Fine pitch flex-on-flex (FOF) assembly using nanofiber solder anisotropic conductive films (ACFs) and ultrasonic bonding method

Nanofiber/solder ACFs are investigated in terms of ball movement and electrical properties depending on their nanofiber polymer types: polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF). For both nanofiber solutions, SAC305 (96.5% Sn, 3% Ag, 0.5% Cu with 217°C melting point) conductive solder balls were incorporated into nanofibers which were fabricated by electro-spinning method. Then, non-conductive films (NCFs) were laminated on top and bottom of the electrospun nanofibers to fabricate nanofiber/solder ACFs. For the bonding method, ultrasonic (U/S) bonding method was conducted to remove solder oxide which allowed fluxless soldering and reduced bonding time. PAN and PVDF nanofiber/solder ACFs successfully provided limitation of solder ball movement resulting in excellent ball capture rate on electrodes.