Modern CPU's generate a great deal of heat. It is vital that this heat is removed from the CPU core to allow the CPU to function properly. Whether you are overclocking or not, it is essential that heat is being removed from your CPU, or you will have stability issues.
Heatsinks are, of course, the primary method of heat dispersal. TIM's, or Thermal Interface Material's help to ensure a good connection between the surface of the CPU, and the base of the heatsink, to ensure the maximum amount of heat can be removed.
This guide takes a look at the best way to apply TIM's, and also points out some things to avoid!
Removing existing TIM's
Just about every heatsink comes with a TIM included. However, many, particularly cheaper ones, are often supplied with a little pad. These are thick and not the best at transferring heat. Furthermore, once used, they melt and become rather difficult to remove!
Before installing a heatsink, make sure that if it has one of these TIM's, it is removed. A razor blade can be good for getting the majority of the material off. Just make sure you don't put any deep scratches into the heatsink surface. Isopropyl alcohol and lint free cloths are good for removing any remaining substances. Try to use isoprophyll alcohol that is as pure as possible - sometimes the non-pure stuff leaves an oily residue.
If you are reinstalling your heatsink, rather than installing a new one, it is good practice to reapply the TIM you are using, as once you've disturbed it you will have eneven surfaces and maybe TIM where it shouldn't be. Isopropyl alcohol and lint free cloths come in to play here too.
The new TIM
Currently, I'm still using Arctic Silver 2. It is a silver based TIM and comes in a little syringe. However, that syringe lasts a long time. Arctic Silver 5 is the latest incarnation, but I aren't buying any more until this tube is used!
There are a vast array of TIM's out there, some better than others. Some also act as epoxy, which is good for sticking RAMsinks on, but not good for CPU heatinks. Some are non-conductive, so getting them on CPU bridges is no worry. Arctic Silver can sometimes become conductive, so if you are using it, make sure if you you get any on something other than the core or heat spreader of the CPU, that you clean it off!
Have a search for reviews of TIM's and ask in our forums for opinions on the best TIM's
Applying the TIM
Now, the general idea is that the TIM is only there to ensure as perfect as possible a mating between the CPU core and HSF. If both CPU core and HSF base were perfectly flat, there would be no need for a TIM.
Due to their metal construction, the base of an HSF will have lots of tiny imperfections (some are better than others.) Work out where the CPU core will meet with the HSF, and put a little blob of TIM on it. Now, using something other than your finger (as your finger is oily and has dead skin on it,) rub the compound into the HSF. This will help smooth out the imperfections in the heatsink base. Wipe off any excess, but don't use and liquid to clean it as you're just taking a step backwards.
Next, put a dot of the compound onto the CPU and spread it out, using something like a credit card or razor blade. The idea is to get a flat surface and use as little TIM as possible, to ensure a thin layer between CPU and HSF. Make sure it is as smooth as possible, so as to avoid getting any air pockets once the heatsink is installed.
Install the HSF
Providing you've done the above, now is time to install the heatsink. When installing it, try not to wiggle it about too much, as this might move the TIM around a lot and make it eneven again.
If you remove the heatsink again in the future, ideally you should reapply the TIM.
Any suggestions, further tips, or queries - please ask.
Above is a link to a guide for applying Arctic Silver. It's similar to what I've written, but also has some nice pictures, and reference to the differences between applying TIM's the CPU core and CPU heat spreaders.