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Flexible interconnects are needed that meet assembly requirements for future applications in flexible consumer electronics products. This work investigates the electrical contact mechanism of ultrathin chip-on-flex (UTCOF) package using anisotropic conductive adhesive (ACA), which is highly flexible. In this paper, a 3-D nonlinear finite element (FE) model, which integrates analytical models of ACA joints and the thermal-mechanical behaviors of the UTCOF, is presented. The model is then applied to simulate the electrical contact mechanism for various ACA joints after thermocompression. Moreover, a “death-birth” meshing scheme is utilized to determine the effect of ACA resin temperature on contact resistance of the ACA joints. Multiple particle models are also generated using the ANSYS program. To validate the suitability of the proposed FE analytical model, contact resistance is measured to determine the bonding quality for 80-μm -pitch UTCOF test samples. The interfaces between the silicon chip and substrate for samples bonded under different bonding pressures are observed using cross-sectional scanning electron microscopy. The contact resistances obtained from the 3-D FE models were in good agreement with experimental results and can be used to predict the effects of thermocompression loading, pressure unloading, and cooling to room temperature on electrical contact mechanism in ACA joints after thermocompression. Generally, the gold bump and the compliant bump joined with multiple conductive particles reached stable contact resistance by capturing five to nine conductive particles. Contact resistance of an ACA-bonded UTCOF can be estimated using a proposed empirical model equation, particularly under loading conditions of L = 40%-60%. Overall, this work improves the design of flip chips for flexible electronics and the understanding of the electrical contact mechanism for ACA joints.