Reflow study of Pb-free Sn-3.5Ag solder with Ni-Sn-P metallization

Electrolessly plated Ni-P has been extensively studied due to its high coating uniformity, selectivity and low coating stress. However, the use of lead-free solders accelerates interfacial reaction because they have higher melting points and higher Sn content than the conventional Pb-Sn solders. In this work, we developed a ternary electroless Ni-Sn-P (6~7 wt.% of P and 15~17 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 (6~7 wt.% of P) metallization. The microstructure of the intermetallic compounds (IMCs) for Ni-P/Sn-3.5Ag and Ni-Sn-P/Sn-3.5Ag soldering reactions were investigated under different reflow durations with a reflow temperature of 260°C. The diffusion mechanisms of solder reaction for both types of solder joints were discussed. In addition, it was found that the formation of microvoids is much more difficult in Ni-Sn-P solder joints than that in Ni-P solder joints, indicating Ni-Sn-P solder joints will have better mechanical properties.