Rigid-flex PCBs integrate rigid boards and flexible circuits into a single assembly optimizing space, weight and electronics connections. Adoption continues growing across aerospace, defense, medical and consumer devices. But properly configuring design tools and planning layer stacks are key to avoiding rigid-flex fabrication issues.
This article provides guidance on critical rigid-flex design considerations including:
- Command settings for flex modeling
- Layer stack arrangement best practices
- Managing transitions between rigid and flex areas
- Implications for fabrication and assembly
Following these guidelines helps prevent problems like stiffener tears, shorts between conductors, or excess flex stresses that can undermine reliability.
Configuring Design Tools for Rigid-Flex
While flexible circuits can be designed standalone, integrating them into larger rigid boards requires configuring circuit board design tools carefully to enable and validate the blended rigid-flex assemblies.
Rigid vs Flex Design Rules
One primary consideration is the very different constraints and tolerances governing rigid portions versus flexible areas:
<div style="text-align:center">| Rigid PCB Requirements | Flex Circuit Requirements | |
|---|---|---|
| Conductor Width | ≥ 5 mils | ≥ 2 mils |
| Conductor Spacing | ≥ 5 mils | ≥ 1 mil |
| Bend Radius | None | ≥ Thickness |
| Annular Rings | 4-6 mils | 2-3 mils |
This disparity means software tools must allow separate rigid and flex design rule sets. Manufacturers like Cadence Allegro, Mentor, and Altium allow assigning different parameters to board sections.
Flex Material Modeling
Another element is modeling the distinct properties and behavior of flexible substrate materials versus typical rigid laminates to prevent creating an unmanufacturable design. This involves defining:
- Flex layer dielectric properties
- Adhesion layers
- Cover layer materials
- Bonding film if any
Assigning suitable flex layer properties enables simulation of behaviors like impedance in flex bends, dielectric gaps, signal losses, etc.
Flex Area Creation
Specialized commands allow designating distinct rigid, flex, and rigid-flex boundary zones within the PCB editor. This provides appropriate rule sets and modeling to unique areas.
Segregating the zones also facilitates defining different layer stacks in the respective regions. This helps optimization like removing unused layers in certain sections to control costs, while retaining sufficient layer count for complex rigid domains.
Design Analysis Settings
In analysis like signal integrity or thermal simulations, the combination of rigid and flexible substrates requires configuring composite material properties and models that represent the blended PCB correctly.
Specifically this entails defining parameters like:
- Composite dielectric constant
- Composite loss tangent
- Composite thermal conductivity
- Composite CTE (coefficient of thermal expansion)
With the design system configured for rigid-flex requirements, let's examine key stack-up recommendations.
Layer Stack Arrangements for Rigid Flex
Carefully planning layer stack-ups in both rigid and flex sections helps avoid problems like finishes getting etched away during exposures or conductors shorting in bend areas:
1. Segregate Rigid and Flex Areas
As highlighted already, separating rigid board and flex circuit layers enables optimizing each area’s stack arrangements rather than forcing a common configuration.
Isolating the regions also allows differing layer counts in rigid vs flex zones minimizing overall layers for cost savings.
2. Bury Flex Layers
Arranging the flexible layer set towards middle of the total buildup helps prevent uniformity issues during lamination which can lead to tom stiffeners or cracked conductive layers.
3. Place Ground Layers Adjacent to Flex Layers
Ground planes next to flex circuits layers helps shield signals from noise and prevents coupling into neighboring layers reducing key rigid-flex integration risks.
4. Assign Thicker Finishes to External Layers
Since exterior fabrications get subjected to most handling stresses and environmental exposure, assigning thicker finishes like ENIG or immersion tin versus HASL helps strengthen resistance to wear and oxidation.
5. Include Bonder Coversheets
Incorporating thin bondsheets around flexible layers prevents resin wicking into the flex area during lamination which can impede flexibility and risk conductor damage.
6. Add Bondflex or Support Layers
Additional thin insulating layers around flexible layers provide more gradual transitions in stiffness between rigid and flex areas minimizing separation or fracturing under bend stresses.
Here is an example 12 layer rigid-flex layer stack-up reflecting these guidelines:
<div style="text-align:center"><img src="https://www.researchgate.net/profile/Eric-Perfecto/publication/277765208/figure/fig3/AS:614305825644547@1523427415785/A-cross-section-view-of-a-typical-Rigid-Flex-PCB-stack-up.png" width="500"> </div>Rigid-Flex Layer Stackup (Source: researchgate.net)
With principles for arrangements covered, let's explore finishing layer options.
Finish Layer Alternatives for Rigid-Flex PCBs
Final metal treatments on external layers demand special consideration for rigid-flex designs where standard PCB finishes can result in poor flex flexibility or wear issues during dynamics:
Immersion Silver
Offers excellent flex durability under continuous bending along with extended shelf life versus copper oxidation over time. But cost is relatively high.
Immersion Tin
More affordable than silver but still provides suitable flex flexibility and oxidation resistance for many apps. However has more limited shelf life versus silver.
ENIG - Electroless Nickel Immersion Gold
ENIG is a popular rigid-flex finish providing good flexibility, shelf life and connection integrity. But lower gold thickness options are needed to control cost.
Additional Flex Encapsulation
For extreme mechanical demands, applying protective shielding like parylene coating or silicone potting over flex areas augments durability though increases materials expenses.
We’ve covered key principles for layer stacks and finish layers, next we’ll explore guidelines for transitions between rigid and flexible regions.
Managing Rigid-Flex Interface Challenges
The intersections between rigid board sections and flexible circuits warrant very careful management in designs to minimize immense stresses that can fracture layers, detach interfaces, or rupture plated through holes:
Maintain Continuous Ground Planes
By passing ground planes continuously across rigid-flex boundaries, this ensures a reliable return path for signals transitioning between sections. Breaks in ground risks noise coupling and erratic function.
Include Strain Relief Features
Specialized rigid-flex design tools facilitate including perforations and geometry adaptations like tongue-and-groove connections along section borders to dissipate stresses gradually and avoid abrupt material transitions.
Reinforce Vias Near Borders
Since drilled plated through holes can represent fracture points as flex circuits bend, adding extra via reinforcement such as conductive annular rings helps strengthen hole reliability.
Verify Layer Registrations
The precision alignment of layers, copper traces, and other features can degrade near borders during dynamic bending. Confirming sufficient margin for registration error in the design helps overcome this like widening spacing slightly.
Proactively addressing these transition points using guidelines like these prevents the bulk of rigid-flex defects that might emerge.
Impacts on Manufacturing and Assembly
While integrating rigid boards and flex circuits produces smaller, lighter, and higher performance solutions, the blended configurations do introduce additional considerations for manufacturing and assembly.
Fabrication Process Effects
Combining distinct processes like rigid laminate pressing along with flexible layer bonding while preserving registration across sections poses challenges for fabricators to deliver quality and yields.
Evaluating manufacturer capabilities specific to rigid-flex like alignment tolerances and minimum bend radii is critical when sourcing to avoid production issues.
Assembly Challenges
With PCB sections at multiple angles and elevation, automated SMT pick and place struggles placing some components consistently. Thorough DFM analysis plus potential use of manual placement are key considerations with rigid-flex designs.
Conclusion
By applying suitable configuration of design tools to handle distinct properties, arranging layer stacks to control transitions, and working closely with manufacturers on capabilities, engineers can effectively harness rigid-flex PCBs to realize tremendous space and weight benefits for advanced devices with confidence.
In closing, we'll address a few frequent additional questions that arise on working with rigid-flex PCBs.
Frequently Asked Questions
What are some example applications using rigid-flex PCBs?
Industries using rigid-flex PCBs heavily include aerospace guidance systems, military radios, medical instruments like endoscopes, and consumer gadgets like foldable smartphones. The technology excels where complex electronics must conform to tight, dynamic physical spaces.
What are typical minimum bend radii and conductor spacing for flex circuits?
As a baseline, minimum bend radii for flex PCBs often range from 0.1mm to 0.25mm or roughly 4 to 10 times the flex layer thickness. Conductor spacing aims for ≥ 2 mils to avoid shorts during bending. So thinner flex materials allow tighter profiles.
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