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The purpose of the present study is to understand the interfacial deformation between pad and wire and the effect of the pad thickness, the pad hardness, and the tool shape on the interfacial deformation. The relationship between the bondability and the interfacial deformation (surface exposure to produce the clean surface) is summarized, because the bondability is largely affected by the interfacial deformation. A simple model of wire bonding is proposed for the numerical analysis. The model is based on the finite element method for rate sensitive materials and applicable to very large deformation processes. The numerical simulation made it possible to visualize the interfacial contacting process which occurs for several milli-seconds. It was suggested that the periphery bond is produced easily as the pad thickness decreases and the pad hardness increases. On the other hand, it was found that the thick pad and the groove tool can help the center bond formation. These results is explained by the distributions of the interfacial extension and the equivalent stress on the bonding interface. Also, the damage to the substrate (Si chip) is discussed, based on the numerical results.

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Wire deformation processes during thermocompression bonding without ultrasonic vibration are simulated by a numerical model which is based on the finite-element (FE) technique. The growth process of interfacial contact between wire surface and lead frame is also analyzed. If the contact interface is fixed, then the lateral wire surface expands simultaneously with folding to the lead frame, producing the perimeter bond. On the other hand, if the contact interface can slide, the center of contact area largely expands and the lateral wire surface does not extend. It follows that the interfacial contact area tends to be fixed, but does not slide when the perimeter bond is produced, even if a strong metallic bond is not achieved at the center. A wire reduction greater than 0.5 is required for ensuring the strong perimeter bond formation. This is supported by the experimental results. The interfacial contact area is governed mainly by the wire reduction. If the reduction is kept constant, then the tool with a groove increases the contact area somewhat larger than the flat tool, although the groove tool decreases the rate of wire deformation. We further discuss the size limit of wire bonding, taking into account the perimeter bonding mechanism.1070-9886/96$05,00 @ 1996 IEEE