Rigid-flex technology is redefining electronics design, merging flexibility with structural reliability.
Think of rigid-flex as the ultimate chameleon of circuit boards, seamlessly blending the robust stability of rigid boards with the dynamic flexibility of flexible circuits. This hybrid approach opens doors that rigid PCBs simply cannot, especially when it comes to adapting to shrinking envelopes that constantly constrain physical hardware configurations and challenge most engineering teams.
If you haven’t explored rigid-flex as a viable alternative solution, you might be missing out on a truly transformative tool in your engineering arsenal. What does rigid-flex enable that rigid PCB does not?
True 3-D interconnects and packaging. Rigid PCBs are inherently 2-D. Rigid-flex, however, allows you to fold, bend and twist your circuit board into complex 3-D shapes, conforming to irregularly shaped enclosures. This is a game-changer for miniaturization.
Figure 1. 2-D flat view of a rigid-flex design layout.
Figure 2. 3-D flat view of a rigid-flex PCB design layout.
Figure 3. 3-D folded view of a rigid-flex PCB design layout.
Space and weight reduction. By eliminating bulky connectors and cables between rigid boards, rigid-flex designs drastically reduce overall system size and weight. Fewer components mean less mass, which is critical for portable devices, aerospace and medical applications.
Enhanced reliability. Connectors are often points of failure. Rigid-flex integrates connections directly into the board structure, reducing the number of solder joints and mating cycles and improving reliability and durability, especially in high-vibration or harsh environments. Also, with the elimination of connectors, the integrity of signal quality and impedance can be maintained throughout the signal trace between connections.
Dynamic flexing. While not all rigid-flex designs are meant for continuous, dynamic flexing, many can withstand multiple bends, making them ideal for applications where parts of the circuit need to move, like camera modules or hinge mechanisms. Whether a static or dynamic bend is needed, rigid-flex provides a greater opportunity to solve for tighter, more condensed envelope constraints.
Rigid-flex is no longer a niche technology; it’s another powerful tool in the engineer’s toolbox. Engineers should no longer be intimidated by it. Modern EDA tools have evolved significantly, offering robust features for rigid-flex design that make implementation easier and more intuitive than ever.
Consider rigid-flex for challenges such as:
- Severe space constraints, when every millimeter counts.
- Weight reduction for portable, airborne, or space applications.
- Eliminating connectors and cables to boost reliability and reduce assembly time.
- Dynamic applications where parts of the circuit need to move or articulate.
- Harsh environments where traditional connectors might fail due to vibration, shock, or debris.
- Complex geometries when the enclosure isn’t a simple box.
Rigid-Flex Design Considerations
While the benefits are clear, rigid-flex design does introduce a few unique considerations. These are entirely manageable with a structured approach. Solvability involves careful planning of the flex sections. You’ll need to consider:
Performance considerations include:
- Thermal management. Heat dissipation can differ between flex and rigid areas due to material properties.
- EMI/EMC. Shielding and grounding strategies are crucial, particularly in dynamic applications, to prevent electromagnetic interference.
- Vibration and shock. Rigid-flex often excels here, but proper design ensures components are secured and stress points are minimized.
Other aspects to consider include material selection. Polyimide is the most common flex material, but choices for adhesives, coverlays and stiffeners are vital.
Likewise, rigid-flex stack-ups are more intricate than rigid ones, involving multiple lamination cycles. In manufacturing, unique processes like laser drilling, specialized etching and precise lamination are used, while handling and assembling flexible circuits require different techniques than those for rigid boards.
Collaborating with the PCB fabricator is perhaps the most critical advice for rigid-flex design. Why is it even more essential here than with standard rigid PCBs?
Rigid-flex fabrication is a highly specialized process. Manufacturers have deep knowledge of material compatibility, process limitations, and best practices that can make or break your design.
What looks good on paper might be challenging or costly to produce. Early DFM feedback from the manufacturer can prevent costly redesigns and delays. They can advise on optimal bend radii, layer transitions, material choices and panelization.
Manufacturers can provide guidance on the most cost-effective and readily available materials that meet performance requirements. And they understand their specific equipment and capabilities, which can influence everything from drill sizes to lamination cycles. Their insights are invaluable for ensuring high yield during production and long-term reliability in the field.
Engaging with your PCB manufacturer at the conceptual stage, even before detailed layout begins, can save immense time, money, and headaches down the line. This early engineering-manufacturing collaboration is today’s best practice not only for rigid PCB design, but especially for rigid-flex PCBs. The manufacturers are your partners in innovation, helping you transform your groundbreaking rigid-flex ideas into reality.
5 Tips for Rigid-Flex Design
Keep these basic principles in mind when starting your first rigid-flex design.
- Keep components and vias out of bend areas. Do not place components, through-holes, or vias in the bend areas, as they are points of high stress that can lead to failure. Place components on the rigid sections and maintain a clearance between the last component and the start of the flex section.
- Design controlled bends with gradual curves. Avoid sharp angles like 90° bends, as they cause high strain and can damage the circuit. Use large, gradual curves for all bends. Curved traces are preferable to sharp angles. For multilayer flex circuits, stagger conductors and align them with the neutral bend axis to minimize tension and compression during flexing.
- Place flex layers in the center of the stackup. Sandwich the flexible layers between the rigid sections to protect them from external stress and simplify manufacturing. This placement also helps with impedance control and prevents damage from outer layer plating processes.
- Maintain a balanced and symmetrical stackup. Ensure an even number of layers for a balanced stackup, which helps prevent warping and improves flexibility. Symmetry in both copper distribution and dielectric materials is also important for consistent performance.
- Consult with your PCB manufacturer early in the design phase to ensure manufacturability and reliability. They can provide crucial guidance on material selection, stackup, and design rules, especially for complex or high-speed applications.
Rigid-flex PCBs are more than just a trend; they are a fundamental shift in how we approach electronic design. By understanding their unique advantages and collaborating closely with manufacturing experts, you can harness this technology to create smaller, lighter, more reliable, and ultimately, more innovative products. Embrace the flexibility!
Stephen V. Chavez is a senior printed circuit engineer with three decades’ experience. In his current role as a senior product marketing manager with Siemens EDA, his focus is on developing methodologies that assist customers in adopting a strategy for resilience. He is an IPC Certified Master Instructor Trainer (MIT) for PCB design, IPC CID+, and a Certified Printed Circuit Designer (CPCD). He is chairman of the Printed Circuit Engineering Association (PCEA); This email address is being protected from spambots. You need JavaScript enabled to view it..
