Heatsink Guide - The Basics of Cooling & Heatsink Technology

Almost all CPU heatsinks require a fan, and in your computer case, there's at least one fan, usually two, even three or more. But when buying a fan, which factors should you consider?

The bearing system

Two kinds of fans are being used on CPU heatsinks: Ball Bearing, and Sleeve Bearing fans.

Sleeve bearing fans are usually less expensive and often quieter, but they are often less reliable. The cheapest kind of sleeve bearing simply consists of a ring made of a porous material, dipped in a lubricant. The fan motor's shaft rotates inside this ring, and is lubricated by the lubricant stored inside the porous material.

Ball bearing fans are a bit more expensive, and sometimes also louder, but they are generally more reliable. Just in case you're not familiar with bearing types: "Ball bearing" means that the rotating shaft is surrounded by tiny balls, which allow smooth rotation with hardly any wear and tear.

Note that if the sticker on the fan says "Ball bearing", then you'll often get a fan that uses both ball bearings and sleeve bearings. A fan needs two bearings, and the popular 50x10mm "ball" bearing fans that come with many heatsinks are usually fans with one ball bearing and one sleeve bearing. Larger fans (60x25mm and up) sometimes have two ball bearings - they are often referred to as "two ball" or "dual ball bearing" fans.

Generally, I would highly recommend buying only ball bearing fans (or dual ball bearing fans in the case of larger fans). Cheap sleeve bearing fans usually fail after only six months or less, whereas high quality ball bearing fans have a very high MTBF (Mean Time Between Failure). But some fan manufacturers also offer high quality sleeve bearing fans with Teflon(tm) bearings; these are just as reliable as ball bearing fans, and quieter - but also quite expensive.

Some words on noise

High airflow always creates noise - this cannot be avoided. For this reason, even a very high quality fan can be quite loud. However, in the case of a good fan, most of the noise it creates comes from the air turbulence, and not from the fan motor. Fan vibrations are a sign of poor quality. If you hold the fan in your hand, you should not feel any significant vibrations.

A larger fan that spins at a lower speed will be less noisy than a smaller fan spinning at a high speed, even if the two provide the same amount of airflow. So, the rule "bigger is better" also applies to fans.

How to specify fan performance

The most common unit for specifying airflow is CFM - Cubic Feet per Minute. In countries where metric system is more widely used, the unity m^3/min (cubic meters per minute) is also common. A conversion table for airflow units can be found here. In addition to this, the air pressure a fan creates can be measured (unlike CFM or m^3/min, this value does not depend on fan size).

50x10mm CPU fans usually move up to 10 CFM, high speed 50x10mm fans with 6000 rpm even more. 60x25mm fans commonly move between about 20 and 30 CFM, and 80x25mm around 30 to 40 CFM. 120mm fans can reach CFM values of over 100 CFM.

Remember, that, when buying a fan, you'll always have to chose between high performance and low noise. The goal is to find a good compromise.