As computers have become more and more advanced they have also increased in terms of the heat they produce. Most computers are fitted with fans and heatsinks as a way to combat this build up of heat. This article covers everything to do with PC fans, heatsinks and air cooling.

Introduction

Most components in a PC produce an amount of heat as a by product of their work, some of the minor chips can be ignored as the heat they product is insignificant and they can be left to run at a slightly raised temperature. However, many other components would fail, even instantly, if left with no form of cooling at all.

Cooling in a PC is usually achieved through the use of either a heatsink (passive) or a combination of a heatsink and a fan (active). Traditionally the CPU and the power supply were the only components that required active cooling but with increases in performance this now included graphics cards and can even extend to memory, motherboard chipsets and hard drives.

Air Cooling Theory

Heatsinks in PCs are usually made from Aluminium and/or Copper and have a flat base with which to make good contact with the chip they are cooling and some kind of fin arrangement to allow the heat to be effectively transferred to the air passing through. This sounds quite simple but in practise the fin arrangements are very varied and can dramatically affect the cooling capacity of the heatsink. Modern heatsinks may also include the use of heatpipes to help speed up the rate of heat transfer from the cooled chip – more on heatpipes later.

In order for the heat to be effectively transferred from the chip into the heatsink some kind of thermal paste or thermal interface material (TIM) needs to be sandwiched between the heatsink and the chip. This is because solid surfaces are never perfectly flat and when the heatsink sits on top of the chip air will become trapped in the tiny holes and grooves. This trapped air acts as an insulator and prevents the correct cooling of the chip. The thermal paste or TIM is more fluid than a solid heatsink and as it is squashed between the chip and the heatsink it flows into these holes and grooves and aids the transfer of heat across to the heatsink.

A tube of Thermal Paste and provided spreading card

The heat in the heatsink fins then passes out into the air around the fins. The transfer of heat into the air from the heatsink is a much slower process than from the chip into the heatsink and if the air around the fins is given time to heat up significantly then the process becomes even less effective. For these reasons to maximise efficiency of cooling the air around the fins of the heatsink needs to be as cool as possible and needs to be constantly replenished by fresh cooler air as it gets hot. This is the job of the fan attached to the heatsink, it blows cool air into the fins of the heatsink which in turn pushes the hot (used) air out from the heatsink and away.

A Passive heatsink mounted on an AMD 4850e with a intake case fan beside

In order for a fan to be effective when fitted to a heatsink it needs to have sufficient force to push air through the fins. Some high-end heatsinks have densely packed fine fins to maximise the surface area of the metal which aids the transfer of heat from the metal to the air but it also means that a more powerful fan is required to move air through the fins because of the narrow gaps between the fins.

As well as cooling CPU and GPU (Graphics Card Chips) heatsinks, fans in PCs are used for removing hot air from the PC case and for providing fresh cool air from outside the case. You will also find a fan fitted in the PC power supply which is to keep the power supply components cool, many of these may be fitted with heatsinks too (usually aluminium).

Some Thermal Conductivity Values

These values give you an idea of why various materials have been chosen for use in heatsinks and thermal paste etc. The thermal conductivity of a material is a measure of it’s ability to transmit heat through itself, i.e. how quickly heat applied at one end spreads through and is experienced at the other end.

Thermal Conductivities k – (W/mK)

Air = 0.024
Aluminium = 250
Copper = 401
Acrylic = 0.2
Silver = 429
Gold = 310

I’ve only included materials that are relevant to the PC building industry, you can see why copper is chosen ahead of aluminium in premium heatsinks and also why silver is used in thermal pastes. I’ve added gold as an example of why it’s not used, it’s thermal conductivity is OK but it’s far more expensive than better metals. Note that acrylic is very poor at conducting heat and so although an aluminium case may allow some heat to pass through the panels an acrylic case just won’t to any appreciable extent.

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