On the thermo-mechanical modelling of a ball bonding process with ultrasonic softening

For an assessment of the stresses occurring during ball bonding of high-voltage CMOS chips in a structure comprising a thin and a thick silicon dioxide layer below the bonding pad, a dynamic model of the process was set up and the materials parameters were calibrated. For a realistic result of the deformation of the bonding ball during the ultrasonic stage, up to 60 ultrasonic cycles were simulated. To reproduce the final height of the bonding ball, dynamically increased friction between the ball and the bonding pad as well as ultrasonic softening of the metals within the model had to be taken into account. For a more sensitive prediction of failure, the conventional failure criterion based on the ultimate tensile strength of brittle materials was complemented by an additional criterion suggested by Christensen which takes the combined effects of perpendicular tensile and compressive principle stresses into account. This yielded a prediction of earlier failure for the thinner oxide layer while no failure was predicted for the thick isolation oxide layer.