heatsinkRFthermaltransistors
I need a thermally AND electrically conductive heat sink compound for attaching power transistors to an aluminium heat sink. I've given Coollaboratory's LiquidPro a try and that oxidized the aluminium. Artic Silver 5 was completely the wrong solution, not conductive at all (I had some exploding FETs on my hands).
Any suggestions?
Typical solutions I've used are either Kapton tape:
Or Sil-Pad:
Sil-Pad 400 is a composite of silicone rubber and fiberglass. The material is flame retardant and is specially formulated for use as a thermally conductive insulator. The primary use for Sil-Pad 400 is to electrically isolate power sources from heat sinks.
If you use a sil-pad the screw will still electrically connect the componet tab with the heat sink. A solution to this is to use a shoulder washer to isolate the screw and the tab:
Just about anything will have better thermal conductivity than air. That's not the whole story though, since most of the cooling in air will typically be from convection.
Typical potting compounds are available from suppliers such as Hysol. If you need guaranteed electrical characteristics, that's a good way to go, but they're often not that easy to source, and it's expensive to buy a 4l or gallon size can. As Eternity said, the dimensional change during curing can rip parts off the board or place them under great stress so they fail very soon with thermal cycling.
Clear silicone (depending on the type) can have highly corrosive (and conductive) acetic acid in it (some types do not). It's not all that good thermally (2 or 3 times worse than epoxy), but it can seal against moisture and is available in translucent. Do not use the acetic acid type on electronics.
Your best bet may be to use a minimal thickness of conformal material such as silicone rubber thermal pad to transfer the heat to an aluminum heatsink.
There are a wide range of these available, with varying prices and performance (including some that are anisotropic- they are more thermally conductive in one direction than another).
Best Answer
I hardly can see why you need a conductive paste? The aluminum is covered with thin insulation layer of Al-oxide and will break the electrical contact anyway with the time, regardless of the paste properties.
If you need the heat sink to be electrically connected to the element - use a wire instead and screw it on the heat sink.
Use copper heat sink and solder the element on it - this way you will get the best electrical and thermal contact possible and very effective heat sink at the same time. (Use very thin solder layer).