Flexibility of anisotropic conductive films (ACFS) bonded CIF(Chip in Flex) package for wearable electronics applications

Abstract

Due to the increasing demand for higher performance, greater flexibility, smaller size, and lighter weight in mobile electronic products, there have been growing needs of wearable electronic products in near future. Wearable watches, glasses, wrist bands, shoes, and clothes have been recently introduced in the market. To realize wearable electronic products, there should be four core electronic hardware technologies - (1) flexible semiconductor, (2) flexible display, (3) flexible battery and (4) flexible packaging & interconnection technologies. In the flexible semiconductor devices, ICs are thinned down to several hundred nanometer thickness, and then transferred to a polymer film. However, much more development is still needed for the real applications of flexible semiconductors in the future. In contrast, flexible displays are already available by introducing OLED (organic LED) realized on polymer films. And flexible batteries are also demonstrated by several companies by adapting solid electrolytes and so on. However, there has been little development on flexible packaging & interconnection areas. As a one of the promising flexible packaging and interconnection technologies, flexible IC packaging such as COF(Chip On Flex)/CIF(Chip In Flex) technologies using FPCs substrates and ACFs bonding materials become very important for realizing wearable electronic products, because ACFs interconnection provides excellent electrical flexibility compared with conventional solder- or socket-based interconnection methods which have a limitation of bending and flexing. [1]-[3] By attaching the cover adhesive film with a polyimide film to COF packages, CIF packages were successfully fabricated. During both convex and concave dynamic bending test[4], the minimum bending radius of CIF was obtained at no chip-crack condition. After fabrication of CIF packages, the effect of ACFs resin properties on the dynamic bending performance of CIF packages up to 160 k cycles was also investigated. It was found that the lowest modulus resin resulted early delamination between the conductive ball and the electrode interface. For the higher resin modulus of 1.04 GPa, the conductive balls and electrodes interface delamination was significantly suppressed due to lower deformation of ACFs. And among the various conductive ball types, metal coated polymer balls showed no electrical and mechanical failure even after 160K dynamic bending cycles, because the metal coated polymer balls had better compliance compared to the solder ball and Ni ball, which eventually prevented chip crack and ACF contact loss to occur.