Designing rigid-flex PCBs combines the challenges of both rigid and flexible circuit design. Here are some common mistakes made in the design of rigid-flex PCB assemblies and how to avoid them:
1. Applying Rigid PCB Design Principles to Rigid-Flex Designs
Mistake: Designers often apply rigid PCB design principles to rigid-flex designs, failing to account for the unique properties of flexible sections.
Solution: Understand the specific requirements of rigid-flex PCBs. Flexible sections need different design rules, such as avoiding sharp corners and ensuring smooth transitions between rigid and flexible areas. Use design tools that support rigid-flex PCB design to simulate and validate your designs.
2. Inaccurate Via Pad Size for Drills
Mistake: Incorrect via pad sizes can lead to manufacturing issues and reliability problems.
Solution: Ensure that via pad sizes are accurately specified for the drill sizes used. Consult with your fabricator to confirm the appropriate sizes and tolerances.
3. Designing Plated Through Holes Too Close to the Transition
Mistake: Placing plated through holes (PTHs) too close to the transition from rigid to flexible circuitry can cause mechanical stress and potential failure.
Solution: Maintain a safe distance between PTHs and the transition areas. Follow guidelines for minimum spacing to reduce stress and improve reliability.
4. Excessive Copper on Flexible Layers
Mistake: Using too much copper on flexible layers can reduce flexibility and increase the risk of cracking.
Solution: Optimize the amount of copper used on flexible layers. Use cross-hatched patterns instead of solid fills to maintain flexibility while providing necessary electrical performance.
5. Non-Cross Hatched Copper Layers
Mistake: Solid copper layers in flexible sections can lead to reduced flexibility and increased risk of delamination.
Solution: Use cross-hatched copper patterns in flexible areas to enhance flexibility and reduce the risk of delamination.
6. Incorrect Adhesive Amounts in Layer Stackup
Mistake: Incorrect amounts of adhesive in the layer stackup can lead to delamination and mechanical failure.
Solution: Follow precise guidelines for adhesive application in the layer stackup. Work closely with your fabricator to ensure the correct amounts are used.
7. Undefined Bend Radius
Mistake: Failing to define the bend radius can result in excessive stress on the flexible sections, leading to cracking and failure.
Solution: Clearly define the bend radius in your design documentation. Ensure that the bend radius is appropriate for the materials and thicknesses used in the flexible sections.
8. Sharp Angles in Trace Routing
Mistake: Sharp angles in trace routing, especially in bend regions, can cause traces to crack or break.
Solution: Use smooth, curved traces instead of sharp angles. Avoid routing traces through bend areas whenever possible.
9. Placing Vias in High-Stress Areas
Mistake: Placing vias in areas that experience high stress, such as bend regions or near stiffeners, can lead to mechanical failure.
Solution: Avoid placing vias in high-stress areas. If vias are necessary, reinforce them with additional support or use alternative routing strategies.
10. Not Capturing SMT and Unsupported Pads
Mistake: Failing to properly capture Surface Mount Technology (SMT) and unsupported pads can lead to pad lifting during assembly.
Solution: Ensure that all SMT and unsupported pads are adequately supported. Use anchoring spurs and pad fillets to increase the strength of the pad-to-trace interface.
Conclusion
By understanding and addressing these common mistakes, designers can improve the reliability and performance of rigid-flex PCB assemblies. Collaborating closely with fabricators and using specialized design tools can help avoid these pitfalls and ensure successful rigid-flex PCB designs.
