A Study on the Fine Pitch Flex-on-Flex (FOF) Assembly using Flux Added Nanofiber Solder Anisotropic Conductive Films (ACFs) and Thermo-compression Bonding Method

Abstract

The advanced electronic device trends are changing to be more and more light weighted and miniaturized. And due to the rapid growth of wearable electronic industry, demands for flexible packaging are growing, and advanced flexible interconnection methods are required. In order to satisfy the needs, fine pitch Flex-on-Flex (FOF) assembly has been introduced, because FOF can reduce not only the package area of the whole devices, but also show excellent electrical performance such as lower contact resistance and higher power handling capability. Anisotropic conductive films (ACFs) are interconnection materials, which have been widely used for the FOF assembly due to their simple and low temperature assembly processes. Previously, nanofiber ACFs were introduced by our research group, which showed remarkable capability of enhancing the conductive ball capture rate along with preventing short circuit in the fine pitch assembly such as COG (Chip On Glass) and FOF by suppressing the movement of conductive particles. In the previous FOF studies, ultrasonic bonding method was used as a solder ACFs bonding method. In the ultrasonic bonding method, native solder oxide layers around solder balls can be physically broken by applying ultrasonic vibrations above the melting temperature of solder balls resulting in excellent solder metallurgical joints between solder particles and metal electrodes. However, there could be potential misalignment and damage issues generated by ultrasonic vibrations for mechanically sensitive and fine pitch applications. Accordingly, there has been need for alternative bonding methods which can remove solder oxide layer to obtain excellent solder wetting without causing misalignment issues by the ultrasonic bonding method. In this study, a flux activator was added in nanofiber / solder ACFs to chemically eliminate solder oxide during the thermo-compression bonding process. For the formation of stable intermetallic compounds (IMCs), optimization process of flux content in nanofiber solder ACFs was investigated. To investigate the flux effect during the thermo-compression bonding process, cross-sectional transmission electron microscopy (TEM) were performed to analyze whether solder oxide layer is certainly removed by the flux or not. In addition, contact resistance was also measured to support this TEM analysis. Eventually, by adding flux in nanofiber solder ACFs, stable solder wetting and excellent alignment could be obtained by using thermo-compression bonding method for the fine pitch FOF assembly.