A novel compliant-bump structure for ACA-bonded chip-on-flex (COF) interconnects with ultra-fine pitch

As the growing demand for high-density electronic applications, a novel COF package with sidewall-insulated Au-coated polyimide (PI) compliant-bumps using a double- layer ACA that can meet the assembly requirement is thus developed for ultra-fine pitch interconnects in this work. The reliability of the ACA-bonded COF package with 20 mum pitch using both Au-bumps and compliant-bumps is examined, respectively. The double-layer ACA consists of an ACA layer with 2.8 mum conductive particles in diameter and a NCA layer selected as an interlayer to bind the silicon chip and the flexible substrate together. Bonding accuracy for ultra-fine pitch is inspected through X-ray works after bonding. To evaluate the bonding quality, both the electrical insulation test and the contact resistance measurement of daisy chain with 606 I/Os around the peripheral of chip are performed. Three types of single-layer ACA with 3 mum, 3.5 mum and 5 mum conductive particles in diameter and a single-layer NCA are also adopted to compare their electrical insulation resistance with those by the double-layer ACA, respectively. The compliant-bump bonded samples using the double-layer ACA show their excellent electrical insulation performance even at 5 mum joint space whereas the Au-bump bonded ones present more than 50% short-circuiting rate no matter what ACA used. It is worthwhile to mention that the compliant- bump assembled with 20 mum pitch COF also passes its reliability evaluation by both the 85degC/85% RH thermal humidity storage test (THST) for 1,000 hours and -55degC ~125degC thermal cycling test (TCT) for 1,000 cycles. The interfaces between silicon chip and substrate for failed samples from the reliability tests are then inspected through the cross-section SEM images. From the results presented, reliable bonding quality and stable contact resistance of the COF package bonded with the compliant-bump structure using the double-layer ACA demonstrate its high potentiality for ultra-fine pitch- applications.