Thin PDMS-on-Sacrificial-PCB Devices

Abstract

Poly(dimethylsiloxane) (PDMS) devices must often be so thin (e.g., <100 μm) and, therefore, so fragile that their mechanical release becomes challenging. As a simple solution, PDMS devices are frequently released by dissolving an underlying sacrificial resist. However, there may be several other issues, including temperature gradients during fabrication, electrical characterization, handling, co-integration with electronics, fabrication time, and cost. Here, we show that conventional printed circuit boards (PCBs) can be ideal sacrificial carriers, which can also perform useful functions during fabrication. The thick (e.g., 35 μm) and thermally conductive PCB copper layer behaves as an excellent heat spreading plate during temperature-sensitive process steps (e.g., PDMS curing and sintering of conductive inks) and enables pre-release electrical stimulations and characterizations of PDMS, which may help during process development. Experiments and finite element method (FEM) simulations confirm that the PCB copper layer can improve temperature uniformity. The UV-protecting film attached to the PCB can be cut to constitute a frame for easy handling. The PCB photoresist enables the straightforward release of PDMS by acetone, which is among the most environmentally friendly chemicals. Contacts can be opened by a simple bump-cut-peel strategy. As proofs of concept, we demonstrate pre-release capacitive characterization of PDMS for evaluating the ability to fabricate thin, but uniform and robust devices and post-release resistive characterization of stretchable strain sensor or interconnects encapsulated in PDMS. The proposed approach is simple, cheap, and environmentally friendly, can improve temperature uniformity throughout the pre-release process steps, enables pre-release electrical characterizations, can simplify co-integration with electronics and can be generalized to other elastomers.