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A new concept of anisotropic conductive film (ACF) called nanofiber/solder ACF combined with an ultrasonic bonding method can overcome limitations of fine pitch flex-on-flex (FOF) assembly using conventional ACF, such as short circuit issues, low current-handling capability, and poor reliability. To fabricate the nanofiber/solder ACF, SAC305 (96.5% Sn, 0.5% Cu, and 3% Ag) conductive solder balls are added to polyacrylonitrile polymer solution and ejected as nanofibers containing SAC305 conductive solder balls by an electrospinning method. This new concept of nanofiber/solder ACF successfully prevents short circuits between neighboring conducting electrodes by limiting the free movement of conducting balls and insulating individual conductive solder balls with coated nanofiber layers. Moreover, SAC305 conductive solder balls in nanofiber/solder ACF are completely melted and made metallurgical alloy contact between FOF electrodes during the ultrasonic bonding, resulting in 41% more electrical power being carried, and showing significant improvement in reliability performance compared with conventional nickel ball ACF.