Development of a Novel Compliant-Bump Structure for ACA-Bonded Chip-on-Flex (COF) Interconnects With Ultra-Fine Pitch

• Published in: IEEE transactions on components, packaging, and manufacturing technology (2011) packaging, and manufacturing technology (2011), 2011-01, Vol.1 (1), p.33-42
• Main Authors: LU, Su-Tsai, LIN, Yu-Min, CHUANG, Chun-Chih, CHEN, Tai-Hong, CHEN, Wen-Hwa
• Format: Article
• Language: English
• Subjects: Anisotropic conductive adhesive (ACA); Applied sciences; Bonding; chip-on-flex (COF); compliant-bump; Contact resistance; Design. Technologies. Operation analysis. Testing; Electronics; Exact sciences and technology; Insulation; Integrated circuits; Joints; reliability; Scanning electron microscopy; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Silicon; Studies; Substrates; ultra-fine pitch
• ISSN: 2156-3950; 2156-3985
• DOI: 10.1109/TCPMT.2010.2101431

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.