Different Construction Types of Rigid-Flex PCBs

In the ever-evolving electronics landscape, rigid-flex PCBs are a game-changer in wearable devices and IoT sensors. By eliminating the need for bulky connectors and cables, rigid-flex PCBs allow designers to pack more functionality into smaller form factors. Additionally, the ability to withstand mechanical stress and vibration makes rigid-flex constructions well-suited for rugged environments, including automotive, aerospace, and industrial applications.
Based on the bonding technique, rigid-flex PCBs can be manufactured in the following 4 ways:

• Traditional rigid-flex board construction
• Short coverlay rigid-flex board construction
• Hybrid laminate rigid-flex board construction
• All rigid material rigid-flex board construction

Whereas designing flexible and rigid sections categorizes the rigid-flex PCB construction into

• Symmetrical
• Asymmetrical
• Odd-layer count
• Integrated ZIF tail
• Flex circuit with air gap construction
• Shielded flex layers

4 rigid-flex constructions based on the bonding technique

1. Traditional rigid-flex board construction

Traditional rigid-flex board construction integrates both rigid and flexible circuitry. This method allows for seamless connectivity and functionality across different sections of the board. The rigid parts of the board are typically made from FR-4, a common and durable PCB material. These sections provide structural support, stability, and a solid foundation for mounting components such as ICs, resistors, and capacitors.

Flexible parts use polyimide substrates, known for their excellent flexibility, high-temperature resistance, and durability. These sections allow the board to bend, fold, or twist, accommodating complex shapes and movements without breaking the electrical connections. The rigid and flexible sections are laminated together using specialized adhesives. These adhesives must maintain strong bonds while allowing flexibility in the designated areas. Advanced bonding techniques ensure that the layers are securely attached, preserving the integrity of the electrical connections and the mechanical strength of the board.

Traditional rigid-flex boards can be designed to fit into tight or oddly shaped spaces, making them ideal for compact and intricate devices. By combining rigid and flexible sections, these boards reduce the need for multiple connectors and interconnecting cables, saving space and simplifying the design.

Applications of traditional rigid-flex PCBs

• Aerospace: Aerospace applications benefit from the durability and reliability of rigid-flex boards, which can withstand extreme conditions and vibrations. The ability to fit into tight spaces and complex shapes is vital for aerospace systems.
• Automotive electronics: Automotive electronics often require components that can handle dynamic movement and vibrations. Rigid-flex boards provide the necessary flexibility and durability.
• Wearable technology: Medical devices, especially wearables, need flexible and durable PCBs that can conform to the human body and endure regular movement.

2. Short coverlay rigid-flex board construction

Short coverlay rigid-flex board construction involves the selective application of coverlay materials over specific areas of a flexible printed circuit board. In short coverlay constructions, the coverlay is not applied over the entire flex area but is instead strategically placed to cover only specific parts. The coverlay, also known as a cover coat or solder mask, is typically a polymer material that protects exposed copper traces, prevents short circuits, and enhances the board’s mechanical strength. It acts as a shield against environmental factors, mechanical damage, and electrical shorts by insulating the copper traces. The areas of the flex section that remain uncovered by the coverlay are designed to bend and flex, enabling the board to navigate tight spaces and dynamic environments.

For instance, in a multi-layer rigid-flex PCB, the top and bottom rigid layers may contain components and connectors. In contrast, the middle flexible layers are selectively covered with a short coverlay.

The construction method balances the need for flexibility and rigidity by allowing the board to flex in designated areas while maintaining structural integrity and electrical insulation in others. Rigid sections of the board are used for component mounting and structural support. These areas are typically reinforced to handle mechanical stress and house critical components

Applications of short coverlay rigid-flex boards

• Consumer electronics: Devices like smartwatches and fitness trackers benefit from the flexibility of short coverlay constructions, allowing them to conform to the user’s body while protecting vital circuits.
• Diagnostic equipment: Short coverlay constructions can be leveraged for optimal performance in portable and wearable medical devices that require both flexibility and durability.
• Automotive applications: Automotive applications, where space is limited and reliability is crucial, benefit from the combined flexibility and protection offered by short coverlay constructions.

3. Hybrid laminate rigid-flex board construction

Hybrid laminate rigid-flex board construction combines the strengths of rigid and flexible laminates to create versatile and durable PCBs. This construction uses different laminate materials to achieve a board that offers structural support in some areas and flexibility in others.

Rigid sections provide structural support and stability. These areas are ideal for mounting components and connectors, ensuring they remain securely attached. Flexible sections allow the board to bend or flex. This is particularly useful for fitting the PCB into complex geometries or tight spaces where traditional rigid boards are unsuitable.

Hybrid laminates consist of traditional rigid laminates (FR-4) and flexible laminates (polyimide). This combination leverages the strengths of both materials to create a single, cohesive PCB. These laminates are carefully bonded together to form a multi-layered board.

Designers can customize the placement of rigid and flexible sections according to the design requirements. This allows for precise control over the board’s mechanical properties and electrical performance. For example, a hybrid laminate PCB might have rigid outer layers for component mounting and a flexible inner layer for connectivity.

Advantages of hybrid laminate rigid-flex boards

• Customizable rigidity and flexibility: The ability to customize the rigidity and flexibility of the board allows for innovative and efficient designs. Designers can tailor the board to the application’s requirements, ensuring optimal performance and reliability.
• Complex board shapes: Hybrid laminate rigid-flex boards can be designed to fit into complex shapes and spaces, making them ideal for compact and intricate devices.
• Reliable electrical connections: Integrating rigid and flexible layers ensures consistent electrical connections. This is crucial for maintaining signal integrity and overall performance. Combining rigid and flexible sections into a single board minimizes the need for additional connectors and cables, reducing potential points of failure.

Applications of hybrid rigid-flex boards

• Aerospace: Aerospace applications benefit from hybrid laminate rigid-flex boards’ lightweight and compact nature. These boards can fit into tight spaces and handle the stresses of flight conditions. Reliable electrical connections and durability are crucial for aerospace systems, where failure is not an option.
• Medical devices: Medical devices often require compact and flexible PCBs that can be worn or carried by patients. Hybrid laminate boards are ideal for these applications due to their flexibility and reliability. Their ability to conform to custom shapes makes them suitable for a wide range of medical applications, from diagnostic tools to implantable devices.
• Wearable electronics: Wearable electronics, such as smartwatches and fitness trackers, require flexible PCBs to move with the user. Hybrid laminate boards provide the necessary flexibility without sacrificing performance. These boards allow for integrating multiple functions and components in a small form factor.

4. All rigid material rigid-flex board construction

All rigid material rigid-flex board construction is primarily made from rigid laminates, such as FR-4. These materials provide the necessary structural support and durability.
Specific areas of the rigid sections are thinned to allow for bending and flexibility. This is done with precision to ensure that the board can flex without compromising the integrity of the internal connections.
Advanced manufacturing techniques, such as controlled depth drilling and selective layer thinning, are used to achieve the desired flexibility in certain areas while maintaining the overall rigidity of the board. This approach provides the robustness of a rigid PCB while incorporating flexibility where needed.
A flexible coverlay, typically made from polyimide, is applied over the thinned areas. This coverlay protects the circuitry in the flexible sections and prevents damage to the copper traces during bending. The coverlay ensures the flexible areas remain insulated and protected, allowing the board to bend and flex as required.

Applications of all rigid material rigid-flex PCBs

• Industrial equipment: The board’s ability to bend and flex allows it to fit into custom enclosures and spaces within industrial machinery.
• Automotive electronics: Automotive electronics often require complex PCB designs that fit into limited spaces while providing robust performance. The rigid-flex construction meets these requirements. The flexible sections help absorb vibrations, reducing the risk of damage to the electronic components in a vehicle.
• Advanced consumer electronics: Space is at a premium in devices like smartphones, tablets, and wearables. Rigid-flex PCBs allow for compact designs that can bend and flex as needed.

6 types of rigid-flex PCB designs

1. Symmetrical rigid-flex PCB construction

Standard rigid-flex PCB constructions feature a symmetrical layout, which is crucial for even impedance control and structural balance. In these designs, the flexible layers are centrally located within the stack-up, ensuring an even distribution of layers across both the rigid and flexible sections. This symmetry helps maintain the PCB’s mechanical stability and electrical performance.

Standard rigid-flex constructions can have 4 to 16 layers or more, depending on the application’s complexity and requirements. Regardless of the total number of layers, the flexible layers typically remain centered.

For example, in a six-layer rigid-flex PCB, there are three rigid layers on the top and three on the bottom, with the flex layers sandwiched in the center. The flexible layers are positioned at the board’s core, providing a balanced structure that aids in consistent impedance control.

Example configuration

• Top rigid layers: layers 1, 2, 3
• Flexible layers: layers 4, 5
• Bottom rigid layers: layers 6, 7, 8

2. Asymmetrical rigid-flex PCB construction

Asymmetrical rigid-flex PCB constructions are used to meet more complex impedance requirements and accommodate varying dielectric thicknesses. In these designs, the flexible layers are not centrally located but are positioned away from the center, either towards the top or bottom of the stack-up. This shift can help meet unique impedance and signal integrity requirements.

Asymmetrical designs allow for a custom stack-up tailored to specific performance needs, such as varying dielectric thicknesses to manage impedance. By adjusting the position of the flex layers, designers can fine-tune the impedance characteristics of the board. This is particularly useful in high-frequency applications where precise control is necessary.

Asymmetrical constructions may be more prone to warping and twisting during assembly. Additional fixtures and careful handling are required to maintain the board’s integrity. Despite the complexity, asymmetrical designs remain manufacturable and reliable with proper techniques and quality control measures.

Example configuration

• Top rigid layers: layers 1, 2, 3, 4
• Flexible layers: layers 5, 6
• Bottom rigid layers: layers 7, 8

3. Odd layer count rigid-flex PCB construction

Odd-layer count constructions in rigid-flex PCBs, though less common than even-layer counts, provide distinct advantages, including enhanced flexibility and cost efficiency. These designs are particularly beneficial for applications requiring precise impedance control and effective RF/EMI shielding. By using an odd number of layers, designers can achieve optimal performance while managing the board’s thickness and flexibility.

Example configuration
A 7-layer board might include 4 rigid layers and 3 flexible layers. The flexible layers can be positioned to provide two-sided shielding in the flex areas. The 3 flexible layers can be used for signal routing and shielding, with one layer acting as a ground plane between two signal layers.

Advantages of odd layer count constructions

• Enhanced flexibility: Using an odd number of layers can result in a more flexible PCB. This is especially useful in applications where the board needs to bend or twist without compromising its structural integrity.
• RF/EMI shielding: In designs with critical RF/EMI considerations, using an odd number of flexible layers allows for two-sided shielding. For instance, in a seven-layer board, three flexible layers can be used to create a shielded sandwich structure, enhancing impedance control and reducing electromagnetic interference.
• Impedance matching: Precise control over the impedance of signal paths is crucial for high-frequency applications. Odd layer constructions can provide the necessary layer stack-up to achieve this, ensuring signal integrity.

4. Integrated ZIF tail construction

Integrated ZIF (zero insertion force) tail constructions represent an innovative approach in rigid-flex PCB design, eliminating the need for separate ZIF connectors. This method offers several advantages, particularly regarding space savings and improved reliability, making it ideal for high-density or thin designs where space and height are critical considerations.

The ZIF tail is designed as an integral part of the rigid-flex board. The integrated ZIF tail connects directly to a corresponding ZIF socket on the device. The zero insertion force design means the connection can be made without applying significant force, reducing wear and tear on both the PCB and the connector.
Integrating the ZIF tail directly into the rigid-flex board eliminates the need for separate ZIF connectors. This significantly reduces the overall space required on the PCB, allowing for more compact and streamlined designs.
Removing additional interconnect points decreases the potential for connection failures. Fewer components mean fewer solder joints, which increases reliability and reduces mechanical or electrical failure risk.

Applications in high-density designs

In applications where PCBs need to accommodate many components in a limited space, integrated ZIF tail constructions are particularly beneficial. They allow designers to maximize the available area for other critical components.
For devices where the overall thickness is a key constraint, such as in portable electronics and wearables, integrated ZIF tails help maintain a slim profile without sacrificing connectivity or performance.

Example configuration

Imagine a rigid-flex PCB with 3 layers. In this example, layer 2 includes an integrated ZIF tail that extends from the rigid section of the board. This tail fits directly into a ZIF socket on the device, providing a secure and reliable connection without the need for an additional connector.

5. Flex circuit with loose leaf/air gap construction

Air-gap construction in flex circuits is a technique for separating flex layers into independent pairs. This separation enhances the circuit’s flexibility and reliability. It is especially advantageous in designs with more than four flex layers.

Flex layers are divided into pairs, each separated from the next by an air gap. This separation allows each pair to flex independently, reducing mechanical stress and improving overall flexibility. The air gaps between the pairs provide space that prevents layers from adhering to each other, which helps to maintain their independent movement and reduce potential points of failure.

Ensuring that no flexible adhesives are present within the rigid areas is crucial. This maintains the structural integrity of the circuit’s rigid parts, preventing unwanted bending or deformation.

Adherence to IPC 2223C guidelines ensures that the flex circuit meets industry standards for quality and reliability. These guidelines dictate the appropriate use of materials and construction methods to optimize performance and durability.

The construction might involve three pairs of layers in a flex circuit with six layers. An air gap would separate each pair, allowing for enhanced flexibility.

Sierra Circuits designed an 8-layer rigid-flex circuit board with 6 flex layers. The design required an overall thickness ranging from 10 to 20 mil with a 2-inch bend radius. Additionally, it must accommodate 253 controlled impedance traces between two connectors.

We transformed the initial flex design into a loose-leaf rigid-flex stack-up to meet these specifications. This innovative configuration utilized a two-leaf structure with an air gap between the leaves, enhancing flexibility and reducing thickness. Each leaf comprised 3 layers, each with a thickness of 10.9 mil. To prevent signal overlap and maintain signal integrity, we routed 253 impedance trace signals across two leaf structures with an air gap between them.

Advantages of flex circuit with air gap construction

• Enhanced flexibility: By allowing each pair of layers to move independently, the circuit can bend and flex more easily without causing stress on the materials. This is particularly beneficial in applications where the circuit needs to navigate tight spaces or undergo repeated bending.
• Improved reliability: Layer separation reduces the likelihood of mechanical failures. Each layer can flex without transferring stress to adjacent layers, which helps prevent cracks and other damage.
• Design versatility: This construction method allows for more complex designs that meet specific application requirements.
  • Consumer electronics: Air gap construction enhances the flexibility and reliability of devices such as smartphones, tablets, and wearable technology.
  • Automotive application: Air gap construction ensures longevity and consistent performance in vehicles where circuits must endure vibration and movement.
  • Aerospace industry: The lightweight and flexible nature of air gap-constructed flex circuits is ideal for aerospace applications, where space and weight are critical considerations.
  • Medical devices: Medical equipment that requires flexibility and durability, such as diagnostic tools and portable monitors, can greatly benefit from this construction technique.

6. Shielded flex layers construction

Shielded flex layers are commonly used in electronic devices with limited space and flexibility. Their lightweight and flexible nature makes them suitable for aerospace applications. Many medical devices require flexibility and reliable shielding, making this construction method ideal.

The key components involved in this construction are as follows:

1. Specialized films: Tatsuta and APlus films are designed specifically for shielding applications. They possess properties that effectively block or attenuate electromagnetic and radio frequency interference. The films are typically composed of materials that provide high conductivity and excellent shielding effectiveness while maintaining flexibility.
2. Conductive adhesives: Conductive adhesives are used to bond the shielding films to the flex areas. These adhesives ensure a strong and reliable electrical connection to the exposed ground circuits on the flex layers. The adhesive must be applied evenly and precisely to ensure maximum conductivity and shielding performance.
3. Lamination process: Laminating the shielding films onto the flex areas involves carefully applying heat and pressure. This ensures that the films are securely bonded to the flex layers. Proper alignment of the films with the ground circuits is crucial. Misalignment can lead to gaps in shielding and reduce the overall effectiveness.

Key takeaways:

• Traditional rigid-flex boards seamlessly integrate rigid and flexible sections, ideal for applications that balance stability and adaptability.
• Short coverlay rigid-flex boards provide selective insulation, enabling flexibility where needed while maintaining rigidity elsewhere.
• Hybrid laminate rigid-flex boards offer unparalleled customization, allowing designers to tailor the board’s rigidity and flexibility according to specific requirements.
• All rigid material rigid-flex boards combine durability and flexibility, making them suitable for industrial and automotive electronics.
• Symmetrical layout ensures even impedance control and structural balance.
• Flexible layers are positioned away from the center (top or bottom) in asymmetrical rigid-flex PCBs.
• Odd layer count rigid-flex PCB construction enhances flexibility and better RF/EMI shielding by using odd-numbered flexible layers.
• Integrated ZIF tail construction eliminates the need for separate ZIF connectors, saving space and improving reliability.
• Flex circuit with air gap construction separates flex layers into independent pairs with air gaps for enhanced flexibility and reliability.
• Shielded flex layers construction uses specialized films (e.g., Tatsuta or APlus) for effective EMI/RF shielding.