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 ºC. 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 ºC.