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Solder voids are one of the most important aging forms of insulated gate bipolar transistor (IGBT) modules. This article presents an online monitoring method of void fatigue in the solder layer of an IGBT module based on common mode (CM) interference signal spectrum. First, the variation law in collector voltage during the switching process of an IGBT module is analyzed. The cause of CM interference signal in the system where the IGBT module is located is studied, and the relationship between solder layer void fraction and CM interference signal is explored. Second, the study examined the influence of the solder layer void fraction on the parasitic parameters of the IGBT module packaging. It was found that the inductance of the solder layer of the IGBT chip increases with the increase in void fraction of the solder layer, reducing the amplitude of CM interference signals. Finally, taking the buck converter composed of an IGBT module as the tested object, the CM interference signals of IGBT modules with different void fraction are measured, and the void fatigue of the solder layer of an IGBT module is monitored online. The experimental results show that, compared with the healthy IGBT module, the amplitude of CM interference signal decreases with an increase in void fraction.