# How Heat Sinks Work

## Choosing a Heat Sink

When you buy a computer, whether it's a Dell, Sony or an HP, the heat sink is already installed. However, if you're building your own computer, there are certain factors you must consider when choosing the right heat sink. Since you have so many options when choosing a processor, just be certain that the thermal output of the processor you buy matches the thermal handling capability of the heat sink you buy. To determine the heat sink performance required by the processor you buy, you need to know three things:

• The maximum allowable case temperature (how hot the actual case of your computer can get without damage)
• The maximum power dissipation of the processor
• The maximum allowable inlet temperature to the heat sink

These figures should be in the owner's manual. Once you have these, they can be plugged into a mathematical formula to determine the heat sink required by your processor. The rate at which the heat sink transfers heat from the processor to the air is known as thermal resistance. To find the thermal resistance needed for the processor, you subtract the maximum inlet temperature from the maximum case temperature and divide that number by the maximum power dissipation of the processor. The thermal resistance is measured in degrees Celsius per watt (C/W).

Let's say, for example, that you buy a processor with a maximum allowable case temperature of 70 degrees Celsius (158 degrees Fahrenheit). Its maximum allowable inlet temperature is 36 degrees Celsius (96.8 degree Fahrenheit) and it allows for a maximum power output of 110. The formula would look like this:

R = (70-36) / 110

R = 0.31 C/W

Therefore, when purchasing a heat sink for this processor, you should be sure that its thermal resistance is no higher than 0.31. It's fine to use a heat sink with this processor that has a lower thermal resistance than 0.31. This will only improve the cooling. However, as is the case with any heat sink, you should never buy one with a higher thermal resistance than is required by your processor.