Thermal paste (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) compound, which is commonly used as an interface betweenheat 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 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[edit]
Thermal grease consists of a polymerizable liquid matrix and large volume fractions of electrically insulating, but thermally conductive filler. Typical matrix materials areepoxies, 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[edit]
| 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) | |
|---|---|---|---|---|
| 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 |
source wikipedia
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