Standards and Design Infractions in Rigid-Flex Circuits as Per IPC-2223
### Title: Adhering to IPC Design Standards for Reliable Rigid-Flex PCBs
In the ever-evolving world of electronics, the design of rigid-flex printed circuit boards (PCBs) has become a critical aspect, particularly in applications that require flexibility, such as satellites, aerospace, and medtech. The IPC-2223E standard serves as a comprehensive guide for ensuring reliable performance under mechanical stress and repeated flexing cycles.
This standard covers key areas such as bend radius requirements, layer stackup configurations, transition zone specifications, conductor routing guidelines, via placement restrictions, and material selection criteria. For instance, it emphasises the importance of maintaining a sufficient bend radius to prevent cracks during bending, and the use of curved traces instead of sharp angles to reduce stress.
One common design violation to avoid is the insufficient design of vias, particularly in flex areas. Not using teardrop-shaped vias can result in pad lifting during flexing, making larger annular rings a recommended solution for added strength. Additionally, the distance between vias and the rigid-flex interface should be at least 20 mils.
Another crucial aspect is the selection of suitable materials. Choosing materials that are not suitable for flexing can lead to mechanical failures under stress. Moreover, following trace-to-pad clearance guidelines and keeping high via aspect ratios within recommended limits can prevent plating reliability issues.
Adhering to IPC standards not only reduces the board size and optimises bending performance but also helps in developing high-quality products. For instance, the bookbinding technique can enable multilayer flex to bend in a small radius without deforming.
In the flex area, the ground plane should be a hatched polygon rather than solid copper planes. Also, a trace entering a pad creates a weak spot where the copper may eventually become worn out. It is advised to taper the pads down near the end where they connect to the traces.
Rigid-flex capabilities at Sierra Circuits include strict adherence to IPC-2223 design standards, ensuring efficient prototype production. The controlled impedance design guide, a part of IPC-2223, provides instructions for applying electrical constraints to the flex and/or rigid circuit board.
IPC-6013D and IPC-6013C offer guidelines for rigid-to-flex areas, deformation anomalies, and specifications for final finishes. The finished hole-to-copper clearance is calculated as the drill-to-copper clearance plus twice the plating thickness.
Common rigid-flex design violations include flexibility, number of layers, traces, trace geometry, drill to copper, and vias of rigid and flex areas. Drill-to-copper clearance for successful product manufacturing is 8 mils.
By following these guidelines and best practices, designers can create robust and reliable rigid-flex PCBs that can withstand the rigours of repeated bending and flexing cycles, making them ideal for a wide range of applications.
Technology, such as controlled impedance, plays a significant role in the design of reliable rigid-flex PCBs, as articulated in the IPC-2223E standard. The controlled impedance design guide provides instructions for applying electrical constraints to the flex and/or rigid circuit board, ensuring efficient prototype production.
