The Physics Behind CPU Cooling with LN2 or Phase Change

[Francio]

Hello everybody,
my question is: do you get an increase in speed/power/performance when you "freeze" computer components such as the CPU with LN2 or phase change cooling, getting to under -100º,-150º.
Because as long as I'm aware CPU´s are made out of semiconductors and as temp lowers the resistivity on these rises. (but i don´t really know the effect of resistivity in performance)
Or is it just a super cool way of making sure the components don´t overheat?

Preferably from a physics point of view

TIA
Fran

 

[russ_watters]

The motherboard of a computer tells the processor exactly what speed to run at. Adding extra cooling allows it to run cooler, which allows you to tell the motherboard to run it faster.

 

[Francio]

Can´t this "mobo telling CPU" be overridden? Cause as far as i know, the mobo knows what temp the cpu is at through a sensor, and this sensor could be tricked, couldn´t it?

 

[Mech_Engineer]

When "overclocking" a CPU (upping the operating frequency beyond manufacturer-set specs) it will tend to generate more heat. As a result, it is necessary to add more cooling power in order to keep the CPU within proper operating temperature. Liquid nitrogen (not normally used for residential applications AFAIK) and vapor phase-change cooling units are the upper extreme of CPU cooling and tend to allow for maximum overcloking potential.

Cooling

Excessive heat can cause system instability, so it’s essential to keep your overclocked CPU cool. To achieve extremely high overclocks, some hobbyists bathe their CPUs in liquid nitrogen. Others use phase-change units (essentially tiny freezers) to push 3GHz chips past the 5GHz mark. The point is, you can’t expect to push your 1.86GHz proc to a reliable 4GHz using a $12 heatsink. Know your overclocking goals and then choose your cooling accordingly. Air cooling is the most modest solution, followed by water cooling, peltier/liquid combinations, phase change, and exotic liquids, such as liquid nitrogen. Also remember that the extra heat produced by overclocking will warm up the rest of your machine, so you may have to upgrade your case’s cooling or the case itself if you experience overheating issues. For our CPU cooling recommendations, see page 34.

Overclocking Article on MaximumPC.com:
http://www.maximumpc.com/article/overclock_your_pc

 

[russ_watters]

Can´t this "mobo telling CPU" be overridden? Cause as far as i know, the mobo knows what temp the cpu is at through a sensor, and this sensor could be tricked, couldn´t it?

The motherboard generally doesn't dynamically adjust the speed of the processor as temperature varies. But most can be manually adjusted to whatever speed you want. No need to trick any sensors.

 

[BryanP]

I used to deal with overclocking a few years back.

Basically you want to give the CPU a cool temperature so you can maximize the potential out of it. There's a certain limit when cooling the chip doesn't promote stability.

In computer chips, the processor can get its proper cooling through air methods (which is what most computers nowadays have). However, cooling heat through heat-sinks doesn't usually get the job done when it comes to more aggressive overclocking.

You would need to use more effective methods such as using a water-cooling rig, peltiers, phase-change, etc.

The point is, it keeps the processor cool enough to keep it stable at high speeds.

Being able to modify the speed is dependent on the motherboard. Again, in general they do not dynamically alter the speed as a function of temperature. This usually only occur in the more recent CPUs that slow down as temperature increases as a safety method to keep you from frying your chip (Pentium 4s did this, and you basically recognize a lag in your computer when it's happening). However, motherboards that are targeted towards enthusiasts usually have a feature to modify the front side bus speed, the FSB multiplier, as well as the voltages.

As you can see, you don't necessarily need a cooling system to be able to tweak your CPU, you just need a motherboard that permits you to.

 

[Francio]

Thank you guys for all your answers!

But I still have one question:
which would be the difference between having your ultra-high-extremely overclocked CPU stable down at -150º or at 25º?

 

[BryanP]

You have more space to work with (varies between processors) in terms of overclocking.

Some chips will fail to work regardless of how low you bring the temperature.

However, depending on the CPU, there could be a lot of bus-speed to extract from the CPU being overclocked when running the CPU @ 25 degrees celsius and 0 degrees celsius (or even lower with peltiers or phase change).

Again it depends on the CPU. Some overclock real well, but some are already clocked to their maxes, so they might become unstable just by messing with the bus-speeds even a little bit.

 

[AhmedEzz]

overclocking also depends on the RAM and the mobo. I think that keeping your PC cooler than it needs won't give anything extra. It may prolong its life but I don't think you get better performance or something, just keep it cool at a good temp.

 

[BryanP]

That is true as well.

Depending on the FSB however, you will need components that can function under those speeds.

That is something I also neglected.

RAM is extremely important because if you want to overclock (extremely), you need to have fast RAM that can handle the bus speeds. If you're providing too much FSB for slow RAM, then you will have an extremely unstable system.