What is  rigid-flex PCB?

A Rigid-flex PCB is a combined circuit board. It has both rigid and flexible boards in the same board or unit. This kind of circuit board has one solid and rigid part, while the other is flexible and can easily bend.

The manufacturer assembles the components in a rigid part of the board. The flexible section connects different parts of devices with less cables and connectors.

The construction of a rigid-flex board makes it a unique and innovative design circuit board. It also helps to handle more complex designs easily. The board easily fits into a compact space, offering an advantage over traditional boards.

So, we now know how the combination of rigid and flexible parts makes the complex yet most effective circuit board. If we look at the types of PCB in the electronics domain, we can find three main types of circuit boards: rigid, flex, and rigid-flex. Each of these technologies and types has unique purposes and properties.

Rigid PCBs are traditional circuit boards made from non-flexible materials like FR4.

On the other hand, flex PCBs have a flexible base. Designers use materials that can easily bend, fold, and twist to a limit for the flexible part. This includes the materials like polyamide.

A rigid-flex circuit board combines the advantages of both rigid and flex design into a single board. It has compact, lightweight, and mechanically robust electronics. These kinds of boards are designed considering the space and flexibility of the system.

Rigid-Flex PCB Stack-Up

Rigid-Flex PCB combines rigid and flexible substrates in one board. So, it offers both strength and flexibility. The most crucial part of this board is the layer stack-up, which involves various layers sandwiching the circuit board. 

Rigid sections provide a stable base for mounting the PCB components on it. The flexible section allows the board to bend and fold. For signal transmission, the board has a conductive copper layer insulated from dielectric materials.

The adhesives, like epoxy, help bond the layers without sacrificing flexibility. Designers can integrate rigid sections alongside flexible ones, making these PCBs ideal for compact electronics and wearables that require rigidity and flexibility.

Manufacturing Process of Rigid-Flex PCB

Before the manufacturing process, it’s a basic rule to have a design of the PCB. The design must also include an electronic component layout. Then, the manufacturing process is carried out as follows:

First, cleaning the base material is crucial. It helps to remove dirt or dust. It ensures smooth processing for the next stages. Then, a pattern for the circuit is made using either photo imaging or screen printing.

Then, the etching process starts, where the chemical process removes the unwanted copper. This leaves only the desired circuit pattern.

Next, holes are drilled. For this process, mechanical or laser drills, also known as vias, are used. These PCB vias allow communication between different layers of the board. The holes are then coated with copper plating so that current can flow between the rigid and flexible layers. The flexible sections have a protective overlay applied over the copper traces so that they can bend without rupture; it is laminated under heat and pressure.

With that, sometimes a stiffener- a much harder material- is added to certain areas to give them extra support. Next, using special tools, such as hydraulic punches or knives, the board is cut carefully to separate it from the larger production panel.

Finally, there's electrical testing to determine if the board is functioning as intended. Electrical testing also includes verifying the board's connectors and whether the flex parts are bendable without necessarily impairing their performance. After these tests have been passed by the board, it is then ready to be assembled into devices such as smartphones, medical devices, or wearable technology.

Advantages of Rigid-Flex PCB

Space Saving

Rigid-flex PCBs eliminate the need for various cables, connectors, and solder joints, simplifying the overall design and helping save space on the circuit board.

Design Flexibility

Rigid-flex PCBs provide greater design flexibility. They allow the creation of unique layouts and help designers create boards that fit into compact spaces. The flexible section can twist, bend, and fold to adapt to the mechanical design.

Improve Signal Integrity

In PCB, the connectors and cables are one of the cases of signal loss. There is no such issue with rigid-flex PCBs as they eliminate cables and connectors.

Reliable and durable

The flexible section in a rigid-flex PCB can absorb mechanical stress, such as vibrations, thermal expansion, and bending. Thus, it is well-suited for applications that require both durability and flexibility.

Rigid-Flex PCB Design and Solutions

Design Challenges in Rigid-Flex PCBs

So, we have already checked the benefits of rigid-flex PCB, including the manufacturing process. Now let’s see some of the common challenges, especially in design areas:

Advanced and Complex Designs

The design of rigid-flex PCBs is more complex and standard than traditional rigid PCB designs. Engineers and designers should know both electrical and mechanical requirements, including the bending radius, materials to use, and more.

Bending and Flexing Stress

Manufacturers design the flex section to move and withstand repeated bending without cracking. Overstressing these areas can cause the flexible circuit part to fail.

Thus, the designer should consider the proper bend radius and maximum number of bends.

Thermal Management

In rigid-flex PCBs, various materials could expand and contract at multiple rates when exposed to heat, creating a reliability problem.

Thus, manufacturers should use the materials for both sections matching thermal expansion rates.

Signal Integrity

Signal transmission from rigid to flexible design could cause signal loss. In the case of a flexible design section, signal integrity could be challenging. The flexibility of the substrate can affect the signal quality.

Thus, the designer should maintain a consistent ground plane between the two sections.

Placement of Connector

The next challenge in rigid-flex PCBs is placing the connector between two design parts. Poor connector placement can lead to reliability issues.

Thus, the designer should carefully plan how the flexible and rigid sections interact, mainly in a place where the connectors are involved,

Addressing Rigid-Flex Design and Fabrication Issues

Techniques for Overcoming Fabrication and Design Issues

Some significant issues with the rigid-flex PCB design are material stress, placement of vias in the wrong place, and poor bendability. Carefully following the designer's specific techniques can avoid such errors. First, keeping an appropriate bend radius in flexible areas protects them from possible damage when bending.

Another critical thing is to avoid placing the via and component in flexible zones because it would weaken the structure. Besides, proper materials are essential. Namely, durable and flexible polyimide would be used for the flexible areas, while FR4 would be used for the rigid sections, which ensures stability.

Another approach is staggering the layers in multilayer constructions to reduce the stress concentration at bends. Proper temperature and pressure controls during lamination result in good bonds between layers with no warping or delamination.

Adherence to particular design rules and fabricating rigid-flex within stringent guidelines allows the manufacturers to ensure the boards will perform well in harsh applications.

Quick turn availability for rapid prototyping.

With quick-turn manufacturing, rigid-flex PCBs can be made in a very short time for prototyping. In fact, if it wasn't for the case of developers who need to test and refine designs before full-scale production, there's no other way around it.

Quick-turn services smooth the process through predefined design standards and heavy reductions in lead times. The materials and methods to be used are thus pre-set to be in hand so that design and production may proceed as fast as possible without waiting on special orders.

All this allows the delivery of rigid-flex prototypes within days, instead of weeks, and hence this process enables engineers to conduct performance tests as quickly as possible. The quick-turn prototyping identifies and detects design flaws upfront. Hence, it saves time with resultant costs in the final phase of production. Fast feedback is critical in industries, such as consumer electronics, that require fast design changes and innovations, where the timeline for completion of a project may be slim. In PCBasic, we consider all of these factors to have a quick turnaround without compromising the quality of a board.

Application of Rigid-Flex PCB

These rigid-flex PCBs have found their increasing application in industries that require flexibility, space efficiency, and reliability. Following are the various areas of applications and product examples that integrate the benefits of the rigid-flex technology. These are described in some detail below:

Aerospace and Defense

Rigid-flex PCBs are an important part of lightweight electronics being designed and developed for use in the aerospace and defense sector.

Examples include instrumentation in aircraft, communication systems, and military-grade equipment.

Medical Devices

These rigid-flex PCBs come in very small sizes. Hence, they find their applications in life-saving machines, too.

Examples: pacemakers, hearing aids, implants, and related medical diagnosis equipment such as ultrasound.

Consumer Electronics

The need for thin and compact gadgets in consumer electronics finds a good combination with rigid-flex PCBs.

Examples: smartphones, smartwatches, laptops, and foldable devices.

Automotive Industry

The electronics involved in automotive must be high-temperature and vibration resistant and must fit into smaller spaces.

Examples of such include Advanced Driver Assistance Systems, dashboard controls, and engine sensors.

Industrial Machinery

The mechanical stress is high in industrial machines, and any circuitry required for them needs to be reliable and space-efficient.

Examples: robotic arms, control systems, and other systems with high-speed data transmission.

High Vibration Environments

Rigid-flex PCBs are suited for installations with high vibration and mechanical stresses.

Examples include those systems that are subjected to a high frequency of movement or higher mechanical stress: drones, satellites, and machinery with continuous mechanical loads.

When to Choose Rigid-Flex Circuit Boards

Some of the instances where rigid-flex PCBs find their applications include:

Space Restrictions

In wearing devices, for example, rigid-flex PCBs become handy when compact designs need to be realized without connectors and cables.

Need for Flexibility

Examples of such applications would involve medical implants and foldable smartphones, which require devices to be in motion or bending at any moment. In these kinds of applications where flexibility can be the major requirement, the capability for bending without breaking and compromising performance is highly important.

Weight Reduction

Drones, satellites, and aerospace applications also want to see weight reductions. Rigid-flex PCBs are lightweight compared to the traditional arrangement of separate boards and connectors, so they're suitable for use in portable or airborne devices.

Reliability under Extreme Conditions

Those applications sensitive to high vibration or temperature, such as automotive electronics and different industrial machinery, are more resistant to the construction of rigid-flex PCBs. In comparison with a normal PCB, rigid-flex PCBs can resist mechanical stress. Hence, they enhance reliability for a longer period of time.

Complex Three-Dimensional Designs

These rigid-flex PCBs connect various components across different planes to facilitate the design process when a device has a complex multi-dimensional layout.

Elimination of Connector and Cable

The rigid-flex PCBs eliminate many connectors and wires. In those applications where space, weight, and reliability become crucial, these PCBs become perfect.

Conclusion

Rigid-flex PCBs combine space efficiency with design flexibility, improving reliability in very compact applications. They reduce connectors and cables, reduce weight, and are resistant to mechanical stress. Some of the drawbacks associated with rigid-flex circuits are design challenges, higher production costs, material selection, and mechanical stresses.

The current trends in rigid-flex PCBs are further miniaturization, a better class of materials used for more durability, and integration of the rigid-flex technology into advanced sectors like IoT, robotics, and wearables. Because of this, rigid-flex PCBs will keep evolving into even more innovative solutions due to the ever-increasing demands for flexible and reliable electronic circuits.