Effects of Polyacrylonitrile Anchoring Polymer Layer Solder Anisotropic Conductive Films on the Solder Ball Movement for Fine-Pitch Flex-on-Flex (FOF) Assembly

In this paper, anchoring polymer layer (APL) solder anisotropic conductive films (ACFs) combined with an ultrasonic (US) bonding method are newly introduced. The effects of APL ACFs properties on conductive particle movement and interconnection stability for ultrafine-pitch chip-on-glass applications were reported earlier. It was found that the APL structure not only suppresses the conductive particles movement during the ACFs bonding process for ultrafine-pitch applications, but also prevents electrical short-circuit formation between neighboring bumps. However, to achieve a stable electrical interconnection, APL ACFs require additional process steps such as an oxygen plasma etch process to remove the APL polymer skins surrounding the conductive particles using a thermocompression bonding method. However, during the US bonding process, it was found that the polyacrylonitrile (PAN) polymer skin coated on conductive particles was successfully removed by vertical directional US vibration. Therefore, stable and metallurgical solder joint formation can be achieved without an additional oxygen plasma etching process.