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Influence of soft sensor chip bonds on the formation of wire bond interconnections

The wedge bonding process on a sensor chip design has been simulated using three-dimensional FEM, across a range of die bond stiffness's, where the results and analysis thereof are discussed in this paper. Comparisons have been made with respect to the influence of the stiffness of the die bond, which has implications for both the sensitivity and accuracy of the sensors. Soft die bonds are preferable, so as to decouple the stiffness of the substrate and the flexibility of the sensor beams. However, an overly soft die bond connection could subsequently result in problems during the wire bonding process, because the die is sub-critically adherent, which can then lead to a partial absorption of the ultrasonic energy that is needed during the welding of the bond wire and pad metallization. The results presented within were obtained using the commercial FEM package LS-Dyna, owing to its validated capabilities of handling rapid deformations which are also highly plastic in behaviour. In order to study the influence of die adhesives on the resulting mechanical stresses in the bond interface a variety of Young's moduli were prescribed. Through further postprocessing of the results information regarding the contact forces was also extracted, for the purposes of assessing the induced ultrasonic energy, which is important for determining the contact formation. The simulation results show reductions of the friction force of up to 40 % which may omit the formation of a wire bond contact, where process conditions are critical. This analysis proves the strong influence of the die bond stiffness on the wire bond process. Both processes have to be optimized in order to create a reliable product with increased functionality.