Effects of anisotropic conductive films (ACFs) resin properties and conductive ball size on bending reliability of chip-in-flex (CIF) packages for wearable electronic applications

Abstract

In this study, the effects of Anisotropic Conductive Films (ACFs) resin properties and conductive ball size on bending reliability in Chip-in-Flex package were evaluated for higher bending reliability of wearable electronics applications. ACFs have an advantage of flexibility, comfortability and simple process. However, ACFs resin delamination and resin crack can occur in the repeated bending test. Since the high reliability of ACFs joint is related with high bending reliability, effects of material properties of ACFs on bending reliability were evaluated. First, effects of ACFs resin modulus were evaluated, secondly, effects of ACFs adhesion strength were evaluated. Finally, effects of difference in the gap height by changing the size of conductive balls on bending reliability were evaluated. In part 2, bending reliability according to ACFs resin modulus was evaluated during a dynamic bending test. Three ACFs with different modulus were prepared and fatigue bending tests were carried out and the fracture mode were observed. Before and after the bending test, the gap height was greatly increased in low modulus ACFs, and through the SEM image, resin delamination was observed between the ACFs resin and the electrode of flexible substrate. In order to analyze, strain analysis using cyclic tensile test was carried out and the value of accumulated plastic strain was predicted by comparison with the value of strain generated in three types of ACF at the same time. As a result, as the modulus increased, it showed the lowest plastic strain and higher bending reliability. In Part 3, the effect of the adhesion properties of the ACFs on dynamic bending reliability was evaluated. In order to change the adhesion property of the ACFs resin only, the chip and the substrate were subjected to the plasma treatment and the bending reliability was evaluated according to the plasma treatment type. In case of ‘one side treatment’, the bending reliability was increased compared to the ‘No treatment’ type, and ‘both sides treatment’ type showed the highest bending reliability. In Part 4, after changing the size of the conductive balls to vary the gap height after bonding, the effect of the gap height on the bending reliability was evaluated. Using the conductive balls of three sizes, the thermos-compression bonding was performed under the same conditions. As a result, the smallest conductive ball had the lowest gap height and then showed the highest bending reliability. In part 5, three variables of the anisotropic conductive film which can be considered to improve the flexibility and bending reliability of flexible package were evaluated. As a result, using small conductive balls was the most effective in improving the bending reliability, followed by the increase of ACFs resin modulus and the increase of the adhesion strength.