Thermal aging of power module assemblies based on ceramic heat sink and multilayers pressureless silver sintering

An assembly based on the deposition and pressureless sintering of successive silver layers on an Aluminum Nitride heat sink is studied in this paper. Sintered silver layers act as die attach, current tracks and adhesion layer on the ceramic. Compared to previous studies where silver sintering is only used as die attach, an additional degree of freedom on the sintering temperature of the ceramic adhesion and track layers is obtained. This allows the investigation of silver sintering at non-conventional temperatures (>400 °C). The presented assembly is expected to endure high operating temperature since all its constituent materials are able to withstand such temperatures. The porosity of the sintered layer is controlled by the dwell time and the sintering temperature, and porosity values between 19% and 41% are obtained. The electrical and thermal conductivities of sintered layers as functions of joint porosity are in good agreement with the literature data and the assembly presents good mechanical properties with a shear strength value higher than 18 MPa. The assembly shows a high robustness during thermal storage at 200 °C for 1000 h. The microstructure and the electrical conductivity of track layers are both stable during the whole aging period. However, a slight improvement of the thermal response and the mechanical properties of the assembly is detected after 200 h of aging followed by a stabilization until the end of the aging tests (1000 h). Such observation is correlated to the microstructure coarsening of the sintered silver under the device and it is shown that this evolution is favored by the presence of the device.