What are Flexible Printed Circuit Boards?

Flexible circuits offer a blend of versatility and functionality in electronic design. Flexible circuits are primarily used to replace traditional wiring connections. These circuits can be made simply as a connector between other devices or circuits or can be designed with any number of electrical components installed as one integrated solution. 

Flexible circuits are made using thin, flexible materials (typically polyimide or polyester film), to allow for intricate designs and space-saving solutions. They can be bent, folded, and shaped to fit into tight spaces, making them ideal for compact and complex electronic devices. Their flexibility is achieved through the use of materials that can withstand bending and twisting without breaking.

Key Parameters of Flexible Circuits

• Substrate Material: The most common substrates for flexible circuits include polyimide (PI) and polyester (PET). Polyimide is renowned for its high thermal stability, chemical resistance, and flexibility, making it ideal for demanding applications. Polyester, while less heat-resistant, offers cost-effectiveness and is used in less demanding environments.
• Conductive Layer: This typically involves copper, which is used to create the circuit paths. Copper is chosen for its excellent electrical conductivity, ease of etching, and high ductility. The thickness of the copper layer, measured in ounces per square foot (oz/ft²), affects the circuit’s current-carrying capacity and overall performance.
• Insulation: Flexible circuits use insulating layers to prevent short circuits and interference. These layers are often made from materials like polyimide or epoxy. The thickness and quality of the insulation affect the circuit’s reliability and durability.
• Thickness: The overall thickness of a flexible circuit, including the substrate, conductive layers, and insulation, impacts its flexibility and strength. Flexible circuits have thicknesses ranging from 0.05 mm to over 1.5 mm.
• Bend Radius: The minimum radius a flexible circuit can be bent without compromising performance or reliability. This parameter is crucial for designing circuits that need to conform to specific shapes or fit into compact spaces. 

Design Considerations

Designing for flexibility requires careful consideration of mechanical stresses, specifically regarding bend radius and related stress points. Excessive bending or twisting can lead to mechanical failure. Designers often use stress relief features and reinforced areas to mitigate these risks.

• Static vs Dynamic Applications: It is critical to know if the flexible circuit will be used in a static, one-time bend-for-installation application, or in a dynamic, constantly flexing application. Design criteria for minimum bend radius, material stack-up, and conductor routing must be carefully considered to prevent premature failure of the flex circuit.
• Thermal Management: Flexible circuits can experience heat buildup, particularly in high-power applications. Effective thermal management strategies, such as heat sinks or thermal vias, are necessary to prevent overheating and ensure reliable operation.
• Component Application Techniques: Soldering to flexible circuits can be challenging due to the substrate’s flexibility. Alternative methods such as conductive adhesives or using connectors specifically designed for flexible circuits are often employed. Adding stiffeners (typically rigid epoxy-glass boards) to component areas of a flexible circuit helps provide stability to added components. Incorporating a rigid substrate into the flexible circuit stack-up (referred to as rigid-flex) also helps stabilize components, and can reduce overall circuit thickness and weight compared to add-on stiffeners. 
• Connectors and Terminations: There is a vast array of connectors and terminals available to allow the mating of flex circuits with other electrical components, whether for permanent installation, or repeated installation/removal cycles. Choosing the correct stock or custom connectors to match the application is critical in achieving proper circuit integrity and performance.
• Layer Stacking: Multi-layer flexible circuits involve stacking multiple layers of conductive and insulating materials. Proper conductor alignment and adhesion between layers are essential to maintain circuit integrity and performance while retaining maximum flexibility.
• Design for Manufacturing: Flexible circuits require careful consideration of manufacturing processes such as etching and lamination. Design decisions should account for these processes to avoid costly production issues.

Advantages of Flexible Circuits

• Space Savings: Flexible circuits can be shaped and molded to fit complex geometries, reducing the need for bulky connectors and wiring. This allows for more compact and lightweight designs.
• Reduced Assembly Time: With flexible circuits, the need for manual wiring and assembly is minimized, leading to faster production times and potentially lower costs.
• Increased Integration: Mounting electrical components directly to a flexible circuit eliminates multiple interconnects and minimizes the number of potential failure points in a system.
• Enhanced Durability: The flexibility and low mass of these circuits help absorb mechanical stress and vibrations, making them less prone to damage compared to rigid PCBs.
• Design Flexibility: Engineers can design circuits with intricate layouts that can conform to 3D shapes, providing more creative solutions in product design.

Limitations of Flexible Circuits

• Cost: While flexible circuits offer many benefits, they can be more expensive to produce than traditional rigid circuits, particularly in low-volume applications.
• Complexity in Design: Designing flexible circuits can be more complex, requiring specialized knowledge and tools to ensure reliability and functionality.
• Thermal Limitations: Although flexible circuits can handle moderate temperatures, they may not be suitable for high-temperature environments without additional thermal management solutions.
• Mechanical Durability: Over time, repeated bending and flexing can lead to fatigue and failure of the circuit. Proper design and material selection are critical to address these issues.

Applications of Flexible Circuits

• Consumer Electronics: Flexible circuits are commonly used in devices such as smartphones, tablets, and wearables due to their ability to fit in compact and curved spaces.
• Medical Devices: In medical applications, flexible circuits are used in devices like flexible sensors and wearable health monitors, where flexibility and conformability are crucial. Implantable devices like defibrillators and pacemakers also contain flex circuits due to their low mass and ability to conform in tight bend areas. 
• Automotive Industry: Flexible circuits are employed in automotive applications for flexible displays, control panels, and internal wiring systems, accommodating the complex shapes of modern vehicles.
• Aerospace and Defense: The aerospace and defense sectors utilize flexible circuits for their lightweight and space-saving properties in critical systems where reliability and performance are paramount. 
• Industrial Equipment: In various industrial settings, flexible circuits can be found in equipment requiring compact and durable electronic solutions, such as robotics and automated systems.

Conclusion

Flexible circuits are a versatile and innovative technology that offers significant advantages in terms of design flexibility and space efficiency. While they come with their own set of challenges, careful consideration of key parameters and design considerations can lead to highly effective solutions for a wide range of applications. As technology advances, the role of flexible circuits is likely to expand, driving further innovation in electronics and beyond.

A Flexible Printed Circuit Board (PCB) is a printed circuit board that can be bent or twisted to conform to a desired shape without damaging the circuit. The substrate material used to build the PCB is responsible for its flexibility. Polyimide, Polyester (PET) or Liquid Crystal Polymer (LCP) are common substrates used to make flexible PCBs. Their adaptability and efficient usage of space make these PCBs very significant in modern electronic devices.

Note: Click here to learn more about the materials used to make flexible PCBs.

Structure of a Flexible PCB

The layers of a flexible PCB are similar to a Rigid PCB - a substrate that acts as a base or core, a conducting layer made of thin copper foil in the shape and dimensions of the electrical circuit and adhesive layers holding the other various layers together. The copper conducting layer is laminated onto the substrate through an electro-deposition process. The thickness of this copper layer is controlled according to the application where a heavier layer of copper is needed for higher currents. Sometimes the copper foil also undergoes a rolling annealing process that elongates the grain structure of the copper in the planar direction. This creates a higher-grade copper foil ideal for flexible PCBs that undergo repeated creasing or movement. The added step of annealing the foil adds to the cost considerably, but the annealed copper can stretch more before fatigue cracking occurs and is springier.

Flexible PCBs also have an additional layer called coverlay which is a type of lamination used to encapsulate and protect the external circuitry of a flex circuit. It is similar to the solder mask layer in a rigid PCB. The coverlay film is a polyimide or polyester film that is coated with a thermoset adhesive. It protects the flexible circuit from corrosion and mechanical damage.

Structure of a Flexible PCB

Commonly used types of flexible PCBs are:

• Single-Sided Flexible Circuits – They are the most common and simple flexible PCBs. These PCBs contain a single conductor layer on a flexible substrate and have a printed conductor layer only on one side. They are the most affordable flex PCBs on the market and are relatively easy to produce.
• Double-Sided Flexible Circuits – They are made by printing a copper conductor layer on both sides of a substrate film which are then connected with plated through holes. Double-sided flex PCBs offer enhanced power-handling capabilities and circuit density. While they are popular for their increased power, they are somewhat more expensive to produce than single-sided flex PCBs.
• Multi-Layer Flex Circuits – These are flexible circuits with three or more layers of conductors. Due to the complexity of their construction, multi-layer flex PCBs are more expensive than their counterparts. They are often used in aerospace and military applications that can justify the cost and capabilities of such a PCB.

Advantages of Flexible PCBs

The bendability of Flexible PCBs solves space constraint problems as they offer a significant reduction in size and weight compared to rigid boards. Flexible circuits are also much more reliable and durable, especially in applications that are subject to continuous vibrations and mechanical stress, such as automotive electronics. As the substrate materials used for flexible PCBs have a good tolerance for temperature changes, they are also suitable for harsh environments. They can replace bulky wiring, often done by hand, in several applications, reducing the total cost of electrical wiring by up to 70%. Flexible PCBs are also significantly lighter than rigid boards and can decrease package weight by up to 75%.

Disadvantages of Flexible PCBs

The fabrication of flexible PCBs costs more than rigid PCBs, not only in terms of cost of materials; as flexible PCBs are generally made for targeted applications, they require specific design and prototype developments, further contributing to the overall manufacturing cost. Repairing and modifying/reworking flexible PCBs are harder than rigid PCBs as one needs to pull off the protective film and then apply it back to the worked part. Being relatively new, all manufacturers may not have the process capabilities to manufacture flexible PCBs.

Applications of Flex PCB

• Commercial Electronics: Most of the sales volume for flexible printed circuits comes from the computer industry, they are used in printers, scanners, smart devices, and more. As the demand for smaller and more advanced handheld technology increases, flex PCBs are preferred to deliver heavy computing power without sacrificing weight restrictions.
• Automotive: This is another major sector for flexible PCBs. Flexible PCBs are selected for their ability to fit in constrained space and their exceptional thermal performance. As cars begin to include more sensors and electronics for everything from climate control and GPS software to dashboards and rear-view cameras use flexible PCBs.
• Aerospace: Thanks to their remarkable ability to handle the stress of extreme temperatures and vibrations, flex PCBs are uniquely suited for aerospace applications such as satellites. They also can take on unusual shapes that are well suited for spacecraft and substitute connectors that are more prone to failure.
• Medical Devices: As medical technology evolves, wearable devices have become a key feature in the past few decades. Flexible printed circuit boards offer greater durability for these devices, as they can mirror shapes found in the body. Hearing aids, vital sensors, etc., all use flexible printed circuits to leverage high performance that can withstand wear and tear.

Click here to see a list of PCB Manufacturers who manufacture flexible boards.