Effects of Anisotropic Conductive Films (ACFs) Gap Heights on the Bending Reliability of Chip-In-Flex (CIF) Packages for Wearable Electronics Applications

The interest in wearable electronics has been rapidly increasing due to the high demands for various wearable devices such as smart glasses and smart watches which satisfy the needs of today's customers. Future wearable devices will require fully flexible chip packaging performance and also maintain stable electrical performance under repeatedly bending environment. To meet these requirements, ultra-thin silicon dies and flexible substrates will be used for flexible electronic packaging and assembly. In addition, among various electrical interconnection methods used in electronic package systems, ACFs interconnection methods will be considered as one of the most promising flexible interconnection methods due to its resilience against bending and flexing environments. ACFs consist of adhesive polymer resin with randomly distributed conductive balls, which form an electrical conduction between the bumps of chip and electrodes of flexible substrates. In our previous studies, ultra-thin chip-in-flex (CIF) packages using ACFs were introduced, and the bending properties have been evaluated in terms of ACF material properties such as adhesion strength and modulus of polymer resins. In this paper, effects of ACFs gap heights on the CIF bending reliability were investigated using polymer conductive balls with various diameters. The dynamic bending reliability of ACFs-assembled CIF packages were evaluated up to 160,000 bending cycles at 6 mm bending radius to determine the dynamic bending performance of CIF packages.