This paper presents a method of structural dynamic analysis with finite element method to identify the cause failure of solder joint caused by the vibration of ultrasonic welding. In this method, explicit FE method and its contact algorithm are used to simulate the energy transmission from vibrational energy to internal energy under some assumptions. Therefore, firstly, its feasibility is verified by 2D coupled thermal–mechanical analysis. Then, based on a certain type of cell phone battery, a 3D FE model is established for analyzing the vibration of the solder joints during ultrasonic welding. According to the simulation, we analyze the cause of the failure from three aspects: the location of the chip, the shape of the PCB and the structure of the housing. Correspondingly, the improvements are presented and applied to trial manufacture. The result from mass production shows that the improvements can decrease the reject ratio by 90% compared with the original design.