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Registered Member #1960
Joined: Thu Feb 05 2009, 11:57PM
Location:
Posts: 48
I'm sick and tired of drilling holes, tapping them and mounting to-247AC (isolated tab) MOSFETS to heatsinks. This led me to think about gluing the transistors directly to the heatsink. I tried with some two part epoxy (araldite) and this works just as good as screws (same operating temperature).
When googling this, it seems this is pretty far from normal practice. One place loosening of the transistors after thermal cycles was mentioned. The question is, will it last over hundreds of thermal cycles? Any opinions?
Registered Member #3271
Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159
@Storken, I have seen some power devices mounted like what you suggest for a catastrophic failures analysis job (ie: fire). The epoxy glue (was supposed to be thermal conductive too) would simply free itself from the heat-sink substrate after a while as a clean break. The device leads would then act as springs and lift it off. The device then would overheat and short out (second engineering failure, no fail-safe design) and run the heaters full blast. Unfortunately they also relied of the heaters thermal safety emergency switch but the dust accumulated over the years caught fire before the emergency switch in the heater module acted (third fail!).
With epoxy glue surface prep is key for good surface adhesion (sanded rough with key ridges for example) but they are still bad for thermal cycling with materials with a different thermal coefficient unless they have a large metal content (conductive epoxies). Overall engineering and material properties are critical for reliability but it is doable and used in aerospace for example.
Yes you might get away with it, in some low power applications or potting pc boards etc... At high powers you better know what you are doing for reliability.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I wouldn't say it's far from normal practice, gamers and overclockers' websites are full of silver loaded or ceramic loaded epoxy for gluing heatsinks onto CPUs. But you need exquisite surface preparation if you're not going to risk debonding after a number of thermal cycles. You might want to check the thermal expansion coefficient of the transisistor tab against that of the heatsink. The great advantage of of the tiny piece of metal flipped over the transistor that Mads points out is that it maintains positive pressure, regardless of any joint movement. You just need a heatsink edge close by, with a backplate that's not too thick, to use them.
I have been known to use bath-sealant (flexible to reduce thermal mismatch strain), and also hot-melt glue (keep the service temperature down) for quick'n'dirty jobs, either are more thermally conductive in a thin layer than air, so worth using.
Registered Member #1960
Joined: Thu Feb 05 2009, 11:57PM
Location:
Posts: 48
Thanks for all the great replies
@Mads Barnkob Those clips are really nice. I would use them if my design allowed it. Possibly I'll try to see if some flat springs can be used instead.
@richnormand The power level is 2kW, it is basically a high performance ebike-inverter (50V40A) (100A max phasecurrent). Hard anodized heatsinks may give the performance needed. At the same time I can swap to non-isolated MOSFET packages, enabling even better thermal path.
I found some aerospace glue, but many products looked like $$.
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Gluing transistors to heatsinks
