# LED Minimum Current ??????????



## Holepuncher (Oct 16, 2008)

New here - First post

A little background on what I'm doing.

I erected a lamp post in front of my house with a simple frosted globe. I designed a circuit board for it with an 8051 flash based microcontroller, 3 sets of LED's - 3 Each of red, green and blue (series connected) , plus some white ones all of which are pulse width modulated by the 8051. The flash memory can reprogrammed via a serial connection to my desktop PC. In this fashion I can have the light slowly fading to various colors or pick any steady color just by compiling a new program and sending it out to the light. Halloweens coming so I guess that night I'll have it making various eerie red and orange colors mixed with some brilliant white flashes. You get the idea.

Anyway I made the mistake of using Opteks 1 Watt Cup series of LED's. Nasty buggers to use and they are failing. I am only driving them with about 200 ma max so I think the failures are soldering related and were probably over heated then. Now I'm looking for something easier to use.

I bought one peice of OSRAM's Platinum Dragon series and it looks like they will be easier to solder without destroying them But I notice their spec sheet stated not to use them below 100ma. Can anyone explain this?

Also maybe you experts here can recommend an ideal Led series for this application. They do need to be small as I am limited to a 3" Diameter printed circuit board. By the way I have 30 years experience designing stuff and soldering things but I just could not solder those little buggers. Took hours.

Thanks


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## R33E8 (Oct 16, 2008)

Did you use mount the LED on a heat sink?


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## Oznog (Oct 16, 2008)

AFAIK the 100mA restriction is due to less efficient operation and spectral content not meeting spec below 100mA. It should not damage the device. They're saying to pulse it at the nominal current- like 300mA- at a 1/3rd duty cycle instead of giving it 100mA.

Maybe you didn't heatsink them properly, maybe the soldering was bad. There are other less likely things that can cause problems, like reverse voltage.


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## Holepuncher (Oct 16, 2008)

Thanks for your replys

The LED's do not have "Ideal" heat sinking but what I have should be adequate. They are on a 4 layer circuit board with plane areas on all 4 layers. I realize the internal layers are probably useless as far as heatsinking. At 200ma forward current I am seeing about a 100 mV drop in forward voltage. A little more or less depending on the LED's color. Unless I'm wrong this should mean about about a 20 to 25 deg C rise in junction temperature. Not enough to burn them out. If my thinking is wrong here please correct me. The LED's that failed will still light if I apply pressure to the top of the package.


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## Oznog (Oct 16, 2008)

Unless you use a specific thermal via construction, only the top layer has any real dissipation, even in a 2-layer board. In a 4-layer board internal planes have no significant effect.

Now consider this. Say I have a very thin copper foil with 10 sq feet of area. If I mount a device in the middle, does it really get the benefit of all that area? Or does it just get hot in the middle?

Well, it turns out the ability to spread heat laterally is easy to calculate for the "simplified" cases. For a 1mm wide trace, to a point down the trace is 70 C/W per mm.

Unless you requested otherwise, this is probably 1oz top copper. That spreads only a few mm from the device's perimeter before the thermal resistance to those points is too high for the copper further out to have any significant effect on total dissipation. If you have 3oz, great, basically adding more copper area will be useful for a pad 3x larger, but the area is still limited.

Also top copper is usually masked, which reduces its dissipation quite a bit.

Unfortunately, I suspect you simply cooked them. It is very difficult to build custom arrays with effective thermal solutions out of these nonisolated thermal pads because of those isolation problems. Now the 100mV drop would only be a 20C temp rise by the spec sheet, but in actuality that measurement is quire difficult to perform accurately. For one, it takes very little time for top copper and die to heat up at 200mA and thus it is easy to obtain the wrong value for the "cold" Vf @ 200mA. Also "temp coefficient of forward voltage" in the spec sheet is only a "typical" value with no max/min listed. Even with an accurate delta-Vf, the calculated value of Tj is only as accurate as the temp coeff of Vf. It must be measured to be of use.

Say, you didn't connect the thermal pads to anything else, or to each other, did you? I didn't see where the Optek spec sheet actually described the thermal path as isolated or not but if it doesn't say then it's safe to assume it's not.


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## Holepuncher (Oct 17, 2008)

Well I guess I got myself in over my head on this one. My women absolutely loves this light and is on me to get it working again. Until now the only thing I have used LED's for is PCB indicator lights.

The 4 layer circuit Boards cost me $0. A few times a year my company has boards made and I "snuck" mine in on the corner of the panel. In a month or so when we make another run I guess I could make some changes to the layout but until then I'll have to figure out something.

The reason I thought it was due to soldering was the first time I powered it up some of the LED's strings would only light intermittently or would light up dim and flicker. These are the ones that have now failed. The other 2 leds in the failed string still seem okay. I guess for now the only thing I can do is lower the drive 25% or so.

So how does one determine the junction temperature of these devices keeping in mind that this is just a hobby project. My fingers tell me they are just slighty uncomfortably hot to the touch.

The thing that gets me about all this power LED stuff is that there seems to be very little in the way of application notes out there (at least that I can find). Guess I should have consulted here first before I dove in head first.

Oznog: You seem to know your stuff but your last statement about isolated thermal paths went way over my head. And yes its 1 oz. copper and is masked but i did place very large pads at the solder points and thus they are unmasked. I tried to do the best I could with my limited knowledge of LED's and the available space I had.


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## Steve K (Oct 17, 2008)

hi Holepuncher,

I've done various small projects with power LEDs, and I think Oznog is on the right track. I wouldn't install a 3 watt LED (for instance) without a moderately large heatsink exposed to ambient air. i.e. 10 square inches of 1/16" aluminum is a good choice, in order to make me sleep better. 

Most power LEDs include a spec for thermal resistance from junction to case, or junction to ambient (assuming some nominal circuit board area). For instance, the Cree XR-E has a thermal resistance of 8 deg C per watt between the junction and "solder point". I'm going to assume that the solder point is where the emitter is attached to. If the LED dissipates 3W, then the junction will be 24 deg C above the temp of the solder point. 

The big question is going to be "what's the thermal resistance of the circuit board?". Without better info, I'd say that you need to be conservative in the design. Use a large enough heatsink that you don't detect any rise in temperature in the LED. Allow plenty of airflow around the heatsink. 

ESD is another good issue to consider. If you have long wires going to the circuit board, use decoupling caps where the wires enter the board (more of an issue for the power and ground wires than the serial data wires). Add a modest zener diode at the LEDs themselves, just for insurance.

The project sounds like fun. With luck, a conservative hardware design will keep it running for a long time. 

Steve K.


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## Oznog (Oct 17, 2008)

Holepuncher said:


> Oznog: You seem to know your stuff but your last statement about isolated thermal paths went way over my head. And yes its 1 oz. copper and is masked but i did place very large pads at the solder points and thus they are unmasked. I tried to do the best I could with my limited knowledge of LED's and the available space I had.



You wouldn't be the first person to underestimate the cooling needs of power LEDs, not at all! We like to keep LEDs as cool as possible. The 125C or whatever is Absolute Max. Actually operating at elevated die temps reduces efficiency and significantly lowers the lifetime.

1 oz copper only has a useful spread of ~2-3 mm past where the device solders to it. Past that, the resistance between the heat source and copper is simply too high to be of much additional value. So say the LED's thermal connection is soldered to a 4mm wide linear trace and that trace is soldered to something like a finned copper heatsink (this is hypothetical, you generally can't actually do something like that) only 3mm down the trace. Well, that's 53C/W of resistance between the thermal lead and the sink so even if the sink is at 25C with infinite heat dissipating capacity (perfect/unobtanium), the LED's junction will be at 25C+2C+53C=80C running at 1W. In reality your pad is not a line leading away from the thermal connection but radiating in all directions, but still, the resistance of a huge pad is only marginally better than a ~3mm radius pad.

Note we're only calculating how well heat transports across the pad, not how well the exposed copper _dissipates_ the heat transported to it. For unmasked bare copper, since it has no fins and has a near zero infrared radiation coefficient, there's only a small amount of thermal convection and it's a very unpredictable degree of convection without a fan. Orientation matters since heated air rises. Even WITH a fan the dissipation would still be inadequate. It is very plausible the Absolute Max die temp was exceeded which destroyed the devices in short order.

Isolated thermal paths. Ah, I misread the Optek spec sheet, they shared the thermal path with the cathode so it's clear. See in most devices (but not Optek Cup), there's an anode, cathode, and a separate thermal slug. Many devices, esp earlier ones, the thermal slug is "not electrically neutral" which may mean tied to anode, cathode, or some unknown mystery internal voltage. It can't be electrically tied to a grounded heatsink, nor can 2 LEDs be electrically connected through the heatsink. And any electrical insulation we put in the way will always reduce the thermal conductivity to the sink by some degree, maybe large, maybe small. Da bomb are "electrically neutral" ("isolated") thermal pads that can be in electrical contact with grounded sinks and shared between devices, especially the solderable pads (some are a non-solderable alloy like aluminum).

Did you place any LEDs or strings of LEDs in parallel, BTW?


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## Holepuncher (Oct 17, 2008)

Thanks Steve and Oznog for your reply. I'm learning a lot here. I'll have to read both of your posts couple more times to fully digest

I think with your help we can salvage this design until the next time my company makes a run of boards and I can sneak in another. I think I'm somewhat safe with the red, green and blues. If they last a few months - Fine. I'm not going to worry about $3 or $4 each to replace them. After all this is a pastime and pastimes cost money. The way I'm using it the R G and B are never fully on for more than a second or so as its just changing and fading through the spectrum of colors so they are probably not seeing too much abuse. At 200 ma the colors are more than bright enough and I can have the firmware cut the max pwm level down a bit and still be bright enough. Where the problem will lie is with the separate white Led's I am using. I want the light to also be a functioning light source for the walkway and the whites will need to be driven hard - harder than I am driving them now.

I'll give you a few more details of the layout. You will probably have a good laugh on this one but my stupidity is what may salvage this design.

As I stated before I have a 3" diameter "main" board with the processor, its support components, voltage regulators, serial level translator and 10 NPN transistors which drive the LED's. The processor is a 40 pin dip so it eats up alot of the board area. No LED's are on the main board but instead there are 10 sockets (5 on each side of the 40 pin dip) for the LED boards to plug into. Now heres where you will roll on the floor. The led boards are 1/2" by 1".

With that in mind there must be some way I can attach some type of heatsink under these boards although I only have about 3/8" space under the boards. The LED boards plug in in such a way that they hang over the processor. I have access to a complete machine shop and a sheet metal shop so maybe I can fabricate some type of heat sink

The other option I guess is, since I have 10 sockets I can just use lots of white LED's but run them at lower power.

Or I can get a new lady of the house that likes to knit rather than look at a pretty color changing light.


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## Oznog (Oct 17, 2008)

It is impossible to sink those boards by attaching them to a sink. The FR4 board provides so much thermal resistance even if the back were perfectly maintained at 25C the LED on front would _still_ overheat.

On 2-layer boards there's a "thermal via" construction that packs in the copper-plated vias so densely that they are very effective at transporting heat to the back. However, that's where the "electrically neutral thermal path" becomes necessary (which you don't have). The pads can't be in electrical contact with one another or the heatsink you want to glue it to and vias will conduct. Placing an electrically insulating thermal pad will cause major problems in this type of construction so that's out.

Best thought, see if you can find an MCPCB will fit those devices. The Cree board construction MIGHT work, but the problem is they made the center nice and wide and the side contacts thin because the center is the thermal pad on the Cree whereas the Optek thermal pad is on the outside.

Or contact Optek about getting th MCPCB designed for those things. Find out for sure if the board actually isolates the thermal path from the back or not.

Note that an MCPCB is NOT a heatsink in itself. It is only a heat _spreader_ and electrical insulator (usually it's an insulator). It can dissipate the heat of a 1W device but only barely. Even 1W devices need a sink.

Honestly? I think these devices are far too difficult to deal with and are a total bust. It will be much cheaper to start over with a new device. I think I'm pretty free of the "hey look the n00b screwed up" arrogance and all, that's just what I see as the reality of the situation. I don't see a way to fix this cheaply. It would be expensive to go looking for boards and try to build a new sinking solution and it looks like the performance will suck- that is, with 1-up MCPCB stars from Optek it sounds like you won't even get one to fit in a housing much less R+G+B+1 white, much less an even more powerful mix you might be able to accomplish with more design. 

Actually the AsianSignals 6x board with the Rebels isn't a bad choice. I tested that one and it performed kinda well for 6x devices, although I'm still annoyed that they failed to take advantage of the space they needed to put vias in. The high bin Rebels are incredibly powerful but 6x 1W devices requires a significant heatsink.


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## Holepuncher (Oct 18, 2008)

Yes I'm the noob that screwed up. When it comes to electronics I have been doing things the hard way all my life. Never went to school for electonics. Learned it all myself and made lots mistakes along the way but had lots of fun doing it.

Anyway I decided to do some destructive testing. I took one of the still working white Cup led's and made connections to it via 30 ga wire-wrap wire. Now I assume that small a wire will offer nothing in the way of heat tranfer. I fed the led 200 mA. I gets quite hot and within 30 seconds I observe a noticeable dimming and a shift in color. Left it like that for a few hours and decided to up the ante. Now I am feeding it 275 mA. It is sizzling hot. In 5 hours or so it will have been going for 24 hours at over .75W with no heat sink and this is an led that has already been "cooked" as Oznog put it. 

My little boards must certainly be providing a small degree of heatsinking because when soldered to the board the devices do not get nearly as hot as with the 30 Ga wire hookup and I do not see any noticeable color shifts or dimming. How long will they last is something I dont now but I'll be willing to bet it will be in the 1000's of hours.

My biggest mistake was choosing the Optek Cup series for a first power led project. I dont think I could have picked a worse one. I have a full set of the Platinum Dragons here now. They seem much more robust and easier to use. Also seem a little more efficient as well and the whites have a nicer color than the Opteks.

I'll keep my destuctive test going and see how long before it pops and post back.

Oznog and Steve K. I realize everything you guys told me is great information and I have a lot to learn about these devices. I appreciate it.


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## Oznog (Oct 18, 2008)

Well the Platinum Dragon still has the same problem- the sink is connected to the anode. So you could get MCPCB-mounted versions and check to see if they provide isolation and if not, provide it with special-order thermal pads... or have a live heatsink and abandon all thought of putting any in series.

Devices running "hot" can lose a substantial chunk of their efficiency thus lower output while hot. They also degrade faster. Not necessarily fail outright, but the output permanently degrades by tens of percent in hours or tens of hours or hundreds of hour, whatever.

In truth people simply cannot perceive differences in brightness well. Looking at a single device, I could turn it up or down by 20% from day to day and most people wouldn't be able to distinguish the difference very well. Taking a device and saying "now turn around"... (reduce output by 15%) "ok turn back" _sometimes_ people can tell the difference. Having 2 devices side-by-side or turning up/down the output while looking at it, people can generally see the difference right off the bat.

Nonetheless there IS of course a benefit to having more light, it's just hard to perceive clearly with the retina's inconsistent response to light under varying conditions.

The point is, by the time you can actually _see_ the light is definitely lower due to high heat, it's probably really really toast.


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## Holepuncher (Oct 19, 2008)

"The point is, by the time you can actually _see_ the light is definitely lower due to high heat, it's probably really really toast."

Well my destructive test Led is still going strong and sizzling hot. I can smell the heat and its color is yellowish with a hint of green. If I disconect it for 30 second or so and reconnect it it seems to go back to normal white color and brightness for 10 seconds or so. I said seems to. I'm sure if I had some way of measuring the output it would be a different story. You are right about the perception of brightness. I seem to have a little trouble with it myself. 

I did this foolish test to prove 2 things. One that my inadequate FR4 boards are keeping the temperature down to an acceptable level though I'm sure I'm losing lots of life. Second - one would get the impression, from reading posts from noob's like myself who destroyed their expensive led's, that these devices are extremely delicate and unforgiving. You have to really push the limits to totally destroy one. I'm sticking to my original reason for my led failures. To much heat from soldering the miniscule Cup package. I'm going along with your theorys about the FR4 not being adequate but only to the extent that I'll get much less life out of them.

I'm going to replace all the Cup's with the Dragons. I dont know if you have ever seen the Cup's but the are simply two peices of thin stamped metal glued together with the die sandwiched in between. In my case the two halves separated slightly causing loss of connection to the bottom of the die. The die is still functional. The Cup's are definetely not for the home hobbyist.

The Dragons construction is much more substatial. In my opinion if I did the same destructive test with the Dragon they would fare much better in free air with no sinking although I'm not willing to try it because I paid $8 each for the white ones.


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## Oznog (Oct 20, 2008)

But what's switching to the Dragons gonna do for you?
Do you have a thermal solution, like are these Dragons mounted on an insulating MCPCB?

If you stick with the FR4 mounting, your heat problem remains mostly unchanged.

See I'd kinda steer you towards one of the more common Luxeon, Cree, or SSC lines. You can get mounted devices or a small number of aftermarket MCPCB board for them. Those are easy.

While your assertion isn't really specific, I would sort disagree with the idea that these LEDs are "tough". Running them hot is pretty significant degradation over time.


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## Oznog (Oct 20, 2008)

Maybe consider this:
http://www.kaidomain.com/ProductDetails.aspx?ProductId=5379
Combined with any whites you might find useful. There's a huge number of whites on KaiDomain/DealExtreme, though some are crap. There's a massive 20W LED for example that people keep thinking will be awesome but its actual lumen/w output is so poor a much cheaper P7 can beat it with much less power and heat.


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## Holepuncher (Oct 20, 2008)

Oznog

For some reason I get the impression you are not reading my posts or totally dismissing anything I am saying.

Back to the beginning. I was running some led's in a light fixture at 1/2 their rated power on FR4 boards on which I made some feeble (or desperate) attempts at providing some degree of heatsinking. 2 leds out of a total of 12 failed. My reasoning for the failure was that it was from soldering the difficult to solder and tiny CUP led's. You are convinced that I fried them due to inadequate heat sinking. Sorry but I disagree.

Then I took a still working led (which at this point should also be nearly destroyed from my inadequate heatsinking) and tried to destroy it by running it with no heatsinking at all, at 1/2 its rated power. At this power level I observed no failure or significant color shift or dimming. I then increased the power to approx 3/4 watts for 24 hours and as I said there was some color shift and dimming which seemed to return to normal after I let it cool. Since then I have increased the power to 1W and then to 1.25W. At 1.25W it went out. Why? The solder melted on the bottom of the package. I think the melting point of solder is around 200C. You can probably do the math of what the actual junction temperature was. I resoldered it and it is now running at 1W. This is the LED that will not die. Yes I'm sure I have done some damage to it and I intend to kill it eventually

So what is switching to the dragons going to do? They are significantly larger, better constructed, more robust and contain a significant amount of copper in the slug and leads. What does this do for me? Well for one they can take the heat of soldering better I assume. Second, running these at 1/2w in free air they do not get as hot as the CUP's. 

That link you posted. Are those things really 4" diameter? The base of my light fixture is only 3.75".


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## R33E8 (Oct 20, 2008)

Is there any reason you refuse to heat sink the LEDs? There are almost no benefits to leaving it in open air. You will get less light and a much shorter life. Just put the LED's on a piece of aluminum from Home Depot and forget about the problems.. I also don't understand how your LED somehow reached 200C because LEDs will fail completely when the junction temp is at 150C... Running the Dragon's at 1/2 watt will generate the same amount of heat as anything else at 1/2 watt.. 1/2 watt of heat will always be 1/2 watt of heat.. Even if the Dragon is slightly more effecient in converting electricity to heat, it's not going make that much of a difference...


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## Holepuncher (Oct 20, 2008)

"Is there any reason you refuse to heat sink the LEDs? "

Come on guys. You talk like 1/2w is some astronomical amount of power and I need to mount these things on a heat sink the size of a toaster. Its fleapower. I am heatsinking them to my FR4 board and running them well below their rated power.

"I also don't understand how your LED somehow reached 200C because LEDs will fail completely when the junction temp is at 150C."

From the dragon Data sheet: Notice the last row where TJ=175C
*Exemplary median Lifetime*[FONT=Arial,Helvetica]2) [/FONT]_[FONT=Arial,Helvetica]page 20[/FONT]_*for median Brightness *

[FONT=Arial,Helvetica]I[/FONT][FONT=Arial,Helvetica]F [/FONT][FONT=Arial,Helvetica]= 500 mA T[/FONT][FONT=Arial,Helvetica]S [/FONT][FONT=Arial,Helvetica]= 25°C[/FONT]
[FONT=Arial,Helvetica]25.000 (blue, deep blue)50.000 (green) [/FONT][FONT=Arial,Helvetica]operating hours[/FONT]

[FONT=Arial,Helvetica]I[/FONT][FONT=Arial,Helvetica]F [/FONT][FONT=Arial,Helvetica]= 1000 mA T[/FONT][FONT=Arial,Helvetica]S [/FONT][FONT=Arial,Helvetica]= 85°C [/FONT]
[FONT=Arial,Helvetica]8.000 (blue, deep blue) 20.000 (green) [/FONT][FONT=Arial,Helvetica]operating hours[/FONT]

[FONT=Arial,Helvetica]I[/FONT][FONT=Arial,Helvetica]F [/FONT][FONT=Arial,Helvetica]= 750 mA T[/FONT][FONT=Arial,Helvetica]S [/FONT][FONT=Arial,Helvetica]= 125°C T[/FONT][FONT=Arial,Helvetica]J [/FONT][FONT=Arial,Helvetica]= 150°C [/FONT]
[FONT=Arial,Helvetica]1.000 [/FONT][FONT=Arial,Helvetica]operating hours[/FONT]

[FONT=Arial,Helvetica]I[/FONT][FONT=Arial,Helvetica]F [/FONT][FONT=Arial,Helvetica]= 750 mA T[/FONT][FONT=Arial,Helvetica]S [/FONT][FONT=Arial,Helvetica]= 150°C T[/FONT][FONT=Arial,Helvetica]J [/FONT][FONT=Arial,Helvetica]= 175°C [/FONT]
[FONT=Arial,Helvetica]100 [/FONT][FONT=Arial,Helvetica]operating hours[/FONT]


"Running the Dragon's at 1/2 watt will generate the same amount of heat as anything else at 1/2 watt.. 1/2 watt of heat will always be 1/2 watt of heat"

The dragons have a much larger metal mass than the CUP's. The heat sink slug also carrys out the side of the package to the anode lead providing another heat transfer point. The larger mass makes them more forgiving to hand soldering problems. The package is significantly larger than the CUPs and the exposed metal surface area is greater meaning that in free air the package can safely dissipate more power than the CUP's. Take the common LM317 linear regulator. If it were dissipating 1/2w in the TO220 package in free air it would be quite warm but in the TO3 package it would be barely warm. 

From the above chart The life stated for a solder point temp of 85C at 1000ma is 8000 - 20000 Hours. With my "alleged" useless FR4 heatsinking I know I am not even close to 85C. I have no way of measuring the temperature but I'm certain my fingers would not like touching 85C. Even If I were reaching 85C, 8000 hours will give several years life of nightly usage. I'll take it.

"There are almost no benefits to leaving it in open air. You will get less light and a much shorter life. Just put the LED's on a piece of aluminum from Home Depot and forget about the problems."

I am not running them in free air. Thats just an experiment. "Just putting" the LED on some aluminum is much easier said than done.


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## Oznog (Oct 20, 2008)

Holepuncher said:


> The larger mass makes them more forgiving to hand soldering problems. The package is significantly larger than the CUPs and the exposed metal surface area is greater meaning that in free air the package can safely dissipate more power than the CUP's.



Well, I wouldn't be so quick to conclude it was all soldering problems. How long did you have the iron on it, anyways? Actually ESD from handling may be significant- also I would not altogether rule out manufacturing defects.

What R33E8 said- don't discount the need for heatsinking, even at 1/2W. With your FR4, and I assume some sort of enclosure, it's _really_ hard to predict what the dissipation will actually be. It could be 30C/W or over 100C/W I really don't know. Convection matters quite a bit so orientation can change the dissipation too. Also note that by 125C you can lose 25% or more of the light output.

The thermal mass of the device isn't going to help you as much as you think. In fact it can make it _worse_ since it takes awhile for it to ramp up and cool down, it will actually expose the die to these high temps for longer. 

If you get solderable pads, you can get some solder paste from DealExtreme (cheap as hell, free shipping, hard to believe but it's true) and some thermal boards, dab, dab, dab on the pads, stick the device basically where it goes, put it in the toaster oven for the prescribed time and bing! it'll even self-center onto the pads. There are people who build reflow controllers for this but honestly even if you just preheat it and stick it in there for a few minutes until the solder melts it's better for the devices than hand soldering.

In any case there is no reason you can't hand solder a Cup, Dragon, or whatever by hand. I'm saying if it WAS due to poor technique, just reviewing what you did for a few minutes and fixing your technique will probably address the problem right there. No reason to use this as a criteria for which devices you're going to buy. The price, heatsinking issues, etc are far more significant.


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## R33E8 (Oct 20, 2008)

Okay, I stand corrected... 

Just wondering, is there any reason you didn't go with Cree LED's?


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## Holepuncher (Oct 21, 2008)

R33E8 and Oznog

I freely admit two things. I screwed up. I dove in head first on this without really doing the proper research. Over confidence in my ability. Second everything you guys say is more or less correct. On the other hand though this is for the most part a hobbyist/experimenter forum. We learn by screwing up. Theres no black and white or right and wrong. We try things, we push the limits and sometimes we pay. There are shades of gray.

I also think you guys over-estimate the amount of heatsinking required. I dont see anything in any of the data sheets that says the devices must be kept at 25C or anything like that. They are power devices and power devices tend to run hot. Yes you want to keep them as cool as possible and stay as far away from the specified limits. In my case maybe I underestimated the heatsinking and I also have space limitations.

Now back to my destructive test. When I had the CUP led wired with .009" wire and fed it 200mA. and notice some dimming and color shift. Then when I increased the power to 1 watt and above the solder melted. At that point I cut the leads off a 1Watt resistor, .035" diam, and soldered them to the CUP package. Now at 200 and even 300ma I see no significant color shift or dimming and it feels much cooler. The point here is that if a .035" wire is capable of removing a good amout of heat then my poorly planned fr4 boards must be doing significantly more.

Oznog. I never heard of solderable pads. See I'm learning. Have to look into that. I dont know if you ever held a CUP led in your fingers. They are tiny! If you seen one you would see how easy it would be to overheat it with a soldering iron. I have an old Ungar miniature Princess low voltage soldering iron, Did I use it. No! I used my regular soldering iron set to 625F.

R33E8. Why didn't I go with CREE. Again I just dove in head first. I was making an order for the company I work for from Digikey and I just bought what they list in their catalog. I get free shipping that way. Right now I dont even know what Cree is.


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## Oznog (Oct 21, 2008)

Some LEDs like the Luxeon emitters (but not Rebels) used aluminum slugs. Aluminum can't be soldered. So the only option was to thermal epoxy the emitter to the board or sink and solder the electrical pads in a separate step. Solderable thermal pads are far superior.


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