Interface Adhesion Effect of Anchoring Polymer Layer Anisotropic Conductive Films on the Bending Reliability for Ultra-fine Pitch Wearable Chip-on-Flex Packages

Nowadays, much research is focused on the fine pitch capability and flexibility of interconnection due to the rapid development of miniaturized wearable electronic devices. In this study, novel APL structure has been introduced into conventional ACFs system for wearable Chip-on-Flex (COF) packages as the interconnection materials. The APL ACFs are consisted of 3 layers structure which is NCF-APL-NCF. Comparing with the conventional ACFs, the APL structure has the conductive particles suppression capability to solve the short circuit problem in ultra-fine pitch electronic packages. Also, the mechanical and thermal properties of APL ACFs were different due to various thermoplastic materials of APL used in ACFs. At the same time, it caused the deformation difference of APL and ACF during the bonding process resulting in different joint morphology. Under the same bonding pressure, due to the tensile strength property of APL material, the thickness change of APL and NCF was different which related to the interface adhesion of COF packages. The interface adhesion energy of APL ACFs structure was confirmed by double cantilever beam test in terms of bonding pressure. Then the interface adhesion effect was investigated to evaluated the dynamic bending property of COF packages. As a result, the thickness of the APL ACFs was decreased as the bonding pressure increasing from 0.5MPa to 0.75MPa and 1MPa resulting in different failure mode of the COF packages after die shear test, the adhesion strength of the COF joint was the highest with 30.9MPa at 0.5MPa bonding pressure due to the thickest adhesive remaining between the joints. Moreover, it was found that at the same 30000 bending cycles, the joints at 0.5MPa had the lowest contact resistance variation of 0.45mΩ due to the highest adhesion strength under 6mm bending radius.