Investigation into stable failure to short circuit in IGBT power modules

This doctoral thesis investigates modes of failure of the IGBT power module and how these modes can be coerced from an open circuit failure mode (OCFM) to a stable short circuit failure mode (SCFM) by using different interconnect technologies and material systems. SCFM is of great importance for a number of applications where IGBT power modules are connected in series string e.g. high voltage modular multi-level converters (M2LC) where one module failing to an OCFM can shut down the whole converter. The failure modes of IGBT samples based on wirebond, flexible PCB, sandwich and press pack structured interconnect technologies have been investigated. Destructive Type-II failure test were performed which concluded that the SCFM is dependent on the energy level dissipating in the power module and the interconnect technology. The higher thermal mass and stronger mechanical constraint of the interconnect enables module to withstand higher energy dissipation. The cross-sections of the tested samples have been characterised with the scanning electron microscope and three dimensional X-ray computed tomography imaging. It was observed that the networked conductive phases within the solidification structure and the Sn-3.5Ag filled in cracks of the residual Si IGBT are responsible for low resistance conduction paths. The best networked conductive phase with lowest electrical resistance and high stability was offered by Ag if used as an intermediate interconnect material on emitter side of an IGBT. To offer a stable SCFM, a module has to be custom designed for a particular application. Hence for the applications which demand a stable SCFM, the IGBT module design becomes an integrated part of the complete power electronics system design