Heat Resistance
The total opposition that a multi-layer structure or interface material presents to the flow of heat from a hot electronic component to a cool heatsink determines the steady-state temperature of the device. This thermal impedance consists of both the bulk thermal resistance of the materials and the contact resistances at their boundaries. In high-power printed circuit board assemblies, a low impedance path is necessary to prevent semiconductor junctions from exceeding their maximum rated temperatures.
Interface Material
When mounting a power component to a circuit board or external heatsink, microscopic air gaps between the mating surfaces act as thermal insulators. To reduce the thermal impedance, designers place thin layers of thermal interface materials, such as grease or phase-change pads, between the surfaces to displace the air. These materials have a higher thermal conductivity than air but are less conductive than the metals they connect.
Therefore, the thickness of the interface layer must be kept as thin as possible to minimize the overall resistance. High clamping force helps distribute the material and eliminate voiding, which would otherwise create hot spots. Ensuring proper application of these materials prevents premature thermal shutdown of power modules.
Substrate Design
Heat dissipation through the printed circuit board substrate is heavily dependent on the layout of copper planes and thermal vias. An optimized layout reduces the thermal impedance by carrying heat away from the surface-mount component to the inner copper layers. This passive cooling technique is essential for high-density surface-mount designs.
