In this study, the influence of different reflow profiles and dosages of SnPb solder paste on the reliability of the SnAgCu/SnPb mixed assembly process was investigated. The interfacial microstructures were carefully examined through scanning electron microscopy, and two failures modes were found. Firstly, when reflowed at 245°C for 95 s, Pb-rich phases were buried within an intermetallic compound (IMC) layer and led to micro-voids and cracks under thermal stress or mechanical stress. However, upon increasing the reflow temperature or time further led to melting the solder ball totally, and the Pb element dissolved into a liquid phase and precipitated into uniform and small granules during the cooling process. Further, among the samples with different dosages of SnPb solder paste, severe shrinkage occurred at the solder joints with the addition of 12 wt.% eutectic SnPb component. Electron backscattered diffraction analysis was conducted in order to determine the mechanism of different shrinkages. The solder ball consisted of several primary Sn grains, and the shrinkage and Pb-rich phase are preferred to appear at the grain boundary. When the solder ball consisted of a single grain, no preferred orientation of the shrinkage and Pb-rich phase were detected. The shrinkage appeared at the final solidification region, which is near the IMC layer. Moreover, the shrinkages connect to each other, and a continuous crack formed on the chip side, leading to the failure.