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Although we can observe that there is no gap between the heat sink and the heat source, in reality, the contact between two solid surfaces nominally in contact only occurs on discrete area elements. The gap between the non-contact interfaces is often filled with air, and since air is a poor conductor of heat, As heat moves between the two, it is slowed down by resistance from the air. This is why the heat sink and the heat source need to be filled with thermal interface materials.
Thermal conductivity interface materials role is largely by filling the gaps between contact, eliminate cracks in the air, thus reducing the contact thermal resistance between, improve the heat transfer rate between the two, the sales of thermal interface materials on the market there are many kinds of, have a solid, a liquid, each kind of thermal interface materials has other characteristics, to make a choice according to different application environment.