Microstructural evolution of lead-free Sn-Bi-Ag-Cu SMT joints during aging

It was reported in a previous study that the Sn-6Bi-2Ag-0.5Cu solder alloy had great potential to replace leaded alloys. This alloy was prepared by mechanical alloying, and had the advantage of providing a high percentage of supersaturate solution of bismuth in tin. In the present paper, the microstructural evolution of surface-mount joints during aging was examined. In the as-soldered joints, small bismuth and Ag₃Sn particles of about 1 mum in size were found to be finely dispersed in a nearly pure tin matrix with a small amount of eta-Cu₆Sn₅ phase in the bulk of solder. During aging, microstructural evolution of solder joints occurred. These include Cu-Sn intermetallic compound (IMC) layer growth at the interface between solder and copper pad on the printed circuit board, as well as bismuth phase and Ag₃Sn phase coarsening. The shear strength of the solder joints decreased parabolically with the increase in IMC layer thickness, such that tau_s=22.22-radic22.05(t-1.88), where tau_s is the shear strength in MPa and t (>1.88) is the total IMC layer thickness in micrometers. The microstructure of solder appeared to be stable under aging at elevated temperatures up to about 160degC. Above this temperature, brittle and porous IMC epsiv-Cu₃Sn appeared at the copper/eta-Cu₆Sn ₅ interface. Fracture was found to occur at the Cu-Sn IMC layer-solder interface and in the bulk of solder