Introduction to Rigid Flex PCBs
A rigid flex PCB (rigid flexible printed circuit board) combines rigid and flexible substrate materials to create a circuit board with both rigid and dynamic flexing capabilities. Rigid flex PCBs contain both rigid board sections (made of materials like FR-4) as well as flexible sections (using polyimide or other polymers). The rigid and flexible materials are bonded together during fabrication to create a single PCB solution.
Rigid flex PCBs provide benefits like:
- Dynamic flexing, bending, and folding in certain areas
- Rigid support for components and connectors
- More compact and efficient 3D layout of electronics
- Ability to integrate complex flexing with high density interconnects
- Withstand vibration, shock, and frequent motion cycles
This article provides an in-depth overview of rigid flex PCB technology, manufacturing processes, design considerations, applications, and benefits.
Construction and Materials
Rigid flex PCBs integrate two types of materials:
Rigid Substrate Materials
The rigid sections of a rigid flex PCB are typically constructed from common rigid PCB materials like:
- FR-4 fiberglass – The most common and cost effective rigid PCB material. Composed of woven fiberglass cloth with an epoxy resin binder.
- High-Tg FR-4 – Improved thermal performance for high temp applications.
- CEM-1/CEM-3 – Woven cotton paper reinforced epoxy laminate materials. Higher performance than FR-4.
- Rogers – High frequency circuit materials. Expensive but highly optimized electrical properties.
- Aluminum – Direct bond aluminum (DBA) placing aluminum traces on an insulating substrate. Excellent thermal dissipation.
Flexible Substrate Materials
The flexible sections of a rigid flex PCB use polymer films like:
- Polyimide (Kapton) – Most common flexible PCB material. High chemical resistance, stable electrical properties.
- Polyester (PET) – Lower cost but lower temperature and chemical resistance than polyimide.
- PEN – An alternative to polyimide with excellent flexibility and heat resistance.
- Fluoropolymers – PTFE-based materials with superb chemical resistance for harsh environments.
Other common flexible circuit materials include acrylic and polyurethane adhesives to bond the layers together.
PCB Layer Stackup
Rigid flex PCBs contain multiple laminated layers of substrates and copper traces:
A typical rigid flex will contain:
- Multiple rigid PCB core layers
- Flexible dielectric layers as needed
- Adhesive bonding all layers together
- Copper traces on outer layers for component pads and routing
- Copper mid-layers for complex circuit routing
- Cover layers or soldermask for insulation and markings
Advanced designs can integrate 10 or more total layers of materials and copper. The layer stackup is carefully engineered to achieve the needed balance of rigidity, flexibility, and circuit density.
Manufacturing Processes
Specialized methods are required to manufacture rigid flex PCBs:
Layer Preparation
The process starts with preparing the rigid and flexible layers:
- Rigid cores – Tooling holes drilled, prepreg layers laminated
- Flex layers – Cut to size, cover film applied
- Adhesive films – Laser cut for openings like vias
Layer Alignment and Bonding
The layers are precisely aligned and bonded:
- Alignment – Rigid and flex layers aligned using tooling holes
- Bonding – Bonded together using heat and pressure
- Adhesives cured – Layers fused into complete board
Via Formation
Vias are created to interconnect layers:
- Mechanical drilling – Holes drilled through adhesive and substrates
- Plating – Copper plated onto via barrels to connect layers
- Microvias – Smaller vias with limited layer spans
Outer Layer Processing
Final board processing:
- Imaging – Photoresist imaging to define trace patterns
- Etching – Copper etched to form defined traces
- Soldermask – Insulating lacquer coating
- Finishing – Labeling, testing, and final checks
This multi-stage process ensures reliable construction of complex rigid flex boards.
Rigid Flex PCB Design Considerations
Designing a reliable rigid flex PCB requires attention to:
Layer Stackup
- Define layer materials – rigid, flexible, adhesive
- Determine total layer count based on routing needs
- Stackup arrangement for optimal performance
Rigid-to-Flex Transitions
- Minimize gaps between rigid and flex sections
- Include stiffeners and frames for robust transitions
- Manage differences in coefficients of thermal expansion
Routing
- Plan routing to efficiently use rigid and flex layers
- Balance trace lengths for timing needs
- Allow for flexing and motion in routing
Component Placement
- Place components in rigid sections for stability
- Avoid component placement on dynamic flex areas
- Watch component clearance through flex bends
Flex Bends
- Design bending sections with sufficient flex radius
- Specify bend directions and locations
- Plan for flex life and motion cycles
Thermal Management
- Use thermal vias, cores, and planes to spread heat
- Focus cooling on high power rigid sections
- Model thermal dissipation through flex areas
Testing
- Simulate dynamic bending and vibration
- Verify electrical performance under flexing
- Accelerate testing for flex life validation
Typical Rigid Flex Constructions
Common rigid flex board designs include:
Multilayer Rigid Flex
- Multiple rigid board layers for complex circuitry
- Thin flexible layers to interconnect rigid sections
- Components mounted on rigid areas
- Flexible “linker” sections between rigids
Rigid Islands Interconnected by Flex Circuits
- Smaller individual rigid sections
- Flexible circuits routing between islands
- Allows compact 3D arrangement of rigids
- Rigids provide component mounting stability
Flexible Folds and Hinges
- Larger flexible areas for folding and hinging
- Mimics bound book style folding motion
- Excellent for compact electronic packaging
- Avoid components on fold areas
Applications of Rigid Flex PCBs
The unique benefits of rigid flex PCBs make them ideal for products like:
- Wearable electronics – Can flex and conform to user movement
- Medical devices – Enable miniature flexible instruments
- Military electronics – Rugged and reliable in harsh environments
- Aerospace avionics – Complex circuits with flexing ability
- Consumer devices – Folding/sliding mechanisms in cellphones, laptops, etc
- Robotics – Stable component mounting plus flexing joints
Any application needing both dense, reliable interconnects as well as repetitive dynamic flexing can benefit from rigid flex PCB technology.
Benefits of Rigid Flex PCBs
Key advantages of rigid flex PCBs include:
Flexible and Dynamic
- Withstand repeated bending and flexing motion
- Enable movable and reconfigurable electronics
Complex and Compact
- High density interconnects with 10+ circuit layers
- Folds into 3D arrangements for compact form
Reliable
- Excellent shock and vibration resistance
- Stable electrical performance over flex cycles
Design Freedom
- New product designs not possible with one PCB type
- Balance rigid and flex needs optimally
Cost Considerations
Rigid flex PCBs are generally more expensive than simpler rigid or basic flex PCBs. Key cost factors include:
- Advanced materials – Both rigid and flex dielectrics required
- More process steps – Additional bonding and via processes
- Lower volumes – Increased cost for smaller batch quantities
- Complex design – Rigid-flex integration requires expertise
However, rigid flex PCBs can enable new product designs and miniaturization not feasible with other PCB types. They should be evaluated based on a total cost of ownership basis.
For simple flexible connections, basic flex PCBs will likely be lower cost. Rigid flex PCBs are specialized solutions for products needing both robust rigid interconnects and repetitive dynamic flexing.
Frequently Asked Questions
Properly designed rigid flex PCBs can reliably withstand hundreds of thousands of dynamic bend cycles with no degradation in performance. Some polyimide-based constructions can endure over 1 million cycles. The flex life is validated through accelerated lifecycle testing.
It is strongly recommended to only mount components onto rigid sections of a rigid flex PCB. Attaching components onto dynamic flexing areas risks fracturing solder joints or creating other reliability issues due to constant motion. Discrete components can be soldered or adhesively bonded to designated rigid regions only.
Rigid flex PCBs enable many compact mechanisms in smartphones, laptops, and tablets. Application examples include foldable/sliding phone and tablet displays, laptop hinged connections, and pop-out phone cameras. Rigid flex PCBs maintain reliable connections under frequent consumer use and abuse.
Rigid flex PCBs integrate both rigid board and flexible circuit technologies into one hybrid solution. Flex PCBs only use flexible base materials without any rigid sections. Rigid flex provides both stability and flexibility where needed; flex PCBs are only dynamic.
Advanced rigid flex PCBs can incorporate 10 or more total layers of interconnected rigid and flex substrates. Typical constructions have 4-8 layers, with additional layers increasing cost and complexity. The design determines the layer count needed to route all required electrical connections.
Conclusion
Rigid flex PCB technology enables electronics designs combining dense, reliable interconnects with dynamic flexing and bending. By integrating the capabilities of both rigid boards and flexible circuits, rigid flex PCBs create unique solutions for wearables, medical devices, consumer gadgets, aerospace systems, and more. As electronic devices continue requiring compact packaging with movable parts, the applications for rigid flex PCBs will grow.
