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The demand for high-density electronic applications is growing. This work develops a novel chip-on-flex (COF) package with sidewall-insulated Au-coated polyimide (PI) compliant-bumps. A double-layer anisotropic conductive adhesive (ACA) material that meets the assembly requirement is adopted for the ultra-fine pitch interconnects. A process for manufacturing 20- μm pitch compliant-bumps is proposed for ACA-bonded COF packages. The double-layer ACA consists of an ACA layer with a diameter of 2.8 μm conductive particles and an NCA layer as an interlayer to bind a silicon chip with a flexible substrate. The bonding accuracy for ultra-fine pitch is determined using X-rays. To evaluate the quality of bonding, the electrical insulation is tested and the contact resistance of the daisy chain with 606 input/output (I/O) around the periphery of the chip is measured. The double-layer ACA material is assembled at different bonding temperatures to study the effects of bonding temperature on the interface adhesion using differential scanning calorimetry (DSC) and a 90° peeling test. The reliability of the fabricated COF interconnects is also evaluated by performing an 85°C/85% relative humidity thermal humidity storage test (RH THST) for 1000 h and a -55°C ~ 125°C thermal cycling test (TCT) for 1000 cycles. The interfaces between the silicon chip and the substrate of the failed samples in the reliability tests are then observed using the cross-sectional scanning electron microscopy (SEM). The compliant-bump-bonded samples with the double-layer ACA provide their excellent electrical insulation performance even at a joint space of 5 μm whereas the Au-bump samples have a short-circuiting rate of more than 50%. Notably, the contact resistance also remains stable and varies by under 3% in both the RH THST for 1000 h and the TCT for 1000 cycles. The presented results show the reliable bonding quality and stable contact resistance- - of the COF package that is bonded with the compliant-bump structure using the double-layer ACA, indicating its great potential for use in ultra-fine pitch applications.