Advanced Flex & Rigid-Flex PCB Capabilities

As flex and rigid-flex circuit technologies advance, designers increasingly need to account for higher switching speeds, tighter packaging, and more demanding interconnect performance.

The capabilities below outline advanced flex and rigid-flex PCB manufacturing options, such as controlled impedance routing, precision laser processing, sculptured copper features, EMI shielding, and strain relief, used to support higher technology parts and improve reliability in critical areas.

At a Glance: Advanced Flex Circuit Capabilities

• High-speed signal integrity support: As switching speeds increase, flex and rigid-flex traces must be treated as transmission lines; controlled impedance routing (stripline/microstrip) is used to minimize reflections and support error-free interconnections.
• Precision feature formation: Advanced laser processing supports drilling vias, micro holes, and slots using UV, CO₂, or hybrid systems, with dimensional/positional tolerances of 5 µm or better and precise depth control for accurate substrate cutting and skiving.

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Controlled Impedance Designs

As flex circuit designs and components become more complex with increasing signal switching speeds, it can be necessary to manage timing so that specific component functions occur before others. With higher processor clock speeds and switching speeds on modern flex circuits, interconnecting paths (traces) must be treated as transmission lines rather than simple interconnections.

To support these higher speed requirements, controlled impedance traces are used to reduce electrical reflections and promote error-free transitions between traces and interconnections. Flex and rigid-flex circuits are well-suited for interconnections engineered for signal integrity using stripline or microstrip techniques.

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Laser Drilled Slots and Holes

Laser hole drilling can be used for vias, micro holes, pinholes, nozzles, orifices, and slots, achieving dimensional and positional tolerances of 5 µm or better. Laser drilling processes can utilize ultraviolet (UV), CO₂, or hybrid systems (both UV and CO₂), selected based on hole diameter and material choice.

A combined UV and CO₂ laser approach is tied directly to flexible and rigid-flex processing. UV lasers are suited for efficient drilling of copper layers, while CO₂ lasers are suited for acrylic layers. Together, UV/CO₂ can support a one-time setup for drilling through flexible and rigid-flex circuits.

Best-in-class laser equipment with precise depth control can accurately cut underlying substrates in flex circuits. Laser beams can also skive circuit openings by removing thin layers with high precision.

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Sculptured Flex Circuits

Sculptured flex circuits are defined by copper conductors that vary in depth and thickness across the circuit. Copper is thinner in the most flexible regions and thicker at interconnection points. This construction supports bare metal connections (like a plug-in interface) while maintaining strength where solder joints form.

Sculptured fingers are presented as a more reliable alternative to mechanically fastened crimp pins. This approach allows thicker copper traces where structural strength or higher current is needed, while thinner trace regions help preserve flexibility.

Sculptured flex circuits can also reduce the need for additional connectors by integrating built-in contacts. This can be done by creating bare or exposed metal contacts beyond or between polyimide regions through selective 3D etching and plating, adding copper in targeted areas, and removing surrounding polyimide, so copper is exposed 360 degrees.

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EMI Shielding

Shielding is used when a flex or rigid-flex application must limit electromagnetic and/or electrostatic interference. Protective shielding can be patterned or made of solid copper or conductive silver ink.

Shielding material selection depends on the application, but the shielding must be conductive and grounded. The approach depends on the amount of EMI present in the device, the conductivity requirements, and how much flexing the circuit will experience. With expanding wireless communication networks, including cellular phones, EMI can be generated from many sources, including sources outside the device, and should be considered for critical flex and rigid-flex designs.

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Ultrasonic Welding

Ultrasonic welding uses high-frequency ultrasonic acoustic vibrations to join objects. This technique does not require bolts, nails, soldering materials, or adhesives to bind materials together, allowing flexible printed circuits to be combined with diverse materials and substances. With pre-assembly of application-specific adhesive systems, the flexible circuit can be permanently bonded with many different materials.

Stiffening effects can be achieved using plastic or aluminum backplates. When bonded to the flexible printed circuit (FPC), backplates can protect electronic components or help dissipate heat. Using this approach, flexible PCBs can also be encapsulated to guarantee certain IP ratings for customer devices.

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Graphic Overlays

Graphic overlays can be placed over flex circuits and are typically used with LEDs, LCD displays, or dome switches mounted on the circuits. This approach supports assemblies where the flex circuit integrates with display or switch interfaces under an overlay.

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Connector and Pin Assembly

For flex circuit assembly, Epec provides specific connector and pin assembly services based on the design. While full SMT services are not provided, flexible PCBs can be supplied with Zero Insertion Force (ZIF) connectors, mechanical crimp-on connectors, Z-axis connectors, and both through-hole and surface mount connectors. Connector styles referenced include circular, D-sub, and pin-and-socket types.

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Eccobond

Eccobond is a two-part epoxy coating applied at rigid-flex interfaces and stiffener-to-flex interfaces to provide strain relief. The most vulnerable area on a rigid-flex design is where a rigid PCB or stiffener meets the flexible circuit. At that transition, the flexible PCB can be bent past the 90-degree mark, which can break circuitry and create an open.

Eccobond is most effective when bonding dissimilar substrates such as FR4 and polyimide and is intended for use where shock and peel resistance are required. It is available in various hardness formulations, in black or clear, and is applied using a fillet technique with specialized dispensing equipment.

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Frequently Asked Questions

Quick Links

• Why do traces on modern flex circuits need to be treated as transmission lines?
• What is the purpose of controlled impedance in flex and rigid-flex circuits?
• What types of features can be produced with laser drilling on flex and rigid-flex designs?
• What makes sculptured flex circuits different from standard flex circuits?
• When is EMI shielding used on flex or rigid-flex circuits?
• How does ultrasonic welding support flex circuit assemblies?
• What problem does Eccobond address in rigid-flex designs?

Why do traces on modern flex circuits need to be treated as transmission lines?

As processor clock speeds and component switching speeds increase, interconnecting paths behave less like simple connections and more like transmission lines. This shifts design focus toward managing reflections and maintaining signal integrity across the interconnect.

What is the purpose of controlled impedance in flex and rigid-flex circuits?

Controlled impedance traces are used to minimize electrical reflections and support error-free transitions between traces and interconnections. Flex and rigid-flex circuits are well-suited for controlled impedance interconnections using stripline or microstrip techniques.

What types of features can be produced with laser drilling on flex and rigid-flex designs?

Laser drilling can be used for vias, micro holes, pinholes, nozzles, orifices, and slots. Processes may use UV, CO₂, or hybrid (UV and CO₂) systems, depending on hole diameter and material selection, with tolerances described as 5 µm or better.

What makes sculptured flex circuits different from standard flex circuits?

Sculptured flex circuits vary copper thickness across the design, thin in highly flexible regions and thicker at interconnection points. This supports flexibility where needed while strengthening areas tied to solder joint formation and higher structural or current demands.

When is EMI shielding used on flex or rigid-flex circuits?

Shielding is applied when an application requires limits on electromagnetic and/or electrostatic interference. The shielding must be conductive and grounded, and the approach depends on interference levels, conductivity needs, and how much flexing the circuit will endure.

How does ultrasonic welding support flex circuit assemblies?

Ultrasonic welding joins materials using high-frequency vibrations without bolts, nails, soldering materials, or adhesives, enabling flexible printed circuits to be combined with diverse materials. Backplates can be bonded for stiffening, component protection, or heat dissipation, and encapsulation can be used to guarantee certain IP ratings.

What problem does Eccobond address in rigid-flex designs?

Eccobond provides strain relief at rigid-to-flex and stiffener-to-flex interfaces, which are described as vulnerable points where bending past the 90-degree mark can break circuitry and cause an open. It is a two-part epoxy intended for bonding dissimilar substrates such as FR4 and polyimide, where shock and peel resistance are desired.

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Harness The Advanced Capabilities of Flex and Rigid-Flex Circuits

From controlled impedance designs through laser-drilled slots, the advanced capabilities described here support precision and signal integrity for high-technology flex and rigid-flex builds. Epec supports expert consultation to help align design needs with the appropriate manufacturing approaches.

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