FLEX AND RIGID-FLEX BEND CAPABILITIES IN PCB DESIGN

 

Introduction

Flexible and rigid-flex printed circuit boards (PCBs) represent a significant advancement in electronic design, offering solutions that traditional rigid PCBs cannot provide. As electronic devices continue to decrease in size while increasing in functionality, the demand for flexible circuit solutions has grown exponentially. These specialized PCBs allow designers to create three-dimensional interconnection schemes, reduce weight and space requirements, and improve reliability through the elimination of connectors and solder joints.

This article explores the comprehensive world of flex and rigid-flex PCB technologies, with a particular focus on their bending capabilities—a critical aspect that determines their functionality, reliability, and longevity in various applications. We will examine the fundamental principles governing bend performance, material considerations, design guidelines, manufacturing processes, and testing methodologies. Additionally, we'll investigate advanced applications and emerging trends that are shaping the future of flexible circuit technology.

Whether you're an experienced PCB designer looking to optimize flex circuit performance or a newcomer seeking to understand the fundamentals of this technology, this comprehensive guide provides the knowledge necessary to leverage the full potential of flex and rigid-flex bend capabilities in your electronic designs.

Basic Definitions and Terminology

Flexible printed circuit boards (flex PCBs) are electronic circuits built on flexible substrate materials, allowing them to bend, fold, and conform to specific shapes. This functionality offers significant advantages over traditional rigid PCBs in many applications where space is limited, weight is critical, or movement is required.

Rigid-flex PCBs, on the other hand, integrate both flexible and rigid circuit board technologies into a single interconnected structure. These hybrid designs typically feature rigid sections for component mounting and flexible sections for interconnection, combining the best qualities of both technologies.

The terminology associated with flex and rigid-flex PCBs includes:

• Flex Circuit: A circuit built on flexible substrate material.
• Bend Radius: The radius of curvature at which a flex circuit can be bent without damage.
• Bend Angle: The angle through which a flex circuit is bent.
• Dynamic Flex: Applications where the circuit undergoes repeated bending during its operational lifetime.
• Static Flex: Applications where the circuit is bent once during installation and remains fixed thereafter.
• Transition Zone: The area where rigid and flexible sections meet in a rigid-flex design.
• Bend Accommodation Features: Design elements incorporated to reduce stress during bending.

Historical Development and Evolution

The concept of flexible circuits dates back to the early 20th century, with patents for "printed wiring" appearing as early as the 1900s. However, the first significant application came in the 1950s when flexible circuits were used in military and aerospace applications.

The evolution of flex and rigid-flex PCB technology can be outlined as follows:

Era
Key Developments
1950s
First practical applications in military equipment
1960s
Introduction in consumer electronics (cameras)
1970s
Development of multi-layer flex circuits
1980s
Standardization of manufacturing processes
1990s
Widespread adoption in computers and mobile devices
2000s
Advanced materials and miniaturization
2010s
Integration with wearable technology and IoT devices
2020s
High-density interconnect (HDI) flexible circuits and stretchable electronics

Advantages and Limitations

Flex and rigid-flex PCBs offer numerous advantages over traditional rigid boards, particularly in terms of their bend capabilities:

Advantages:

1. Space and Weight Savings: Flex circuits can be designed to fit into three-dimensional spaces, eliminating the need for multiple interconnected rigid boards.
2. Elimination of Connectors: By replacing connector interfaces with flexible sections, reliability is improved and assembly costs are reduced.
3. Improved Reliability: Fewer solder joints and connector points result in fewer potential failure points.
4. Enhanced Thermal Management: Flexible circuits can dissipate heat more effectively across three-dimensional spaces.
5. Vibration Resistance: The ability to absorb vibration and movement makes them ideal for high-reliability applications.
6. Design Freedom: The ability to bend allows for innovative form factors and packaging options.

Limitations:

1. Cost: Flex and rigid-flex PCBs generally cost more than equivalent rigid boards.
2. Design Complexity: Designing for optimal bend performance requires specialized knowledge.
3. Manufacturing Challenges: More complex manufacturing processes with tighter tolerances.
4. Material Constraints: Not all electronic components are suitable for flexible circuits.
5. Limited Bend Cycles: Even well-designed flex circuits have finite bend cycle lifetimes.

Types of Flex and Rigid-Flex Constructions

Various construction types exist for flex and rigid-flex PCBs, each with distinct bend capabilities:

1. Single-Sided Flex: Conductors on one side of a flexible substrate. These offer the greatest flexibility and lowest cost but are limited in complexity.
2. Double-Sided Flex: Conductors on both sides of a flexible substrate, with plated through-holes connecting the layers. These provide more routing options but reduced flexibility compared to single-sided designs.
3. Multi-Layer Flex: Multiple conducting layers separated by dielectric layers. These support complex circuitry but have reduced flexibility proportional to the number of layers.
4. Rigid-Flex: Combination of rigid and flexible substrate areas. The rigid areas typically contain multiple layers for component mounting, while flexible areas provide interconnection.
5. Sculptured Flex: Specialized flex circuits with varying conductor thicknesses, allowing for both fine traces and power-carrying capabilities.

Material Considerations for Bend Performance

The selection of materials for flex and rigid-flex PCBs significantly impacts their bend capabilities, reliability, and performance in various environmental conditions.

Substrate Materials

The flexible substrate forms the foundation of any flex circuit and directly influences its bend performance. Common substrate materials include:

Polyimide (PI)

Polyimide is the most widely used flexible substrate material due to its excellent combination of properties:

• Temperature resistance: Can withstand temperatures from -269°C to +400°C
• Chemical resistance: Excellent resistance to most chemicals and solvents
• Dimensional stability: Maintains shape under thermal stress
• Dielectric properties: Low dielectric constant (3.2-3.5) and loss tangent
• Moisture absorption: Relatively high (2-3%) which can affect electrical properties
• Bend capability: Excellent bend performance with minimal mechanical degradation

Polyester (PET)

Polyester offers a lower-cost alternative to polyimide:

• Temperature resistance: Limited to about 105-115°C
• Chemical resistance: Good for most applications but inferior to polyimide
• Cost: Significantly lower than polyimide
• Bend capability: Good flexibility but less suitable for dynamic bending applications
• Dielectric properties: Comparable to polyimide
• Transparency: Available in transparent options

Liquid Crystal Polymer (LCP)

LCP is growing in popularity for high-frequency applications:

• Dielectric properties: Excellent high-frequency performance
• Moisture absorption: Very low (0.02-0.1%)
• Temperature resistance: Good (up to 190°C)
• Bend capability: Good but requires careful design consideration
• Cost: Higher than polyimide
• Dimensional stability: Excellent

Conductor Materials

The conductor material affects not only electrical performance but also bend reliability:

Copper

Various types of copper are used in flex circuits, each with different bend characteristics:

Copper Type
Characteristics
Bend Performance
Rolled Annealed (RA)
Fine grain structure, highly ductile
Excellent; preferred for dynamic applications
Electrodeposited (ED)
Less ductile than RA
Good for static applications, limited dynamic use
High-Temperature ED
Improved thermal stability
Moderate bend performance
Modified RA
Enhanced properties for specific applications
Very good to excellent

Alternative Conductor Materials

For specialized applications, alternative conductor materials may be used:

• Aluminum: Lighter weight, but less conductive and more prone to fatigue
• Silver: Higher conductivity but cost-prohibitive for most applications
• Copper-clad aluminum: Balance between weight and conductivity
• Conductive polymers: Emerging technology for extreme flexibility requirements

Coverlay and Adhesive Systems

Coverlay (the flexible equivalent of solder mask) and adhesive materials significantly impact bend performance:

Coverlay Materials

• Polyimide: Provides the best temperature resistance and bend performance
• Polyester: Lower cost but reduced temperature and bend capabilities
• Liquid photoimageable (LPI): Offers precise feature definition but inferior bend performance

Adhesive Systems

Adhesives bond the conductors to the substrate and the coverlay to the circuit:

Adhesive Type
Temperature Range
Bend Performance
Key Characteristics
Acrylic
-40°C to +125°C
Good
Good chemical resistance, moderate cost
Epoxy
-40°C to +150°C
Moderate
Excellent chemical resistance, rigid
Modified Acrylic
-40°C to +150°C
Very good
Balance of properties
Pressure-Sensitive
-40°C to +85°C
Excellent
Limited temperature range
Adhesiveless
-269°C to +400°C
Excellent
Premium cost, best performance

Material Stack-Up Considerations

The overall material stack-up design significantly impacts bend performance:

1. Symmetric Construction: Balancing materials on either side of the neutral bend axis minimizes stress during bending.
2. Layer Thickness: Thinner materials generally offer better bend performance.
3. Neutral Bend Axis: Positioning critical traces near the neutral bend axis minimizes strain during bending.
4. Adhesiveless Systems: These often provide superior bend performance by eliminating the relatively rigid adhesive layer.
5. Selective Bonding: Strategic application of adhesives can create "loose leaf" areas with enhanced flexibility.

Bend Radius: Theory and Practical Limitations

Understanding Bend Radius Fundamentals

The bend radius is a critical parameter in flex circuit design, defined as the radius of curvature at which a flex circuit is bent. It directly influences the strain experienced by the conductors and substrate materials during bending.

Smaller bend radii create higher strain, which can lead to material fatigue and failure. The theoretical basis for understanding this relationship comes from material mechanics, where strain (ε) at any point in a bent material can be calculated using:

ε = t / (2R)

Where:

• t is the material thickness
• R is the radius of curvature

This equation highlights two key principles:

1. Strain increases linearly with material thickness
2. Strain decreases with increasing bend radius

Minimum Bend Radius Calculations

The minimum bend radius (MBR) represents the smallest radius to which a circuit can be bent without causing damage. It varies depending on whether the application involves static (one-time) or dynamic (repeated) bending:

For Static Applications

The industry standard formula for minimum bend radius in static applications is:

MBR = 6 × t

Where t is the total thickness of the flex circuit in the bend area.

For Dynamic Applications

For dynamic applications where the circuit undergoes repeated flexing:

MBR = 12 × t

This more conservative value accounts for material fatigue over multiple bend cycles.

Example Calculation

Consider a single-layer flex circuit with:

• Polyimide substrate: 25μm
• Copper foil: 18μm
• Coverlay: 25μm
• Adhesive layers (2): 15μm each

Total thickness: 25 + 18 + 25 + (2 × 15) = 98μm

For static applications: MBR = 6 × 98μm = 588μm or 0.588mm For dynamic applications: MBR = 12 × 98μm = 1,176μm or 1.176mm

Factors Affecting Practical Bend Radii

While theoretical calculations provide a starting point, several factors affect practical bend performance:

Material Properties

1. Copper Type: Rolled annealed copper can achieve smaller bend radii than electrodeposited copper.
2. Substrate Material: Polyimide generally allows for tighter bends than polyester.
3. Adhesive System: Adhesiveless systems permit smaller bend radii than those using adhesives.
4. Coverlay Flexibility: Coverlays with higher flexibility enable tighter bends.

Design Factors

1. Copper Thickness: Thinner copper allows for tighter bends.
2. Trace Orientation: Traces perpendicular to the bend axis experience more strain.
3. Plated Through-Holes (PTHs): Proximity of PTHs to bend areas increases failure risk.
4. Layer Count: More layers increase overall thickness and reduce bend capabilities.

Environmental Factors

1. Temperature: Lower temperatures reduce material flexibility.
2. Humidity: Can affect adhesive properties and substrate flexibility.
3. Aging: Material properties change over time, often reducing flexibility.

Industry Standards for Bend Radii

Several industry standards provide guidelines for bend radii in flexible circuits:

Standard
Static Bend Recommendation
Dynamic Bend Recommendation
Notes
IPC-2223
6 × thickness
12 × thickness
Most widely used standard
MIL-P-50884
6 × thickness
10 × thickness
Military applications
JPCA-MB01
5 × thickness
10 × thickness
Japanese standard
Company-specific
4-8 × thickness
10-15 × thickness
Varies by manufacturer

Practical Limitations and Constraints

Even with careful design, practical limitations exist:

1. Manufacturing Tolerances: Material thickness variations affect actual bend performance.
2. Handling Damage: Improper handling during assembly can damage circuits at bend areas.
3. Stress Concentration: Features like pads and vias create stress concentration points.
4. Combined Stresses: Thermal cycling combined with bending creates more severe conditions.
5. Minimum Manufacturable Radius: Tooling limitations may prevent achieving theoretical minimums.

Design Guidelines for Optimal Bend Performance

Designing flex and rigid-flex PCBs for optimal bend performance requires careful consideration of numerous factors. Following these guidelines maximizes reliability and longevity in applications requiring bending.

Trace Routing Strategies

The orientation and design of conductive traces significantly impact bend performance:

Trace Orientation

1. Perpendicular vs. Parallel: Traces should run parallel to the bend line whenever possible. Perpendicular traces experience significantly higher strain during bending.
2. Angled Crossings: When traces must cross a bend area at an angle, use angles of 45° or less to the bend line to reduce strain.
3. Trace Staggering: Avoid placing traces directly on top of each other in multi-layer designs, as this creates stress points. Instead, stagger traces across different layers.

Trace Geometry

1. Trace Width: Narrower traces generally perform better in bend areas. For critical applications, reduce trace width in bend zones by 25-30% if current-carrying capacity allows.
2. Trace Shape: Use curved traces rather than sharp corners in bend areas. Rounded corners distribute stress more evenly.
3. Teardrop Reinforcement: Add teardrop reinforcements at pad connections within or near bend areas to reduce stress concentration.

Trace Design Approach
Benefit
Implementation Notes
Serpentine Traces
Absorbs mechanical stress
Add minimal length to avoid impedance issues
Hatched Ground Planes
More flexible than solid copper
Use in bend areas only to maintain EMI shielding
Varied Trace Width
Optimizes for both electrical and mechanical needs
Gradually transition width changes
Trace Thinning
Reduces copper thickness in bend areas
Apply selectively to critical traces

Component Placement and Consideration

Component placement relative to bend areas is critical:

1. Bend Area Clearance: Maintain a minimum clearance of 1.5mm between components and the start of any bend area. For large or heavy components, increase this distance to 3mm or more.
2. Component Orientation: Orient rectangular components parallel to the bend line when placing near bend areas.
3. Component Types: Use smaller, lighter components near bend areas. Surface-mount components generally perform better than through-hole components in proximity to bends.
4. Stiffeners: Apply stiffeners in component areas to prevent flexing that could damage solder joints or components.

Pad and Via Design

Pads and vias represent discontinuities in the material that can concentrate stress:

1. Via Location: Keep vias at least 1mm away from the beginning of any bend area. In critical applications, this distance should be increased to 2mm or more.
2. Via Design: Use smaller vias with teardrop reinforcement when they must be placed near bend areas.
3. Pad Shapes: For pads near bend areas, use oval or teardrop shapes oriented parallel to the bend line rather than circular pads.
4. Annular Rings: Increase annular ring width for pads and vias near bend areas to provide additional strength.

Structural Reinforcement Techniques

Several design techniques can reinforce flex circuits at critical points:

1. Stiffeners: Apply polyimide, FR4, or metal stiffeners to areas requiring rigidity. Common stiffener materials include:

Stiffener Material
Advantages
Disadvantages
Polyimide
Compatible CTE with substrate, lightweight
Limited rigidity
FR4
Cost-effective, good rigidity
CTE mismatch with polyimide
Aluminum
Excellent heat dissipation
Heavier, requires special adhesives
Stainless Steel
Very rigid, thin profiles possible
Heavy, more expensive

2. Selective Adhesive Application: Creating "loose leaf" or unbonded areas in regions where maximum flexibility is required.
3. Relief Cuts and Slits: Strategic cuts in the circuit reduce stress during bending. Common patterns include:
   • Stress relief holes at the edges of rigid-flex transition zones
   • Accordion or expansion patterns in areas needing extra flexibility
   • Contoured edges parallel to the bend line
4. Coverlay Design: Use specific coverlay patterns in bend areas:
   • "Windows" (openings) in the coverlay at bend areas
   • Thinner coverlay materials in bend regions
   • Selective application to leave bend areas uncovered

Layer Stackup Optimization

The arrangement of layers significantly affects bend performance:

1. Symmetrical Construction: Balance material thicknesses on either side of the central plane to prevent twisting or curling during bending.
2. Critical Signal Placement: Position critical signal layers closer to the neutral bend axis where strain is minimized.
3. Copper Weight Distribution: Use thinner copper (lower weight) in layers that will experience more strain during bending.
4. Layer Count Transitions: When transitioning from multi-layer to fewer layers, taper the transition gradually rather than creating abrupt steps.

Design for Manufacturing (DFM) Considerations

Practical manufacturing constraints must be considered:

1. Registration Targets: Include registration targets to ensure proper alignment of layers during manufacturing.
2. Bend Area Marking: Clearly define bend lines and minimum bend radii on design documentation and fabrication drawings.
3. Panel Design: Consider how the circuits will be panelized and how this might affect bend areas during manufacturing.
4. Handling Features: Add handling features or tabs that can be removed after assembly to protect delicate bend areas during production.
5. Tooling Holes: Position tooling holes away from bend areas to prevent distortion.

Manufacturing Processes and Their Impact on Bend Capabilities

The manufacturing processes used to produce flex and rigid-flex PCBs significantly impact their bend performance and reliability. Understanding these processes and their effects is essential for achieving optimal results.

Copper Processing Methods

Rolled Annealed vs. Electrodeposited Copper

The method used to produce copper foil fundamentally affects its grain structure and flex properties:

Rolled Annealed (RA) Copper:

• Manufactured by mechanically rolling copper and annealing
• Features a refined, elongated grain structure
• Superior ductility and fatigue resistance
• Preferred for dynamic flex applications
• Typically 5-10 times more bend cycles before failure

Electrodeposited (ED) Copper:

• Produced by electroplating copper onto a drum
• Exhibits a columnar grain structure
• Less expensive than RA copper
• Adequate for static flex applications
• Lower elongation before breaking

The following table compares key properties:

Property
Rolled Annealed Copper
Electrodeposited Copper
Elongation
12-25%
3-10%
Grain Structure
Refined, elongated
Columnar, vertical
Bend Cycles to Failure
High
Moderate to Low
Cost
Higher
Lower
Typical Applications
Dynamic flexing, medical devices
Static flex, consumer electronics

Lamination Processes

The lamination process bonds the copper to the substrate material, significantly affecting flexibility:

Adhesive-Based Lamination

Traditional lamination uses adhesive layers:

• Acrylic or epoxy adhesives bond copper to substrate
• Adhesive layer increases overall thickness
• Creates a less flexible structure
• Differential expansion between materials can cause stress

Adhesiveless Lamination

Advanced processes eliminate separate adhesive layers:

• Copper is directly bonded to polyimide
• Reduces overall thickness by 25-50%
• Significantly improves bend performance
• Higher cost but superior flexibility
• Better thermal performance

Lamination Parameters

Process parameters during lamination affect bend performance:

• Pressure: Higher pressure can reduce voids but may compress materials unevenly
• Temperature: Affects adhesive flow and cure properties
• Time: Impacts complete curing and bond strength
• Cooling rate: Can create internal stress if too rapid

Etching and Surface Treatments

Copper Etching Process

The etching process affects conductor profile and flexibility:

• Underetching creates wider-than-designed traces that may crack during bending
• Overetching creates thinner traces prone to breaking
• Uneven etching creates stress concentration points
• Chemical etching provides more uniform results than mechanical processes
• Plasma etching can produce the most precise results for fine features

Surface Finishes

Surface finishes protect copper but impact flexibility:

Surface Finish
Flexibility Impact
Notes
ENIG (Electroless Nickel Immersion Gold)
Moderate reduction
Nickel layer is relatively stiff
Immersion Tin
Minor reduction
Thinner layer with good flexibility
Immersion Silver
Minor reduction
Good flexibility but susceptible to corrosion
HASL (Hot Air Solder Leveling)
Significant reduction
Not recommended for flex circuits
OSP (Organic Solderability Preservative)
Minimal impact
Excellent flexibility but limited shelf life
Hard Gold
Moderate to high reduction
Used for contact areas, avoid in bend regions

Drilling and Mechanical Processing

Mechanical vs. Laser Drilling

The method used to create vias and holes affects structural integrity:

Mechanical Drilling:

• Creates stress zones around holes
• Minimum practical size larger than laser drilling
• May require entry/exit materials that affect thickness
• Less precise positioning

Laser Drilling:

• Minimal mechanical stress
• Smaller hole sizes possible (down to 25μm)
• More precise positioning
• May create heat-affected zones
• Higher cost

Edge Finishing Techniques

Edge quality impacts crack initiation during bending:

• Routing: Creates micro-cracks that can propagate during bending
• Die Cutting: Clean edges but can create stress concentrations
• Laser Cutting: Precise but heat-affected zones may be problematic
• Chemical Etching: Smoothest edges with minimal stress concentration

Coverlay Application

The method used to apply coverlay affects overall flexibility:

Vacuum Lamination

• Provides more uniform adhesion
• Reduces void formation
• Better conformity to irregular surfaces
• Minimizes trapped air that can create stress points

Press Lamination

• More widely available
• Lower cost
• May create more internal stress
• Less uniform adhesion in complex geometries

Screen Printing vs. Photoimageable Coverlay

Screen Printed Coverlay:

• Limited resolution for openings
• Variable thickness control
• Lower cost
• Adequate for most applications

Photoimageable Coverlay:

• Superior resolution for fine features
• More consistent thickness
• Higher cost
• Better electrical performance
• Usually less flexible than traditional coverlay

Rigidization Processes for Rigid-Flex

The methods used to create rigid areas in rigid-flex designs affect the transition zones:

Selective Layer Buildup

• Adds rigid layers only where needed
• Creates more gradual transitions
• Better bend performance at transition zones
• More complex manufacturing process

Full Panel Lamination and Routing

• Simpler manufacturing process
• Creates abrupt transitions
• Higher stress concentration at rigid-flex interfaces
• Less expensive but potentially less reliable

Stiffener Application

• Post-process addition of rigid areas
• Can be selectively applied
• Various thickness options
• May use different adhesives than main lamination

Manufacturing Controls for Bend Performance

Several manufacturing controls directly impact bend reliability:

1. Copper Grain Direction: Aligning the grain direction of rolled copper with the bend axis improves performance.
2. Clean Room Environment: Prevents contamination that could create stress points or delamination.
3. Temperature and Humidity Control: Maintains consistent material properties during processing.
4. Handling Procedures: Minimizes mechanical stress during fabrication and assembly.
5. Tooling Design: Purpose-built tooling for flex circuit handling reduces mechanical damage.
6. Inspection Methods: Advanced inspection techniques identify potential weak points before deployment.

Types of Bends and Their Applications

Different applications require various types of bends, each with specific design considerations and limitations. Understanding these bend types helps designers optimize their flex and rigid-flex PCBs for specific use cases.

Static Bends

Static bends occur when a flexible circuit is bent once during installation and remains in that position throughout its service life. While these bends experience less mechanical stress than dynamic bends, they still require careful design consideration.

Single Axis Bends

The simplest form of bend involves flexing around a single axis:

• Common Applications: Connecting boards at fixed angles, folding into device housings
• Design Considerations:
  • Can use smaller bend radii (typically 6× material thickness)
  • Trace orientation parallel to bend axis is critical
  • Can use standard (non-RA) copper in most cases
• Examples: Folded interconnections in smartphones, camera-to-mainboard connections

Compound Bends

Compound bends involve multiple bends along different axes but remain static after installation:

• Common Applications: Conforming to complex enclosures, 3D interconnects
• Design Considerations:
  • Each bend radius must be calculated separately
  • Special attention to areas where bend stresses might compound
  • More complex strain relief features needed
• Examples: Automotive dashboard electronics, medical device implants

Z-Fold Configurations

Z-fold designs incorporate two parallel bends in opposite directions, creating a Z-shaped profile:

• Common Applications: Space-constrained devices requiring connections between stacked boards
• Design Considerations:
  • Minimum distance between folds must be maintained (typically ≥3× combined thickness)
  • Symmetric construction reduces twisting forces
  • Consider neutral bend axis for each fold
• Examples: Mobile phone hinges, laptop display connections

Dynamic Bends

Dynamic bends occur when the flexible circuit undergoes repeated flexing during normal operation. These applications require specialized design approaches to ensure reliability over the required number of bend cycles.

Continuous Flex

Continuous flex applications involve constant or near-constant movement:

• Common Applications: Print heads, robotic joints, continuous rotation connections
• Design Considerations:
  • Requires rolled annealed copper
  • Larger minimum bend radius (typically 12-15× material thickness)
  • Special attention to fatigue resistance
  • May require strain relief features
• Examples: Inkjet printer heads, CNC machine tool connections

Intermittent Flex

Intermittent flex involves occasional but repeated bending:

• Common Applications: Folding displays, hinged device connections
• Design Considerations:
  • Intermediate bend radii (typically 10-12× material thickness)
  • Cycle count requirements determine copper and construction type
  • Consider flex frequency and environmental factors
• Examples: Flip phone hinges, laptop lid connections, foldable displays

Bend Type
Typical Bend Radius
Recommended Copper
Typical Cycle Life
Continuous Flex
12-15× thickness
RA Copper
>1,000,000 cycles
Intermittent Flex
10-12× thickness
RA Copper
10,000-500,000 cycles
Static Z-Fold
8× thickness
ED or RA Copper
1-100 cycles
Static Single Bend
6× thickness
ED or RA Copper
1-10 cycles

Bend Angle Considerations

The angle through which a circuit is bent affects design requirements:

0-90° Bends

Most common bend angles fall within this range:

• Design Considerations:
  • Standard design rules apply
  • Strain increases proportionally with angle
  • Conductor failure usually not the limiting factor

90-180° Bends

Sharper bends create higher strain:

• Design Considerations:
  • Increased bend radius recommended
  • More critical attention to trace orientation
  • Consider double-sided designs with traces on outside of bend
  • May require specialized bend relief features

>180° Bends

Extreme bend angles are challenging but possible:

• Design Considerations:
  • Specialized construction required
  • Consider spiral wrap approaches
  • May require sections with different flexibility
  • Often requires advanced manufacturing techniques
• Examples: Roll-to-roll electronics, wrapped cylindrical batteries

Application-Specific Bend Types

Certain industries and applications have developed specialized bend configurations:

Medical Device Bends

Medical applications often require unique bend types:

• Catheter Integration: Extremely small bend radii with dynamic flexing
• Implantable Devices: Bio-compatible materials with compound bends
• Wearable Monitors: Conforming to body contours with variable flexibility
• Design Considerations:
  • Biocompatibility of all materials
  • Sterilization process compatibility
  • Extended reliability requirements
  • Moisture and chemical resistance

Automotive Fold Configurations

Automotive applications face harsh environments:

• Dashboard Integration: Complex 3D routing with multiple bend axes
• Door Panel Connections: Dynamic bends with environmental exposure
• Engine Compartment Links: High-temperature applications with vibration
• Design Considerations:
  • Temperature cycling resilience
  • Vibration resistance
  • Resistance to automotive fluids
  • Extended service life (10+ years)

Aerospace and Defense Bends

These applications have the most stringent requirements:

• Satellite Deployment: One-time deployment with extreme temperature variation
• Aircraft Wing Connections: Vibration-resistant dynamic connections
• Missile Guidance Systems: High-acceleration applications
• Design Considerations:
  • Qualification to military standards
  • Radiation resistance
  • Extreme temperature range operation
  • High reliability requirements

Bend Relief Features

Special design features can be incorporated to enhance bend performance:

Accordion Patterns

• Description: Series of S-curves in the circuit that act as mechanical springs
• Benefits: Reduces strain on individual sections, increases effective length
• Applications: High-flex applications, tight bend radii
• Design Considerations: Takes more space, requires specialized tooling