With the deployment of 5G and the outlook toward 6G, flexible printed circuits (FPC) are facing more stringent requirements in high‑frequency, high‑speed signal transmission, and dielectric constant (D_k) and dielectric loss factor (D_f) are key parameters determining signal attenuation and time delay. This paper reviewed the current state and application challenges of key insulating materials for FPC, focusing on the performance advantages, modification strategies, and industrialization progress of modified polyimide (MPI) and liquid crystal polymer (LCP) as high‑frequency dielectric substrates. It also analyzed the challenges in formulation design, filler selection and process compatibility of modified polyimide and polyolefin systems used for high-frequency pure adhesive films. In addition, the potential of low‑D_k/D_f polymers such as fluorinated resins was evaluated, the trade-offs among electrical properties, processability and cost were discussed, and suggestions for future research and industrialization pathways of high-frequency FPC materials were proposed.

With the rapid development of AI technology, AI servers have raised higher requirements for the dielectric properties of polyphenylene ether (PPO) materials. Conventional PPO has a high dielectric constant, making it difficult to meet the requirements of high-frequency signal transmission in AI servers for low dielectric constant, low dielectric loss, and excellent thermal stability. This paper reviewed the research progress of low-dielectric and low-dielectric loss PPO materials in recent years from both physical and chemical modification perspectives, summarized the application and development of modified PPO in core components of AI servers, and provided an outlook on its future development prospects.

Inspired by the molecular architecture of liquid-crystalline polyester resins that exhibit inherently low dielectric loss at high frequency, we incorporated ester moieties and fluorinated substituents for reducing dielectric constant, into polyimide backbones to develop polyimides with good processability, high mechanical performance, and low-dielectric characteristics. Firstly, effect of the number of ester linkages per repeat unit, their connection topology, and their chemical identity (phenyl vs. naphthyl esters) on the dielectric properties, mechanical properties, thermal properties, and water absorption of polyimides were systematically explored. On this basis, -CF₃ pendent groups were subsequently introduced into ester-containing diamines to evaluate their influence on the dielectric, mechanical, and thermal properties of the resulting polyester-imides. The results demonstrate that the rigid and linear topology of ester-bearing monomers enhances the chain packing and crystallinity of polyimides, leading to a substantial reduction in dielectric loss of polyimides together with low water absorption, high tensile strength and excellent thermal properties. Concurrently, the incorporation of trifluoromethyl substituents does not disrupt the dense chain packing and crystallinity of polyester-imide chains, thereby preserving the low dielectric loss, low water absorption and high thermal properties of polyester-imides while further reducing the dielectric constant and improving mechanical robustness. These findings can provide theoretical guidance for the molecular design of high-performance polyimides with low dielectric constant and low dielectric loss for high-frequency applications.

Four fluorene-containing polyimide (PI) resins without per- and polyfluoroalkyl substances (PFAS), including PI-I_a (BPAF-BAOFL), PI-II_a (BPFDA-BAOFL), PI-I_b (BPAF-FDAADA), and PI-II_b (BPFDA-FDAADA) were prepared from two fluo-rene-containing dianhydrides 9,9-bis[(3,4-dicarboxy)benzene]fluorene dianhydride (BPAF, I) and 9,9-bis[4-(3,4-dicarboxyphenoxy)phenyl]fluorene dianhydride (BPFDA,II) and the fluorene-containing diamines 9,9-bis[(4-aminophenoxy)phenyl]fluorene (BAOFL,a) and 9,9-bis[4-(4-aminobenzamide)phenyl]fluorene (FDAADA,b), respectively via two-step chemical imidization procedure. The corresponding PI films were prepared by the solution processing procedure. The results show that the derived PI resins show good solubility in polar N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc) and cyclopentanone (CPA). The incorporation of fluorene substituents endows the PI films good thermal resistance with the glass transition temperatures (T_g) over 298℃. The BAOFL-based PI films (PI-I_a and PI-II_a) exhibit low dielectric features with the dielectric constants (D_k) and dielectric dissipation factors (D_f) in the range of 2.87 to 3.20 and 0.006 6 to 0.012 3, respectively at 10 GHz. While the FDAADA-based PI films (PI-I_b and PI-II_b) shows a bit deteriorated dielectric feature due to the existence of polar amide units (-CONH-). However, the high-temperature dimensional stability of the FDAADA-based PI films is improved with the linear coefficient of thermal expansion (CTE) values as low as 38.9×10^-6 K^-1.

Two fluorine-containing polyimide (PI) resins, including PI-1(6FDA-BATFB) and PI-2(6HDA-BATFB) were prepared from two aromiatic dianhydrides 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 4,4′-(isopropylidene)diphthalic anhydride (6HDA) and an aromatic diamine 1,4-bis[(4-amino-2-trifluoromethyl)phenyl]benzene (BATFB), respectively via two-step chemical imidization procedure. For comparison, two referenced PI resins, including PI-ref1 (6FDA-DMBZ) and PI-ref2 (6HDA-TFMB) were prepared using the dianhydrides 6FDA and 6HDA and the diamines 2,2′-dimethylbenzidine (DMBZ) and 2,2′-bis(trifluoromethyl)benzidine (TFMB) as the monomers, respectively. The corresponding PI films were prepared by the solution processing procedure. The testing results show that all the PI resins have good solubility not only in the polar aprotic solvents of N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF), but in the common solvents of cyclopentanone (CPA) and tetrahydrofuran (THF). The incorporation of trifluoromethyls endow the PI-1 and PI-2 films good dielectric features. The dielectric constants (D_k) of PI-1 and PI-2 at 10 GHz are 2.97 and 3.01, respectively, and the dielectric dissipation factors (D_f) of PI-1 and PI-2 are 0.007 7 and 0.006 3 for PI-2, respectively, which are superior to those of the referenced PI films. As for the thermal properties, the glass transition temperatures (T_g) of the terphenyl-bridged PI-1 and PI-2 films is over 309℃, and the linear coefficient of thermal expansion (CTE) value is lower than 50.0×10^-6 K^-1, which are a bit inferior to those of the referenced PI films. Meanwhile, the PI-1 and PI-2 films exhibit good optical transparency with the optical transmittance higher than 75% at the wavelength of 450 nm.

To address the stringent requirements of 5G/6G high-frequency and high-speed communication technologies for printed circuit board (PCB) substrates, a polytetrafluoroethylene (PTFE)-filled modified bismaleimide-triazine (BT) resin/glass fiber (GF) composite was designed and fabricated in this study. The glass fiber fabric was surface-modified with silane coupling agent KH570 to enhance interfacial bonding, and low-dielectric PTFE filler was introduced into the glass fiber cloth pores via high-temperature and high-pressure lamination process. The influence mechanism of PTFE content on the comprehensive properties of the composite was systematically investigated. Results demonstrate that when the PTFE mass fraction is 5%, the composite achieves the optimal comprehensive performance. The dielectric constant and dielectric loss are 4.3 and 0.006 at 1 MHz, and 3.4 and 0.003 at 10 GHz, respectively. The glass transition temperature increases to 250℃ (approximately 25℃ higher than that of the pure BT system), and the residual char yield at 800℃ reaches 76.63% (an increase of about 17.55%). The flexural strength is maintained at 574.7 MPa, and the water contact angle increases to 86.5°. Mechanism analysis reveals that the introduction of PTFE significantly improve the high-frequency dielectric properties of the composite by reducing the polarization degree, filling pores to decrease interfacial polarization losses, and restricting the orientation movement of polar groups. Simultaneously, the high bond energy of C-F bonds endows the composite with enhanced thermal stability. However, the high coefficient of thermal expansion (CTE) of PTFE raises the CTE of composite to approximately 45×10^-6℃^-1, which requires further optimization. This material system achieves significant improvement in dielectric properties while maintaining the high heat resistance and good processability of BT resin, providing a cost-competitive substrate solution for mid-to-high frequency applications such as 5G millimeter-wave antennas and radio frequency front ends.

To address the bottleneck that thermal conductivity and dielectric properties are difficult to be synergistically enhanced for high-frequency and high-speed electronic packaging materials, silica (SiO₂) was modified via surface grafting with methylvinyldimethoxysilane. The modified SiO₂ (m-SiO₂) particles were incorporated into thermosetting polyphenylene ether (MPPO) through solution blending, and m-SiO₂/MPPO composites with low dielectric performance and high thermal conductivity were successfully prepared. The results demonstrate that the thermal conduction capability and thermal stability of the composites are remarkably improved, while its favorable dielectric properties are well maintained. At an m-SiO₂ mass fraction of 70%, the thermal conductivity of the composite reaches 1.18 W/(m·K), which is 4.92 times higher than that of neat MPPO. Moreover, the dielectric constant is 3.11 at 10 GHz, and the dielectric loss factor is 0.004 4. Thermogravimetric analysis reveals that the thermal decomposition temperatures (at 5% and 10% weight loss) of the composite under nitrogen atmosphere are increased by 224.1℃ and 181.4℃, respectively compared with neat MPPO. It is verified that m-SiO₂ can effectively inhibit the thermal degradation of polymer segments through bonding reaction with the resin matrix.

In this study, tetramethylbisphenol A (TMBPA) was used as the bisphenol source, and a dihydroxy-terminated oligomeric polyphenylene ether resin (TMBPA-MPPO) was synthesized via monomer copolymerization. Further, through a nucleophilic substitution reaction, TMBPA-MPPO was end-capped with chloromethylstyrene (VBC) to prepare a crosslinkable polyphenylene ether resin MPPO-VBC. The product structure was characterized by FT-IR, and the dielectric properties, thermal properties, and moisture absorption of the cured product were systematically evaluated. The results show that MPPO-VBC has good curing activity, and after curing it exhibits excellent dielectric properties (dielectric constant is 2.60, dielectric loss factor is 0.004 5 at 1 MHz) and heat resistance (T_g=200℃). On this basis, MPPO-VBC was blended with triallyl isocyanurate (TAIC), coated onto glass fiber cloth, and a composite was prepared. Its dielectric loss factor further decreased to 0.001 4, the glass transition temperature increased to 221.4℃, and the tensile strength and flexural strength reached 212 MPa and 267 MPa, respectively, demonstrating application potential in high-frequency copper-clad laminates.

To address the problem that it is difficult for high-frequency and high-speed electronic packaging materials to exhibit both high thermal stability properties and low dielectric properties, a novel biobased benzoxazine (M-f) was synthesized from 4-vinylguaiacol and furfurylamine using both solvent and solvent-free methods, and then the benzoxazine monomer was chemically grafted onto polyphenylene oxide oligomer (SA9000) through free radical copolymerization to fabricate a series of IPN resins. The structure, thermal, and dielectric properties of all resins were characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), dynamic thermo-mechanical analysis (DMA), and microwave network analyzer. The results demonstrate that both M-f and M-f/SA9000 IPN resins display relatively high glass transition temperatures (170-245℃), and high frequency low dielectric properties (dielectric constant<3, dielectric loss factor≤0.008) under 5 and 10 GHz, which is satisfied with the requirement of high frequency communications. In particular, the M-f and M-f/SA9000 IPN resins prepared via the solvent-free method exhibit superior high-frequency low dielectric properties compared with the corresponding samples fabricated by the solvent method. Therefore, the work provides a new strategy for the structural design and preparation method of high-performance electronic packaging biomaterials.

In order to solve the problems of poor dispersibility and weak interface bonding of nano hexagonal boron nitride (BN) in epoxy resin (EP), and to improve the comprehensive performance of EP composite, we used polydopamine (PDA) to modify the surface of BN, and prepared EP/m-BN composites. The microstructure and dispersibility of the modified BN was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), atomic force microscopy (AFM) and other methods. The thermal conductivity, mechanical properties, dielectric properties, and thermal expansion characteristics of EP and EP/BN composites were systematically tested. The results show that PDA forms a uniform coating layer on the surface of BN through self polymerization, introducing active functional groups such as hydroxyl and amino groups, which significantly inhibits the aggregation of BN and improves its dispersibility in EP. Performance tests indicate that the composite achieves optimal overall performance at an m-BN loading of 10%. Compared with pure EP, the thermal conductivity of EP/m-BN composite increases from 0.21 W/(m·K) to 0.47 W/(m·K), with an increase of 123.8%. The coefficient of linear thermal expansion decreases from 59.2×10^-6℃^-1 to 45.5×10^-6℃^-1, with a decrease of 23.1%. The tensile strength increases from 42.6 MPa to 58.6 MPa, with an increase of 37.6%. The elongation at break increases from 18.8% to 23.6%, with an increase of 25.5%. And within the frequency range of 10-10⁶ Hz, all EP/m-BN composites maintain a low dielectric constant of 3.5-4, and the composites with 5%-10% of m-SiO₂ show a dielectric loss close to pure EP and excellent stability.