Fatigue Performance of Aged SAC-Bi Solder Joint under Varying Stress Cycling

SAC solder alloys have been widely used in electronic assembly, and its fatigue resistance is a critical factor that affects the reliability of electronics. Bi is one of the candidate doped materials for SAC solder alloys to improve the fatigue properties of solder joints. Much research using accelerated test was proposed to investigate the fatigue property of SAC-Bi solder alloys. However, it has been proved that the fatigue property of the standard tested bar performs differently from the actual solder joints. Additionally, solder joints are expected to fail earlier under realistic service conditions with varying amplitude cycling. In this study, three kinds of individual solder joints (SAC305, SAC-Q, and SAC-R) are aged for 10 hours and 1000 hours, and then cycled to total failure under both single and varying stress amplitude conditions. It indicates that the life of all solder joints is degraded after aging. However, SAC-Q and SAC-R showed more aging resistance than SAC305 under single stress amplitude conditions. In varying cycling with two stress amplitudes, the inelastic work of lower stress always rises after switching from high to low stress cycles. This leads to more damage per cycle and, thus, faster damage accumulation and early failure.