The Mechanics of Gold Beam Leads During Thermocompression Bonding

An analysis of the stress and deformation in gold beam leads during thermocompression bonding is presented. The analysis isolates two particular bonding processes for study. One has the beams on opposite sides of the chip bonded simultaneously and the other sequentially. Since strains are encountered that are two orders of magnitude greater than the elastic limit, the beam leads are modelled as a work-hardening rigid-plastic material. The independent variables are yield strength, plastic modulus, beam dimensions, chip length, and chip rise (or, bugging height). The dependent variables include strain, beam-centerline geometry, and the force and moment transmitted to the chip. A parametric variation is performed, which shows that increases in yield stress will significantly increase the maximum transmitted force and moment. Increasing beam thickness significantly increases the transmitted force and moment as well as the maximum compressive and tensile strains. Increasing the beam length or decreasing the chip rise will significantly decrease the maximum strains. The results are of interest in the analysis of device damage which occurs during the bonding process. Furthermore, the solution for the beam configuration will be useful in elastic and plastic analyses of power and thermal cycling problems involving goldbeam-lead devices.