Rigid Flex PCB Design Guidelines and Rules

A rigid-flex PCB design offers various benefits, such as weight and space savings. However, it is a complex board that requires more effort than a traditional rigid type. It has special considerations, especially in the transition and bending areas, or it will not meet the desired requirements. So, how do you avoid the pitfalls? This comprehensive design guide highlights the dos and don’ts.

What is Rigid-Flex PCB?

A flex-rigid PCB is a circuit board with bendable or foldable parts. That way, you have one section of the design that is bendable and another that is rigid. One of the main reasons for implementing the design is due to space constraints and the lack of room for components like connectors or cables.

In cases where weight is a concern, the bending sections help reduce mass by replacing bulky connection components. The design also allows the circuit to withstand shock and vibrations, making it applicable in harsh operating environments.

Many electronic products today require rigid-flex circuits. They include the hinging section of laptops, the moving ribbons of printers, and compact applications like wearable tech devices and communication gadgets.

Rigid Flex PCB Design

Implementing a flex and rigid-flex PCB requires a careful approach at every process stage. The aim is to meet the final product’s form fit and function, which differ from that of a stiff board in a few ways.

For instance, the bendable circuit parts must bend without breaking, sometimes even thousands of times in the product’s lifetime. There are also the size and weight requirements to balance and a lot more.

Its design creation involves a few significant steps: stack-up creation, component placement, and routing. If well-designed, rigid-flex PCBs offer excellent reliability and durability in their end-use product or system.

In other words, the board will have a high potential for return on investment (ROI). With that said, how do you ensure you’re doing it right when creating it? In the following section, we delve into its essentials.

Rigid Flex PCB Design Guidelines

Given its complex construction, a few rules apply when designing a rigid and flexible printed circuit board. The rules range from the materials to use to the number of layers, bend angle, stiffener type, routing considerations, and via placement considerations.

Material Selection

Choose your materials based on the board’s electrical and mechanical requirements. The stiff sections are typically FR4, while the foldable parts are a polyimide film. For stiffeners, you may use FR4 or thicken the flexible layer. If needing greater strength, use metals such as aluminum and steel.

Depending on whether the flex sections are dynamic or static, you may opt for rolled-annealed or electrically-deposited copper traces. The copper must not exceed 2 ounces per square foot for higher reliability.

Stackup Layers

A rigid-flex PCB stack-up has two layers: the stiff and bendable parts. The designer must determine the correct number of layers in these sections based on the requirements of each.

The count in the stiff sections will depend on the board’s complexity and routing requirements. On the other hand, the number will depend on the required flexibility levels.

Use an even number of layers to improve the stack-up performance. It’s also necessary that the flexing board be inserted in the middle to prevent it from detaching. To increase flexibility, consider reducing the number of layers in the bending circuits.

Bend Ratio and Radius

Do not let the bending section bend more than it should. A tighter bend (small radius) will stress it more and increase the chances of damage. Likewise, a thick layer will flex less and, if subjected to a dynamic application, have a higher chance of breaking.

To determine the bend radius, base it on the thickness of the board. The recommended bend ratio for single-layer boards is 10:1 for static applications and 100:1 for dynamic flexing.

If the board is two-sided or double-layer, use a 10:1 ratio for static bending and 150:1 for dynamic flexibility needs. Multilayered boards do not suit repeated bending during use. They need a 20:1 ratio for static applications.

Use of Stiffeners

Do you need the bendable part stiff for mechanical strength or more flexible to achieve higher bendability levels? A dynamic board must be thin and flexible, bending repeatedly without breaking.

Use stiffeners if you must include components in the flex board for support or if you want to make it more robust for installation. Using a stiffener is less costly than using a fully stiff FR4 board.

Routing Traces

Rigid-flex PCB design rules mainly concern the flexible layers and how to route them for optimum performance and reliability. The traces should be able to bend without breaking. This requirement calls for thin, narrow traces.

When routing copper traces to vias or through holes, use a teardrop design. It will help reduce stress points. Keep them straight and perpendicular to the bend line to avoid twisting forces when the board flexes.

Via Design and Placement

Rigid and flex PCB circuit boards have special requirements for via placement. It would be best if you did not design them on the edges of the stiff sections. Instead, maintain a distance of 50 mils minimum.

More importantly, avoid any vias in the bendable sections. If you must use them, reduce the count. The holes can cause the flexing part to weaken and can lead to early board failure.

Challenges When Designing Rigid Flex Circuit Boards

Despite their necessity and advantages, rigid-flex PCBs are more challenging to design than other boards. The designer encounters various complexities that require different approaches and careful considerations, or the board may need to be revised.

One of the main challenges designers face is ensuring a seamless transition where the flexible part joins the stiff board, especially if you’re designing a high-speed board. The combination can lead to mechanical instability or electrical performance problems.

Ensuring the board will fit its enclosure while meeting the mechanical requirements of the application environment is another major challenge. It calls for a deeper analysis of the material properties and stack-up thickness.

Given its complexity and fragility, the board design costs more time and money. In addition to using costly materials, design errors can lead to expensive fabrication errors.

Rigid Flex PCB Design Optimization Tips

When creating a PCB design for rigid-flex boards, the aim is to optimize it for fabrication and usability in the intended product. These tips can help you ensure that.

• To start with, your CAD software should have design tools that allow you to create a representation of the board, including simulating it.
  
• Do not use sharp corners to place the copper conductors (traces) on the bendable regions. Instead, curve them to reduce stress when the ribbon bends.
  
• Avoid having components on the flex parts of the board. They will weaken and make it less reliable. Do not drill holes, too. If you should, use anchoring to make them more robust.
  
• Lastly, ensure your fabricator can manufacture your rigid-flex design, especially if it’s multilayered and complex.
  

Conclusion

A rigid-flex PCB design is more complex, requiring specialized software and hours of effort to create. However, with a careful approach, the designer can produce a board that will achieve high performance, reliability, and durability in the application environment. The tips in this guide should help you start with its design and fabrication steps.