Microstructure/properties and resistance to electrochemical migration of electrodeposited Ag-Sn alloy thin films

In 2.5D packaging, silver is widely used in electronic packaging due to its excellent electrical and thermal conductivity. However, the plastic deformability and electrochemical migration (ECM) resistance of pure silver severely limit its solid-state interconnects and long-term reliability. The plastic deformability and ECM resistance of the material can be effectively improved by alloying. Ag-Sn alloys were prepared by direct current electrodeposition technique, and their microstructure, mechanical properties, resistance to electrochemical migration and mechanism were investigated. It was found that the characteristic peaks of Ag-Sn alloys were shifted to the left as a whole with the increase of Sn content, the crystal plane spacing and lattice constants increased, and a single-phase solid solution structure was presented, and their hardness and elastic modulus decreased with the increase of Sn content. In addition, the anti-electrochemical migration properties of Ag-Sn at different voltages were explored by water drop test and compared with pure Ag. Phase field simulation and first principles were used to reveal the anti-electrochemical migration mechanism. The short-circuit failure time of Ag was 46 s at a bias voltage of 3 V. When the tin content was 16 wt. %, the short-circuit failure time of the specimen increased dramatically to 704 s, which was 15.3 times that of pure Ag. The inclusion of Ag and Sn elements in the dendritic products confirms their participation in anodic dissolution, ionic migration to the cathode and deposition at the cathode, and the formation of dendritic products. The Ag-Sn short-circuit failure time calculated in conjunction with nonlinear phase field simulations does not differ much from the experimental values, further verifying the accuracy of the experiments. The antimigration mechanism of Ag-Sn system is that under the bias voltage, the Sn atoms preferentially dissolve with the anode due to the low standard potential and react with the ionized OH^- to form SnO/SnO₂ covering the anode surface to inhibit the dissolution of Ag at the anode, which protects the anode, and the concentration of Ag⁺ in the vicinity of the cathode decreases due to the limitation of dissolution. The ECM “gestation period” and “growth period” are extended, thus prolonging the short-circuit failure time of the electrode. The discovery of the anti-electrochemical migration mechanism is of significant importance for Ag-Sn alloys in improving long-term reliability, and lays a theoretical foundation for subsequent research on silver-based alloys.