Evaluation of the corrosion performance of Cu-Al intermetallic compounds and the effect of Pd addition

Copper wire has become a mainstream bonding material in fine-pitch applications due to the rising cost of gold wire. In recent years, palladium-coated copper (Pd-Cu) wire is being increasingly used to overcome some constraints posed by pure Cu wire. During wire bonding with aluminum bond pads, different intermetallic compound (IMC) phases that have been identified at the bond interface are typically CuAl₂, CuAl and Cu₉Al₄. Copper wire bonds were reported to fail humidity reliability tests and Cu₉Al₄ was observed to be the corroded IMC layer. However, the corrosion susceptibility of the individual IMCs has not been investigated. There has also been conflicting results on the effect of palladium on the reliability of copper wire bonds reported in literature. This paper compares the corrosion performance of the three types of Cu-Al IMCs in a chloride medium by employing electrochemical potentiodynamic polarization. A comparison was made with pure Cu and Al. The effect of Pd on the IMC corrosion performance was also studied. Among the three Cu-Al IMCs, Cu₉Al₄ was observed to have the largest corrosion rate followed by CuAl₂ and CuAl. For the metals, Cu was observed to have the lowest corrosion rate and Al is the most easily corroded. The addition of a few percent of Pd slightly improves the corrosion resistance of the metals and IMCs.