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Flexible PCB Design Guide: Key Considerations

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flex PCB design software
Using flex PCB design software
Resource: https://www.youtube.com/watch?v=1N6ekEDXEYg

To ensure the functionality and durability of the final product, the flexible PCB design process involves several crucial considerations. These include the board’s stackup, type of material, and bending radius, among others. Read more about these flexible PCB design rules below.

Flexible PCB Design

Flexible PCB design means the process of creating printed circuit boards that can bend, or a board that you can mold to fit tight spaces and conform to unique shapes. This starts with making a schematic diagram of the circuit design or, in other words, a flex PCB layout. To do that, engineers use specialized software tools, as explained below.

Flexible PCB Design Software

This is a special type of software that allows engineers to produce a detailed layout of the flex PCB, while taking into account the unique constraints of the flexible substrate material.

Essentially, the flexible PCB software offers features such as routing options, bend area constraints and, most importantly, a 3D visualization of the PCB.

Flex PCB Layout

The flex PCB layout is a representation of the arrangement of electronic components, traces, and other features on a PCB. This is a crucial step in the design and manufacturing of electronic circuits as it determines the electrical and mechanical properties of the final product.

It also guides for the placement of components, as well as the routing of traces. This helps avoid problems later. Let’s now dive into the rules or, in other words, the flexible circuit board design guide.

Flexible circuit board design process
Flexible circuit board design process
Resource: https://www.youtube.com/watch?v=uzK0_C1NWU4

Flexible PCB Design Guidelines

When designing flex PCBs, engineers must observe best practices. This is crucial to ensure a reliable product that performs optimally, as these PCB types have unique design needs. The flexible PCB design rules include the considerations for board layers, size, vias, bend radius, and impedance.

1. Flex PCB Layers

Also called the PCB stackup, this refers to the arrangement of the various layers of the flexible PCB. The number and arrangement of layers depend on the complexity of the circuit design and the required electrical and mechanical properties.

Flex PCB layers are optimized for the specific application, usually based on whether the board is required for dynamic or static usage situations. Generally, thinnest pcb flex more than thicker ones. Flexible PCB design guidelines when it comes to the number of layers include the following:

  • Ensure a balanced stackup by using even number of layers
  • Choose materials based on the required mechanical flexibility, and the application needs.
  • Determine the thickness individual flex PCB layers with regard to the required impedance

2. Flex PCB Board Size

This is an important consideration during the flexible PCB design process because, while flexible PCBs offer the advantage of being able to bend and twist, their size can impact their flexibility and durability.

For example, a wearable device may require a small PCB, a larger electronic device a large board. Additionally, the size of the PCB may impact the number and placement of components. When determining the flex PCB size, these factors are considered:

  • Circuit complexity: The PCB should be large enough to accommodate the required number of components and traces without overcrowding.
  • Electrical performance: The flex PCB board size can affect its electrical performance, and must be optimized for the desired electrical characteristics.
  • Thermal management: The required thermal performance should inform the PCB size during the design stage.

3. Flexible PCB Vias

PCB vias mean the small holes that are drilled in the board, plated with a conductive material (such as copper), and used to connect different layers of the board together. A flexible PCB via creates a continuous electrical path between the different layers.

The size and placement of vias is a critical consideration in when designating a flexible circuit board, as they can affect the electrical performance of the board. The following flex PCB guidelines for vias will, therefore, usually apply:

  • Avoid placing flex PCB vias near the edges of stiffeners as that can increase the risk of cracking, or in the bending regions.
  • Consider using teardrop vias to reduce stress points that could damage the board.
  • Use large annular rings around vias to improve transmission between the vias and copper traces.

4. Flex PCB Bend Radius

Flexible PCBs are designed to bend and flex, and the bending radius is a crucial factor to ensure the board’s functionality and durability. The flex PCB bend radius is the extent or degree by which the board can safely bend.

This is mainly determined by the thickness and stiffness of the materials used as well as the number of layers and the location of any rigid areas (if any). The design guidelines when it comes to flex PCB bending radius are:

  • Avoid the use of sharp (90-degree) bends and instead make use gradual curves
  • Use of cutouts of the PCB can also help reduce the bend radius
  • Reduce the thickness of copper traces, too, so the board can flex better

5. Flex PCB Impendence

When it comes to the flex PCB impedance, its proper control is crucial for ensuring that the signals transmitted through the PCB remain within the required limits. But this may also increase the board’s thickness, especially if higher current carrying capacity is also a required, and reduces its bendability.

Engineers must, therefore, use proper measures to ensure optimal signal quality for the finished board without affecting the flexibility of the PCB. The following are some key requirements and considerations for impedance control during a flex PCB design stage.

  • Use the surface micro-strip design to ensure a thin PCB when flexibility is paramount
  • If flexibility is not a critical factor, the strip-line configuration is a more viable option
  • Adhesive-less flex PCB is preferred if designing a high-speed circuit board
  • Consider using a more uniform flex PCB material, such as polyimide, which also offers lower DK values.

Conclusion

Before manufacturing the actual flex PCB, a lot goes on during the design stage. Overall, proper flex PCB design requires careful consideration of various factors, including the board stackup and material or bending radius and impedance. By taking these factors into account, engineers can create flexible PCBs that meet the required specifications and provide reliable performance over time.

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