Effect of immersion Ag surface finish on interfacial reaction and mechanical reliability of Sn–3.5Ag–0.7Cu solder joint

In this study, we investigated the interfacial reaction and joint reliability of immersion Ag-plated Cu substrate with the most promising Sn–3.5Ag–0.7Cu solder. We first evaluated the interfacial reactions of the solder joint and also successfully revealed a connection between the interfacial reaction behavior and mechanical reliability. During reflow, the Ag plating layer was dissolved completely into the molten Sn–Ag–Cu solder and some of the Cu layer was also dissolved into the molten solder. The dissolved Ag and Cu were precipitated as Ag3Sn and Cu6Sn5 IMCs in the solder matrix. Upon reflow, the Sn–Ag–Cu solder exhibits an off-eutectic reaction to produce the eutectic phase and precipitate (Cu6Sn5 and Ag3Sn). The Cu–Sn IMC layer was formed at the solder/Cu interface after reflow and the IMC layer grew during aging treatment. During the shear tests, the failure mode switched from a bulk-related failure to an interface-related failure. After aging for 500 h, the joint failed partially at the solder/Cu6Sn5 interface. The brittle fracture was linked to the formation of thick Cu–Sn IMC layer.