Microstructural, thermal and mechanical properties of Co added Sn-0.7Cu lead-free solder alloy

This study investigated the effects of cobalt microalloying addition on the microstructural features, thermal characteristics and mechanical behavior of eutectic Sn-0.7wt%Cu lead-free solder alloys. The results show that minor cobalt addition of similar to 0.05 wt% causes significant grain refinement of beta-Sn, facilitates the formation of fine fibers (Cu,Co)(6)Sn-5 phases and preventing the formation of eta '-Cu6Sn5 phases, whereas a large amount of Co (similar to 0.5 wt%) additions accumulated in the (Cu,Co)(6)Sn-5 IMCs and clearly changed into coarse fibers. The precipitation strengthening mechanisms of fine fibers (Cu,Co)(6)Sn-5 in the beta-Sn matrix increased the ultimate tensile strength (UTS) and Young's modulus (Y) of the alloy from 30.5 MPa and 15 GPa to 44.6 MPa and 22.3 GPa, respectively, but the ductility decreased from 60 to 45.7%. The coarse fibers (Cu,Co)(6)Sn-5 in eutectic alloys is of interest from not only increased UTS and Y to 38.7 MPa and 16.3 GPa but also maintaining the ductility at the same level, allowing for unique microstructure design. Furthermore, 0.05wt% of Co significantly reduce the onset, eutectic temperatures and undercooling, although pasty rang has been slightly raised, which may enhance the thermal characteristics. This presumably has important implications for the reliability of solders as well as their performance in electronic service.

Automated summary

This study investigates the effects of cobalt microalloying addition on the microstructural features, thermal characteristics, and mechanical behavior of eutectic Sn-0.7wt%Cu lead-free solder alloys. The addition of cobalt refines the grain of beta-Sn, promotes the formation of fine fibers (Cu,Co)(6)Sn-5 phases, and prevents the formation of eta'-Cu6Sn5 phases. Precipitation strengthening mechanisms increase the ultimate tensile strength and Young's modulus of the alloy but decrease its ductility. Additionally, the inclusion of cobalt significantly affects the thermal characteristics and may have important implications for the reliability and performance of solder in electronic service.