Modeling and Experimental Study of the Kink Formation Process in Wire Bonding

Looping is a key technology for the modern wire bonder. A kink is a critical structure in a wire loop. In order to understand the kink formation mechanism, a 2-D dynamic finite element model is developed using ANSYS/LS-DYNA, in which the air tension force, friction between capillary and wire, and real capillary trace are considered. The simulated kink formation process was verified by an experiment. With this model, the strain distribution on a gold wire was calculated, and the effects of wire material properties and capillary trace parameters on the kink number, position, and loop profiles were studied. Simulation results show that a minute average plastic strain of 0.14 is needed to form a distinct kink in a wire. Similarly, an elastic core with an average plastic strain of less than 0.08 at the center of a kink provides stiffness and sag/sway resistance for loops. A kink is a wire segment with plastic deformation outside and an elastic core inside, and the number of kinks and their positions are mainly affected by the capillary trace. In contrast, wire material properties only slightly influence the kink properties. This paper may provide helpful insights into loop design for modern microelectronics packages.