Microstructural Evolution in Solid-State Sn-Zn/Ag/Cu Interfacial Reactions at 170 °C

Ag is widely used as a surface-finish material in electronic packaging, while Sn-Zn is a representative lead-free solder system. This study investigates the interfacial reactions and microstructural evolution in Sn-Zn/Ag/Cu couples during solid-state aging at 170 °C. Unlike the conventional approach, in which Ag is electroplated onto Cu substrates followed by reflow soldering, Ag (~2.0 μm) and Cu (~50 μm) layers were sequentially electroplated onto three Sn-Zn solders containing 1, 3, and 9 wt.%Zn. By eliminating the reflow process, the Ag layer remained at the interface, allowing the reactions to proceed in the Sn-Zn/Ag/Cu couples and preserving the sequential interactions among these elements. The effects of Zn concentration on interfacial phase evolution, elemental diffusion, and reaction mechanisms were systematically investigated. For the Sn-1Zn and Sn-3Zn couples, Ag₃Sn preferentially formed near the solder/Ag interface during the early stage, accompanied by Ag₅Zn₈ in the inner reaction region. With prolonged aging, the Ag-Zn phases were progressively redistributed into the solder, while the reaction at the Cu side evolved from Cu₅Zn₈ to CuZn, and eventually to Cu₆Sn₅ as Zn was progressively depleted. In contrast, the Sn-9Zn couple exhibited a distinct reaction pathway, initially dominated by Ag₅Zn₈, AgZn₃, and Cu₅Zn₈, with no continuous Ag₃Sn layer detected. The higher initial Zn content provided a greater supply of Zn for interfacial reactions, leading to substantial Zn consumption and accelerating the transition from Cu-Zn to Cu-Sn reactions, with localized Cu consumption and subsequent Cu₆Sn₅ formation. Comparison with the Sn-Zn/Ag couples, in which AgZn was formed, further demonstrated that the limited Ag supply from the thin Ag layer in the Ag/Cu couples favored Ag₅Zn₈ formation and promoted the continuous redistribution of Ag into the solder.