Xelmon over at the PDXLAN Forums asked me a very good question, to answer it i jumped in head first to give him a good explanation on LN2 Overclocking.
Please Check out the Wiki Article here:
Liquid Nitrogen - Overclocking Wiki
WR Overclocking 101
Xelmon wrote: "I could see voltage limiting, because mobos can only do so much.
The heat, @ -15C though, I really cant see it. Care to explain?"
[edit] Temperature
-15c only feels cold in relation to our normal body temperature, the real point at which something is cold is absolute zero or -273C anything above that is simply heat. and most materials begin to show signs of superconducting properties around absolute zero. superconducting is the ability to transmit large amounts of power at very high voltage with out the risk of burning up the lines or having any migration of the electricity (jumping out of the cable into near by objects)
so the colder you get a CPU in theory the more voltage you can pump into it with minimal damage to the circuits.
for example if you were to apply 1.9 Volts to a CPU (lets say a P4 631) with stock cooling the CPU would overheat very fast by overcoming the thermal capacity of the stock cooler. your CPU has a device in it (a thermal couple or diode) that will sense when the core temperature exceeds its safe zone (about +100C) this will tell the CPU to shut down in order to prevent fatal damage.
So why not put a really big copper cooler of dewm on it and pump 1.9 Volts through her?
If you manage to keep the CPU below its thermal threshold using water cooling or Air (lets say we have her at 1.9 Volts and its a nice 30C) an evil bugger known as Electron Migration will come into play.
[edit] Electron Migration
is something Chip manufacturers have been battling since the dawn of time. When you have a very small and very complex electrical component such as a CPU both heat and voltage are directly responsible for instability and damage to the component.
When we look a the very inner working of a CPU it is similar to the traces of a motherboard but on a Scale of (lets stick with the 631) 65nm meaning the transistors that open and close billions of times per second in effect computing lines of code (so you can have your Uber Interweb and... stuff) are actually "65 Nano Meters" wide! this means really really small. well because they are so small there is also not much distance between the "traces" and transistors.
[edit] Thermal Electrical Resistance
When you heat something up you are in effect adding to (in this case) the electrical resistance (meaning electricity has to push harder to get to the other side) so when we think of a CPU if you were at a theoretical (lets forget "minimal kinetic energy" for a moment) absolute zero (-273c) electricity would have no problem moving through the "maze of traces" in a CPU and if we were at say +100C electricity would have to work a lot harder to make it through the "maze".
With electricity being the lazy ass it is, its always looking for the easy way out. (remember those two words Electron Migration?) If two traces are very close to each other in the core of a CPU and one is in essence "Closer to the maze exit" and Mr. electricity is working his ass off to run through at +100C he will simply cheat and migrate (Jump) over the insulator (wall) to the other trace this causes the CPU to destabilize resulting in a flawed computation ending in a blue screen or shutdown.
Imagine there are hundreds of millions of transistors in a CPU core that is about 1x1cm x 1mm thick and when the migration starts there are usually thousands jumping over causing fatal errors and extreme instability.
The damage electron migration can cause is the death of your CPU or a permanent return of corrupted information. This happens when a Higher voltage trace jumps into a near by lower voltage zone due to its lower electrical resistance and the result is a fried portion of the chip rendering it useless...
[edit] Hardware
If we want to say make our CPU (631) run at 8GHZ we have to take into account every thing listed above and a few more things like our budget Sad !st off we know we need to keep it cool, so we get a nice Team NexGen Rev 2.5 Copper Liquid Nitrogen Pot and a frosty 160L Dewar of LN2 from the local A-L compressed gas company. we solve the problem of water condensation with nice Neoprene tape and heating elements. We have a good ole NexGen Full modded ASUS COMMANDO board capable of supplying more voltage than needed. we have a CPU that is a good steeping and can take us to 8GHZ and the Micron D9GKX that can handle at least 1200MHZ at 4-4-4-12 @ 2.7V we have a nice PSU that can push 800W of clean power. a crappy GPU that is PCI (less stress on the FSB) and pulls minimal power from the board.
so now we know why we need the CPU cold but we don't know why we need the extra voltage
[edit] Voltage
The CPU speed is a Frequency! measured in Hz, or kHz (kilohertz, 10^3 Hz), MHz (megahertz, 10^6 Hz), GHz (gigahertz, 10^9 Hz) & THz (terahertz, 10^12 Hz) (thanks wiki). (Hertz not the car place, but rather cycles per second) now in order to have a CPU operate at a frequency and be stable enough for use we have to match a voltage (voltage is acts as the stabilizer for the CPU speed) to carry the signal through the CPU and successfully compute and output information. Intel does a good job of this when developing faster chips they spend a great deal of resources finding the optimal voltage for a set frequency by testing possible environmental temperatures (home, office, server room....) so if we are to increase the frequency of a CPU we need to in crease the voltage to stabilize the faster signal but with the increase in voltage we have an increase of heat in the CPU and heat will destabilize the CPU with electron migration. when you double the Voltage of a CPU you quadruple the heat generated.
This is where the LN2 comes into play, If we want a speed of 8GHz or 266% of the stock 3GHz we need a lot of voltage. and to prevent the CPU from heating up and causing electron migration we cool it to about -150c (LN2 is -196c) this allows the CPU to operate with less electrical resistance and hopefully prevent migration when running such a high voltage like 1.9 Volts.
The colder you can get the CPU the more voltage it can take allowing a higher clock frequency.
