Deformation analysis of wire bonding on soft polymers

Abstract

The paper describes a three-dimensional, dynamic finite element simulation of a wire bonding process on a soft polymeric substrate (PTFE). Wire bonding between the chip and the substrate can be used to improve the interface performance in multi-GHz applications and provides high quality inductors for inductive peaking. For such high speed applications special high frequency substrates are required, such as PTFE. Wire bonding on those substrates is not easy, because they are very soft. Thus, such substrates can absorb a lot of the ultrasonic energy that is needed for the welding of wire and metallization. The simulations presented here compare the mechanical stresses generated on two pad geometries of this high frequency setup. These geometries represent a ground pad and signal pad structure. The results of the explicit dynamic simulations show little differences in the mechanical stresses generated during the initial compression of the bond wire. The contact force on the bond pads, the substrate penetration and the final bond wire diameter show little differences. But in all cases the ground pad structure causes slightly higher mechanical loads. Some noticeable differences can be found in case of the mechanical stresses caused in the bond pads. However, if serious problems appear during the wedge bonding on PTFE substrates, these issues cannot be solved by the little reinforcement of the ground pads by the copper clamp.