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4 Vital Design Considerations When Configuring Rigid-Flex PCBs

Rigid-flex PCBs are composed of both rigid and flexible board materials. Layers of flexible substrate are attached to rigid boards by internal or external means. Rigid-flex PCB designs offer more options to designers and eliminate the need for costly, unreliable connections between rigid and flexible substrates.

Rigid-flex PCB design requires attention to detail if you want to ensure the integrity of the PCB and the success of subsequent manufactured boards. Here are four design considerations that must be addressed when creating rigid-flex board designs.

1. Stack Management

The stackup of your PCB design should be a precise template for the finished product. Designers must use appropriate software to create rigid-flex layouts that work. If rigid-flex stackups are flawed, inefficiently managed, or improperly communicated to the fabricator, expect delays and issues with your PCB performance.

Use ECAD software and tools that support board-specific stackups when designing rigid-flex PCBs. Region-specific stackups require more changes to fine-tune the board designs. Board-specific tools allow you to easily alter the board outline when designing the stackups rather than making multiple complicated changes.

Use ECAD/MCAD integration tools that save time and offer precise stackups when configuring rigid-flex boards. An ECAD program that supports 3D design is best since you can verify the placement and integrity of bends and other sensitive components.

2. Ground-Plane Integrity

Are you designing dynamic-flex or flex-to-install PCBs? Dynamic-flex PCBs must be able to bend and fold repeatedly whenever a device is used. Flex-to-install PCBs must be bent only when the PCB is placed in the device or product during assembly. Flex-to-install boards are easier to design since ground planes are only mechanically stressed during placement of the PCB.

While ground-plane and overall signal integrity are key concerns with dynamic-flex PCBs, the emphasis should be placed on the reliability of the substrates and other materials used in the PCB versus the signal integrity of the design.

If you plan to use solid copper ground planes to route high-speed circuits in a dynamic-flex PCB, understand that unbroken copper layers on flexible components are subject to cracking and failure.

Gold and nickel plating are also prone to fracture and mechanical stress when used on flexible components. One solution to increase the integrity of ground planes on flexible parts is using annealed copper. However, annealed copper is more costly than standard copper. To reduce costs, place annealed copper only on ground planes where you must have consistent signal integrity.

A second solution is the use of cross-hatched polygon ground planes. The cross-hatch design increases flexibility but does also negatively impact high-speed signals. One workaround for this issue is adding solid return paths below the high-speed traces. Localized solid return paths should be 5 to 10 times wider than your signal traces.

3. Bend Management

Trace routing is only one concern when you carry ground planes or power on the flexible components of a PCB. Surface-mounted pads and through-holes are also at risk in a bend that repeatedly folds. Use additional coverlay to anchor surface-mounted pads. Also, use through-hole plating to increase the strength of your PCB.

Don’t design a rigid-flex board to have its components or vias in close proximity to bends. Constant mechanical stresses can impact both PCB components and vias that are too close to the bend area.

When designing bends, you should avoid the following issues:

  • Increased material thickness at bend location
  • Too-tight bending
  • Any stretching of bend material

Bends that repeatedly compress or stretch are under increased mechanical stress and are more likely to fail than bends that are properly configured.

4. Trace Management

When designing your traces, route them perpendicular to the bend. This routing configuration will reduce the stress on the traces in a rigid-flex design. Another trick is to offset traces on double-sided rigid-flex circuit. When you stagger the traces on the top and bottom of the circuit, the PCB is stronger and stiffer and can withstand repeated bends with greater consistency.

Never bend traces at a 90-degree angle. When traces are bent, the corners of the traces are under more stress than traces designed with straight paths. Design your traces to curve, or use piecewise-linear curves to configure your traces and reduce the chances of delamination.

If you use pads in a rigid-flex design, choose teardrop pads rather than circular pads. Teardrop pads beef up a flexible substrate and make drilling easier. Support any circular or teardrop pads with anchor stubs when there is concern about copper adhesion to a flexible substrate.

When designing your trace layouts, specify that high-speed signal-path impedance should remain constant for the entire length of the path. Your ECAD tool can then alter the width of traces to accommodate the signal on both rigid and flexible components.

Receive more expert rigid-flex design tips for your next important PCB project by contacting Streamline Circuits today. We offer outstanding rigid-flex PCB manufacturing services that include top-notch engineering and technical support for your board designs and their intended applications. Because we were recently acquired by Summit Interconnect, we can now offer even more value to our customers, thanks to our staying on top of industry equipment and trends and our expended services.