Effects of intermetallic compounds on properties of Sn-Cu lead-free soldered joints

The environmentally conscious manufacturer is moving toward ´green products´ for electronic devices and components. In this paper we investigate the interfacial reactions of Sn0.7Cu solder on electrolytic Ni layer for extended times reflow. It is found that during as-reflowed, the formation of Cu-rich Sn-Cu-Ni ternary intermetallic compounds (TIMC) at the interface of Sn0.7Cu solder with electrolytic Ni is much quicker, resulting in the entrapment of some Pb (which is present (about 0.02wt%) as impurity in the Sn-Cu solder) rich phase in the TIMC. During extended time of reflow, high (>30at %), medium (30-15at %) and low (<15at %) Cu TIMC are formed at the interface. Cu-rich TIMC have higher growth rate and consume more Ni layer. Less than 3 micron of the electrolytic Ni layer was consumed during molten reaction by the higher Sn-containing Sn0.7Cu solder in 180 minutes at 250°C. The shear strength of Sn0.7Cu solder joint is stable from 1.982-1.861 kgf during extended time reflow. Cu prevents the resettlement of Au at the interface. The shear strength does not depend on the thickness of intermetallic compounds (IMC) and reflow time. Ni/Sn-Cu solder system has higher strength and can be used during prolonged reflow.