High-frequency, low-loss FPCs (flexible printed circuit boards) are the key medium for signal transmission in 5G communication equipment; their material properties directly determine the transmission speed, stability and coverage of 5G signals. As 5G technology evolves from the Sub-6GHz band to the millimetre-wave band, and application scenarios expand from consumer devices to industrial internet, aerospace and automotive radar, the performance limitations of traditional FPC materials—such as low loss, high stability and high integration—are becoming increasingly apparent in high-frequency signal transmission.
The realisation of 5G’s core advantages—ultra-high bandwidth, ultra-low latency and ultra-wide connectivity—relies on the technical support of high-frequency, low-loss FPCs. Unlike the 4G era, signal transmission in the 5G millimetre-wave band (30GHz–300GHz) imposes stricter requirements on materials in terms of dielectric properties, dimensional stability and heat dissipation capabilities.
Traditional PI (polyimide) materials, due to their relatively high dielectric constant and large loss factor, are prone to signal attenuation and distortion in high-frequency scenarios and can no longer meet the demands of high-end 5G equipment. Based on the application scenarios and technical characteristics of 5G, the new requirements for high-frequency, low-loss FPC materials have gradually become clear, becoming the core direction for technological upgrading in the PCB industry.
Upgraded Dielectric Properties
In 5G high-frequency signal transmission, the dielectric constant (Dk) and loss factor (Df) are key indicators for evaluating FPC material performance, and represent the most critical requirements of 5G communications for FPC materials. The higher the dielectric constant, the more severe the signal attenuation; the larger the loss factor, the greater the energy loss. This directly impacts the communication range and signal quality of 5G devices, a factor that is particularly pronounced in the millimetre-wave band.
Traditional PI materials typically have a Dk ranging from 3.2 to 3.8 and a Df between 0.005 and 0.01. Whilst these values can still meet basic communication requirements in the Sub-6GHz band, signal loss in the millimetre-wave band can reach tens of decibels per metre, making it difficult to achieve high-speed, stable data transmission.
5G communications set clear targets for the dielectric properties of high-frequency, low-loss FPC materials: Dk should be controlled below 3.0, Df below 0.003, and dielectric properties must remain stable across the entire 5G operating frequency band, with a temperature coefficient of less than 50 ppm/°C to avoid signal fluctuations caused by temperature changes.
These requirements have driven the evolution of FPC materials towards lower dielectric constants and lower loss, with LCP (liquid crystal polymer) and modified PI becoming the mainstream choices. LCP can achieve a Dk as low as 2.9–3.1 and a Df of just 0.002–0.003. It also has an extremely low water absorption rate, with no significant drift in the loss-frequency curve before and after moisture absorption, effectively reducing high-frequency signal attenuation.
It is widely used in core components such as 5G terminal antennas and base station AAUs (Active Antenna Units). Modified PI, on the other hand, optimises Dk to below 3.0 through the addition of ceramic fillers and other methods, balancing flexibility with low loss to meet the cost requirements of mid-to-low-end 5G devices.
Improved dimensional stability
The trend towards miniaturisation and high integration in 5G devices places greater demands on the dimensional stability of FPC materials. Whether in consumer devices such as 5G smartphones and wearable devices, or industrial equipment such as base stations and automotive radar, there is a need to integrate more RF modules and antenna units within limited space. This necessitates FPCs with thinner profiles and finer circuit patterns, whilst maintaining excellent dimensional stability during processing and use to prevent circuit displacement or board warping caused by thermal expansion or moisture absorption.
Dimensional stability is primarily reflected in three aspects: coefficient of thermal expansion (CTE), moisture absorption rate and dimensional shrinkage rate. Traditional PI has a relatively high CTE (approximately 50–70 ppm/°C), making it prone to thermal expansion and contraction during high-temperature processing or long-term use, which can cause circuit misalignment or board deformation. The operating temperature range for 5G equipment is typically -40°C to 85°C, imposing stricter requirements on material thermal stability: the CTE must be controlled below 25 ppm/°C, post-etching curl must be ≤1.0%, and dimensional stability (E-0.5/150) must be ≤±0.05%.
Water absorption is also a key factor affecting dimensional stability. When materials absorb water, they expand and deform, leading to a decline in dielectric properties. The water absorption rate of high-frequency, low-loss FPC materials for 5G applications should be controlled below 0.8% to ensure stable dimensional and dielectric properties in humid environments (such as outdoor base stations and in-vehicle scenarios). At the same time, the dimensional shrinkage rate must be kept within 0.1% to ensure that the board exhibits no significant deformation following processes such as etching and lamination, thereby meeting the demands of high-density wiring.
To meet these requirements, the industry is adopting a dual-pronged approach involving both material modification and process optimisation. For example, adding reinforcing fillers such as glass fibre to LCP can reduce the CTE to 15–20 ppm/°C; modified PI reduces water absorption and enhances thermal stability through molecular structure optimisation. Furthermore, the use of adhesive-free flexible copper-clad laminates helps to minimise dimensional deviations caused by adhesive layers, thereby further improving the dimensional accuracy of FPCs.
Heat Dissipation and Reliability Upgrades
5G equipment generates significant heat during high-frequency, high-power operation, particularly in devices with limited space and heat dissipation capabilities, such as millimetre-wave base stations and vehicle-mounted radars. If the thermal performance of FPC materials is inadequate, it can lead to increased equipment temperatures, which in turn affects signal stability and shortens the service life. At the same time, the application scenarios for 5G equipment are becoming increasingly complex (indoor, outdoor, in-vehicle, etc.), placing higher demands on FPC reliability metrics such as temperature resistance, moisture resistance and vibration resistance.
In terms of thermal dissipation performance, the key lies in improving the thermal conductivity of the material. The thermal conductivity of traditional FPC materials is typically 0.3–0.5 W/m·K, which is insufficient to meet the demands of high-power 5G equipment. Currently, the thermal conductivity of high-frequency, low-loss FPC materials for 5G applications needs to be increased to 1.0 W/m·K or higher.
This can be achieved by adding thermally conductive fillers such as graphene and aluminium oxide to the substrate, or by embedding metal substrates or thermally conductive adhesive layers.
Accelerated Domestic Substitution
The rapid development of the 5G communications industry has driven a surge in demand for high-frequency, low-loss FPC materials; however, the high-end materials market remains dominated by Japanese and American firms. Core materials such as LCP film are primarily reliant on imports, which are expensive and subject to supply risks, thereby affecting the autonomy and control of China’s 5G industrial chain.
Consequently, promoting domestic substitution and improving the cost-effectiveness of materials have become another key requirement for FPC materials in the 5G communications sector.
The core objective of domestic material development is to break the monopoly of imported products and achieve mass production and cost optimisation, whilst meeting requirements for dielectric properties, dimensional stability, and thermal dissipation. Currently, domestic enterprises have begun to invest in the research, development and production of high-end FPC materials such as LCP and modified PI.
Companies such as Kingfa Science & Technology have participated in drafting national standards for LCP films, driving the standardisation and regulation of domestic materials and gradually narrowing the performance gap with imported materials.
At the same time, the widespread adoption of 5G equipment also demands materials with a higher cost-performance ratio. Applications such as mid-to-low-end 5G terminals and civilian base stations have relatively modest performance requirements, so there is no need to blindly pursue high-end imported materials. Domestic modified PI materials, which offer good performance, have become the preferred choice for such applications due to their cost-performance advantage.
This requires domestic enterprises to optimise production processes and reduce production costs whilst improving performance, thereby achieving ‘performance compliance and reasonable pricing’.
Furthermore, the green development trend in 5G communications imposes environmental requirements on FPC materials: materials must comply with environmental standards such as halogen-free and low VOC emissions, reduce pollutant emissions during production, and align with global environmental policies. This is also a key direction for the upgrading of domestic materials, driving the industry’s transition towards green manufacturing.
The iterative upgrades in 5G communications have not only reshaped the landscape of the telecommunications industry but have also driven a new round of technological innovation and demand upgrades for high-frequency, low-loss FPC materials. Low dielectric constant and low loss, excellent dimensional stability, efficient heat dissipation, reliable environmental adaptability, and the development focus on domestic production and cost-effectiveness collectively constitute the core requirements for high-frequency, low-loss FPC materials in the 5G era, whilst also charting the course for material R&D and technological upgrades within the PCB industry.
