A Study on Polyvinylidene Difluoride (PVDF) Anchoring Polymer Layer (APL) Solder Anisotropic Conductive Films (ACFs) for Fine-Pitch Flex-on-Flex (FOF) Interconnection

Fine-pitch flex-on-flex (FOF) assembly has been introduced for various electronic packaging applications because electronic products require higher functionality, flexibility, and I/O density. In this article, novel polyvinylidene difluoride (PVDF) anchoring polymer layer (APL) solder anisotropic conductive films (ACFs) were investigated for fine-pitch FOF applications. The capture rates of conductive particles and the insulation rates between neighboring electrodes have been significantly enhanced using PVDF APL solder ACFs as a result of suppressing the movement of conductive particles by the high tensile strength of the PVDF APL layer. For good solder wetting on electrodes, flux activators were added into the PVDF APL layer to remove the solder oxide layer. However, the tensile strength of the PVDF layer decreased as the flux activator content in the APL increased. The capture rates of conductive particles decreased as more flux activators were added because of the lower tensile strength of PVDF. Furthermore, optimized flux activator content in ACFs with 0.5 wt% in the APL and 2-wt% flux activators were also added in nonconductive films (NCFs) to obtain good solder wetting. Using the optimized APL solder ACFs, excellent contact resistances and insulation resistances were obtained before and after a pressure cooker test (PCT) and 85 °C/85% relative humidity (RH) reliability tests. As a conclusion, the PVDF APL solder ACF assembly can provide an excellent interconnect solution for fine-pitch FOF assemblies used for portable electronic product applications.