Interfacial reactions of Sn–Cu solder with Ni/Au surface finish on Cu pad during reflow and aging in ball grid array packages

Abstract

The interface between the solder and the under-bump metallization (UBM) affect the reliability of the solder joints. In this paper an investigation is reported on the interfacial reactions of Sn0.7Cu (wt.%) solder on electrolytic Ni layer for different times of reflow and solid-state aging. It is found that during reflow, the formation of Sn–Cu–Ni ternary intermetallic compounds (IMCs) at the interface of solder joints is much quicker, resulting in the entrapment of some Pb (which is present as impurity in the Sn–Cu solder) rich phase in the ternary IMCs. The growth rate of IMCs during reflow is higher than that in aging. Less than 3 μm of the electrolytic Ni layer was consumed by the higher Sn-containing Sn0.7Cu solder with 180 min molten reaction at 250 °C and with 16 days aging at 175 °C. Ni and Au can diffuse into the Sn–Cu solder in both solid and liquid state and form stable quaternary compounds in the solder. The diffusion of Au into the high Ni-containing ternary and quaternary compounds is found to be low. The shear strength of solder joint is relatively stable from 1.98 to 1.86 kgf during long time reflow with high Ni and negligible amount of Au in the ternary IMCs at the interface. The shear strength during aging increases up to 4 days and than decreases with the increase of aging time up to 12 days. It is due to the formation of quaternary compounds with relatively higher amount of Au.