Copper versus palladium coated copper wire process and reliability differences

Fine pitch copper wire bonding presents challenges to both first and second bond processes. Corrosion of the first bond copper-aluminum (Cu-Al) intermetallic compound bond interface layer can be induced by mobile chlorine in the epoxy mold compound, and the second bond process window can be narrower than with gold wire. Palladium-coated copper wire is believed to overcome these two problems, however, the actual benefits and challenges of the wire need to be considered by the semiconductor industry. The price of palladium-coated copper wire is 2.5-3 times higher than bare copper wire. The mechanical properties of palladium-coated copper wire increase the risk of damaging bond pad structures if not bonded correctly. The addition of a thin palladium layer can increase electrical resistivity, which can be a concern for high frequencies and smaller diameter wire applications. Others promote palladium-coated copper wire with reports of improved biased highly accelerated stress test (HAST) results versus bare copper wire. As a consequence, debate over the choice between palladium-coated and bare copper wire is common. Some semiconductor suppliers and original equipment manufacturers (OEMs) incorrectly believe that palladium-coated copper wire is a panacea for all historical concerns with the use of bare copper wire. A study has been conducted to assess advantages and disadvantages of bare and palladium-coated copper. This paper shows bare copper wire can provide the same level of chlorine-induced corrosion resistance as palladium-coated copper wire if the copper-aluminum intermetallic bond is properly formed and the mold compound is correctly formulated. The basis for the belief that palladium-coated copper wire provides better resistance to chlorine-induced corrosion is explained and the electrical performance difference between palladium-coated and bare copper wires is discussed. High temperature (175°C) storage life testing, up to 7000 hours, was conducted with- both wire types to determine the end-of-life failure mechanism. Bond interface cracking, initiating in the bond periphery, was observed. The time dependence of copper-aluminum intermetallic phase transformation for both wire types will be presented. It is shown that choice criteria for each wire type can be defined by product field application requirements and not by perceived advantages. The reported work shows bare copper has equal performance to palladium-coated copper wire under Automotive Electronic Council (AEC) reliability grade 1 in specified package types when the bonding process, substrate / lead frame design and mold compound have been correctly optimized [1].