Abstract
Abstract For future advanced applications in consumer electronic products, the need of high-density and flexible interconnects increases rapidly. In this study, the reliability and flexibility of ultra-thin chip-on-flex (UTCOF) interconnects using anisotropic conductive adhesive (ACA) were investigated. A rigorous three-dimensional (3-D) numerical analysis model was performed using the ANSYS program. Moreover, experimental and parametric analyses of UTCOF interconnects were evaluated to establish the design rule of ACA-bonded UTCOF interconnects. Due to shrinkage of ACA resin and elastic-plastic characteristic of conductive particles, the effect of contact behavior in ACA joints on the contact resistance of the fabricated UTCOF interconnects after thermo-compression process was also estimated using a three-dimensional finite element model. The contact resistances obtained from the analysis results were in good agreement with experimental results, and can be used to predict the reliability performance in ACA joints. A process for manufacturing 20 □m-pitch compliant-bumps was proposed for COF structure using 2-layer ACA material. The reliability of the fabricated COF interconnects was evaluated by performing an 85°C/85％ relative humidity temperature-humidity storage test (RH THST) for 1,000 hours and a -55°C~125°C thermal cycling test (TCT) for 1,000 cycles. For UTCOF interconnects, the effect of chip thickness (25 □m, 35 □m, 50 □m) on the fracture strength of the ultra-thin silicon chip was estimated. Both the ACA-P and ACA-F materials were assembled at different bonding temperatures to study the temperature effects on the curing percentage and adhesion by differential scanning calorimeter (DSC) measurement and peeling test, respectively. Meanwhile, the relationship between curing conditions and electrical contact resistance was examined. Moreover, the bonding pressure effects on the electrical performance and deformation degree of conductive particles within the ACA resin were also explored. The contact resistance of daisy chain was measured to examine the bonding quality through the 80 □m pitch dummy test samples using micro Au (gold) bump (GB) and compliant bump (CB). The reliability of the fabricated UTCOF interconnects bonded with selected ACA joints was evaluated by long-term 85°C/85％ RH THST for 7,000 hours and their flexibility was performed by static bending under various testing environments and 4-point bending tests. The interfaces between ultra-thin silicon chip and substrate for failed samples from THST and 4-point bending tests were then inspected through the cross-section scanning electron microscopy (SEM) works. Finite element analysis (FEA) was also conducted to interpret the failure mechanism of the UTCOF interconnects under 4-point bending test. Based on the results achieved, the UTCOF with ACA joints presented in this work would be reliable for serving as flexible interconnects for consumer electronic products. Also, the manufacturing technology for high-density and flexible UTCOF interconnects with ACA joints was thus established.