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Rigid Flex PCB: how to choose the Design tool

Benefits of Rigid-Flex PCBs, challenges, and criteria to choose the right software tool

What is a Rigid Flex PCB

Rigid Flex PCBs are printed circuit boards that are flexible (flex) or flex and rigid at the same time. They consist of a combination of rigid and flexible boards without the need for physical connectors. The design is composed of multiple layers of flexible circuit substrates attached internally and/or externally to one or more rigid boards. By combining the advantages of the two technologies, and eliminating the physical connectors, designers have more options when working with dense designs that must conform to a specific form factor.

Rigid Flex PCB

Benefits of Rigid Flex PCB

The benefits of Rigid Flex PCB technology include some that are obvious and others that are less obvious.

The obvious benefits include:

  • Offering more options when working with dense designs that must conform to a specific form factor
  • Reducing cost and increasing reliability through the elimination of the physical connectors used in the traditional “design-separately-then-assemble” approach
  • Reducing space requirements as parts can be placed, and traces can be routed, in three dimensions.

The less obvious benefits include:

  • Improving signal integrity through the elimination of cross-sectional changes to the conductors when the physical connectors and their associated solder connections are removed
  • Improving electromechanical functionality. This includes dynamic bending, vibration and shock tolerance, heat resistance, and weight reduction. The fact is Rigid Flex designs provide the mechanical stability that is required by most applications

The current generation of Rigid Flex designs is well suited for many of today's modern products, especially in the consumer electronics space. Rigid Flex is a truly enabling technology that lets product development teams cost-efficiently apply greater functionality to a smaller volume of space.

You can find Rigid Flex designs in mobile phones, televisions, digital cameras, and laptops, but also in wearable devices, which are leveraging the benefits of Rigid Flex technology. The bottom line is whenever a product needs to be compact and/or lightweight and/or flexible, then Rigid Flex technology will most likely be applied.

Rigid Flex PCB benefits

Rigid Flex PCB Challenges

Rigid Flex design software must enable the engineer to meet certain design challenges that are typical of this technology. One of these challenges is the management of the stackups for the PCB.

Stackup management

Some ECAD tools do not support multiple stackups within a single design, as such a “master stackup” must be created that satisfies all of the requirements for all of the PCB’s and then the actual stackup requirements must be manually conveyed to the fabricator, usually as notes within the fabrication drawing. This is of course inefficient and potentially confusing, both during design and during fabrication. A Flex PCB Design Software tool that supports region-specific stackups (not ideal) or board-specific stackups (preferred) will help simplify this complex task.

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Board outline management

The multiple boards in a rigid-flex design need to be properly configured and managed throughout the design process. Rigid-flex designs are in fact electromechanical projects that require collaboration between the electrical and mechanical domains. The ability to import and automatically create the multiple and potentially complex board outlines from mechanical CAD data through the import of DXF data or utilities that provide mechanical integration will save time and reduce the potential for errors.

Signal and power integrity analysis

Signal integrity and power integrity analysis can also be a challenge during rigid-flex design. Most signal integrity and power integrity tools assume a single PCB with a uniform stackup. As previously seen, the stackups for the rigid and flex PCBs will vary in nearly every rigid-flex design. As such, for rigid-flex designs, the analysis tool(s) must recognize flex-specific layers and local stackups in order to ensure correct analysis results.

Bend area management

All rigid-flex designs include one or more bendable regions. The bendable regions are critical to the form, fit, and function of the final product. The Flex PCB Design Software tool being used during rigid-flex design must be able to manage critical bend area items such as:

  • The bend location
  • How the bend is formed including the radius, angle, and origin
  • And the bend-specific rules for placement, routing, vias, and planes

Design, visualization, and verification

The last challenge is the ability to properly design, visualize and verify the rigid-flex system. The ability to define the bend parameters, edit the design in 3D, view the design in the context of the enclosure in 3D, and perform 3D rigid-flex aware design rule checks (DRC), cannot be overstated. 3D design, visualization, and verification ensure that the design team is taking advantage of open real estate in all three dimensions while at the same time, identifying potential design issues.

Rigid Flex PCB Design

Rigid Flex PCB Design for Manufacturing (DFM)

The flex circuit’s thinness makes them more delicate, and they are more susceptible to accidental damage during manufacturing. This negatively affects the yield, increasing the per-item cost. Running concurrent DFM is even more important with flexible circuitry than for rigid PCBs.

Flexible circuits are typically built using a combination of material types and methods. These various materials present a challenge for manufacturing. When combined with typical FR4 PCB materials, the complexity increases. The greatest challenge is posed by differences in the thermal expansion characteristics of the various materials. These characteristics need to be addressed in DFM reviews.

Designs that have both rigid and flexible circuitry contain all circuit construction elements; separate rigid circuits are connected by flex circuits to form a single assembly, eliminating the possibility of mating and signal routing errors. While rigid flex can be more costly, in actuality, they may be more cost-effective and increase yield with a reduction of touch steps during assembly.

Rigid Flex PCB DFM
Interface region issues found thanks to Design for Manufacturing: trace width changes, via proximity and trace angle changes.

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