Heat sink thermal grease (also called thermal grease, thermal compound, CPU grease, heat paste, heat sink paste, and thermal interface material) is a sticky paste that's used as an interface between CPU heat sinks and heat sources.

Heat sink thermal grease is used to fill gaps between the CPU (central processing unit) or other heat generating components and the mechanical heat sink. The mechanical heat sink, a passive component made of a conductive metal, is placed over the CPU. Heat is drawn from the CPU through the mechanical heat sink to its fins, where a fan blows air through to dissipate the excess heat.

Inefficiencies in the thermal transfer occur because two flat surfaces, e.g. CPU and mechanical heat sink, never join together perfectly. Inevitably, there is an air gap because of imperfect surfaces. Because air is a relatively bad thermal conductor, this gap can lead to a reduction in heat dissipation, and result in an overheating and failing device. Heat sink thermal grease is used to fill that gap and is designed to efficiently transfer the heat from the heat generating component to the heat dissipating device. Although the heat sink thermal grease does not have the thermal conductivity of metals like copper and silver, its conductivity improvement over the air will increase thermal responsiveness.

Heat sink thermal grease does not mechanically connect devices like thermal adhesive does. The mechanical heat sink must be firmly attached using screws or other means to minimize the space between the sink and the source.

Heat sink thermal grease is an older formula more appropriate for use in electrical applications. Silicone is used in combination with zinc oxide to create a paste that applies easily but does not flow into unintended areas. However, silicone oil can migrate from the heat sink compound and cause both solderability issues and conformal coating dewetting.

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