In this study, solder ball incorporated polyvinylidenefluoride (PVDF) nanofiber was added into the ACF system to overcome short circuit issues of fine pitch flex-on-flex (FOF) assembly. Also, in order to improve the thermal mismatch of the flexible substrate which can lead to electrode misalignment during the bonding process, low melting temperature Sn58Bi solder balls were used with vertical ultrasonic (U/S) bonding method. When performing FOF assembly using PVDF nanofiber/Sn58Bi solder ACF and vertical ultrasonic bonding, PVDF nanofiber/Sn58Bi solder ACFs showed 34% higher solder capture rate on an electrode compared to conventional Ni ACFs and conventional Sn58Bi solder ACFs. Additionally, PVDF nanofiber/Sn58Bisolder ACFs showed 100% insulation between neighboring electrodes where conventional Ni ACFs and conventional Sn58Bi solder ACFs showed 75% and 87.5% insulation. Other electrical properties such as contact resistance and current handling capability as well as reliability test of PVDF nanofiber/Sn58Bi solder ACFs showed improved results compared to those of conventional Ni ACFs, which proves the formation of stable solder joint of PVDF nanofiber/Sn58Bi solder ACFs.
Low Temperature Flex-on-Flex Assembly Using Polyvinylidene Fluoride Nanofiber Incorporated Sn58Bi Solder Anisotropic Conductive Films and Vertical Ultrasonic Bonding