The Microstructure and Crystal Orientation of Sn-Ag and Sn-Cu Solders Affected by their Interfacial Reactions with Cu and Ni(P)

Recently, it has been reported that the crystal orientation and grain size of the β-Sn phase in Sn-rich solders have profound effects on the reliabilities of Pb-free solder joints, such as thermo-mechanical fatigue, electromigration, and among others. Additionally, it is also known that the microstructure of Sn-rich solders is strongly affected by their alloy composition. In this study, the grain size and orientation of the βSn phase are investigated in terms of their alloy composition and interfacial reactions with two different under bump metallurgy (UBM), Cu vs. Ni(P). Solder balls (380μm in diameter) of pure Sn, Sn-0.5Cu, Sn-0.5Ag and Sn-1.8Ag (in wt%) were reflowed on Cu and Ni(P) UBM. After reflowed at 250°C for 2min, the microstructure of solder joints is analyzed by cross polarization light microscopy and electron backscatter diffraction (EBSD). For the compositional analysis of solder joints, electron probe micro analyzer (EPMA) was used, and thermodynamic calculations were performed to compare with.