Study on bubble formation in rigid-flexible substrates bonding using anisotropic conductive films (ACFs) and their effects on the ACF joint reliability

Because of downsizing of electronic products and cost effectiveness, rigid substrate-flexible substrate (RS-FS) bonding technology using ACFs becomes more important as an alternative to socket type connectors and rigid/flex substrates. However, formation of process related bubbles, entrapped inside the ACF layer during bonding processes, is strongly influenced by process variables, such as a bonding pressure and a bonding temperature. These bubbles can reduce adhesion strength of ACFs joints, and induce moisture penetration path and entrapment location during reliability tests in humid environments. However, the causes of bubbles formation during the ACF bonding process and the effect of bubbles on ACFs joints reliability have not been fully understood. Bonding process variables, such as bonding temperature, bonding pressure and flexible substrate (FS) types, were changed in order to investigate their effects on bubbles formation. According to the results, the tendency of bubbles formation was closely related to these three factors. The bubble area increased as the bonding temperature increased. Moreover, same tendency was observed against the bonding pressure changes at fixed bonding temperature conditions. Two different FSs, which have different surface roughness and energies, were used and the bubbles formed only at the FS with larger roughness and lower surface energy. According to the results from surface energy measurement of FSs by using goniometry, the FS with higher surface energy is favorable for bubble free assembly because higher surface energy provides better wettability. Therefore, Ar and O2 plasma treatments were performed on the FS with lower surface energy to improve the wettability, and bubbles were significantly removed. Finally, two types of test vehicles (TVs), without (type 1) and with bubbles (type 2), were assembled to investigate the effects of bubbles on the ACFs joints reliability in humid environments, such as PCT (pressure cooker test). All type 2 TVs, with bubbles, were electrically failed after 72 hours of PCT because the process related bubbles acted as a moisture penetration path and entrapment sites. However, all type 1 TVs survived even after 120 hours of PCT