A Comprehensive Guide to FPCs: Learn More About FPC Flexible Circuit Boards
Flexible printed circuits (FPCs) are an essential component of the modern electronics industry. Their flexibility, compactness, and durability make them ideal for applications where traditional rigid PCBs are unsuitable, such as wearable devices, medical devices, and mobile phones. This article will provide you with a comprehensive overview of the basics, manufacturing process , advantages, and applications of flexible printed circuit boards .
What is FPC ?
FPC (Flexible Printed Circuit) is a type of PCB, also known as “flexible board”. FPC is made of flexible substrates such as polyimide or polyester film. It has the advantages of high wiring density, light weight, thinness, bendability and flexibility. It can withstand millions of dynamic bends without damaging the wires. It can move and stretch arbitrarily according to spatial layout requirements, realize three-dimensional assembly, and achieve the effect of integrated component assembly and wire connection. It has advantages that other types of circuit boards cannot match.
Types of FPCs
PCBs are now indispensable components in electronic devices. PCBs come in various types due to different structures and applications. Based on the mechanical properties of the substrate’s flexibility, PCBs are divided into two main categories: rigid boards and flexible boards, as well as rigid-flex PCBs (a hybrid of rigid and flexible). They are also classified by the number of conductor layers: single-sided boards, double-sided boards, and multilayer PCBs. Flexible printed circuit boards (FPCs) include conventional PCBs and IC packaging carriers (COB: Chip on Flex). FPCs also include single-sided flexible circuit boards, double-sided flexible circuit boards, and multilayer flexible circuit boards. Currently, multilayer boards include conventional through-hole interconnect multilayer boards and stacked multilayer boards (HDI boards), and FPCs also include these two types of multilayer boards with different structures and processes.
Single-layer FPC
It features a chemically etched conductive pattern layer, with the conductive pattern layer on the flexible insulating substrate being rolled copper foil. The insulating substrate can be polyimide, polyethylene terephthalate, aramid fiber ester, or polyvinyl chloride. Single-layer FPCs can be further divided into the following four subcategories:
1. Uncovered single-sided connecting wire patterns are on an insulating substrate, with no covering layer on the surface of the wires. Their interconnection is achieved by soldering, fusion soldering, or pressure welding, and was commonly used in early telephones.
2. Single-sided connection with a cover layer: Compared to the previous type, this type only has an additional cover layer on the surface of the conductors. When using the cover layer, the pads must be exposed; for simpler designs, the end areas can be left uncovered. This is the most common and widely used type of single-sided flexible PCB, found in automotive instruments and electronic devices.
3. The double-sided connection interface without cover can be connected to both the front and back of the conductor. A through hole is made in the insulating substrate at the pad. This through hole can be made by punching, etching or other mechanical methods at the required position in the insulating substrate.
4. Unlike the previous type with a double-sided connection, this type has a cover layer on the surface. The cover layer has through holes, allowing it to be terminated on both sides while still maintaining the cover layer. It is made of two layers of insulating material and one layer of metal conductor.
Double-sided FPC
Double-sided FPCs have etched conductive patterns on both sides of the insulating base film, increasing the wiring density per unit area. Metallized vias connect the patterns on both sides of the insulating material to form conductive paths, satisfying the design and functional requirements of flexibility. A cover film can protect single- or double-sided conductors and indicate the placement of components. Depending on the requirements, metallized vias and cover layers are optional, and this type of FPC is less commonly used.
Multilayer FPC
Multilayer FPCs are made by laminating three or more layers of single-sided or double-sided flexible circuits together . Metallized vias are formed through drilling and electroplating, creating conductive paths between the different layers. This eliminates the need for complex soldering processes. Multilayer circuits offer significant functional differences in terms of higher reliability, better thermal conductivity, and easier assembly.
FPC production process
| Cutting | FPC material is manufactured in roll form. In order to meet the different size requirements of products, the optimal utilization rate must be planned and designed according to the different product sizes, and the material is cut into the required size according to the planning results. |
| drilling | Drill holes of the required diameter on the surface of the part according to customer requirements to facilitate insertion, circuit conduction, welding, or drilling of protective film openings and positioning holes. |
| Platinum | A layer of copper is deposited on the surface of the FPC, and the vias are metallized so that they can act as conductors during subsequent electroplating for via metallization. |
| Electroplated copper | The copper layer on the FPC and the walls of the vias is thickened to a thickness sufficient to resist subsequent processing and environmental impact. |
| Copper foil surface treatment | To improve the adhesion of the film, the surface of the copper foil must be cleaned before applying the film. |
| screen protector | After the through-holes are plated, a dry film is bonded to the cleaned material using a heating and pressurizing method to serve as an etching inhibitor. |
| exposure | The material with the film applied is transferred to the dry film by using image transfer, which transfers the circuit pattern designed on the film negative to the dry film through ultraviolet exposure. |
| development | After exposure, the dry film in the area of the material exposed to ultraviolet light undergoes a polymerization reaction. The unexposed areas are then washed away by the developing chemicals, exposing the copper layer of the material. |
| Etching | After the material is developed, it is rinsed with etching solution, which removes the copper layer from the exposed parts that are not protected by the dry film, leaving the protected circuitry. |
| Film removal | After etching, the board surface still has a hardened dry film. Chemical solutions are used to completely separate the dry film from the copper layer, allowing the circuitry to be fully exposed to the copper layer. |
| Apply cover layer | A protective film (an insulating material) that meets customer requirements is applied to the surface of the circuit board. This insulating layer prevents the circuit from being oxidized and scratched, thus providing protection. |
| Hot pressing | The bonded materials are then subjected to high temperature and high pressure through hot pressing to melt the adhesive of the cover film, which fills the gaps between the circuits and tightly bonds the copper foil material and the cover film. |
| Apply reinforcement | Reinforcing plates are used to enhance the hardness of welded or gold-finger areas on flexible circuit boards. They are typically attached to the bottom of the welded or gold-finger areas. Commonly used reinforcing materials include P-based substrates, FR4-based substrates, PET-based substrates, and steel sheets. |
| character | The text, trademarks, or part numbers required by the customer are screen-printed onto the board surface, and then the ink is hardened by heating or ultraviolet radiation. |
| Surface treatment | After the material is hot-pressed, the exposed copper foil areas must be plated with different metals such as tin, silver, or nickel-gold by electroplating or chemical plating according to the customer’s requirements to protect the exposed parts from further oxidation and ensure that they meet performance requirements. |
| punching | To meet customers’ precision requirements for outer angles, as well as the testing needs of other high-quality and high-requirement products such as precision, fineness, and density, the positioning holes are made by punching to ensure that the positioning holes are consistent with the graphic lines, etc. |
| test | Electrical testing of FPCs is performed to ensure that the quality meets customer requirements. |
| punching | We use steel molds, die-cutting tools, or laser cutting to shape the customer’s design, separate unwanted waste materials and circuit boards, and perform FPC drilling, FPC groove processing, and related parts finishing. |
| FQC | The FPC is comprehensively tested according to the customer’s specific requirements or IPC inspection standards to meet the customer’s quality standards. |
| FQA | From the customer’s perspective, we conduct sampling inspections of products to ensure their reliability and special characteristics. |
| Package | When shipping FPC, the packaging method is determined according to the external dimensions to ensure that the product is not damaged or defective during transportation. |
Introduction to FPC Flexible Circuit Board Surface Treatment Process
| process | advantage | shortcoming |
| FPC flexible circuit board immersion gold | It is not easily oxidized, can be stored for a long time, has a smooth surface, and is suitable for soldering components with fine-gap leads and small solder joints. It can be repeatedly reflowed without significantly reducing its solderability. | The cost is high, and the welding strength is relatively poor. Because it uses an electroless nickel plating process, it is prone to blackening issues. The nickel layer will oxidize over time, which raises concerns about long-term reliability. |
| FPC flexible circuit board immersion silver | Simple manufacturing process, suitable for lead-free soldering and SMT. Very smooth surface, low cost, suitable for very fine circuitry. | Storage conditions are demanding, and it is prone to contamination. Welding strength is susceptible to problems (microvoid issues). Electromigration and galvanic etching of the copper under the solder mask are also likely to occur. Electrical testing is also problematic. |
| Tin-plated FPC flexible circuit board | Suitable for horizontal line production. Suitable for fine line processing, lead-free soldering, and especially suitable for crimping technology. Excellent flatness, suitable for SMT. | Good storage conditions are required, ideally no more than 6 months, to control solder whisker growth. Not suitable for contact switch designs. The manufacturing process requires high precision in the solder mask application; otherwise, the mask may peel off. For repeated soldering, N2 gas protection is recommended. Electrical testing is also an issue. |
| Organic solderability preservative (OSP) for FPC flexible circuit boards | The process is simple, the surface is very smooth, and it is suitable for lead-free soldering and SMT. It is easy to rework, production operations are convenient, and it is suitable for horizontal line operations. It is low-cost and environmentally friendly. | Limited reflow soldering cycles; unsuitable for crimping and wire bonding techniques. Inconvenient for visual inspection and electrical testing. Requires N2 gas protection during SMT. Not suitable for SMT rework. Requires stringent storage conditions. |
| FPC flexible circuit board with nickel-gold plating | Long storage time >12 months. Suitable for contact switch design and gold wire bonding. Suitable for electrical testing . | Higher cost due to thicker gold plating. Additional conductive lines are required during gold plating of fingers. Inconsistent gold thickness can lead to brittle solder joints and reduced strength during soldering due to excessive gold thickness. There are also issues with the uniformity of the plating surface. The nickel-gold plating may not completely cover the edges of the wires. |
Advantages of FPC Flexible Printed Circuit Boards
Flexible printed circuit boards (FPCs) have many advantages in the electronics industry, mainly in the following aspects:
| Flexibility and bendability | FPCs can be bent, folded, and twisted, making them suitable for applications requiring three-dimensional cabling. They can adapt to complex internal structures of devices, reducing the use of connectors and cables. |
| Reduce weight and save space | FPC flexible printed circuit boards are very thin (typically less than 0.2 mm) and lightweight, which helps reduce the overall weight of electronic devices and save internal space, making them especially suitable for thin and portable electronic products. |
| High-density cabling | Flexible printed circuit boards (FPCs) enable high-density circuit routing, allowing more functionality to be accommodated within a limited space. They can be used in high-density interconnect (HDI) technologies to support small-sized components and complex circuit designs. |
| Reduce contact points and improve reliability | Because FPC flexible boards can directly connect various components, they reduce the number of plug-in connectors and solder joints commonly found in traditional rigid board connections, thereby reducing the risk of contact failure and improving system reliability. |
| Good thermal management | Flexible printed circuit boards (FPCs) have good thermal conductivity, enabling effective heat dissipation and making them suitable for high-power-density electronic devices. Furthermore, their flexible material better accommodates thermal expansion and contraction, reducing the impact of thermal stress on the circuitry. |
| Lightweight and low cost | Although FPCs have high initial design and manufacturing costs, in mass production they can reduce overall costs by reducing the use of connectors and cables, especially in complex circuits and small devices. |
| Excellent dynamic performance | FPCs can operate in dynamic environments and are suitable for applications that require frequent movement and bending, such as printers, cameras, and foldable mobile phone screens. |
| Easy to install and maintain | Due to their flexibility and lightweight nature, FPC flexible printed circuit boards (FPCs) are easier to install and wire. Furthermore, they can be rolled and stacked as needed, simplifying the installation process and reducing maintenance difficulty. |
| Multifunctional integration | Flexible printed circuit boards (FPCs) can integrate multiple functions such as signal circuits, antennas, and shielding layers onto a single board, reducing system complexity and improving the overall design’s compactness and functionality. |
| Strong environmental adaptability | FPC flexible printed circuit boards can maintain good performance in high temperature, high humidity and corrosive environments, making them suitable for harsh working conditions such as automotive electronics, aerospace and military equipment. |
FPC International’s Quality Management Standards
| International IEC | IEC60326-7(1989) Flexible Printed Circuit Boards with No Through-Connection Single or Double-Sided Printed Circuit Boards IEC60326-8 (1989) has through-connection single and double-sided flexible printed circuit boards IEC60326-9(1991) Flexible Multilayer Printed Circuit Boards IEC60326-10(1991) Rigid-flex double-sided printed circuit boards IEC60326-11(1991) Rigid-flex multilayer printed circuit boards |
| China GB GJB | GB/T14515(1993) provides for through-connection single and double-sided flexible printed circuit boards. GB/T14516(1993) Flexible Printed Circuit Boards with No Through-Connection Single and Double-Sided Printed Circuit Boards GB/T4588.10(1995) Standard for through-connected rigid-flex double-sided printed circuit boards GB/T13555(1992) Flexible copper-clad polyimide film for printed circuits GB/T13556(1992) Flexible copper-clad polyester film for printed circuits GB/T14708(1993) Coated Polyester Film for Printed Circuit Boards GB/T14709(1993) Coated Polyimide Films for Printed Circuit Boards GJB20604(1996) General Specification for Flexible and Rigid Printed Circuit Boards GJB2830 (1997) Requirements for Flexible and Rigid-Flexible Printed Circuit Board Design |
| JapanISJPCA | JISC5016 (1994) Test Methods for Flexible Printed Circuit Boards JISC5017 (1994) Flexible Single- and Double-Sided Printed Circuit Boards JISC 6471 (1994) Test methods for copper-clad laminates for flexible printed circuit boards JISC6472 (1994) Copper-clad laminates for flexible printed circuit boards – Polyester film, polyimide film JPCA-BM03(2003) Copper-clad laminate for flexible printed circuit boards (Chinese version of CPCA/JPCA-BM03 available) JPCA-FC-01 Flexible Single-Sided Printed Circuit Board JPCA-FC-02 Flexible Double-Sided Printed Circuit Board JPCA-FC-03 Flexible Printed Circuit Board Appearance Standard |
| US IPC | IPC 2223 (1998) Specification for Flexible Printed Circuit Board Design IPC4202(2002) Flexible Insulation Substrate for Flexible Printed Circuit Boards IPC4203(2002) Flexible cover film and adhesive sheet for flexible printed circuit boards IPC4202(2002) Flexible Copper Clad Laminate for Flexible Printed Circuit Boards IPC4562 (2000) Printed Circuit Copper Foil IPC 6013A (2003) Qualification and Performance Specification for Flexible Printed Circuit Boards (Chinese version available from CPCA/IPC 6013A) IPC/JPCA6202(1999) Performance Guidelines for Single- and Double-Sided Flexible Printed Circuit Boards MIL-P-50884C(1993) Flexible and Rigid-Flexible Printed Circuit Boards IPC-A-600F (1999) Printed Circuit Boards (4.1 Flexible and Rigid-Flex Boards) |
Common applications of FPC
| Computers and peripherals | HDDs, laptops, transmission cables, printers, scanners, keyboards, PDAs, calculators, electronic notebooks, etc. |
| Communication and office equipment | Mobile phones, multi-function telephones, fax machines, photocopiers, fiber optic switches, laser communication devices, etc. |
| Consumer electronics devices | Cameras, video recorders, mini radios, CD players, VCD and DVD players, game consoles, LCD and plasma TVs, etc. |
| Automotive electronics | Display instruments, engine electronic controller, ignition and circuit breaker system, exhaust controller, anti-lock braking system, brake components, audio and video system, in-vehicle mobile phone and satellite positioning system, etc. |
| Industrial Instruments and Equipment | Sensors, electronic instruments, nuclear magnetic resonance analyzers, X-ray machines, laser or infrared light control instruments, electronic weighing instruments, etc. |
| medical devices | Cardiac physiotherapy devices, pacemakers, electro-vibrators, endoscopes, digital signal processing hearing aids, ultrasound diagnostic instruments, nerve activation devices, diagnostic equipment, and programmable controllers, etc. |
| Aerospace and Military | Artificial satellites, spacecraft, rocket and missile controllers, remote sensing and telemetry devices, radar systems, navigation devices, gyroscopes, intelligence and reconnaissance equipment, anti-tank rocket artillery weapons, etc. |
| integrated circuit | IC packaging substrate, IC magnetic card core board, etc. |
The future development direction of FPC: to help products become smaller, more sparse, and lighter.
Flexible printed circuit boards (FPCBs) have evolved from single-layer to double-sided, multi-layer, and flexible designs, and continue to develop along their respective trends. Driven by continuous advancements in precision, density, and reliability, along with reductions in size, cost, and performance, flexible PCBs maintain strong viability in the future development of electronic devices.
Geyuan Electronics ‘ description of future trends in flexible printed circuit board (FPCB) manufacturing technology is largely consistent: development is moving towards higher density, higher precision, smaller apertures, finer conductors, finer pitch, higher reliability, multilayering, high-speed transmission, lighter weight, and thinner profiles. Simultaneously, production is moving towards increased productivity, lower costs, reduced pollution, and adaptability to multi-variety, small-batch production. The technological development level of flexible printed circuits is generally represented by the linewidth, aperture, board thickness , and aperture-to-diameter ratio on the FPCB .
According to the latest market information , flexible printed circuits (FPCs) are widely used due to their thin and light characteristics, especially in the field of smartphone modules. They are used in display modules, touch panel modules, camera modules, LiDAR modules, battery modules, and antenna modules, and it is not uncommon for a single model to use 10 or more FPC modules.
Furthermore, in the automotive sector, the application of flexible printed circuits (FPCs) is continuously advancing with the development of electrification, and this trend is expected to continue. Currently, FPCs are mainly used in automotive navigation systems, headlights, switches, and tire pressure sensors.
One of the main advantages of using flexible printed circuit boards (FPCs) lies in their function as printed circuit boards (wiring materials). By significantly simplifying and optimizing wiring and connection work, the time required for wiring inspection and repair can be reduced. Furthermore, since FPCs are only tens to hundreds of micrometers thick, they contribute to the reduction of the final product size. This not only allows for more efficient use of space within the final product’s housing but also reduces weight per unit volume.
Request an FPC manufacturing quote now
Ready to elevate your electronics projects with high-quality FPCs? Contact Geyuan Electronics today for a custom quote for your next FPC manufacturing project. Our professional team will provide full support to ensure your designs are realized accurately and exceptionally.
Summarize
In recent years, the consumer electronics market, led by mobile electronic devices such as smartphones and tablets, has experienced rapid growth, with an increasingly pronounced trend towards miniaturization and thinner designs. Consequently, traditional PCBs can no longer meet the demands of these products. As a result, major manufacturers have begun researching new technologies to replace PCBs, and FPC (Flexible Printed Circuit) is becoming the most favored technology and is emerging as a key connection component for electronic devices.
Furthermore, the rapid rise of emerging consumer electronics markets such as wearable smart devices and drones has brought new growth opportunities for FPC products. Simultaneously, the trend towards displays and touchscreens in various electronic products has allowed FPCs to enter a wider range of applications through small and medium-sized LCD screens and touchscreens, leading to increasing market demand. A recent report indicates that flexible electronics technology will drive a trillion-dollar market in the future, representing an opportunity for China to achieve leapfrog development in its electronics industry and potentially becoming a pillar industry of the nation. If you need a professional printed circuit board manufacturer for mass production of FPCs, please contact Geyuan Electronics for assistance.