Thermal grease

From left to right: Arctic Cooling MX-2 and MX-3, Tuniq TX-3, Cool Laboratory Liquid Metal Pro( Liquid Metal based), Shin-Etsu MicroSi G751, Arctic Silver 5, Powdered Diamond. In background Arctic Silver grease remover

Silicone thermal compound

Metal (silver) thermal compound

File:Thermalgrease.jpg

Metal thermal grease applied to a chip

Surface imperfections

Thermal grease (also called CPU grease, heat paste, heat sink compound, heat sink paste, thermal compound, thermal gel, thermal interface material, thermal paste, or grey goo) is a kind of thermally conductive (but usually electrically insulating) adhesive, which is commonly used as an interface between heat sinks and heat sources (e.g., high-power semiconductor devices). The grease gives a mechanical strength to the bond between the heat sink and heat source, but more importantly, it eliminates air gaps or spaces (which act as thermal insulator) from the interface area.

Composition

Thermal grease consists of a polymerizable liquid matrix and large volume fractions of electrically insulating, but thermally conductive filler. Typical matrix materials are epoxies, silicones, urethanes, and acrylates, solvent-based systems, hot-melt adhesives, and pressure-sensitive adhesive tapes are also available. Aluminum oxide, boron nitride, zinc oxide, and increasingly aluminum nitride are used as fillers for these types of adhesives. The filler loading can be as high as 70–80 wt %, and the fillers raise the thermal conductivity of the base matrix from 0.17–0.3 watts per meter Kelvin or W/(mK), up to about 2 W/(mK).

Silver thermal compounds may have a conductivity of 3 to 8 W/(m·K) or more. However, metal-based thermal grease can be electrically conductive and capacitive; if some flows onto the circuits it can cause malfunctioning and damage.

Filler properties

Compound	Thermal conductivity (ca. 300 K) (W m−1 K−1)	Electrical resistivity (ca. 300 K) (Ω cm)	Thermal expansion coefficient (10−6 K−1)	Reference
Diamond	20 ‒ 2000	1016 ‒ 1020	0.8 (15 – 150 °C)
Silver	418		1.465 (0 °C)
Aluminum nitride	100 ‒ 170	> 1011	3.5 (300 – 600 K)
β-Boron nitride	100	> 1010	4.9
Zinc oxide	25.2

See also

• Computer cooling
• Phase-change material
• Thermally conductive pad
• Thermal adhesive
• List of thermal conductivities