The Ultimate Guide to Flexible PCB: Types, Design, and Applications

A flexible printed circuit board (FPC or flex PCB) is a type of PCB that is designed to be flexible, allowing it to be bent, folded, or twisted. An FPC features a combination of several printed circuits as well as components that are positioned on a flexible substrate. They are typically made from a polyimide (PI) film material, which guarantees high flexibility and thermal stability.Their miniaturized design drives innovation across major electronics sectors that increases innovations and applications in main electronics sectors, such as consumer, automotive, medical devices, wearables, telecommunications, and aerospace.

A flexible circuit board requires less space and is more reliable. You can bend it up to 360 degrees, most of which are designed for 500 million flex cycles. The technology has been used for interconnecting electronic devices since the 1950s in one form or another. It is now one of the most important interconnection technologies in use for the manufacture of many of today's most advanced electronic products.

Types of Flexible Circuit Boards:

1) Single-sided Flex PCB:

The single-sided flexible circuit board is the most basic among the types of flex PCBs. It consists of a substrate layer, conductive copper layer, overlay, and silkscreen.

It only has a single layer of flexible polyimide or polymer film, and the conductive copper layer is only accessible on one side of the PCB. As a result, the other side can be used to put various electronic components. Its simple design makes it suitable for various applications and more affordable. Hence, it is also the main reason why it is the most widely used flex circuit.

2) Double-sided Flex PCB:

Unlike the single-sided flex circuit, a double-sided flexible circuit board allows the creation of trace patterns on both the top and bottom copper layers. You can connect each side using the copper-plated through-holes wherever you want. It also permits electrical connections via the layers.

One advantage of the double-sided flexible circuit board is that it can be used in more complex electronic products to achieve more functions. Manufacturers have found it useful in making dashboard components, lighting, and other industrial areas.

3) Multi-layered Flex PCB:

Several copper layers in multi-layered flexible PCB boards are joined together by plated through holes, similar to those in double-sided boards. Multilayered flexible PCBs contain a number of individual copper layers and dielectric layers.

One of the benefits of using a multi-layered flexible circuit board is freedom in design with multiple functions. Manufacturers can use it for those with complex design requirements. It is utilized to make components for mobile phones, satellites, cameras, and hearing aids.

For now, the services at JLCPCB fabrication manufacturing unit are capable of manufacturing 1 to 4-layer Flex PCB with high precision and quality control.

How much can you bend a PCB?

The question of “How much can you bend a PCB?” may seem like an easy one to answer, but it is not as simple as that. Bending a PCB can compromise the strength of the circuit board, copper tracks, and solder pads. The maximum bend radius for a PCB depends on the design material and thickness of the PCB. A 0.4mm is enough to bend a PCB 90 degrees. Some PCBs can be bent up to 20 times, but it is best not to bend the circuit board more than once. The formula for the maximum bending angle of a PCB based on the thickness of the substrate is given here:

For example, performing a 90-degree bend increases the chance of circuit damage. Lastly, make sure the bending radius does not exceed one millimeter. You can bend PCBs up to 180 degrees with a flex material. However, if you try to bend it past its recommended limits, you could end up destroying your circuit board.

Structure of Flexible PCB:

The three Flexible PCBs include; single-layer, double, and multi-layer circuits. The major element of a single-layer Flex PCB are:

1) Dielectric substrate film: It can be considered the base material of PCB. The prevalent material includes polyimide(PI), characterized by high-temperature resistance.

2) Electric Conductors: These include traces of the circuit, copper foil is used as the surface conductor layers in the production of single/multilayer printed circuit boards.

3) Protective Finish: It is composed of organic film and adhesive. The function of the cover film is to protect the completed flexible circuit conductor part.

4) Adhesive Material: A type of sticky film has different adhesive types and thickness specifications. The sticky film is used for interlayer adhesion and insulation of multilayer boards.

Role of Stiffener in Flex PCB design:

A stiffener in a flexible printed circuit (FPC) is a rigid layer, typically made of materials like polyimide or FR4, added to specific areas of the flexible PCB. It provides mechanical support and reinforcement to prevent bending or flexing in delicate regions, such as connector areas or mounting points. Stiffeners improve durability, enhance reliability in high-stress areas, and help maintain the overall form factor of the FPC in critical sections.

JLCPCB Flexible PCB Manufacturing Capabilities Table

Comprehensive Stiffener Selection & DFM Guidelines:

In practical FPC engineering, three main types of stiffeners are selected based on application requirements:

1. PI Stiffener (0.1mm – 0.25mm): Ideal for ZIF connector insertion areas, increasing localized thickness (e.g., reaching standard 0.3mm) without losing edge precision.

2. FR4 Stiffener (0.1mm – 1.6mm): Applied underneath SMT component clusters or IC packages to provide mechanical rigidity and protect solder joints from fracturing.

3. Stainless Steel Stiffener (0.1mm – 0.3mm): Offers ultimate physical strength nd grounding capability within ultra-slim structural envelopes.

Designing Tips for Flexible Printed Circuit Boards:

1. Keep the max bending radius in mind while working with FPC of different thicknesses.

2. In areas where the substrate or overlay bends, avoid discontinuities.

3. Teardrops can be added to make the through holes more stable and rigid.

4. Use a stiffener under the main electronics circuitry of the PCB.

5. Make the corners round and a little bigger to avoid any corner tear of the PCB.

6. Around stiffeners, it allowed etched markings, while on the other hand, silkscreen markings are not.

For more design-related information, see a full article covered on an Arduino board fully flexed PCB.

Advantages of Flexible PCB:

Flexible circuit boards offer many benefits, which is why they have become a popular choice among electronics product manufacturers regarding PCBs. Here are some key advantages of using flex circuits in your electronics components:

Flexibility:  These types of PCBs can be easily bent and twisted. It can thus be moved and expanded in 3D space, making it easier to integrate the component with an effective wire connection.

Lightweight: The size and weight of FPC are less as compared to rigid PCBs, which are suitable for high-reliability and high-density products.

Durability: Flex circuits are bendable up to 360 degrees. They can survive the impact of vibrations and shocks, which enhances performance in any application.

Thermal Stability: Due to thinner substrate and lightweight it can easily dissipate heat.  Flex circuits can be exposed to high temperatures of up to 400°C.

Signal Integrity: It can provide high mechanical resistance and resistance to high atmospheric agents with better EMI(electromagnetic immunity).

Hatched Ground Planes for Flexible Regions: Avoid solid copper pours in active bend zones. Utilize cross-hatched copper patterns to balance EMI shielding while retaining maximum mechanical flexibility.

Staggered Trace Routing: For 2-layer FPC designs, avoid overlapping top and bottom traces directly over one another in the bend area. Stagger the traces to prevent the "I-Beam effect," which drastically increases flex fatigue and risk of trace cracking.

Disadvantages of Flexible PCB:

Although flexible circuit boards offer numerous advantages in modern electronics, there are some concerns to consider:

● The process of Flex PCB assembly is complex.

● It is difficult to repair any work and faults that arise in the PCB board.

● There is a high probability of damage while handling.

● Flex PCB is a bit more expensive compared to traditional PCB design.

● Flex PCB is sensitive to scratching.

● It requires proper storage conditions in sulfur-free plastic.

Applications of Flexible PCB:

Flexible PCBs are used in various sectors due to their versatility, reliability, and space-saving features:

Consumer Electronics: Flex PCBs are used in compact, lightweight designs to connect components efficiently. Used in smartwatches, fitness trackers, and other wearable tech for flexibility and durability. Integrated into compact digital and analog mixed-signal units for high-performance connectivity.

Automotive industry: Ensures efficient communication between sensors and control units of cars with Advanced Driver Assistance Systems (ADAS). Provides reliable connections in car entertainment systems. Used in LED headlights and interior lighting for compact design and flexibility.

Medical and Healthcare Devices: FPC high-density connections are helpful in reducing the sizes of devices like MRI and CT scanners. Offers compact and reliable connections in small, intricate designs including hearing Aids and other wearable monitoring devices providing comfort and flexibility.

Industrial Applications: Provides flexibility and durability in robotic arms and machinery with moving parts. Ensures efficient and compact connections in automated systems containing industrial sensors and control units for reliable performance.

Why not use a Flexible PCB all the time?

Flexible PCBs, while offering advantages like design versatility and space savings, aren't ideal for every application due to their higher cost compared to traditional rigid PCBs. They are also more complex to manufacture and can be prone to damage if not handled carefully. Flexible PCBs may not be necessary for simpler designs that don't require bending or folding, making rigid PCBs a more economical and durable choice in such cases. Additionally, flex circuits often have limitations in terms of high-power handling and mechanical stability.

FAQ about Flexible PCB

Conclusion

Undoubtedly, flexible circuit boards have become essential in making products that are more powerful but portable in many industries. Other conventional options cannot compare with the flexibility and accuracy of flex circuits. Components for aircraft, medical devices, cars, and even those used in daily life rely on flexible circuit boards to become lightweight without compromising capabilities. As technology improves and products continue to innovate, flex PCBs will remain the primary choice in making circuit boards for products created by various industries.