Effect of La₂O₃ Doping on the Hardness, Microstructure and Void Formation of Sn-Ag-Cu (SAC305) Lead-Free Solder Alloys

Abstract

The transition to lead-free solders in response to environmental regulations has established Sn-Ag-Cu (SAC) alloys as the industry standards. However, their reliability is limited by coarse microstructures, excessive intermetallic compound (IMC) growth, and susceptibility to void formation. This study investigates the microalloying of SAC305 solder with lanthanum oxide (La₂O₃) to enhance micromechanical properties and refine microstructure. SAC305 pastes doped with 0.5–5.0 wt.% La₂O₃ were prepared, reflowed on copper substrates, and characterized. Nanoindentation tests revealed substantial improvements in hardness and elastic modulus, with the most pronounced enhancement at 1.5 wt.% La₂O₃, where hardness and modulus increased by approximately 2408.76% and 1767.25%, respectively, compared to the undoped alloy. Microstructural analysis confirmed that La₂O₃ acts as a potent grain refiner, promoting heterogeneous nucleation for finer β-Sn grains and stabilizing IMC phases such as Cu₆Sn₅ and Ag₃Sn. Additionally, La₂O₃ doping suppressed void formation by improving wetting and facilitating gas escape during solidification. These findings demonstrate that La₂O₃ is an effective reinforcement for SAC solder, enabling superior mechanical performance through microstructural control and void reduction. These results help in advancing the development of high-reliability lead-free solder systems for demanding applications in electronic packaging, where joint integrity under thermal and mechanical stress is critical.