The Crystal Orientation of β-Sn Grains in Sn-Ag and Sn-Cu Solders Affected by Their Interfacial Reactions with Cu and Ni(P) Under Bump Metallurgy

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 and electromigration. Additionally, it is also known that the microstructure of the Sn-rich solders is strongly affected by their alloy composition. In this study the grain size and orientation of the β-Sn phase were investigated in terms of their alloy composition and interfacial reactions with two different under bump metallurgies (UBMs), Cu and Ni(P). Solder balls (380 μm in diameter) of pure Sn, Sn-0.5Cu, Sn-0.5Ag, and Sn-1.8Ag (in weight percent) were reflowed on Cu and Ni(P) UBMs. After the reflow at 250°C for 120 s, the microstructure of the solder joints was analyzed by cross-polarization light microscopy and electron backscatter diffraction. For the compositional analysis of solder joints, electron probe micro-analysis was used and thermodynamics calculations were also performed. During reflow on Cu or Ni(P) UBM, Cu and Ni atoms were dissolved quickly and were saturated to their solubility limits in the solders, causing changes in composition and β-Sn grain orientation.