Mark Hanson via EV wrote:
Heat sink grease should be NON conductive electrically but thermally conductive white grease. You can also use thermosil less thermally conductive but less messy or Kapton.
"Conductive" heat sink compounds usually aren't all that conductive (i.e. they won't carry high currents). It just means you can't depend on them as an insulator.
When you really do want high electrical conductivity, they solder the chip to the heatsink. Note that "solder" can mean many different metal alloys. Many of them melt at far lower temperatures, so you can solder without damaging the semiconductor.
Another technique is to use a soft metal washer. Lead has traditionally been used, though copper is occasionally found if the semiconductor can stand the needed clamping pressure.
Still another is to use an amalgam. This is a pair of metals that you mix just before using, and they chemically react to form a solid. Like a 2-part epoxy glue; but metallic. The mercury amalgams that used to be common for filling teeth are an example. But for semiconductors, amalgams made with bismuth are used. (Don't ever try a mercury amalgam on aluminum!)
All this may all be academic when attaching to an aluminum heatsink. The aluminum is going to have an insulating oxide layer no matter how hard you try. It takes *substantial* clamping force to break this oxide layer; more that most semiconductors can stand.
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