Thermal compound (also called thermal gel, thermal grease, thermal paste, heat paste, heat sink paste or heat sink compound) is a viscous fluid substance, originally with properties akin to grease, which increases the thermal conductivity of a thermal interface by filling microscopic air-gaps present due to the imperfectly flat and smooth surfaces of the components; the compound has far greater thermal conductivity than air. In electronics, it is often used to aid a component's thermal dissipation via a heat sink.
For comparison, the approximate thermal conductivities of various materials relevant to heatsinks in W/mK are:
1. Air 0.034
2. Water 0.58
3. Thermal grease about 0.5 to 10
4. Unbranded grease typically 0.8; some silver-and graphite-based greases claim about 9
5. Aluminium oxide (surface layer on aluminium) 35
6. Steel About 40, varies for different types
7. Aluminium 220
8. Copper 390
9. Silver 420
These figures vary slightly between sources, and depend upon purity, etc. of the material. Other units are sometimes used, obviously giving different numerical values.
These are bulk thermal conductivities; the thermal resistance of a particular interface (e.g., a CPU, a thin layer of compound, and a heatsink) is given by the thermal resistance, the temperature rise caused by dissipating 1 W, in K/W or, equivalently, °C/W. For example, a thermal pad of specified area and thickness will be rated by its thermal resistance. A typical value for a pad for a microprocessor is roughly 0.2 °C/W per square inch, dependent upon thickness and decreasing at high pressure.
Purpose of Thermal Compound
Thermal grease is primarily used in the electronics and computer industries to assist a heat sink to draw heat away from a semiconductor component such as an integrated circuit or transistor. Thermally conductive paste improves the efficiency of a heat sink by filling air gaps that occur when the imperfectly flat and smooth surface of a heat generating component is pressed against the similar surface of a heat-sink, air being approximately 8000 times less efficient at conducting heat than, for example, aluminum (a common heatsink material). Surface imperfections and departure from perfect flatness inherently arise from limitations in manufacturing technology and range in size from visible and tactile flaws such as machining marks or casting irregularities to sub-microscopic ones not visible to the naked eye. Thermal conductivity and "conformability" (i.e., the ability of the material to conform to irregular surfaces) are the important characteristics of thermal grease.
Reference: Wikipedia & T-Global Technology Research Team