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

There is growing interest in copper (Cu) wire bonding due to its significant cost savings over gold wire. However, concerns regarding package reliability and corrosion susceptibility have driven the industry to develop alternative materials. Recently, palladium coated copper (Pd-Cu) wire has been widely used in fine pitch applications 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 intermetallic compounds (IMCs) has not been studied in detail. In this paper, bulk alloys of Cu and Cu-Al alloys with different concentrations of Pd were studied. Among the three IMCs studied, CuAl (H ∼ 8.5 GPa) was found much harder than CuAl₂ and Cu₉Al₄ (H ∼ 6.0 GPa). Young's moduli of CuAl and Cu₉Al₄ IMCs were around 155 GPa and are much stiffer than the one for CuAl₂ (∼110 GPa). The addition of a few percent Pd only slightly increases the hardness and Young's modulus of the IMCs.