The influence of low level doping of Ni on the microstructure and reliability of SAC solder joint

In this paper, the behavior of BGA solder joints microstructures was studied as a function of Ni doping in SAC solder. Three kinds of solder compositions were selected including Sn3.0Ag0.5Cu, Sn1.0Ag0.5Cu and Sn1.0Ag0.5Cu0.02Ni to value the influence the effect of Ni doping, OSP and Au/Ni pad was employed on the PCB side. Emphasis was placed on studying the effect of low level doping with Ni on the joint microstructure and subsequent reliability. Both solder composition and PCB surface finish had a notable effect on the interfacial microstructure, the Ni addition can give rise to needle like NiCuSn IMC formation and reduce the grain size locally at solder/NiAu pad interface after one time reflow according to top-view interface analysis, and had no obvious effect on the IMC evolution of solder/OSP pad, the phenomena was investigated from the perspective of metallurgy. Bending and drop tests were conducted to evaluate the effect of solder composition and pad finish on the joint reliability. It was found that the decrease of Ag concentration and Ni addition in SAC solder could significantly improve the drop test performance when NiAu pad was used. In bending test, OSP pad show better performance than Au/Ni pad. The correlation between joint microstructure and joint reliability was discussed in detail. The work can give some directions on the solder alloy design and choice of pad finish in electronic packaging.