Effect of Pr addition on properties and Sn whisker growth of Sn–0.3Ag–0.7Cu low-Ag solder for electronic packaging

In this study, trace amount of rare earth Pr was added into Sn–0.3Ag–0.7Cu low-Ag solder to enhance properties of solders. Experimental results indicated that optimal amount of Pr addition (~0.06 wt%) can improve properties of wettability, shear force and ductility of Sn–0.3Ag–0.7Cu low-Ag solder. This is because that solder with optimal Pr addition not only had a refined microstructure but also owned a regular and thin interfacial IMC layer after soldering on Cu substrate. Meanwhile, we have explained the change of morphology and thickness of interfacial IMC layer after Pr addition based on Kim and Tu’s kinetic model of interfacial IMC growth. However, it was found excessive Pr addition led to the formation of PrSn₃ phase, which was easy to be oxidized and became a great deterioration on the properties referred above. Besides, the fracture mode of solder joint also gradually changed from ductile fracture to cleavage fracture. Moreover, this oxidized PrSn₃ became the birthplace of Sn whisker since it provided abundant supply of Sn sources and compressive stress for Sn whisker growth. By the in-situ observation of Sn whisker growth in 0.5 wt% Pr-doped solder joint, we obtained that the incubation period of Sn whisker is very short and its growth rate may have a great decrease with time extending. Besides, it was found after 1 day at room temperature, spindly and rod-like Sn whiskers also grew in 0.5 wt% Pr-doped solder joint besides dot-shaped Sn whisker that grew in 0.5 wt% Pr-doped solder matrix. We suggest this may be related with the extra compressive stress provided by interfacial IMC layer growth in 0.5 wt% Pr-doped solder joint.