The advanced electronic packaging trends are changing to be more light weighted and miniaturized. Demands for flexible packaging are rapidly growing due to the rapid growth of wearable electronic industries, and advanced flexible interconnection methods are required. In order to satisfy these needs, fine pitch flex-on-flex (FOF) assembly has been introduced. FOF assembly can provide smaller package size, lower contact resistance, higher power handling capability, and flexibility. Anisotropic conductive films (ACFs) are interconnection materials, which have been widely used for the FOF assembly due to their simple and low-temperature processes. Previously, nanofiber ACFs were introduced by our research group, which showed remarkable capability of enhancing the conductive ball capture rate and preventing short circuit in the fine pitch assembly by suppressing the movement of conductive particles during the bonding process. Conventionally, an ultrasonic bonding method was used for a solder ACF bonding, because native solder oxide layer around solder balls can be physically broken by applying vertical ultrasonic vibrations, resulting in excellent solder metallurgical joints between electrodes. However, there are potential misalignment and damage issues generated by vertical ultrasonic vibrations. Therefore, the alternative bonding method is needed that can remove solder oxide layer to obtain excellent solder wetting without misalignment and damage issues by the ultrasonic bonding method. In this paper, thermocompression (TC) bonding and nanofiber/solder ACFs with addition of flux activators were used to chemically eliminate solder oxide. For the formation of stable intermetallic compounds, the contents of flux activators in nanofiber/solder ACFs were optimized. To investigate the flux effect during the TC bonding process, cross-sectional transmission electron microscopy was performed to analyze whether solder oxide layer is certainly removed by the flux. With the TC bonding and nanofiber/solder ACFs with addition of flux activators, stable solder wetting and excellent alignment were demonstrated for the fine pitch FOF assembly.