How to Design the Perfect Flex Circuit

Flex circuits were created to fill the market’s need for compact devices. Flex circuits allow for 3D designing of a circuit, saving a lot of space in the process. They also let the designer create devices without worrying too much about PCB assembly, since Flex circuits can be designed and shaped to fit a particular device. Flex circuits also provide high reliability despite repeated installations, harsh, and high vibration environments.

Its flexibility of structure shouldn’t fool you into thinking that it’s design will be just as flexible. Usually, designing a Flex circuit is more complex than designing a conventional Rigid PCB. You have to consider the mechanical implications of your design as much as electrical. Dimensions, choice of the conductive and base material, flex requirements, fabricator’s guidelines, and arrangement of components are just a few considerations for designing the perfect Flex circuit.

Designing a Flex Circuit

Designing a Flex circuit is different from the start. Unlike a conventional rigid board, you have to think in 3D while designing a Flex circuit. The right software will allow you to design considering the circuit’s installation in the device, the dimensions and bends you have in the Flex circuit. But like everything else, a good design starts with the right materials.

Materials for a Flex Circuit

The most important material in a Flex circuit is its substrate. This is what allows the circuit its flexibility. Most commonly used substrate for flex circuit is polyamide film. It’s preferred because of its relatively better electrical, mechanical, and thermal properties. Unless your design requires choosing something else, it’s the most cost-effective and easy way to incorporate substrate. It can be adhesive-less or adhesive. Ideally, adhesive-less is better since it reduces the thickness of the Flex circuit.
For conduction, copper is typically used. Normally, copper traces are kept as thin as possible, for improved flexibility, and preventing possible fractures in the copper lines. Instead of using a thicker layer, using a wider copper layer is better for consistent conduction. Manufacturers allow copper layers ranging from 18 micrometer to 140 micrometer in thickness. Copper can come from rolled or annealed foils, as well as electrodeposited copper for thinner foils.

3D Considerations of Design

If you are having trouble realizing the dimensions directly in the software, it is prudent to choose the more conventional approach of using paper templates. By mimicking the actual circuit design (Keeping in mind the Flex limitations and guidelines from your fabricator), you will know how the circuit can be folded and you can calculate the most optimized dimensions and fit.
Another factor to keep in mind is the heat dissipation. It’s different in Flex circuits compared to Rigid circuits.

Bend or Flex of the Circuit

This is the most important part of the Flex design. And it’s different for dynamic flex and static flex. For dynamic flex, the circuit that will be repeatedly bent, single, or at most, two flex layers are recommended. A Flex circuit with more layers is more suited for static Flex, a circuit that will only bend at the time of installation. The bend radius depends upon the number of layers.
For single layer, the radius shouldn’t be less than six times the circuit thickness, 10 times for double side Flex, and 15 times the thickness for multilayer Flex PCB. For dynamic Flex, the bend radius shouldn’t be less than 25 times. Sharp bends should always be avoided.

Bends of 90 degrees or more cause serious distress in the copper lines. If a bend of such angle is necessary, it’s important that it’s made only once, at the time of installation.

Annealed or rolled copper is preferable at the bend area, compared to electroplated copper. The later is more prone to cause fracture lines when bent. Vias in bend areas should also be avoided. A via’s presence in a bending area weakens the whole surroundings around it. IPC doesn’t recommend any components on the bend area as well, but some manufacturers allow components in bends over 100mm. This is only for static Flex.

Conductor width should be uniform throughout the flex circuit and it shouldn’t be increased near the bend areas. It’s important to identify the neutral bend axis. A neutral bend axis is an area that sees no external forces because of the bend in the circuit. Conductor and material thickness, as well as weight, should be balanced around the neutral bend line.

Other felx circuit Design Considerations

Even though it’s a smart way to minimize Electromagnetic interferences, but conductors should run on top of each other in multilayer Flexes. This increases the flex thickness, and reduces its bending capabilities. It’s an example of how mechanical and electrical consideration may clash while designing a Flex circuit. Conductor lines should always be staggered from layer to layer.

A major mechanical design consideration is that there are no sharp 90 degree points in the Flex body, as they are major tear starting points. And once a tear starts, it’s likely to propagate in a Flex circuit. Tear shaped stops or arc shaped reliefs in the corners are always preferable over sharp right angles.

Consult your fabricator’s design guide about stuff like bonding sheet design, conductor thickness, raw material tolerance and coverlay, and solder resistive formative dimensions. Designing of pads larger than access holes is also recommended, but if you can’t compromise on space than use hold-down tabs. Pads fillets are also recommended to provide structural relief.

Conclusion

It is important to consider that the boundaries of this technology are still being pushed. We are already past most hindrance in Flex design, and we are still pushing forward. If you want something beyond the normal guidelines and practices in Flex design, we might be able to accommodate you. This is one of the reasons why it’s always recommended to consult your fabricator before finalizing your design.

A flex circuit is supposed to be more reliable than a Rigid PCB. But its reliability depends mostly on good design, with all mechanical, electrical and electromagnetic characteristics kept in mind. This is especially important when you work with multilayer flex. With each added layer, you will have to be more careful about the design and fabrication steps. We offer unparalleled services in this regard. We have years of experience in fabricating and improving flex designs, and can easily work with as many flex layers as you can throw at us.

We can accommodate much more extensive flex lengths then market standards. And even with a higher number of layers and extended lengths, you don’t need to worry about heat dissipation or impedance controlling of the circuit. We are equipped to deal with any level of sophistication in flex circuit design, are pride ourselves in long-life, durable, and rugged flexes for a wide variety of applications.