Effects of Tellurium addition on the microstructural evolution and properties of Sn-0.7Cu lead-free solder alloy

Recently, tellurium (Te) has become known for its low melting point and high affinity with Sn. Micro-alloying Te has been anticipated to modify Sn-0.7Cu solder in different aspects. This study investigated the effects of Te on the thermal behaviors, microstructure, wettability, post-soldering interface reactions, and tensile properties of Sn-0.7Cu solder. The results indicated that the addition of Te slightly impacted the melting behaviors of the Sn-0.7Cu solder but significantly enhanced wettability. The β-Sn phase transformed from dendrite to equiaxed or “petal-like” dendrites with some coarsening after the addition of Te. Moreover, Cu₆Sn₅ IMCs exhibited a tendency towards “rod-like” and “dot-like” structures when the Te content increased to 0.10 wt%, displaying the most significant refinement. This is attributed to the excessive precipitation of Te in the eutectic region, which combine with Sn to form the SnTe phase, thereby impeding the further growth of the Cu₆Sn₅ phase. Furthermore, the post-soldering IMCs layer transformed from serrated to scallop-shaped structure, and the Cu₆Sn₅ IMCs in the post-soldering matrix evolved from “plate-like” or “letter-like” shapes with cracks to a structurally stable “nut-like” or hexagonal shapes. When the amount of Te added was 0.10 wt%, the ultimate tensile strength (UTS) and elongation (EL) reached maximum values of 36.42 MPa and 30.14 % respectively, and the UTS increased by 27.4 % compared with Sn-0.7Cu solder. This enhancement is attributed to the dispersion distribution of the Cu₆Sn₅ phase and fine-grain strengthening.