Multilayer Rigid-Flexible PCB: A Comprehensive Guide

October 10, 2023

Introduction

Multilayer rigid-flexible PCBs represent an advanced evolution in PCB technology that combines the benefits of both rigid and flexible PCBs into one integrated product. With their increased functionality and versatility, multilayer rigid-flex boards are widely adopted across various industries, including consumer electronics, automotive, aerospace, and medical.

In this comprehensive guide, we will cover everything you need to know about multilayer rigid-flex PCBs, including:

What is a multilayer rigid-flex PCB?
Key benefits and applications
Materials and construction
Design considerations
Manufacturing process
Assembly and testing
Cost factors
Future trends

Equipped with a detailed understanding of this innovative PCB technology, you can effectively incorporate multilayer rigid-flex into your product designs and achieve maximum benefits. Let’s get started!

What is a Multilayer Rigid-Flexible PCB?

As the name suggests, a multilayer rigid-flex PCB combines rigid and flexible PCB technologies into a single circuit board solution. It contains rigid sections for mounting electronic components and flexible sections that allow dynamic flexing and bending.

The rigid portions provide mechanical support and structure just like a standard multilayer PCB, while the flexible portions enable freedom of movement. The two portions are seamlessly integrated into the same board through careful design and advanced manufacturing processes.

Key Features:

Contains alternating layers of rigid and flexible PCB materials.
Offers benefits of both rigid stability and flexible movement.
Components mount on rigid portions while flexible portions connect and route.
Layers interconnected through plated through holes or other vertical connections.
Can fold, bend, and flex dynamically in flexible areas.
Rigid-flex transition areas can be straight or sculpted.

In short, multilayer rigid-flex PCBs provide the ideal combination of support, connection, and flexibility in a single printed circuit board solution.

Benefits and Applications

Key Benefits

Here are some of the significant benefits that multilayer rigid-flex PCBs offer over conventional PCB technologies:

Space and weight savings: By integrating multiple rigid PCBs connected through flexible interfaces, rigid-flex boards provide more efficient use of space and lower weight.
Increased component density: Components can be densely packed on both sides of the rigid portions of the PCB without interference.
Fewer connectors: Integrating connections into the flexible substrate eliminates discrete wires and connectors.
Dynamic flexing: Products can be designed with moving and bendable parts to fit various morphologies and spaces.
Improved reliability: Rigid-flex PCBs offer improved thermal performance, signal integrity, and reliability through high layer count and integration.
Design freedom: Rigid-flex allows three-dimensional configuration and folding, enabling innovative package designs.
Lower assembly cost: Automated assembly is enabled, leading to faster fabrication and lower associated costs.

Key Applications

Due to the unique benefits that rigid-flex PCBs offer, they are being rapidly adopted across diverse products and industries where reliability, compact size, and dynamic flexibility are critical requirements.

Consumer electronics – Laptops, smartphones, tablets, wearables, etc. Rigid-flex allows compact and flexible circuitry to fit into small, irregular spaces.

Medical devices – Hearing aids, pacemakers, imaging equipment. Rigid-flex allows circuits to bend and flex with body movements.

Automotive electronics – Navigation systems, engine control units, sensors. Rigid-flex withstands vibration and allows the integration of rigid and flex circuits.

Aerospace and defense – Avionics, radars, missile guidance. Rigid-flex can withstand extreme environments and integrate multiple rigid and flex layers.

Industrial equipment – Robotics, instrumentation, control systems. Rigid-flex enables compact, 3D circuit layouts and flexing interconnects.

Communications – Mobile base stations, routers, switches. Rigid-flex allows high-density circuitry and component mounting.

Materials and Construction

Rigid-flex PCBs require specialized materials engineered to deliver mechanical flexibility and resilience while retaining electrical and thermal conductivity. Here are some critical materials used:

Substrate Materials

Rigid layers: FR-4 glass epoxy and high Tg materials like Isola, Arlon, and Nelco are commonly used for higher temperature ratings. Ceramic-filled PTFE composites, polyimide, and aluminum are options for further ruggedization.
Flexible layers: Polyimide films like Dupont Kapton or UBE Upisel are widely used for their superior mechanical and electrical properties. PET and PEN films provide lower-cost options. PEEK offers radiation resistance for space applications.
Bonding films: Acrylic, epoxy, nitrile phenolic, and thermoset adhesives are used to laminate the layers under heat and pressure securely. The flexibility of the adhesive film itself is tailored to match the design.
Cover layers: Dry film solder masks with openings for soldering, connectors, etc. Liquid photo-imageable overlays (LPI) provide a permanent protective coating of traces.

Stiffening and Reinforcement

Metal stiffeners: Plated copper, steel, or beryllium copper inserts control flexibility and provide shielding in required locations.
Non-conductive stiffeners: Glass or aramid fiber-reinforced stiffeners prevent flexible layer fractures. Kapton, aluminum, and FR4 are also options.
Encapsulation: Epoxy, silicone, and urethane coatings protect components and fragile areas from environmental damage.

PCB construction can deliver maximum functionality across electrical, thermal, mechanical, and environmental operating needs by selecting the right combination of materials engineered for rigid-flex requirements.

Layer Stackup

Here are some of the properties of layer stack-up

Rigid-flex PCBs have both rigid sections and flexible ribbon interconnects. The rigid sections can have a typical multilayer stack up with alternating signal and power/ground layers.
The flex ribbon is thinner than the rigid sections. It should have a similar stack up to the inner layers of the rigid PCB sections.
Flex ribbons typically need to carry signals between rigid sections, so they need at least one signal layer. They also need a ground plane for return currents.
To extend power or ground planes across the flex ribbon, use a cross-hatched copper pattern instead of a solid plane. This improves flexibility.
Components can be mounted directly on the flex ribbon if there is a signal layer below the coverlay. Solder lands need to be in the signal layer below the coverlay.
An alternative for mounting components is to use plated through-hole vias under the components, sometimes called button plating.
Avoid placing components or critical features directly in the flex ribbon bend areas.
It’s best to separate analog and digital signals onto different layers, with their own ground planes, in both the rigid and flex sections.
The rigid and flex sections must be designed together to ensure proper layer alignment and stacking across the interface.

In summary, rigid-flex layer stack-up requires carefully planning the layer arrangement in both rigid and flex sections while considering signals, grounding, and component placement.

Design Considerations

Designing a high-performance and reliable multilayer rigid-flex PCB requires extensive pre-planning and simulation to account for the complex interplay of electrical, mechanical, and thermal factors. Here are some fundamental design guidelines to follow:

Layer Stackup Planning

Define the placement of components on rigid and flexible sections based on stability and routing needs. High pin count ICs should be placed on rigid portions.
Plan the layer stack-up sequence carefully, with signal layers sandwiched between power and ground layers. Avoid placing signal layers together.
Use a minimum of 4-6 layers for good signal trace routing. Higher layer counts accommodate more complex routing.
Incorporate thermal dissipation layers paired with ground layers for heat spreading.
Assign power and ground layers next to each other to form decoupling capacitors.

Trace Routing

Use minimum 45-degree angle traces at rigid-flex layer transitions to avoid cracks from flexing stress.
Route critical high-speed traces only through rigid portions, avoiding dynamic flex areas. Multilayer routing can minimize crossovers.
Use wider trace widths in flexible sections to account for flexing loads.
Incorporate teardrop pads at junctions to prevent cracking.
Match trace impedances for high-frequency signals.

Flexible Area Design

Allow sufficient bend radius to prevent metal fatigue and fractures over product lifetime flex cycles.
Incorporate polyimide stiffeners and spacers in flexible sections to provide mechanical support during movement.
Avoid long unsupported spans of flexible layers to prevent undue stress and material deformation.
Eliminate flexing immediately over plated through holes and vias, which can crack soldiers.

Thermal Management

Incorporate thermal vias and pads connected to ground layers to conduct heat away from hot components.
Allow sufficient air gaps between components for ventilation and cooling airflow.
Use thermally conductive rigid layer materials like aluminum or metal-based PCBs.
Ensure no high-power or thermally critical components are placed in flexible sections.

Signal Integrity

Incorporate closely spaced ground vias adjacent to high-speed routing to limit EMI.
Match trace impedances to eliminate signal reflections that corrupt data.
Minimize abrupt transitions between flexible and rigid sections.
Use blind and buried vias, which offer better signal performance over through-hole vias.

Simulating and optimizing across these multilayer rigid-flex design parameters allows the layout to achieve maximum electrical performance, long-term reliability, and mechanical resilience.

Manufacturing Process

A high-reliability multilayer rigid-flex PCB requires specialized fabrication processes and equipment to handle rigid and flexible materials through complex layer stack-ups. Here is an overview of the critical manufacturing steps:

Materials Preparation

Sheets of rigid laminate materials like FR-4 and high Tg epoxy and flexible materials like polyimide, PEN, and PET are ready.
Rolls of electrodeposited copper foils, liquid photo imagable overlay, bonding films, and stiffeners are prepared, matching the stack-up requirements.
All materials are cleaned, baked, conditioned, surface treated, and inspected before lamination.

Layer Bonding

The materials are laid on buildup fixtures per the layer stacked with copper foils, prepregs, cores, and bonding films.
Books and subassemblies are collated and loaded into lamination presses.
The layers are bonded under high pressure and temperature, and the bonding films cure to join the layers.

Circuit Formation

Dry film or liquid photoresists are applied on the outer copper layers.
The resist is exposed and developed using lithographic imaging to define the circuit patterns.
Chemical etching selectively removes the unprotected copper to create the conductor traces.

Hole Formation

Mechanical drilling and laser ablation create through holes, blind and buried vias spanning multiple layers.
Walls are plated with copper electroplating to form interconnections between layers.

Outer Processing

Soldermask layers are laminated, and openings are defined for soldering.
Exposed copper is plated with ENIG, immersion silver, or other surface finishes.
Identifications, markings, and legends are screen-printed.
Panels are routed into individual PCB units.

Combining specialized materials, fabrication tools, and processes allows the construction of highly complex rigid-flex configurations within tight tolerances.

Assembly and Testing

To harness the benefits of multilayer rigid-flex boards, the assembly and testing need to be adapted:

PCB Assembly Considerations

Components can be placed on both sides of rigid portions for 2x density.
Stencil design must account for rigidity differences.
Adhesives augment solder joints for high-flex areas.
Flexible sections require folding and shaping post-assembly.
Careful handling is needed to prevent damage.

Product Testing Factors

Both rigid and dynamic flexing tests need to be performed.
Temperature and vibration resilience should be verified.
Continuity testing ensures end-to-end connections.
Cyclic fatigue testing validates flex endurance.
Thermal analysis determines hot spots.
Shock and drop testing confirms durability.

Rigid-flex PCB performance can be fully validated before product integration with adapted assembly processes and thorough testing methods.

Cost Factors

Here are some of the considerations that impact the cost of multilayer rigid-flex PCBs:

Materials: Flexible substrates and bonding films carry a premium over rigid laminates.
Layer count: Higher layer rigid-flex PCBs proportionately increase costs.
Sequential lamination: Separate pressing of sub-assemblies takes more production time.
High mix processes: Frequent material changeovers reduce efficiency.
Low volume production: Lack of economies of scale leads to higher per unit cost.
Advanced processing: Add-ons, like blind/buried vias controlled depth drilling, add cost.
Testing needs: Rigorous inspection and qualification require investment.
Yield optimization: Process refinement is needed to improve yields over time.

Costs can be contained by designing optimized rigid-flex configurations and working with competent manufacturers while maximizing benefits.

Future Trends

Here are some of the advances on the horizon that will expand rigid-flex PCB capabilities:

Materials Evolution

New flexible materials with better electrical and thermal properties.
Improved bonding films enable thinner and lighter constructions.
Adhesives and surface finishes tailored for rigid-flex needs.

Manufacturing Innovation

Additive direct imaging eliminates etching and hazardous waste.
Sequential lamination and modular assembly for quick turnaround.
Automation through reel-to-reel processing and direct integration.

Design for Flexibility

3D modeling tools specifically for multilayer rigid-flex simulation.
HDI technologies allow higher component densities.
Built-in flex testing and quantification of dynamic characteristics.

Applications Expansion

Growth opportunities in wearables, robotics, defense, and space electronics.
Embedding actuation mechanisms into flexible portions.
Development of hybrid PCB technology merging flex, rigid, and cores.

In summary, rigid-flex PCBs are poised for greater diversity and capabilities driven by next-generation materials, processes, and designs.

Conclusion

Multilayer rigid-flex PCB technology represents a significant advancement, combining the stability of rigid boards with the dynamic flex capability of flexible circuits. With benefits like increased reliability, component density, smaller size, and lighter weight, rigid-flex boards enable innovative product designs across industries.

While requiring careful design considerations, rigid-flex PCB fabrication leverages specialized materials, processes, and testing methods to deliver maximum performance. As the technology evolves with new materials and capabilities, multilayer rigid-flex circuits are poised to expand into diverse applications that need compact, high-speed solutions to withstand vibration, shocks, and temperature extremes.

With this comprehensive guide, you should have a detailed understanding of this powerful PCB technology. By leveraging the capabilities of multilayer rigid-flex PCBs, you can take your products to new levels of technological sophistication and market competitiveness.

Frequently Asked Questions (FAQs)

What are some typical applications of multilayer rigid-flex PCBs?

Some major applications are in consumer electronics (mobile devices, laptops), automotive (instrument panels, LED lighting), aerospace (avionics, in-flight entertainment), medical (hearing aids, medical imagers), and industrial electronics (robotics, automation).

How many conductive layers are typically present in a multilayer rigid-flex board?

A typical construction would have 6 to 12 conductive copper layers, but some complex designs may have up to 20 layers or more. At least 2 layers are recommended for good signal routing.

What are some key design factors to consider for rigid-flex PCBs?

Critical design considerations include layer stack-up planning, trace routing for flex tolerance, minimizing conductive lines through flex areas, thermal management, signal integrity, impedance matching, and mechanical reinforcement.

What are some ways to control flexibility in rigid-flex PCBs?

Incorporating stiffeners, limiting unsupported flexible overhangs, avoiding flex over plated through holes, and helping sufficient bend radius control flexibility. Careful penalization also reduces fractures.

How are the rigid and flexible layers bonded together in multilayer constructions?

Bonding films such as acrylic or epoxy adhesives join the layers under heat and pressure. The bonding technique depends on materials and the sequence of lamination.

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