A Study on the Bonding Conditions and Nonconductive Filler Contents on Cationic Epoxy-Based Sn–58Bi Solder ACFs Joints for Reliable Flex-on-Board Applications

Although cationic epoxy is reported as the best adhesive candidate for Sn-58Bi solder anisotropic conductive films (ACFs) reliability, ACFs bonding conditions have not been optimized yet. In this paper, ACFs bonding parameters, such as pressure and time at 200 °C, have been optimized to produce flex-on-board (FOB) applications with a higher reliability, using the cationic epoxy-based Sn-58Bi solder ACFs joints with 10-$\mu \text{m}$ average sizes Al2O3 spacers. It was difficult to achieve above 90% cure degree by less than 20 s at 200 °C, and Sn elements started to diffuse from micrometer-sized Sn-58Bi joints after 10 s. Both 80% and 90% cure degree can provide 520-kgf/cm peel adhesion strength. To obtain a larger amount of Sn phase remaining at solder joints for lower contact resistance and higher reliability and adhesion strength, 10 s at 200 °C bonding condition was selected. As large amounts of Al ₂ O ₃ spacers were added into ACFs resin, the reliability of joints became lower. Direct contacts of two electrodes were obtained at 1.5 wt% Al ₂ O ₃ spacers, however, no direct electrode contact was observed at 3 wt% Al ₂ O ₃ spacers. As a summary, optimized bonding conditions at 200 °C for 10 s at 1-MPa pressure and solder ACFs with 3 wt% 10-μm diameter Al ₂ O ₃ spacers were suggested for nondirect contacts of electrodes interconnection and higher reliability of cationic epoxy-based Sn-58Bi solder ACFs joints for FOB applications.