Reverse-offset printed flexible chips assembled on thermoplastic polyurethane: studies on mechanical tolerance

Reverse-offset printing (ROP) is a high-resolution printing method that can deliver µm-level linewidths. ROP could be used to fabricate flexible high-performance devices and integrated circuits (ICs) to replace, in the future, some of the conventional Si ICs e.g. in wearable electronics applications, potentially improving their form-factor and sustainability. Here, we fabricated test chips with Cu conductors on thin, flexible polyimide substrate using two ROP-based processes: i) ROP of Cu nanoparticles, and ii) ROP of polymer resist and lift-off of vacuum deposited Cu. Both types of samples were attached onto screen-printed Ag conductors on stretchable thermoplastic polyurethane (TPU) substrates using conductive adhesives and mechanically supported either by edge adhesive or glop-top encapsulation. The electromechanical behavior of the assembled test chips was examined by performing compression and expansion bending tests for five test chip samples of each fabrication variation, showing good mechanical tolerance of the ROP-based Cu and the interconnections. Although the expansion bending tests reveal weaknesses in the Ag structure on the TPU, the compression bending tests indicate encouraging robustness. The conductor at the edge of the more rigid adhesive is found to be the weakest point and preliminary tests using a more flexible underfill support material suggest potential mitigation pathways.