Nanofiber Anisotropic Conductive Films (ACF) for Ultra-Fine- Pitch Chip-on-Glass (COG) Interconnections SANG-HOON LEE, 1 TAE-WAN KIM, 1 KYUNG-LIM SUK, 1 and KYUNG-WOOK PAIK 1,2 1.—Department of Materials Science and Engineering, KAIST, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea. 2.—e-mail: kwpaik@kaist.ac.kr Nanofiber anisotropic conductive films (ACF) were invented, by adapting na- nofiber technology to ACF materials, to overcome the limitations of ultra-fine- pitch interconnection packaging, i.e. shorts and open circuits as a result of the narrow space between bumps and electrodes. For nanofiber ACF, poly(vinyli- dene fluoride) (PVDF) and poly(butylene succinate) (PBS) polymers were used as nanofiber polymer materials. For PVDF and PBS nanofiber ACF, conductive particles of diameter 3.5 lm were incorporated into nanofibers by electrospin- ning. In ultra-fine-pitch chip-on-glass assembly, insulation was significantly improved by using nanofiber ACF, because nanofibers inside the ACF sup- pressed the mobility of conductive particles, preventing them from flowing out during the bonding process. Capture of conductive particles was increased from 31% (conventional ACF) to 65%, and stable electrical properties and reliability were achieved by use of nanofiber ACF. Key words: Anisotropic conductive films (ACF), ultra-fine-pitch, chip-on- glass (COG) interconnection, nanofiber, electrospinning INTRODUCTION The demand for smart electronic devices, for example telephones and tablet personal computers, has risen dramatically in recent decades. The dis- play industry requires more demanding technology, for example ultra-high-definition, to fulfil the needs of customers who require high performance, multi- functionalization, and miniaturization. As a conse- quence, more input/output pins are placed within the same space and the space between bumps has decreased, resulting in fine-pitch interconnection. Fine-pitch packaging technology has therefore be- come very important in assembly processes. 1 In flip-chip packaging, anisotropic conductive films (ACF), well known interconnecting adhesives which consist of thermosetting resin and conductive particles in a film format, have been widely used to provide interconnections and attachment between bumps and electrodes. 2 However in ultra-fine-pitch interconnections, the pitch and the space between bumps and electrodes have become extremely small, resulting in such interconnection problems as short circuits and open circuits, which did not occur with conventional ACF bonding. As a result, a novel ACF concept, called ‘‘nanofiber ACF’’, was introduced to solve these issues. 3–5 It was proposed that nanofiber ACF would be a promising interconnection material for ultra-fine-pitch assembly, because they could suppress the mobility of conductive particles and eliminate electrical interconnection problems at ul- tra-fine-pitch. In this study, nanofiber ACF were investigated for ultra-fine-pitch chip-on-glass (COG) intercon- nections in which the assembly structure was established by a flip-chip bonding, by use of nano- fiber ACF. In this process, an electrical path was formed via conductive particles captured between a bump of the silicon chip and a thin-film electrode of the glass substrate. Nanofiber ACF incorporating conductive particles were investigated to determine conductive particle capture and electrical properties for 20 lm ultra-fine-pitch COG interconnections for which the bump gap was only 7 lm. Poly(vinylidene fluoride( (PVDF) and poly(butylene succinate) (PBS) (Received March 2, 2015; accepted August 27, 2015; published online September 11, 2015) Journal of ELECTRONIC MATERIALS, Vol. 44, No. 11, 2015 DOI: 10.1007/s11664-015-4021-0 Ó 2015 The Minerals, Metals & Materials Society 4628