Reliability of ultra-thin chip-on-flex (UTCOF) with anisotropic conductive adhesive (ACA) joints

For the future advanced applications in consumer electronic products, the need of flexible interconnects increases rapidly. The reliability study of ultra-thin chip-on-flex (UTCOF) interconnects using anisotropic conductive adhesive (ACA) is thus reported herein. When the flexible device is bent, since the interface between ultra-thin chip (<50 mum in thickness) and substrate may create the major possibility for delamination, the flexibility of ultra-thin silicon chip is first evaluated by 3-point bending test. The ACA-P and ACA-F materials are assembled at different bonding temperatures to study the temperature effects of bonding on the interface adhesion for substrate-adhesive and adhesive-passivation layer by differential scanning calorimeter (DSC) measurement and peeling test. The contact resistance of daisy chain with 188 I/Os is measured to evaluate the bonding quality through the 80 mum pitch dummy test samples. The flexibility of the UTCOF interconnects bonded with ACA joints is then performed by the static and 4-point bending tests. The reliability of UTCOF interconnects with ACA is also evaluated by the 85 degC/85%RH thermal humidity storage test (THST) for 1,000 hours. To demonstrate the techniques developed, the interface between ultra-thin silicon chip and substrate is finally inspected through the cross-section SEM works. As expected, the average maximum allowable deflections of the UTCOF interconnects by using ACA-P and ACA-F materials increase 26% and 153% as compared with those obtained by COF ones with 670 mum chip thickness, respectively. Moreover, the contact resistance remains stable and varies under 10% with the bending curvature 30 mm for 1,000 hours. Based on the results achieved, the UTCOF with ACA joints presented in this work would be reliable for serving as the flexible interconnects by choosing an appropriate ACA material and optimum curing conditions.