Complete elimination of Ag₃Al in Ag wire bonds on Al by alloying In into Ag

During the electronic packaging process, when the silver (Ag) alloy wire bonds on aluminum (Al) pads, it is often desirable to avoid the formation of Ag₃Al because of its low fracture toughness and low corrosion resistance. The current work reports a method to eliminate the Ag₃Al through alloying indium (In) into Ag. The corresponding mechanism is studied through transmission electron microscopy (TEM) and thermodynamic modeling. The experimental results show that the formation of Ag₃Al leads to the repelling of In atoms into unreacted Ag solid solution when the In concentration is low, but the Ag₃Al can be fully eliminated and replaced by an Ag–In–Al ternary phase when the concentration of In is at 12%. We establish an analytical thermodynamic model for the nucleation process of Ag₃Al that considers the effects of In doping, and successfully explains the underlying reason for the elimination of Ag₃Al: it has to do with the necessity of repelling In into unreacted region. Reaction kinetics are further discussed and the deceleration in Ag–Al inter-diffusion is attributed to the solute–vacancy attraction. A study of defects in Ag–Al bonding wire system is conducted and the growth rate of voids is found to be low due to small volumetric change during phase transformation, which also reduces the growth rate of alumina. Compared to Ag₃Al, Ag–In–Al ternary phase is experimentally proven to be softer, tougher, and more corrosion resistant, which would contribute to enhanced wire bond reliability and perhaps an increase the market share of Ag alloy bonding wires.