Influence of cooling conditions on the estimated power cycling lifetime of a large-area substrate solder joint in power modules

The efficiency of wind power systems is linked to accurate lifetime modelling of the power modules used. Typical power-cycling lifetime models depend on junction temperatures, which might lead to inaccuracies in modules that are limited by substrate solder degradation during power cycling, especially if the cooling conditions change. This influence is examined in this publication. For this purpose, a lifetime model based on substrate solder temperatures is derived by substituting junction temperatures in an existing junction-based model. Then, the substrate solder and junction temperatures are calculated for three different cooling conditions from the mission profiles. Lifetimes are calculated using each lifetime model and the results are compared. It is observed that the lifetime estimates derived with the substrate solder model and junction-based model differ. Furthermore, the lifetimes estimated with substrate solder temperatures develop differently with respect to the cooling condition compared to those obtained with junction temperatures. It is shown using multiple examples that in these cases higher loads occur in the substrate solder under weaker cooling conditions compared to the junction. • A Physics-of-Failure based lifetime model based on substrate-solder temperatures is derived by substitution. • The effect of cooling conditions on substrate solder and junction temperatures is studied for power cycling and application. • Lifetime estimates for mission profiles with varying cooling conditions can be more accurate using the PoF-lifetime model • Here, lifetime estimates increase with better cooling conditions if they are estimated using substrate solder temperatures.