Dynamic Strain and Chip Damage during Ultrasonic Flip Chip Bonding

The distribution of strain under a bump generated by ultrasonic flip chip bonding was dynamically measured using two-directional strain gauges. The strain gauges were designed to be sensitive in both the horizontal and vertical directions. Dynamic strains generated at the surfaces of the substrate and the chip were measured. The distribution of strain during Au bump formation was also measured and compared with that during flip chip bonding. Strain variations with time and position have been successfully measured during the bonding process composed of pre-press, ultrasonic and post-press steps. The strain field during ultrasonic application of flip chip bonding is found to be divided into two regions, while the strain field during Au bump formation is divided into four regions. Generation of chip damage such as crack and pad delamination, which was sometimes observed in ultrasonic flip chip bonding, is explained in term of the strain field in the bump area. It is shown that a high pressing load is effective to avoid damage of the chip surface.