Electromigration behavior of Cu/Sn–58Bi–1Ag/Cu solder joints by ultrasonic soldering process

Sn–58Bi–1Ag solder ribbon was prepared by twin-roll rapid solidification technology, and the ultrasonic soldering process was used to prepare Cu/Sn–58Bi–1Ag/Cu linear solder joints. Electron probe microanalysis (EPMA) and energy dispersive X-ray spectroscopy (EDS) were used to study the interface morphology of intermetallic compounds (IMC), Bi segregation, and solder joint matrix microstructure evolution with the current density of 1 × 104A/cm2 (25 °C). The result shows that the morphology of the anode IMC layer changes from scallop-like to hilly-like and then to flat-plate-like with the electrification time, and the thickness of IMC layer increases gradually. Bi is segregated toward the anode to form a Bi-rich layer. The cathode IMC layer is changed from a scallop to a zigzag, and its thickness is firstly increased and then decreased. The Sn is segregated toward the cathode to finally form a β-Sn-rich layer. Coarsening of eutectic microstructure (β-Sn+Bi) in solder joint matrix was caused by prolonged time. Based on linear fitting, the kinetic index n of the IMC layer of the anode and cathode is 0.432 and 0.491, respectively, and the growth mechanism is supposed to be volume diffusion. Ultrasonic soldering techniques can refine the Bi phase and increase the solubility of Bi in β-Sn, which slows down the formation of Bi-rich layers. The addition of Ag forms wedge-shaped and granular Ag3Sn intermetallic compounds, which hinders the growth of the Bi-rich layer and the Cu6Sn5 IMC, thus effectively inhibiting the electromigration process.