Manufacturing Elastic Multilayer Printed Circuits for Flexible Hybrid Electronics

Printing technologies enable additive manufacturing of wiring for electronic systems on flexible and stretchable substrates. Integrating high-performance electronics on such conformable printed wiring boards (PWBs) forms applications with advanced form factors. However, the number of layers over the flexible printed circuit boards (PCBs) is typically limited and constrains realization of complex PCB designs.
This study presents an innovative approach to manufacture elastic multilayer printed circuits (EMPCs) in a sheet-based process. The concept is based on stacking pre-perforated thermoplastic polyurethane (TPU) films on top of each other, and screen-printing and curing conductive traces layer by layer. Moreover, layered wiring is connected by filled vias forming conductive paths between separated signal layers. The material stack is based on TPU substrate system including hotmelt adhesive layer, and adding a new layer to the stack is conducted by heat press process. The experimental part includes investigation of stacked and staggered vias between signal layers printed by a stretchable silver paste. Another design variable is structure of vias as both single large opening and multiple smaller openings per via are included in the study. Thus, about 200 x 200 mm sheet included four different multilayered test structures. Furthermore, several options for manufacturing steps are studied by altering the order of via filling and signal layer prints, as well as forming conductive vias only by printing a signal layer.
The results show feasibility of 4-layer elastic multilayer printed circuits with the proposed approach. Laminating a pre-perforated TPU film with hotmelt adhesive on lower layer with printed conductors and printing a new signal layer on top of the laminated film comprises a repetitive process to stack EMPCs. Electrical characterization proves that preparing stacked and staggered vias with both via topologies forms conductive paths between different layers with the stacking method. Conducted 3D X-ray imaging illustrates quality of via filling and enables evaluation between different design variables. Findings from the experimental study form a basis for design rules for elastic multilayer printed circuits. The successfully demonstrated manufacturing concept proves that 4-layer EMPCs are reality also in conformable electronics domain. Applying multilayer structures enables utilization of mature design tools and simulators well-known from rigid PCBs also to flexible hybrid electronics (FHE).