Effect of palladium on the mechanical properties of Cu and Cu-Al intermetallic compounds

There is a growing interest in copper (Cu) wire bonding due to its significant cost savings over gold wire. However, concerns on package reliability and corrosion susceptibility have driven the industry to develop alternative materials. Recently, palladium coated copper wire (Pd-Cu) wire has seen rapid entry into the market as it is believed to improve reliability of copper wire bonds on aluminum (Al) pads. However, the effect of palladium on the mechanical properties and corrosion resistance of Cu and Cu-Al intermetallics has not been studied in detail. In this paper, bulk alloys of Cu and Cu-Al with different concentrations of Pd were prepared under controlled conditions to simulate the intermetallics (IMC) formed between the copper ball bond and aluminum pads during bonding. Material properties such as elemental composition and phase formation were analyzed. Hardness and Young´s modulus of the alloys were characterized by nanoindentation. It was observed that CuAl intermetallic is the hardest and stiffest. Palladium was observed to slightly increase the modulus and hardness of the alloys.