Process Window Optimization for Roll‐to‐Roll Slot‐Die Coating of High‐Resolution Dry Film Solder Resists for <scp>HDI PCB</scp> Applications

ABSTRACT As electronic devices trend toward high‐density interconnection (HDI) structures, ensuring thickness uniformity in solder resists has become a critical challenge for electronics engineers. This study focuses on continuous roll‐to‐roll slot‐die coating and aims to establish the hydrodynamic coating process window that enables stable film formation. Although initial trials exhibited hydrodynamic instabilities such as ribbing and streaking, a stable coating regime was identified within a coating gap of 65–80 μm and a pump flow rate of 7.0–8.0 mL min −1 , where the balance between viscous forces, capillary forces, and substrate motion stabilizes the coating bead. Under the optimized condition, a highly uniform functional layer with a target thickness of 26 μm (with a uniformity of ±3.74%) was achieved without observable coating defects. Crucially, atomic force microscopy showed a nanoscale average roughness of approximately 1.16 nm, and thermogravimetric analysis confirmed excellent thermal stability with a primary decomposition temperature of 412°C. Photolithographic evaluation further confirmed clear pattern formation and complete micro‐via opening without residual resist. These results demonstrate that identifying the hydrodynamic coating window is essential for stable slot‐die coating and provides practical guidelines for scalable manufacturing of high‐resolution dry‐film solder resists.