FE analyses and power cycling tests on the thermo-mechanical performance of silver sintered power semiconductors with different interconnection technologies

Abstract

The paper reports on thermo-mechanical performance analyses of power semiconductors. Realistic transient temperature loadings as well as mechanical stresses were simulated by fully coupled electro-thermal-mechanical finite element analyses for power cycling loads. Power cycling tests were run in parallel to the theoretical investigations. The failure modes observed by testing were analyzed and adjusted to FE results. Some of the failures need a sophisticated evaluation strategy, as failure initiates at bi-material free edges, which obey a mechanical stress singularity. Damage mechanical modelling by means of the cohesive zone method (CZM) was adopted along with the coupled finite element analysis (FEA) in those cases. Applications of the methodology are presented for a SiC Mosfet testing sample operating at medium power and a high voltage inverter module with insulated gate bipolar transistors (IGBTs) and diodes, operating at high power. Both modules use silver sintering technology on directly bonded copper (DBC) substrates. Top interconnects are made by wire bonding for the Mosfet test sample but by an electroplating based planar technology for the inverter. Considering electro-thermal results it was calculated that stacks with planar copper interconnects outperform the wire bonded versions by 15–30% dependent on layout and current concerning thermal performance. For the die bonds, networks of cracks in the DCB copper and the silver layer replace the creep-ratchetting mechanism dominant for soft-soldered dies. This failure mode could be attributed to high cyclic in-plane normal stresses leading to subcritical crack growth at high power cycle numbers. The failure mode wire bond lift-off, characteristic for heavy Al wires, was investigated by CZM. The CZM methodology was also adopted to evaluate planar metallization delamination. For the latter, a parametric study has been made to optimize the materials choice and the layout of the metallization.