The Effect of Ultrasonic-Assisted Soldering on the Microstructure and Mechanical Properties of Cu/Sn58Bi-0.3Co/Cu Solder Joints

Ultrasonic-assisted soldering technology offers an efficient and highly promising solution for achieving high-reliability interconnections in space applications. Through analysis, investigated the impacts of ultrasonic soldering time (300W, 0s, 4s, 8s, 12s) on the microstructural modification, grain orientation, and mechanical properties of Cu/Sn58Bi-0.3Co/Cu solder joints were studied. The results demonstrated that the introduction of ultrasonic energy significantly optimizes the microstructure of the composite joints. It effectively eliminated the agglomeration of intermetallic compounds (IMCs) in the matrix and promoted the uniform distribution of elements. As ultrasonic treatment duration increased, the IMC layer thickness at the interface showed a clear increasing trend. Notably, the IMC thickness at the cold end consistently exceeds that at the heat end, which may be related to the temperature gradient induced by ultrasonic cavitation. Electron backscatter diffraction analysis revealed that ultrasonic processing markedly refined the grain size of the solder joints. The grain size of (Cu, Co)6Sn5 decreased from 0.504 μm to 0.339 μm. The Sn and Bi phases transition from an initially random orientation to a preferred crystallographic orientation, while the (Cu, Co)6Sn5 phase retains its random orientation. Shear tests showed that the ultrasonically treated welds exhibited higher strength, with a maximum value of 61.24 MPa, representing a 24% increase compared to the untreated solders. Additionally, fracture consistently occurs within the matrix, displaying a fracture with ductility morphology, indicating that ultrasonic treatment enhanced strength. The findings of this study deepen the understanding of ultrasonic-assisted soldering mechanisms but also offered fundamental theoretical for optimizing composite soldering processes.