By Matt Stevenson
January 9, 2024
Flex and rigid-flex PCBs represent exciting technology for designers. Suddenly, boards no longer need to exist along one plane—or along a flat plane at all. Designs can now conform to specific shapes or be bent during use, and this opens new possibilities for applications with space constraints or flexibility requirements.
Built using materials that can bend, a flex PCB is usually constructed with materials like polyimide and polyester film. These PCBs have many properties that designers might find useful beyond their flexibility. They have improved resistance to both vibration and movement. They are also significantly thinner and lighter than traditional rigid PCBs.
Flex PCBs do have some drawbacks, however. They can be more expensive to manufacture, and extra care needs to be taken for large or heavy components that may not be able to handle the flex. When these drawbacks seem problematic, rigid-flex PCBs are probably the best way to go.
Rigid-flex PCBs combine traditional rigid PCBs with flexible connectors. This approach offers better durability and mechanical stability compared to flex PCBs. While designs may lose some of the versatility of flex PCBs, combining traditional rigid PCBs and flexible elements brings the best of both approaches.
Of course, adapting to these new materials requires learning about their capabilities and restrictions, as well as learning a new set of design rules and best practices. Here are five best practices to get you started on your first flex or rigid-flex PCB design.
Before exploring these best practices, consider whether your PCB will be used in a static flex application—that is, it just needs to flex once during installation—or if it will be in a dynamic flex application. This important distinction can impact your design decisions.
Sharp angles can cause weak points in your traces across flex PCB segments. They tend to concentrate mechanical stress along the angles, so if you use curved paths for flex points, you will avoid cracking and weakening connections. This is particularly important in dynamic flex applications, where traces and conductors are subjected to repeated bending.
A widened fillet can help reduce stress at connection points between traces and pads, greatly increasing the robustness of your design. Teardrop and rounded trace designs can reduce stress concentration points and prevent potential cracking or peeling around connections.
Unlike rigid PCBs, flex PCBs require an additional coverlayer to protect circuits and maintain flexibility. It is important that the coverlayer leaves room for various features of the PCB and for the interaction between the coverlayer and the flex board.
First, the coverlayer needs to be drilled to create access to the pad for soldering. However, it is important that the adhesive isn't squeezed out onto the pads. To accommodate for this, coverlayer drill holes need to be 0.005–0.010" larger than the land area.
In addition, this same allowance needs to be made for clearance holes in the coverlayer around through-holes in the flex PCB.
Conductors need to be evenly staggered across areas of the board that will see regular flexing. If the circuit is repeatedly flexed perpendicular to the conductors, stress will occur in the same location. This will cause any isolated conductors to crack prematurely.
However, when conductors are routed and staggered evenly, an isolated stress condition cannot develop. This ensures that no premature failure will occur.
In addition, be careful to avoid traversing conductors in the bend area. This can cause additional mechanical stress and can lead to cracking and failing traces.
