Analysis of Transmission Characteristics of Three-Layer Flexible Printed Circuit Board

A FPCB (Flexible Printed Circuit Board) has been used in various electronic products such as smart phones, wearable devices, and tablet PC's, etc. In high performance electronic devices with multiple modules to be connected, high speed signal data usually needs to be transmitted through FPCB between the modules. Due to the laminated structure of the multilayer FPCB, however, an air gap could be introduced between the FPCB layers when it is bent, which deviates the designed characteristic impedance of the FPCB. For the prediction of the bent FPCB characteristics, a 3D electromagnetic model has been used, but the calculation time is too long, especially due to the meshed ground employed in FPCB. In this paper, we propose a circuit model which efficiently predicts the characteristic impedance of the bent FPCB with an air gap inside layers. The validity of the proposed model was verified between the EM simulation and measured results.