Thermal grease
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Image:Wärmeleitpaste Thermal Compound.jpg Image:Thrermal grease.jpg Image:Thermalgrease.jpg Image:Cpuimperfections.jpg Thermal grease (also called Thermal compound or Thermal paste) is a substance used to help the conduction of heat between two (usually metal) surfaces. In electronics, it is often used to aid the thermal dissipation of components via a heatsink.
Today, thermal grease is produced by many companies. There are several types of thermal grease:
1) Silicone-based thermal grease has moderate thermal conductivity, is usually white, and is often shipped with CPU heatsinks.
2) Ceramic-based thermal grease contains ceramic particles suspended in other thermally conductive ingredients. Ceramic thermal grease conducts heat better than most silicone greases, but poorer than metal greases.
3) Metal-based thermal grease contains metal particles (usually silver) and other conductive ingredients. Metal thermal grease has a better thermal conductivity (and is more expensive than silicone-based grease).
Thermal grease's primary purpose in electronics is to substitute for the constructional imperfections of a heatsink's surface. When applied in appropriate quantities, it fills the tiny pits and grooves (particularly on today's CPUs that have a complicated surface), thereby increasing the amount of surface-to-surface contact with the CPU die.
The most important parameter of the thermal grease is the thermal conductivity, measured in watts per metre-kelvin (W/(m·K)). The typical silicon/zinc oxide thermal compound has thermal conductivity between 0.7 and 0.9 W/(m·K) (in comparison, copper is 401 W/(m·K) and aluminum is 237 W/(m·K)). The silver thermal compounds may have the condictivity of around 2 to 3 W/(m·K) or even more. The compound must also be smooth, easy enough to apply in a really thin layer.
Applying and removing
The grease is applied on the both surfaces with a small plate (or, sometimes, just with finger). Even silver compounds do not conduct electricity well, so several minor drops on the chip pins usually do not damage the device.
Because the thermal conductivity of heatsink compounds is so poor in comparison to the metals they couple, it's important to use no more than is necessary to exclude any air gaps. Excess grease separating the metal surfaces further will only degrade conductivity.
It is recommended you remove thermal grease from a component or heatsink using isopropyl alcohol.