Coupled Electro-Thermo-Mechanical Analyses on Power Cycling Induced Loadings in Sintered Silver IGBT-Modules with and without Overmolding

Abstract:
The reliability challenges of inverter modules with insulated-gate bipolar transistors (IGBTs) and diodes silver sintered to DBC substrates and copper wire bonded were investigated in a project called “ProPower”. It was one aim to find out possible failures when the modules are subjected to different kinds of power cycling. Coupled finite element (FE-) analyses are reported to understand the thermo-mechanical stresses that cause these failures. Initial parametric FE-studies on passive thermal cycles revealed that missing decoupling between the mismatched stacked materials, which previously was guaranteed by the soft solder, leads to coupling of thermo-mechanical stresses not only dependent on stack geometry but also on mounting conditions. Hence, a system view on the stress situation is required and was realized in FEA by including the full assembly with heatsink mounting, bonding wires, by including intrinsic stress due to processing and by different types of active loading, and in particular also of combined temperature and power cycling. Different parametric variations were studied, e.g. the effect of overmolding. Power cycling induced failure modes from testing are related to critical stresses and proposals for “physics of failure” based lifetime modeling are made.