Microstructure and Shear Properties Evolution of Minor Fe-Doped SAC/Cu Substrate Solder Joint under Isothermal Aging

Different amounts of Fe (0.005, 0.01, 0.03, 0.05, and 0.07 wt%) were added to SAC305 to study the shear behavior damage of Fe-doped SAC solder joints under thermal loading (170 °C, holding time of 0, 250, 500, and 750 h). The results show that during isothermal aging at 170 °C, the average shear force of all solder joints decreases with increasing aging time, while the average fracture energy first increases and then decreases, reaching a maximum at 500 h. Minor Fe doping could both increase shear forces and related fracture energy, with the optimum Fe doping amount being 0.03 wt% within the entire aging range. This is because the doping Fe reduces the undercooling of the SAC305 alloy, resulting in the microstructure refining of solder joints. This in turn causes the microstructure changing from network structure (SAC305 joint: eutectic network + β-Sn) to a single matrix structure (0.03Fe-doped SAC305 joint: β-Sn matrix + small compound particles). Specifically, Fe atoms can replace some Cu in Cu₆Sn₅ (both inside the solder joint and at the interface), and then form (Cu,Fe)₆Sn₅ compounds, resulting in an increase in the elastic modulus and nanohardness of the compounds. Moreover, the growth of Cu₆Sn₅ and Cu₃Sn intermetallic compounds (IMC) layer are inhibited by Fe doping even after the aging time prolonging, and Fe aggregates near the interface compound to form FeSn₂. This study is of great significance for controlling the growth of interfacial compounds, stabilizing the microstructures, and providing strengthening strategy for solder joint alloy design.