Predicted Stresses in Wire Bonds of Plastic Packages During Transfer Molding

An analytical stress model is developed for the evaluation of flow induced stresses in wire bonds of plastic packages during molding. We limit our analysis to the stresses acting in the plane of a wire bond. These stresses can possibly result in liftoff of the ball bond from the bonding pad of the integrated circuit. The main purpose of the analysis is to evaluate the effect of the wire bond configuration. It is shown that the stresses in wire bonds are proportional to the square of the ratio of the wire-bond span to the diameter of the wire. This explains the difficulty in molding assemblies with long wire bond spans. We also showed that wire configurations, characterized by nonzero slope angles at the ends, result in lower stresses than conventional wire shapes, where the wedge bond to the electric lead forms a zero angle. The obtained results are useful when designing plastic package assemblies and/or choosing the appropriate wire bond loop height and span.