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
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, or thermal paste) is a kind of thermally conductive (but usually electrically insulating) compound, which is commonly used as an interface between heat sinks and heat sources (e.g., high-power semiconductor devices). The main role of thermal grease is to eliminate air gaps or spaces (which act as thermal insulator) from the interface area so as to maximize heat transfer. Thermal grease is an example of a Thermal interface material.

As opposed to thermal adhesive, thermal grease does not add mechanical strength to the bond between heat source and heat sink. It will have to be coupled with a mechanical fixation mechanism such as screws, allowing for pressure between the two, spreading the thermal grease onto the heat source.

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/(m·K), up to about 2 W/(m·K).[1]

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

CompoundThermal 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) [2]
Silver 418 1.465 (0 °C) [3]
Aluminum nitride 100 ‒ 170 > 1011 3.5 (300 – 600 K) [4]
β-Boron nitride 100 > 1010 4.9 [4]
Zinc oxide 25.2 [5]

See also

References

  1. ↑ Werner Haller; et al. (2007), "Adhesives", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley, pp. 58–59
  2. ↑ Otto Vohler; et al. (2007), "Carbon", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley
  3. ↑ Hermann Renner; et al. (2007), "Silver", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley, p. 7
  4. 1 2 Peter Ettmayer; Walter Lengauer (2007), "Nitrides", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley, p. 5
  5. ↑ Hans G. Völz; et al. (2007), "Pigments, Inorganic", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley
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