High stability and aging resistance Sn-1Ag-0.5Cu solder alloy by Fe and Bi minor alloying

Abstract

The current work investigates the effect of Fe and Bi addition, 0.05 wt% Fe and 1 or 2 wt% Bi, on the mechanical and microstructural properties of the Sn-1Ag-0.5Cu (SAC105) solder alloy after aging at 125 °C for 30 days. Microstructural and mechanical results revealed stabilized properties of Sn-1Ag-0.5Cu-0.05Fe-xBi (x = 1 or 2) after aging treatment. UTS (ultimate tensile strength) and yield strength of SAC105 decreased by aging due to coarsening of intermetallic compounds (IMCs). However, UTS, Yield strength, and total elongation of SAC105-Fe-Bi remained approximately constant by aging treatment. These are due to Bi within the β-Sn matrix reducing the activity of Sn involved in the chemical reaction between Sn and Cu or Sn and Ag, hence reduces the amount of Cu6Sn5 and Ag3Sn IMC particles. Furthermore, partial of Fe atoms solute in the Sn matrix in the eutectic regions reduces the diffusivities of Ag, which is the consequence of the Ostwald ripening process. Therefore, Fe reduces the supply of Ag required for the Ag3Sn coarsening. Moreover, partial substitution of Fe in Cu6Sn5 due to Darken-Gurry ellipse decreased microstructure coarsening rate. The nanoindentation results showed a remarkable increase in Er (reduced elastic modulus) and hardness of β-Sn and IMCs which can be attributed to the solid solution effect and distortion of the lattice parameter of β-Sn by Bi and substitution of Fe with Cu in Cu6Sn5.