Phase analysis and kinetics of solid-state ageing of Pb-free Sn-3.5Ag solder on electroless Ni-P substrate

Growth kinetics of intermetallic compound (IMC) layers formed between Sn-3.5Ag solder and electroless Ni-P/Cu substrates were investigated at temperatures ranging from 100 to 170degreesC for 0 to 60 days. The IMC formed at the interface was mainly Ni3Sn4, and a P-rich Ni (hereafter Ni3P) layer was also observed as a by-product of Ni3Sn4 formation, that was localized between the Ni3Sn4 IMC and the electroless Ni-P deposit layer. This Ni3P layer formed because of the phosphorous accumulation at the interface between the electroless Ni and the IMC layer. It was found that both the Ni3Sn4 IMC and the Ni3P layer were formed due to the solder reaction-assisted crystallization. Also, another layer with Ni-Sn-P was observed between the Ni3P layer and the Ni3Sn4 IMC layer. The layer thickness of Ni3Sn4 and Ni3P were approximately 1.83 and 1.56 mum after 50 days of ageing at 150degreesC, respectively. Copyright (C) 2004 John Wiley Sons, Ltd.