Rigid-Flex PCB Design Considerations and Key Issues

2025-09-19 17:22:03

Rigid-flex PCBs are ideal solutions for applications involving multiple rigid PCBs with SMT components on both sides that require interconnection between them.

The most familiar products may be smartwatches connected to smartphones and fitness trackers also worn on the wrist. However, beyond these consumer products, wearable devices have made significant inroads into medical devices and military applications. Now, the emergence of smart clothing has almost eliminated the possibility of using rigid PCBs. So, what does it take to successfully design flexible and rigid-flex PCBs to keep up with the market?

Before designing a rigid-flex circuit, ensure it is indeed what you need. If the circuit has only a few layers, stiffeners are a more cost-effective alternative to rigid-flex PCBs.

Constructing rigid-flex boards with an even number of layers is the most cost-effective. All rigid sections of the circuit should have the same number of stack-up layers.

The biggest challenge in designing rigid-flex PCBs is ensuring all components fold in the correct manner while maintaining good flex circuit stability and service life. The next major issue to address is communicating the design to manufacturers so that they clearly understand the design intent and produce the product the designer/engineer wants. Rigid-flex PCBs require additional cutting and lamination stages, and the manufacturing process demands more specialized materials. Therefore, the cost of rework and failures is much higher than that of traditional rigid boards. To mitigate the risks and costs associated with rigid-flex design and prototyping, it is advisable to outline the rigid-flex PCB design process to ensure proper form and fit. Additionally, absolutely clear manufacturing documentation must be provided to manufacturing and assembly facilities.

1. Basic Specifications of Rigid-Flex PCBs

- Part number (including revision number) for easy tracking
- PCB thickness (including flex section thickness, thickness of each stiffener area, and total thickness of rigid sections)
- Substrate type (adhesive-free polyimide substrate, polyimide substrate with adhesive, FR4, high-temperature FR4, Rogers, Teflon, etc.). Polyimide substrate with adhesive and FR4 are standard.
- Number of layers
- Surface finish (OSP, immersion gold, etc.). Immersion gold is standard.
- Colors of solder mask and coverlay. Yellow coverlay and green are standard colors.
- Outer layer copper weight (1 oz, 2 oz, etc.). 1 oz is standard.
- Inner layer copper weight (0.5 oz, 1 oz). Both are standard.
- Stiffener material and thickness (FR4, polyimide, stainless steel, copper, etc.)
- Minimum trace and space width in the design
- PCB dimensions marked on the mechanical layer
- Whether you want the substrates to be supplied in panelized form or as individual pieces
- Gerber files, drill files, IPC-356A (optional)

2. Via Placement

For multi-layer flex areas, vias may sometimes be needed to transition between layers. If possible, it is recommended not to place vias, as they fatigue quickly during bending movements. Additionally, a minimum gap of 35 mils must be maintained between the copper ring of the nearest via and the rigid-flex interface. The board edge clearance rules in PCB CAD editors can handle this automatically.

Regarding whether vias are necessary: if vias must be placed in the flex circuit, use "rooms" to define areas you know will not bend, and use the PCB editor’s design rules to allow via placement only in these fixed areas. Another method is to use the layer stack manager to define "rigid" sections—these are ultimately flexible but bonded with rigid dielectric stiffening materials.

3. Define Stacks by Area

Undoubtedly, the most important document you can provide to the manufacturer is the layer stack design. Furthermore, if you are manufacturing a rigid-flex board, you must provide different stacks for different areas and mark them clearly. A simple method is to duplicate your PCB outline on the mechanical layer, then place a layer stack table or diagram with a pattern-filled legend for the areas containing different layer stacks.

4. Define Drills by Layer

Undoubtedly, the most important document you can provide to the manufacturer is the drill information. Moreover, if you are manufacturing a rigid-flex board (of any layer count) or a rigid-flex board with blind and buried vias, you must provide different drill data and information for different layers and mark them clearly. A simple method is to duplicate your PCB layer stack and insert a drill drawing with a pattern-filled legend to present the drill information.

5. Bonding Fillets (Transition Zone)

For rigid-flex printed circuits (R-FPCs), the area connecting rigid and flexible materials (transition zone) may sometimes contain defects. While these defects may be acceptable, they can affect the effectiveness of the final component. Defects in the transition zone may include any of the following:
- Adhesive squeeze-out
- Protruding dielectric material
- Cracking
- Haloing

In "rigid/flex" circuits and circuits requiring rigid stiffeners, the area where the flex section intersects the rigid section is called the transition zone. This area typically has unsmooth material edges. If the flex circuit bends sharply, these rough edges may damage the conductor paths. To prevent this, it is highly recommended to place a bead of epoxy material in this transition zone, as shown in the figure below.

6. Minimum Spacing from Outer Layer Copper/Pads to Flex Transition Zone = 0.040”

Measurements show that the distance between the flex transition zone and the outer layer copper or pads is less than 40 mils. Ensure there is sufficient space for reliable outer layer imaging. In the production panel configuration and before the final lamination process, the rigid layers must be removed from the flex areas. This creates internal edges resulting from the height difference between the rigid and flex areas, and the outer layer image transfer film must transition over these edges.

If you have any questions or concerns about rigid-flex PCBs, we would be happy to hear from you! During the design phase, feel free to contact us—our engineers are ready to assist you.

As one of the early enterprises engaged in the R&D and manufacturing of high-difficulty rigid-flex PCBs, Unice Circuit, founded in 2005, has long focused on the process implementation of high reliability and high precision for R-FPCs. It has developed a technology-driven path to continuously optimize processes, improve operational efficiency and product first-pass yield, ultimately maintaining distinct advantages in process capability and delivery lead time for Unice’s small and medium batch R-FPCs.