Substrate Solder Crack Evaluation and Modelling of Power Module under Passive Cycling

Accelerated passive cycling test is a typical and effective way to evaluate the substrate solder layer degradation of power modules due to thermal stresses. The main failure mechanism is the initiation and penetration of cracks inside the solder layer and contributes to the overall thermal resistance rise of the module. Based on passive cycling test results of eight modules with 48 substrates in total under two testing conditions, this paper models the behaviour of the solder crack growth and its impact on the thermal resistance of power module on device-level and module-level. By dividing substrate solders into several groups, results show that solders in the middle of the power module under test withstand the severest thermo-mechanical stress and have higher crack length growth rate in a Logarithmic law based crack model. It indicates that diodes on top of the highly stressed solder areas have much lower lifetime and suffer from high reliability concern under passive cycles.