Effective number of accelerated thermal cycles (ATCs) for accurate prediction of damage and fatigue life of solder joints in IGBT power module

Insulated gate bipolar transistor (IGBT) power module is vital to efficient functioning of several critical systems in electric vehicle (EV), photovoltaic (PV) module, and more/all electric aircrafts (MEA/AEA). As reliability qualification of IGBT immensely depends on system's modelling which employs thermal cycling, identification of effective number of accelerated thermal cycles (ATCs) is important to ensure accurate reliability prediction. This investigation advises on the effective number of ATCs for modelling damage and fatigue life of 96.5 % tin, 3 % silver, and 0.5 % copper (SAC305) solder joints in IGBT module. SolidWorks software is used to create four realistic 3-D finite element (FE) models of typical IGBT module. The IEC 60068-2-14 thermal cycle test and Anand's time independent visco-plastic constitutive model are implemented in static structural package in ANSYS mechanical software to simulate the response of the models to 6 ATCs, 12 ATCs, 18 ATCs and 24 ATCs. Thirty ATCs produced magnitudes of von-Mises stress, equivalent plastic strain and accumulated strain energy density which are almost invariant with application of greater number of ATC. Deployment of four fatigue life constitutive equations from Morrow, Coffin-Manson, and Syed generated polynomial model which produced minimum and consistent lives of the IGBT modules at about 30 ATCs. Based on this finding, 30 ATCs are proposed as the effective number. The generated models establish the relationship between the number of ATCs and fatigue life predicted for IGBT module. It is proposed to be used to evaluate the effect of number of ATCs employed and the resultant predicted fatigue life of IGBT module.