The Study on Electromigration of Solder Joints under Thermal Cycling Load

As the main components of aerospace, home appliances and smart equipment, electronic packaging devices are gradually developing towards miniaturization, lightweight and multi-function, which leads to more and more reliability problems. BGA solder joints play the role of mechanical support and electrical connection between the chip and the PCB board. It is prone to electromigration failure under the condition of high current density, which is one of the most important problems restricting the development of chips. In this study, the electromigration failure of solder joint in WLCSP package with BGA interconnects is investigated by establishing the multi-physical fields. Based on the finite element simulation software COMSOL, the current density, temperature, stress, and concentration on the electromigration behavior of solder joints are taken into account by fully-coupled transient analysis. And the birth and death element method is applied to simulation of voids. Meanwhile, the effects of thermal cycling load and thermal conductivity of voids are considered. According to mass diffusion theory and voids formation and expansion criterion, the thermal characteristics and failure behavior of solder joint under the thermoelectric condition were studied. At the last, the failure time of solder joint is determined according to the failure criterion of electromigration. The result shows that the temperature, stress, and atomic concentration of solder joint change periodically with the thermal cycling load. During the ramp rate stage, the temperature and stress of the solder joints change accordingly, while there is no remarkable effect on atomic concentration and voids generation rate. At the high-temperature dwell time, the maximum temperature and stress of the solder joint occur on the cathode side. Its atomic concentration drops rapidly, which is the main stage of voids generation. Voids mainly formed at the cathode corner of the solder joint, which will cause the current density continue to increase and stress concentration. Thus, electromigration is accelerated and the failure life of solder joint became shorter. When the current is 1.5A, the failure time of the solder joint is about 516 cycles.