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 pins or the electrical pathways on the circuit board