Effects of the Mechanical Properties of Polymer Resin and the Conductive Ball Types of Anisotropic Conductive Films on the Bending Properties of Chip-in-Flex Package

Ultrathin chip-in-flex (CIF) packages using anisotropic conductive film (ACF) as an interconnecting material were demonstrated as one of the flexible electronic packages for wearable electronics applications. In this paper, the effects of ACF resin material and conductive ball type on the CIF package bending properties were investigated. Various ACFs with different moduli were fabricated by adding silica particle to polymer resin. For the conductive ball type, solder ball, metal-coated polymer ball, and Ni ball were used. To quantify the bending properties of the CIF packages, dynamic bending test was performed. It was found that the lowest modulus resin (0.54 GPa) resulted early delamination between the conductive ball and the electrode interface. For the resin modulus of 1.04 GPa, the conductive balls and electrodes interface delamination was significantly suppressed due to lower deformation of ACFs. However, when the modulus was increased to 1.3 GPa, chip crack failure occurred presumably due to the internal stress increase at the chip area. Therefore, it is suggested that the optimal modulus of the ACF resin was around 1 GPa. In addition, among the various conductive ball types, metal-coated polymer balls showed no electrical and mechanical failure even after 160k dynamic bending cycles. The metal-coated polymer balls had better compliance compared with the solder ball and Ni ball, which eventually prevented chip crack and ACF contact loss to occur.