Issue link: https://resources.pcb.cadence.com/i/1524390
Stress and Strain on Circuitry Sharp Bends and Corners: Avoid sharp bends and corners in trace routing. Gradual bends are easier on the copper traces and reduce the risk of damage. Trace Placement: Whenever possible, keep conductors smaller than 10 mil within the neutral bend axis as they tolerate compression better than stretching. Plated Through-Holes (PTHs): Avoid PTHs within bend areas as they can break due to stress. Trace Orientation: Traces should run perpendicular to the bend axis to minimize stress. Component Placement Unsupported Pads: Component pads on the outer layers are more prone to lifting due to the flexible nature of the substrate. To prevent this, incorporate anchors or spurs encapsulated in the coverlay to provide mechanical support. Solder Joints: Solder joints can weaken if components are placed in bending areas. For surface-mount components on the flex area, consider using solder bumps on the pads for better control over the connections. If through-hole components are necessary in a flex area, add stiffeners for support. Overcoming Challenges Design for Flexibility Minimize Layer Count: Whenever possible, use a single-layer design for dynamic flex PCBs. Gradual Bends: Design with smooth, gradual bends instead of sharp corners. Trace Routing: Route traces perpendicular to the bend area and keep smaller traces within the neutral bend axis. Teardrops: Utilize teardrops at the junction of pads and traces, especially when trace width changes. This helps reduce stress concentration points. Larger Pads: Use larger component pads whenever possible for better mechanical stability. Stiffeners: Strategically place stiffeners in areas requiring additional support for components or to manage stress on the flex area. Material Selection Rolled Annealed Copper: Use rolled annealed copper for its improved bendability. Adhesive-Less Core (for 2-layer boards): If using a two-layer design, choose a thin, adhesive-less core material. Manufacturing Considerations Minimum Bend Radius: Clearly specify the minimum bend radius requirement in the design specifications for the manufacturer. Component Placement: Indicate component placement restrictions in the design, especially for areas that should not have components due to flexing. When Should You Use Flex Instead of Rigid? Given all the challenges and extra accommodations we need to make for flex PCB design, when is it the right time to use flex PCB material as opposed to rigid PCB material? Simply it's when your application won't allow for a standard rigid PCB solution. All of PCB design is driven by the end goal. The end goal sets the needs and requirements and rigid PCB will either meet those requirements or fall short. In that case, flex PCB designs become the next best solution. This is the same scenario where printed wiring boards became too cumbersome for more modern applications. Now, standard PCBs, while still used everywhere, are no longer able to meet the tight physical constraints required of new, cool technology. However, you don't need to wait to implement something in flex. Maybe you want to take advantage of fewer discrete wires and components, or opting for a lighter design, or creating a more portable version of an existing product simply because it's possible. All these options make flex PCB a fair choice. Then at that point, you're in the realm of flex PCBs. To give you an idea of how and where flex PCBs are implemented and where you would use them, read the next section where we highlight the popular technologies that help push the boundaries of PCB design. 12 www.cadence.com Rigid Flex Design Guide