A study on the 50 μm fine pitch Flex-on-Flex (FOF) assembly using conventional and nanofiber anisotropic conductive films (ACFs)

Abstract

As advanced flexible electronic devices have become indispensable, fine-pitch devices are widely utilized due to their lightweight and miniaturized structures. The increasing demand for flexible packaging and flexible interconnection methods, such as Flex-on-Flex (FOF), can be attributed to the rapid development of wearable devices. An essential component of FOF assembly is the utilization of anisotropic conductive films (ACFs) as interconnection materials. Previously, conventional ACFs were extensively employed in such systems. However, these ACFs incorporated metal-coated polymer conductive particles, which could only establish physical contact between electrodes, resulting in poor reliability. In an attempt to address this issue, conductive particles were substituted with solder particles within the ACF system. Nevertheless, this change presented a new problem, as the conductive particles tended to aggregate between neighboring electrodes, leading to electrical short circuits in fine-pitch FOF ACF assemblies. To overcome these challenges, nanofiber ACFs have been introduced as interconnection materials due to their remarkable ability to resolve the short-circuit problem. In this study, we investigate the performance of 50 μm fine-pitch FOF assemblies using both conventional and nanofiber ACFs. Our evaluation focuses on the electrical reliability and joint formation of these assemblies, with a specific emphasis on measuring insulation resistance and ACF joint contact resistances. The results reveal that, when employing nanofiber ACFs, the insulation resistance of 50 μm pitch FOF assemblies achieved a remarkable 100%. Furthermore, lower contact resistances were achieved through the optimization of the polymer resin utilized in the nanofiber ACF system. Additionally, we conducted thermal cycling (T/C) and 85 °C/85%RH reliability tests to assess the joint reliability of the 50 μm FOF assembly. Remarkably, our findings demonstrate that the 50 μm FOF assembly exhibits exceptional performance with 1000 cycles of T/C and 500 h of 85 °C/85%RH reliability when employing nanofiber solder ACFs. This marks the first reported instance of nanofiber solder ACFs delivering outstanding electrical properties and reliable ACF joint formation in the context of 50 μm pitch FOF assembly.