Microstructural and mechanical characterization of 95.5Sn-4Ag-0.5Cu solder balls by nano-indentation

Elimination of Pb usage in electronic assemblies accelerates the research on lead-free solder alloys. The Sn-Ag-Cu (SAC) ternary system draws attention among other lead-free candidates due to its superior creep and fatigue strength and slow-coarsening behavior in its near-eutectic alloys. Previous studies show that solder connections exposed to thermo-mechanical loading exhibit weak interfaces. Particular attention needs to be paid to operative damage mechanisms at dendrite boundaries, colony boundaries and Cu-pad solder interface. Knowledge concerning damage mechanisms can be used later to predict reliability and life time of solder connections with the help of a numerical model. Such a model must incorporate the microstructure and simulate the initiation and evolution of damage. It can be used to design solder joints and microelectronic products/systems. This study concentrates on the material characterization of 95.5Sn-4.0Ag-0.5Cu solder balls. E-SEM is employed for the characterization of the initial microstructure. The nano-indentation technique is used to determine the mechanical properties of individual phases. Solder balls mounted on FR4 board are also examined to determine the interfacial behavior between the solder and copper pads.