Microstructure and mechanical properties of Sn58Bi composite solders reinforced with Ag-coated ZnO nanoparticles

With the development of advanced electronic packaging technologies, higher requirements have been placed on the service reliability of low-temperature solders. However, Bi-rich phase coarsening and excessive growth of interfacial intermetallic compound (IMC) in Sn58Bi solder restrict its engineering applications. In order to improve the microstructure and mechanical reliability of Sn58Bi solder, Ag-coated ZnO (ZnO@Ag) nanoparticles were successfully fabricated by electroless silver plating and Sn58Bi-xZnO@Ag (x = 0, 0.3, 0.5, 0.7, and 1.0 wt.%) composite solders were prepared by ball milling. The effects of ZnO@Ag addition on the wettability of solder joints, matrix microstructure, interfacial IMC growth, and shear properties were systematically investigated. The results showed that the addition of 0.3 wt% ZnO@Ag exhibited the optimal strengthening effect. Compared with the unreinforced Sn58Bi solder, the wetting angle decreased from 39.18° to 36.32°, the Bi-rich phase became finer with the number fraction of fine Bi-rich phase regions (0.1 < A ≤ 1 μm 2 ) reaching 31.78%. Meanwhile, the average thickness of the interfacial IMC decreased from 0.85 μm to 0.67 μm. Ultimately, the shear strength of the solder joints increased from 51.49 MPa to 69.62 MPa. The performance improvement may be attributed to the synergistic effects of heterogeneous nucleation, grain-boundary pinning, and interfacial diffusion blocking induced by the ZnO@Ag nanoparticles. This study demonstrates that the appropriate addition of ZnO@Ag is an effective approach for improving the reliability of Sn58Bi low-temperature solder, which can provide a reference for the design of high-reliability low-temperature lead-free composite solder.