Flex and Rigid Flex PCB: Design and Manufacturing

Flex PCB and rigid-flex PCB technology combines flexible polyimide circuits with rigid FR-4 sections to create compact, lightweight, and highly reliable electronic interconnections. Compared with conventional rigid PCBs connected by wires or connectors, a rigid flex PCB reduces assembly complexity, improves signal integrity, increases vibration resistance, and saves installation space. Successful rigid flex PCB manufacturing depends on proper material selection, stackup design, bend radius control, copper balancing, sequential lamination, drilling accuracy, and strict quality verification. Following IPC-2223, IPC-2221, IPC-6013, and IPC-A-600 throughout design and fabrication ensures long-term mechanical and electrical reliability.

What Is Flex PCB and Rigid-Flex PCB?

A Flex PCB, also called a flexible printed circuit, is built on polyimide film instead of rigid fiberglass. It can bend repeatedly while maintaining electrical continuity.

A rigid-flex PCB combines rigid FR-4 sections with flexible polyimide layers into a single integrated structure.

Unlike cable assemblies, rigid-flex technology eliminates connectors between rigid boards, reducing electrical interfaces and improving reliability.

Typical constructions include:

Typical production parameters

IPC-2223 specifically defines design requirements for flexible printed boards, while IPC-6013 specifies qualification requirements for flexible and rigid-flex PCB fabrication.

Official IPC standards are available through the IPC website:https://www.ipc.org

Rigid-Flex PCB Benefits

Space Savings

One rigid-flex PCB replaces:

• Ribbon cables
• Board-to-board connectors
• Multiple rigid PCBs
• Wiring harnesses

Example:

A handheld medical scanner originally required:

• Three rigid PCBs
• Two FPC cables
• Four connectors

After redesign:

• One rigid-flex PCB
• Zero board connectors

PCB volume decreased by approximately 28%.

Higher Reliability

Every connector removed eliminates a potential failure point.

Compared with traditional cable assemblies:

• Fewer solder joints
• Lower contact resistance
• Better vibration resistance
• Better thermal cycling performance

Typical connector failure resistance increases after 500 thermal cycles.

Light Weight

Polyimide weighs considerably less than FR-4.

Typical reduction:

• Weight reduction: 25–45%
• Assembly components: reduced by 20–40%
• Wiring length: reduced by 15–35%

For aerospace electronics, every gram saved improves payload efficiency.

Main Uses

Smartphones and Cameras

Rigid-flex PCB technology allows:

• Folding structures
• Compact camera modules
• OLED displays
• Foldable devices

Typical BGA pitch:

0.35–0.40 mm

Typical HDI routing:

50/50 μm

Medical Devices

Applications include:

• Endoscopes
• Ultrasound probes
• Portable monitors
• Implantable electronics

Requirements:

• High reliability
• Small size
• Continuous flexing
• Biocompatible materials where applicable

Aerospace

Rigid-flex PCBs are widely used in:

• Satellite electronics
• Avionics
• Radar
• Flight control

Typical operating temperature:

−55°C to +125°C

Typical vibration testing follows product qualification requirements in addition to IPC workmanship criteria.

Rigid-Flex PCB Material

Material selection directly determines mechanical life.

Common rigid materials:

• FR-4 Tg170
• High-speed laminate
• Low-loss laminate

Flexible materials:

• Polyimide
• Adhesiveless polyimide
• Rolled annealed copper
• Electro-deposited copper

Comparison

For dynamic applications exceeding one million bend cycles, rolled annealed copper is generally preferred because of its superior fatigue resistance.

Rigid-Flex PCB Stackup

Typical stackups include:

2 Layer Flex

1+2+1

2+2+2

2+4+2

4+2+4

Example:

8-layer rigid-flex

L1 Signal

L2 Ground

L3 Signal

Flex Core

L4 Signal

L5 Power

L6 Ground

L7 Signal

L8 Signal

Important stackup considerations:

• Symmetrical copper balance
• Controlled impedance
• Neutral bend axis
• Flex thickness
• Sequential lamination compatibility

Poor stackup design frequently causes board warpage after multiple lamination cycles.

Bend Radius and Layout Rules

Safe Bending

Minimum bend radius depends on copper layer count.

Typical recommendations

Dynamic applications generally require larger bend radii.

No Sharp Turns

Signal traces should use:

• Curved routing
• 45° transitions
• Teardrops

Avoid:

• 90° corners
• Acute angles
• Narrow necks

Curved traces distribute mechanical stress more evenly during repeated bending.

Keep Holes Away

Typical production requirement:

Hole edge to bend line

≥1.5 mm

Microvias should remain inside rigid areas whenever possible.

Plated through holes located directly in bend areas frequently develop barrel cracking after repeated flexing.

No Parts in Bends

Components should remain inside rigid sections.

Avoid placing:

• BGAs
• Connectors
• Large capacitors
• Heavy inductors

inside dynamic bend regions.

Rigid-Flex PCB Design Guidelines

Professional rigid-flex PCB design begins with defining the mechanical folding sequence before component placement.

Important design controls include:

• Bend direction
• Neutral axis
• Copper balancing
• Dynamic versus static flex
• Coverlay openings
• Stiffener position
• Via placement
• Controlled impedance
• Return current path

Typical HDI rigid-flex values

Good rigid-flex PCB design reduces manufacturing rework while improving assembly consistency.

Rigid-Flex PCB Cost

Cost depends primarily on:

• Layer count
• Flex layer quantity
• HDI structure
• Sequential lamination
• Material grade
• Laser drilling
• Copper filling
• Yield

Comparison

Although fabrication cost is higher, total system cost may decrease because connectors, cables, brackets, and assembly labor are reduced.

Rigid-Flex PCB Manufacturing Process

Flex Layer Etching

Copper foil laminated on polyimide undergoes:

1. Cleaning
2. Dry film application
3. Exposure
4. Development
5. Etching
6. AOI inspection

Typical etching tolerance:

±10 μm

Coverlay Lamination

Unlike rigid PCBs using solder mask, flexible circuits generally use polyimide coverlay.

Typical parameters:

Temperature

170–190°C

Pressure

1.5–2.5 MPa

Coverlay protects copper during repeated bending.

Sub-Assembly Lamination

Rigid cores and flex cores are laminated together through multiple press cycles.

Critical controls:

• Resin flow
• Registration
• Copper balance
• Flex alignment

Artwork compensation is usually calculated separately for each lamination cycle because polyimide and FR-4 expand differently under heat.

Drilling and Plating

Processes include:

• Mechanical drilling
• Laser drilling
• Desmear
• Electroless copper
• Electroplating

Typical plating thickness:

20–25 μm

Microvia filling requires complete copper filling before additional build-up layers are laminated.

Routing and Profiling

Final profiling uses:

• CNC routing
• Laser cutting
• Steel rule die

Critical tolerances:

Outline

±75 μm

Flex edge quality directly affects long-term fatigue life.

Quality Control

Production verification includes:

• AOI
• X-ray
• Cross section
• Flying probe testing
• Impedance testing
• Peel strength testing
• Bend testing
• Thermal cycling

Typical inspection values

Dynamic bend samples are commonly tested through thousands to millions of flex cycles depending on the product specification.

Factory Case Study

A medical imaging customer required a compact rigid-flex PCB.

Specifications

• 10 layers
• 2+6+2 stackup
• 0.35 mm BGA
• 75 μm laser microvia
• 50/50 μm routing
• Board thickness 1.2 mm
• RA copper
• Polyimide 25 μm

Initial production issues

• Coverlay wrinkles
• Registration shift 48 μm
• Two cracked plated holes after bend testing
• Yield 91.6%

Engineering improvements

• Optimized lamination pressure profile
• Increased hole-to-bend distance from 0.8 mm to 1.8 mm
• Balanced copper distribution
• Added independent artwork compensation for flex core
• Replaced ED copper with RA copper in dynamic bend areas

Final production results

The largest improvement came from relocating plated holes outside the bend zone rather than changing the plating process itself.

Common Design Errors

Copper Crossing the Bend

Wide solid copper planes increase bending stress.

Cross-hatched copper provides better flexibility.

Vias Inside Bend Areas

Through holes and microvias inside bend regions become stress concentration points.

Incorrect Bend Radius

Using a bend radius smaller than six times the flex thickness often leads to copper fatigue.

Symmetry Ignored

Uneven copper distribution causes board warpage after sequential lamination.

Components Too Close to Flex Transition

Heavy components should remain inside rigid zones with adequate clearance from the rigid-to-flex interface.

FAQ

What is the difference between a Flex PCB and a Rigid-Flex PCB?

Question: Are Flex PCB and Rigid-Flex PCB the same?

Answer: No. A Flex PCB is entirely flexible, while a Rigid-Flex PCB integrates rigid FR-4 sections with flexible polyimide circuits into one structure, eliminating cable connections between boards.

What material is commonly used in a Rigid-Flex PCB?

Question: Which materials provide the best reliability?

Answer: High-Tg FR-4 is commonly used for rigid sections, while adhesiveless polyimide with rolled annealed copper is widely selected for dynamic flex applications because of its superior fatigue resistance.

Why are bend radius rules important?

Question: What happens if the bend radius is too small?

Answer: A bend radius below the recommended value increases copper strain, leading to cracked conductors, plated-hole fatigue, and reduced service life. Static designs typically use at least six times the flex thickness, while dynamic applications require larger radii.

Why does a Rigid-Flex PCB cost more than a standard PCB?

Question: What drives the manufacturing cost?

Answer: Sequential lamination, polyimide materials, laser drilling, tighter registration control, coverlay processing, and more extensive inspection increase fabrication cost. However, eliminating connectors, cables, and assembly steps often lowers the overall system cost.