Insulated gate bipolar transistor (IGBT) module is a component with high current density and considerable power dissipation in a power system. Any void in the solder layer below the chip will raise the junction temperature dramatically. The increment of junction temperature can cause many issues that affect the reliability of the device, and even directly leads to device failure. In the paper, a three-dimensional lumped-element thermal network model of the IGBT module is constructed to study the effect of voids on the junction temperature. The model consists of many basic elements that are interconnected, and the element is modeled as a block composed of a central node surrounded by six thermal resistances. The thermal resistance is determined by geometrical dimensions and material property of the element. The thermal network model is revised and verified by the results of thermal measurements and finite element (FE) simulations. The voids are simulated by deleting corresponding elements, and some failure criteria are drawn by analyzing the results of simulation studies. The failure criteria can be used for screening procedures and are significant for reliability assurance of IGBT modules.