Behaviors of palladium in palladium coated copper wire bonding process

Because of its high thermal conductivity, great electrical property and low cost, copper wire is considered to replace the conventional gold wire and becomes widely used in IC assembly processes recent years. However, copper wire bonding also has its limitations. Copper oxidation, its high hardness and yield strength are two main disadvantages that manufacturers concern most. The application of copper wire coated with palladium is a solution to prevent copper oxidation during the bonding process. Nevertheless, Pd coated copper wire brings in new possible influences to bonding interface and its reliability. This paper gave a systematic study on behaviors of palladium in palladium coated copper wire during the bonding process. SEM and EDS were used to analyze Pd distributions in copper wire, FAB (free air ball), bonded ball and its interface. It was shown that Pd distribution changed with bonding process going on and the factors that might cause these changes were discussed.