A rigid flex PCB (printed circuit board), also called a rigid flex circuit board, is a hybrid circuit used in high-reliability, high-density applications. They offer several advantages over traditional hardboards and flex boards, but their higher cost often raises questions about when and where they should be used. In this blog, we’ll provide an overview of rigid flex circuit boards, their applications, and the advantages of rigid flex PCB use.
Understanding Hybrid Circuit Boards
First things first: What is a rigid flex PCB, exactly? As we briefly touched on above, these circuit boards are a hybrid construction that combines elements of both rigid and flexible circuits into a single board. It is constructed of flexible sections interconnected with rigid sections, providing an advantageous combination of flexibility and structural integrity. These boards are used in applications that require the circuitry to bend or conform to fit into a specific form factor while maintaining the necessary stability and reliability.
Rigid flex PCBs are different from flexible circuit boards with stiffeners, and the main differences lie in their construction and performance characteristics. In terms of construction, these hybrid PCBs integrate both rigid and flexible components on a single board, while flex boards with stiffeners are essentially flexible circuits reinforced with additional stiffeners to provide rigidity in specific areas. Because of these differences, rigid flex PCBs provide the electronics designer with much higher circuit routing density in a package that is more durable, more reliable and yet still flexible. A downside to these hybrid PCBs is their higher manufacturing cost — flex boards with stiffeners, being a simpler construction, tend to be more cost-effective.
Other significant construction characteristics of hybrid PCBs are the plated through holes (PTH) and mounting components included in their design. PTH provide electrical interconnections between the flexible and rigid sections of the board, allowing for seamless integration of circuitry and components throughout the entire assembly. Then, by leveraging a combination of vias, proper component placement, and design considerations for the flexible sections, the PCB can incorporate mounting components on both sides of the board. This allows for increased component density, improved functionality, and optimized use of space in a compact and flexible electronic assembly.
Rigid Flex PCB Applications
Generally, rigid flex PCBs applications are used across various industries for which a combination of flexibility, space efficiency, and high reliability is required. Rigid flex PCBs are often seen in aerospace and defense applications, like aircraft instrumentation and military equipment, as well as in the medical industry, where they are utilized in devices like implants and diagnostic equipment. Automotive electronics, industrial machinery and many other applications that demand an adaptable circuitry solution will also use these hybrid PCBs to great effect.
On a more granular level, rigid flex PCB applications are ideal for situations that present challenges to the circuitry during installation or operation. Flex-to-install applications, in which the circuit is designed to be flexed or bent during the installation process, call for this type of PCB due to the application’s space constraints and/or the need for conformability to irregular shapes. In dynamic flex applications, the circuit will be continuously flexed during operation, so they are specifically designed to withstand and accommodate the ongoing mechanical stress without compromising their electrical performance or reliability. PCBs that follow the manufacturer’s recommended minimum bend radius offer hundreds of thousands of flex cycles without failure.
Applications in high-vibration environments present their own challenges that hybrid PCBs overcome. Vibration generates mechanical stress that can impact circuits, causing them to flex and bend repeatedly during operation. This dynamic flexing can raise various concerns, including material fatigue, interconnect reliability issues, and component integrity failures.
To mitigate these risks, PCBs designed for high-vibration environments are subjected to specific design and manufacturing considerations, like reinforcement to enhance their rigidity and mechanical stability or selecting a material with high flexural strength for their construction.
Finally, this type of PCB is also commonly used in applications that handle high currents and generate heat, like high-speed data communication, industrial equipment, or power electronics. These PCBs generally have a higher current carrying capacity than rigid boards with flex circuit connections, which makes them a compelling choice for these applications — but that higher capacity comes at a cost.
In a phenomenon known as thermal expansion, the heat generated by a high current passing through a conductive trace or copper plane on a PCB causes the circuit material to expand and contract repeatedly. The flexible sections may undergo stretching or elongation, while the rigid sections experience minimal expansion; when the current stops or decreases, the PCB cools down and contracts. This presents several potential problems for the integrity of the circuit board, including the risks of mechanical stress and fatigue, as well as the risk of electrical performance issues.
But as with rigid flex PCBs designed for high-vibration applications, circuit board manufacturers can implement certain design and manufacturing considerations to help mitigate these risks. Choosing materials with good thermal stability and low coefficient of thermal expansion (CTE), implementing thermal management techniques like heat sinks and thermal vias, and optimizing the trace widths, thickness, and copper plane configurations can all help ensure the reliability and long-term performance of the PCB in high-heat environments.
Advantages of the Rigid Flex Design
Rigid flex PCB applications offer a wide range of advantages, especially in environments that demand reliable, durable solutions capable of bending to fit complex shapes. These applications benefit from the unique combination of flexibility and strength, making them ideal for compact, high-performance designs. Some of the most compelling advantages include:
- Space Efficiency. Thanks to their hybrid design, these PCBs allow for compact and space-efficient designs. They can be bent or folded to fit into tight or irregularly shaped spaces, saving valuable space in electronic assemblies.
- Reliability. The design creates an enhanced mechanical stability, reducing the risk of fatigue, cracking, or failure in high–shock and high-vibration environments and improving the overall reliability and durability of the circuit.
- Higher Component Density. This technology supports component mounting on both sides of the board, increasing the component density for more compact and sophisticated designs. This is especially advantageous in miniaturized electronic devices where real estate is at a premium, and there is no longer room for connectors.
- Design Versatility. This advanced design approach offers greater versatility than traditional rigid or fully flexible options. Its ability to bend, flex, and fold while maintaining structural support in key areas opens the door to more creative and innovative product designs.
- Simplified Assembly. This solution streamlines assembly by minimizing the need for complex interconnects, connectors, and cables. Fewer interconnection points lead to simpler builds and greater system reliability. In designs with multiple interconnected sections, it can even reduce overall production costs.
- Budget-Friendly. While upfront manufacturing costs may be higher, this design approach can lead to significant long-term savings. Its compact structure, reduced need for interconnects, and enhanced reliability help lower maintenance expenses, minimize system failures, and extend product lifespan. It’s also a cost-effective choice for complex assemblies involving multiple interconnected sections.
Choosing The Right Solution
While rigid-flex circuit boards have many significant advantages, the suitability of this PCB type for your specific application depends largely on the application itself and the construction and performance requirements therein.
As covered previously, you can confidently choose rigid flex technology if your application calls for a high degree of reliability, durability, and flexibility, especially when space constraints or complexities are in play. Medical imaging equipment, military missile guidance systems, cameras, and video cameras, along with wearable electronics, vehicle infotainment systems, and industrial robotics would all be appropriate rigid flex PCB applications.
However, there are situations in which these PCBs may not be the best choice.
- Cost Constraints: These PCBs often have higher manufacturing costs compared to alternative solutions. If cost is a primary consideration for your project and the design does not require flexibility, alternative options may be more cost-effective.
- Simple Designs: If the design does not require complex interconnections, bending or folding capabilities, or high-density component mounting, a traditional solution may suffice.
- Low-Volume Production: Rigid flex technology often requires specialized fabrication techniques and expertise, making them better suited for medium- to high-volume production runs. For low-volume production or prototyping, it may be hard to justify the cost and time associated with this type of PCB fabrication.
Ultimately, it’s best to weigh the advantages and disadvantages for your specific application and make your decision accordingly.
The Bottom Line
Whatever type of PCB you choose, All Flex is here to help make the process simple and the board top-quality. We’re here for all your specialty printed circuit boards and value-added assembly needs. Contact us today to learn more about All Flex’s quality solutions!