Thermal grease is primarily used in the electronics and computer industries to assist a heatsink to draw heat away from a semiconductor component such as an integrated circuit or transistor.
Thermally conductive paste improves the efficiency of a heatsink by filling air gaps that occur when the irregular surface of a heat generating component is pressed against the irregular surface of a heatsink, air being approximately 8000 times less efficient at conducting heat (see Thermal conductivity) than, for example, aluminium, a common heatsink material. Surface imperfections 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.
As such, both the thermal conductivity and the "conformability" (i.e., the ability of the material to conform to irregular surfaces) are the important characteristics of thermal grease.
Both high power handling transistors, like those in a conventional audio amplifier, and high speed integrated circuits, such as the central processing unit (CPU) of a personal computer, generate sufficient heat to require the use of thermal grease in addition to the heatsink. High temperatures cause semiconductors to change their switching properties to the point of failure while CPU power dissipation overheating causes logic errors as heat raises electrical resistance on the multi-nanometer wide circuits of the CPU core.