Evolution of the microstructure of Sn-Ag-Cu solder joints exposed to ultrasonic waves during solidification

Cu/SAC405/Cu solder joints were fabricated using a modified reflow-soldering procedure. The samples were first maintained at 260 °C for 320 s, following the conventional reflow-soldering methodology. The reflow process was then interrupted and the samples were exposed to ultrasonic waves (USW) while they were cooling in air. Compared with a sample reflow-soldered conventionally, the solidification of the Sn-Ag0.04-Cu0.005 (SAC405) solder filler metal alloy under the influence of USW resulted in significant changes to the microstructure of the solder joints. The thickness of the Cu6Sn5 intermetallic layer at the Cu/SAC interface of the USW-solidified soldered joint decreased by as much as 76%. The β-Sn dendrite width was also reduced by as much as 67%, and the SAC matrix was filled with bundles of acicular Cu6Sn5 crystals. The formation of Ag3Sn plates was also prevented, and the size of the rod-like SAC ternary eutectic matrix was reduced from 700 nm to 50 nm. This behaviour is attributed to the effect of cavitation and liquid metal streaming induced by the USW on nucleation and the whole solidification process. The presence of Cu6Sn5 bundles and the refined eutectic and β-Sn dendrites in the matrix led to an average improvement in the hardness of the solder bulk by 45%.