Best Practices for Via Placement in Rigid-Flex Circuits

Flex Plus Tech team
Jul 14
4 min read

Rigid-flex PCBs combine the structural stability of rigid boards with the dynamic versatility of flexible circuits. As applications continue to demand compact, high-speed, and reliable electronic assemblies, via placement in rigid-flex circuits becomes a crucial design factor that directly affects product performance and manufacturability.

This article outlines essential best practices for via placement in rigid-flex circuits, addressing design, mechanical reliability, and fabrication guidelines to ensure optimal performance in demanding environments.

Why Via Placement Matters in Rigid-Flex Circuits

Vias play a fundamental role in connecting different layers within a PCB. In rigid-flex circuits, these connections often span between rigid and flexible sections, increasing the potential for mechanical stress. Poor via placement in rigid-flex circuits can lead to cracking, delamination, or signal integrity issues.

As miniaturization trends grow and layer counts increase, improper via positioning becomes a leading cause of early failure. That’s why thoughtful via placement in rigid-flex circuits is essential not just for functionality, but also for long-term reliability.

Types of Vias in Rigid-Flex Circuits

Understanding via types is key to mastering via placement in rigid-flex circuits:

Through-Hole Vias: Extend from the top to bottom layers, common but may induce stress if placed over flex areas.
Blind Vias: Connect outer layers to inner layers, minimizing space while improving routing.
Buried Vias: Connect inner layers only, typically used to avoid crossing flexible layers.
Microvias: Laser-drilled and used in HDI rigid-flex designs, suitable for high-density applications.

Selecting the right via type is the first step in optimizing via placement in rigid-flex circuits.

1. Avoid Vias in Dynamic Flex Areas

The most critical rule for via placement in rigid-flex circuits is to never place vias in zones that will bend, flex, or move repeatedly. These areas are prone to mechanical fatigue, and any via—especially through-hole—becomes a failure risk.

Instead, concentrate vias in the rigid sections, or at least in flex zones that are static or protected by stiffeners.

2. Maintain Sufficient Via-to-Via Spacing

Tight via clusters can lead to resin starvation and weakened laminate strength during lamination. Best practice spacing recommendations for via placement in rigid-flex circuits:

Minimum spacing: 0.25 mm (10 mil)
Ideal spacing: 0.5 mm (20 mil) or more for high-reliability applications

This spacing ensures proper resin flow, heat dissipation, and mechanical support.

3. Staggered vs. Stacked Microvias

In HDI rigid-flex applications, via placement in rigid-flex PCB boards often involves multiple stacked microvias. While stacking can save space, it increases the risk of drill misalignment and plating failure. For better reliability:

Use staggered microvias when possible
Limit stack height to two levels if stacking is necessary

Staggered arrangements reduce stress concentrations and improve manufacturing yields.

4. Proper Via Pad Design

Via pads must accommodate drilling tolerances and ensure robust electrical connections. Key design tips for via placement in rigid-flex circuits include:

Oversized pads for through-hole vias to account for drill wander
Annular rings no smaller than 0.1 mm (4 mil)
Use teardrop pads for vias near traces to reduce stress

Proper pad design supports via reliability under both thermal and mechanical stress.

5. Avoid Vias Near Bend Zones

Even if a via is technically in a "rigid" portion of the circuit, it should not be placed too close to a bend transition. For safe via placement in rigid-flex circuits, maintain a clearance of at least:

1.5 mm (60 mil) from the edge of a bend zone
More clearance for thicker flex stacks or aggressive bending

This buffer protects vias from flex-induced strain during operation or installation.

6. Consider Via Fill and Capping for Signal Integrity

Open vias can create signal stubs and negatively impact impedance in high-speed designs. For best via placement in rigid-flex PCB boards:

Use filled and capped vias in controlled impedance layers
Avoid via-in-pad unless specifically designed for it
Use backdrilling to eliminate stub lengths when necessary

These practices enhance electrical performance, especially in RF and high-speed digital circuits.

7. Simulate Mechanical and Thermal Stress

Advanced simulation tools can model how via placement in rigid-flex circuits responds to repeated flexing, thermal cycling, and vibration. Use these tools to:

Identify high-stress zones
Optimize via positions and sizes
Validate material choices and stackups

Simulations are especially useful in aerospace, medical, and automotive applications where failure is not an option.

8. Collaborate Early with Your Fabricator

No amount of CAD expertise can replace early involvement from your rigid-flex PCB fabricator. An experienced supplier can guide you on:

Manufacturable via placement in rigid-flex circuits
Layer stack-up feasibility
Cost-effective alternatives to complex via configurations

Your fabricator’s DFM (Design for Manufacturability) feedback is essential for ensuring success from prototype to production.

Conclusion

Proper via placement in rigid-flex circuits is a critical factor in building reliable, high-performance electronic assemblies. Following the best practices above can prevent mechanical failures, improve signal integrity, and reduce manufacturing risks.

If you’re designing a new rigid-flex PCB or want expert advice on via placement in rigid-flex circuits, our engineering team at Flex Plus is here to help. We specialize in high-reliability FPC and rigid-flex manufacturing with advanced capabilities in HDI, microvias, and multi-layer flex designs.

Looking for a Reliable Rigid-Flex PCB Manufacturer?

We’ve supported global OEMs across automotive, medical, and industrial sectors with precision-engineered flex and rigid-flex solutions. Contact us today for a free DFM review or custom quote.

Email: info@flexplusfpc.com

Website: www.flexplusfpc.com