In industries where failure isn’t an option, such as aerospace, medical electronics, industrial automation, and high-speed communication, electromagnetic interference (EMI) is a top-tier design concern. When your application relies on signal clarity, controlled impedance, and mechanical flexibility, rigid-flex PCBs offer a unique solution.
But with that innovation comes a challenge: how do you maintain electrical integrity while managing the EMI risks inherent to dense, multi-layered, mixed-material constructions? As rigid-flex PCBs become more complex and compact, EMI mitigation strategies must evolve beyond rigid board norms.
In this blog, we’ll explore three key shielding technologies, copper layers, silver ink, and shield films, as well as design-level best practices that can reduce EMI from the ground up. Whether you’re in early-stage layout or refining a legacy design, understanding the EMI tools available for rigid-flex is essential for achieving signal reliability in critical applications.
Why EMI Is a Bigger Deal in Rigid-Flex Designs
Rigid-flex circuits, by nature, are designed to route high-density signals across 3D geometries, often in devices where internal space is limited and reliability is mission-critical. These designs frequently support high-speed communication protocols like Ethernet, USB, CAN, or RF, where signal degradation or crosstalk due to EMI can result in data corruption, system errors, or outright failure.
Shielding solutions and layout strategies that work in rigid boards don’t always translate well to the flexible regions of a rigid-flex assembly. The flex portions introduce mechanical stress, variable impedance, and restrictions in layer stack-ups. As a result, EMI control must be carefully tailored to both the rigid and flexible zones of your PCB.
Copper Shielding Layers: The Gold Standard for EMI Protection
When it comes to EMI shielding, copper layers remain the most effective option available. By adding dedicated ground or shield planes, either on one side of the signal layer or encapsulating it between two, you can significantly reduce EMI by creating tight return paths and full shielding enclosures.
Flexible circuit board with copper shielding layers.
This method is ideal for controlled impedance environments, which are common in medical imaging, avionics, and high-speed industrial networks. Copper planes are also highly compatible with stitching via strategies, often implemented in a "picket fence" pattern to create a pseudo-Faraday cage around critical signal areas.
However, copper shielding is less favorable in flex regions. Adding copper increases layer count and stiffness, which contradicts the need for bendable areas and adds both cost and risk during dynamic flexing. Still, for rigid sections of your design, copper is the go-to for shielding and is easily integrated into most rigid-flex stack-ups.
Silver Ink: Legacy Tech with Limited Modern Application
Once a staple in flexible circuits, silver ink shielding has largely been phased out in today’s rigid-flex applications. While it offered an inexpensive and flexible EMI solution, it came with a host of downsides: messy application, poor impedance control, incompatibility with complex stack-ups, and minimal support for high-density designs.
As newer technologies have emerged, silver ink has become obsolete, especially for designs with tight tolerances, high signal integrity requirements, or those needing multi-layer flexible routing. If EMI is a concern and your design still relies on silver ink shielding, it may be time to evaluate more modern options such as shield films.
Shield Films: The Flexible, Cost-Effective Middle Ground
Shield films are a newer, cleaner alternative to silver ink that offer significant EMI protection without the rigidity of added copper layers. These films are laminated over the flex circuit, using a metallic core and conductive adhesive to establish a continuous grounded surface.
Flexible circuit board with shielding film.
While not as robust as copper layers for shielding, shield films strike a good balance for applications where EMI protection is needed in flex regions without sacrificing flexibility. They’re commonly used in consumer electronics, wearables, and compact medical devices where space is tight and dynamic bending is required.
Do note: shield films may not be suitable for high-density routing or applications with stiffener conflicts. In these cases, copper shielding may be required in flexible areas, with careful planning to isolate the EMI-sensitive areas.
Design-Level Practices to Reduce EMI
In addition to physical shielding, EMI mitigation starts in the design and layout phase. Several best practices can greatly reduce EMI exposure even before shield materials are introduced:
- Use stitching vias to connect ground planes and create Faraday cage effects around critical signals.
- Keep signal traces short and direct. Shorter runs reduce the chance of noise insertion or signal loss and are easier to impedance-match.
- Route high-speed signals on internal layers whenever possible, using adjacent ground planes to minimize EMI.
- Centralize noisy components and move them away from sensitive analog or RF traces.
- Separate impedance-controlled lines to avoid crosstalk, especially critical in differential pair routing.
- Use dedicated ground and power planes instead of shared planes for improved isolation and reduced loop area.
- Select high-frequency materials when needed. Consult your PCB manufacturer for material recommendations that offer better signal integrity and EMI resistance.
Summary
Electromagnetic interference can cripple signal performance and compromise reliability in today’s rigid-flex PCB designs. Whether you’re designing for a surgical robot, flight control module, or industrial data concentrator, ignoring EMI is not an option.
By combining smart layout practices with tailored shielding technologies, like copper planes in rigid zones, shield films in flex, and via stitching throughout, you can ensure your design performs reliably in the field. And when even more advanced control is needed, partnering with a manufacturer that understands rigid-flex fabrication, impedance tuning, and EMI shielding is your strongest line of defense.
At Epec, we help engineers develop rigid-flex PCB solutions that meet both mechanical and electromagnetic demands. With deep experience in layout consulting, stack-up development, and material selection, we’ll help you find the balance between performance, cost, and reliability.
Key Takeaways
- Copper layers offer the strongest EMI shielding but are best limited to rigid sections due to flexibility and cost constraints.
- Shield films provide lightweight EMI protection in flex zones and are ideal for space- and cost-sensitive designs.
- Silver ink is outdated and not recommended for modern rigid-flex applications with high-density or impedance-controlled requirements.
- Design strategies like via stitching, internal signal routing, and ground plane isolation are essential for EMI reduction, shielding alone isn’t enough.
- Early engagement with your PCB manufacturer ensures materials, stack-ups, and shielding choices align with your performance and budget goals.
