Abstract: Many studies have been reported on the reliability of solder layers based on accelerated tests and mathematical models, but few of them provided insights into the differences and failure mechanisms among solder layers. This paper efficiently evaluated the thermal fatigue reliability of different solder layers of insulated gate bipolar transistor (IGBT) module by designed thermal cycling (TC) accelerated tests with the assistance of scanning acoustic microscope (SAM). The results showed that the reliability of the busbar solder layer was the lowest and that of the substrate was the highest under the IGBT packaging system. Under the action of thermal stress and chemical potential, the relative content of phosphorus in the layer increased, and the intermetallic compounds at the interface grew thicker, which has enhanced the incompatibility of the interface structure and resulted in the initiation and growth of cracks. During this process, the change in interface structure has played a key role.