The Importance of Rigid-Flex PCB Design Guidelines

Key Takeaways

• Learn what rigid-flex PCBs are.

• Find out why material considerations are so important for rigid-flex designs.

• Explore design guidelines for rigid-flex PCBs.

A rigid-flex PCB is space-saving and shock-resistant

I have the tendency to try to replicate the delicacies I’ve ordered at restaurants in my own kitchen. One of my latest attempts at creating restaurant-worthy dishes was a Korean pancake that’s crispy on the outside but soft on the inside. With my amateur cooking skills, it proved to be an impossible task—I could either make only a hard pancake or one that was total fluff. 

While I’m still struggling to figure out the trick to bringing together the different textures of a Korean pancake, I’ve had more success in bringing together the hardboard elements and flexible PCB elements of a rigid-flex PCB. Compared to making Korean pancakes, striking the right balance of flexibility and rigidity on a rigid-flex PCB is easy if you abide by rigid-flex PCB design guidelines. 

What Is a Rigid-Flex PCB?

For those who have spent their careers designing conventional PCBs on the Fr4 substrate, a rigid-flex PCB may be unfamiliar territory. As the name implies, a rigid-flex PCB is a PCB that combines both elements of a hardboard and a flexible PCB in a single piece. 

A rigid-flex design is typically made up of two or more rigid areas that are interconnected by a flexible strip. The synergistic property of the rigid and flex elements allow the PCB to be bent or folded in applications. 

Rigid-flex PCBs are getting more attention in recent years, due to the demand for more compact, shock-resistant, and robust electronics. A rigid-flex design eliminates the need for connectors and interconnecting cables. Rigid-flex PCBS are also easier to install, as the entire design is manufactured on a single PCB. 

Material Considerations for Rigid-Flex PCB Design

Talk to your manufacturer to ensure you select the right material for your rigid-flex design

You’ll want to consult your PCB manufacturer before starting a design. Depending on whether the PCB is meant for dynamic bend or stable bend, the choice of copper type, the number of layers, bend radius, and coverlays may differ.

A dynamic bend rigid-flex PCB is installed in an environment where it will constantly be subjected to bending. Therefore, it is recommended to use no more than 2 layers and ensure that the bending radius is at least 100 times the material thickness. 

Meanwhile, it’s possible to have up to 10 to 20 layers for a rigid-flex PCB that’s meant for stable-bend installations. It is not subjected to repeated bending force and that means a smaller bending radius of about 10 times its material thickness is also possible. 

Rigid-Flex PCB Design Guidelines

Use curved corners and teardrops for the flex area of a rigid-flex PCB

For all the advantages of a rigid-flex PCB, they also bring about challenges for PCB designers. Not only do designers need to deal with the electrical aspects of a design, but they also need to consider the mechanical dynamics of the PCB. 

If you happen to be working on a rigid-flex PCB, the following guidelines will save you from committing costly mistakes. 

Avoid Plated Through Holes on the Bending Areas

Avoid placing pads and vias on the bending area of the flex part of a PCB. Areas near the bending line will deliver mechanical stress that could jeopardize the structure of the plated holes.

Pads and vias can be placed on areas that are not subjected to bending, although it’s not recommended. In such cases, use anchors to strengthen the plated holes. Additionally, use a teardrop to connect the trace to the plated hole for stronger joints. It’s also a good practice to use larger pads and vias. 

Pay Attention to Routing Across the Bending Area

Traces across the bending line should be kept as straight, perpendicular lines. It’s better to use narrower traces that are spread evenly across the flexible area. Adding in dummy traces can help to increase mechanical sturdiness that protects the traces from breaking. For a double layer design, traces should be routed alternately on both the top and bottom layer.

Avoid making any corners with the traces on the bending area. If the traces need to change direction on a flex PCB, use curves rather than sharp 45° or 90° corners. 

Use a Cross-Hatched Ground Plane

If you’re pouring the ground plane as a solid area of copper, you’ll risk putting a huge amount of stress on it and reducing its flexibility. Instead, use a cross-hatched ground plane on the flex area of the PCB.

It goes without saying that using a PCB design software that supports rigid-flex PCB design guidelines helps in creating an error-free prototype. OrCAD enables stack-up by zones and inter-layer checks, which are handy in rigid-flex design. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.