Bubbles formation in rigid-flexible substrates bonding using anisotropic conductive films (ACFs) and their effects on ACFs joints reliability

Rigid-flexible (R-F) substrates bonding technology using ACFs becomes more important as an alternative to connectors and rigid/flex substrates. ACF interconnection is a well-established technology but the quality of ACF interconnection is very dependent on bonding process variables. Especially, formation of process bubbles, which are entrapped inside the ACF layer during bonding processes, is strongly influenced by bonding process variables. These bubbles can reduce mechanical property, such as adhesion strength in ACFs joints, and induce moisture penetration and entrapment location during reliability tests in humid environments. Therefore, trapped bubbles also can reduce the reliability of ACFs interconnection joints. Bonding process variables, such as bonding temperature, bonding pressure and flexible substrate types, are controlled in order to investigate their effects on bubbles formation. According to the results, bubbles formation is closely related to these three factors. The ratio of bubble area increased as the bonding temperature increased. Moreover, same tendency was appeared against the bonding pressure change at fixed bonding temperature conditions. Two different flexible substrates, which have different surface roughness and energy, were used and bubbles formed only at flexible substrates with larger roughness and lower surface energy. Furthermore, a reliability test in humid environment, such as a PCT (pressure cooker test), has been performed to investigate the effect of process bubbles on ACFs interconnection joints reliability. According to the results of PCT, process bubbles acted as important moisture penetration and entrapment sites. Therefore, the daisy resistance of the samples which have process bubbles increased abruptly and finally, all bubble formed samples were failed after PCT