Mechanisms of deformation in high-ductility Ce-containing Sn-Ag-Cu solder alloys

Rare-earth-containing Pb-free solders have gained widespread attention due to their superior ductility relative to conventional Pb-free alloys. Our previous work has shown that new Ce-based alloys are also extremely oxidation resistant compared to La or Y-containing alloys. In this paper, we report on a mechanism-based model for the large increases in ductility with small addition of rare-earth element to Sn-3.9Ag-0.7Cu. The mechanisms of ductility enhancement by Ce were observed in a scanning electron microscope, in interrupted shear-tests, where CeSn₃ particles served as microscopic fracture and void nucleation sites. Micro-mechanical modeling using the finite-element method was used to examine the plastic strain field in solder affected by the particles. The concentrated deformation band was seen to be disturbed by the particles, resulting in a more uniform deformation pattern with reduced strains and thus enhanced ductility of the lap-sheared joint.