Introduction to Rigid Flex PCBs

Rigid flex PCBs are a specialized type of printed circuit board that combines the best features of rigid and flexible circuit boards. They consist of rigid PCB sections connected by flexible circuits, enabling 3D configurations not possible with conventional rigid boards.

The flexible circuits are made from thin, flexible, insulating polymer substrates like polyimide with conductive copper traces. The rigid sections use standard FR-4 epoxy glass laminate. Rigid flex PCB Stackups intersperse flexible polyimide layers with rigid FR-4 layers.

Key Advantages of Rigid Flex PCBs

• Enable 3D packaging of electronics
• Eliminate need for connectors between PCB sections
• Reduce size, weight, and thickness of electronics
• Improve reliability by reducing solder joints
• Allow dynamic flexing for movable/hinged devices

Applications Well-Suited for Rigid Flex PCBs

• Aerospace and Military Electronics
• Medical devices
• Smartphones, tablets, laptops
• Wearable devices
• Robotics
• Automotive electronics
• Test and measurement equipment

Rigid Flex PCB Materials

Flexible Circuit Materials

The flexible layers in rigid flex PCBs are most commonly made from polyimide (PI) film. PI is available in different thicknesses (0.5-5 mils) and is prized for its excellent insulating properties, temperature resistance, dimensional stability, and mechanical strength.

Polyester (PET) film is sometimes used for low-cost consumer products. Polyethylene naphthalate (PEN) and liquid crystal polymer (LCP) are specialty materials used where low moisture absorption or precise dielectric constant control is needed.

The conductive traces are formed by laminating copper foil to the flexible polymer substrate. Rolled annealed (RA) copper is used for its high ductility. Electrodeposited (ED) copper with lower ductility is unsuitable for dynamic flexing.

Adhesives for Flexible Circuits

Flexible circuit materials are bound together using acrylic or epoxy adhesives. Adhesiveless materials with the PI film directly cast onto the copper foil are also available (e.g. DuPont Pyralux AP). The elimination of adhesives increases flexibility and thermal resistance.

Coverlay Films

Coverlay films are used as the outer insulating layer on flexible circuits, protecting the copper traces. Polyimide and polyester coverlay materials are common. Photoimageable solder masks can be used in place of coverlay films.

Rigid PCB Materials

The rigid sections of rigid flex PCBs use standard PCB materials such as:

• FR-4 epoxy glass laminate
• High Tg FR-4 for lead-free assembly
• Polyimide glass for higher temperatures
• Teflon (PTFE) laminates for RF/MW

Bonding Flexible and Rigid Sections

The rigid and flexible sections of the PCB are joined using specialized acrylic or epoxy adhesives. The adhesive must be carefully selected to provide good adhesion and minimize stresses on the materials during fabrication and use.

Rigid Flex PCB Stackups and Configurations

Simple Rigid Flex Stackups

The simplest rigid flex stackups have a single flexible layer laminated between two rigid layers. This allows the PCB to be bent or folded once. More complex designs use multiple flexible layers laminated between rigid layers.

Bookbinder Construction

Bookbinder rigid flex stackups have one or more long flexible layers laminated between rigid sections. This allows the board to be folded like a book, with the flexible circuit acting like the spine. Bookbinder construction is commonly used to stack rigid PCBs on top of each other.

Origami Construction

Origami-style rigid flex PCBs have multiple flexible layers that allow the board to be folded into a compact 3D shape, similar to origami paper folding. This enables dense packaging of electronics in a small space.

Flex Tails and Flex Extenders

Flex tails and extenders are short flexible circuits used as interconnects between rigid PCB sections or connected to other devices/subsystems. They eliminate the need for connectors or wire harnesses.

Shielded Flex Layers

For applications requiring shielding, ground planes can be placed on flex layers above and below sensitive signals. This creates a stripline controlled impedance structure that shields the signals from interference.

Rigid Flex PCB Layer Count and Thickness

Rigid flex PCBs can range from 3 layers (1 flex + 2 rigid) to 20+ layers for complex designs. The maximum number of layers is limited by the stresses on the materials during lamination and the minimum bend radius.

Overall thickness depends on the layer count, material thicknesses, and copper weights specified. Rigid flex PCBs are typically 0.062-0.125″ thick, but can exceed 0.250″ for high layer counts.

Designing Rigid Flex PCBs

Choosing Materials

The first step in rigid flex PCB design is selecting the appropriate materials. The material properties must meet the mechanical, electrical, and environmental requirements of the application.

Key material properties to consider include:

• Dielectric constant (Dk) and loss tangent (Df)
• Tensile strength and elongation
• Flexural strength
• Glass transition temperature (Tg)
• Moisture absorption
• Flammability rating

IPC-4101 specifies electrical and mechanical properties for various rigid and flexible laminates. Consult with your PCB fabricator on available material options.

Determining Layer Stackup

The layer stackup should be defined early in the design process. The number of layers and arrangement of rigid and flex sections will determine the mechanical configuration possibilities.

Rigid sections should use an even number of layers to prevent warpage. Use symmetric stackups to minimize stresses. Flexible layers should be placed near the center of the stackup when possible.

Consider using dual-sided flexible layers in rigid sections to improve adhesion. Avoid burying flex layers in the rigid stackup, as this increases stresses during bending.

Placing Components

Component placement is critical in rigid flex design. Heavy or tall components should be placed on rigid layers only. Avoid placing components near the rigid-to-flex interface, as this area experiences the most stress during bending.

Routing Signals

Copper traces on flexible layers should be routed perpendicular to the bend axis to minimize stresses. Avoid running traces along the bend radius.

Use curved traces and teardrops to improve reliability. Avoid acute angles in traces.

Increase annular rings and pad size to improve flexibility. Use additional through-holes and microvias to provide mechanical anchoring between layers.

Defining Keep-Out Areas

Keep-out areas should be defined in the mechanical CAD data to specify regions where components, traces, and vias are not allowed. This includes:

• Rigid-to-flex interface area
• Bend areas requiring access holes
• Flex circuit areas to keep free of components

Creating Cutouts and Curves

Cutouts are needed in flex and rigid layers to allow folding/bending of the PCB. Use generous bend radii to minimize stresses – typically at least 10x material thickness.

Curves should be used instead of sharp corners to reduce stress concentrations. Provide strain relief openings at the rigid-to-flex interface.

Adding Shielding

For designs with EMI susceptibility, shielding can be incorporated by placing ground layers above and below critical signals on flex layers. This creates a stripline structure that shields the signals. Exercise caution when joining ground layers between rigid and flex sections.

Panelization Considerations

Rigid flex PCBs are commonly fabricated in panelized format. Flex areas should be oriented in the same direction if possible. Use routed tabs with curved corners to restrain flex areas during assembly. Provide sacrificial material in flex areas for clamping during plating and processing.

Rigid Flex PCB Fabrication and Assembly

Fabrication Process Overview

Rigid flex PCB fabrication involves additional steps and processes compared to standard rigid PCBs, including:

1. Imaging and etching of flexible layers
2. Coverlay or solder mask application
3. Lamination of rigid and flex sections
4. Drilling and plating of through-holes
5. Cutting of flex circuit geometries
6. Outline routing and singulation

Key Fabrication Challenges

The main challenge in rigid flex fabrication is managing the stresses on the materials during lamination and processing. The dissimilar properties of the rigid and flexible materials can cause warping, delamination, or damaged plated through-holes if not properly addressed.

Dimensional stability is also critical, particularly for the flexible layers. Special handling and fixturing are required to prevent stretching or distortion of the flex layers during processing.

Lamination Considerations

Careful control of lamination parameters (pressure, temperature, time) is required to bond the dissimilar materials while minimizing stresses. Adhesive flow and squeeze-out must be tightly controlled.

Flexible layers must be adequately supported during lamination to prevent distortion. For inner flex layers, a “hard plate” such as a polished steel plate is used between flex layers in the lamination press.

Coverlay Application

Flexible coverlays are applied using a hydraulic press or autoclave. Photoimageable coverlay (PIC) may be used for finer feature resolution.

If a flexible solder mask is used instead of coverlay, a “bump” process may be required to encapsulate the traces before applying the liquid photoimageable solder mask (LPSM).

Cutting Flex Circuits

Flexible circuit outlines are cut using a variety of methods, including:

• Steel rule die
• Laser cutting
• Routing
• Water jet cutting

The choice of cutting method depends on material, thickness, minimum bend radius, and feature size requirements.

Assembly Process Considerations

Assembly of rigid flex PCBs requires specialized processes and fixturing to prevent damage to the flex layers. Key considerations include:

• Supporting flexible sections during component placement and soldering
• Use of low-stress soldering profiles
• Careful control of reflow temperatures to avoid damaging flex materials
• Underfilling of components to improve reliability
• Conformal coating to protect assembled circuits

Testing and Qualification of Rigid Flex PCBs

Continuity and Isolation Testing

Continuity and isolation testing are performed to verify the electrical integrity of the rigid flex PCB. This includes:

• Net list verification to check continuity
• Isolation resistance testing between nets
• High potential (HiPot) testing

Structural Integrity Testing

Structural integrity testing verifies the mechanical robustness of the rigid flex construction. Key tests include:

• Cross-section analysis
• Microsection analysis
• Thermal stress testing
• Accelerated thermal cycling
• Bending/flexing cycling tests
• Adhesion testing of coverlay and rigid-to-flex bonds

Environmental Testing

Environmental testing ensures the rigid flex PCB can withstand the conditions of its intended application. Relevant tests may include:

• Thermal shock
• Humidity exposure
• Vibration and shock
• Salt spray
• Fungus resistance

Functional Testing

Functional testing exercises the rigid flex PCB in its final assembled configuration to verify proper operation. This may involve:

• In-circuit testing (ICT)
• Flying probe testing
• Boundary scan testing
• System level functional test

Advantages of Rigid Flex PCBs

Size and Weight Reduction

One of the primary advantages of rigid flex PCBs is the ability to significantly reduce the size and weight of electronics packages. By folding the PCB into a 3D configuration, the footprint can be minimized. Eliminating connectors between PCB sections also saves space.

Improved Reliability

Rigid flex PCBs offer several reliability improvements over conventional PCB assemblies:

• Reduction in number of solder joints
• Elimination of connectors
• Lower interconnect resistance
• Reduced opportunity for contact fretting, shock damage

Studies have shown rigid flex PCBs can offer a 10x improvement in reliability compared to wire harnesses or connectorized PCBs.

Increased Flexibility

The incorporation of flexible layers allows the PCB to be dynamically flexed or folded during use. This enables applications not possible with rigid boards, such as fold-out displays, hinges, and wearables.

Better Signal Integrity

Rigid flex PCBs enable shorter interconnects between circuits, which reduces signal loss, crosstalk, and EMI. The ability to shield signals on flex layers also improves signal integrity.

Disadvantages of Rigid Flex PCBs

Higher Cost

Rigid flex PCBs have higher material and fabrication costs compared to rigid PCBs. The specialized materials, processes, and expertise required drive up the unit cost, particularly for low volumes. However, the total system cost may be lower by eliminating connectors and improving reliability.

Longer Lead Times

The fabrication cycle time for rigid flex PCBs is longer than rigid boards due to the additional processing steps required. Typical lead times are 3-5 weeks versus 1-2 weeks for rigid PCBs.

Limited Supplier Base

Fewer PCB fabricators have the capabilities to manufacture rigid flex PCBs, which can limit sourcing options and increase costs. Ensure your supplier has a proven track record with rigid flex construction.

Greater Design Complexity

Rigid flex PCBs require careful attention to material selection, stack-up design, and mechanical constraints. The designer must have a good understanding of the materials and fabrication processes to ensure a successful design.

Frequently Asked Questions

Q1: What is the minimum bend radius for flexible circuits?

A1: The minimum bend radius depends on the thickness and material properties of the flexible layers. In general, the minimum bend radius is 6x the total thickness of the flexible circuit. For dynamic flexing applications, a bend radius of 10-12x total thickness is recommended.

Q2: Can rigid flex PCBs be reworked?

A2: Rigid flex PCBs can be reworked, but special care must be taken not to damage the flexible layers. Low-temperature soldering techniques and heat shielding are typically used. Localized repairs to damaged flex circuits are possible but require specialized expertise.

Q3: What is the maximum operating temperature for rigid flex PCBs?

A3: The maximum operating temperature depends on the materials used. Polyimide films are typically rated for operation up to 150-200°C. Solder mask and coverlay materials may have lower temperature limits. Consult with your fabricator on the temperature ratings of specific material combinations.

Q4: How do rigid flex PCBs compare in cost to rigid PCBs?

A4: Rigid flex PCBs are typically 2-5x the cost of rigid PCBs on a per-unit basis, depending on complexity, materials, and volume. However, the total system cost may be lower when the elimination of connectors and improvements in reliability are considered.

Q5: Are there any limitations on component types that can be used on rigid flex PCBs?

A5: Most standard surface mount and through-hole components can be used on rigid flex PCBs. However, heavy or tall components should be placed on rigid layers only to avoid stressing the flexible layers. Fine-pitch BGA and QFN packages are commonly used but require careful layout and assembly processes.

Conclusion

Rigid flex PCBs offer a powerful solution for electronics packaging, enabling significant reductions in size, weight, and complexity while improving reliability. By combining the benefits of rigid and flexible circuits, they allow designers to create products not possible with conventional PCBs.

However, rigid flex PCBs also come with trade-offs in terms of higher costs, longer lead times, and greater design complexity. Designers must carefully weigh the benefits and drawbacks to determine if rigid flex is the right choice for their application.

As technology continues to advance, rigid flex PCBs will play an increasingly important role in a wide range of industries, from consumer electronics to aerospace. By understanding the capabilities and limitations of rigid flex technology, designers can unlock new possibilities for innovation.