The IGBT solder fatigue is a frequently reported failure mechanism attributed to package wear-out. It can aggravate other type of failures and degradations of the device and lead to ultimate failures. Thermal impedance can be used as a degradation indicator to detect this type of failure mode and issue early warnings for the aged IGBT In this research, different power dissipation conditions and ambient temperatures are discussed. Their impacts on the thermal impedance has been studied with FEA simulation and validated by the experimental results. This reveals the interplay between thermal impedance and these conditions. Furthermore, the health conditions of DCB substrate solders of the IGBT modules are altered by the aging test. The changes of junction temperature and thermal impedance are compared for solder layer in both health and fatigue conditions. This confirms the validity of the proposed condition monitoring method for vehicle-ready IGBT power modules.