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Mounting a Heatsink to an uneven surface

Posted on 2007-03-18
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Last Modified: 2008-02-01
What is the most efficient method to mount a heatsink to an uneven surface?

I am converting all my lights to LED's they are cheaper on power and bulbs produce better light etc.. etc..

I just replaced 320watts of Fluorescents in a Display cabinet with 66watts of LED Tubes !! and they look Brighter and more evenly illuminated as a bonus!

Anyway I am finding out that one problem with LED's is heat. yes they produce far far less heat but it also takes less heat to DAMAGE them so I want to make a heatsink for them but therein lies the problem.

HOW do I mount it. ie what material do I use to interface the PCB with the Heatsink. go here

http://www.nerys.com/LED

06 is a picture of the backside of the LED TUBE
07 is my drawing illustrating the problem. ie something has to go between the heatsink and the PCB (Red and Tan) I can not lap the surfaces for better mating like I can with a CPU since those solder nubs are required :-)

I have seen some CPU's mounted with what was more like double stick tape (not foamy though ie it was not "liquid" like artic silver is for example.) basically it was more solid than liquid but flexible enough to mate the objects.

Would something like that work efficiently ? IF so where could I get a large quantity of it (like a roll of it like a roll of tape thankfully these are pretty narrow about 3/4inch wide.

Basically I need to keep the led's below 85' F if possible or as best I can to extend there lifespans. These will run never ending at minimum 18 hours a day so they will not haver an opportunity to cool down. I want to retain the poly carb tube if I can in order to keep it airtight (the other danger with led's is the metal components corroding or rusting before the bulbs wear out) is there something I can POUR over the led's (the bottoms ie between them and the PCB to prevent moisture from getting to anything metal and how would this affect heat transfer ? I imagine if I use epoxy for example it would work great but actually act as an insulator increasing the heat problem. if I could protect it from water I would just discard the Polycarbonate tube all together further assisting heat dissipation. This is especially important for the outdoor tubes I want to use (protecting them from the elements) impact and direct element attack are not an issue they will be inside an enclosure. its humidity and condensation ie indirect moisture attack that concerns me. Also if I solve this for the outdoor sign led's I can then use the entire sign as one giant heatsink :-)

Any suggestions or help would be GREATLY appreciated!
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Question by:nerys71
7 Comments
 
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by:☠ MASQ ☠
ID: 18744892
As long as you can place a thin insulating layer on the back of the PCB you could simply pack out the unevenness using someting as basic as aluminium foil and then mount a heatsink on the flat surface this produces.

The insulating layer could be somthing like an epoxy resin spray if you wanted to keep it really thin, the drawback is that epoxy resins are also quite good thermal insualtors as well but the thin section should mean the electrical insualtion happens while very little thermal insulation does, allowing the heat into the aluminium and onto the heatsink
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by:scrathcyboy
ID: 18745011
I dont see how you can heatsink LEDs without cutting down the light output dramatically.
Why not consider an in-cabinet fan?  Blown across the LEDs, it should keep them below 85.
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Author Comment

by:nerys71
ID: 18745846
I am considering a fan but any fan small enough to push air through a tube that small will be a maintenance issue and cost electricity. plus I do not think a fan that size would be able to move enough air the heatsink would be on the REVERSE side of the array (the rear exposed PCB side) plus the usualy way to do this is to apply a heatsink and THEN fan the heatsink I was planning to oversize the heatsink to use ambient and therefore maintenance free and failure free cooling. See the pictures in the link pictures 6 and 7 in particular 6 is the rear side of the PCB LED Array while 7 is an illustration to show what I am trying to do.

The Dust issue is even more a concern in my home where I want to set and forget this stuff thats kind of the point of paying the money and expending the labor to install this kind of lighting low power ever lasting light (in a home environment an LED that lives to its full rated 80,000 should outlast me outlast my kids and give there kids a run for there money :-)
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Expert Comment

by:CarlosMMartins
ID: 18765535
Yes, indeed your best option would be to find some thermal conductive tape - similar to the "thermal" compound you mention that comes in some CPUs - and attach your heatsink there.

I've also used some led lighting, and heat is definitely the most important issue if you want to make last long.

I have no idea where you can find that thermal tape though. :(
Some googling provided:
http://www.thermaflo.com/interface.shtml
and
http://www.surplussales.com/RF/ThermalConductiveTape.html
$10 per foot  (1.96" wide)
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Author Comment

by:nerys71
ID: 18766118
That looks interesting Carlose even if very very expensive at $32 per light (more than the lights cost!!) my concern is the titanium and aluminum. IE it sounds electrically conductive to me - I am going to send a message to the company and ask them and see what they say.
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Accepted Solution

by:
andyalder earned 500 total points
ID: 18766309
I think you would be best to get a sheet of mica, cut out holes for the solder bumps and put that between the heatsink and the PCB, then another sheet of mica to complete the electrical insulation. Thermal paste rather than a liquid lke arctic silver will fill in the  small bumps of the PCB tracks and still have fairly good conductive properties. Mica is a reasonable thermal conductor but also a good electrical insulator, it's what you put in between power transistors and the heatsink to stop the heatsink becoming live.

There is a calculation at www.kpsec.freeuk.com/components/heatsink.htm which suggests to allow an extra 2 degrees/watt for the mica sheet.
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Author Comment

by:nerys71
ID: 18787066
While possibly not practical andyalder provided the solution that most closely met most of my needs so I am accepting that - I will however look into alternative solutions. I am going to try filling inbetween the LED's with hot glue to seal the front side then epoxy the rear side to waterproof the back end and then water cool the whole thing and see what happens.

For home use I would not even have to pump the water just filling it with water may provide enough dissipation for a few hours use at a time.

thanks guys!
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