Optimizing performance is something that can be found in every aspect of life. From cars to computers, and from tools to PCBs, optimizing performance can lead to better reliability, less risk, and ultimately a more robust product.
However, enhancing the performance of a rigid-flex PCB isn’t as straightforward as some other things. In many cases, it will rely on the design itself, incorporating design techniques such as extracting the most efficiency out of stack-ups, laying out circuitry and vias such that it can withstand bending, and setting up mechanical situations that aren’t as taxing on the flexible portion of the board.
Determining Bending and Mechanical Layout
Everything starts somewhere, and with rigid-flex PCBs, it usually starts with the mechanical layout. Setting up components and determining via sizes all can be important for a rigid-flex or flexible PCB. But most critically, the bend radius of the design needs to be determined as well. Without doing so, a flexible circuit board that is intended to bend ten thousand times may not last for a tenth of that.
Rigid-flex circuit board manufactured to bend multiple times.
In another scenario, a flexible circuit board may end up bending in such a way as to accidentally crease a flexible section, sometimes even against the rigid board, resulting in a deformed part.
The best considerations for this line in three different things:
- Determining the bend radius.
- Making all bends smooth and even.
- Making sure the bend does not press against the board or any other portion of the housing.
While these aren’t always black-and-white solutions, sometimes it can help by adding strain relief, preventing those extreme bends and creases against the rigid circuit board, or even by optimizing the stack-up to best suit the bend requirements.
Rigid Flex PCB Stack-up Determination
Usually, the stack-up is one of the most important elements in determining the reliability and performance of a rigid-flex circuit board. While it can be key in determining something as critical as the bend radius, it can also have minor changes to allow for a design. Something as simple as material selection can even aid, such as selecting rolled annealed copper for the flexible section can help with bending. In another example, if a stack-up can allow for it, it is always best to situate the copper in the middle of the flexible portion of the stack-up, thus putting it on the neutral bend axis, which would cause the copper to receive the least amount of stress during repeated bends.
In the rigid section, the best practices for a rigid circuit board apply to rigid-flex. By creating a stack-up with symmetry and choosing similar materials on each side, a rigid circuit board would be less prone to warp and twist, thus allowing better reliability and performance.
Most importantly, it’s always best to situate a board along the grain direction of the copper. Doing so allows the board to bend with the grain, rather than against it, creating peace of mind in knowing that a board will be that much more optimized and reliable.
Laying out the Circuit Board
Laying out the circuit board can start with one of the aspects of the previous paragraph by laying out circuitry to follow the grain direction and length of the board. By doing so, this ensures that traces are never bent at the edges of the copper, but rather along the length of them. Additionally, along these same lines, it’s always important to ensure that traces, if they need to wind or curve in a flexible section, that they do so gradually, rather than at sharp angles, as this can lead to similar issues.
Furthermore, it’s important to remember via placement. Should a design have vias in the flexible section, it’s always best to either place them out of the bending area or if applicable allow them to have redundant connections in other places. This can help to ensure that no vias will be cracked or broken during bending and that if they had to be placed there, the circuit board can still function even if the via no longer could.
Summary
A rigid-flex PCB design is tough enough as it is without even factoring in ways to best optimize it. But by doing so, you can save yourself headaches and possibly even money in the future with less need to replace boards or be able to further expand their lifespans.
While these solutions cannot apply to every circuit board, it’s always best to incorporate them where you can. And if there are ever any concerns or questions, it’s always best to consult a PCB manufacturer from the very beginning of design phases, allowing input to best achieve performance and success with your product. As part of an ongoing way to aid customers, Epec performs such services, getting in on the design phases to give DFMs and feedback to best suit your designs.
Key Takeaways
- Mechanical Layout is Critical: Optimizing the bend radius, avoiding sharp creases, and ensuring bends don’t press against rigid sections or housings are essential for ensuring flexible PCB durability.
- Stack-Up Optimization: Aligning copper layers with the neutral bend axis and using rolled annealed copper in flexible sections minimizes stress and improves bending reliability. Symmetry in rigid sections prevents warping.
- Copper Grain Direction Matters: Designing along the grain direction of copper layers ensures smoother bends, reducing the risk of cracks or breaks and enhancing long-term reliability.
- Circuitry Layout for Longevity: Traces in flexible sections should curve gradually, not at sharp angles, to prevent stress points. Avoid placing vias in high-bend areas, or use redundant connections to ensure functionality.
- Strain Relief Solutions: Incorporating strain relief techniques, such as optimizing stack-ups or adding protective features, can reduce mechanical stress on the board during bending.
- Consult Early with Manufacturers: Engaging PCB manufacturers during the design phase provides valuable insights into stack-ups, materials, and design improvements, ensuring better performance and manufacturability.
- Epec’s Support Services: Epec offers free DFM analysis and design consultations to help customers achieve optimized rigid-flex PCB designs with higher reliability and cost savings.
