(A) study on the 50μm Fine Pitch Flex-on-Flex (FOF) assembly using conventional and nanofiber Anisotropic Conductive Films (ACFs)

Abstract

As advanced flexible electronic devices have become essential, fine pitch devices are used widely due to its light weight and miniaturized structure. And the demands for flexible packaging as well as flexible interconnection methods such as Flex-on-Flex (FOF) are growing faster due to the fast development of wearable devices. In order to achieve the FOF assembly, the anisotropic conductive films (ACFs) are used as the interconnection materials. Conventional ACFs as one of the types of ACFs was widely used before, however, the metal coated polymer conductive particles used in conventional ACFs system only could form the physical contacts between the electrodes resulting in poor reliability. Then the conductive particles were changed to solder particles into the ACFs system, however, conductive particles have the tendency to agglomerate between the neighboring electrodes resulting in electrical short circuit at the flexible fine pitch FOF ACFs assembly. Therefore, the nanofiber ACFs are used as the interconnection materials because of its excellent capability to solve the short circuit problem. In this study, 50μm fine pitch FOF assembly using conventional and nanofiber ACFs was investigated. In order to demonstrate the electrical reliability & joint formation of 50μm pitch FOF assembly, the insulation and ACF joint contact resistances were measured. As a result, the insulation resistance of 50μm pitch FOF assembly was 100% using nanofiber ACFs and lower contact resistances were achieved through the optimization of polymer resin used in nanofiber ACF system. Finally, the thermal cycling (T/C) and 85℃/85%RH reliability tests were performed to investigate the joint reliability. As a result, 50μm FOF assembly showed excellent 1000 cycles T/C and 500hours 85℃/85%RH reliability using nanofiber solder ACFs. It was the first reported result that the nanofiber solder ACFs can provide the excellent electrical properties and reliable electrical ACFs joint formation at 50μm pitch FOF assembly.