Flex PCB Designing for Flexibility and Reliability

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Flex circuits meet the needs of modern day electronics and support the on-going miniaturization of devices. They are light, compact, and if designed properly, extremely flexible reliable. Unlike rigid circuits, flex PCBs have specific requirements because they bend. Designers must consider all aspects of the circuit including material selection, via placement, PCB architecture, and the number of layers.

Other factors that must be considered include the degree to which the circuit will bend, the radius and angle of the bend, how the bend is to be formed, and how frequently the PCB will bend.    

By carefully strategizing the design and application priorities, the designer can build a truly reliable and flexible PCB while realizing the full potential of their device.

Flexible circuits material selection

Material selection must take into account the flexibility of the circuit. It is recommended to choose copper that is thinner than 2 OZ and under.

If the circuit requires more thickness than the copper can provide, the designer has a few choices:  

  • Order special materials with thicker copper
  • Increase the thickness of the copper with the aid of surface plating
  • Manufacture core materials in-house

Special order materials can be costly and time consuming, but they offer the added advantage of using an adhesiveless build. This can reduce the overall flex thickness.

Surface plating is another solution that allows copper thicknesses of up to 3 OZ (consisting of 1 OZ of added plated copper and a 2 OZ base copper). The only disadvantage is that the added plating copper has a lower degree of bend capability because it is electro deposited. The base copper, however, can be of the preferred rolled annealed format.

Finally, making the material in-house may be a more viable solution. It requires the designer to purchase the required material separately in sheet form. They can use a layer of flexible adhesive to laminate the copper to a layer of polyimide.

The benefit of using this method is that the design won’t utilize any additional surface plating. The disadvantage is that it will increase the core thickness because of the adhesive layers. An adhesive layer will be required for each copper layer in the flexible circuit.

Use Curved Traces

A common design tip is to lay out copper traces at 90° angles. This may not be possible in some applications.

In this case, try to maintain curvature in the track by using conical shaped bends. Try to avoid sharp right angle trackworks, especially hard corners at 45°. This will minimize copper stressing and increase the reliability of the flexible circuit.

Pads Should be Supported

It is likely for copper on a flexible PCBs to disconnect from the polyimide substrate because of added stress in bending and the adhesion characteristics of copper with the substrate.  Exposed copper can be supported using vias .

Vias are recommended because thru-hole plating provides a convenient mechanical anchor between flex layers. It is strongly recommended to allow for more thru-hole plating for flex PCBs and rigid flex PCBs, while also using traditional plating in inflexible PCBs.

Other measures you can use are non-plated-thru pads and surface mount pads to prevent copper detachment.

Via Placement

Via placement may be necessary for multilayer flex circuits to connect the different layers. You should, however, avoid placing vias, if you can help it. This is because vias can greatly suffer fatigue in flexing movements.

If you must use vias, try to define regions with the help of ‘rooms’ to identify areas that do not have bends. Use your PCB editor software to place vias in stationary areas only.

Corners and Cutouts in Flexible Circuits

If you want to place a cutout or corner in the flex area of the PCB, the cutout should be properly terminated. The IPC suggest using a minimum radius of 1.55mm for circular sections. This will minimize the risk of tearing flex substrate materials at corners.      

Use tangential curved corners with a minimum radius of 1.5 mm if the corner has an angle less than 180°. In case of acute corners less than 90°, punch out a circular curve. The same applies to slits and slots in the flex areas.

Just make sure that there is a relief hole with a diameter of 3mm at either end.

Widening Should be Gradual

Try to use wide conductors because they are more robust and are better suited to bending areas than small conductors. It is a good idea to widen small conductors in the bend area if the bend is pushing the minimum bend ratio limits.

Note that the forces from a bend can create stresses beyond the bend zone. Make sure that the widening of the conductors is gradual and not abrupt.

Hatched (Lattice) Polygons

Often in flex PCBs, a power plane or ground plane may be needed. You can use solid copper fills as long as this doesn’t reduce the circuit’s flexibility.

A better solution is to use hatched (lattice) polygons to maintain the circuit's flexibility. Note that hatched polygons may stress copper in the 90°, 45°, and 0° angle directions because of the alignment of the Xes and hatch traces.

You can use a hexagonal hatch pattern. This can be done using an anti-pads arranged in a hexagonal pattern and a negative plane layer.

Double-Layer Flexible Circuits

It is not recommended to lay traces over each other in the same orientation if you are using double sided flexible PCBs. A better solution is to stagger traces in a way that they don’t overlap. This will reduce the stress on the traces because the copper is distributed more evenly between layers.

Layers with overlapped traces will be under increased stress during bending because the layers are pressing against one another. Staggering layers is a great way of spreading out the stress throughout the substrate and uniformly distributes the stress.

Wrapping Up

Flex circuits are one of the most useful types of PCBs in electronics. They can be used in the miniaturization of technology and can be just as effective as bigger circuits.

Make sure to get in touch with the best manufacturer who can provide you with high quality PCBs at your budget. Get in touch with the consultants at Hemeixin PCB to learn more

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