One of the most important factors when building a PC is managing heat output. A modern CPU packs huge numbers of transistors into increasingly tiny pieces of silicon and when power is applied this has the potential to generate massive amounts of heat that need to go somewhere.
This is where heatsinks come in. They are designed to take the heat generated in that tiny piece of silicon and take it away, keeping the processor from overheating and the silicon itself from being damaged. This heat transfer is important, and most of the time PC and laptop overheating can be traced back to poor heatsink efficiency.
One of the major causes of this is the interface between the heatsink and the processor. Most modern processor packaging includes a heatspreader. This is a metal cap that sits over the silicon itself and helps to move the generated heat to a larger area. Heatspreaders first appeared on Intel’s Pentium 4 (and a couple of later generation Pentium3 CPUs) and were introduced by AMD with the Athlon 64.
Even with a smooth base, there are tiny air gaps that form between the CPU and heatsink, which TIM fills.
Previous to this, heatsinks needed to be mounted directly onto the silicon die of the processor. However, as die size continued to shrink so did the silicon size, which caused two problems. The first was that destroying a CPU while trying to mount a heatsink became easier and easier (smaller dies like ‘Barton’ core Athlons were notorious for this) and the second was that it became incredibly thermally inefficient to try to transfer heat from such a small area.
An integrated heatspreader allows the contact area between the processor and the base of a heatsink to be larger, which increases the potential amount of heat transfer. It also completely removes the ability to destroy a silicon die while installing a heatsink, making life easier for all.
You want to ensure that your heatsink and CPU have as close a contact as possible. Heatsink brackets are designed to put downwards pressure on the heatsink in order to achieve this, but the mating still isn’t perfect. The reason for this is that, while the base of a heatsink and the top of a heatspreader look to be incredibly smooth to the naked eye, this doesn’t translate at a microscopic level.
When applying thermal paste, always start with a clean CPU.
Microscopic surface imperfections lead to air gaps between the two surfaces. Air is a horrible conductor, so these gaps decrease the heat transfer to the heatsink. The way to minimize this impact is to use a Thermal Interface Material (TIM).
There are various kinds of TIM out there, but the most relevant for PC builders is thermal paste. This is a goo made out of some kind of conductive material suspended in grease. The theory is that when you apply this between the heatspreader and heatsink you fill in those microscopic gaps with a material that is more suitable than air for conduction.
This is important because metal to metal heat transfer is the ideal situation. While you want to eliminate air gaps your thermal paste isn’t supposed to get in the way of this direct metal heat transfer. There are several ways to do this – serious overclockers use a technique called Lapping, where both the heat spreader and base of a heatsink are smoothed out with fine sandpaper, the theory being that the eventual mirror finish that emerges give a much better contact between the two.
Use paste sparingly and smear it across the CPU with the included applicator or a plastic card.
Lapping is overkill for 99% of the PC-owning population though, and the best way to maximise thermal transfer is to apply thermal paste in the correct way – sparingly.
DOS AND DON’TS
If you are installing a retail packaged processor and heatsink the tough work is already done. Retail Intel processors come with a small amount of preapplied TIM on the stock heatsink. It is a relatively small amount in three stripes. This gives the TIM room to spread out under heat and pressure, making for a good mating.
If you don’t have pre-applied TIM you’ll need to apply thermal paste yourself. Remember that you want to fill microscopic air gaps, which means a small amount of paste. Thermal paste is an efficient conductor, but nowhere near as efficient as bare metal. So if you use too much paste you can end up in a situation where it sits in a thick layer between the heatsink and CPU and reduces the ability to transfer heat.
This is how not to use TIM. Not only will it kill heat conductivity, it will also likely work its way in and damage the socket as well
The other potential side effect with too much paste is that it oozes out the side of the heatspreader and into the socket. This can range from inconveniently messy to potentially disastrous(some thermal pastes will use metals like Silver, which are electrically conductive as well). What you want to do is use as small an amount as possible.
Some more advanced metallic thermal pastes can be applied in a few drops in the centre of the CPU, and the pressure will spread it. But ideally you want to apply a small smear across the top of the processor. Our preferred means of application in the PC & Tech Authority Labs is a plastic-coated business card, but some thermal pastes will also come with applicators. What you want to do is ensure a relatively thin smear across the surface of the heatspreader – toss out any excess that protrudes over the sides. As long as the coverage is thin then don’t worry too much if it looks a bit uneven – the pressure from the heatsink will help with that.
You should leave only a thin layer on the processor; any excess should be carefully removed and junked.
DE-GOOIFYING YOUR CPU
Removing thermal paste from a processor can be an annoying process. It is good practice to clean off any old paste before installing a new heatsink, as over time the efficiency will reduce. The first thing to do is remove the processor from the motherboard. With the processor on some sort of absorbent material, you can use a variety of substances to loosen the paste.
Our personal preference is Lighter Fluid, although Acetone and Rubbing Alcohol work as well. Keep in mind that these are flammable substances best kept away from naked flames and running electronics. Also make sure you are buying the pure fluid– some nail polish removers for example add nail strengtheners and other chemicals besides acetone, which aren’t good to use on your processor.
Loosen the goop with one of these liquids and use a soft cloth to mop it up. These liquids will evaporate relatively quickly, which is one of the advantages of using them, but you need to make sure the paste is gone before this happens. Once the surface is clear of paste you are ready to reapply it and start the process all over again.
This article was brought to you by Thermaltake.