That being said, remember that there's really two reasons that the carriers are made of ceramics. One, because ceramics can withstand superhigh temperatures, far above what metals can do without melting. On the other hand, we're building chips, not a blast furnace, so that's not necessarily their main advantage ;)
But the second reason is that ceramics are electrical insulators - they're nonconductive. If you had chip casings and carriers made of metal, you've introduced more complexity into the design, because now you have to figure out some way of insulating the pins, traces, silicon, et cetera, from the chip casing. And this is another potential point of failure for the chip - this insulator has to withstand high temperatures without ever failing. It has to be perfectly insulated as it rolls off the assembly line, and it can never fail, or the chip is again dead, because the electrical impulses that are supposed to be traveling along the silicon and out the pins will leak into the carrier itself. And thermal expansion will again rear its ugly head - this insulator has to be such that you can squeeze it in between the silicon and the metal case surrounding it, and it has to be of a material that won't expand so much that it pops the carrier. You could leave an airspace within for it to expand somewhat, but that just increases the size of the whole package, naturally. But if that insulation ever cracks or wears down, it's game over.
It seems to me that an external system is a reasonable trade-off - put an external temperature probe on the thing, along with support circuitry, and monitor the chip for excessive heat. If the temperature exceeds some preset limit, the support system kicks in and either throttles back or shuts down the chip entirely. And that's exactly what many systems do ;)
Yes, the ability of the heat sink to withstand temperatures high enough to destroy the chip is not a particularly important feature.
But the second reason is that ceramics are electrical insulators - they're nonconductive. If you had chip casings and carriers made of metal, you've introduced more complexity into the design, because now you have to figure out some way of insulating the pins, traces, silicon, et cetera, from the chip casing. And this is another potential point of failure for the chip - this insulator has to withstand high temperatures without ever failing. It has to be perfectly insulated as it rolls off the assembly line, and it can never fail, or the chip is again dead, because the electrical impulses that are supposed to be traveling along the silicon and out the pins will leak into the carrier itself. And thermal expansion will again rear its ugly head - this insulator has to be such that you can squeeze it in between the silicon and the metal case surrounding it, and it has to be of a material that won't expand so much that it pops the carrier. You could leave an airspace within for it to expand somewhat, but that just increases the size of the whole package, naturally. But if that insulation ever cracks or wears down, it's game over.
Power semiconductors are routinely heat-sinked using mica insulators. While there would be slightly different issues in heat sinking a chip, the way I see it the ceramic case of the chip is a fairly thick electrical insulator which is a poor conductor of heat; it has to be able to withstand exposure to the outside world. If the chip were to use a metal case, then the insulator could be much thinner. I would also expect that having the chip fabricated in a clean room would allow for a better heat sink bond than that produced by a ham-fisted guy with a tube of sinking goo.
It seems to me that an external system is a reasonable trade-off - put an external temperature probe on the thing, along with support circuitry, and monitor the chip for excessive heat. If the temperature exceeds some preset limit, the support system kicks in and either throttles back or shuts down the chip entirely. And that's exactly what many systems do ;)
Yes, but if the heat sink fails, it's possible for part of the chip to critically overheat before the temperature sensor reaches its trip point. BTW, I saw that happen with a "thermal overload protected" motor driver chip. Thing lit up like a magnesium flare when the output was shorted out, and got hot enough to short the power and ground planes of the board on which it was sitting.