Experimental Dynamic Characterization of Rigid-Flex PCB Systems

Rigid-flex printed circuit boards (PCBs) are used in many applications that range from mobile electronic devices to low-volume medical tools. Dynamic characterization of a rigid-flex system is difficult because the system exhibits large deformation in the joints, panel-to-panel contact, and other sources of dynamic nonlinearities and complexities not present in traditional PCBs. To date, rigorous test methods for the dynamic characterization of rigid-flex PCB systems have not been proposed. The objective of this paper is to present a case study of a rigid-flex PCB system from the NASA Jet Propulsion Laboratory (JPL), demonstrating nonlinear characterization and a modal testing protocol that sufficiently characterizes the frequencies, mode shapes, damping, and nonlinear properties of the structure. Additionally, single panels and a rigid-flex assembly without aluminum components are tested to draw more generalized conclusions regarding appropriate test protocols for other rigid-flex systems. The nonlinear characterization of the rigid-flex structures was performed using the test for homogeneity, an examination of energy-dependent resonance and damping, and the use of the Hilbert transform. Based on the results of this study, it is recommended to first use the Hilbert transform to efficiently identify the presence of a nonlinearity then investigate energy-dependent frequency characteristics and nonlinear damping, which are likely to be influenced by the rigid-flex system’s geometry. All of the rigid-flex systems exhibited energy-dependent frequency characteristics and damping nonlinearities that increased with increased range of motion of the PCB panels about the Nomex hinges and increased with an increase in the number of hinges activated in a particular mode. However, the addition of the aluminum reinforcing components reduced both forms of nonlinearity. To enable modal testing of the rigid-flex structure with aluminum components, it was necessary to understand the resonance replicability and the sensitivity to suspension and excitation orientation to separate noise from local and global resonances.