Embedding PVDF-Based Force Sensors in Stacked Printed Circuit Boards

Force sensing plays a significant role from industrial applications to medicine, and active sensors, such as piezoelectric transducers that actively output a signal themselves, are already widely used. Typically, these are individual components that have to be connected to a measurement circuit using wires or solder joints potentially adding multiple processing steps. This work reports on embedding these sensors directly into the stack of a printed circuit board (PCB) to eliminate the need for complicated wiring, further integrate the transducer into the measurement circuit, and enable the measurement of the forces exerted onto the PCB itself. The transducer material was chosen to be polyvinylidene fluoride (PVDF) as it can easily be printed onto various surfaces and has sufficient piezoelectric sensitivity for the measurement of small forces. Poly(vinylidene fluoride–trifluoroethylene) (P(VDF-TrFE)), a copolymer of PVDF, is stencil printed onto commercially fabricated two-layer PCBs. The boards already incorporate the electrodes that are needed for the electrical connection of P(VDF-TrFE) with the remaining circuitry. Two PCBs with the applied P(VDF-TrFE) layers are stacked and bonded using the thermoplastic properties of PVDF to create the finished sensor. Measurements show a mostly linear behavior of these force sensors with a sensitivity of around 10 pC/N. The linear measurement range strongly depends on the P(VDF-TrFE) layer thickness, where 70 µm allow for measurement up to at least 3.5 N. In addition, a limit of detection of approximately 10 mN was observed. This concept shows the possibility of embedding sensors further into the measurement circuitry and enabling a much more compact force-sensing package as well as structural monitoring of electronic circuits.