Application and Development Trend of Low Dielectric Loss Materials in Flexible Printed Circuit Boards Field (Part II)

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Industry Landscape and Emerging Players

Constrained by this, the number of companies capable of mass-producing and supplying LCP film is also very small, with Japanese firms such as Murata and Kuraray leading the way, resulting in a highly concentrated industry landscape.

However, it is worth noting that in recent years, companies such as Kingfa Science & Technology, Prite, Water Co., Ltd., and JuJia New Materials have been actively investing in the R&D and industrialization of LCP resins and films.

In this process, Prite, leveraging its technological expertise and product advancements, currently leads the industry; its LCP film products are closest to mass production levels, signaling a promising path toward substitution.

Material Characteristics and Processing Complexity

Table 2 presents the molecular structures and melting points of typical LCPs. For LCP materials, processing difficulties exist not only during the material synthesis stage but are also significantly evident in downstream manufacturing processes. Both FCCL manufacturers and FPC manufacturers face corresponding process challenges.

Table 2: Typical structure of LCP molecule and its

FCCL Manufacturing Challenges and Process Routes

For FCCL manufacturers, the core challenge lies in how to fully leverage the thermoplastic properties of LCP to achieve a stable and reliable bond with copper foil, while simultaneously ensuring uniform film thickness and overall dimensional stability.

Achieving these performance metrics relies heavily on the precision of FCCL production equipment, the optimization of process parameters, control over the forming process, and the selection of appropriate copper foil.

Each of these steps directly impacts the quality and reliability of the final product, placing high demands on a company’s comprehensive manufacturing capabilities.

In addition to the common method of preparing FCCL by laminating LCP film, some manufacturers currently use modified LCP solutions and a coating process to produce FCCL.

This process is relatively simpler in terms of manufacturing and can achieve a stronger bond between the LCP and the copper foil.

However, to improve the applicability and film quality of the LCP solution coating process, it is often necessary to introduce corresponding modifying components into the material formulation, which to some extent compromises the excellent dielectric properties and low moisture absorption characteristics of LCP itself.

FCCLs produced via these two process routes differ in several key performance aspects; a detailed comparison of their characteristics is shown in Table 3.

Table 3. Performance Comparison of Different Types of LCP FCCL

FPC Processing Challenges and Structural Innovation

For manufacturers of flexible printed circuits (FPCs), processing LCP-based flexible copper-clad laminates also presents several technical challenges. First, the thermoplastic nature of LCP material itself results in relatively limited thermal stability.

Typical LCP materials have a melting point of around 300°C, a characteristic that restricts their use in high-temperature processes such as hot bar bonding.

Additionally, thermal processing steps during manufacturing—such as lamination and baking—pose challenges to interlayer adhesion, dimensional stability, and the consistency and uniformity of basic performance.

Second, the bond strength between LCP and copper foil is typically lower than that of traditional PI substrates, requiring manufacturers to pay special attention to soldering reliability.

At the same time, the interfacial bond strength between LCP and pure adhesive film after lamination is also affected by material properties, often necessitating specialized surface treatment processes and lamination parameter settings.

In addition to traditional FPC structures using pure adhesive film, the industry is also exploring all-LCP structural solutions that leverage LCP’s inherent thermoplastic properties (as shown in Table 4).

Table 4 Stacking design of different LCP FPC

This structure eliminates the need for a pure adhesive film, instead relying on the self-bonding properties of LCP in its molten state to achieve interlayer adhesion.

However, this approach requires lamination temperatures to exceed the melting point of LCP (approximately 300°C), placing higher demands on interlayer alignment accuracy, post-lamination performance stability, and consistency, and consequently testing the capabilities of FPC manufacturers in terms of equipment capacity and process control.

It is worth noting that manufacturers such as Murata have achieved mass production of multi-layer all-LCP FPC structures through specialized stacking designs and printed sintering processes.

However, this process currently faces limitations, including difficulties in controlling trace flatness and insufficient uniformity in interlayer LCP thickness.

Performance Advantages and Commercialization Barriers

In summary, compared to modified polyimide (MPI) solutions, the core advantage of liquid crystal polymer (LCP) flexible printed circuit (FPC) solutions lies in the stability of their dielectric properties resulting from lower moisture absorption, a benefit that is particularly evident in terms of signal transmission loss at high speeds.

The extremely low moisture absorption rate of LCP materials effectively reduces the drift in dielectric parameters (Dk/Df) caused by fluctuations in ambient humidity, thereby maintaining more stable impedance control and signal integrity in high-frequency applications.

However, the large-scale application of LCP solutions still faces significant challenges. LCP raw materials are inherently expensive, and the high concentration of the global supply chain results in relatively limited resources.

In terms of manufacturing processes, LCP films have a narrow processing window, imposing more stringent requirements on processes such as thermal lamination, drilling, and metallization, significantly increasing the process difficulty compared to MPI.

These factors collectively limit the scope of LCP-FPC applications in cost-sensitive scenarios or those with limited manufacturing capabilities.

• Fluorinated Resins

Fluorinated materials demonstrate significant potential in the field of high-end electronic packaging, primarily due to their unique molecular structure and chemical properties.

› Intrinsic Advantages of Fluorinated Materials

The -CF₃ group, as a typical strong electron-withdrawing group, not only possesses extremely high electronegativity (approximately 3.98), but its rigid backbone and large steric hindrance also effectively suppress molecular chain motion, thereby reducing polarization loss during dielectric response.

At the same time, the extremely low polarizability of the C-F bond (approximately 1.54 D) and weak intermolecular interactions confer excellent hydrophobic properties on the material, keeping its equilibrium moisture absorption below 0.05%.

This characteristic is crucial for maintaining the stability of high-frequency signal transmission. From a materials chemistry perspective, the C–F bond energy is as high as approximately 485 kJ/mol, conferring exceptional thermal stability (thermal decomposition temperature > 400°C) and chemical inertness.

It can withstand the corrosion of strong acids, strong bases, and organic solvents, maintaining stable dielectric properties (D_k ≈ 2.1–2.4, D_f < 0.001) even under extreme conditions.

This suite of properties makes fluororesins ideal candidate materials for high-frequency applications such as 5G millimeter-wave communications and aerospace electronics.

› Mechanical Limitations and Composite Solutions

However, pure fluororesin films, such as PTFE, exhibit significant shortcomings in mechanical properties, making it difficult to meet the mechanical reliability requirements of flexible printed circuits.

Consequently, in practical applications, systems combining MPI with fluororesin are often employed to enhance mechanical strength; however, this approach typically comes at the cost of sacrificing some electrical properties, as illustrated in the comparative data in Table 5.

Table 5. Comparison of Key Properties of Fluororesin Composite Films

› Processing and Manufacturing Challenges

On the other hand, conventional fluororesins have extremely low UV absorption, resulting in a narrow process window during microfabrication processes such as laser drilling.

This necessitates the development of specialized processing parameters and optimization of the resin system to improve processability.

Additionally, the high stability and low surface energy of fluororesins result in significant surface inertness, making it difficult to achieve stable bonding with other materials.

This also poses severe challenges during post-drilling cleaning and electroplating processes, placing extremely high demands on FPC manufacturers’ process expertise and process control capabilities.

› Regulatory Pressure and Future Material Trends

More critically, increasingly stringent global environmental regulations (such as the EU PFAS ban and RoHS 2.0) are restricting the production and use of perfluoroalkyl substances (PFAS).

Although fluororesins can achieve a D_k/D_f of 2.15/0.0008 at 10 GHz, far exceeding that of traditional PI materials, their raw material and processing costs are high, they are difficult to process, and they face supply chain sustainability challenges.

These technical bottlenecks, combined with policy restrictions, are hindering the large-scale adoption of fluororesins in high-end electronics, driving the industry to accelerate the development of new, low-dielectric, and easily processable non-fluorinated alternative materials.

• Pure Resin Films for High-Frequency FPCs

In the conventional manufacture of FPCs, epoxy resin (EP) is typically used as the pure resin film for bonding CVL to the FPC or bonding different FCCLs together.

The epoxy system also includes additional curing agents and fillers; the curing agents are used to form a cross-linked network, and common curing agents are typically polyfunctional primary or secondary amines.

Fillers (such as SiO₂, Al₂O₃, etc.) in the pure adhesive film are primarily used to improve the microstructure and final properties after curing, such as increasing modulus, improving thermal stability, and adjusting electrical characteristics.

Additionally, the viscosity and flowability of the adhesive film during the curing process can be controlled by selecting appropriate particle sizes and surface treatments, controlling the filler content, and adding dispersing and rheological additives.

› Limitations of Epoxy Adhesives in High-Frequency Applications

However, conventional epoxy adhesives have poor D_k/D_f characteristics and cannot meet the electrical performance requirements of high-frequency FPCs.

To address high-frequency transmission needs, high-frequency pure adhesive films must be selected, typically based on MPI or polyolefin systems, with the introduction of epoxy groups for modification.

This achieves a balance between low Dk/Df and adhesive functionality, thereby reducing signal transmission loss in FPCs. We collected and compared the transmission losses of conventional film-FPC and high-frequency film-FPC; the results are shown in Figure 5.

Fig.4 Comparison of transmission loss between

Fig.5 Comparison of transmission loss between conventional

› Material Performance Comparison and Structural Differences

Conventional epoxy resin films, high-frequency polyolefin resin films, and modified polyimide films exhibit significant differences in mechanical properties, electrical characteristics, and heat resistance, as shown in Table 6.

Taking polyolefin resin as an example, polyolefins are composed of nonpolar carbon-carbon and carbon-hydrogen bonds. They exhibit high molecular symmetry, low polarization in an electric field, and minimal energy loss;

In contrast, epoxy resin molecules contain strongly polar groups such as ether bonds and hydroxyl groups. These groups undergo significant dipole relaxation in an alternating electric field, resulting in higher polarization and energy loss.

Consequently, polyolefin materials exhibit extremely low D_k and D_f values, demonstrating excellent signal integrity in high-speed, high-frequency applications.

› Processing Challenges and Reliability Optimization

However, this molecular chain structure also results in a high elongation rate of the film, making it prone to punching-related issues during the precision processing of FPCs.

Furthermore, the interfacial adhesion between polyolefin films and copper foil or substrates, as well as their heat resistance after lamination, is significantly weaker than that of epoxy film systems containing polar groups.

To overcome these limitations, specific substrate pretreatment processes—such as plasma activation, copper foil surface roughening, and substrate baking—must be incorporated into actual FPC manufacturing.

These steps enhance interfacial adhesion and improve heat resistance, thereby ensuring the long-term reliability of the final FPC product in high-frequency applications.

Table 6. Common Pure Adhesive Films and Their Performance

Other Materials

In addition to the materials mentioned above, there are many other polymer film materials with excellent electrical properties; however, due to issues related to resource costs and processing characteristics, they have not been widely adopted in the high-frequency, high-speed FPC sector.

• PPO and BMI: High-Performance Engineering Resins

For example, polyphenylene oxide (PPO) is a high-performance engineering plastic that offers outstanding mechanical strength, thermal stability, and electrical insulation properties. Modified PPO, in particular, exhibits superior dielectric properties.

Bis-maleimide (BMI) resin is renowned for its exceptional heat resistance, electrical insulation, wave transmission, radiation resistance, flame retardancy, as well as good mechanical properties and dimensional stability.

Its molding process is similar to that of epoxy resin, leading to its widespread application in high-tech industries such as aerospace, mechanical manufacturing, and electronics.

• PEEK: Excellent Properties with Processing Limitations

Polyetheretherketone (PEEK), as a semi-crystalline thermoplastic, has been repeatedly proven to possess excellent dielectric strength, dielectric constant, and low dielectric loss.

Although its mechanical properties and glass transition temperature can be optimized by adjusting the annealing process, crystallinity, and molecular structure, PEEK still faces adaptability issues under the harsh process conditions—such as high temperatures and strong tensile forces—encountered during the manufacturing of flexible printed circuits.

• COCs: Low Dielectric Materials with Practical Constraints

Cycloolefin copolymers (COCs) are a class of advanced amorphous thermoplastics.

These materials are typically produced via metallocene-catalyzed copolymerization of cyclic monomers with linear olefins; their cyclic, non-polar repeating unit structure confers extremely low dielectric loss and water absorption on the material.

However, high monomer costs, low tensile strength, and low glass transition temperatures have limited their widespread practical application.

• Porous Structures: Ideal Dielectric vs. Mechanical Trade-offs

Air, as an ideal dielectric material with a dielectric constant close to that of a vacuum, has demonstrated that porous MPI materials possess excellent electrical properties, and various preparation methods have been reported.

However, the porous structure inevitably leads to issues such as reduced mechanical strength and weakened high-voltage resistance.

At the same time, this unique structure imposes higher demands on downstream manufacturing processes, thereby limiting its scope of application.

Conclusion

The increasingly stringent requirements for signal transmission speed and integrity in 5G/6G communication technologies necessitate the use of advanced dielectric materials with low D_k/D_f values in FPCs to reduce signal loss.

Currently, the high-frequency FPC materials sector is characterized by a landscape where “MPI dominates practical applications, while LCP represents the direction of performance.”

MPI has captured the mainstream market due to its excellent comprehensive performance and mature supply chain, whereas LCP, despite its top-tier performance, has a relatively limited scope of application due to cost and manufacturing complexity.

Future development will focus on further optimizing the comprehensive performance of MPI, overcoming raw material and manufacturing bottlenecks for LCP, developing non-fluorinated, eco-friendly alternative materials, and resolving process compatibility issues between high-frequency pure adhesive films and substrates, all to jointly drive the evolution of next-generation high-speed communication electronics.