The Solder Layers Degradation of 6.5kV IGBT Modules for Railway in Power Cycling Tests Considering Moisture Intrusion

Due to the particularity of its application, the 6.5kV IGBT modules for railway must face the actual working conditions of coupled thermal mechanical stress and humid environment stress. Therefore, in this article, power cycling tests (PCT) considering moisture intrusion is conducted for the 6.5kV IGBT modules. Whether it is conventional PCT or PCT considering moisture intrusion, its failure mode is manifested as an increase in thermal resistance with solder layer degradation. In conventional PCT, the failure mechanism of the module is chip solder aging, while for modules with moisture intrusion, the chip solder only shows slight aging, indicating that there are other factors causing the increase in module thermal resistance. Through comparison with scanning acoustic microscope (SAM), it was found that modules with moisture intrusion had uneven thickness of the system solder layers after PCT. This is because the high temperature during the PCT process increases the expansion stress of moisture, causing deformation of the system solder layer, then accelerating its ageing process. Furthermore, by comparing the transient thermal impedance (Z_th) before and after moisture intrusion, it was found that the curve separated around the system solder layers inside the module.