Electronic – Why are electrically insulating heatsinks so rare? Is it just cost

heatsinkthermal

Edit: It seems my initial question (Why are there no insulating heatsinks?) was based on a false premise, and there are in fact insulating heatsinks–I just wasn't able to find them with a cursory search. So instead, I'm changing this to ask about their rarity instead.

Heatsinks seem to be almost universally made of aluminum, copper, or some combination thereof. This makes sense; aluminum and copper are easy to work and have high thermal conductivity. But diamond has one of the highest thermal conductivities of any known substance–it's obvious, of course, that diamond of the type suitable for use as a heatsink would be inordinately expensive to say the least, as it would probably have to be a single gem-quality crystal, but would it not be possible to use, for example, cubic boron nitride, which has a similar thermal conductivity?

And yes, the manufacturing difficulties with making a large single crystal of c-BN would probably be about the same as making a large single crystal of diamond, but I expect the end price wouldn't be as much because there's no De Beers group to come after you for boron nitride. And there are surely other nonmetallic compounds that have good heat conductivity, and some of them would presumably be better suited to manufacture. I doubt they'd be able to even approach the price point of extruded aluminum, but sometimes you do need higher performance.

So, in summary, my question is: Is it only cost that makes nonmetallic heatsinks so rare, or are there some other drawbacks that make them less desirable outside of the most esoteric of applications?

Best Answer

One thing that other answers don't seem to have covered is that you only need a very thin layer of electrical insulator (at modest voltages) while the heat spreader part of a heat sink works best if it's thick. So it's more efficient to use a thin electrically-insulating barrier followed by a thick, cheap, and easily-made metal heatsink than it would be to use a single piece of a material that's thermally conductive and electrically insulating. The few materials that do exist (such as diamond) can't be extruded or otherwise easily formed into a heatsink shape. Some can be sintered but sintering can't generally reach the thermal conductivity of bulk material. The engineering effect of all this materials science is that we end up doing what we've always done.

Another factor is stock policies: by stocking bulky robust heatsinks and a smaller number (as they're not always required) of small delicate insulating pads your stock occupies less storage space and capital than if you stocked two types of bulky heatsink. Cost and performance are both better without the insulator when it's not required.