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 Au sub(0.5)Ni sub(0.5)Sn sub(4) redeposits and grows at the interface between eutectic Pb-Sn solder and Ni/Au metallization during aging at 150 degree C. The present work verifies the existence of the Au sub(0.5)Ni sub(0.5 )Sn sub(4) phase by examining the Sn-rich corner of the Au-Ni-Sn ternary phase diagram. The reconfiguration mechanism of the AuSn sub(4) from the bulk solder is also discussed, with detailed observations of the Au sub(0.5)Ni sub(0.5)Sn sub(4 ) microstructure. The results show that the Ni solubility limit in the AuSn sub(4) phase is approximately 12 at.% at 150 degree C and thus, the Au sub(0.5)Ni sub(0.5)Sn sub(4) phase is a ternary AuSn sub(4)-based compound with high Ni solubility. Due to the slight solubility and the fast diffusion of Au in the eutectic Pb-Sn at 150 degree C, the AuSn sub(4) intermetallics in the bulk solder can reconfigure to form a Au sub(x)Ni sub(1-x)Sn sub(4) compound at the interface where Ni is available. The Au sub(x)Ni sub(1-x)Sn sub(4) compound layer consists of nanocrystals arranged in a larger grain-like morphology. It appears that the inherent lattice strain of the Au sub(x)Ni sub(1-x)Sn sub(4) compound and the volume change due to its formation results in a nanocrystalline microstructure.