This paper presents a method to obtain the effects of small ΔT<sub>j</sub> on the power modules so that we can enhance the reliability and lifetime model. Bonding wire fatigue and die-attach solder fatigue have been identified as one of the main root causes of power electronic module failures. Power cycling rig is designed to obtain the process of power module failure and account for the thermal fatigue life behavior. It is proved that solder fatigue is earlier than bonding wire fatigue, therefore ageing tests which produce initial crack in the solder layer before do small ΔT<sub>j</sub> test are designed. The experiment results show that not only small ΔT<sub>j</sub> still cause the module damage only for aged module but also the higher the junction temperature is the faster the crack degrades.