Microstructure and mechanical properties of Ag nanoparticles-modified Sn–58Bi/Cu solder joints during liquid-state reaction

Alloying various elements into the solders or the Cu substrate has been conducted to prevent embrittlement of Sn–Bi/Cu solder joints, caused by Bi segregation and growth of Kirkendall voids, via changes in the intermetallic compounds (IMCs) and interfacial microstructure of the solder joints. In this study, reactions between Cu and Sn–58Bi alloyed with Ag nanoparticles (NPs) were investigated by varying number of reflow. Even after 9 times of reflow, formation of Cu 3 Sn and Kirkendall void was not much observed at the interface, contrary to what is expected in Sn–58Bi/Cu solder joints after solid-state aging. The experimental results revealed that alloying a large amount of Ag was not helpful to strengthening the solder joint, whereas small addition of Ag NPs (0.5 wt.%) effectively improved the mechanical reliability via refinement of the eutectic microstructure of Sn–58Bi solder and suppression of growth of the Cu 6 Sn 5 grains at the interface. With increasing amount of Ag NPs, shear strength decreased because of increase and coarsening of Ag 3 Sn particles and formation of weak interfaces of solder/Ag 3 Sn and Ag 3 Sn/Cu 6 Sn 5 , where brittle fracture was observed. For liquid-state reaction, addition of a small amount of Ag into Sn–58Bi solder was successful to enhance mechanical reliability without formation of Kirkendall voids at the reaction interface.