Best Tips for Designing Your Flexible PCB for Manufacturability

Flexible PCBs, the go-to solution for applications that require a thinner, lighter and more adaptable products, have been transforming the landscape of modern electronics. Whether it be medical wearables, automotive sensors or consumer electronics, this type of PCB is perfect for applications where traditional rigid boards would not fit or perform as well. But, just like for everything else, a quality is always counterbalanced by a corresponding drawbacks, and the unique properties of flexible PCBs require special care and design choices to ensure manufacturability, reliability and cost-efficiency. In our article for today, we will be going over these 10 best practices that can optimize your flexible PCB design for your application.

1. The choice of an Optimal Substrate

The choice of the correct base material is the base of a reliable flexible PCB. Polyimide has been the substrate of choice for most flexible PCB designs, offering the dual qualities of thermal stability and flexibility. For projects under a tighter budget and for which thermal resistance is less of a critical issue, PET is also a valid choice.

• Single- and double-sided boards are generally easier to manufacture than multilayered boards that require more complex processing.
• Rolled Annealed or RA copper is prized for its ductility, resisting cracking under repeated flex.
• For applications that involve regular bending, a thinner copper would be the better choice.

2. Trace Routing

Trace Routing is a vital aspect of all PCB designs. But for flexible PCBs, poor routing can cause stress concentration, signal loss and mechanical failure:

• Communicate with your manufacturer to adhere to their limits on minimum trace width and spacing. Standard designs use around 75 – 100 µm. Finer lines are both more difficult and more expensive.
• Try to always use gradual and curved traces in the areas that will be bending. Sharp corners are prone to cracking.
• Keep a good distance between your traces and the edges of your board to avoid possible delamination.
• Serpentine or meander patterns can help distribute mechanical stress and extend the lifespan of your flexible PCB. This is especially important for dynamic applications.

3. Minimum Bend Radius

Make sure your flexible PCB will not be bending beyond the minimum bend radius. Exceeding this radius can damage both the copper and the substrate.

• For a static application, the bend radius is usually about 10x the board thickness.  
• In a dynamic application that will be seeing repeated flexes, it is not a bad idea to increase this radius to 20x or more
• Ensure no vias, pads or stiffeners are present in areas that will be bending frequently.

4. Stiffeners

You will want to reinforce the connector zones and component pads of your flexible PCB to accommodate the bending:

• Stiffeners are usually made of FR-4, polyimide or stainless steel where rigidity is a requirement.
• Limit your stiffeners to essential regions of your board and maintain flexibility elsewhere.
• The choice of a correct adhesive is crucial to prevent stiffener delamination during changes in temperature.

5. Component Placement and Pad Design

For the assembly process of your flexible PCB, position your components to avoid flex-induced stress.

• Teardrop-shaped pads can reduce stress at pad-trace junctions.
• Use stiffeners to better support heavier components.
• Apply fillets to pad corners and relief cuts in stiffeners to prevent tearing.

6. Via Design and Placement for Flexible PCBs

The design, placement, and types of vias will end up affecting manufacturability of your flexible PCB:

• An excessive number of vias can reduce the mechanical reliability of your board.
• Stacked through-holes are stress points. Use instead microvias and staggered vias to relieve stress.
• Keep your vias away from the bend areas for good measure.

7. Optimizing Panelization and Tooling

Smart panelization and tooling strategies can make your production process much more efficient:

• Design your flexible PCB layouts to maximize panel use so as to reduce material waste.
• Add fiducials for alignment and tooling holes for handling during the assembly process.

8. Coverlays and Solder Mask Parameters

Coverlays protect the circuitry on a flexible PCB. It is usually a polyimide film with adhesive.

• Carefully specify the cutouts for pads, vias and test points in your design.
• Avoid excess adhesive which can cause problems for your product.

9. Communication with your Manufacturer

Engage with your manufacturer early on in the design process of your flexible PCB. This will not only save time but also reduce errors. Manufacturers should know the optimal stack-ups, materials and layout rules from experience, and can give you good advice for making a more manufacturable board.

10. Prototyping and Testing for Flexible PCB

Always prototype and test your prototypes. Simulate real-world flexing, thermal cycling and assembly processes so as to catch potential issues before pushing on to production.

Manufacturability is much more crucial to the design of flexible boards than our conventional rigid boards. In order to endure repeated bending or twisting, a flexible PCB relies on thin, bendable materials like polyimide or polyester film. These materials open new design possibilities but also present unique challenges for the manufacturing process. A designer must keep these problems in mind when designing a flexible PCB, and do their best to optimize for an efficient and defect-free fabrication.

In our article for today we went over 10 tips for designing your flexible PCB in the mindset of DFM, tips that will help your product remain robust in high-volume production, meet reliability standards and avoid unnecessary cost drivers. Following these best practices and collaborating closely with your manufacturer, you’ll ensure that your flexible PCB project is the success that it should be: delivering innovative products that perform as intended and are manufacturable at scale.