The Importance of Rigid-Flex PCB Design Guidelines

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When designing a printed circuit board, reliability should be at the top of your requirements. Rigid-flex PCBs are the go-to option for various electronic applications because they use the best of both flexible and rigid board technologies.

 

A rigid flexible circuit board is designed through a selective layer stack-up of flexible and rigid PCBs plated through holes to establish a connection between layers. This eliminates the need for interconnecting cables and connectors.

 

Furthermore, rigid-flex printed circuit boards simplify the assembly process because you only require one simplified PCB assembly. This allows circuit designers to customize the PCB to odd enclosures and provide unique form factors that are impossible without rigid-flex PCBs.

Why are Rigid-Flex PCB Design Guidelines so Important?

But all these advantages also bring about various challenges for circuit board designers. Designers must understand the compromise between rigidity and flexibility to ensure they don’t lower quality standards. To this end, not only do designers have to deal with the unique mechanical dynamics of the PCB, but they also need to consider the electrical aspects of the board.

 

Moreover, routing between multiple boards on a rigid-flex PCB spreads out the components over a larger area. This means that the enclosure needs to be based on the mechanical shape of the board. The device's form factor requires designers to consider whether the flex elements will be static, creased, or dynamic. Each option comes with its own challenges.

 

This is why it is important to understand and follow tried rigid flex PCB board design guidelines.

The Most Important Factors When Designing Rigid-Flex PCB

When designing rigid-flex PCBs, you’ll have to consider various factors such as:

  • The layer count
  • Detailed analysis of the material layup
  • Heat sinks to dissipate heat-buildup

Important Guidelines for Designing Rigid-Flex PCB

If you decide to switch to rigid-flex PCBs, consider the following guidelines for best results.

Avid Plates Thru Holes on the Bending Areas

Avoid placing vias and pads on the bending area of the flex part of the PCB. This is because specific areas on the bending line could stress the structure of the plated holes. Although you can place vias and pads on areas of the board that don’t have bends, it is not recommended. A better alternative is to use anchors to strengthen the holes. You can create a stronger joint with the help of a teardrop and establish a connection between the trace the plated hole.

Bending Lines Must be Kept Straight and Perpendicular

The terraces across the bending line need to be kept as straight and perpendicular as possible. A better alternative is to use narrow traces that are evenly spread across the flexible area. Consider adding dummy traces to increase the mechanical sturdiness of the board and protect the traces from breaking.

 

In the case of a double layer design, traces can be routed alternatively on the top and bottom layers. Avoid making 90 or 45-degree angles with the traces. A better idea is to use curves when changing the direction of the trace.

Implement PCB Design Software

It is highly recommended to use your design software to keep track of all the activity under the surface of the rigid-flex PCB. You will need software that lets you define a full layer stack up and the link between layers in a rigid flex circuit board. Your design tools should be compatible across all materials, including FR4, ceramic, and MCPCB to help you customize everything from your layer stack to board footprint.

 

The best PCB design software comes with a full layer stack manager that gives you a graphical representation of the board’s layer arrangement and the links between the boards.

Consider the Layer Count

Rigid-flex PCBs are designed with alternating layers of rigid and flexible PCB materials. Make sure to determine how many layers you will need for your specific purpose. Next, get in touch with your manufacturer to ensure that the board can meet your specific requirements.

Heat Sinks

The flow of electricity through the circuit creates heat. The amount of heat generated depends on various factors such as power, PCB  design, and device characteristics. Excessive heat can severely affect the device’s performance and may even result in damage. This is why you should take measures to reduce the amount of heat dissipated.

 

The rigid-flex PCB should sue heat sinking to dissipate the heat to the surroundings. Also, allow for airflow within the enclosure to make the heat sink more effective.

Layup of Materials

The material layup is an important factor when choosing rigid-flex boards. It is important to contact your board’s manufacturer and discuss various factors, including:

  • Materials
  • Bend radii
  • Mechanical stress
  • Impedance control
  • UL flammability rating

The use of materials can have a major impact on your device's performance, cost, and assembly. Do your research to find the best layup of materials.

 

Although it is possible to use boards with varying layers, make sure they are similar in thickness and observe rigid-flex design PCB rules. This will minimize any manufacturing problems from derailing the project.

Work With Your Supplier

Each supplier may have its own manufacturing process for rigid-flex design. Their choices will affect various factors such as line spacing and circuit identity. Ask your PCB manufacturer to provide you with as many details as possible, including material selection. In addition to this, the material should be well suited to the application in which the rigid-flex PCB will operate.

Testing

While prototyping goes a long way in helping you course correct any problems with the PCB, the process is error-prone and requires too much trial and error. A better idea is to make rigid-flex paper doll models to help you get things right the first time.

Wrapping Up

Both flexible and rigid PCBs have their pros and cons. But you can get the most out of both design technologies by switching to rigid-flex PCBs. the benefits of rigid-flex PCBs include:

  • Affordability
  • Lower weight
  • Easier assembly
  • Increased reliability
  • Higher circuit density

 

Hemeixin is an experienced PCB designer with years of expertise in rigid-flex manufacturing. Get in touch with us for more information on how we can help you with your next design.

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