Modified Ni/Pd/Au-finished DBA substrate for deformation-resistant Ag–Au joint during long-term thermal shock test

An Ag–Au joint developed by Ag paste joining on electroless nickel/electroless palladium/immersion gold (ENEPIG)-plated direct-bonded aluminum (DBA) substrate was employed in SiC (Silicon carbide) power modules. The ENEPIG plating was modified to possess high mechanical strength by thickening the Ni layer to 20 μm. The reliability of SiC/DBA die-attached module by Ag–Au joining was evaluated during a long-term thermal shock test (TST) from − 50 to 250 °C up to 2000 cycles. The shear strength of as-sintered Ag–Au joint was evaluated to be 37.6 MPa, but it showed a significant decrease after 1000 thermal cycles and maintained stably from 1000 cycles to 2000 cycles. Based on the microstructural evolution via EBSD observation, it was confirmed, by modifying ENEPIG, the Ag–Au interfacial deformation derived from Al plastic deformation was successfully suppressed even after 2000 cycles. In this case, the mechanism for shear strength degradation was found to be the interfacial delamination between Ag paste layer and ENEPIG substrate. This can be ascribed to the large thermal stress caused by different coefficients of thermal expansion (CTE) of power module components. This study not only realized the deformation-free structure for Ag–Au joint during long-term thermal cycling but also provided fundamental insights for fracture behaviors of Ag–Au sinter joint.