Several have suggested the driver is generating extra heat because the XML vF is lower, but when I open the light after it's been running, I can barely tell the driver is warm. 

Also wanted to mention by increasing my installation time and pressure (with thermal tape) my preliminary tests look a lot like I'm beating AS-5 with screws...more to come on that! Before you go crazy, I know that doesn't make sense on paper...but that's why I'm doing testing  I need to figure out if my test is screwed up or if the real world is not a perfect reflection of the theoretical world.


----------



## Harold_B (Jun 3, 2011)

I didn't but I do now: http://i1083.photobucket.com/albums/j390/Harold_LF/MC-E.jpg

I tested the XM-L at 2.0A so it wasn't maxed but then tested the MC-E at 700mA which is the rated max for that LED. Also, XM-L has a copper MCPCB and the MC-E has an Aluminum MCPCB. Perhaps those differences are why my results are the inverse of yours.


----------



## MikeAusC (Jun 3, 2011)

I wonder what these IR temperatures are telling us about the chip temperature - the silicon is covered by the phosphor.


----------



## Harold_B (Jun 3, 2011)

Mike - Phosphor was removed along with the silicone dome (that's the bare die in the XM-L photo) for the XM-L and the MC-E is a "Dental" blue (no phosphor in the first place). One thing the images are telling us is that the copper star is spreading the heat better than the aluminum.


----------



## archer6817j (Jun 3, 2011)

Harold_B said:


> One thing the images are telling us is that the copper star is spreading the heat better than the aluminum.


 
WOW, science makes me giddy. Any discussion around this? Looks fairly clear cut to me but obviously it would be even more conclusive if the drive current was the same. Is the MCE wired parallel? The ones I use are series wired so I can push 2.8A through it. I use the same driver for the XML.


----------



## Oznog (Jun 3, 2011)

Harold_B said:


> Mike - Phosphor was removed along with the silicone dome (that's the bare die in the XM-L photo) for the XM-L and the MC-E is a "Dental" blue (no phosphor in the first place). One thing the images are telling us is that the copper star is spreading the heat better than the aluminum.


 
Neat... but were these heatsinked or what? How would you compensate for the difference in thermal mass?
Copper's about 40% greater thermal mass than aluminum, for the same volume. In actual installed usage, it is not significant in helping performance. In short-term off-the-sink tests, a device on a star with the same dissipation will stay notably cooler on a material with a greater thermal mass.


----------



## Harold_B (Jun 3, 2011)

OK, the thermal images are tough to see detail but yes they are heat sinked. Same heat sink with a heat pipe capable of dissipating 7W of heat. The LEDs were run at their current limit or within the rated limits per die. Artic Silver between the star and heat sink. Current limited supply.

My assertion that the copper is doing a better job stems from the images: the surface of the XM-L copper star is at a higher temperature relative to the die as compared to the temperature gradients on the surface of the MC-E aluminum star. The other supporting data given that the test set-up is consistent (relatively) is the disparity between the die temperature and the ambient temperature which is higher for the MC-E.


----------



## HarryN (Jun 4, 2011)

Interesting images - thanks. The heat sink that you are using has impressive heat movement capability.

I am still confused on the aspect of different results though between testing of the MC-E vs XM-L. (as far as heat generation, not just the heat removal aspect) For purely speculation purposes, here are a few wild ideas:

a) One large die vs multiple small die 
- The muliple die MC-E package allows a firm to do more selective matching of the die properties, and a potentialy weak region of the wafer can be eliminated. When you make a single large die, it is either yes or no on the overall spec. In summary - maybe on average a die is good, but perhaps some sections are better than others?

b) Vf variation between the packages.
- Cree makes nice parts, but AFAIK, they don't bin by Vf. Vf x amps = heat, not just amps. Since the driver is trying to produce a constant current, then it will attempt to drive amps, not power. In theory, this Vf difference can be measured.

c) Maybe they are not all real Cree parts
- It isn't easy to tell a "real" vs "counterfeit" Cree part, and we all know they are out there. It might be nearly impossible using normal methods, so even a good supplier of boards / stars / lights can be fooled. The difference might not show up until a test like this one.

d) Other ?

Just ideas to ponder.


----------



## Harold_B (Jun 4, 2011)

It's a nice heat sink for bench testing but not very practical if I wanted to use a bunch of them for a product. Too expensive (don't have the pricing at hand but just recalling the project for which they were purchased). I can put bullet "C" to rest though: the LEDs were samples direct from Cree through Arrow.

In the context of the OP the only way I could really nail that question quickly with the thermal images would be to test a few samples of the same bin of one selected model of the LEDs on both copper and aluminum stars. I just don't have those samples handy. It may be in our test plan for our own product and if so and we don't consider it proprietary then I'll post the results.


----------



## archer6817j (Jun 24, 2011)

new twist...is it better for the mcpcb to be thick...or thin?


----------



## HarryN (Jun 24, 2011)

Figures you would ask a question like that. Now you are really into the thermal modeling aspect to obtain a correct answer.

In general, you are trying to spread the heat to the walls. That means that the overall thermal path (including the mcpcb and the heat spreader behind it) are involved in the thermal path.

The largest impact on resistance to thermal spreading on most MCPCBs happens in the first 0.1 - 0.3 mm vertically and 1-2mm horizontally from the LED thermal pad. If that resistance was zero, then the optimum thickness of the "total" would be very close to the same as the distance from the LED to the sidewall. Since it isn't anywhere near zero, but in fact - substantial, then numbers more like 10 - 20% of that distance are more appropriate.

I did a board caculation for a multi rebel setup, and at 1.5 mm thick copper (total) the lateral thermal resistance from the thickness was in the noise.


----------



## Oznog (Jul 4, 2011)

CKOD said:


> Nope, not so lucky. Yes you can solder to aluminum with the right flux, but reworking it isnt so easy, it doesnt stay tinned like copper does, its wierd. Ive soldered on aluminum shielded rigid coax that comes coated with a flux so you can solder it with plain electrical solder. Putting on RF connectors, and it may go on ok the first time around, doesnt turn out as nice as copper, or silver plated stuff, but if you ever try to re-work it, it goes to crap. It wont take the solder and most the time you have to cut it back and do it on a fresh area, or just trash the whole cable assembly you cant cut it back. If you want to play around with plating, you can "zincate" the aluminum part, then plate it with copper according to caswell plating, but by time you mill an aluminum slug, put on a little FR-4 electrical contact, mess with plating, and soldering, and given that the thread starter wanted to get boards made, you might as well go with what I linked in the first reply,


 
I did some playing around with the aluminum flux. It most certainly DOES stay tinned!

However, there did seem to be complexities and problems. It seems that the flux activates as a specific temp, at which point the solder must be in contact with the aluminum to alloy into it. The problem is, there may be spots where the solder blob bridges over an unreacted spot and excludes the flux so it won't ever wet the aluminum. Heating it up again and removing most of the solder through suction, wicking, or flicking showed that. 

Granted, I only played with it a little bit, and my efforts were crude- it's difficult to heat up the part. The soldering iron's heat transfer is very low until the solder wets the part, but it won't EVER wet the metal until it gets hot enough to activate the flux, and of course the part has to reach the melting point of solder to wet too.


----------



## CKOD (Jul 5, 2011)

Well in that case, I hate that aluminum rigid coax even more now... lol


----------

