Before the Intel 486, microprocessors were cooled largely by the airflow inside the case created by the system fan. This was called radiant cooling. Any heat radiated by the pro cessor was cooled by air being sucked into the system case by the fan in the power supply.
Beginning with the 486, processors were cooled with a heat sink or processor cooling fan (see Figure 3 7) or both, attached directly to the surface of the processor. In addition, the system fan was reversed to extract the heated air from inside the computer case and force it out.
The Pentium processor is meant to operate at 185 degrees Fahrenheit (85 degrees Celsius). The Pentium III processor should operate at 100 degrees Celsius (about 212' Fahrenheit), which is pretty hot. It is very important that a processor's cooling system is kept at or near its designed operating temperature. At too high operating temperatures, processors begin to perform poorly, shut down, or become permanently damaged. Heat sinks, like the one shown in Figure 3 8, and fans (see Figure 3 7) are designed to draw the heat up and out of the processor's packaging and carry it away on the tines of the heat sink and the airflow of the fan.
On the 486, Pentium, and Pentium Pro processors, heat sinks and fans are either clipped to the processor or attached with a dielectric gel, also called thermal grease or both. Later Pentium models, including the Celeron, the Pentium 11, and the Pentium 111, all of which use SECC (single edge contact cartridge) packaging, include mounting points for fans and heat sinks as part of their design.
Commonly, the processor is not the only high heat device inside the computer case. Other high performance devices, such as accelerated video cards and high speed hard disk drives, can also produce significant heat. Computer case designs should provide for enough ventilation to allow cool air to be drawn in and hot air to be expelled. Otherwise, the computer's lifespan will be dramatically shorter.
