Investigation of ultrasonic palladium coated copper wire wedge bonding on different surface finish

Abstract

In semiconductor packaging, 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. It has been widely used in various electronic packages, such as chip scale package (CSP) and ball grid array (BGA). 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 wire material, surface finish hardness and bonding machine parameter. In this study, 0.7mil Pd coated Cu wire was bonded on two kinds of surface finish, as electro-plating Nickel and Gold, and Electroless Nickel, Electroless Palladium and Immersion Gold (ENEPIG). Its purity is 4N. The thickness of Pd coating was less than 0.2um. The surface finish characteristics were examined using a scanning electron microscope (SEM). The noncontact optical profiler was used to measure surface finish roughness. Hardness was measured, using microhardness test. Thermosonic Pd coated Cu wire wedge bonding was preformed on a wedge bonder equipped with a kit to forming gas. The wire bonding process window for each surface finish was established using various combination of bond force and power. Bonder machine alarm rate, wire pull test and wedge bonding appearance were performed to measure the quality for Pd coated Cu wire bonded on three kinds of surface finish. Cross section sample was prepared Focused Ion Beam (FIB). Then, it was observed using scanning electron microscope to discuss wedge bonding mechanism.