Enhancement of the impact toughness in Sn–Ag–Cu/Cu solder joints via modifying the microstructure of solder alloy

• Short-time annealing in the post-reflow process can improve the impact toughness of solder joints.
• The annealing step changes the Sn grain structure from interlaced type toward larger-grain type.
• The elemental distribution relates to the grain structure and microstructure in the solder.
• The softer solder can absorb more impact energy to enhance the impact toughness of solder joints.

In this research, the impact toughness of Sn–3.0Ag–0.5Cu/Cu (wt%, SAC305/Cu) solder joints is efficiently improved via controlling the solidification step of reflow process. During solidification step of reflow process, the solder joints were annealed under various heating time and temperature. A high speed shear tester was used to evaluate the impact reliability of these solder joints. Noteworthily, the annealing at higher temperature (210 °C) for a short time (100 s) can enhance the average impact toughness of SAC305/Cu solder joints up to 30%. The improvement of impact toughness was attributed to the evolution of microstructure in the solder alloys. In the annealed solder, the phase precipitation within the solder varied from a dot type toward a network type. Additionally, the β-Sn grain structure was altered from interlaced grains to larger grains due to the redistribution of Cu in the solder.