Electrical modeling of anisotropically conductive adhesive interconnections for microwave applications

This paper gives results from electrical modeling of microwave interconnects using anisotropically and isotropically conductive adhesives. We study several models, on both micro- and macroscopic levels, based on physical considerations and phenomenological observations. The models are compared to measured data from two test structures, a flip-chip bonded Si test chip and a Cu foil used to bridge a microstrip line gap. The Cu foil is adhesively bonded at both ends to adjacent connecting lines. The test structures are implemented on various substrates, from rigid FR-4 and flex-board to a duroid HF substrate. SEM studies of adhesive interconnect microstructures provide a physical basis for electrical modeling and describe individual conducting particle behaviour. The models studied are an electrical model based on physical considerations for single particles and their distribution, an empirical electrical model describing the frequency dependence of transmission characteristics, a model based on 3D FEA simulation of the interconnect structure, and a statistical model taking into account the stochastic conducting particle distribution in the epoxy. Comparison between models and measurements is made to evaluate model strengths and weaknesses in order to establish design rules for adhesive microwave interconnects. Results from power testing, where 250 W, pulsed at 10% working factor, is transmitted through the Cu bridge interconnect, and climate testing, where test chips are subjected to 985 temperature cycles over a -55°C to +125°C range, are combined with the models to identify possible failure modes