Lithography Free, Soft, Flexible Vias for 2.5D Fabrication of Ultra-Flexible Circuits

To achieve significant complexity in circuit boards, signals must be able to cross without interfering, requiring multilayer circuits. Unlike in standard PCBs, in ultrathin and flexible circuits these vertical interconnects (or vias) between these layers cannot be achieved by drilling and hole plating. Here we demonstrate a rapid, low-cost, lithography-free route to creating vias in ultrathin, conformable circuit boards. Several materials are evaluate for the long-term behaviour of such devices under repeated bending to demonstrate the durability of these vias.<br/>Starting from thin, 5 µm layers of Parylene C, we create simple two layer circuits with low resistance, flexible vias. Circuits are fabricated by metallisation and laser ablation to create each layer, followed by opening of via holes, down to 200 µm diameter, through a second laser ablation step, before finally aligning and stacking the layers and backfilling the vias with flexible conductive adhesive.<br/>In addition to demonstrating this simple, low cost process, we present evaluation of several commercially available conductive adhesives in terms of their suitability for this process. Vias filled with these adhesives are evaluated in terms of the electrical performance of the both in the as-fabricated state, and throughout cyclic bending. We consider the total resistance of vias in the initial flat state, the bending radius at electrical failure, and the change in resistance with repeated bending.<br/>Compared to alternative materials such as low temperature solder paste and conductive inks, conductive adhesives are shown to provide significantly more robust interconnections, making this approach to vias well suited to highly flexible PCB fabrication.