Impact of Microstructure Evolution on the Long-Term Reliability of Wafer-Level Chip-Scale Package Sn–Ag–Cu Solder Interconnects

The interaction between the continuous microstructure evolution during thermal cycling and the long-term reliability of wafer-level chip-scale packages (WLCSPs) with Sn-1.0Ag-0.5Cu (wt%) (SAC105), Sn-3.0Ag-0.5Cu (wt%) (SAC305), and Sn-3.9Ag-0.6Cu (wt%) (SAC396) solder ball interconnects were investigated. Three different body-sized WLCSP with three different solder alloys on three different board thickness were thermally cycled from 0 °C to 100 °C with 10 min of dwell time, and the microstructure evolution and their impact to the life cycle numbers were identified. Based on both experimental and calculated data, higher Ag contained solder alloys perform better in thermal cycling. However, the comparison between the calculated life cycle and the experimental results revealed mismatch, which is due to the localized recrystallization areal fraction differences. Smaller die WLCSP with 4×4 mm
²
and 3.2×3.2 mm
²
exhibited a large difference in expected life cycle numbers. The calculated life cycles expected a lower cycle number with thicker boards for both SAC105 and SAC396 WLCSPs, but the experimental data revealed an increase with SAC105 and a similar level of life cycle time with SAC396, for thicker boards. A widely distributed areal fraction of damage accumulation through the solder rows were observed in SAC105 compared with higher Ag solder alloy joints, which show localized damage accumulation at corner joints. The difference of areal recrystallization distribution explains the difference between SAC105 and SAC305/396 thermal cycling behavior between the calculated and experimental thermal cycling results.