Introduction
Printed circuit boards (PCBs) provide the foundation for electronics, with copper traces on laminated substrates forming the circuits between components. While traditionally PCBs have been completely rigid, new "rigid-flex" designs incorporate both rigid sections and flexible layers enabling devices to bend and fold.
This opens up new use cases like wearables or medical devices where conformability is required. However, rigid-flex boards bring unique structural challenges compared to purely rigid PCBs. The combination of stiff and pliable materials adds complexity in maintaining physical integrity across bending.
In this guide, we’ll explore considerations around structural integrity when designing rigid-flex PCBs. We’ll look at common failure points and methods to improve reliability. We’ll also cover best practices when laying out rigid-flex boards using EDA tools.
Background on Rigid-Flex PCBs
Let’s start with some background on what makes a rigid-flex PCB unique from traditional rigid boards.
Materials
Rigid-flex PCBs combine standard rigid FR-4 substrates with flexible polyimide films bonded together. Rigid sections provide structure while flex layers enable bending.
Flex Bends
Thin flex layers can fold into 3D shapes with tight bend radii. Bends may be dynamic folds or static formed bends.
Layer Transitions
An additional consideration is handling the transition between rigid and flex where the layer stack changes.
Applications
Use cases demanding flexibility include wearables, medical devices, robotics, folding mechanisms, and more.
Understanding these core characteristics sets the stage for then exploring structural integrity challenges.
Stress Points in Rigid-Flex Designs
Anytime a rigid material is combined with flexible layers, stresses arise when bending that can lead to potential failures. Here are common stress points in a rigid-flex PCB that warrant attention:
Flex Bends
The most obvious stress location is right at the bend itself on the flex layer. Repeated or tight bending can fracture copper traces.
Layer Transitions
The interface between rigid and flex sees shear stress between mismatched materials. Delamination can occur.
Component Edges
board or component edges. Cracks can form leading to separation.
Vias
Mismatched expansion of rigid and flex materials at vias can put strain on plated through holes possibly causing cracks.
Interconnects
High strain between points connecting the rigid and flex sections. Traces can detach.
Paying attention to these areas prone to stress and failure is crucial when designing rigid-flex PCBs. Next we’ll explore methods to mitigate these risks.
Improving Structural Rigidity
Many strategies exist to improve structural integrity when working with rigid-flex PCBs. Here are some best practices to minimize stress points:
Flex Stiffening
Areas of the flex that will see repeated bending can be stiffened to prevent fracturing. Strategies include adding more laminate layers, using thicker copper, or even attaching a rigid stiffener.
Corner Reinforcement
Use teardrops or fillets to round sharp corners on the board outline and cutouts. This reduces crack initiation sites.
Pad Geometries
Careful pad shapes and spacing at layer transitions reduces shear stress between mismatched copper and dielectric layers.
Bend Geometry
Optimize the bend radius and distribute stress with techniques like folded flex layers or slotted pads around bends.
Trace Routing
Route traces perpendicular to bend axis and use zig-zag or sine patterns to reduce strain on traces.
Vias Optimization
Minimize/avoid vias in high flex zones. Use capped, filled, or staggered vias to reduce stress.
Purposefully implementing such strategies throughout the design improves structural robustness and longevity.
Layout Considerations and Rules
To produce a physically robust rigid-flex board, the PCB layout software must account for the complexities involved. Here are some key layout considerations and rules to follow:
Defining Rigid vs. Flex
Outline rigid and flex sections as board outlines or mechanical layers. This defines stackups.
Bend Lines
Add bend lines to indicate intended folds on the flex layers.
Layer Transitions
Follow layout rules at transitions e.g. teardrops, conquered pads, no traces through.
Flex Clearance
Increase clearance around traces on flex layers. Allow space for bending.
Mirrored Traces
Mirror and offset matched impedance traces across bends.
No Rigid Traces
Avoid routing traces on rigid layers under flex bends. Use flex layers only.
Pad and Via Spacing
Give extra spacing around pads and vias in flex areas.
Following rigid-flex specific guidelines when laying out boards helps design reliability into the product versus relying solely on materials alone.
Modeling and Simulation
Due to the complex interplay of stresses, performing modeling and simulation on rigid-flex designs is highly recommended. Finite element analysis tools like ANSYS and COMSOL can analyze stresses and deformation. This allows validating designs before reaching final fabrication. Simulation highlights potential weak points needing improvement. while modeling provides insight on optimization tradeoffs. Rigid-flex designs in particular benefit from these advanced numerical tools.
Example Rigid-Flex PCB Design
To illustrate some of these concepts, let’s step through an example rigid-flex PCB design:
We’ll design a board with both a rigid MCU/connector section and a flex section with folded display and sensor components. The rigid portion has 4 conductive layers while the flex uses 2 layers.
Layer Stackup
We define the unique layer stackups for the rigid and flex portions. For the rigid section we’ll use a symmetrical stackup of Signal-GND-PWR-Signal layers. The flex will use a simple 2 layer stackup.
Layer Transitions
Where the rigid and flex sections meet, we create conquered pad connections between layers. Dense stitching vias provide vertical connections through the section. Teardrop shapes relieve stress.
Bend Areas
The flex area has defined bend lines between the display and sensor components. We assign them appropriate bend radii.
Routing
Traces are routed on the flex layers exclusively in the bend areas. Mirroring is used on matched pairs. Teardrop pads ease traces into bends.
Clearances
We assign larger clearances on flex layer traces compared to the rigid sections. Pads and vias have increased spacing in flex zones.
This example illustrates how an EDA tool can be used to layout a rigid-flex design meeting structural requirements. SIMULATION
Summary
Rigid-flex PCBs provide exciting capabilities for electronics through their blend of rigid and flexible materials. However, the mismatch does introduce structural concerns that must be mitigated through careful design. Areas of focus include flex bends, layer transitions, component edges, and interconnects.
Methods such as wise layout, geometry improvements, routinging techniques, spacing rules, layer buildup, among others can strengthen problematic areas prone to stress and failure. Rigid-flex demands advanced layout skills combining both electrical and mechanical considerations to produce robust, reliable PCBs able to survive bending motion.
By understanding the science behind inducing and relieving stresses plus following best practice design guidelines, engineers can take full advantage of rigid-flex PCBs in their products. The future will continue seeing amazing innovations built with these unique boards.
Frequently Asked Questions
How many times can a rigid-flex PCB be bent without failure?
The bend life depends on many factors like flex material, trace design, bend radius, etc. but is typically 100,000+ cycles for a well designed board. Rigid-flex PCBs can achieve hundreds of thousands of bend cycles if properly designed.
What are some key differences designing rigid-flex vs. rigid PCBs?
Rigid-flex requires considering mechanical factors like bend radii, layer transitions, stresses, flex routing, teardrops, and spacing. It involves a joint electrical/mechanical design perspective for reliability.
Does rigid-flex allow more or fewer PCB layers compared to traditional boards?
Rigid-flex theoretically enables more total layers by combining separate rigid and flex layer stacks. But cost and yield considerations normally limit overall layer counts comparable to traditional PCBs.
Can components be placed on both rigid and flex sections?
Yes, components can be surface mounted across both rigid and flex areas. The main requirements are tracing back to the rigid section for stability and handling layer transitions.
What are the typical board thickness and spacing rules for flexible PCBs?
Typical flex thickness is around 2 mil, with 5 mil trace/space. Fine lines, increased dielectric thickness, and tighter clearance help enable robust flexible circuits.
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