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I.

INTRODUCTION

THE ultrasonic wire bonding (UWB) process is widely used in the electronics industry to make interconnections between integrated circuits (IC) and package metallization. Wire bonding was introduced in the late 1950s, and today, more than a trillion wire bonds are made annually. In the UWB process, a fine (25- to 100-mm diameter) wire is drawn under a tool (driven by a transducer), which is then lowered to apply a vertical force and ultrasonic vibrations for a short period of time (typically in the millisecond range) to create a bond to the substrate.[1] Two variations of this process are commonly used today: ball bonding and wedge bonding. Ball bonding uses gold wire and is carried out at slightly elevated temperatures (150 7C to 200 7C), whereas wedge bonding is done at room temperature with aluminum wire. Further discussions and complete schematic illustrations of these wire bonding processes can be found in Reference 2. Figure 1(a) shows wedge bond on a copper foil substrate; note that in the locations where the wire is bonded, the wire undergoes significant plastic deformation. A well-known feature of wedge bonding is the impression or ‘‘footprint’’ left on the substrate. This is commonly observed by etching the wire in aqueous sodium hydroxide so that it is completely removed. A footprint for a bond from Figure 1(a) is shown in Figure 1(b) and exhibits a torus-shaped region typical of these types of bonds. It is clear that the substrate undergoes some type of permanent deformation, which appears somehow different for the torus-shaped and central regions. The objective of this article is to understand the nature of the deformation in the NIKHIL MURDESHWAR, formerly Research Assistant, Mechanical Engineering Department, University of New Hampshire, is Staff Development Engineer, IBM, Endicott, NY 13760. JAMES E. KRZANOWSKI, Associate Professor, is with the Mechanical Engineering Department, University of New Hampshire, Durham, NH 03824. Manuscript submitted March 20, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A

bond footprint and how this deformation relates to the overall mechanism of wire bonding. Despite the wide acceptance of UWB, the basic mechanisms of bond formation and the effects of bonding on wire and substrate microstructures are not well understood. In a recent article,[3] we demonstrated that ultrasonic bonding of aluminum wire to aluminum substrates can significantly modify the materials bonded, to the extent that recrystallization can occur. In the present study, this work is extended to bonding of aluminum wires to substrates of Cu, Ni, and various metallic alloys. In Section II, we review previous studies of microstructural effects in wire bonding, including our own recent studies. We then present the results of our electron microscopy investigation of deformation mechanisms in wire-bonded metal substrates. II.

BACKGROUND

A. Early Studies of UWB Early studies on UWB were conducted to investigate bonding mechanisms, the temperature rise due to bonding, bond interface stru