It’s a challenge to reconstruct the complex structure of fatigued solder layer. A multi-parameter random generation-growth method, quartet structure generation set (QSGS), is proposed to set up a model to simulate the microstructure of voids, which the fractal dimension is introduced to characterize the complexity of the voids. In order to study the effect of voids complexity on the junction temperature and junction-to-case thermal resistance, an improved three-dimensional finite element model is established for IGBT package configuration, which the reconstructed model is used to replace the solder layer. The result shows that junction temperature is related to the complexity of voids, the thermal resistance decreases with the increase of fractal dimension.