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FPC Circuit Board Materials and Application

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FPC Circuit Board Materials and Application

The development of Flexible Printed Circuit Board (FPC) has undergone rapid advancements in recent times, which have resulted in a significant increase in market share and technological progress. The introduction of novel flexible PCB manufacturing technologies has facilitated the advantages of FPC, such as low weight, thinness, and flexibility, enabling them to find widespread applications across multiple industries.

As the performance of the substrate material is integral to the fundamental properties of printed circuit boards (PCBs), enhancing substrate performance becomes crucial to improving the technical performance of the PCB, including its flexible variant.

FPC, a type of flexible circuit board, is similar to traditional PCBs in its function. FPC soft boards find extensive use in a variety of applications, including FPC antennas, flex PCB connectors, wireless charging coil arrays, and more. The composition of FPC primarily consists of flexible material that features high thermal stability and good electrical insulation properties

Material Composition of an FPC PCB

● Insulating Substrate: The base substrate material is a crucial component in the manufacturing of both rigid and flexible printed circuit boards (PCBs). It provides the principal foundation upon which the entire PCB structure is built. For rigid PCBs, the most commonly used substrate material is FR-4, while for flexible printed circuit PCBs, polyimide (PI) film and PET (polyester) film are the prevalent materials. Other polymer films, such as PEN (polyethylene nphthalate), PTFE, and Aramid, are also available.

PI, a thermosetting resin, is currently the predominant material used in the production of Flex PCBs. It is highly regarded for its excellent mechanical properties, such as good tensile strength, high thermal stability (-200°C to 300°C), chemical resistance, exceptional electrical properties, exceptional robustness, and heat resistance. Moreover, it possesses unique flexibility features, unlike other thermosetting resins, even after thermal polymerization. However, PI resin exhibits relatively low tear strength and moderate moisture absorption.

Conversely, PET resin boasts adequate electrical and mechanical properties; however, it is unsuitable for direct soldering as it exhibits poor heat resistance. PEN, another substrate material used in Flex PCBs, provides medium-level performance, surpassing PET while not matching the quality of PI in terms of critical mechanical and electrical properties. Throughout the manufacturing process, Flex PCB manufacturers should employ the appropriate substrate material to ensure that the eventual PCB can function seamlessly for its intended application.

FPC PI Stiffener

● Liquid Crystal Polymer (LCP) Substrate: Liquid Crystal Polymer (LCP) substrate is rapidly emerging as a popular substrate material in Flex PCBs. LCP is gaining immense popularity due to its ability to mitigate the limitations of Polyimide (PI) substrate while retaining all its features. LCP boasts a remarkable moisture or humidity resistance level of 0.04% while exhibiting a dielectric constant of 2.85 at 1GHz, making it ideal for high-speed digital circuits and high-frequency RF circuits.

LCP substrate’s ability to address the inadequacies of other substrate materials has made it a staple in the Flex PCB manufacturing industry. Furthermore, the melted version of LCP, known as TLCP, can be injection molded and pressed to create a flexible PCB substrate that is readily recyclable.

Given the exceptional performance features attributed to LCP, Flex PCB manufacturers must consider integrating it as their preferred substrate material to manufacture high-performance Flex PCBs to fulfill the demands of modern industries.

● Adhesive: The adhesive sheet serves to bind the film to either the metal foil or to the film in the case of the cover film. Different adhesive sheets can be used for different film substrates. For instance, adhesive sheets designed for polyester substrates differ from those used for polyimide substrates, with the latter being classified into epoxy or acrylic variants. The bonding sheet is typically chosen based on its fluidity and thermal expansion coefficient. Additionally, copper clad polyimide laminates with no adhesive sheets require less maintenance, possess superior chemical resistance, and exhibit excellent electrical properties.

The use of acrylic adhesive sheets poses a challenge as their low glass transition temperature leads to significant contamination during the drilling process, which is difficult to eliminate, thereby negatively impacting the quality of metallized holes. Subsequently, other adhesive materials are deemed inadequate for interlayer bonding sheets of multilayer flexible circuits. Hence, polyimide materials are commonly employed, which exhibit a consistent coefficient of thermal expansion (CTE) when paired with polyimide substrates, eliminating the issue of dimensional instability in multilayer flexible circuits. Furthermore, polyimide adhesive sheets possess exceptional mechanical and electrical properties.

Therefore, Flex PCB manufacturers must choose the right adhesive sheet for a given substrate type to ensure that the resulting PCB possesses the necessary mechanical, electrical, and dimensional stability to function correctly in its intended application.

● PI Cover Layer: The use of traditional PI/adhesive coverlay has not always sufficed in satisfying the needs of modern flexible PCBs in terms of high density, dimensional stability, and environmental protection. Consequently, a more suitable alternative known as photo-imageable coverlay (PIC) with high flexibility resistance and comparable attributes to solder mask oil has been developed.

Thus far, liquid or film-type PIC based on modified epoxy or acrylic resin has been extensively studied and applied due to its excellent binding force, high resolution, and flexibility. However, the limited dimensional stability of PIC based on modified epoxy or acrylic resin when used in high-density PCBs and its inferior Tg and heat resistance are notable shortcomings.

Flex PCB manufacturers must consider the factors affecting their desired outcome to choose the most appropriate coverlay material for the intended application. Consequently, the selection of coverlay materials should be conducted carefully with a focus on achieving the desired flexibility, environmental protection, and dimensional stability for the final product.

● Flex PCB Stiffener: Stiffeners for flexible circuit boards are commonly categorized as follows: PI stiffener, FR4 stiffener, Steel stiffener, FED stiffener, and others. The thickness of PI stiffeners may vary and is denoted by two values following “PI”, which indicate the thickness of PI and adhesive, respectively, both measured in Mil. These values can be adjusted according to the specific requirements of the customer. For instance, PI stiffeners may come in thicknesses ranging from PI1/2 1/2 to PI11, PI21, PI31, and up to PI91 depending on the customer’s needs.

● Copper Foil: Copper foil is a conductive coating that is applied to insulating substrate before selectively etching conductive lines. Rolled copper foil and electrolytic copper foil are the two dominant types of copper foils used. Rolled copper foil features higher ductility and bend resistance compared to electrolytic copper foil, with elongation levels ranging from 20% to 45%, whereas electrolytic copper foil has an elongation of 4% to 40%. The most commonly employed copper foil thickness is 35um (10z). However, there are thinner foils such as 18um (0.50z), and thicker ones like 70um (2oz) and even 105um (30z) foils.

Electrolytic copper foil is formed via electroplating. The copper particle’s crystal state is such that it has a vertical needle-like shape, which makes it easier to produce vertical line edges during etching, which is advantageous in the manufacture of precision circuits. However, when the bending radius is less than 5m or when dynamic deflections are frequent, the use of needle-shaped copper particles is unsuitable. Rolled copper foil is usually used for flexible circuit substrates due to its malleable structure, featuring copper particles with horizontal shaft-like shapes that can accommodate numerous windings.

Therefore, Flex PCB manufacturers must carefully select the appropriate copper foil type considering several factors, such as flexibility and durability, among other aspects, to ensure the resulting design meets the specific application’s needs.

FPC Copper Foil

The Characteristics of Different Copper Foil Types Differ

The use of either rolled copper foil or electrolytic copper foil in Flex PCB manufacturing depends on several factors, including the desired properties of the final product. Although these two types of copper foils are produced using different methods, determining the superior choice can prove challenging.

When choosing the right type of copper foil, Flex PCB manufacturers must consider various design requirements, including flexibility, durability, and precision. Rolled copper foil’s superior ductility and bend resistance, coupled with its horizontal shaft-like copper particle shape, make it particularly suitable for flexible circuit substrates that accommodate numerous windings. In contrast, electrolytic copper foil features a crystal structure consisting of vertical needle-shaped copper particles, which makes it ideal for manufacturing precision circuits requiring vertical line edges during etching.

Ultimately, the selection of copper foil type is dependent on the specific application’s requirements, as the qualities of each type of copper foil offer distinct advantages in particular circumstances. Therefore, Flex PCB manufacturers must conduct careful assessments to determine the most suitable copper foil type and ensure that it meets the demands of the intended application, resulting in a successful end-product.

Two Types of Copper Foil

In some cases, Copper Sheet or Cu Copper may be referred to as a flexible circuit board material by older designers. However, it is worth noting that these terms represent distinct types of copper foil used in Flex PCB manufacturing.

Rolled Annealed Copper Foil, also known as RA Copper Foil, is one type of copper foil employed in the manufacturing of Flex PCBs. The other commonly used type of copper foil is Electrodeposited Copper, or ED Copper Foil.

● ED Vs RA Copper Foil Cost: In regards to the cost of flexible circuit boards, we have found that the average cost of manufacturing using ED Copper Foil is lower than that of RA Copper Foil. However, if there are differing opinions, we welcome input and invite interested parties to reach out to JarnisTech directly.

● ED Vs RA Copper Foil performance: ED Copper Foil is comparatively fragile when compared to RA Copper Foil, and therefore more prone to breakage during the manufacturing process. In contrast, RA Copper Foil offers greater flexibility, making it the preferred choice for FPC copper foil.

When selecting the appropriate copper foil type for Flex PCB production, it is essential to consider several factors, such as durability, flexibility, and precision requirements, among other considerations. As such, Flex PCB manufacturers must conduct comprehensive evaluations to determine the most suitable copper foil type for the intended application, which will ultimately result in the desired outcome for the finalized Flex PCB.

One-Stop FPC Circuit Board Manufacturer – JarnisTech

JarnisTech is a comprehensive provider of flexible PCB manufacturing services, including assembly, design, component sourcing and end-product assembly. Established in 2002, JarnisTech operates two state-of-the-art PCB production bases in Jiangsu and Jiangxi, as well as a flexible PCB assembly plant in Shenzhen.

We offer a broad range of flexible PCBs, including yellow polyimide PCBs with 1-12 layers, clear PCBs with 1-10 layers, single/double-sided flexible PCBs, flexible aluminum PCBs, aluminum LED PCBs, and HDI flexible PCBs.

Here are some of the benefits of having JarnisTech produce your flexible PCBs:

● JarnisTech provides turnkey manufacturing services for flexible printed circuit boards (FPC). Our services include FPC design, manufacture, PCB assembly, testing, and box-build assembly.

● We ensure high-quality flexible PCB production that meets strict industry standards such as ISO 9001, IATF 16949, UL, RoHS, and REACH.

● Our customers can benefit from our free and professional one-on-one engineering/design assistance for flexible PCBs.

● For high volume flexible PCB/PCBA orders, we offer complete samples and PCBA functional testing to ensure the highest standards of quality.

● We stand behind our manufacturing services and provide quality money-back/free-rework assurance and traceable flexible PCB/PCBA fabrication.

Clean Flexible PET Circuit Board

FPC Design Guidelines and Considerations

Here are some possible FPC design guidelines and considerations:

● Bend Radius: FPCs should be designed with bend radius in mind to avoid cracking or breaking of the material. Typically, the minimum bend radius should be a multiple of the thickness of the FPC.

● Trace Width and Spacing: The trace width and spacing are critical factors to consider as they affect the electrical performance and can impact manufacturability. They are typically determined by the specific application requirements and should meet the manufacturer’s specifications.

● Material Selection: FPCs can be made from different materials, each with unique properties such as thermal conductivity, flexibility, and dielectric strength. Material selection should be based on the specific application requirements.

● Connection Points and Pads: Connection points and pads should be designed with the specific components and connectors that will be used. The size and shape of these areas should allow for strong, reliable connections.

● Solder Mask and Silk Screen: The solder mask and silk screen layers should be designed to ensure that they cover the appropriate areas of the FPC and do not interfere with the electrical performance or manufacturability.

● EMI Shielding: FPCs can be susceptible to electromagnetic interference (EMI). Adding EMI shielding to the design can be important to minimize the impact of EMI on the electrical performance.

● Thermal Management: FPCs can generate heat, which can impact electrical performance and reliability. Thermal management should be designed into the FPC to avoid overheating and damage to the components.

● Testing and Validation: It is important to test and validate the FPC design to ensure it meets the application requirements and is manufacturable. Testing can include electrical performance, mechanical stress testing, and environmental testing.

The Distinction Between PET and FPC

● Polyethylene terephthalate (PET) is a common polymer used in manufacturing flexible printed circuit boards (FPCs). PET’s high transparency and low thermal expansion make it ideal for solar panels and display panels. FPCs, on the other hand, are highly adaptable devices that are suitable for a variety of applications, including high-performance display panels and indoor applications.

● Flexible Printed Circuit Boards (FPCs) are a cost-effective solution that saves valuable transit space. The size of traditional rigid PCBs can become unwieldy when populated with numerous components. In contrast, FPCs are easy to fabricate and assemble, thanks to their flexible design.

● FPCs are hybrids of integrated circuits (IC) and thin-film, printed circuit traces. Used in creating flexible circuit boards and electronic devices, FPCs incorporate IC chips into thin, flexible plastic sheets. These circuits can be utilized in various applications, including solar panels, electric cars, and aircraft, as well as emerging technologies such as aerial drones and wearable electronics.

● Applications of FPCs continue to expand, and high-speed electrical circuits demand consistent electrical characteristics. As a result, FPCs are relied upon for their ability to maintain signal integrity for a variety of applications, including cutting-edge technologies like aerial drones.

Flexible Printed Circuit Board (FPC) Applications

Flexible printed circuit boards (FPCs) have broad applications across various electrical and electronic sectors. Despite FPCs offering versatility and adaptability, their cost is relatively higher than rigid PCBs. Nonetheless, FPCs have found widespread use in various industries, including:

● Automotive Electronics: Flexible circuit boards are used extensively in automobile systems such as the anti-lock brake systems, airbag controllers, dash systems, and instrument panels. Their flexibility and capacity to fit in small spaces make them a preferred choice for engineers. Additionally, automotive components must endure exposure to heat, cold, and vibrations, which FPCs can handle without compromising on performance.

● Aerospace Components: The aviation industry uses flexible PCBs in a myriad of applications, including holographic waveguide helmet-mounted displays. These displays offer high-resolution 3D imaging. Moreover, FPCs are used in satellites to replace connectors prone to connectivity faults. Their ability to fold and bend several times, carry components, and interconnect different components make them ideal for aerospace applications.

● Computer Electronics: Limited space in computers necessitates the use of flexible PCBs to transmit signals. Desktop computers typically employ 4 and 6 layers of FPC boards, while laptops require 8-layer FPC boards to function effectively.

● SmartphonesSmartphones are multipurpose, portable, and compact, and flexible PCBs provide interconnectivity between components inside them. The FPCs fit comfortably into small spaces, making them safer and more durable. These boards are also heat and environment-resistant, making them ideal for use in smartphones.

● Medical Devices: Flexible PCBs are ubiquitous in medical devices such as gastroscopy, swallowable pill cameras, and glass hearing devices. Their thin, compact, foldable, and robust design make them suitable for medical equipment that requires high precision in confined spaces.

In addition to the industries mentioned above, flexible PCBs are also used in wearable devices, digital cameras, and other applications. These FPCs offer significant advantages, including their adaptability, high performance, and versatility, making them ideal for use in high-end devices.

Advantages of FPC Board

Flexible printed circuit board (FPC) technology offers several advantages that make it highly sought after for various applications. Some of the advantages of FPC technology include:

● Greater Flexibility: FPC technology allows the boards to be bent and shaped into various forms, offering new design possibilities and increased flexibility for circuit designers.

● Space-Saving: The flexibility of FPCs allows for smaller devices and tighter device packaging since they can be bent and folded to take up less space.

● High Reliability: With FPC technology’s high tensile strength, the boards can be deformed or bent extensively without breaking, making them highly reliable in various applications. Furthermore, FPCs can bend up to 360 degrees, making them highly adaptable in demanding applications.

● Enhanced Capabilities: FPCs utilize flexible flat cables (FFC), which can bend without breaking and take up less space than typical wires. FFCs provide increased design flexibility, lower weight, and improved signal speed and integrity.

● Cost Savings: FPC technology has also allowed for significant cost savings due to reduced material and packaging requirements, lower part replacement costs associated with their high reliability, and lower assembly faults that prohibit repairs.

In summary, FPC technology offers greater flexibility, space-saving, higher reliability, enhanced capabilities, and cost savings, making them highly desirable for various complex electronic device applications.

FPC Coverlay

Comparison of FPCs With Other Types of Circuit Boards

Here are some possible points of comparison between FPCs and other types of circuit boards:

● Flexibility: Unlike rigid circuit boards, FPCs are flexible, making them easier to fit into unique and curved designs.

● Size and Weight: FPCs are typically thinner, lighter and smaller than other types of circuit boards, which can be beneficial in applications where space is limited.

● Durability: FPCs can be more durable than traditional circuit boards because they are less prone to damage from vibration or bending, as long as they are designed and manufactured appropriately.

● Complexity: FPCs can be designed to carry multiple electrical signals, resulting in more complex designs and allowing for more advanced devices.

● Manufacturing cost: The manufacturing cost of FPCs can be higher than other types of circuit boards due to the process and equipment needed, although costs have been decreasing over time.

● Assembly process: Compared to traditional circuit boards, the assembly of FPCs typically requires a more specialized process and skillset.

● Resistance to high temperatures: FPCs can handle higher temperatures than traditional circuit boards, making them suitable for use in high-temperature applications such as automotive and industrial sectors.

Ultimately, the choice of circuit board type depends on the specific application requirements and design constraints.

Future Prospects of FPC Technology

The future prospects of FPC technology include the following:

● Miniaturization: FPCs will continue to become smaller and thinner, allowing them to fit into even more compact devices.

● Integration: FPCs will increasingly be integrated with other technologies such as sensors, RFID, and LEDs, leading to the creation of more sophisticated and intelligent devices.

● Durability: FPCs will continue to improve in durability, making them more suitable for harsh environments and long-term use.

● Cost Reduction: Development of more cost-effective manufacturing processes will bring down the cost of FPCs, making them more accessible to a wider range of applications.

Increased Applications: FPC technology will be utilized in more areas including the medical sector, wearable technology, and the automotive industry, leading to an expansion in the overall market for FPCs.


Flexible Printed Circuit (FPC) is an adaptable electronic component with a broad range of functions. Its versatility makes it a suitable choice for various applications that require a durable and cost-efficient embedded component. Its flexibility and capabilities open up a plethora of possibilities to designers to incorporate it into their electronic designs. FPC circuits are a perfect choice for a wide variety of products, including cell phones and solar cells, owing to their high level of functionality and dependability.