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Instant glue for glueing LED PCBs to aluminium heatsink

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grizedale

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Hello,

We are seeking a fast setting, single part glue for use on our production line to glue 20mm by 30mm FR4 LED PCB’s onto aluminium heatsinks.
(the PCBs are FR4 but obviously the outer surface that would be glued is covered in solder resist)

Also, it must be easy/quick for the production staff to use…we don’t want them screwing the top on and off all the time.

-so preferably there would be applicators available for production line use.

Do you know of anything, no instant glue that i can find has production-type applicators.

Theres Loctite 435, but this needs the top screwing on/off all the time.

(The PCBs will be at around 120degC).. average current is 180mA in each LED (they are in parallel).....so its ~1.8W per PCB
-they are double sided PCBs
 

SunnySkyguy

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3M double-sided thermal tape is the fastest solution.
Al or Ag filled 5 minute epoxy is best for low Rth and time can be reduced with ventilated bake flow oven
You can get dual injectors that mix with a pressure control gun to dispense this. and clamp or better a self-tap into FR4 would be faster labor-wise if poss. IDEALLY you want the heatsink Rth to be << than Rth of chip. right?

I suppose a pressure spot weld would be great too,.. fast but requires jig and current step-up or voltage step-down transformer with AC 1000:1 to get 1KA or more. WIth gap fill epoxy & spot weld even bettter.

Loctite 435 is both an electrical and thermal insulator. very bad choice for LEDs.
 
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grizedale

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Yes its a bad choice, but it does set very quickly which means cheap , fail-safe production line.
-as long as we can get it through junction temp testing then thats satisfactory for us.

Ive been trying to get a price for 3M 8805 tape but cant find it yet, say for 4000 pieces of about 20mm by 30mm.
 
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SunnySkyguy

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SunnySkyguy

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Loctite 435 has 5x the R factor or insulation factor of Air and melts @ 130'C .. good luck. ( read Glass transition temperature spec and thermal conductivity spec)

Whereas 3m 8805 has 6x the thermal conductivity of air or 30x the conductivity of Loctite 435. So if it only fills air gaps in 50% of the surface and the rest is metal to metal contact, you may be ok otherwise, it may melt off.

8805 has a thermal impedance of 0.5 C-in^2/W and since 20x30mm is about 1 square inch the impedance is only 0.5'C/W which is much better than the heatsink or the LED Jcn to SP. So it is worth the price of 0.38$/ LED although there is wastage of 5mm and extra cutting step, consult with 3M or CREE about getting custom tape pads for that device for $0.25/pad in volume.
 

grizedale

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I take your point about the 130degC.

I like the idea of a pad, since especially if it was slightly bigger than the PCB, then we could be sure the back of the PCB is well covered, rather than glue where who knows if the assemblers have pasted it fully over the back of the PCB.

These LED PCB's stand vertically up against a flat aluminium heatsink, and we glue them to that with the Loctite 435, then the two solder points are soldered at the bottom of the LED PCB, so that the LED PCB is electrically contacted to the Driver PCB.
.......it worries me that if we use a pad, then as the solder cools, it may kind of force the PCB away from the vertical aluminium post, such that the pad also becomes ripped away from the aluminium post....but this may or may not be realisitic?

.(..a similar phenomenon is that of tomb-stoning, where cooling solder stands chip resistors vertically like tombstones)
 
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SunnySkyguy

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Verify 3m pad temp specs, it may prohibit soldering post after placement. You may need to apply a constant force to prevent lifting on thermopad until set. Do a vibration test. The Rth-ja is not simply the result of one sample but the result of n samples measured with 6 sigma added to the result. I would spec 15deg/w for a 3W LED as a cheap target with 6 sigma. That gets you up to 90'C with a 45'C ambient. But then shared heatsink will raise ambient, so only you know the specs.

There are other great adhesives I found for you.
 
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