Dynamic three dimensional simulation of wire bonding processes on soft polymeric substrates for high frequency applications

The paper describes a three-dimensional, dynamic finite element simulation of a wire bonding process on a soft polymeric substrate (PTFE). Wire bonding interconnects on PTFE substrates can be used to improve the circuit performance in multi-GHz applications. Opposite to on chip inductors, which are limited to frequencies up to 5 GHz, the use of external wire bonds as inductors enables the realization of high frequency circuits up to 100 GHz. However such GHz-system-on-package applications can be realized on high-frequency dielectrics like PTFE only. Wire bonding on those substrates is not easy, because they are very soft. Thus, these substrates can absorb a lot of ultrasonic energy that is needed for the welding of wire and metallization. Systematic problems of sensitive process steps can be investigated by numerical simulations, e.g. by the finite element method (FEM). The FEM simulations presented in this paper are carried out in order to analyze the influence of the soft PTFE substrates on the bonding process. These simulations apply the commercial code LS-Dyna in order to account for the high deformation speed and the high plastic deformations of the bond wire. The formations of the wedge contacts on the substrate pads are investigated by complete 3D meshes made of solid elements. The bonding process is divided into two steps. In the first step the capillary is moved in a vertical direction in order to press the bond wire on the pad. In the second step the capillary starts the ultrasonic vibration in a horizontal direction. This ultrasonic vibration has a frequency of 100 kHz. In order to analyze the different process conditions on different substrate materials and substrate geometries, different parameters were extracted from the FEM simulation results. Such parameters include substrate deformations, contact force and plastic deformations of wire and metallization. Simulation results show that there are high penetrations of the soft PTFE substrate during wedge- formation. These penetrations are 2 magnitudes higher than for a silicon substrate. Furthermore, the contact forces between substrate pad and bond wire hardly differ for different geometries on the PTFE substrate. These contact forces are lower than for a bonding process on a silicon substrate.