Effect of Rare Earth Pr on Creep Behavior of Sn-0.3Ag-0.7Cu-0.5Ga Low-Ag Solder Alloys

For development of low-Ag lead free solder alloys for microelectronic packaging, the correlation of creep properties with microstructure of novel Sn-0.3Ag-0.7Cu-0.5Ga (SAC-Ga) solder alloys bearing Pr has been investigated using nanoindentation. The results show that the creep deformation of SAC-Ga, SAC-Ga-0.06Pr, SAC-Ga-0.5Pr is 1717, 1144, and 1472 nm, respectively, which indicates that Pr addition could significantly enhance the creep resistance of SAC-Ga solders due to the refinement and uniform distribution of Cu₆Sn₅ intermetallic compounds (IMCs). However, compared with the SAC-Ga-0.06Pr solder alloy, the SAC-Ga-0.5Pr alloy shows poorer creep resistance which is mainly attributed to the surface oxidation of excess rare earth Pr. In addition, Dorn model has been used to describe the creep behavior and to obtain stress exponents of the SAC-Ga solder alloys bearing Pr. The strengthening mechanism of creep resistance in SAC-Ga solder alloys bearing Pr is that when encountering refined and well-distributed Cu₆Sn₅ IMCs, a dislocation line cannot climb through the IMCs but bypass the IMCs, thus leading to a decrease in the creep deformation of the solder alloys bearing Pr.