All are designed to conform to surface irregularities, thereby eliminating air voids and improving heat flow through the thermal interface. Some have secondary properties and functions, as well. Types of Thermal Interface Materials Thermal greases are made by dispersing thermally conductive ceramic fillers in silicone or hydrocarbon oils to form a paste. Sufficient grease is applied to one of the mating surfaces such that when pressed against the other surface, the grease flows into all voids to eliminate the interstitial air. Excess grease flows out past the edges and the thinnest possible thermal joint is formed as the two surfaces come into contact at their high points.
Joint integrity has to be maintained with spring clips or mounting hardware.
Greased joints can also dry out with time, resulting in increased thermal resistance. The effect of thermal grease on heat transfer through an interface can be seen from the data in Table 1. 7.9 Elastomer 0.010 5.0 1.8 8.9 AdhesiveFilm 0.009 0.7 2.7 9.6 Table 1. Case-sink and junction ambient thermal resistance for various interface materials. This data was generated using a P54C Pentium Thermal Test Die powered to six watts in an AMP Socket 5.