Introduction

A rigid-flex PCB, also known as a flex-rigid PCB or flex circuit, is a printed circuit board that consists of rigid and flexible substrates laminated together. The rigid sections provide mechanical support while the flexible sections allow dynamic flexing, bending or folding of the assembly.

Rigid-flex PCBs provide solutions for electronic devices and components that require movement, vibration damping, compact 3D packaging, or the ability to conform to irregular shapes. They are commonly used in aerospace, military, medical, consumer electronics and automotive applications.

Advantages of Rigid-Flex PCBs

Rigid-flex PCBs offer several benefits compared to rigid PCBs or flexible circuits alone:

• Flexible bending and folding – The flex sections allow 3-dimensional conformability and dynamic flexing which is not possible with a fully rigid PCB. This helps fit PCBs into tight spaces and complex shapes.
• Vibration resistance – The flexible sections act as a damper which absorbs vibrations and mechanical stresses. This improves reliability and durability.
• Miniaturization – By combining multiple rigid PCBs onto a single flex-rigid assembly, the overall product size can be significantly reduced. The flexible sections eliminate the need for discrete connectors between separate PCBs.
• Lighter weight – Rigid-flex PCBs have a lower mass compared to using multiple rigid PCBs, connectors and cables. This provides weight savings.
• Increased reliability – The bonded connection between the layers eliminates solder joints and connectors which can fail over time. Rigid-flex PCBs have fewer potential failure points.
• Design freedom – Rigid-flex allows flexible layouts and 3D configurations that cannot be achieved with multiple rigid PCBs. Components can be placed on both sides of the flex sections.
• Reduced assembly cost – Automated assembly and soldering is simplified compared to assembling separate rigid boards and connectors manually.

Parts of a Rigid-Flex PCB

A rigid-flex PCB consists of four key elements:

Rigid Sections

The rigid sections of the PCB provide mechanical support and house surface mount components. They are made from standard FR-4 circuit board laminates which are rigid and do not flex. Common rigid layer thicknesses are 0.4mm (1/64″), 0.8mm (1/32″) or 1.6mm (1/16″).

Flexible Sections

The flexible sections allow dynamic bending and flexing of the circuit board. These are made from polyimide films such as Kapton or UPILEX which can be bent and flexed repeatedly without damage. Typical flex layer thicknesses range from 25μm (0.001″) to 100μm (0.004″).

Coverlay

The coverlay is a protective layer laminated over the flexible circuitry to prevent shorting and damage. The coverlay has openings for components and vias. Common coverlay materials include LCP, polyimide, or acrylic. The coverlay thickness is typically around 25-50μm.

Bonding Adhesive

Acrylic or epoxy adhesives are used to bond the rigid and flexible layers together into a complete circuit board. The adhesive must withstand flexing, temperature extremes, humidity, and other environmental conditions.

Stiffeners

Optional stiffeners made from thicker FR-4 can be added to strengthen and support the flexing regions. Stiffeners help control the bend radius and prevent damage.

Manufacturing Process

Rigid-flex PCBs require specialized manufacturing processes to laminate the rigid and flexible materials together. Here are the main steps:

1. Fabricate the individual rigid and flexible circuit layers separately.
2. Print solder masks and coat surfaces.
3. Drill component and tooling holes.
4. Align layers and sandwich adhesive between them.
5. Use a thermal-pressure lamination process to bond layers together.
6. Repeat to laminate additional layers.
7. Automated optical inspection.
8. Route outer circuit shape with precision routing tools.
9. Test for continuity and shorts.
10. Apply solder paste and place components for SMT assembly.
11. Run through solder reflow oven to solder components.
12. Conformal coating as needed.
13. Final testing and inspection.

The process requires careful layer alignment and registration to ensure the conductors on each layer correctly interconnect. Highly specialized presses are used for the lamination steps.

Rigid-Flex PCB Design Considerations

Designing rigid-flex PCBs requires attention to details such as:

• Layer stacking – Ensure adequate rigid layers are used to avoid flexing issues under components. Follow minimum bend radius rules.
• Routing – Avoid routing traces perpendicular to the flex area. Use teardrop pads for better adhesion.
• Component placement – Position tall components wisely to avoid collisions when flexed. Use flexible solder joints.
• Transitions – Taper the rigid sections near bends to reduce stresses. Allow sufficient adhesive fillets.
• Flex folds – Design precise fold lines and incorporate rolled or sliding flex folds where needed.
• 3D modeling – Simulate the PCB flexing and motion in 3D design tools. Check for collisions.
• Thermal management – Use thermal reliefs, zone filling, and/or thermal vias in the rigid sections. The flex areas have lower thermal conductivity.
• ESD protection – Use proper grounding and ESD protection measures in the flexing regions.

Following DFM guidelines and working closely with your PCB manufacturer is key to a successful rigid-flex design. Prototypes are highly recommended to test the complex assemblies.

Rigid-Flex PCB Applications

The unique properties of rigid-flex PCBs make them suitable for products such as:

Wearable Electronics

• Smart watches
• Health/fitness trackers
• VR/AR headsets
• Body sensors

Rigid-flex allows the PCB to conform to the user’s wrist or body comfortably. The flexible sections absorb mechanical stresses that would damage a rigid PCB.

Portable & Handheld Devices

• Laptops
• Tablets
• Cellphones
• Cameras
• Portable ultrasound

Rigid-flex PCBs provide reliable interconnections while saving space and allowing motion. The boards can be folded into complex 3D shapes to fit into thin, constrained enclosures.

Vehicle Electronics

• Auto instrument clusters
• Backup cameras
• Blind spot detection
• In-vehicle entertainment
• Telematics

Rigid-flex circuits withstand vehicle shock, vibration and temperature extremes while providing movement needed for adjustable displays and mechanisms.

Medical Electronics

• Hearing aids
• Pacemakers
• Endoscopes
• Patient monitoring
• Surgical robots

The boards can flex repeatedly to accommodate body movement. Rigid sections provide strength for components while thin, flexible interconnects improve reliability and save space.

Defense & Aerospace

• UAVs
• Satellites
• Missiles & munitions
• Soldier-worn systems
• Radars

Rigid-flex is ideal for high-reliability applications that undergo vibration and mechanical stresses. The boards can integrate multiple rigid PCBAs into one assembly.

Consumer Electronics

• Digital cameras
• Game controllers
• Keyboards
• Larger flexible OLED displays

Foldable rigid-flex PCBs allow new form factors for mobile tech. Custom shapes, bending displays and hinged assemblies are possible.

With innovative design, rigid-flex PCBs can enable applications that would not be possible using only rigid boards or flexible circuits alone. As manufacturing processes and base materials continue improving, we can expect wider adoption across many industries that require reliable, moving electronics.

Rigid-Flex PCB Materials

There are a wide range of materials that can be used for the construction of rigid-flex PCBs, with the most common being:

Rigid Substrates

• FR-4 – Most common rigid PCB substrate. Woven fiberglass with epoxy resin. Low cost but more limited in high frequency/high speed uses.
• Polyimide – Flexible at higher temperatures. Very dimensionally stable over temp. Excellent high frequency performance.
• CE Series High-Tg Card – Specialized polyimide and cyanate ester cards for flex-rigid.
• Rogers RO4350B – High frequency circuit material. Expensive but has excellent electrical properties.
• Arlon 85N – High performance rigid laminate with very low moisture absorption.
• I-Tera MT40 – Woven fiberglass-reinforced ceramic-filled PTFE composite. Extreme rigidity and temperature resistance.

Flexible Substrates

• Kapton – Polyimide film from DuPont. Most common flex circuit material with high chemical resistance.
• UPILEX – Polyimide film similar to Kapton but with higher temperature rating (400°C vs 300°C).
• Apical – Thin avimid polyimide films as thin as 25um (1mil). Low dielectric constant.
• LCP – Liquid Crystal Polymer films have excellent chemical resistance. Stable electrical properties.
• PEEK – Polyether ether ketone thermoplastic has high temperature and chemical resistance.

Bonding Films

• Acrylic – Most common adhesive. Fast curing and low cost. Limited chemical and temperature resistance.
• Modified Acrylic – Improved acrylics with enhanced resistance to high temp, humidity, and chemicals.
• Epoxy – Stronger bonds but slower curing. Withstands higher temperatures and humidity.
• Polyimide – High temp adhesive films. Can bond at up to 400°C (750°F). Excellent chemical resistance.

The choice of materials impacts the rigid-flex PCB’s electrical performance, flexibility, temperature rating and cost. Consult closely with potential PCB manufacturers when selecting materials.

Rigid-Flex Design Software Tools

To aid in the design of rigid-flex PCBs, various software tools are available:

• Cadence Allegro Rigid-Flex – Provides extensive rigid-flex design capabilities for layer stackup, 3D modeling, bend simulation, documentation, etc. Integrated with Allegro PCB Editor.
• Mentor Xpedition – Tools for rigid-flex board layout, 3D modeling, documentation. Real-time DRC checking tailored for flex.
• Altium – Layer stack manager and rigid-flex capabilities built into Altium Designer. Allows defining rigid/flex areas and simulating bend.
• Zuken CR-8000 – Rigid-flex design functionality including 2D/3D modeling, autorouting, analysis, and documentation.
• Siemens Solid Edge – MCAD software with tools for modeling flex circuit 3D motion and collision detection.
• ANSYS – Simulation software to model stresses and perform FEA analysis of rigid-flex bending.
• KeyCreator – Popular MCAD tool with intuitive 3D modeling capabilities to visualize rigid-flex designs.

Proper rigid-flex design software with flex modeling capabilities is highly recommended. This helps validate designs early and avoids costly errors due to improper layer configs or mechanical aspects.

Rigid-Flex PCB Cost Considerations

Some key factors that impact the costs of rigid-flex PCBs:

• Number of layers – Costs increase with more layers due to additional materials, lamination cycles, and process complexity.
• Board size – Larger boards require more raw materials driving higher cost. Panel utilization also becomes lower.
• Flex layer count – More flex layers means additional flexible dielectric films, adhesives, and handling.
• High speed design – Tight impedance control, advanced materials, and extra fabrication steps add cost.
• Small batch quantities – Low volume or prototype orders cannot take advantage of panel efficiencies.
• Advanced materials – Exotic rigid/flex materials and adhesives cost more than standard FR-4 and acrylic.
• Tight tolerances – Holding very tight layer-to-layer registration and feature tolerances adds cost.
• Testing – Rigid-flex designs may require extensive inspection, testing, and qualification due to the criticial nature of many applications.
• Special processing – Unique PCB fabrication requirements drive additional cost.

Working with a rigid-flex PCB supplier, you can evaluate design options to achieve the optimal functionality and cost. Just like any technology, costs continue to decrease over time with improving manufacturing capabilities and economies of scale.

Rigid-Flex PCB Examples

Here are some examples of real-world products using rigid-flex PCBs:

Wearable Health Tracker

This health monitoring wristband houses the rigid PCB mainboard and display in the central module. Thin flexible interconnects extend to the curved ends allowing it to wrap comfortably around the wrist.

Drone Gimbal Assembly

Drones use flex circuitry to interconnect the flight controller board with the gimbal assembly which provides stabilized camera pointing and control.

Hearing Aid Circuit

Hearing aid packaging requires ultra-compact, lightweight PCB solutions. Rigid-flex allows the components to be assembled on a folded 3D assembly.

IV Pump

Medical infusion pumps use rigid-flex PCBAs that can withstand repeated flexing to improve reliability. Safety is critical in these applications.

Automotive Center Console

Sliding and folding mechanisms in car interiors are interconnected using rigid-flex circuits.

With innovative product design, many other applications can benefit from the unique capabilities of rigid-flex PCBs. As fabrication processes and materials continue improving, adoption is expected to accelerate across industries.

Frequently Asked Questions

Here are some common questions about rigid-flex PCB technology:

What are the major applications for rigid-flex PCBs?

The most common applications are aerospace, military, medical devices, automotive, and consumer electronics. Any products that need to combine rigid PCBS with dynamic flexing or folding can potentially benefit from rigid-flex technology.

What types of materials are used in rigid-flex PCB construction?

The rigid sections use typical PCB substrates like FR-4, polyimide, or Rogers materials. The flexible layers use polyimide films such as Kapton or UPILEX. Bonding is done with acrylic, epoxy, or polyimide adhesives.

Can components be mounted on the flex section?

Yes, but the parts must be specially processed with flexible terminations. Common flexible components include small passives, LEDs, buttons, and connectors. Larger ICs can sometimes be fixtured onto flex areas.

How many times can a rigid-flex PCB be flexed before failing?

Properly designed rigid-flex PCBs can endure hundreds of thousands of flex cycles. The flex life depends on the materials used, trace layout, bend radius, and other factors.

How small can the bend radius be with rigid-flex?

Typical minimum bend radius is about 6-10 times the total board thickness. With special materials and structures this can potentially be reduced for very tight folding applications.

Can rigid-flex PCBs be double-sided or multilayer designs?

Yes, rigid multilayer and double-sided flex configurations are very common in rigid-flex PCBs. Complex designs are possible with 10, 12 or more metal layers.

Are there any limitations on rigid-flex PCB size?

There are no inherent size limitations. Rigid-flex PCBs have been fabricated in sizes ranging from tiny hearing aid boards up to 18″x24″ aerospace assemblies.

Can vias be used in the rigid-flex sections?

Yes, plated through holes (PTHs) and buried or blind vias can be used in both the rigid and flex areas. Microvias are also possible but require additional process steps.

Is electrical testing/ICT required for rigid-flex PCBs?

Yes, all rigid-flex PCBs should undergo meticulous electrical testing. Flying probe testers are commonly used since fixtures can damage the flexing regions. ICT and boundary scan provide the highest test coverage for critical boards.

Conclusion

Rigid-flex PCB technology leverages the unique benefits of combining rigid circuit boards and dynamic flexible circuits into one integrated assembly. With creative design, they can enable innovative electronic products with movement, compact 3D packaging, ruggedness, and improved reliability compared to assemblies made only with rigid PCBs.

As rigid-flex PCB fabrication matures, the costs continue to decline and the technology is becoming accessible to a broader range of industries and applications beyond military and aerospace. With the right design partner, rigid-flex PCBs can help bring visionary products to reality by combining the functionality of multiple rigid boards and connectors into one optimized flexible circuit assembly.