Interface reaction of Pb-free Sn-3.5Ag solder with Ni-Sn-P metallization

The electrolessly plated Ni-P has been extensively studied due to its coating uniformity, selectivity and low coating stress. However, the use of lead-free solders accelerates interfacial reaction because its higher melting points and higher Sn content than the conventional Pb-Sn solders. In this work, we developed a ternary electroless Ni-Sn-P (7~8 wt.% of P and 1.4 wt.% of Sn) alloy to be used as the soldering metallization. Besides having good solderability, the presence of Sn in electroless Ni-Sn-P changes the diffusion process during soldering reflow. Comparison was made with the results obtained from commercial binary Ni-P (7~8 wt.% of P) metallization. The microstructure of the interfacial IMCs for Ni-P/Sn-3.5Ag and Ni-Sn-P/Sn-3.5Ag solder joints were investigated under different reflow durations at 260°C. The diffusion mechanisms of solder reaction for both types of solder joints were discussed. In addition, it was found that the consumption rate of plated Ni-Sn-P layer is faster than that of Ni-P layer up to 30 cycles of reflow at 260°C.