Rigid-flex PCB design is transforming how we build our electrical systems and appliances as they have the benefits of both rigid and flexible PCBs.
For instance, these boards can fit in compact spaces and withstand vibrations due to the flexible section.
At the same time, the rigid part provides structural strength, has more copper layers (higher circuit density), and holds more components.
The combined construction makes rigid-flex PCBs harder to design, although eliminating connectors slightly simplifies its design.
Overall, these PCBs are suitable for smart, compact devices like smartphones, and here’s a step-by-step guide to their design process.
Table of Contents
- What is a Rigid-Flex PCB?
- What Should You Consider in Rigid-Flex PCB Design
- Rigid-Flex PCB Design Guidelines
- How To Design Rigid-Flex PCB: Step-by-Step Design Process
- How To Control Quality While Designing and Manufacturing Rigid-Flex PCBs
- Applications of Rigid-Flex Circuit Boards
- Conclusion
What is a Rigid-Flex PCB?
As stated earlier, rigid-flex PCBs have rigid and flexible sections. The rigid section has non-bendable core materials, such as FR4, metal (aluminum), and Teflon, but FR4 is the most common.
On the other hand, the flexible part has a bendable substrate, with polyimide being the most commonly used material.
These two sections are joined permanently during manufacturing using orderly and selective layer stacking of the two sides, so there are no removable connectors or joints between them.
Types of Rigid-Flex PCBs
Rigid-flex PCBs come in three primary types, which are:
Single-sided vs. double-sided vs. multilayer PCBs
- Single-sided: These simple boards have only one conductive copper layer.
- Double-sided: With two copper layers (one on each side of the substrate), this PCB has a medium density and can accommodate more components and functionality than the single-sided type.
- Multilayer: These boards are the most complex to build and design because they have at least three copper layers.
Rigid-flex PCBs can also be tailored to meet application-specific requirements, such as high-frequency circuits for wireless communication and high-density interconnections for compact devices, such as smartphones.
Rigid vs. Flex vs. Rigid-Flex PCBs
Rigid, flexible, and rigid-flex PCBs primarily differ in these six areas.
PCB Types Comparison: Rigid vs. Flex vs Rigid-Flex PCBs
Flexibility
Flexible PCBs provide the ultimate flexibility, so they can be bent, creased, or folded to fit in the end product, such as in wearable electronics.
Rigid boards cannot be bent at all, but rigid-flex PCBs have this property in some sections, making them more adaptable to modern electronics that have compact form factors.
Connectivity
Flexible boards provide connectivity between rigid boards and can even replace ribbon cables. But they need connectors to achieve this.
Rigid boards cannot interconnect other boards, but rigid-flex PCBs have this connectivity element built in.
The rigid boards are two independent circuits joined by the flex region, which is a circuit on its own.
Weight
Rigid boards are the heaviest of the three types, while flexible PCBs are the lightest. Therefore, their use determines the overall weight and size of the final device.
For instance, drones and unmanned vehicles often use flexible or rigid-flex boards to reduce their weight, which extends their range.
Durability
Rigid PCBs are strong and provide mechanical strength to the built-in circuit and assembled components. But they are not good at absorbing shock and vibrations.
Therefore, they are not the best for applications exposed to regular vibrations, such as automotive, robotics, missile guidance, and aviation.
For such, flexible or rigid-flex boards are the most suitable.
Assembly
Rigid-flex PCBs might be more challenging to design, but their assembly process is significantly easier because they eliminate the need for wire harnesses and connectors.
Essentially, they are self-contained circuits.
Environmental Resistance
Although they provide mechanical strength, rigid PCBs are more at risk of damage due to external factors, such as chemicals, heat, and radiation.
But flexible boards and rigid-flex PCBs by extension, are more environmentally resistant, meaning better durability.
For instance, polyimide can withstand higher heat levels than FR4, and the thin structure of the board dissipates heat faster.
What Should You Consider in Rigid-Flex PCB Design
Challenges and Solutions in Rigid-Flex PCB Design
Some of the typical challenges faced during rigid-flex PCB design include:
Maintaining Impedance Requirements
Impedance matching and maintenance are more challenging in rigid-flex boards than in regular PCBs because factors like dielectric and copper thicknesses affect them.
As the copper lines cross over from the rigid to the flexible board, the stackup changes and this affects the impedance.
The best way to maintain it is to vary the properties of each copper trace, such as widening it in the flexible section, then balance the stackup design to cater to the difference.
Trace Routing
Copper traces must be placed carefully in flexible PCBs while considering component locations, heat management, and spacing requirements.
So, as a designer, you should route them without interfering with design requirements like impedance and via holes.
Trace routing on a PCB
Component Placement
Component placement must be considered at the very beginning due to the bending properties of the PCB.
This will help to adhere to standards like IPC-2221, which requires no vias (through holes) to be placed near the transition point.
These components should also be placed carefully to avoid colliding with the flexible board as it flexes.
Preserving Bend Requirements
Rigid-flex PCBs must be able to bend to meet their requirements, which can be for one-time installation or multiple bend cycles throughout their lifespan.
The design of this part should follow the IPC-2223 guidelines, and you can preserve the bending requirements by:
- Increasing the length of the flexible part
- Reducing the number of layers or thickness of the flexible stack
- Applying epoxy strain relief on the transition edges
Design Considerations for Rigid-Flex PCB
Layer Stack-Up Configurations
When defining the layer stack-up configurations, place the flexible layers in the middle to prevent slippage and pick the most suitable materials based on the PCB’s design and operating environment.
Additionally, you should implement an even number of layers to create a balanced stack and mention the impedance requirements and thickness of each layer.
Don’t forget to use the air gap layout method to reduce adhesive use, which helps to increase via and bending reliability in high layer-count boards.
A rigid-flex PCB layout built using the air gap construction method (with two air gaps)
Trace Width and Spacing Guidelines
Trace width changing is necessary along the transition to maintain impedance requirements.
However, this should be done gradually (tapered down) to avoid creating weak spots along the joint.
To accommodate this change, incorporate teardrops on vias and pads in the flexible circuit. But if you use a stiffener, then it’s okay to change widths abruptly.
Typical trace widths range from 5–10 mils in the flexible section and 5–20 mils in the rigid section.
Although the copper width in the flexible section should be wider, excessively broad traces can affect flexibility and increase the risk of delamination.
As for the spacing in the flexible section, the minimum distance between traces should be 6–8 mils.
Via Types and Placement
Rigid-flex PCBs can have any via type, but these must be placed carefully, especially in multilayer flex areas, to prevent fatigue.
Ideally, the design should have at least a 20-mil spacing between a via’s copper annulus and the rigid-flex joint.
Materials Used in Rigid-Flex PCB Design
- Rigid materials (FR4 most common) for the rigid substrate
- Flexible materials (Polyimide most common) for the flexible core
- Copper (electrodeposited or rolled annealed)
- Adhesives (some flexible PCBs are adhesive)
- Coverlay (polyimide) or LPI for flexible boards
- Solder mask for rigid boards
The most critical materials are the cores, and they should be selected based on the rigid-flex PCB application.
Polyimide alternatives include PEEK, PET, LCP, polyester, and TPU, while FR4 alternatives are ceramic, metal, and PTFE (Teflon).
Each has unique dielectric properties and thermal performance that are suitable for specific applications.
Design Tools and Software
The recommended CAD tools for designing rigid-flex PCBs are:
Some features to look for when picking a design software include providing a smooth workflow, productivity enhancement tools (constraints and design rules management, automated trace routing, etc.), and continual improvements/upgrades.
Rigid-Flex PCB Design Guidelines
(Rigid PCB with a flexible cable)
Layout Guidelines
- First, keep the trace spacing and width as wide as possible in the flexible sections of your PCB.
- Second, do the solder pad and track connections roundly like a teardrop.
- Third, keep the soldering surfaces and annular rings wide as possible.
- Fourth, add stiffeners to your PCB to reach a 0.2 to 1-millimeter thickness.
Coverlay
Add coverlay to your printed circuit board to protect the conductors from environmental hazards.
Furthermore, the coverlay holds the pads to make your work easier when soldering.
Pads and Vias
Use pads that have tie-downs to ensure that the base material and copper metal foil attach during assembly.
Moreover, fillet the pads to lower the chances of breakage and stress points during flexing.
Concerning the vias, you can go with plated vias or blind vias.
Blind vias connect the outer layer to an adjacent layer and stop there.
On the other hand, plated vias run through the PCB, connecting all layers.
Defining Bending Radius
The flexible parts of the PCB have limits when it comes to how much you can bend them to ensure that they don’t break.
Make the bend radius at least 0.05 inches from a plated via to be safe.
Furthermore, it should be 10 times the thickness of the material.
Ground Plane
Crosshatch the ground planes to reduce overall design weight and improve circuit flexibility.
Moreover, keep the opening size proportional to the requirements to retain the shielding.
How To Design Rigid-Flex PCB: Step-by-Step Design Process
Project Planning and Requirements
Before getting into the actual design process, you have to plan the project by determining the PCB’s requirements. The electrical parameters are quite important for this stage in the development process, so list down the following to form the foundation for the project.
- Maximum voltages and currents
- Capacitance restrictions
- Impedance properties
- Shielding
- Type and location of components (in both sections)
- Wire and schematic listing
Select Materials and Components
The data gathered from the project planning and requirements phase will help you pick the most suitable materials, specifically the core or substrates of the rigid and flexible boards.
This data will also help you select the most appropriate components and their values to make the circuit function as expected.
Schematic Design
A schematic design is the board’s overall logical and visual representation, and it should follow the standard schematic guidelines (including net labeling), as well as symbol standardization.
These should make the design well-structured and error-free.
It is also important to consider the board’s flexibility at this stage, which can be static or dynamic. The former bends only a few times during its operational lifespan, while dynamic boards should withstand multiple bending cycles.
Layer Stack-Up and Design Rules
The next step is to create the stackup, which is basically the layer-by-layer arrangement. This arrangement should take into account the bends and transitions, so some of the design rules include:
- Keeping vias in the stiff sections at least 20–50 miles away from the rigid-flex joint
- Using teardrops to route traces to these vias to create stronger connections
- Make vias in the flexible section be at least 10 mils in diameter with a 10-mil anchoring point to the polyimide coverlay
- Placing the flexible section of the board at the center of the stack-up design to prevent breakage
In a nutshell, remember to consider rigid-flex IPC design standards when setting up these design rules to ensure the PCB passes the checks later quickly to proceed to manufacturing.
PCB Layout Design
Layout designs detail the exact arrangement of components and traces on the PCB to show how the final PCB will look. So this design should factor in signal integrity, component placement, and impedance to ensure the PCB runs power and signals efficiently throughout the circuit.
Rigid-Flex PCB Design
Design Rigid-Flex Transitions
Design transitions between rigid and flexible areas carefully to avoid mechanical stress and ensure reliability. Consider factors like the tapered-down traces when joining these two sides.
Prototyping and Testing
Before finalizing the design for mass production, you have to develop a prototype for testing purposes to ensure the PCB works as required. You might have to develop a few prototypes after each design iteration to perfect the layout and stack-up.
Finalize Design Files
Finalize the rigid-flex PCB design based on the final working prototype, then send the design files to your manufacturer for mass production.
How To Control Quality While Designing and Manufacturing Rigid-Flex PCBs
For quality control, you should be guided by IPC standards, specifically IPC-2221, IPC-2223, IPC-FC-234, IPC-6013, IPC-A-600, IPC-A-610, and IPC-J-STD-001.
Once you complete your board and circuit designs, your contracted manufacturer should check them to ensure they meet the required electrical performance and IPC standards.
Your PCB manufacturer should also adhere to the relevant IPC standards when manufacturing the board and use various testing methods to check if everything is in order. These include:
- AOI
- Visual inspection
- Functional testing
- Electrical testing (flying probes, bed of nails, etc.)
- Solderability testing
- Contamination testing
- X-ray inspection
Applications of Rigid-Flex Circuit Boards
(Smart industrial power distribution switch gear)
During its earlier days, engineers used rigid-flex PCBs primarily in military and aerospace applications.
Over time, they are gaining popularity in consumer products across homes and business applications, whereby durability is a factor.
Here are some applications of the rigid-flex PCBs.
Industrial Application
Rigid-flex PCBs are better than rigid circuit boards in industrial applications that operate under immense stress or vibrations.
For instance, they are popular in power distribution systems and communication technologies.
Additionally, their compact size accommodates smaller, more effective sensors in irregular spaces.
Wearable Devices
The primary reason for using rigid-flex PCBs in medical and wearable devices is that they are lightweight and reliable.
Therefore they are popular circuit boards in wearable devices like smart clothes, fitness trackers, and smartwatches.
Doctors who monitor patients over long durations can’t use heavy monitoring devices in the medical field.
Device manufacturers opt for rigid-flex PCBs to reduce weight and allow doctors to monitor patients effectively.
Electronic Commerce
There’s a great demand for rigid-flex circuit boards in the E-commerce sectors like online retail businesses.
For instance, logistics companies use lightweight, powerful electronics to scan and track the movement of goods.
Modern-day trackers have rigid-flex PCBs to ensure longevity, especially if tracking vehicles subject them to stress.
Conclusion
The planning phase is critical to the success of any project, and this is what the design step does to rigid-flex PCB manufacturing.
If handled well while adhering to the relevant IPC standards and in partnership with a reputable manufacturer, everything will work as properly as required at the end of the partnership.
This is where FlexiPCB comes in. We collaborate with clients to provide the following services during rigid-flex PCB design.
- Concept development
- Material selection
- Design optimization
- Stack-up creation
- Signal and power integrity analysis
- Thermal analysis
- DFM and DFA analysis
- Mechanical analysis
So once your prototype is tested and the final design files are ready, you can be sure of getting reliable, high-quality rigid-flex PCBs for your project at reasonable prices.
Plus, we can provide quick-turn, low-volume, medium-volume, high-volume, and turnkey solutions so you can pick the most suitable option for your project and budget.
Contact us today for more information and to get started on our rigid-flex design and manufacturing partnership.
