Flex PCB designs and rigid flex PCB designs present many advantages over standard circuit boards traditionally based on FR-4 or other rigid substrates. Interconnecting displays and other hardware connected directly with the flex circuit cuts down on a lot of space that would be usually devoted to connectors and cables. There are many areas of flex layout though that will be different or new to a PCB designer if you’ve not worked with it before, including the following:
1. Types of Flex Circuit Design
2. Flex Circuit Stackups
3. Pads and Holes Used in Flex Circuits
4. Component Placement on Flex Circuits
5. Flex Circuit Routing
6. Ground Planes on Flex Circuits
7. High Speed Considerations with Flex Circuits
Let’s take a quick look at these different areas so that you will be better prepared for your next flex PCB design layout.
Flex PCB Design, Stackups and Component Placement
First let’s look at the different flex types and stackups of a flex circuit, followed by some placement considerations:
Flex circuit types: There are too many distinctions between flex circuits; those that are designed for minimal flexing which are referred to as “static,” and those that flex regularly called “dynamic.” Static flex circuits usually only bend when they are being installed or serviced, while dynamic flex circuits are used for devices like laptops that open and close on a regular basis. The thicker that a flex circuit is, the less flexible it will be. It is important therefore to choose materials appropriate for the intended purpose of the circuit, and for dynamic flex circuits to keep bending areas as thin as possible.
Stackups: Just as printed circuit boards can have differing layer stackups, so can a flex circuit. Single and double layer flex circuits are composed of a flexible dielectric film layer with a conductive polymer layer on one or both sides. Multi-layer flex circuits are used for dense circuitries, while rigid flex circuits are a combination of both flexible and rigid substrates with the flex circuits spanning both. Flex circuits are usually covered by a layer of insulating material called “coverlay” to protect the conductor patterns of the circuit.
Pads and holes: All of the pads on a flex circuit are susceptible to lifting off the substrate due to the flexible nature of the circuit, and should be anchored. This is done by extending ties from the pad that are encapsulated by the coverlay. It is also a good idea to make pad sizes larger to help with stress relief, especially on single sided designs where there isn’t a plated thru-hole to help secure the pad.
Component placement: Due to the flexible nature of the circuit, you will need to confirm not only the X, Y, and Z spacing of the components like in a regular PCB, but also all of those spacings when the circuit is being bent. This calls for the use of 3D flex PCB design tools that allow you to both see and check the flex circuit while it is laid out flat, and while it is bending.
Next comes the routing of the flex circuit, and there are several considerations here that you need to be aware of as well.
Screenshot from 3D CAD tools of a flex circuit laid out flat
Routing Your Flex Layout
Although routing traces is something that every PCB designer is familiar with, there are some unique challenges when routing a flex design:
Routing: You need to be careful routing in the bend areas. Traces need to cross those areas perpendicular to the bend line, and vias and pins should be excluded from the bend areas to prevent damage to the holes. Be careful of stacking layers with traces running adjacent on top of each other. This will add undesired stiffness to bend areas of the flex circuit, and you should stagger the traces instead. The layer structures of flex circuits are unique, especially in rigid designs, and not all of the layers may be available in every section of the board. The designer needs to be careful not to route on a layer that may suddenly drop out. You should use curved corners and arc routing instead of orthogonal.
Ground planes: A solid layer of ground plane can reduce the flexibility of the circuit, as well as add weight and make the circuit more fragile. When possible, use a cross-hatch pattern in the ground planes to reduce the possibility of these problems.
High speed design: Due to the layer stackup of some flex circuits, impedance control is a little more challenging, but there are still ways to do this. Single layer flex circuits can use a co-planar stripline construction, where signal traces alternate with ground strips, but are still unfortunately susceptible to EMI. Two layer flex stackups will give you a microstrip structure that works well for 50 ohm circuits. Higher layer counts give you the ability to create a standard stripline construction, but the extra layers will adversely affect the circuits ability to flex.
At this point, you are mostly through the design of your flex circuit. The next major step will be to get it to the manufacturer. One important point here is that flex circuit materials may have longer lead times, plus the unique challenges of the flex circuit make it imperative that you work with your manufacturer early on in the design phase of the circuit.
While the image files (Gerbers) are similar to a regular PCB design, the organization and content of those files can be more complex. In addition, there are many other drawings and files that accompany a flex design to manufacturing. This makes the use of an intelligent data format for file transfer, like IPC-2581, a much more desirable method of sending data to your manufacturer instead of the traditional individual fab and assembly files.
The same flex circuit as above, but folded up within the CAD system for checking
The Right Flex PCB Design Tools Can Make the Difference
For decades PCB designers have done a great job of using 2 ½ D tools to design their boards, and these tools will work for flex circuits as well. With the unique bending nature of a flex circuit however, using a tool that is designed to operate in 3D for visual verification and automated checking is a much more productive choice. In addition, flex design tools are configured to service the unique requirements of the flex PCB design guidelines that we’ve outlined above with flex specific features and functionality.
An example of a PCB design system that has the features and capabilities to design flex circuits, including real-time 3D design and analysis, is OrCAD PCB Designer from Cadence. With OrCAD you will have the high powered professional design tools that you need to successfully design your flex and rigid flex circuits.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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