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Flex Circuit Design Rules

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • Flex PCBs can be easily bent without breaking. 

  • Flex PCBs can be classified into rigid-flex PCBs and high-density interconnect (HDI) rigid-flex PCBs. 

  • The number of layers of flex PCBs vary between 1 and 6. It is preferable to limit the number of layers to a max of 2 for greater cost benefits and mechanical flexibility.

Flex Circuit Design Rules

Flex circuit design rules are different from rigid PCB design rules

Whenever board design needs to be flexible and capable of withstanding vibrations, flex circuit design rules are applied by PCB designers. In addition to bendability and vibration resistance, flexible PCBs are helpful for saving space. Lightweight flex PCBs improve air circulation and thermal performance and reduce manufacturing costs.

Basically, there are two types of PCB boards based on their flexibility: rigid boards and flexible boards. For certain applications, installing a rigid PCB is tough. Bringing the rigid PCB to the exact position required might even become impossible when there are space constraints and access difficulties. In such circumstances, flexibility is required.

Flexible PCBs can be easily bent without breaking and are more easy to manuever into place. Flexible boards are made out of thin, flexible materials that offer high tensile strength. Usually, flexible PCBs are employed when electronic circuits are required in confined areas or movable sections of a product. They are preferred when designs are required to be lightweight and small while maintaining the same performance, efficiency, and reliability. 

Flex PCB Classifications

Flex PCBs can be classified into rigid-flex PCBs and high-density interconnect (HDI) rigid-flex PCBs.

Rigid-Flex PCBs

High-Density Interconnect Rigid-Flex PCBs

When the PCB is made of both rigid and flexible materials, it forms rigid-flex PCBs. They are extremely bendable and foldable to any shape.

Characterized by closer spaces, a higher number of layers, and denser wiring. Offers faster connections at a reduced size.

Flex PCB Advantages

Improved Reliability

It is possible to customize the routing path. By doing so, the routing path becomes more dependable due to the reduction in integrity issues. Reliability can be enhanced with flex PCBs.

Withstands High Temperatures and Vibrations

The circuits used in aerospace, avionics, medical, and military applications have to withstand high temperatures and vibrations. Circuits need to withstand high strain without breakage or loss of strength. Considering the temperature and vibration aspects, flex PCBs endure both and are ideal for harsh environments.

Saves Costs and Space

As flex circuits cut down the amounts of wires and cables, this results in space saving and weight reduction. Flex circuits offer seamless integration of various designs into a single compact circuit. Assembly time and cost is therefore reduced.

Flex Circuit Design Steps

The following flow chart introduces the steps involved in designing a flex PCB. 

Flex circuit design steps

Flex Circuit Design Rules

1

The number of layers of flex PCBs vary between 1 and 6. It is preferable to limit the number of layers to a max of 2 for greater cost benefits and mechanical flexibility.

2

Within the flexible area of the rigid-flex PCB, the trace width and spacing should be maintained as wide as possible.

3

The solder pads and tracks need to be connected in a round or teardrop-like shape.

4

Make the soldering surfaces and annular rings as large as possible.

Standards Regulating Flex Circuit Design Rules

Flex circuit design rules are mainly based on IPC standards: IPC 2223, IPC-FC-234, IPC-6013, and IPC-600 set the acceptability criteria and rules for designing flex circuit boards. Cadence’s OrCAD software offers PCB design tools that ensure flex circuit boards meet the design rules.

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