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Long-Term Reliability Evaluation of Power Modules With Low Amplitude Thermomechanical Stresses and Initial Defects

In renewable energy and grid applications, solder-attached power modules are subject to fatigue stress cycles of low amplitudes, but the devices will be in service for decades. This article aims to understand the slow aging process by focusing on the initial defects and their growth under the stress cycles of concern. A finite-element analysis (FEA) is performed to obtain the thermomechanical stress distribution around the defects, which is combined with the solder material’s property to give a lifetime model. Effort is made to validate the model by power cycling plus microscale computed tomography (CT) scanning. It is found that a void in the solder layer may first transform into a crack on the material boundary, which then grows progressively more rapidly until device failure. A numerical example is shown for evaluating the lifetime of an IGBT power module in a “soft-open-point” (SOP) converter designed for an 11-kV power network.