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The interaction between isothermal aging and the long-term reliability of wafer-level chip-scale packages with Sn-3.0Ag-0.5Cu (wt%) solder ball interconnects is investigated. On isothermally aging at 100 and 150°C for 500 h and then thermally cycling from 0 to 100°C with 10 min of dwell time, the lifetime of the package is reduced by approximately 29%, depending on the aging condition. The microstructural evolution is observed during thermal aging and thermal cycling using orientation image microscopy. A Sn grain orientation structure transformation is observed. Different mechanisms after aging at various conditions are identified, and their impacts on the fatigue life of solder joints discussed.