Effect of Varying Amplitude Cycling on SAC-Bi Solder Joint Fatigue

Realistic service conditions of electronics involve varying loading scenarios. The fatigue behavior of lead-free solder alloys under varying stress amplitude cycling is still not well understood. Service life predictions based on the common linear damage accumulation rules such as Miner's rule, ignore the effect of varying stress amplitude cycling. This can result in a large error and misunderstanding of solder alloys' fatigue performance. No research work has investigated the Bi-doped solder alloys under varying stress amplitude cycling. In this study, individual solder joints of three different types of Sn-Ag-Cu based solder alloys (SAC305, SAC-Q, and SAC-R) are cycled at room temperature under single and varying stress amplitude. The results indicate that the linear damage accumulation rules overestimate the fatigue life (cycles) of solder alloys under varying stress amplitude cycling. There is a significant effect of varying stress amplitude cycling on Bi-doped solder joints. SAC-Q shows more fatigue resistance than SAC305 and SAC-R in both single and varying stress amplitude cases. Also, the result shows that the average inelastic work of mild stress cycling is increased after every switch from mild to harsh stress amplitude. After the crack initiation, it will dramatically increase after each switch under varying amplitude cycling.