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As the use of anisotropic conductive films (ACFs) in flex-on-board bonding has increased recently, the need for high-speed bonding for higher productivity has developed. For high-speed bonding, a fast-curable acrylic resin has been used instead of an epoxy resin, and ultrasonic (US) bonding is introduced to allow for short bonding time at high bonding temperatures. However, it is possible for fast curable acrylic ACFs at 250°C to be fully cured before the resins flow out sufficiently between the electrodes. In such cases, the ACF joints could show poor electrical characteristics due to there being no conductive particle capture. Therefore, in this paper, in order to understand important factors for high-speed bonding, the effects of the bonding parameters, substrate geometry, and ACF material property on the ACF joint characteristics, such as resin flow and joint resistance, are investigated using acrylic-based ACFs and US bonding. The fast-curable acrylic resin is fully cured at a bonding temperature of 250°C within 1 s. However, the ACF joints bonded at 250°C show joint gaps larger than the diameter of conductive particles, i.e., 8 μm, due to insufficient resin flow and open failure due to no particle capture. As the bonding pressure increases from 2 to 4 MPa, the ACF joint gap at 250°C is smaller than the conductive particle size, and the joint resistance reaches about 20 mΩ within 7 s when using 3-mm-long electrodes and a high-viscosity resin. However, in order to use a shorter bonding time and the common bonding pressure of 3 MPa, the effects of substrate geometry and resin property on ACF joint characteristics are investigated. As the electrode length of the substrates are decreased from 3 to 1 mm, the ACF joints show stable ACF joint gaps and joint resistance at 3 MPa despite the bonding temperature of 250°C applied within 1 s. In terms of the ACF resin properties, when a resin with low minimum viscosit- is used, the ACF joint gaps are smaller than the diameter of the conductive particles. Furthermore, the joint resistance of low-minimum-viscosity ACF is more stable than that of the ACF joints bonded with the high-minimum-viscosity resin at 250°C within 1 s, even though a 3-mm-long electrode is used. These results indicate that the substrate geometry of the printed circuit board and the ACF resin viscosity are significant factors for high-speed bonding to obtain stable ACF joints at a high bonding temperature of 250°C within 1 s.