Oxidation and corrosion of Au/Al and Cu/Al in wire bonding assembly

As a result of increasing gold (Au) price, the use of copper (Cu) bonding wire has gained popularity for mass production. The different properties in hardness, oxidation and electrochemical potential between Au and Cu have a significant impact on the wire bonding process and reliability. This paper focuses on the oxidation and corrosion of Au/Al and Cu/Al ball bond for the wire bonding processes and their bond reliability in pressure cooker test (PCT) and halides environment. A design of experiment was conducted to optimize the wire bonding process prior to subjecting the bonded samples to oxidation and corrosion studies. It is noted that a higher ultrasonic energy is necessary for Cu wire bonding. This is likely due to higher energy necessary to break the oxides of copper and aluminum to result in formation of bonds between the wire and the bonding pads. In PCT analysis, the Au ball shear test results remained stable, while the Cu ball shear test results improved with soaking. CuO blisters are found on the surface of the Cu with corrosion apparent on the circumference of the Cu ball. The corrosion resulted in delamination between Al and Cu and generated undesired interface failure modes. A significant drop in stitch pull strength with large results deviation was observed for Cu wire bonding due to oxidation and stress resulting in cracks of the stitch. In halide environment, an increase in failure rate was observed for Cu/Al pad. This is a result of galvanic corrosion due to the very limited intermetallic formation beneath the Cu ball which allowed electrolyte to seep in to initiate galvanic corrosion. This paper provides an insight to the failure mechanisms due to oxidation and corrosion which can weaken the Cu/Al bond as compared to Au.