Low Cost Printed Flexible Multilayer Substrates

For high-volume products, such as mobile terminals, low-cost techniques for multilayer polymer-based thick film wiring board manufacturing are needed. Screen-printing is a cost-efficient technology candidate to build up approximately 6 conductor layers on both sides of a flexible substrate, for example. In order to experimentally evaluate the feasibility of screen-printing technique, the printing resolution was tested on different substrate materials, such as, polycarbonate (PC), polyethylene terephtalate (PET), polyimide (PI) and liquid crystal polymer (LCP). Conventional screen printed polymer thick film pastes were characterized on polymer substrates to form multilayer fine-line patterning and through-hole vias. The final demonstrator was a double-sided PI substrate having two conductor layers separated by dielectric layers on both sides of the substrate and through substrate vias. The screen-printed conductor material was an Ag-based nano particle ink and the dielectric layer was a polyimide-based material. Several challenges were identified that might hinder the applicability of the technology for mass-production. The stability of the polyimide substrate is a problem if the curing temperature of the printed materials is above 200 degrees C. Layer-to-layer alignment tolerances are feasible if the printed area is small, 5"x5", in our case. The flatness of the substrate, however, is not very good after printing several layers on each other. The tested nano particle ink is a promising conductor system; however, lowering of the curing temperature from 230 degrees C below 200 degrees C would have a major impact on production friendliness. Another way to realise multilayer structures is to utilise lamination methods. In this study PET and PC sheets were also used. The focus in the processing development was on the deposition of adhesive layer over the PET sheet and the lamination process. Vias and conductors needed in multilayer structures were realised by utilising conventional thick-film processes. Vias were punched and then filled by stencil-printing and conductors were screen-printed. The lamination parameters were optimised for each material system. Good adhesion was achieved and the alignment accuracy between the layers was <+/- 15 mu m.