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In this paper, a thermosonic flip-chip bonding process using lateral ultrasonic vibration is proposed. To enhance the reliability of the specimen after the lateral thermosonic process, a sliding mechanism is adopted with investigation of equivalent stiffness of the anisotropic conductive film (ACF) joint. By a tensile test, it is shown that the equivalent stiffness of the ACF joint gradually increases as curing proceeds. Based on these results, the sliding point where the vibration amplitude of the chip specimen begins to decrease can be adjusted by the applied pressure. Thanks to the sliding mechanism, forced excitation to the sufficiently cured chip specimen can be naturally avoided. In addition, the robustness of the degree of cure against the bonding time variation can be improved in spite of the short bonding time. To demonstrate the feasibility of the proposed sliding mechanism in practice, experiments are conducted with a commercialized driver chip assembly of a liquid crystal display with an ACF.