Numerical analysis of the interfacial contact process in wire thermocompression bonding

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.