A new approach to low-temperature bonding for fine pitch chip-on-flex technology

In this work, plasma treatment, using argon and hydrogen mixed gas, was studied as pretreatment of Au electrode surface for chip-on-flex (COF) interconnection. Ion bombardment and chemical reactions of Ar/H₂ plasma with Au electrode surface were used for both surface activation and removal of surface contaminants. Contact angle measurement was used to evaluate the cleaning effect of the plasma treatment. Relationships of plasma treatment, bonding temperature, bonding pressure and peeling tests were also studied. Optimization of the plasma treatment time was achieved according to surface roughness on electrodes measured by atomic force microscopy (AFM) and verified by peeling tests. The criteria of peeling strength were also made by the failure mode of the bonding interfaces. After optimizing the plasma treatment time and bonding parameters, reliable joints with lower temperature and appropriate pressure were obtained compared with traditional COF bonding technology. After that, underfill was dispensed in the gap of COF bonding structure. After thermal shock reliability test (-55°C/+125°C, in liquid environment, 500 cycles), completive COF assemblies were used in LCD panel lighting-up test for inspecting the bonding quality. Cross section view on bonding interfaces were also observed by scanning electron microscopy (SEM). According to the results, it is possible to get reliable bonding quality and stable contact resistance of fine pitch COF interconnection at low temperature (150°C) using surface activated method.