The Flexible PCB, also known as a soft board or FPC, is a highly reliable and impressive flexible printed circuit board. It is constructed using polyimide or polyester film as the base material. One of its distinguishing features is its unique design, characterized by high wiring density, lightweight construction, and thin thickness. The Flexible PCB finds extensive application in various products, including handheld computers, notebook computers, PDAs, digital cameras, and LCMs.
What is Flexible PCB？
The Flexible PCB, also referred to as a “soft board,” is a type of PCB used in flexible electronics. It is commonly known by various names such as Flexible Printed Circuit (FPC), Flexible Circuit Board (FPCB), or Flex PCB. Flexible PCBs are assembled by mounting electronic devices on flexible plastic substrates like polyimide, PEEK, or transparent conductive polyester film. They can also be made with a silver circuit screen printed on polyester. These flexible electronic components offer the same functionality as rigid PCBs but with the added advantage of conforming to desired shapes or bending during use. They find application in a wide range of devices such as handheld computers, notebook computers, PDAs, digital cameras, and LCMs.
Flexible Printed Circuits (FPCs) are produced using photolithographic techniques, which involve the use of light to transfer circuit patterns onto flexible substrates. Another method for creating flexible foil circuits or flexible flat cables (FFCs) is by laminating ultra-thin copper strips (typically 0.07 mm) between two layers of PET (Polyethylene Terephthalate). These PET layers are usually 0.05 mm thick and are coated with a thermoset adhesive. The adhesive is activated during the lamination process, resulting in a secure bond between the copper strips and PET layers.
With the increasing production ratio of flexible PCBs and the growing popularity of rigid-flex PCBs, it has become more common to incorporate flexibility, rigidity, or a combination of both in PCBs, resulting in what is known as a few-layer FPC.
Typically, FPCs made with a soft insulating substrate are referred to as flexible FPCs, while rigid-flex composite PCBs are called rigid-flex PCBs. These PCB types cater to the requirements of modern electronic products, which demand high density, reliability, compactness, and lightweight design. They also fulfill the strict economic requirements and the competitive needs of the market and technology.
Flexible PCBs have been widely utilized overseas since the early 1960s, while in China, their production and application began in the 1960s. In recent years, with the advancement of global economic integration and market openness, the adoption of flexible PCB technology has steadily increased.
To meet the growing demand for flexible PCBs, some small and medium-sized rigid FPC factories have adopted the strategy of incorporating flexible and rigid techniques. They leverage existing facilities, improve tooling and processes, and transform their production capabilities to meet the rising demand for flexible PCBs. To gain a deeper understanding of PCBs, this article provides a research and discussion introduction to the flexible PCB process.
FPCs and FFCs offer several advantages in various applications:
- Tightly Assembled Electronic Packages: They are suitable for applications that require electrical connections in three axes, such as cameras in static applications.
- Dynamic Applications: Components that require flexing electrical connections during normal use, like folding phones, benefit from FPCs and FFCs.
- Replacement for Wiring Harnesses: FPCs and FFCs can replace heavier and bulkier wiring harnesses, providing electrical connections between subassemblies in automobiles, rockets, and satellites.
- Space Constraints: When board thickness or space limitations are a factor, FPCs and FFCs offer a solution for efficient electrical connections.
Advantages of FPCs:
- Potential for Board and Connector Replacement: FPCs have the potential to replace multiple rigid boards or connectors, reducing complexity and saving space.
- Dynamic and Flexible Applications: Single-sided circuits in FPCs are well-suited for dynamic or highly flexible applications.
- Versatile Configurations: FPCs can be stacked in various configurations to meet specific design requirements.
Disadvantages of FPCs:
- Increased Cost: FPCs tend to be more expensive than rigid PCBs, which can impact overall production costs.
- Higher Risk of Damage: FPCs are more susceptible to damage during handling or use due to their flexible nature.
- Complex Assembly Process: Assembling FPCs can be more challenging compared to rigid PCBs, requiring specialized techniques and equipment.
- Limited Repairs and Rework: Repairing or reworking FPCs can be difficult or even impossible in some cases, which may increase manufacturing costs.
- Reduced Panel Utilization: FPCs often have lower panel utilization, leading to increased material waste and potentially higher costs.
Flex PCB Manufacturing Process
Up until now, the majority of FPC manufacturing processes have predominantly utilized the subtractive method, also known as the etching method. In this method, a copper-clad laminate is typically used as the starting material. A resist layer is formed through photolithography, and the unwanted copper surface is subsequently removed via etching, resulting in the formation of circuit conductors. However, the subtractive method has limitations in processing fine circuits due to issues like side etching.
Due to the difficulties associated with the subtractive method and the challenges in maintaining high-standard microcircuits, many consider the semi-additive method to be an effective alternative. Various semi-additive methods have been proposed by experts. An example of microcircuit processing using the semi-additive method involves using a polyimide film as the initial material. A liquid polyimide resin is first applied (coated) onto a suitable carrier to form a polyimide film.
Next, a crystal planting layer is created on the polyimide base film using the sputtering method. Subsequently, the photolithography method is employed to form a resist layer pattern known as the anti-plating layer, which serves as the reverse pattern of the circuit. Conductor circuits are then formed by electroplating in the exposed areas. The resist layer and unnecessary crystal planting layer are removed, leaving behind the first layer circuit.
The process continues by applying photosensitive polyimide resin onto the first layer of circuit. Photolithography is used to create holes, ensuring that the layer remains an insulating layer for the second layer of circuit. A crystal planting layer is then deposited on top, acting as the base conductive layer for the second layer circuit. By repeating these steps, a multilayer circuit can be formed.
Using the semi-additive method, ultra-fine circuits with a pitch of 5um and via holes of 10um can be successfully processed. The key to fabricating such ultra-fine circuits lies in the properties of the photosensitive polyimide resin used as the insulating layer.
Flexible PCB Application
Flexible circuits are frequently utilized as connectors in various applications where flexibility, space savings, or production constraints make rigid circuit boards or hand routing unsuitable.
Most flex circuits are passive wiring structures that interconnect electronic components such as integrated circuits, resistors, and capacitors. However, some flex circuits are solely employed for direct interconnections between other electronic components or through connectors. Consumer electronics extensively employ flex circuits in devices like cameras, personal entertainment devices, calculators, and motion monitors. Industrial and medical devices that necessitate numerous interconnections within a compact package also rely on flexible circuits. Cell phones are another prominent example of the widespread utilization of flexible circuits.
Here are some specific applications of flex circuits:
- Input Devices: Flex circuits are commonly used in computer keyboards, where they serve as switch matrices.
- LCD Monitors: Flexible plastic or metal foil substrates can replace rigid glass substrates in LCD manufacturing, allowing for flexible displays.
- OLED Displays: Organic Light Emitting Diodes (OLEDs) are used to create flexible displays by replacing the backlight.
- Automotive Circuits: Flex circuits find application in instrument panels, under-hood controls, cabin headliners, and ABS systems within the automotive industry.
- Printers: Flex circuits are utilized in computer printers for connecting signals to the moving printhead and the arm carrying the disk drive’s read/write head.
- Solar Batteries: Flexible thin-film solar cells have been developed for lightweight and easy-to-deploy power generation, making them suitable for satellites and portable applications like integrating them into backpacks or coats.
- Skin-Like Circuits: Engineers have developed stretchable and skin-like semiconductor circuits, which may be used in wearable electronic devices to wirelessly transmit health data to doctors in the future.
These examples illustrate the versatility and potential of flexible circuits in addressing specific needs in various industries and applications.
An FPC board is a PCB that can have a cylindrical or rectangular shape and can vary in size based on application requirements. FPCs are categorized as rigid or flexible. Rigid FPCs mechanically connect parts but cannot bend, while flexible FPCs are double-sided PCBs capable of withstanding bending forces.
Flexible FPCs are primarily used for electrical interconnection applications. They eliminate the need for solder joints and offer ease of modification and customization. The flexibility of FPCs allows for various retrofit solutions and reduces the cost of printing new boards for specific applications. FPCs are commonly used in devices that require flexibility and conductivity, such as cell phones, digital cameras, and walkie-talkies. They can also be used in larger devices like peripherals and power supplies.
However, FPCs are not suitable when mechanical strength is needed. Due to their thin and lightweight nature, they are ideal for portable devices.
What are the 3 types of Flexible PCB?
- Single-layer FPC: It consists of a conductive pattern on one side of a flexible insulating substrate, typically made of materials like polyimide, polyethylene terephthalate, aramid, or polyvinyl chloride. Single-layer FPC can be further categorized as follows:
- No cover layer: The wire pattern is exposed without any covering layer. Interconnections are made through soldering, fusion welding, or pressure welding, commonly used in early telephones.
- Single-sided connection with a covering layer: Similar to the previous type, but with an additional covering layer on the wire surface. The connection pads are exposed, while the end area remains uncovered. Widely used in vehicle instruments and electronic spectrographs.
- Double-sided joint signature without covering layer: The connection pads are available on both sides of the wire, with via holes allowing interconnection. Via holes can be created through mechanical means.
- Double-sided joint signature with covering layer: Similar to the previous type, but with a covering layer that has via holes, enabling termination on both sides while maintaining the covering layer. It consists of two layers of insulating material and one layer of metal conductor.
- Double-sided FPC: This type has conductive patterns etched on both sides of the insulating base film. Metallized holes connect the patterns on both sides, forming a conductive path. The use of cover films helps with component placement. Double-sided FPCs are less commonly used.
- Multilayer FPC: It involves laminating three or more layers of single-sided or double-sided flexible circuit boards together. Metallized holes and conductive paths are formed through drilling and electroplating. Multilayer FPCs provide higher reliability, better thermal conductivity, and easier assembly. They are advantageous for their lightweight substrate, low dielectric constant, and improved functionality. Multilayer FPCs can be further classified as:
Multilayer flexible PCB can be further divided into the following types:
Types of Multilayer Flexible PCBs:
- Flexible Multilayer PCB: This type of PCB is formed on a flexible insulating substrate and is highly flexible. It consists of multiple single-sided or double-sided microstrip flexible PCBs bonded together, with the inner core part remaining unbonded. To achieve desired electrical properties, such as impedance performance matching with rigid PCBs, each circuit layer has pre-set signal lines on the ground plane. A thin, conformable coating like polyimide can be used instead of a thicker laminated cover layer to maintain flexibility. This type is suitable for designs requiring flexibility, high reliability, and high density.
- Flexed-End Multilayer PCB: This PCB is formed on a flexible insulating substrate and can be flexed at the end. It is made of a soft insulating material, such as polyimide film, which is laminated into a multilayer board, preserving its inherent flexibility. This type is suitable when special properties of film insulation, such as low dielectric constant, average thickness, light weight, and continuous processing, are required. For example, a polyimide film-based multilayer PCB is significantly lighter than a rigid epoxy-glass cloth PCB.
- Formable Multilayer PCB: This PCB is made of soft insulating materials and is designed to be formable rather than continuously flexible. It is limited by electrical requirements, such as the need for thick conductors to meet resistance specifications or the need for thick conductors between signal and ground layers for impedance or capacitance requirements. The term “formable” refers to a multilayer flexible PCB that can be shaped as required but cannot flex during application. It is used in wiring inside avionics facility units. This type requires low resistance, minimal capacitive coupling, and a smooth 90° flex at the interconnect end. Polyimide film-based multilayer flexible PCBs are commonly used for such applications due to their high-temperature resistance, flexibility, and special electrical and mechanical properties.
To achieve full interconnection in these devices, the traces can be further divided into multiple multilayer flexible circuit devices and assembled with adhesive tapes to form a printed circuit bundle.
Rigid-Flexible Multilayer PCB
Rigid-flex multilayer PCBs combine rigid PCBs with flexible PCBs to form a single unit. Flexible PCB layers are laminated within rigid multilayer PCBs, either for specific electrical requirements or to extend beyond rigid circuits. This type of PCB is commonly used in electronic equipment that prioritizes weight and size compression, requiring high reliability, high-density assembly, and good electrical properties.
The rigid-soft multilayer PCB design involves bonding multiple single-sided or double-sided flexible PCBs to a rigid section, while leaving the middle section flexible. The rigid section utilizes metalized via interconnects. These PCBs are increasingly used in applications with strict size limits that demand ultra-high packaging density, excellent electrical properties, and high reliability.
Hybrid multilayer flexible PCB devices have been developed for military avionics, where weight and size are crucial considerations. To meet weight and size limits, the internal packing density must be extremely high. Shielding is essential to minimize crosstalk and noise, but using shielded cables can be economically challenging. Hybrid multilayer PCBs address this by incorporating a flat stripline flexible PCB with shielded signal lines as part of the rigid PCB, providing effective interconnection.
In demanding operating conditions, these PCBs can form a 90° S-shaped buckling, allowing for the simplest surface interconnection path. This design reduces stress on solder joints caused by surface-oscillating stresses in the x, y, and z directions.分享
What are the differences between Flex and Rigid boards?
Materials:
Rigid boards are typically made from FR4 (glass-epoxy compounds), while flexible circuits are made from polyimide. Although there are cases where rigid boards are constructed with polyimide, it is less common.
Coverlay:
Flex PCBs can have a flexible mask or overlay, whereas rigid boards usually utilize a solder mask. In the case of an overlay, the openings are routed or laser cut. Then, an adhesive, typically 1 or 2 mils thick, is applied to attach the overlay to the flexible board.
Stiffeners:
Flexible printed circuit boards often incorporate FR4 or polyimide stiffeners to reinforce specific non-flexible areas. Stiffeners are either laminated to the flex or attached using a PSA (pressure-sensitive adhesive). Rigid boards do not require stiffeners.
Permittivity:
Rigid board materials exhibit a wide range of relative permittivity (dielectric constants), whereas flexible polyimide material typically has a value of 3.4.
Outlook on the Application of Flexible Circuit Boards
Flexible circuit boards (FPCs) play an essential role in the printed circuit board industry, especially in circuit sheets printed on flexible substrates typically made of PET or laminated silica. FPCs are highly suitable for PCBs due to their ability to withstand high temperatures and easy integration into circuit boards, offering excellent flexibility. They find applications in various fields that require durable and cost-effective embedded components.
The flexibility and versatility of FPC circuits make them ideal components for projects such as solar cells and cell phones. Audemars Piguet (iPcb®) is a professional R&D and manufacturer of high-precision PCB circuit boards capable of mass producing 4-46-layer PCB boards, circuit boards, high-frequency boards, high-speed boards, HDI boards, and more.
The mass production of folding screens poses several technical challenges, with the application of FPC being one of them. Folding screen mobile phones have driven the growth of the FPC industry, and size-related issues are a focal point for research and development. With the continuous advancement of smartphone technology, flexible screen mobile phones have emerged as a potential future design trend after full-screen displays. FPCs, with their lightweight, thin profile, and bendability, have become indispensable components in consumer electronic products, especially smartphones.
However, the application of FPC in smartphones still faces various technical difficulties, which manufacturers are actively working to overcome. The homogenization of smartphones has driven manufacturers to seek innovation, leading to the emergence of foldable screen phones. Companies such as Huawei, Samsung, Royole, Xiaomi, OPPO, and vivo have released or showcased folding screen mobile phones. Achieving mass production of folding screens requires overcoming several technical challenges, including the application of FPC.
FPC possesses characteristics such as lightweight, thinness, bendability, winding, foldability, and high wiring density, making it an ideal fit for the development trend of lightweight, thin, and miniaturized devices. However, the development of foldable PCBs and corresponding electronic materials poses significant challenges and requires extensive research and development.
FPC finds applications in various industries, including mobile terminals, consumer electronics, automotive electronics, industrial control, medical, aerospace, and military. Mobile terminals, particularly smartphones, represent the largest application field for FPC and also demand the highest technical capabilities. The trend towards miniaturization and intelligence will drive the future development of flexible mobile phones.
In mobile phones with flexible screens, the flexible circuit board inside the device needs to endure multiple folds, which increases the requirements and usage area of FPC. However, FPC production is typically limited by the size of the manufacturing equipment. Solutions to address this limitation include extending through the middle socket and reinforcing the FPC with pull-out gold fingers.
The increasing application of FPC in mobile phones is a well-established trend, driving a favorable outlook for the FPC market. However, challenges remain, particularly in the manufacturing of large-sized FPCs for folding screen mobile phones. Solutions to these problems exist but often come with increased costs. Mass production or technological advancements are necessary to better control the cost of FPC.