A damage-based time-domain wear-out model for wire bond interconnects in power electronic modules

In the Physics-of-Failure based approach to reliability design and assessment, which aims to relate lifetime to the identified root-cause of the potential failures, the development of effective failure mechanism models is a crucial task. The extent and rate of wire bond degradation depends on both the magnitude and duration of exposure to the loads. In the existing physics-of-failure models for wire bond interconnects, lifetime is accounted for by loading amplitude alone and is usually derived based on a regular thermal cycle of a known duration. They are not ready to predict life of arbitrary mission profiles and the effects of time at temperature on the wear-out behaviour are not addressed, leading to substantial errors for thermal cycling regimes with high peak temperatures. In this thesis, a new lifetime prediction model for wire bonds is proposed based on some phenomenological considerations, which accounts not only for the damage accumulation processes but also the damage removal phenomena during thermal exposure. The methodology discards the usual cycle-dependent modelling methodology and is instead based on a time domain representation so that it can more accurately reflect the observed temperature-time effects and related phenomena. This new time-based presentation allows estimation on the bonding interface damage condition at regular time intervals through a damage based crack propagation model which includes the effect of temperature and time dependent material properties. Meanwhile, bond degradation state is indicated in the form of crack growth and shear force reduction that are predicted by the total interface damage as a function of time. The model is calibrated and validated by the experimental results from wire material characterization tests and accelerated thermal cycling which demonstrates the advantages over the cycle-based methodologies.