Flex and rigid-flex printed circuit boards (PCBs) are reliable solutions for electronic devices and components, but it’s important to understand how they differ. Assessing the unique designs, features, and advantages of flex and rigid-flex PCBs helps you select the best option for each project.
What Are Flex PCBs?
Flex PCBs are built on flexible plastic substrates, which allow them to twist and bend. Because of this structure, they can adapt to various shapes and tight spaces within devices.
The following types of flex PCBs are available for various electronic components:
- Static flex: A static flex PCB is also known as a flex-to-install PCB. It can bend once during assembly, but it cannot move again following its installation.
- Dynamic flex: Dynamic flex PCBs can withstand millions of flex cycles following assembly. They are best for moving parts such as laptop hinges and printer heads.
What Are Rigid-Flex PCBs?
Rigid-flex PCBs are integrated circuit boards that combine flexible and rigid circuit technologies within a unit. A rigid-flex PCB features rigid portions for mounting components and connectors, which provide support and stability. These rigid sections are interconnected by flexible points, allowing the entire unit to fold or bend in specific areas. This design offers a unique combination of dynamic flexibility and structural integrity for complex electronic packaging.
Flex vs. Rigid-Flex PCB
Flex and rigid-flex PCBs differ in the following categories:
Structure
Structure is a fundamental difference between flex and rigid-flex PCBs. Flex PCBs consist entirely of flexible dielectric materials, such as polyimide. This enables the entire circuit to conform to various shapes and configurations.
Rigid-flex PCBs have a more sophisticated hybrid structure. Manufacturers fabricate them by laminating multiple flexible circuit layers with rigid FR4 board sections. This produces a single, unified board that provides the stability of rigid sections and the dynamic flexibility of bendable sections. A rigid-flex PCB’s flexible sections enable it to route signals through three-dimensional spaces.
Features and Benefits
A flexible PCB is thin, lightweight and highly compact due to its polyimide construction. A rigid-flex PCB provides enhanced strength and durability, which often reduces the need for external connectors. While flex PCBs are best for small spaces and lightweight applications, rigid-flex PCBs are ideal for applications requiring stable component platforms alongside flexible interconnections. A rigid-flex PCB can reduce the need for external connectors and improve overall system reliability.
Component Mounting and Stability
Components typically mount directly onto a rigid-flex PCB’s designated rigid sections. These sections are specifically designed to provide stability and support heavier or more sensitive components.
While flex PCBs can also support small and lightweight components, they often require the use of external stiffeners such as stainless steel or FR4. The stiffeners are installed under components to prevent stress on the flexible substrates. While they are functional, stiffeners may not offer the same integrated durability and mechanical stability as dedicated rigid sections found in rigid-flex designs.
Applications
Flexible PCBs are ideal in applications that prioritize minimal weight, compact size, and dynamic flexing. They are often necessary in devices such as wearable technology, compact camera modules, and sensors that require continuous movement.
Rigid-flex PCBs excel in more demanding environments. They typically perform best in high-vibration environments, complex electronic assemblies, or high-density applications that require robust component mounting and reliable interconnections.
Reliability
Rigid-flex PCBs offer the greatest reliability in harsh operating environments involving significant vibration, high heat, or sudden shock. Their integrated rigid sections provide structural integrity, and their flexible components can absorb mechanical stress and reduce points of failure better than traditional circuitry.
Pure flex PCBs offer durability in dynamic bending, but they may reduce long-term reliability under constant or high-load mechanical conditions. Components are especially prone to failure if they lack adequate support or if the PCB’s flexing exceeds design tolerances.
Assembly
Since rigid-flex PCB assemblies eliminate the need for connectors between board sections, they simplify assembly. A rigid-flex PCB’s integrated design reduces the number of individual parts that need to be procured and connected. The lack of connectors helps save valuable space and improve signal integrity due to shorter, more direct electrical paths. They also result in fewer potential points of impedance mismatch or signal degradation.
Flex and Rigid-Flex PCB Manufacturing Processes
The manufacturing processes for flex and rigid-flex PCBs differ in the following categories:
- Materials and substrate: Flex PCBs consist of rolled copper and polyimide. Rigid-flex PCBs integrate these materials with FR4 substrate, which requires a specialized low-flow pre-impregnated dielectric material to avoid unwanted adhesive flow during lamination.
- Lamination: Unlike flex PCBs, rigid-flex PCBs require a multistage lamination process to bond rigid outer layers to flexible inner layers.
- Drilling and plating: Flex and rigid-flex PCBs require precise drilling and copper plating. However, rigid-flex PCBs require thicker plating for greater durability and interconnection under stress.
- Coverlayer application: Manufacturers apply a protective polyimide coverlay to flex PCBs, while rigid-flex PCBs only require this layer on flexible sections.
Flex and Rigid-Flex PCB Cost Factors
Flex circuits are typically more cost-effective for applications that don’t require high-strength rigid sections. Rigid-flex PCBs tend to have higher manufacturing costs because they require more precise and specialized manufacturing.
Materials and steps involved in PCB manufacturing can significantly affect expenses. Understanding the most impactful cost drivers for flex and rigid-flex PCBs enables you to adjust your design for a more economical board that delivers the performance you need.
Low-Influence Cost Factors
Some of the factors that only have a minor impact on your PCB’s cost include:
- Routing: As with rigid boards, more complex routing will increase manufacturing costs, as it requires greater precision.
- Panel thickness: Thinner, more common panel sizes generally save you money.
- Via plugging: Via plugging requires an extra step of completely filling vias with resin or solder mask. If you can limit instances of via plugging, you can save on costs.
- Strain relief: Similarly, reducing the need for strain relief and its associated materials can help you save money.
- Adhesive: PCBs can use adhesive or adhesiveless cores to bond the stiffener to the board. The most affordable adhesive material will vary depending on your stiffener, but adhesiveless materials are the more expensive option overall.
Moderate-Influence Cost Factors
These next factors more significantly influence PCB costs:
- Aspect ratio: A larger aspect ratio between the thickness of the PCB and the diameter of a drilled hole makes plating more difficult and time-consuming, which can increase the PCB’s cost. You’ll need to either keep the drilled holes larger or reduce the PCB’s size to achieve a good plating ratio.
- Trace and space size: Tight traces/spaces are more difficult to work with and also add to the complexity and price of a PCB.
- Stiffeners: Stiffeners or rigidizers will increase costs due to the additional material requirements. Its thickness will also affect the price. If you need a stiffener, you can reduce costs by sticking to the common and versatile option, FR4. Polyimide, aluminum or stainless steel typically cost more.
- Button or selective plating: You’ll increase the price of a PCB if you use any kind of button or selective plating due to its added complexity.
- Controlled impedance: If you opt for controlled impedance, the manufacturer uses additional checks and coupons that require more labor and materials.
- Annular rings: Your annular rings should be large enough to create a reliable connection with the via, but if they’re too big, you’ll likely pay more for them.
High-Influence Cost Factors
Finally, the factors that play the biggest role in the price of your PCB include:
- Advanced technologies and custom tooling: If you opt for more advanced technologies or specialized tooling in your PCB, you’ll face higher costs.
- Buried vias: Buried vias require more drilling, plating and lamination, driving up the cost. They might also call for more expensive materials.
- A high number of layers: Layer count is a significant cost driver for flex PCBs. It adds to the complexity of lamination, requiring more time, materials and expertise.
- Panel utilization: Perhaps the most important cost driver is panel utilization. Since manufacturers typically charge by the panel, packing in more components per panel can help you reduce costs.
- Selective plating: The process for selective plating is time-intensive and results in material waste. It will generally make your PCB more expensive.
- Dual surface finish: Multiple surface finishes again add complexity and cost to the process.
How to Choose Between Flex and Rigid-Flex PCBs
To choose between flex and rigid-flex PCBs, you must consider the following factors:
- Weight and space constraints: Flex PCBs are ideal for three-dimensional, extremely tight, or abnormally shaped spaces. Rigid-flex PCBs are best when the goal is to save space and reduce weight by integrating multiple circuit boards and eliminating bulky external wiring and connectors.
- Mechanical movement and reliability: Opt for flex PCBs if a design requires constant or dynamic bending. Select rigid-flex if the board is bent once at installation and remains fixed afterward.
- Materials: A flex PCB is the optimal choice if your design benefits from polyimide’s inherent flexibility. A rigid-flex PCB is the better option if you need to integrate diverse materials into your application.
- Manufacturing: For a flex PCB, you must be prepared for more delicate handling during assembly. Rigid-flex PCBs require early and close collaboration with manufacturers to define critical parameters.
- Serviceability and rework: Rigid-flex PCBs are mounted on stable rigid sections, allowing them to withstand maintenance and component replacements. Flex PCBs are better for maintenance-free solutions or applications where the cost and complexity of reworks are prohibitive, regardless of PCB type.
Best Applications for Flex and Rigid-Flex PCBs
Learn more about the uses of flex and rigid-flex PCBs below.
Flex PCBs are ideal for the following types of applications because they are compact, lightweight, and offer dynamic bending:
- Internet of Things (IoT) devices and wearable technology: Flex PCBs can bend and conform to the irregular shapes of wearable devices such as fitness trackers. They are also perfect for fitting the small spaces and innovative designs of IoT devices.
- Medical implants and devices: Medical devices such as pacemakers, surgical tools, and hearing aids require flex PCBs to fit constrained spaces and deliver high-density interconnect capabilities in lightweight designs.
- Components with dynamic movement and bending: Components such as smartphone camera modules and inkjet printer print heads operate best with flex PCBs because they can withstand ongoing flex cycles without breaking. This provides reliable electrical connections where fatigue would otherwise cause traditional wiring or rigid boards to fail.
- Flexible displays and optoelectronics: Flexible displays in automotive dashboards and smartphones rely on the bendability of flex PCBs because they enable designs that fold, curve, or roll up.
The following applications operate best with rigid-flex PCBs:
- Avionics and aerospace systems: Rigid-flex PCBs provide robust mounting for heavy avionics and aerospace components while providing flexible sections for complex routing and stress relief. They also enhance durability, protecting components from vibration, expansion, contraction, and shocks.
- High-end consumer electronics: Rigid-flex PCBs are essential for modern consumer electronics such as laptops and smartphones. They offer stable mounting on components and flexible connections in complex geometries while enhancing shock resistance.
- Industrial equipment: Complex industrial equipment often requires PCBs that can withstand harsh operating environments. Flex PCBs deliver the necessary stability and dynamic connections for industrial applications.
- Diagnostic and medical imaging equipment: Medical imaging and diagnostic machines, such as MRI and ultrasound equipment, demand high reliability in irregularly shaped housing. Rigid-flex PCBs can maintain stability and navigate intricate internal layouts in these applications.
Why Trust Millennium Circuits Limited for Flex and Rigid-Flex PCBs?
With over 15 years of experience, Millennium Circuits Limited (MCL) specializes in navigating complex flex and rigid-flex PCB builds. We build PCBs to match your precise design needs and deliver high-performance results.
Our skilled engineers thoroughly examine your designs and address critical details before production, saving you time and money. We are dedicated to providing reliable PCBs. Our ISO 9001 certification is a testament to our rigorous quality management systems, and we strictly adhere to IPC Class 2 and Class 3 standards. This means that our boards are suitable for high-reliability products where consistent performance is essential.
At MCL, we become an invaluable extension of your engineering team. We are dedicated to your project’s success, and our experts understand the dynamic nature of product design and development. You can depend on our rapid prototyping capabilities to empower your iterative design and testing processes. Trust the MCL team to provide maximum flexibility, responsive support, and high-quality PCBs for your applications.
Learn More About Flex and Rigid PCBs for Your Next Project
Flex and rigid PCBs offer various advantages, and choosing the right option for your technology is essential. MCL develops flex and rigid-flex PCBs for various industries, helping you optimize design and cost to meet your precise needs.
Contact us to request a quote and discuss your specific project requirements.