Effects of Ca on grain boundary cohesion in Au ballbonding wire

Measurements of the tensile properties of Ca doped Au ballbonding wires show that Ca significantly increases the strength of Au and improves ductility. The strengthening mechanism is poorly understood but the limited solubility of Ca in Au and the large atomic misfit factor precludes solid solution strengthening and there is insufficient Ca present for conventional precipitation hardening to be effective. However, Ca segregation would be a possible mechanism. The effects of Ca segregation are investigated by calculating the cohesive strength of a Ca-containing Au grain boundary relative to a free surface using a first-principles calculation within the framework of density functional theory. The simulation results support the theory that the tensile strength and ductility increase with Ca doping may be due to enhanced Au grain boundary cohesion.