Thermal grease, also called thermal paste or thermal compound, is a substance used to promote better heat conduction between two surfaces and is commonly used between a microprocessor and a heatsink . The top surface of most microprocessors is not perfectly flat. Some have microscopic grooves and others might even have a slight curve, which produces air gaps between the microprocessor and the heatsink and reduces the cooling performance of the heatsink. The air gaps are filled by applying a thin layer of thermal grease to the top of the microprocessor and the base of the heatsink.
There are two types of thermal grease: non-conductive and conductive. The non-conductive thermal grease types includes silicone and zinc thermal greases. The conductive types includes silver, copper, and aluminum-based greases. These are considered superior, but must be used carefully or they can cause electrical shorts if applied to the microprocessor's pin s or the electrical pathways on the circuit board .