The effect of cross-section geometry of bonding wire on wire sweep for semiconductor packages

Wire diameter reduction is an effective way to reduce cost, as gold content accounts for the majority of bonding wire costs. Replacing gold wire with copper wire is another way to reduce cost. The advantages of using Cu instead of Au wire are the lower price and the higher electrical conductivity. However, the oxidation problem of copper wire during the bonding and plastic encapsulation process needs to be overcome. In this study, we propose an alternative way to increase the sweep stiffness of a wire bond through the transformation of cross section of the bonding wire. We investigate the effect of cross section geometry of bonding wire on the moment of inertia of a wire bond. According to the authors´ previous studies, the sweep deflection of wire bond can be divided into the combined contributions of the bending moment and the twisting moment applied on the wire bond. The bending moment-induced sweep deflection per unit drag force is closely related to the moment of inertia of a wire bond. Similarly, the twisting moment-induced sweep deflection per unit drag force is suggested to be inversely proportional to the polar moment of inertia of a wire bond. By manipulating the cross section geometry of bonding wire, we can optimize the moment of inertia and the polar moment of inertia of a wire bond based on the design request from wire sweep resistance during the transfer molding process for encapsulating semiconductor devices. Then, we can obtain better performance of wire sweep without changing the measure of area of bonding wire. This methodology can be applied to both gold wire and copper wire without any doubts.