Although cationic epoxy was optimized for low-melting SnBi58 solder ACF joints with the lowest coefficient of thermal expansion (CTE) in terms of reliability, cationic epoxy also showed a faster curing property than any other types of adhesives. In fact, solder joint shapes at 250 degrees C bonding are very different from those shapes at 200 degrees C bonding. In this paper, four adhesive film types were investigated in terms of Sn-3Ag-0.5Cu solder ACF joint shapes on the electrical performances and reliability in a pressure cooker test (PCT). Thermal stability of adhesive films was tested to be first. Resin curing speeds were measured in a 250 degrees C isothermal mode differential scanning calorimetry, resin viscosities were checked by a parallel-plate rheometer, and adhesive thermomechanical properties, such as modulus and CTE, were characterized. Then, four different types of ACF resins containing the same weight percentages of Sn-3Ag-0.5Cu solders were assembled by the same thermocompression bonding parameter (250 degrees C 10s 2MPa on bump) on a 500-mu m-pitch flex-on-board (FOB) application, and different solder joint morphologies were verified. Various bonded solder ACFs joints were compared in terms of solder wetting areas, electrical performances by a four-point-probe method, and the reliability of PCT (121 degrees C 100% humidity 2atm) for 120 h. This paper aims at optimizing the best adhesive film candidate for SAC305 solder ACF joints of FOB application.