Evolution of microstructure and mechanical properties of Cu/SAC305/Cu solder joints under the influence of low ultrasonic power

Cu/SAC305/Cu solder joints were fabricated by a modified ultrasonic-assisted reflow soldering technique under ambient atmosphere. Ultrasonic vibration (USV) with low ultrasonic power ranging from 10 to 30 W was used while the evolution of microstructure and mechanical properties of the solder joints under the influence of such USV were investigated in this study. Results showed that ultrasonic power promoted the formation of eutectic phase and the coarsening of β-Sn phase in the solder matrix of the solder joints. When the samples were treated with longer USV time, the β-Sn phase changed from dendritic structure (10 W of USV) to globular structure (20 and 30 W of USV). The observed morphological change of β-Sn and eutectic phases in the solder matrix led to the variation in the solder matrix hardness and the formation of interfacial intermetallic compounds (IMCs) in the samples treated with USV at increasing ultrasonic power and time. Overall, the shear strength of the ultrasonic-treated samples increased with increasing ultrasonic power, regardless of the USV time.