Instore Clinic

CPU Cooling

All CPUs require a heat sink and fan assembly for proper operation, and to avoid premature failure from overheating. Boxed products usually include this assembly. If the CPU you have selected for installation in your system did not come with a heat sink cooling assembly, obtain one approved for the CPU's thermal requirements.

Alone, a heat sink is not enough to carry all of the heat away from the CPU, and, as the size of the exposed CPU surface decreases, cooling becomes a critical issue. Any irregularity or gaps between the surface of the CPU and the heat sink can result in overheating and failure of the processor. To improve the cooling efficiency between the heat sink surface and the CPU, some type of thermally conductive material must be used between the surfaces. The purpose of thermal compound is to fill any gaps or uneven places between the two, and to conduct heat away from the processor into the metal of the heat sink.

Thermal tape, thermal grease, and phase change compound are the three basic forms of thermal compound available to improve the efficiency of the heat sinks.

Thermal tape, like its name implies, is a rubber tape or film that may be sticky on both sides. The tape is usually applied to the heat sink, sometimes with a protective film that needs to be removed before placing the heat sink in place on the CPU. Thermal tape is not recommended for use on processors much over 200-250 MHz, because it will not transfer heat as effectively as the other materials. There is a possibility of small air bubbles being trapped between the tape and the heat sink or processor. Heat will not conduct as well in these areas, resulting in localized hot spots in the CPU.

Phase change compound appears to be almost putty-like in appearance or as a dry waxy film where the heat sink contacts the processor. When it has been pre-applied to the heat sink, a plastic film is used to protect the surface from damage.

A heat sink with phase change compound and protective film cover in place. Remove the film before installation.

Phase change compound changes from solid to a liquid state as heat is applied, either externally, or as the processor heats up when in use. As it softens, phase change compound can flow to fill uneven areas, and eliminate any gaps or small air bubbles between the heat sink and CPU. Once it has cooled again, it cannot be removed without destroying the seal between the heat sink and CPU. Both AMD and Intel warn against reusing heat sinks with phase change compound once it has been removed from a CPU. Although the material itself may be able to be reused indefinitely, the resulting gaps will not be filled during normal use, and the material should be replaced to avoid permanent damage to the processor.

If you must reuse a heat sink that came with phase change compound, first remove all traces of the compound from the heat sink and processor. One the heat sink is clean, apply a thin coat of thermal grease over the area where contact with the CPU is made.

Heat sink with thermal grease
Apply a small amount of thermal grease to the heat sink. Use a toothpick or paper clip to spread a thin layer over the area where the heat sink and processor touch. Excess grease will be forced out around the edge of the processor when the heat sink is locked down. Too much grease can act as an insulator, blocking airflow around the CPU.

Thermal grease is a thick white cream that is lightly applied to the surface of the heat sink before attaching to the CPU. Since the grease is a liquid, small bubbles and gaps are not an issue, as these are eliminated as the heat sink is locked down in place. Any excess material is forced out, creating a uniform seal between the heat sink and processor. Intel recommends using thermal grease over tape, or even phase change compounds, on its high-end processors. You can obtain thermal grease from the Micro Center peripherals department, Radio Shack, electronics suppliers, or the store where you purchased your CPU and heat sink. Some heat sinks include a small packet of thermal grease in the package.

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