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Anisotropic conductive films (ACFs) composed of an adhesive polymer resin and fine conductive particles such as metal-coated polymer conductive particles are interconnect materials for fine-pitch flex-on-flex (FOF) assembly technologies due to their fine-pitch handling capability, low-temperature bonding, and stable electrical properties due to the compliance of polymer conductive particles. For the ultrafine pitch FOF assembly, the polymer conductive particle content becomes more important due to electrical storage problem between electrodes. To design better quality ACFs materials for 50- \mu \textm FOF assemblies, the effects of polymer conductive particle contents on the electrical stability and reliability of FOF ACFs joints were investigated. It was found that the bonding temperature should be less than the deformation temperature of polymer conductive particles, and proper conductive particles content should be applied to obtain the stable joint contact resistance and ACF joint morphology. When the conductive particle contents increased more than 4wt%, the gap width between the top and bottom electrodes increased and the deformation of conductive particles became less, resulting in higher contact resistance. Therefore, 4wt% of conductive particle ACFs bonded at 175 °C, 2 MPa for 10 s showed the lowest contact resistance of 36.2 \textm\Omega and excellent thermal cycle (T/C) reliability up to 1000 cycles. As a result, the optimum polymer conductive particles contents and the bonding conditions such as temperature, pressure, and times were achieved to obtain the best performance and T/C reliability of 50- \mu \textm pitch FOF assembly using ACFs. [ABSTRACT FROM AUTHOR]