Abstract: Polyimide (PI)-based multilayer flexible circuits are widely used in aerospace, flexible displays, and new energy sources due to their high space utilization, excellent optoelectronic properties, and high-temperature resistance. However, current fabrication methods face challenges including process complexity, high costs, and stress concentration in the vertical interconnect access (VIA) structure, significantly limiting frequent design iterations and personalized customization of multilayer flexible circuits. Here, we present an all-printed fabrication method for VIA-free PI-based multilayer flexible circuits. The process utilizes direct ink writing to print the substrate layer, the middle dielectric layer with hole structures, and the encapsulation layer for filling the hole structures. Electric-field-driven printing is employed to produce high-precision wires, which can achieve continuous deposition at the locations of the dielectric layer and hole structures. These wires can cross-connect with wires in other layers at the hole structures. Since the high-precision wires at the crosspoints are in the same plane, this approach achieves nearly VIA-free electrical interconnection, significantly reducing stress concentration. Finally, the encapsulation layer fills the hole structures, further enhancing the overall mechanical properties. The method achieves low-cost, integrated rapid prototyping of multilayer flexible circuits to meet customization requirements. Furthermore, application cases involving multilayer infrared display devices provide compelling evidence that PI-based multilayer flexible circuits possess stable interlayer electrical interconnect, excellent mechanical stability, and high-temperature resistance. The all-printed fabrication method provides a novel solution for the microscale and simple preparation of PI-based multilayer flexible circuits.

Key words: All-printed, VIA-free, PI-based, Multilayer flexible circuits