Synchrotron radiation real-time in situ study on dissolution and precipitation of Ag3Sn plates in sub-50 μm Sn–Ag–Cu solder bumps

Synchrotron radiation real-time imaging technology was used to in situ study the dissolution and precipitation behavior of Ag3Sn plates in sub-50 μm Sn–4.0Ag–0.5Cu flip chip solder bumps on Ni–P under bump metallization (UBM). The dissolution rate of large Ag3Sn plates was as slow as 0.1–0.2 μm/s at the heating stage, while the precipitation rate exceeded 5 μm/s at the cooling stage with an undercooling of 31–55 °C. The random precipitation behavior of Ag3Sn plates was in situ observed and explained from the perspective of cluster. This study shows an effective method to investigate the microstructural evolution and the undercooling of phases in real ultra-fine solder joints.