Thermal and mechanical design optimization of a pressure-mounted base-plate-less high temperature power module

We discuss the mechanical and thermal design of a high temperature pressure-mounted base-plate-less power module for application in a continuous high temperature (150°C) ambient. The thermal resistance of potential thermal interface materials (TIMs) has been measured as a function of contact pressure and applied power with the samples aged under pressure at a temperature of 170°C. The results show significant ageing with all of the TIMs having broadly similar thermal resistance after 90 days. As anticipated the thermal resistances measured at the lower contact pressure of 1.4MPa are somewhat higher than those measured at 3MPa whilst measurements at a power of 200W and 350W show no significant temperature dependence. A high temperature silver loaded thermal grease was selected for the application. An active metal bonded silicon nitride substrate was used as it offers good mechanical and thermal performance. High temperature Sn5Pb solder was used for die attach and 99.999% aluminium wire bonds were used to connect to the top side of the devices. The pressure mount was designed with the aid of a FEA model. A preliminary model suggested that the required contact pressure of ~3MPa could not be achieved by clamping the substrate at the edges and a design was developed to contact to the unpopulated areas of the substrate. The FEA model suggested that a profile of ~4° was required to account for the flexure of the mount on tightening and achieve the required contact pressure. Measurements using a pressure sensitive film confirmed the pressure distribution but showed a slightly lower pressure than predicted. A thermal resistance of 2.6±0.2 °C/W was measured for one of the SiC Schottky diodes. A FEA model predicts a significantly lower value of 1.9°C/W for a uniform TIM thermal resistance. However a modelled thermal resistance of 2.9°C/W in good agreement with the measured value was observed when the thermal contact area of the TIM was localized to the areas with highest contact pressure.