Characterization of packaging warpage, residual stress and their effects on the mechanical reliability of IGBT power modules

Abstract

Injection-moulded power electronics are inevitably subjected to warpage and residual stress from anisotropic contraction and thermal mismatch during the assembly, which deteriorates their reliability in service. In this work, the packaging warpage and residual stress of an epoxy resin encapsulated Insulating Gate Bipolar Transistor (IGBT) module were characterized with X-ray Computerized Tomography (CT) and blind-hole drilling, supported with Finite Element Analysis (FEA). Their effects on the bonding quality after silver-sintered to an external heat sink were investigated with Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and mechanical tests. The module exhibited a maximum warpage of 420 μm, which comprised a flexed midsection and localized bending at the edges of the internal circuit board. The residual stress distribution had high magnitudes up to −39 MPa near the edges and a nearly stress-free interior. The warpage had little effect on the chemical composition across the module-silver interface, but the area fraction of the sintering defects increased by 37.5%, and the mean shear strength dropped by 7.4 MPa compared to the warpage-free counterparts. This work characterized the negative influences of packaging warpage and located the critical geometrical features that dictated the warpage behaviours, providing valuable information for future design optimization.