Thermal Management of Microelectronic Equipment
9 Thermal Interface Resistance
It is common practice to use brackets, heat sinks, and circuit boards for mounting components that are to be cooled by conduction. The primary heat transfer mode inside a component is also heat conduction. Cooling by conduction often requires the transfer of heat across various materials and interfaces that may be laminated, bolted, riveted, clamped, or bonded together. A high temperature gradient may occur if a large amount of heat is transferred across these interfaces.
Since there is no perfect surface in the real world, an actual surface always contains peaks and valleys. The surface irregularities are an inherent result of manufacturing processes. Surface irregularities with large wavelength are usually referred to as waviness. The length of the waves can vary from 0.1 to 1 cm, while the height can vary from 20 to 40 µm.
On the other hand, the depth due to the peaks and valleys is often called surface roughness and can vary from about 0.05 µm for a very smooth surface to 25 µm for a very rough surface. As shown in Figure 9.1, only a fraction of points on two nominally flat contacting surfaces are actually contacting each other. Therefore, additional thermal resistances, including constriction and spreading resistances, are created because of reduction of heat transfer area at the interface of two bodies.