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I was wondering why the colder a CPU is, the further you can overclock it...
And by colder I don't mean more watts of heat taken away from it at the same die temperature, I mean lower temperature at the same heat flow.
Well, one of the limiting factors of overclocking is square signal becoming insufficiently square. That's because of stray capacitance. But a capacitor alone can't round the edges of a square signal - it needs some resistance on where the signal is coming from.
1/
Idk how stray capacitance reacts to temperature. But if it's anything like ceramic capacitors, it should have more capacitance when it's cold, right? So that can't be it.
And what about resistance?
Well, semiconductors have higher resistance when they're cold, and lower when they're hot, right? So that shouldn't help either.
But! the silicon is only a small part of the chip. The rest is wires. Layers upon layers of tiny metal wires. And in metals, resistance increases with temperature.
2/
So my guess is that the reason overclocking benefits from sub-ambient temperatures is that the metal wires in the CPU have less resistance when they're cold.
@ivesen yeah, the cold bug
> When you're throwing a workload at it, you can go to, say, -160C, but when you reboot, there's no more workload, all of a sudden you can't boot anymore. Then you have to increase to -140C to boot."
https://www.pcgamer.com/overclocking-a-cpu-to-7-ghz-with-the-science-of-liquid-nitrogen/
I'm guessing at that point there isn't enough free charges in the silicon for it to (semi)conduct.
But IIRC there is a sweet spot and it's below zero... lemme do some seraching
@ivesen I mean T_die.
The aggregate of the readings of the dozens of temperature sensors built into the CPU silicon.
@ivesen but yeah I guess it can be confusing.
I just opened GamersNexus's last livestream where the AMD guys visited him to do some XOC, skipped to the part when they started running some benchmarks, and they were saying their target with that particular CPU is -120°C, because at -130°C it won't boot, and at -125°C it only sometimes works.
And it wasn't clear if they meant Tdie or Tpot