A Multicomponent Visible-Light Initiating System for Rapid and Deep Photocuring through UV-Opaque Polyimide Films

Polyimides (PIs) play a crucial role in modern electronics, especially in flexible printed circuit boards (FPCBs). However, these FPCBs are vulnerable to physical damage, which can compromise the copper wiring. To protect this wiring, encapsulation with resin is commonly used. Traditional UV-curable resins, however, are ineffective due to the UV opacity of PI films, which hinders proper curing. In this study, we present an advanced multicomponent photoinitiating system, activated by visible light, that enables rapid and deep curing of acrylic resins even through UV-opaque PI films. This system, which integrates camphorquinone/amine (a well-established type II system) with a highly efficient triplet-generating photoredox catalyst and co-initiators, offers exceptional curing speed and depth. When tested on a real FPCB, the system achieved a curing depth exceeding 1 cm within just 10 s under 480 nm LED light, highlighting its practical value for intricate FPCB designs. We believe that this system will play a key role in a variety of applications that require fast and deep photocuring, such as three-dimensional/four-dimensional printing.