The Effect of Extended Dwell Time on Thermal Cycling Performance of Hybrid Low Temperature Solder Joints

Accelerated temperature cycling (ATC) with either standard 10-minute or extended 60-minute dwell times is used to compare the performance of the hybrid Bi-Sn ball grid array (BGA), low temperature solder joints to homogeneous SAC305 solder joints. Dwell or soak time is important for accelerated testing because the creep process is incomplete during typical accelerated testing, which uses short dwell times uncharacteristic of service conditions. For SAC solders, increasing the dwell time from 10 minutes to 60 minutes in a 0 to 100 degrees C profile results in substantial reductions in cycles to failure. Similar dwell time data are lacking for Bi-Sn solders, particularly for the challenging case of hybrid BGA assembly. In this study, increasing the dwell time reduces the reliability of the hybrid joints but also changes the failure mode. These findings are significant because dwell times in service generally are longer than 10 minutes, so testing with the standard 10-minute dwell could overestimate performance. Two components are tested, a 192CABGA and an 84CTBGA. Previous test results show that the 84CTBGA always outperforms the 192CABGA by a substantial margin, independent of alloy or thermal cycling profile. In this study, the performance of the two hybrid BGA assemblies is effectively identical when tested with the 60-minute dwell time. This is attributed to greater ingress of Bi into the unmelted SAC region of the hybrid 84CTBGA during thermal cycling, which in turn strengthens the SAC solder and alters the failure mode from fatigue in the solder to fracture at the soldered package interface. Results are presented using Weibull statistics, microstructural characterization, semi-quantitative measurement of Bi content using SEM-EDS, and failure mode analysis.