FEM wire bonding simulation for sensor chip applications

The paper describes a three-dimensional dynamic finite element simulation of the wedge bonding process on a sensor chip design. The stiffness of the die bond connection has a strong influence on the sensitivity and accuracy of sensors. Soft die bonds are preferred in order to decouple the substrate stiffness from the flexibility of the sensor beams. However, a very soft die bond connection may cause trouble during the subsequent wire bonding process because the die is not sufficiently fixed to the substrate which may result in a partial absorption of the ultrasonic energy that is required for the welding of bond wire and pad metallization. The FEM simulations presented in this paper were done by the commercial code LS-Dyna which is capable of high deformation speeds and high plastic deformations. The study investigates the effect of die adhesives with different Young´s moduli on the resulting stresses and strains in the bond interface. Based on the simulation results contact forces are evaluated, too, in order to assess the induced ultrasonic energy which is essential for the contact formation. The simulation results show big differences in the resulting force peaks of up to 20 %. The bond pad stresses even differ by up to 70 %. The simulations prove a remarkable influence of the die adhesive stiffness on the mechanical contact loads during the wire bond process.