Au-Ni-Sn intermetallic phase relationships in eutectic Pb-Sn solder formed on Ni/Au metallization

Recent work has shown that a Au-Ni-Sn ternary compound with a nominal composition of Au0.5Ni0.5Sn4 redeposits and grows at the interface between eutectic Pb-Sn solder and Ni/Au metallization during aging at 150 degreesC. The present work verifies the existence of the Au0.5Ni0.5Sn4 phase by examining the Sn-rich corner of the Au-Ni-Sn ternary phase diagram. The reconfiguration mechanism of the AuSn4 from the bulk solder is also discussed, with detailed observations of the Au0.5Ni0.5Sn4 microstructure. The results show that the Ni solubility limit in the AuSn4 phase is approximately 12 at.% at 150 degreesC and thus, the Au0.5Ni0.5Sn4 phase is a ternary AuSn4-based compound with high Ni solubility. Due to the slight solubility and the fast diffusion of Au in the eutectic Pb-Sn at 150 degreesC, the AuSn4 intermetallics in the bulk solder can reconfigure to form a A(x)Ni(1-x)Sn(4) compound at the interface where Ni is available. The AuxNi1-xSn4 compound layer consists of nanocrystals arranged in a larger grain-like morphology. It appears that the inherent lattice strain of the AuxNi1-xSn4 compound and the volume change due to its formation results in a nanocrystalline microstructure.