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Rigid Flex PCB Design: 5 Key Considerations

The rigid flex PCB design process requires a careful consideration of several key factors. These include industry rules, size and shape constraints, board stackup requirements, and possible design optimization solutions. The aim of this guide is to shed light on these considerations and help you determine the best way to design a rigid flex PCB.

Rigid Flex PCB Design

Rigid flex PCB design is the process of creating a printed circuit board that combines the rigidity of a standard rigid PCB with the flexibility of an interconnecting flex circuit. This combination allows for more efficient use of space and easier connections. It also reduces PCB weight when compared to traditional rigid designs.

During the design process, engineers use software tools to create a model of the PCB. This model is used to represent the board stack up, size and shape requirements, and all other details necessary for a successful rigid flex PCB design.

Given their unique features, designing rigid flex PCBs requires adherence to industry rules and other requirements. Below, we take you through the main factors when it comes to PCB design for rigid flex boards.

Rigid flex PCB layer stackup
Rigid flex PCB layer stackup
Resource: https://www.researchgate.ne

Rigid Flex PCB Design Guidelines

How do you ensure a successful rigid flex PCB design? While specific design rules can vary depending on the application, some general guidelines apply and should be followed. These include rules for the rigid flex PCB materials, stackup, bend radius, impedance, and via features.

Rigid Flex PCB Material

Rigid flex PCB materials include materials for the rigid and flexible parts. Additionally, there are material options for each part. According to IPC standards, the materials must meet certain requirements. Generally, the considerations during material selection are as follows:

The choice of rigid flex PCB material depends on the application, cost and performance needs. For the rigid parts, typically FR4 and polyimide are used. These materials provide excellent insulation, thermal resistance and flame retardancy.

For flexible parts, the popular choice is polyimide or Kapton film owing to the materials’ very low Dk and CTE values. Additionally, they provide good thermal stability and chemical resistance.

Rigid Flex PCB Stackup

A PCB stackup refers to the way in which the board layers are arranged. When designing a rigid flex PCB stackup, engineers must take into account the number of layers, the type of materials for each layer, and other design elements that are necessary for a successful circuit board.

One of the rules when it comes to a rigid flex PCB stackup is to optimize it so it matches the application requirements. This includes ensuring the correct board thickness for the different sections, number of layers, and the type of materials.

The rigid flexPCB thickness determines its bendability in the flexible sections and strength in the rigid sections. For successful performance, the thickness must meet certain requirements. Generally, it’s not recommended to have more than 2 layers if the flexible section is to bent repeatedly during use.

Rigid Flex PCB Bend Radius

The bend radius is a critical factor when it comes to rigid flex PCB design. It determines how much space is required for bending the flexible sections of the board. It also dictates the board’s suitability for either static or dynamic flexing applications.

To ensure successful performance, the rigid flex bend radius must remain within certain limits, usually by using a ratio of the bend radius to the thickness of the board. The bend ration is gotten by dividing the bend radius by the board thickness.

Depending on the application, your manufacturer should be able to provide a minimum and maximum ratio for the rigid flex PCB bend radius.

  • For 1 layer rigid flex PCB sections, a bend ratio of 10:1 for static applications and 100:1 dynamic applications is recommended.
  • For 2-layer boards, a 10:1 ratio applies to static board applications, and 150:1 for dynamic usage situations.
  • If using a multiple layers in the flexible parts, a 20:1 bend ratio is used for static applications. Owing to the increased thickness, this type of board should not be used for dynamic application PCBs.

Rigid Flex PCB Impedance

Impedance is an important factor during the rigid flex PCB design process. It refers to the opposition of a circuit or component to alternating current. High impedance in flexible circuitry can lead to signal reflections, loss of signal integrity, and other issues.

For optimal performance and reliability of high-frequency circuits, engineers must ensure controlled impedance at the design level. To control rigid flex PCB impedance, the factors that impact it must be taken into account. These include trace width, dielectric material thickness, and copper thickness or weight.

Engineers can use the design software to automatically calculate the exact impedance of a rigid flex circuit. This will ensure that the impedance is properly matched in the circuit, leading to improved performance and signal integrity.

Rigid Flex PCB Vias

Vias are metalized holes that connect different layers of a board. During the rigid flex PCB design process, it’s important to follow certain rules when adding vias, as these can affect both the performance and durability of the finished product.

  • Avoid using vias in the flexible regions of a rigid flex PCB – or use them sparingly
  • Use teardrops to route copper traces to a via. Teardrop shapes provide additional strength and reduce stress
  • In the rigid sections, avoid placing vias near the edge. Instead ensure a distance of at least 50 mils

With the right combination of materials, stackup, bend radius, impedance and vias, when carrying out PCB design for rigid flex boards, engineers can create boards that are optimized for their application requirements and have superior reliability, flexibility, and thermal performance.

Conclusion

Rigid flex PCB design must take into account several factors to ensure the board is suitable for its intended application, given the product’s unique construction and requirements. This includes design rules based on the thickness and bend radius, – as well as impedance and vias.

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