Advancing technology has impacted rigid-flex PCB designs in the same manner as rigid printed circuit boards. Increasing interconnect requirements and smaller form factors require higher circuit densities and increased layer counts. The impact of these elements on rigid-flex circuit designs is potential reliability issues both electrically and mechanically.
In this blog, we discuss the specifics of both the electrical and mechanical concerns and how to eliminate them by utilizing an air-gap design and construction method in your rigid-flex PCB.
Impact Of Higher Flex Layer Counts
The typical method of addressing higher layer counts in a design is to laminate them into one finished structure. This works for rigid circuit boards but creates both electrical and mechanical reliability concerns in a rigid-flex circuit board design.
The first issue is mechanical bend capability. The thicker a flex area, the less the bend capability due to the added strain placed on the copper circuitry. A common minimum bend guideline for a 1- to 2-layer flex, in a static application, is 10X the flex thickness. This increases to 20-30X for a laminated 4-layer flex section. If the minimum bend capability is exceeded, the circuitry may crack, creating either complete or intermittent open circuits. A manufacturer is incapable of modifying the physical properties of the materials. This can only be addressed through design and construction methods.
The second issue of electrical reliability stems from the materials used in a rigid-flex PCB construction. Beyond the FR4 based rigid and pre-preg layers, used in the rigid areas, flex layers are laminated together using either a flexible acrylic or epoxy-based adhesive. Given that the flex layers extend throughout the rigid areas, so do the lamination adhesives. A negative property of both is that they have a much high co-efficient of thermal expansion than either FR4 or polyimide materials. When the rigid-flex circuit board is assembled or long term as the temperature fluctuates on the finished assembly in the field the adhesives expand and contract and place vertical stress on the via hole plating. This can also result in the formation of cracks in the copper plating and either complete or intermittently open circuits.
6-layer rigid-flex PCB stack-up with 4 laminated flex layers.
Air Gap Rigid-Flex PCB Design & Construction
To address the above concerns the industry, combined with IPC, developed a design and construction method referred to as an air-gap construction. In this configuration, the flex layers are retained as multiple individual sets, containing no more than 3 layers per set, as opposed to one lamination containing all layers. The preferred configuration is 2 layers per set, but IPC 2223 does allow for 3 layers if a design has controlled impedance lines in the flex area that has to be configured as a stripline for 2-sided shielding purposes. A surface microstrip configuration in the flex layers requires only 2 layers. The most common design configuration has 2 sets of flex layers, but it can have more. Note that a higher number of flex layers sets will ultimately constrain the bend requirements.
6-layer rigid-flex PCB stack-up with 4 air-gap flex layers
8-layer rigid-flex PCB stack-up with 6 air-gap flex layers
Via Reliability
An air-gap construction completely addresses the via reliability concern and results in a via structure as reliable as that of a rigid only printed circuit. With the flex layers configured as independent pairs, or sets, the flex layers are separated only by FR4 based pre-preg within the rigid areas. This results in both a higher bond strength, between layers, and eliminates all flexible adhesives from within the rigid areas and with it the high co-efficient of expansion issue.
The reliability of an air-gap construction has been proven through testing using IPC’s Interconnect Stress Test method. This test rapidly temperature cycles a sample of a specific construction. An air-gap construction exceeds the number of cycles for a pass result while a rigid-flex with 4 flex layers laminated together will fail.
An air-gap construction does require additional process steps as each flex pair requires coverlay on both sides, but this is easily outweighed by the saving from improved reliability and after assembly yields.
Mechanical Flexibility & Bend Reliability
Mechanically, the bend capability and reliability are significantly improved by an air-gap construction. The improvement is due to a combination of two elements. The first is that for each individual flex pair the flex thickness is significant reduced as compared to that of a completely laminated structure. This reduces what is referred to as the “I-Beam” effect. The second is that each flex pair is allowed to bend on its own based on its specific thickness without significant interference from the rest of the flex pairs. That is within reason. When bent, the individual flex pairs are allowed to take their own natural radius but at a point the flex pairs may come in contact with one another as the air-gap spacing between pairs is consumed. Under a severe “U”-shaped bend requirement and due to the slight difference in bend radius between pairs, the flex pair on the outside of the bend may constrain the inside flex pair to the point where it may start to buckle. This situation must be avoided as any buckling of a flex pair will exceed the material capabilities resulting in cracked circuits.
If the bend requirement is that of an “S” shape this issue does not occur as the 2 bends comprising the S shape cancel out the differences in bend radius between pairs.
severe u-shaped bend with buckled inside flex pair
Summary
An air-gap construction is the preferred solution to higher layer count rigid-flex PCB designs. It provides the ability to reliably achieve tighter bend requirement and eliminate any via plating reliability concerns. Bend the few specific examples in the blog there are wide variety of air-gap configurations available that address many different design challenges.
Please feel free to contact QFPCB to discuss your specific design needs and to have your construction requirements reviewed to ensure the reliability of your design.
QFPCB has 15 years of experience in the field of Rigid-flex PCB production, choose QFPCB, you will get a reliable supplier in China. Contact us today for your Rigid-flex PCB solutions.
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Introduction
A rigid-flex printed circuit board (PCB) is a PCB that contains both rigid and flexible substrate materials. The rigid sections provide mechanical support and component mounting, while the flexible sections enable connections between different rigid sections and allow the PCB to bend and flex.
Rigid-flex PCBs provide a space-saving and lightweight solution for electronic devices with complex interconnections and 3-dimensional form factors. They are commonly used in consumer electronics, automotive, aerospace, medical, and industrial applications.
This article will provide an in-depth overview of rigid-flex PCB technology, covering topics such as:
- What is a rigid flex PCB and how does it work?
- Rigid-flex PCB design considerations
- Rigid-flex PCB manufacturing process
- Rigid-flex PCB cost factors
- Rigid-flex PCB applications
- Advantages vs traditional PCBs
- Design guidelines and best practices
Understanding rigid-flex PCBs enables designers and engineers to effectively utilize this technology for products that require dynamic flexing, folding, wrapping, and complex 3D shaping.
What is Rigid Flex PCB?
A rigid-flex PCB consists of rigid PCB material combined with flexible PCB material, connected by a flexible adhesive material. The rigid sections provide mechanical support for components and the flexible sections allow dynamic movement and shaping.
The rigid sections are typically made of materials like FR-4, while the flexible sections use polyimide or other flexible dielectric materials. The layers are bonded together using advanced adhesion techniques. Conductive traces can be etched on both rigid and flexible layers to route signals between components.
Vias and component holes are plated through both rigid and flexible layers to enable vertical interconnections. Components are mounted on the rigid portions only, while the flexible portions provide dynamic interconnects between the rigid segments.
Rigid-flex PCBs enable:
- Dynamic flexing, bending, folding, and wrapping
- Interconnections between multiple PCBs
- Complex, dense, and compact electronics packaging
- Rugged devices with flex-resistant cabling
- Lighter and thinner products by eliminating wires and connectors
- 3D shaping and conformability
- Reduced assembly cost by combining multiple PCBs
Compared to using discrete rigid PCBs with external cables or flex circuit interconnects, rigid-flex PCB technology provides a fully integrated solution with design, performance and reliability advantages.
How Does Rigid Flex PCB Work?
A rigid-flex PCB integrates rigid board material (like FR-4) with flexible board material (like polyimide), allowing the PCB to be dynamically flexed and shaped. Here is how the technology works:
- Rigid segments provide mechanical structure for component mounting and heat dissipation.
- Flexible segments enable out-of-plane interconnections between rigid segments.
- Conductive layers are etched on both rigid and flexible materials.
- Plated through holes connect layers vertically across both rigid and flexible areas.
- Components are mounted on the rigid portions only.
- Flexible portions are dynamically folded, bent, wrapped, etc.
- The rigid-flex PCB can be shaped into 3D geometries.
This configuration allows rigid support where needed, while enabling flexible interconnects between sections that can be dynamically shaped as required.
The rigid and flexible materials are bonded into a single integrated PCB solution through advanced lamination processes. The materials must be properly adhered to withstand thermal and mechanical stresses during assembly and operation.
Proper design techniques are essential for rigid-flex PCBs to function correctly. Special considerations include:
- Trace routing on flexible layers to enable dynamic bending
- Minimizing rigid-to-flex transition stress
- Maintaining proper bend radius control
- Allowing for thickness changes between rigid and flex areas
When designed properly, rigid-flex PCBs can achieve extreme flexibility and 3D shaping while maintaining signal and power integrity across both rigid and flex sections.
Rigid Flex PCB Design Considerations
Designing a reliable rigid-flex PCB requires attention to details that are not relevant for standard rigid PCBs. Here are some key rigid-flex design considerations:
1. Layer Stackup
The designer must define an appropriate layer stackup with the necessary mix of rigid and flexible layers. Component density, routing requirements, dynamic flexing needs, and other factors influence the choice of stackup.
Typical rigid layers use FR-4 or other glass-reinforced laminates. Typical flex layers use polyimide or other flexible dielectric materials. Adhesive layers bond the rigid and flex layers into an integrated structure.
The stackup must balance factors such as flexibility, rigidity, thickness, impedance control, and manufacturability. More flex layers improve dynamic shapeability but reduce rigidity. Thicker rigid layers provide better heat dissipation but increase thickness.
2. Rigid-to-Flex Transitions
The intersections between rigid and flexible sections require careful management to avoid cracking or peeling stresses. The design should optimize factors like:
- Transition geometry – Generally gradual tapers or rounded corners
- Annular ring dimensions – Wider annular rings reduce stress
- copper pulls backs – Minimize uncovered copper near transitions
- Avoid acute angles – Use gradually curving corners instead
3. Bend Radius
Flexible segments must be designed to avoid exceeding the minimum bend radius. Too tight of a bend can fracture copper traces, delaminate layers, or damage components.
The bend radius depends on factors such as flexible material, layer stackup, copper thickness, and dynamic flexing requirements. Typical minimum bend radius values range from 3mm to 10mm.
4. Component Placement
Since flexible layers cannot support components, all components must be placed in rigid areas. There should be a keepout area around the edges of rigid sections to avoid component leads overlapping flexible areas.
High stress components like connectors should be placed away from rigid-flex transitions. Thermally critical components require sufficient copper area for heat dissipation.
5. Routing Considerations
Traces on flexible layers require design adjustments such as:
- Wider trace widths and spacing to withstand dynamic bending
- Avoiding acute angles
- Routing along the neutral bend axis where possible
- Balancing copper density across flexible layers
Rigid Flex PCB Manufacturing Process
Producing a reliable rigid-flex PCB requires advanced manufacturing techniques and processes. Here is an overview of rigid-flex fabrication:
1. Materials Preparation
The process starts with raw rigid and flexible core materials being prepared. The materials are cleaned, coated, and conditioned to optimize adhesion and processing performance.
2. Imaging
The individual inner and outer layers are imaged with the conductive pattern using lithography and etching processes. This forms the traces, pads, and features on each layer.
3. Hole Formation
Holes are drilled or punched through the core layers. This includes component holes, tooling holes, and through vias between layers. The holes are plated with copper to form the vertical interconnects.
4. Lamination
The rigid and flexible core layers are precisely aligned and bonded using high temperature and pressure. The bond line between materials is critical to performance. Advanced adhesives are used.
5. Outer Layer Imaging
After lamination, the outer layer traces and features are imaged using lithography and etching. This completes the conductive patterning on the PCB.
6. Solder Mask and Finish
Solder mask is applied to exposed copper surfaces. The edges of pads and traces are exposed. The PCB surface is finished with treatments like ENIG, immersion tin, or OSP.
7. Singulation
The large panel is precision routed to separate into individual PCBs. Rigid-flex PCBs often have complex board shapes.
8. Testing
Each completed PCB goes through electrical testing and inspection to verify function and workmanship. Automated optical inspection checks feature accuracy.
9. Assembly
Components are mounted on the rigid portions using soldering or adhesive attachment. The PCB can then be dynamically flexed into the 3D shape as required.
Rigid-flex PCB fabrication requires expertise in advanced processes like:
- Surface preparation and adhesion promotion
- High-accuracy imaging and pattern transfer
- Controlled lamination of dissimilar materials
- Mitigation of thermal and mechanical process stresses
- Precise techniques for routing, drilling, and hole plating
- DFM techniques for optimizing manufacturability
Rigid Flex PCB Cost Factors
Rigid-flex PCBs tend to cost more than standard rigid PCBs due to the specialized materials, processes, and lower fabrication volumes. Here are some of the key factors that influence rigid-flex PCB pricing:
1. Layer Count
Adding more conductive layers increases material costs, lamination complexity, and fabrication difficulty. High layer count rigid-flex PCBs cost exponentially more than 2-4 layer versions.
2. Panel Utilization
Rigid-flex PCB panels often have lower utilization due to complex board geometries. Less PCB area per panel drives up cost. Tight panel layout Optimization is critical.
3. Finishing and Coatings
The specialized solder mask, coverlay, and surface finish add cost compared to baseline FR-4 finishing. Thick copper, buried vias, and other techniques also increase cost.
4. Flexible Material Type
The flexible dielectric material choices like polyimide, LCP, PEN drive cost. More durable and heat-resistant flex materials are more expensive.
5. Registration Accuracy
The precision alignment of layers and drilling/routing accuracy requirements affect cost. Tighter tolerances require advanced equipment and processes.
6. Design Complexity
Dense routing, high component counts, HDI features, and impedance control requirements increase fabrication difficulty and cost.
7. Low Volume
The overall smaller market for rigid-flex PCBs prevents economies of scale. Shorter fabrication runs increase cost per board.
8. Testing
Rigid-flex PCBs require extensive inspection and electrical testing to validate quality. This adds cost compared to basic PCB qualification.
In addition to fabrication costs, there are engineering costs associated with specialized rigid-flex design, simulation, prototyping, documentation, and qualification.
Rigid Flex PCB Applications
Here are some of the major application areas taking advantage of rigid-flex PCB technology:
Consumer Electronics
Wearables, smartphones, laptops, tablets, cameras, and other connected consumer gadgets utilize rigid-flex PCBs to achieve folding, compact size, rugged cabling, and dynamic positioning.
Automotive Electronics
Automotive camera systems, powertrain electronics, LED lighting, and in-vehicle infotainment systems employ rigid-flex PCBs for shape conformability and vibration resistance.
Medical Equipment
Hearing aids, blood analyzers, surgical tools, and implants use rigid-flex PCBs to fit within tight, curved spaces and provide dynamic interconnections.
Industrial
Industrial rigid-flex applications include robotics, motion control, telemetry devices, and navigation systems that require ruggedized, shape-conforming PCBs.
Military/Aerospace
Avionics systems, guidance controls, communications, and navigation electronics in defense and aerospace applications utilize rigid-flex PCBs for compact, rugged performance.
Advanced HDI
Rigid-flex PCBs are commonly used as integrated carrier boards for ultra-high density interposers and multi-chip modules (MCMs) for applications like ASIC packaging.
From small wearable gadgets to large avionics systems, rigid-flex PCB technology enables solutions not possible with standard rigid boards – supporting dynamic motion, flexing, wrapping, and 3D shaping.
Advantages vs Standard PCBs
Here are some of the key benefits provided by rigid-flex PCBs compared to traditional rigid PCBs:
Dynamic Flexing and Motion
The flexible portions of rigid-flex PCBs enable motion, vibration absorption, twisting, bending, and wrapping unavailable with rigid PCBs.
Lightweight and Compact
By eliminating external cables and connectors, rigid-flex PCBs achieve significant size and weight reduction for portable and space-constrained designs.
Enhanced Reliability
The integrated rigid-flex structure avoids interconnection failures associated with cables and connectors. System reliability improves.
Complex Shaping
Conformal and complex 3D shapes can be achieved with rigid-flex PCBs to fit product enclosures and mechanics. Rigid PCBs are planar and 2D.
Simplified Assembly
Components can be pre-mounted on the rigid portions, enabling system assembly by simply folding the rigid-flex PCB into shape.
Design Flexibility
Rigid-flex PCBs allow optimal placement of rigid areas only where mechanically required, improving design flexibility.
Cost Savings
By reducing parts count, assembly steps, and handling, rigid-flex PCBs can achieve net cost savings compared to hybrid rigid/flex solutions.
For applications requiring dynamic motion, vibration resistance, light weight, or complex 3D assembly, rigid-flex PCB technology provides performance and reliability benefits over traditional rigid PCBs. The advanced integration reduces cost and complexity compared to hybrid solutions.
Design Guidelines and Best Practices
Here are some key design guidelines and best practices to follow when working with rigid-flex PCBs:
Manage Layer Stackup Carefully
- Use sufficient layers and thickness for rigidity, heat dissipation, and signal integrity.
- Minimize flexible layers to what’s required for dynamic bending motions.
- Balance properties like impedance control, flex/rigid transition stresses, and manufacturability.
Optimize Rigid-Flex Transitions
- Use gradual tapers and teardrop shapes at transitions.
- Keep components and vias back from transition intersections.
- Adjust solder mask pullbacks methodically.
- Analyze transition points using FEA stress simulation.
Maintain Proper Bend Radius
- Do not exceed minimum bend radius specifications.
- Keep bends in the central neutral axis of flex layers where possible.
- Dynamic flexing stresses can decrease bend radius over product lifetime.
Follow Flexible PCB Design Rules
- Use wider trace/space dimensions on flex layers.
- Avoid acute angles on traces and planes.
- Balance copper density across flex layers.
- Route critical signals near neutral bend axis.
Allow Adequate Board Tolerances
- Account for greater alignment variability between layers.
- Provide tolerance for dynamic shape variations during use.
- Ensure critical features have design margin.
Simulate PCB Performance
- Verify shape and motion with 3D CAD simulation.
- Perform stress analysis at rigid-flex transitions.
- Thermally modelComponents on rigid areas with sufficient copper.
- Verify signal integrity, especially across bends.
Facilitate Manufacturing and Testing
- Use standard materials when possible.
- Account forthickness variations between rigid and flex areas.
- Provide testpoints to access critical signals.
- Follow fabrication shop DFM guidelines.
With attention to these design principles, rigid-flex PCBs can be optimized to achieve the desired balance of dynamic flexing, shapeability, reliability, and manufacturability.
Rigid Flex PCB Design Software Tools
To facilitate working with rigid-flex PCB designs, engineers use various software tools for tasks like:
PCB Layout
- Altium Designer
- Cadence Allegro
- Mentor Xpedition
- Zuken CR-8000
3D Modeling
- Cadence OrCAD 3D Modeler
- Altium 3D PCB Visualization
- SolidWorks PCB 3D Modeling
Simulation and Analysis
- Ansys Q3D Extractor (for signal integrity)
- Ansys HFSSTM (for antenna design)
- Cadence Clarity 3D Solver (for thermal and mechanical analysis)
- Mentor Valor Process Preparation (for manufacturability verification)
Documentation
- AutoCAD
- PDF 3D
- Illustration and diagramming tools
Using these sophisticated design automation tools enables engineers to design and analyze complex rigid-flex PCBs with confidence. The tools help optimize parameters like layer stackup, trace routing, 3D shape modeling, signal and power integrity analysis, thermal analysis, and manufacturability validation.
Conclusion and Summary Points
Key points in summary:
- Rigid-flex PCBs contain both rigid board material (like FR-4) and flexible dielectric material (like polyimide), providing dynamic flexing and 3D shaping.
- Careful design is required to manage parameters like layer stackup, rigid-to-flex transition geometry, bend radius, trace routing, and component placement.
- Rigid portions provide mechanical structure while flexible portions enable out-of-plane interconnections between rigid sections.
- Rigid-flex PCBs achieve space, weight, complexity, and reliability benefits compared to traditional PCBs for many applications.
- Cost is higher than rigid PCBs but can be offset by system miniaturization, consolidation, and assembly benefits.
- Consumer electronics, automotive, aerospace/defense, medical devices and industrial automation are leading application areas.
- Following design guidelines for flex/rigid transitions, bend radius, trace routing, and modeling ensures reliable performance.
Rigid-flex PCB technology provides unique benefits for products requiring dynamic flexing, vibration resistance, lightweight and conformal assembly. By understanding the design, manufacturing and cost considerations, engineers can effectively apply rigid-flex PCBs as a solution.
Frequently Asked Questions
Here are some common questions on rigid-flex PCB technology:
Q: What are the main benefits of rigid-flex PCBs?
They allow dynamic flexing and interconnections, consolidation of multiple PCBs, lighter weight assembly, and improved reliability.
Q: What are some typical applications of rigid-flex boards?
Consumer products, computer components like drives, medical devices, robotics, defense, automotive electronics are common applications.
Q: What are the major design considerations for rigid-flex PCBs?
Stackup symmetry, adequate bonding widths, copper relief cuts, component layout, controlling flex stresses, and smooth impedance transitions are key considerations.
Q: How are rigid-flex PCBs manufactured?
Specialized lamination, drilling and routing processes are required along with precise process control.
Q: What drives the higher cost of rigid-flex PCBs?
Low volume production, additional process steps, advanced materials, complex designs and more inspection all contribute to increased costs.
