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

6G is expected to become commercially available around 2030 and will deliver a quantum leap in performance.

Transmission speeds will increase to 50 times those of 5G, latency will decrease to one-tenth of 5G levels, and key metrics will achieve significant breakthroughs in connection density, mobility, and positioning accuracy.

6G-Driven High-Speed FPC Design and Material Selection

This technological evolution will strongly drive the implementation of cutting-edge applications such as native AI, VR devices, Level 5 autonomous driving, and collaborative robots, while also placing higher demands on the performance of underlying hardware.

As a key signal carrier, flexible printed circuit boards (FPCBs) play a crucial role; enhancing their transmission performance at high frequencies and speeds while reducing transmission losses is particularly important.

• Microstrip vs. Stripline Structures in High-Speed FPCs

The most common stack-up designs for high-frequency, high-speed FPCs typically employ microstrip or strip-line configurations, as shown in Figure 1. In a strip-line design, the transmission line is fully encapsulated by the dielectric substrate, with reference ground planes above and below.

Fig.1 Design of microstrip and stripline

This configuration offers strong signal immunity to interference and minimal electromagnetic radiation, making it generally suitable for frequencies above 10 GHz.

In contrast, a microstrip is a semi-exposed transmission line with a reference ground plane only on the underside.

It is susceptible to external interference and tends to radiate energy outward, making it generally suitable for frequencies below 10 GHz.

• Signal Loss Mechanism and Key Dielectric Parameters

In general communication technology, signal transmission loss (TL) is divided into conductor loss (TLC) and dielectric loss (TLD). The relationship between dielectric loss (TLD) and the dielectric constant (Dk) and dielectric loss (Df) of the dielectric material is as follows:

Formula 1

In this equation, K is a coefficient, f is the frequency, and c is the speed of light. The relationship between signal transmission delay T_d and the dielectric constant D_k of the dielectric material in communication technology is given by: T_d = KD_k^0.⁵, where K is a coefficient.

In the two mainstream flexible printed circuit (FPC) designs—microstrip and strip-line—the key electrical performance parameters of the insulating dielectric material in direct contact with the metal conductor layer, such as the dielectric constant (D_k) and the dielectric loss factor (D_f), play a decisive role in signal transmission quality and the degree of signal loss.

Fig.1 Design of microstrip and stripline

• FPC Material Systems and Their Impact on Performance

Common FPC materials primarily include flexible copper-clad laminate (FCCL), cover film (CVL), and bare film (BS), with their typical laminate structures shown in Figure 2.

Therefore, the dielectric properties of the dielectric layer in FCCL, as well as those of CVL and BS, directly affect the overall signal transmission performance of the FPC.

Fig.2: Two typical FPC stack-up designs

Against this backdrop, the selection of advanced materials with low dielectric constants and low dielectric loss has become one of the most effective ways to reduce signal dielectric loss and transmission delay.

Since the advent of FPC technology, research institutions and materials manufacturers have been continuously developing and optimizing dielectric layers and pure adhesive films.

In terms of material systems, there is a wide variety of traditional low-dielectric-loss polymers, including polyimide (PI), liquid crystal polymers (LCP), fluorinated resins, polyolefins (such as polypropylene and cycloolefin copolymers), polyphenylene oxide (PPO), polyetheretherketone (PEEK), bis-maleimide-triazine (BT), and benzocyclobutene (BCB).

However, most of these materials have not achieved widespread application in this field due to limitations in mechanical properties, temperature resistance, processability, or cost.

Currently, the two materials that have been successfully used as insulating dielectric films in high-frequency, high-speed FPCs are primarily modified polyimide (MPI) and liquid crystal polymers (LCP).

As for pure film materials, modified polyimide (MPI) and polyolefin polymers have become the mainstream choices for high-frequency applications due to their excellent comprehensive properties.

The Application Status of Low-Dielectric-Loss Materials in the Flexible Printed Circuit Board Industry

• Modified Polyimide (MPI)

Since DuPont introduced the first commercial polyimide (PI) film, this material has been widely and extensively used in the flexible printed circuit (FPC) industry due to its excellent overall performance.

However, faced with the increasingly stringent requirements for high-frequency and high-speed transmission in modern electronic products, traditional PI materials have reached their performance limits.

To overcome these limitations, industry and academia have undertaken systematic modification research, thereby significantly driving the evolution and development of modified polyimide (MPI) with superior performance.

Key Strategies for Reducing Dielectric Constant (Dk) and Loss (Df)

Modification methods primarily focus on reducing the material’s D_k and D_f. Currently, the technical approaches for lowering D_k have become diverse, and the relevant theoretical frameworks are maturing.

Common methods include increasing the free volume between molecular chains, reducing the polarity of molecular chains, and introducing functional fillers with low D_k. The nature of D_f stems from dielectric relaxation phenomena occurring under alternating electric fields, as well as energy loss caused by the rotation of polarizable units.

Therefore, the core approach to reducing D_f lies in suppressing the rotational response of polarizable units at different frequencies, with the key being to limit the contributions of dipole moment polarization and electronic polarization. Figure 3 lists common PI and MPI molecular structures.

Fig.3 Typical PI and MPI molecular structure

Material Design Innovations: Fluorination and Molecular Engineering

In material design, incorporating fluorine is an effective strategy for enhancing the overall performance of materials.

Fluorine atoms can significantly reduce the moisture absorption of polymers and improve their electrical insulation properties; consequently, several research teams have attempted to introduce fluorinated groups, such as -CF₃, into the PI backbone.

HE Jianhao et al.successfully developed PI films with a D_k value below 3.0, achieving low dielectric properties while effectively balancing the potential decrease in thermal stability caused by the introduction of fluorinated groups.

The team led by LIU Yiwu, on the other hand, constructed sub-nanoscale free-volume pores by introducing rigid, non-planar conjugated side chains with high steric hindrance into the rigid PI main chain, thereby effectively reducing the material’s D_k.

The prepared films exhibited significantly superior dielectric properties compared to commercially available PI products. The aforementioned methods all demonstrate innovative material design concepts and have successfully led to the preparation of MPI materials with excellent performance.

Approaches to Lower Dielectric Loss and Moisture Absorption

To reduce the water absorption and D_f of MPI films, the team led by LU Qinghua precisely controlled the density of ester and ether bonds in the molecular backbone to modify intra-chain interactions, thereby achieving the preparation of MPI films with D_f < 0.002.

Although this approach had some impact on thermomechanical properties, it provided a key strategy for molecular modification.

On the other hand, the addition of high-bandgap inorganic fillers (such as SiO₂ and Al₂O₃) is a common method for improving dielectric properties; however, polarization often occurs at the filler-matrix interface due to electrical mismatch, leading to an increase in the dielectric constant.

To address this, the team led by Yang Jing-hui effectively suppressed interfacial polarization and enhanced dispersion by in situ assembling pyrrole polymers on the MoS₂ surface, thereby achieving more effective control over dielectric properties.

Regulatory Considerations: PFAS Restrictions

However, it is worth noting that on January 13, 2023, five EU member states (Denmark, Germany, the Netherlands, Norway, and Sweden) jointly submitted a proposal to the European Chemicals Agency aimed at comprehensively restricting the production, placing on the market, and use of perfluoroalkyl substances within Europe.

The proposal defines PFASs very strictly, explicitly stipulating that: Any chemical containing a fully fluorinated methyl or methylene structure, where the carbon atom is not bonded to hydrogen, chlorine, bromine, iodine, or other such atoms, falls within the scope of the restriction.

Although the proposal is currently still in the legislative process and has not yet officially taken effect, potential regulatory compliance risks must be closely monitored when conducting future research on fluorinated modifications of MPI materials to enhance their dielectric properties.

Performance Advantages of MPI over Traditional PI

Compared to traditional polyimide (PI), modified polyimide (MPI) materials not only have lower Dk and Df values but also exhibit superior moisture absorption properties, making them more effective at maintaining signal integrity in high-frequency, high-speed applications.

Therefore, we fabricated FPCs using both MPI FCCL and PI-FCCL and compared their transmission losses; the results are shown in Figure 4.

As shown in Figure 4, the signal transmission loss of the FPC fabricated using MPI material is lower than that of traditional PI (all transmission loss data were collected under conditions of 25°C and 50% RH; the same applies below).

Industrial Adoption and Market Landscape

From an industrial chain perspective, the manufacturing complexity of MPI films in the flexible printed circuit (FPC) production process is comparable to that of traditional PI materials.

This means that both upstream material suppliers and downstream FPC manufacturers can achieve a smooth transition and rapid adoption using existing, mature process equipment, significantly lowering the barriers and costs associated with technology switching.

Fig.4 Comparison of transmission loss between

At the same time, the market landscape is quietly shifting. In recent years, numerous MPI film suppliers have emerged worldwide.

In addition to early international manufacturers such as Kaneka, PIAM, UBE, and Damai, domestic PI manufacturers, including Ruihuatai, domestic PI manufacturers such as Juxian, Guofeng, and Times Huaxin have also begun to focus on the development of MPI materials.

Table 1 lists the key performance characteristics of major manufacturers and their products (all D_k/D_f data were collected under conditions of 25°C and 50% RH; the same applies below).

Table 1. Performance Comparison of PI/MPI from Mainstream Manufacturers in the Market

Increasingly fierce competition has led to a growing abundance of material resources for FPC substrates (FCCL) and cover films (CVL), driving cost optimization and providing strong momentum for the widespread adoption of MPI.

Technical Challenges and Future Development Directions

However, it must be noted that although MPI materials have become an important choice in the field of high-frequency, high-speed FPC applications, their further development still faces a series of technical bottlenecks that urgently need to be overcome.

First, at the process level, the ability to fabricate thicker MPI materials is key to meeting specific circuit design requirements.

Second, regarding core performance, further reducing and stabilizing D_k and D_f across a wider range of frequencies and temperatures is the central challenge in achieving optimal signal integrity.

Third, the hygroscopic nature of the material is a persistent issue affecting the stability of its electrical properties; therefore, developing effective moisture-resistant strategies to mitigate the detrimental effects of water molecules on electrical performance is of paramount importance.

Finally, as the forms of flexible electronics diversify, how to further enhance the film’s bend resistance and fatigue resistance has also become a critical factor in determining the boundaries of its applications.

• Liquid Crystal Polymer (LCP)

LCP is a thermoplastic organic material whose molecular chains maintain a certain degree of ordered arrangement even in the molten state. This property gives it characteristics such as low dielectric constant, low dielectric loss, and low water absorption.

In particular, it exhibits a remarkably stable dielectric constant across a wide frequency range, making it one of the most promising polymer materials for 5G high-frequency applications. Apple began incorporating LCP antennas into its smartphones in 2017.

Manufacturing Challenges and Material Limitations

However, the molecular structure of LCP dictates that in its molten state, it adopts an ordered arrangement resembling matchsticks (the liquid crystal state), rather than the random “tangled strands” typical of ordinary polymers.

This inherent order is the source of its superior performance, but it also poses significant manufacturing challenges. During film formation, these “matchsticks” are highly prone to alignment along the stretching direction.

If not properly controlled, this leads to significant differences in mechanical properties and thermal shrinkage between the machine direction (MD) and transverse direction (TD) of the film—that is, excessive anisotropy—causing the film to warp or tear during subsequent processing due to uneven stress distribution.

Additionally, the resin used to produce the film must be a high-molecular-weight, high-purity “film-grade” LCP resin, which is difficult to synthesize.

Currently, a handful of international chemical giants (such as Celanese and Polymer) primarily control high-quality film-grade LCP resins, leading to a scarcity of upstream materials and persistently high costs.

Processing Methods and Industrial Constraints

The film formation processes for LCP films primarily involve two methods: blown film and cast film.

To improve film formability, adjustments to the resin formulation or process may be necessary. However, regardless of the process used, the resulting LCP green films generally suffer from poor performance uniformity.

Therefore, in actual production, manufacturers typically introduce an annealing process to improve overall performance and enhance uniformity.

Furthermore, globally, only a few Japanese manufacturers can stably supply LCP resin raw materials.

Continue reading: Part 2 «