Study of Complex Looping With Five Kinks in Thermosonic Wire Bonding by Using Variable-Length Link-Spring Model

Complex looping with five kinks has been commonly used in the stacked die 3-D packaging and simulation models were desired to help control the loop profile in modern thermosonic wire bonding. The traditional finite-element model was time consuming and cost intensive, and the simple link-spring analysis model cannot provide precision simulation of the complex looping as it ignores the stage when the capillary is moving upward from the first bond to the highest point. In this paper, a variable-length link-spring model is proposed to deal with the above problems. In this model, the wire segments and moment equilibrium equations at the opening end of the wire are dynamically added during the capillary upward movement stage, to simulate the wire feeding and loop formation process. The experiment was carried and the complete wire loop profile in the model agree well with the experimental results. Based on this verified model, the effects of various parameters on loop profiles with five kinks were studied. It seems that the first kink height has little effect on the final loop profiles, while the second kink height can change the loop profiles and sweep resistance obviously.