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Rigid-Flex PCBs Design Guidelines

The performance and shelf life of an electrical system directly depends on the integrity of rigid-flex PCBs. Hence, manufacturers must pay utmost attention when designing and manufacturing the PCBs. Any design or manufacturing flaw considerably affects the performance and productivity of the boards.

Best Practices in Rigid-Flex PCBs Manufacturing

Following the right manufacturing practices and standards helps in producing rigid-flex PCBs with unmatched quality and functionality. There are some standard guidelines to be considered when designing and manufacturing rigid-flex circuit boards.
The following guidelines can act as a checklist to produce rigid-flex circuit board without any design and manufacturing defect.

  • Decide on the manufacturing processes and materials in advance, based on how much ‘flex’ is needed. This can be understood with an example. If the boards you are designing are only folded during the manufacturing process, and then kept in a fixed position without any bend, you are free to decide on the:
    • Type and grade of copper
    • Manufacturing method
    • Number of layers

In contrast, if the board is for applications that experience frequent moves, shocks, and vibrations, you must be particular when choosing

    • Copper coils
    • Adhesives
    • Number of layers
  • It is advisable to position copper traces to the right angle of the rigid flex. This is because, copper traces work superiorly when they are placed at the right angle of the board. Hence, avoid bending at the corners as it strains the copper traces. However, there are some PCB design considerations that call for bending at the corners. In such cases, you may use conical radius bends.
  • Due to repeated stress caused by frequent bending, copper on the circuitry is more likely to detach from the polyimide substrate. To avoid this, provide the exposed copper traces with enough pad support. In most cases, usage of surface mount pads is found to be ineffective for this purpose. However, you can increase the reliability of the pads by using coverlay “mask” openings and anchoring stubs.
  • To retain the board’s flexibility, consider using hatched polygons instead of solid copper pours. Usage of solid copper pours may result in reduced flexibility, and buckling of the copper under tight radius bends.
  • It is suggested to leave at least ½ mm clearance between copper annulus and adjacent vias to avoid fatigue due to frequent bending. Place the vias in stationary areas of the board, which are not subjected to frequent bends and movements.
  • Contact your raw material supplier during the design stage in case you are planning to use non-standard base dielectric materials and adhesiveless laminates. Since they come in standard sizes by special ordering, it may take time to deliver the same in your door steps. Hence, it is suggested to avoid using these materials, until the design conditions really demand using them.

Rigid-flex PCBs manufacturing processes are laborious, requiring close attention to detail. There are several factors to be considered to produce rigid-flex PCBs with outstanding quality.

Even though, the above mentioned tips/rules seem basic, they help producing PCBs with unmatched reliability and service life.

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