Mechanical Properties of Arrayed Pb-Free Tin Bump and Its Interfacial Reaction with Ni-P UBM during Reflow Process

In this paper the array and peripheral structured pure tin solder bump with electroless Ni-P UBM (under bump metallization) as barrier layer was fabricated on 4 inch wafer. The solder uniformity at wafer level was good after reflow. The effect of different reflow temperature on solder shear strength was studied and the bump failure mode and the interfacial reaction between tin bump and Ni-P UBM layers were analyzed. The morphology and constitute of intermetallic were analyzed by optical microscope and SEM. The results show that the needle-like Ni₃Sn₄ intermetallic was formed at the interface between tin bump and UBM layer. The reflow temperature had little effect on bump shear strength. The failure of solder shear test was ductile fracture. With the reflow temperature and time increasing, needle-like Ni₃Sn₄ intermetallic began ripen and spall-off into the solder due to compressive stress. At the same time fine Ni₃P intermetallic was formed between electroless Ni-P layer and Ni₃Sn₄ layer