Microstructural evolution of the Sn-51Bi-0.9Sb-1.0Ag/Cu soldering interface during isothermal aging

Development of lead-free solders with a melting point lower than that of the most widely used Sn-3.0Ag-0.5Cu (SAC305) is crucial for coping with the welding defects of ultra-thin microchips. The Sn-58Bi solder with a low melting point of 138 °C is a promising one. However, some severe issues of this solder, such as the brittle failure of soldering interface and the serious coarsening of the intermetallic compound (IMC) in the solder/Cu matrix interface, have hindered its applications in microelectronic assembly industries. In this work, the morphologies and layer thicknesses of interfacial intermetallic compounds between the Sn-51Bi-0.9Sb-1.0Ag solder and Cu substrate during the isothermal aging at 125 °C have been investigated. The results show that the scallop-like intermetallic compound layers of different curvature radii are formed at the solder joint interfaces. The main phase composition of the IMC layer is Cu6Sn5. After the isothermal aging treatment, the IMC layer gradually becomes a flat layer. The IMC thickness of the Sn-51Bi-0.9Sb-1.0Ag/Cu soldering interface at the stable state is thinner by 33% compared to that of the Sn-58Bi/Cu interface. By fitting the experimental data, the growth rate constant of the IMC layer in the Sn-51Bi-0.9Sb-1.0Ag/Cu solder joint is 2.060 × 10–18 m2·s−1, while it is 7.302 × 10–18 m2·s−1 in the Sn-58Bi/Cu solder joint. The present study indicates that the addition of Sb and Ag elements can substantially suppress the IMC layer growth.