Characterization of interfacial thermal resistance for packaging high-power electronics modules

In high-power electronics modules, the heat generated by the power devices is transferred to the ambient environment by attaching a heat spreader to the semiconductor package. Once the heat spreader is selected, it has to be attached optimally to the semiconductor package to ensure efficient thermal management through the thermal interface. In most power electronics modules, solder or thermally conductive epoxy is used to eliminate the air gaps from the module-heat spreader thermal interface by conforming to surface irregularities; however, these bonding agents introduce interfaces and/or interlayers of finite thickness. While strong interfaces are necessary to ensure mechanical integrity of the bonded components, they are responsible for lowering the electrical and thermal conductivities of the assemblage. Thermal resistance of the joint is directly proportional to the interface thickness and inversely proportional to the thermal conductivity of the interface material as well as to the area of the heat transfer. Moreover, these layers are quite nonuniform and consist of voids, which introduces anomalous thermal spreading. In this paper, interfacial thermal resistance of a few common attachment materials (solder and thermally conductive epoxy with conductivity as high as 60 W/m-K) between the power module and the heat spreader are characterized. Processing parameters such as time, temperature, and pressure conditions of the solder reflow and epoxy curing are varied to fabricate test samples for subsequent comparison of the resulting interfaces.