Rigid Flex PCB Design Guidelines

Most designers cut their teeth designing rigid printed circuit boards due to their simplicity (this is relative). Because the design is static from a mechanical standpoint, the designer needs only indicate the component locations for assembly. A flex circuit offers numerous mechanical advantages over a rigid PCB but also requires additional considerations for materials, dynamic bending, and density (components, traces, layer count, etc.) Some designs benefit from including rigid and flex materials to leverage their strengths where necessary. Rigid-flex PCB design guidelines focus on how to organize designs before and during layout to optimize performance.

Building Rigid-Flex PCB Design Guidelines

Overall, rigid-flex PCBs support greater innovation due to their unique mechanical properties, which require additional constraints during layout. Broadly, designers must contend with three distinct design-constraint areas: rigid, flex, and transition. In other words, design strategies that work best for rigid do not necessarily work best for flex (and vice versa). The earliest stages of rigid-flex PCB projects require a greater emphasis on the “behind-the-scenes” mechanical aspect than a traditional rigid board:

• Floorplanning – Like with a standard rigid PCB, designers will want to begin the layout by grouping components according to the schematic. Where rigid-flex becomes more complex is designing around the bending areas (these will need to be free of components). The engineer may define the flex sections in the design documents, or the layout designer may have some leeway to alter them.
• Layer count – In a flex or rigid-flex stackup, additional layers are necessary to process flex materials and mechanical integrity (i.e., stiffeners, adhesives, etc.) The flex ribbon has to occupy the center of the stackup; this constraint can complicate routing in high-speed or HDI designs. In general, rigid-flex PCB stackups cannot accommodate the layer density of rigid boards and are significantly less cost-effective per layer.
• Dense pinout packages – Flex ribbon sections of the design may be suitable for simple passive devices (e.g., chip packages). However, specific ICs, BGAs, etc., require more internal layers for signal breakout and smaller via structures that are incompatible with flex materials. These components will require a rigid section or sections for breakout before routing across the flex section(s).

Design Rules and Features for Rigid-Flex PCBs

The type of bending action will also significantly impact the design. A static bend (in other words, the flex section is bent and remains in a fixed position during assembly) produces greater options in the stackup than a device with dynamic bending capabilities. Static bend assemblies support additional copper layers (and types of copper) for denser routing, while dynamic bending designs must reduce flex layer counts to prevent stress-strain damage. For static bend radii, a minimum bend radius of ~10x the flex thickness is reasonable, while dynamic applications use a bend radius of ~100x the flex thickness.

Some additional rigid-flex PCB design rules to keep in mind:

• Bend location and trace geometry – Bends in the flex layers should always be perpendicular to the trace orientation, ideally during straight planar runs of the traces. Traces should gently redirect trace sections using arcs instead of corners (even 45-degree mitered corners add significant strain to the flex sections).
• Strain relief fillets – Fillets accomplish two critical tasks – they provide a more gradual bending action between the rigid-flex transition areas, and their tapered width reduces the mechanical fatigue between the rigid and flex trace sections.
• Pad support – Copper adheres less to flex materials like polyimide than FR-4, as the former’s smoothness precludes strong mechanical interlocking during lamination. Two potential options are shrinking the coverlay layer and adding stubs to pads for additional grip strength. Additional plated copper thickness on through holes can further support the structural integrity. SMT pads require oversizing on the coverlay layer for anchoring on two sides of the pad.
• Multilayer flex routing – When routing traces in the flex layers opposite one another, use a staggered trace arrangement to prevent stress from concentrating on one side of the flex section during bending. This structure will also reduce broadside coupling between the traces (a potential electromagnetic compatibility issue).
• Plane-layer hatching – A full pour of copper (e.g., for a reference plane) performs poorly during bending; designers can use a hatched polygon design to better distribute some of the strain.

Your Contract Manufacturer for Rigid, Flex, and Rigid-Flex Builds

Rigid-flex PCB design guidelines are more complex and constraining than traditional rigid rules, but the numerous advantages offered during assembly can make these challenges worthwhile. With fewer discrete components, assembly turnaround times are faster, testing is simpler, and the overall box build becomes lighter and denser (the last two points are highly salient to portable devices). For designers new to rigid-flex designs or looking to optimize their builds, VSE is here to help. Our engineers are committed to building electronics for our customers, including complex flex and rigid-flex designs that can replace hundreds or thousands of distinct wires. We’ve been realizing life-changing and life-saving designs for over forty years with our valued manufacturing partners.

If you are looking for a CM that prides itself on its care and attention to detail to ensure that each PCB assembly is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.