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 degrees 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 m average sizes Al2O3 spacers. It was difficult to achieve above 90% cure degree by less than 20 s at 200 degrees 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 degrees C bonding condition was selected. As large amounts of Al2O3 spacers were added into ACFs resin, the reliability of joints became lower. Direct contacts of two electrodes were obtained at 1.5 wt% Al2O3 spacers, however, no direct electrode contact was observed at 3 wt% Al2O3 spacers. As a summary, optimized bonding conditions at 200 degrees C for 10 s at 1-MPa pressure and solder ACFs with 3 wt% 10-mu m diameter Al2O3 spacers were suggested for nondirect contacts of electrodes interconnection and higher reliability of cationic epoxy-based Sn-58Bi solder ACFs joints for FOB applications.