Investigation of ultrasonic Pd coated Cu wire wedge bonding on different surface finish

In IC, wire bonding is the main technology for electrical connections between chip and leadframe or substrate. Gold wire bonding has the advantages of a fast bonding process, excellent electrical property, and stable chemical property. But, gold prices have risen significantly over the last few years. Many manufactures have been investigating ways to replace the conventional gold wire various new material. Copper wire bonding is an alternative interconnection technology. Compared with gold wire, Cu wire is better than gold with respect to electrical conductivity. The inherent stiffness of the copper wire also makes long wire with small diameters more resistant to wire sweep during molding. There are also some problems with Cu bonding process: (1) Copper easily oxidizes in air. The application of copper wire coated with palladium is a solution to prevent copper oxidation during the bonding process. (2) The higher hardness of wire generally requires higher ultrasonic power and bond force to bond on metal. It also lead to high risk of cratering for ball bonding and tearing for wedge bond. This paper reports a study on the influence of surface finish type, deposition thickness and bonding machine parameter for wedge bonding. In this study, 0.7mil Pd coated Cu wire was bonded on three kinds of surface finish, as electro-plating Nickel and Gold, electro-plating Nickel, palladium & gold, Electroless Nickel, Electroless Palladium and Immersion Gold (ENEPIG). The noncontact optical profiler, microhardness tester and scanning electron microscope (SEM) were used to examine surface finish characteristics. Bonder machine alarm rate, pull strength and failure mode after wire pull test were performed to confirm the quality for Pd coated Cu wire bonded on three kinds of surface finish. Then, wedge bonding mechanism of Pd coated Cu wire was discussed in this paper.