Rigid-flex printed circuit boards feature the best of both rigid and flexible PCBs. This hybrid approach has made the boards quite popular because the circuits provide mechanical stability and adaptability in a compact and reliable package.
In this ultimate rigid-flex PCB guide, we will explain the differences between this board and its flexible and rigid counterparts (including cost-wise), the PCB’s stackup, design considerations, and more. Let’s get down to business.
Table of Contents
- Introduction to Rigid-Flex Printed Circuit Boards
- What Are the Differences Between Rigid, Flexible, and Rigid-Flex PCBs?
- How Much Does Rigid-Flex PCB Cost?
- What Is Rigid-Flex PCB Stackup?
- What Are the Design Considerations for Rigid-Flex PCBs
- How To Manufacture Rigid-Flex PCBs
- Where Are the Rigid-Flex PCBs Used
- Why Choose Us As Your Rigid-Flex PCB Manufacturer
- FAQ
- Conclusion
Introduction to Rigid-Flex Printed Circuit Boards
To define rigid-flex printed circuit boards, we have to look at their constituent parts.
Rigid PCBs are single, double, or multilayer circuit boards with non-flexible substrates made of materials like FR4, metal, or ceramic. On the other hand, flexible PCBs have bendable and flexible substrates, which can also have single, double, or multiple copper layers.
Rigid-flex circuit boards combine rigid and flex circuit sections into a single circuit board, giving you the benefits of both worlds, which include:
- Reduced chances of failure due to fewer board-to-board connectors and solder joints
- Lower overall cost when considering logistics, assembly, size, and connections
- Mechanical stability
- Increased routing space
- Better packaging flexibility
- Suitability for high-density applications
- High shock and vibration resistance
- Easy assembling and testing
What Are the Differences Between Rigid, Flexible, and Rigid-Flex PCBs?
Rigid PCBs
As stated earlier, rigid PCBs have inflexible and unbendable substrates, the most common one being FR4.
This solid structure is only suitable for applications with fixed form factors, but it is widely used in various devices because it is durable, mechanically stable, and cost-effective.
Flexible PCBs
Flexible circuit boards feature a flexible substrate that conforms to fit inside tiny or uniquely shaped packages, such as smartwatches and smartphones.
Most have polyimide substrates and are costlier to manufacture due to the specialized material selection and fabrication process. Besides the size advantage, flexible circuit boards are also lightweight.
Rigid-Flex PCBs
Rigid-flex PCBs are hybrids that have rigid and flexible sections in one board.
This structure provides mechanical stability in the required parts and flexibility in others, allowing you to design complex circuits that can fit in compact spaces.
Of the three, rigid-flex PCBs are the most expensive to design and manufacture.
However, their automated testing costs are lower before assembly, and the assembly costs are low, as well, because you’ll need fewer mechanical connectors and solder joints.
Differences Between Rigid, Flexible, and Rigid-Flex PCBs
How Much Does Rigid-Flex PCB Cost?
The general cost of rigid-flex PCBs is roughly $0.35-0.45 per square inch per layer. So the cost of a basic 4-layer board measuring 10 x 5 inches will be:
0.45 x 10 x 5 x 4 = $90
This cost can vary depending on these factors.
- Layer count
- Material (polyimide, polyester, peek, PI composite, LCP, etc.)
- PCB Thickness
- Copper Weight
- PCB Size
- Surface Finish
- Tolerance
- Flexibility
- Reliability
- Testing and Certification
- Order Quantity
Generally, rigid-flex PCBs can cost anywhere from $10–500+ per piece, making them significantly cheaper as the orders increase in the thousands.
But prototypes, which are necessary before mass production, will be on the higher side of this scale.
In comparison, flexible boards cost around $10–500 per board, while rigid boards go as low as up to $50 per piece.
As experts in flexible and rigid-flex PCB manufacturing, we can deliver high-quality prototypes and small to high-volume orders for your project.
Our team consists of experienced engineers and technicians who can collaborate with you to create custom rigid-flex PCBs that provide optimal performance and high reliability. Contact us for more information about rigid-flex PCBs.
What Is Rigid-Flex PCB Stackup?
Obviously, the flexible section in rigid-flex PCBs is thinner and has fewer copper layers to maintain its flexibility. But how thick can it get?
The answer is 6 copper layers. Here are the types and examples of the rigid-flex PCB stack-ups you can get.
- 4 layers (2 rigid and 2 flexible)
- 6 layers (4 rigid and 2 flexible)
- 6 layers (2 rigid and 4 flexible)
You can use the above stack or the one below with no flow prepreg.
- 6 layers with an air gap (2 rigid and 4 flexible)
Instead of using adhesive to bond the two inner polyimide coverlays, this stack has an air gap.
- 8 layers with an air gap (4 rigid and 4 flexible)
- 8 layers with two air gaps (2 rigid and 6 flexible)
- 10 layers (7 rigid and 3 flexible)
- 12 layers with an air gap (8 rigid and 4 flexible)
What Are the Design Considerations for Rigid-Flex PCBs
Complex Design Rules
Rigid-flex PCB designers need to understand the mechanical constraints and electrical performance of the two sections, which requires detailed planning while considering the layer stackup, material constraints, and bend radius.
Material Selection
Selecting the most suitable materials for both sections is challenging because they must be compatible when considering factors like thermal expansion.
Bend Radius Considerations
The flexible section of the board should be able to bend without causing mechanical failures or signal integrity issues, so the calculations must be spot on.
EMI and Signal Integrity
Balancing signal integrity with minimal or zero EMI on the flexible areas is challenging but doable with careful routing, grounding, and shielding techniques.
Layer Transition
Transitioning the signal layers from the rigid to flexible sections should be done carefully to avoid misalignments that can cause degradation or impedance mismatches.
Thermal Management
The thermal behaviors of the rigid and flexible sections are different, so you have to employ different cooling solutions. Thermal vias are excellent for the flexible part because you can’t use traditional methods, such as heat sinks. But the rigid part can have both vias and heat sinks.
Mechanical Reliability
Guaranteeing the reliability of the flexible section to withstand multiple bending and flexing cycles requires careful trace layouts and selection of the material and protective coatings.
Manufacturing (DFM) and Assembly (DFA)
The hybrid PCB structure requires extensive DFM and DFA analysis to ensure the board can be manufactured and assembled reliably and quickly. It requires extra work to ensure the handling along the production lines does not weaken the transition zones.
Environmental Factors
Rigid-flex PCBs might be exposed to harsh conditions, such as heat, humidity, and corrosive chemicals, so designers must consider these factors when considering the substrate materials and protective coatings.
Design Verification
The final design requires rigorous simulation and verification processes to ensure the final PCB meets all the required performance specifications.
How To Manufacture Rigid-Flex PCBs
rigid-flex PCB manufacturing
The rigid-flex PCB manufacturing process is nuanced because it combines techniques used in rigid and flexible PCB fabrication. It includes these steps.
- Material Selection and Preparation
The process begins with careful material selection for both sections, cleaning them, and then laminating them with the copper layer.
- Circuit Patterning
Circuit design transfer from the Gerber files to the copper layer requires either photolithography (using UV) or laser direct imaging, but all need a photoresist over the copper.
- Etching
This process requires a chemical etching bath or spraying solution to form the circuit pathways.
- Drilling
Drilling uses mechanical or laser methods to create the holes for vias and through-hole components.
- Copper Plating
Copper electroplating using VCP (Vertical Continuous Plating) deposits an even copper thickness on the hole walls.
- Coverlay Application
The overlay material, usually polyimide and adhesive, is imprinted on the PCB surface via screen printing.
- Coverlay Lamination
Lamination uses specific heat and pressure parameters to bake the PCB in a vacuum. If the rigid-flex PCB design requires localized stiffeners, they should be added before the lamination process.
- Flex Board Cutting
The board is cut from the production panel using hydraulic punches or specialized blanking knives to separate it for inspection and testing.
- Inspection and Testing
The inspection and testing processes are carried out according to the relevant quality standards using methods like AOI, X-ray, flying probe, contamination, and solderability tests.
Where Are the Rigid-Flex PCBs Used
Consumer Electronics
Rigid-flex PCBs are common in portable devices like smartphones because they optimize space consumption and make the products lightweight while providing the required functionality.
Additionally, these boards are shock and vibration-resistant, which enhances the durability of these portable products, such as when they fall.
Wearable Tech
The primary requirement of wearable tech is compactness. Using a rigid-flex PCB provides this advantage, coupled with multiple functions enabled by the multilayer structure of the rigid section.
These boards are also lightweight and can fit inside the device’s ergonomic design, making them comfortable for daily wear.
Aerospace
Rigid-flex PCBs provide the required strength to withstand shock and vibrations in aviation and aerospace applications. These include cockpit instrumentation boards and those for satnav.
Medical Devices
Wearable health monitors and diagnostic equipment rely on rigid-flex circuit boards to conform to ergonomic designs and withstand demanding medical environments.
Automotive
Car ADAS, infotainment, and dash display systems are exposed to shock and vibrations as you drive but benefit greatly from the shock and vibration resistance provided by rigid-flex PCBs to maximize durability.
Why Choose Us As Your Rigid-Flex PCB Manufacturer
Careful planning during the design process is critical if you want a durable and reliable rigid-flex PCB. At FlexiPCB, we collaborate closely with our clients to provide:
- Concept development
- Design optimization
- Material selection
- Stackup selection
- DFM and DFA analysis
- Signal integrity analysis
- Power integrity analysis
- Thermal analysis
- Mechanical analysis
These services are critical to verifying the design for fabrication.
During manufacturing, we accommodate rigid-flex PCB projects of all sizes, which include quick turn, low volume, medium volume, and high volume production. We can also simplify your entire project by providing turnkey solutions to our customers. Our team can handle:
- Material sourcing and procurement
- PCB manufacturing and assembly
- Quality control
- Packaging and shipping
- After-sales support
- Maintenance services
Our goal is to ensure you have a smooth experience straight from the ordering step to PCB design and verification, manufacturing, and assembly.
FAQ
Which materials are used to develop rigid-flex PCB?
First, we use substrate materials like woven fiberglass, epoxy resin, polyimide, and polyester. Moreover, we apply copper materials as the conductor materials.
Furthermore, we include adhesives to improve the board’s service life. Finally, we apply cover layers and coats to offer a protective coating.
Are rigid-flex PCBs worth it?
Generally, using rigid-flex PCB is worth it and even better than traditional options.
Surprisingly, you can bend and fold these PCBs to fit in smaller and tighter spaces. Also, the cost of using these PCBs is lower than that of traditional boards.
Conclusion
To summarize, rigid-flex PCBs have several advantages that make them suitable for current and future electronics where miniaturization, lightweight designs, flexibility, and durability are key.
Although costly and complicated to design and manufacture, they are cheaper considering the overall costs.
We have tailored our rigid-flex PCB design and manufacturing processes to reduce the cost and complexities involved with these products to remove the hurdles for adoption.
Additionally, our production service is quite flexible to suit your requirements, so reach out today to get a quote for rigid-flex PCB manufacturing or assembly!
