Having set up the vibration acceleration failure test platform, the failure test of solder joints was completed. On this basis, solder joints were observed by electron microscope, and failure modes under vibration load were studied. The results showed that the solder joint on the outermost corner of the chip was a key part and the cracks degree distribution of solder joints had a tendency of increase from inside to outside. There was almost no cracks extension indication in internal solder joints, and the cracks were concentrated in the outermost corners of the solder joints. Cracks were distributed in the PCB board side and the chip side of the IMC (intermetallic compound) layer area, where the chip side withstanded greater stress. The crack was easily sprouted on the loose parts of the solder joint and extended along the Cu₆Sn₅ grain in the IMC layer. There were two kinds of extension modes:transgranular and intergranular. When the crack extension tended to the penetration, the crack was prone to momentary brittle fracture.