Manufacturing Challenges and Signal Integrity Considerations in High-Speed Rigid-Flex PCB Design

Rigid-flex boards are a combination of rigid and flexible printed circuit boards (PCBs). These PCBs can be bent and twisted allowing them to be used as interconnects joining multiple rigid PCBs in a compact environment. Their usage can be found in a myriad of applications such as ear bud headphones to complex military systems. Rigid flex PCBs are mainly used for low frequency signal applications consisting of a harmonic content that are not heavily disturbed by the changes in the flex PCB geometry. However, recently rigid flex PCBs have been increasingly employed in connections for extremely high bitrate signals consisting of harmonic bandwidth greater than 100 GHz. At these frequencies, the signal wavelengths are easily disturbed by slight changes in manufacturing tolerances or geometric changes in the flex angles of the PCBs. In this paper, a study is conducted to quantify the effects of manufacturing tolerances and mounting various bend angles on the performance of signals up to and higher than 100GHz. The studied model cases are simulated using 3D full wave EM solver and compared to measurements. Various studies of the bend angle and the impact on the electric field propagation as well as Time domain reflectometer (TDR) responses are key factors used to demonstrate the impact on overall design performance.