Silver-assisted copper wire bonding using solid-state processes

The copper (Cu) wire has become a new alternative of wires in advanced packaging technology. In this research, we selected large Cu wires to develop a new bonding process based on solid-state process. Upon thorough investigation, we chose silver (Ag) as the bonding medium between Cu wires and silicon (Si) chips, for its ductility and superior physical properties. The end of Cu wires is cut and polished at an angle to form a flat bonding surface. This surface is plated with 50um thick Ag, following annealing to make it more ductile to facilitate solid-state bonding. For demonstration, Cu substrates and Si chips were used. Other chips such as SiC can be used too. The bonding was performed at 300°C with 1,000psi (6.9MPa) pressure in 0.1 torr vacuum for a few minutes. This pressure is less than 1/10 of what used in industrial thermo-compression processes. The breaking force of resulting wire-bonds was evaluated by pull test. For 1mm Cu wire-bonds on Si chips, the average breaking force is 17kg. The fracture surfaces were examined and analyzed in details. The limit on the operating temperature is the eutectic temperature of the Ag-Cu system, 780°C. Our target operating temperature is 350°C for 1,000 hours, still being verified. At this temperature, no encapsulating material or mounding compound is available on the market to reinforce the wire-bonds, except lead-containing glass. Most likely, the wire-bond has to stand as it is. Accordingly, high breaking force is an essential requirement.