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Heat and Cooling Problems

Heat can be a problem in any high-performance system. The higher-speed processors normally consume more power and therefore generate more heat. The processor is usually the single most power-hungry chip in a system, and in most situations, the fan inside your computer case might not be capable of handling the load without some help.

Heat Sinks

To cool a system in which processor heat is a problem, you can buy (for less than $5, in most cases) a special attachment for the CPU chip called a heat sink, which draws heat away from the CPU chip. Many applications may need only a larger standard heat sink with additional or longer fins for a larger cooling area. Several heat-sink manufacturers are listed in the Vendor List, on the CD.

A heat sink works like the radiator in your car, pulling heat away from the engine. In a similar fashion, the heat sink conducts heat away from the processor so that it can be vented out of the system. It does this by using a thermal conductor (usually metal) to carry heat away from the processor into fins that expose a high amount of surface area to moving air. This allows the air to be heated, thus cooling the heat sink and the processor as well. Just like the radiator in your car, the heat sink depends on airflow. With no moving air, a heat sink is incapable of radiating the heat away. To keep the engine in your car from overheating when the car is not moving, auto engineers incorporate a fan. Likewise, there is always a fan somewhere inside your PC helping to move air across the heat sink and vent it out of the system. Sometimes the fan included in the power supply is enough, other times an additional fan must be added to the case, or even directly over the processor to provide the necessary levels of cooling.

The heat sink is clipped or glued to the processor. A variety of heat sinks and attachment methods exist. Figure 3.25 shows various passive heat sinks and attachment methods.

Figure 3.25 Passive heat sinks for socketed processors showing various attachment methods.


According to data from Intel, heat sink clips are the number-two destroyer of motherboards (screwdrivers are number one). When installing or removing a heat sink that is clipped on, make sure you don't scrape the surface of the motherboard. In most cases, the clips hook over protrusions in the socket, and when installing or removing the clips, it is very easy to scratch or scrape the surface of the board right below where the clip ends attach. I like to place a thin sheet of plastic underneath the edge of the clip while I work, especially if there are board traces that can be scratched in the vicinity.

Heat sinks are rated for their cooling performance. Typically the ratings are expressed as a resistance to heat transfer, in degrees centigrade per watt (°C/W), where lower is better. Note that the resistance will vary according to the airflow across the heat sink. To ensure a constant flow of air and more consistent performance, many heat sinks incorporate fans so they don't have to rely on the airflow within the system. Heat sinks with fans are referred to as active heat sinks (see Figure 3.26). Active heat sinks have a power connection, often using a spare disk drive power connector, although most newer motherboards now have dedicated heat sink power connections right on the board.

Figure 3.26 Active heat sinks for socketed processors.

Active heat sinks use a fan or other electric cooling device, which require power to run. The fan type is most common but some use a peltier cooling device, which is basically a solid-state refrigerator. Active heat sinks require power and normally plug into a disk drive power connector or special 12v fan power connectors on the motherboard. If you do get a fan-type heat sink, be aware that some on the market are very poor quality. The bad ones have motors that use sleeve bearings, which freeze up after a very short life. I only recommend fans with ball-bearing motors, which will last about 10 times longer than the sleeve-bearing types. Of course, they cost more, but only about twice as much, which means you'll save money in the long run.

Figure 3.27 shows an active heat sink arrangement on a Pentium II/III type processor. This is common on what Intel calls its "boxed processors," which are sold individually and through dealers.

Figure 3.27 An active (fan-powered) heat sink and supports used with Pentium II/III–type processors.

The passive heat sinks are 100 percent reliable, as they have no mechanical components to fail. Passive heat sinks (see Figure 3.28) are basically aluminum-finned radiators that dissipate heat through convection. Passive types don't work well unless there is some airflow across the fins, normally provided by the power supply fan or an extra fan in the case. If your case or power supply is properly designed, you can use a less-expensive passive heat sink instead of an active one.

Figure 3.28 A passive heat sink and supports used with Pentium II/III–type processors.


To function effectively, a heat sink must be as directly attached to the processor as possible. To eliminate air gaps and ensure a good transfer of heat, in most cases, you should put a thin coating of thermal transfer grease on the surface of the processor where the heat sink attaches. This will dramatically decrease the thermal resistance properties and is required for maximum performance.

To have the best possible transfer of heat from the processor to the heat sink, most heat sink manufacturers specify some type of thermal interface material to be placed between the processor and the heat sink. This normally consists of a zinc-based white grease (similar to what skiers put on their noses to block the sun), but can also be a special pad or even a type of double-stick tape. Using a thermal interface aid such as thermal grease can improve heat sink performance dramatically. Figure 3.29 shows the thermal interface pad or grease positioned between the processor and heat sink.

Figure 3.29 Thermal interface material helps transfer heat from the processor die to the heat sink.

Most of the newer systems on the market use an improved motherboard form factor (shape) design called ATX. Systems made from this type of motherboard and case allow for improved cooling of the processor due to the processor being repositioned in the case near the power supply. Also, most of these cases now feature a secondary fan to further assist in cooling. Normally the larger case-mounted fans are more reliable than the smaller fans included in active heat sinks. A properly designed case can move sufficient air across the processor, allowing for a more reliable and less-expensive passive (no fan) heat sink to be used.

See "ATX."

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