Variable-Length Link-Spring Model of Wire-Bonding Looping Process

Looping is a key technology in modern thermosonic wire bonding. To provide insight into the loop formation mechanism, a variable-length link-spring (VLLS) model is proposed. In this model, the wire segments and moment balance equations at the opening end of the wire are dynamically added during the capillary upward movement stage, to simulate the wire feeding and kink formation process. The complete looping process is analyzed by solving nonlinear equations iteratively and using Newton's method. Using this model, the wire profile evolution process and kink number, position, and deformation during looping are obtained and verified by experimental results. The effects of the upward loop trace parameters (reverse motion and kink height parameters) on the final loop profile are studied. The results show that the VLLS model, which considers the upward loop trace, is more suitable for looping process analysis.