A microwave-absorbing, thermally conductive, and flame-retardant composite thin film with synergistic macro-micro-nano structures for 6G chips, its preparation method, and its application.

Abstract

This invention provides a macro-micro-nano synergistic microwave absorbing, thermally conductive, and flame-retardant composite film for 6G chips, along with its preparation method and applications. The composite film uses modified silicone rubber or polyimide as the matrix. Through a three-level synergistic design strategy—constructing a low-impedance core-shell interface structure at the nanoscale, building a three-dimensional interpenetrating thermally conductive network at the mesoscale, and controlling rheological properties at the macroscale—it achieves excellent comprehensive performance with a thermal conductivity ≥15 W / m·K and radar wave return loss ≤-30 dB. The preparation process includes four steps: nano-interface modification, eddy current dispersion based on rheological simulation, orientation coating based on scraper leveling simulation, and gradient curing. This invention, through a specific microstructure construction method guided by a theoretical model, overcomes the technical bottlenecks of discontinuous thermal conduction paths, uneven absorber distribution, and high interfacial thermal resistance in traditional thermally conductive and microwave absorbing materials. It is suitable for integrated electromagnetic compatibility and thermal management solutions for 6G high-frequency communication chips.

Description

Technical Field

[0001] This invention belongs to the field of functional composite materials technology, specifically relating to a microwave-absorbing, thermally conductive, and flame-retardant composite thin film with macro-micro-nano structural co-design for 6G communication chips, its preparation method, and applications. More particularly, it relates to a composite thin film material possessing high thermal conductivity, strong electromagnetic wave absorption capability, and excellent flame-retardant properties, along with its microstructure control strategy and preparation process. This composite thin film is mainly used in integrated solutions for thermal management and electromagnetic compatibility of high-frequency electronic components in 6G communication base station RF front-end modules, power amplifier packaging, millimeter-wave antenna arrays, and terminal devices. Background Technology

[0002] With the rapid development of 5G and even 6G communication technologies, the operating frequencies of communication chips have crossed millimeter waves (30~300GHz) and even entered the terahertz band (0.1~10 THz). The sixth-generation mobile communication system (6G) is expected to be commercially available around 2030, and its hallmark features include: full-band coverage (from the Sub-6G band below 6 GHz, to the millimeter wave bands of 28 GHz, 39 GHz, and 73 GHz, and then to the terahertz bands of 140 GHz, 220 GHz, and even higher frequencies), intelligent reflector (RIS) assisted communication, terahertz communication, and new technologies such as sensing communication and integrated communication-sensing computing (ISAC). As the core hardware carrier of these new technologies, 6G chips have operating frequencies, power consumption, and integration levels far exceeding those of the 5G era, placing unprecedentedly stringent requirements on the comprehensive performance of packaging materials.

[0003] From a thermal management perspective, the power density of 6G chips has reached several hundred W / cm². 2 The power density of 6G millimeter-wave base stations is on the order of magnitude, and due to the use of Massive MIMO antenna technology, a single base station module integrates dozens or even hundreds of RF front-end channels, each of which generates a large amount of heat from its power amplifier. It is estimated that the effective heat dissipation power density of 6G millimeter-wave base stations is 3 to 5 times that of 5G base stations. At the same time, for every 10°C increase in chip operating temperature, its reliability and lifespan decrease by approximately 50%. Therefore, 6G chip packaging materials must have a high thermal conductivity (≥10 W / m·K, preferably ≥15 W / m·K) to ensure that heat can be quickly and effectively dissipated from the active area of ​​the chip to the heat dissipation structure.

[0004] From an electromagnetic compatibility (EMC) perspective, 6G systems operate in extremely high frequencies, with electromagnetic waves having wavelengths on the order of millimeters or even sub-millimeters (corresponding to frequencies of 300 GHz to 3 THz). Their propagation characteristics are closer to light waves than traditional radio frequency (RF) waves. The skin effect and near-field coupling effect of high-frequency electromagnetic waves are more pronounced, leading to more severe electromagnetic interference (EMI) problems between different functional modules within a chip, between different chips, and between the chip and the PCB circuitry. Simultaneously, 6G communication systems place higher demands on the noise figure and dynamic range of the RF front-end. If the encapsulation film generates electromagnetic reflection or secondary radiation, it will significantly degrade the receiver's sensitivity. Therefore, 6G chip packaging materials must possess wide-bandwidth (covering 8–300 GHz) and strong absorption (return loss ≤-20 dB, preferably ≤-30 dB) electromagnetic wave attenuation capabilities to convert interfering electromagnetic energy into trace amounts of heat rather than reflecting it back.

[0005] From a safety and reliability perspective, 6G base stations and terminal equipment operate for extended periods in enclosed spaces (such as indoor micro base stations and vehicle-mounted terminals), imposing stringent requirements on the fire safety performance of materials. The UL94 flammability rating test for electronic equipment is an internationally recognized standard for evaluating the flame-retardant performance of materials. The V-0 rating requires the material to self-extinguish within 10 seconds and prohibits any burning droplets. Furthermore, 6G base station equipment is typically installed in high-rise buildings, bridges, streetlights, and other locations where a fire could have extremely serious consequences. Therefore, the encapsulation materials must also possess low smoke density and low toxic gas release characteristics.

[0006] However, existing thermally conductive and microwave-absorbing composite materials suffer from the following inherent technical bottlenecks, which severely restrict their practical application in the field of 6G chip packaging: (1) Structural defects: Traditional thermal conductive and microwave absorbing materials generally adopt a "sea-island" structure, with fillers randomly distributed in the matrix, resulting in discontinuous thermal conductive paths and uneven distribution of microwave absorbing agents. If the microwave absorbing filler is not evenly dispersed, the propagation path and attenuation behavior of electromagnetic waves inside the material are difficult to predict, making it difficult to achieve effective electromagnetic wave attenuation over a wide frequency band.

[0007] (2) Interface compatibility problem: The interface compatibility between the filler and the matrix is ​​poor, resulting in high interfacial thermal resistance.

[0008] (3) Difficulty in performance synergy: In existing technologies, the improvement of thermal conductivity often comes at the cost of sacrificing microwave absorption performance, and vice versa, forming an irreconcilable technical contradiction. In addition, the sharp increase in viscosity under high filling amount also leads to the deterioration of processing performance, making it difficult to prepare thin film products.

[0009] (4) Process adaptability issues: Traditional preparation processes (such as ball milling, stirring, three-roll milling, etc.) lack the ability to precisely control the microstructure and cannot convert the optimal filler distribution structure calculated by the theoretical model into a physical material.

[0010] Therefore, how to accurately construct a continuous thermally conductive network at the mesoscale, achieve the design of a low-impedance interface at the nanoscale, ensure good rheological processing performance at the macroscale, and simultaneously optimize the synergistic effect of wave absorption and thermal conductivity are key technical challenges that urgently need to be solved in the field of 6G chip packaging materials. Summary of the Invention

[0011] Based on the deficiencies of existing technologies, the first objective of this invention is to provide a microwave-absorbing, thermally conductive, and flame-retardant composite film with synergistic macro-micro-nano structures for 6G chips; the second objective of this invention is to provide a method for preparing the microwave-absorbing, thermally conductive, and flame-retardant composite film with synergistic macro-micro-nano structures for 6G chips; and the third objective of this invention is to provide the application of the microwave-absorbing, thermally conductive, and flame-retardant composite film as a packaging material in high-frequency electronic components of 6G communication base station RF front-end modules, power amplifier packaging, millimeter-wave antenna arrays, and terminal devices.

[0012] The core innovation of this invention lies in proposing the "core-shell-network" microstructure design concept, which decomposes the structural design of composite materials into three scales: nanoscale, mesoscale, and macroscale, and optimizes them in a coordinated manner. The three scales are interconnected and support each other, forming a complete structural and functional system.

[0013] First, at the nanoscale, an organic-inorganic hybrid transition layer (shell thickness 1-10 nm) is constructed on the filler surface using a silane coupling agent. This shell layer simultaneously possesses the following functions: (a) it forms a chemical bond with the filler core, ensuring strong adhesion; (b) the molecular structure of the shell layer contains active functional groups (such as epoxy, amino, acryloyloxy, etc.) that can react with the matrix, forming chemical bonds with the matrix during curing; (c) the thermal expansion coefficient of the shell layer is between that of the filler and the matrix, serving as a buffer layer to alleviate thermal stress caused by the mismatch in thermal expansion coefficients; (d) the phonon transport characteristics of the shell layer are superior to those of the original filler-matrix interface, reducing phonon scattering and decreasing interfacial thermal resistance. The general molecular formula of the silane coupling agent is YR-SiX3, where X is a hydrolyzable group (such as methoxy, ethoxy), Y is a functional organic functional group, and R is an alkylene chain. Preferred silane coupling agents include KH-560 (containing epoxy group), KH-550 (containing amino group), KH-570 (containing methacryloyloxy group), and phenyl-TMOS (containing phenyl group), whose molecular structures correspond to different reactivity and interfacial affinity properties.

[0014] Secondly, at the mesoscale, the eddy current stirring process parameters (rotation speed 1000~3000 rpm, time 5~30 min) were optimized through fluid dynamics simulation to form a three-dimensional interpenetrating network structure in the matrix of the thermally conductive filler. The core features of this network structure include: (a) the network node spacing is controlled in the range of 5~50 nm, at which point the thickness of the matrix interlayer between adjacent filler particles is comparable to the mean free path of phonons (approximately 1~10 nm in polymers at room temperature), resulting in the highest phonon heat transfer efficiency; (b) the network is isotropic or anisotropic, allowing for directional design according to heat dissipation requirements; and (c) "thermal hotspots" exist at the network nodes, and precise control of the node spacing can prevent local overheating. The microwave absorbing filler is designed to be dispersed in the mesh of the thermally conductive network, rather than participating in the construction of the thermally conductive network. This is the key to the decoupling design of thermal conduction and microwave absorption in this invention—the thermally conductive network is constructed by a dielectric filler with high thermal conductivity, while the microwave absorption function is undertaken by magnetic microwave absorbing filler uniformly distributed in the mesh. The two do not interfere with each other and each performs its own function. The particle size of the absorbing filler is multi-level (micrometer / submicrometer / nanometer). The micrometer-sized filler (1~10 μm) is mainly responsible for the absorption of the low frequency band (8~18 GHz), the submicrometer-sized filler (0.1~1 μm) is responsible for the absorption of the mid frequency band (18~60 GHz), and the nanometer-sized filler (10~100 nm) is responsible for the absorption of the high frequency band (60~300 GHz). The three levels of fillers work together to achieve broadband absorption.

[0015] Finally, on a macroscopic scale, the blade gap (10~100 μm) and coating speed (5~20 m / min) are calculated using a blade coating leveling model, combined with magnetic field-assisted orientation (0.1~2 T), to control the uniformity of wet film thickness and surface smoothness. The magnetic components (carbonyl iron, nickel-zinc ferrite) in the microwave absorbing filler exhibit magnetocrystalline anisotropy and align along the magnetic field direction under an applied magnetic field, forming a layered or chain-like ordered structure. This ordered structure has a better absorption effect on obliquely incident electromagnetic waves because there is an optimal matching angle between the polarization direction of the electromagnetic wave and the ordered layer. The gradient curing process after coating is crucial for eliminating internal stress and ensuring the dimensional stability of the film—temperature changes during curing cause matrix shrinkage and a mismatch in the thermal expansion coefficients of the filler and matrix. If the heating rate is too fast, an uneven temperature field and thermal stress will be generated inside the film, leading to warping or cracking. This invention employs a multi-stage gradient heating curing program, holding the film at each temperature step for a sufficient time to homogenize the temperature field, thereby minimizing residual stress.

[0016] The objective of this invention is achieved through the following technical solution: On the one hand, the present invention provides a microwave absorbing, thermally conductive and flame-retardant composite film with macro-micro-nano structure synergy for 6G chips, the composite film comprising a matrix and a filler system filled in the matrix; The packing system is selected from thermally conductive packing and / or microwave absorbing packing; the surface of the packing is modified with a silane coupling agent to form a nanoscale low interfacial thermal resistance transition layer.

[0017] In the above-mentioned microwave-absorbing, thermally conductive, and flame-retardant composite film, preferably, the matrix is ​​selected from modified silicone rubber matrix and / or polyimide matrix.

[0018] In the aforementioned microwave-absorbing, thermally conductive, and flame-retardant composite film, preferably, the modified silicone rubber matrix is ​​a product of addition-type silicone rubber or condensation-type silicone rubber modified with methacryloxysilane or epoxysilane.

[0019] In the aforementioned microwave-absorbing, thermally conductive, and flame-retardant composite film, preferably, the polyimide matrix is ​​selected from polyimide prepared by thermal imidization or chemical imidization of 3,4,3',4'-biphenyltetracarboxylic acid dianhydride and 4,4'-diaminodiphenyl ether, or is a fluorinated polyimide.

[0020] This invention selects modified silicone rubber or polyimide as the matrix, both of which exhibit excellent comprehensive performance in the 6G chip operating environment.

[0021] First, the advantages of the modified silicone rubber matrix are: (a) the molecular backbone is composed of Si-O-Si bonds with high bond energy (approximately 460 kJ / mol), giving the material excellent high and low temperature resistance (operating temperature range -60~250℃), which matches the temperature range of 6G chips from extremely cold outdoor environments to high-power operating temperatures; (b) silicone rubber has good flexibility and elasticity (elongation at break can reach 200%~500%), which can buffer the interfacial stress caused by the mismatch of thermal expansion coefficients and prevent the film from cracking; (c) silicone rubber itself has flame-retardant and self-extinguishing properties, and the SiO2 carbonized layer formed during combustion can block the transfer of heat and oxygen; (d) addition-cure silicone rubber (cured by vinyl hydrosilylation reaction) has no by-products during curing and low volume shrinkage (<1%), which is beneficial to maintaining the dimensional accuracy of the film; (e) this invention selects methacryloxysilane or epoxysilane to further modify the grafted addition-cure silicone rubber, so that its surface contains active functional groups that can react with the shell layer of silane coupling agent, further enhancing the interfacial bonding force.

[0022] Secondly, the advantages of polyimide matrix are: (a) polyimide has a glass transition temperature (Tg) as high as 300~400℃, which can maintain mechanical stability and dimensional accuracy in extremely high temperature environments; (b) polyimide has excellent dielectric properties (dielectric constant 3.0~3.5, loss tangent <0.005), which will not have a negative impact on microwave absorption performance; (c) fluorinated polyimide can further reduce the dielectric constant to 2.5~2.8 by introducing trifluoromethyl (-CF3) groups into the molecular chain, while improving hydrophobicity and flame retardant properties; (d) polyimide has self-flame retardant properties, forming a dense char layer during combustion, which effectively inhibits heat release and smoke generation.

[0023] In the aforementioned microwave-absorbing, thermally conductive, and flame-retardant composite film, preferably, the silane coupling agent is selected from one or more combinations of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, methacryloyloxypropyltrimethoxysilane, and phenyltrimethoxysilane.

[0024] In the aforementioned microwave-absorbing, thermally conductive, and flame-retardant composite film, preferably, the thermally conductive filler is selected from boron nitride and / or alumina, and the microwave-absorbing filler is selected from iron carbonyl and / or nickel-zinc ferrite. The advantages of iron carbonyl (CIP) are: (a) iron carbonyl powder is prepared by chemical vapor deposition (CVD), exhibiting high sphericity, high purity (≥99.5%), and excellent magnetic properties; (b) iron carbonyl possesses high saturation magnetization (Ms≈170~200 emu / g) and high permeability, demonstrating excellent magnetic loss characteristics at high frequencies; (c) iron carbonyl has a high Snoek limit, maintaining high permeability in the 1~10 GHz frequency band, making it the preferred material for X-band and Ku-band microwave absorption; (d) iron carbonyl is a conductive filler with high real and imaginary parts of its complex permittivity, which is beneficial for achieving strong absorption in the low-frequency band. This invention uses plate-like or spherical carbonyl iron with a particle size distribution of micrometer-level (1~10μm) and submicrometer-level (0.1~1μm) in combination. The advantages of nickel-zinc ferrite are: (a) Nickel-zinc ferrite is a spinel-structured soft magnetic material with moderate saturation magnetization (Ms≈ 50-80 emu / g) and high resistivity (ρ ≈ 10). 5 ~10 8 (a) The skin depth is large, and electromagnetic waves can penetrate deep into the material and be absorbed; (b) The dielectric constant of nickel-zinc ferrite is moderate (about 10~15), which is conducive to achieving good impedance matching, so that more electromagnetic waves can enter the material instead of being reflected by the surface; (c) Nickel-zinc ferrite has a wide operating frequency band, which can cover the S-band to the Ka-band (2~40 GHz), and can achieve a wider frequency band absorption when combined with carbonyl iron.

[0025] In the aforementioned microwave-absorbing, thermally conductive, and flame-retardant composite film, preferably, the mass ratio of boron nitride to alumina in the thermally conductive filler is 1:0 to 0:1; the volume fraction of the thermally conductive filler in the matrix is ​​5% to 40%. The advantages of alumina (Al2O3, especially α-Al2O3) are: (a) moderate thermal conductivity (30~40 W / m·K), good chemical stability, wide availability, and low cost; (b) dielectric constant of approximately 9.5, with relatively controllable influence on electromagnetic waves; and (c) precisely controllable particle size distribution, suitable for large-scale industrial production. In this invention, alumina can be used in any proportion with h-BN, and by adjusting the mass ratio of the two, an optimal balance can be achieved between cost, thermal conductivity, and dielectric properties.

[0026] In the above-mentioned microwave absorbing, thermally conductive, and flame-retardant composite film, preferably, the mass ratio of carbonyl iron to nickel-zinc ferrite in the microwave absorbing filler is 1:0 to 0:1; and the volume fraction of the microwave absorbing filler in the matrix is ​​5% to 30%.

[0027] In the aforementioned microwave absorbing, thermally conductive, and flame-retardant composite film, preferably, the particle size of the microwave absorbing filler is distributed in a three-level gradation: micron-sized particles with a diameter of 1~10 μm account for 20%~40% of the total microwave absorbing filler mass; submicron-sized particles with a diameter of 0.1~1 μm account for 30%~50% of the total microwave absorbing filler mass; and nano-sized particles with a diameter of 10~100 nm account for 20%~40% of the total microwave absorbing filler mass.

[0028] In the aforementioned microwave-absorbing, thermally conductive, and flame-retardant composite film, preferably, the thermally conductive filler is hexagonal boron nitride nanosheets with a diameter of 0.1~10 μm and a thickness of 10~200 nm, arranged in a two-dimensional layered structure. The advantages of hexagonal boron nitride (h-BN) are: (a) its crystal structure is similar to graphite, being a layered hexagonal structure with extremely high in-plane thermal conductivity (high-quality single-crystal h-BN can reach 300~600 W / m·K), and low inter-plane thermal conductivity (approximately 2~3 W / m·K), exhibiting significant anisotropy in thermal conductivity; (b) the dielectric constant of h-BN is approximately 4.0~4.5, and the loss tangent is extremely low (<10). -4(c) h-BN has good chemical stability and oxidation resistance, and is not easily oxidized and deteriorated at high temperatures; (d) This invention preferably uses h-BN nanosheets (BNNS) with a diameter of 0.1~10 μm and a thickness of 10~200 nm. Compared with micron-sized h-BN particles, BNNS has a larger specific surface area and a larger contact area with the substrate, which is conducive to building a denser thermally conductive network. Moreover, the planar orientation characteristics of BNNS are more conducive to forming an ordered thermally conductive path in the thin film plane. In this invention, the volume fraction of h-BN is controlled in the range of 5~40 vol.%. When the volume fraction is 20~25 vol.%, BNNS begins to form a percolation thermally conductive network, and the thermal conductivity shows a step increase.

[0029] In the above-mentioned microwave absorbing, thermally conductive, and flame-retardant composite film, preferably, the thickness of the composite film is 10~200μm, the thickness uniformity is ≤±5%, the thermal conductivity is ≥15 W / m·K, the radar wave return loss is ≤-30 dB, and the vertical combustion rating reaches V-0 level.

[0030] On the other hand, the present invention also provides a method for preparing the above-mentioned microwave absorbing, thermally conductive, and flame-retardant composite film, the method comprising the following steps: Step 1, Nanoscale interface modification: The thermally conductive filler and / or microwave absorbing filler are surface modified by sol-gel method or in-situ polymerization to construct a core-shell structure on the filler surface, wherein the shell is an organic-inorganic hybrid transition layer formed by silane coupling agent, and the shell thickness is 1~10 nm.

[0031] Step 2, Dispersion process based on rheological simulation: The surface-modified filler is mixed with the matrix and dispersed using eddy current stirring technology. The shape of the stirring blade and process parameters are controlled according to the fluid dynamics simulation results. The stirring speed is 1000~3000 rpm and the stirring time is 5~30 min, so that the filler achieves a homogeneous dispersion state without agglomeration in the matrix.

[0032] Based on the Navier-Stokes equations and the particle population equilibrium (PBM) model, a hydrodynamic model of the eddy mixing process was established to simulate the fluid velocity field, shear force field, and particle dispersion behavior under different mixing parameters (rotation speed, impeller shape, and mixing time). The model considered factors such as fluid viscosity, packing volume fraction, and particle density difference, and determined the optimal combination of mixing parameters through computational fluid dynamics (CFD) simulation. The optimization objective was to maximize the shear stress on the packing particles to break up agglomerates, while avoiding excessively high local flow velocities that could lead to bubble introduction. Simulation results show that when the Reynolds number Re is in the range of 1000–3000 (transitional flow to the initial stage of turbulence), eddy mixing can achieve the best balance between dispersion efficiency and energy consumption. At this point, the packing agglomerate size can be reduced to below 100 nm, and the dispersion index (DI value) can reach above 0.95.

[0033] Step 3, coating process based on doctor blade leveling simulation: The mixture obtained in Step 2 is coated by doctor blade coating. The doctor blade gap and coating speed are calculated according to the film leveling model. The doctor blade gap is 10~100 μm and the coating speed is 5~20 m / min. With magnetic field-assisted orientation, a wet film with specific thickness and flatness is formed.

[0034] Step 4, gradient curing: Set a gradient heating program according to the heat conduction model to cure the wet film. The curing temperature is 120~300℃ and the heating rate is 1~10℃ / min. Eliminate internal stress and obtain the composite film.

[0035] Based on the lubrication approximation theory and capillary flow equations, a leveling dynamics model of the wet film during blade coating is established to simulate the effects of blade gap, coating speed, liquid viscosity, capillary force, and gravity on film thickness uniformity and surface waviness. The model outputs the optimal combination of blade gap and coating speed, resulting in a wet film thickness standard deviation (σ) of less than 1 μm and a surface waviness Ra of less than 0.5 μm. The model also considers the influence of filler settling and flocculation on the rheological behavior of the wet film, minimizing these adverse factors by optimizing coating parameters.

[0036] Based on Fourier's law of heat transfer and thermoelasticity theory, a model of the temperature field and thermal stress distribution inside the film during the curing process is established. The model considers the exothermic curing reaction (for addition-cure silicone rubber, the addition reaction is exothermic, ΔH≈50~80 kJ / mol), matrix curing shrinkage, and the mismatch in thermal expansion coefficients between the filler and the matrix (CTE≈2×10⁻⁶ for h-BN). -6 / K, CTE of silicone rubber ≈ 2~3×10 -4 Factors such as / K (difference of approximately two orders of magnitude) are considered to determine the optimal combination of gradient heating rate and holding time, resulting in residual stress inside the film being less than 0.5 MPa after curing.

[0037] In the above preparation method, preferably, in step one, when using the sol-gel method for surface modification, the silane coupling agent is mixed with anhydrous ethanol and deionized water at a volume ratio of 1:(10~50):(1~10), the pH is adjusted to 4~6 with acetic acid, and the mixture is stirred and hydrolyzed at 25~80℃ for 2~24 h to form a sol; then the thermally conductive filler and / or microwave absorbing filler are added to the sol, and the mixture is stirred and refluxed at 40~120℃ for 1~6 h, and the surface-modified filler is obtained after filtration, washing and drying.

[0038] In the above preparation method, preferably, in step two, after dispersion is completed, vacuum degassing treatment is performed, wherein the vacuum degree is ≤-0.09 MPa and the time is 5~30 min, in order to eliminate the bubbles introduced during the mixing process.

[0039] In the above preparation method, preferably, in step three, the magnetic field-assisted orientation uses a permanent magnet or electromagnet to provide a stable magnetic field with an intensity of 0.1~2 T, and the direction of the magnetic field is perpendicular or parallel to the coating direction, so as to achieve the orientation arrangement of the microwave absorbing filler in the wet film.

[0040] In the above preparation method, preferably, in step four, the gradient heating program includes: heating from room temperature to 120°C at 3~8°C / min and holding for 10~30 min, heating from 120°C to 200°C at 1~5°C / min and holding for 10~30 min, and heating from 200°C to the curing temperature at 1~3°C / min and holding for 10~60 min.

[0041] In another aspect, the present invention also provides the application of the above-mentioned heat-absorbing, thermally conductive, and flame-retardant composite film as a packaging material in high-frequency electronic components of 6G communication base station radio frequency front-end modules, power amplifier packaging, millimeter-wave antenna arrays, and terminal devices.

[0042] The beneficial effects of this invention are: The wave-absorbing, thermally conductive, and flame-retardant composite thin film with macro-micro-nano structure synergy for 6G chips of the present invention has the following advantages compared with the prior art: (1) Excellent thermal conductivity: Due to the three-dimensional interpenetrating network structure formed by the thermally conductive filler in the matrix, the network node spacing is precisely controlled at 5~50 nm, which effectively shortens the phonon heat transfer path; at the same time, the nanoscale low impedance interface transition layer (shell thickness 1~10 nm) on the surface of the filler significantly reduces the interface thermal resistance. Compared with the unmodified filler, the interface thermal resistance is reduced by 60%~80%, so that the thermal conductivity of the composite film reaches 15~25 W / m·K, which is 50~80 times higher than that of the matrix material (<0.3 W / m·K), which can meet the technical requirements of high power heat dissipation of 6G chips.

[0043] (2) Excellent absorption performance: The uniform distribution of the absorbing filler in the thermally conductive network and the multi-level gradation design of the particle size (covering the micron, submicron, and nano scales) achieve broadband and strong absorption of electromagnetic waves in the 8~300 GHz wideband. Micron-sized carbonyl iron is responsible for the 8~18 GHz band (absorption peak -35 to -45 dB), submicron-sized carbonyl iron / nickel-zinc ferrite is responsible for the 18~60 GHz band (absorption peak -28 to -35 dB), and nano-sized carbonyl iron / nickel-zinc ferrite is responsible for the 60~300 GHz band (absorption peak -20 to -30 dB). The overall radar wave return loss can reach -30~-50 dB, and the effective attenuation rate (EA) ≥99.9%.

[0044] (3) Reliable flame retardant performance: The modified silicone rubber and polyimide matrix itself have self-flame retardant properties, and the filler modified with silane coupling agent has good interface bonding with the matrix, effectively suppressing heat release and smoke generation during combustion. The limiting oxygen index (LOI) of the modified silicone rubber-based composite film is ≥30%, and the vertical burning rating reaches UL94 V-0 level (3.0 mm thick sample, self-extinguishing within 10 seconds, no dripping); the LOI of the polyimide-based composite film is ≥38%, also reaching V-0 level, and the smoke density rating (SDS) during combustion is ≤50, which is much lower than that of ordinary polymer materials.

[0045] (4) Performance Synergistic Optimization: Through the synergistic design of the macro-micro-nano three-level structure, strong wave absorption is achieved while ensuring high thermal conductivity, thus solving the technical contradiction of difficulty in balancing thermal conductivity and wave absorption in traditional materials. In the optimal ratio scheme of this invention (h-BN 25 vol.% + carbonyl iron 15 vol.%), the thermal conductivity reaches 18.6 W / m·K, while the minimum return loss can still reach -38.2 dB, realizing bidirectional decoupling optimization of thermal conductivity and wave absorption.

[0046] (5) High process repeatability: The design of preparation process parameters based on theoretical models makes it possible to accurately replicate the microstructure and carry out large-scale industrial production. The product performance stability and repeatability are significantly better than those of traditional process routes. Validation in 8 batches of continuous production shows that the batch-to-batch standard deviation (σ) of the main performance indicators (thermal conductivity, return loss, thickness uniformity) is less than 5%, and the Cp and Cpk values ​​are both greater than 1.33, meeting the process capability requirements of industrial production.

[0047] (6) Wide range of applicable frequency bands: The composite film of the present invention covers the full 6G frequency band of 8~300 GHz, including Sub-6G (3~6 GHz), 5G millimeter wave (24~52 GHz), 6G low frequency millimeter wave (52~110 GHz), 6G high frequency millimeter wave (110~220 GHz) and even terahertz frequency band (220~300 GHz), which is a comprehensive solution for the 6G era.

[0048] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention will be described in detail below. Detailed Implementation

[0049] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. The processes, conditions, reagents, experimental methods, etc., for implementing the present invention, except for the contents specifically mentioned below, are all common knowledge and general knowledge in the art, and the present invention does not have any special limitations.

[0050] Example 1: Preparation of modified silicone rubber-based composite film This embodiment provides a method for preparing a microwave-absorbing, thermally conductive, and flame-retardant composite film using modified silicone rubber as the matrix and hexagonal boron nitride nanosheets (BNNS) and carbonyl iron (CIP) as fillers. The preparation method includes the following steps: (1) Nanoscale interface modification: γ-glycidoxypropyltrimethoxysilane (KH-560) was mixed with anhydrous ethanol and deionized water at a volume ratio of 1:20:3. The pH was adjusted to 5.0 with acetic acid, and the mixture was stirred and hydrolyzed at 60°C for 4 h to form a uniform and transparent silane sol. During the hydrolysis reaction, the methoxy group (-SiOCH3) was gradually replaced by the hydroxyl group (-OH) to form silanol (Si-OH), which is a prerequisite for the chemical bonding between the silane coupling agent and the surface of the inorganic filler.

[0051] With an average particle size of 5 μm and a specific surface area of ​​8 m², 250 g of hexagonal boron nitride powder ( / g) was added to the above sol and stirred and refluxed at 80 °C for 3 h. During reflux, silanol molecules formed Si-O-Si chemical bonds with hydroxyl groups on the surface of h-BN (h-BN nanosheets contain a small number of -OH functional groups due to oxidation) through dehydration condensation reaction. Simultaneously, condensation reactions also occurred between silane molecules, forming a cross-linked shell structure. After the reaction, the mixture was filtered and washed three times with anhydrous ethanol to remove physically adsorbed silane molecules. The mixture was then vacuum dried at 120 °C for 4 h to obtain the surface-modified boron nitride filler. FTIR spectroscopy analysis showed that the modified filler exhibited good performance at 1100 cm⁻¹. -1 (Si-O-Si stretching vibration) and 2900 cm -1 The appearance of a new absorption peak at (CH stretching vibration) confirms the successful grafting of the silane coupling agent.

[0052] Meanwhile, 30 g of carbonyl iron powder with an average particle size of 2 μm was taken and surface modified using the same sol-gel method. The amount of KH-560 coupling agent was 3 wt.% of the filler mass. A thin oxide layer (Fe2O3) will form on the surface of the carbonyl iron powder in air, which contains -OH functional groups and can undergo bonding reactions with silanols.

[0053] (2) Dispersion process based on rheological simulation: Take 100 parts (by weight) of addition-type silicone rubber modified with methacryloyloxysilane (vinyl content 0.5 mol%, viscosity 3000 mPa·s, Shore A 40) as the matrix, and add 30 parts of surface-modified BNNS filler and 20 parts of surface-modified carbonyl iron filler.

[0054] The mixture was added to a vacuum mixer and stirred in a vortex at 1500 rpm for 15 minutes under normal pressure to ensure thorough dispersion of the packing material. Based on computational fluid dynamics (CFD) simulation results, an asymmetric blade impeller (blade inclination angle 45°, blade width to container diameter ratio 1:3) was selected. At 1500 rpm (Reynolds number Re≈1500), the fluid exhibited a transition from laminar to turbulent flow, with the maximum shear rate at the impeller blade tip approximately 5000 s⁻¹. -1 This is sufficient to break up soft agglomerates with a particle size of up to 10 μm. Simulations also show that the velocity gradient distribution in the stirring zone is uniform, with no obvious dead zones or local turbulence zones, which is beneficial for the uniform dispersion of the packing material.

[0055] After dispersion, degassing is performed for 10 min under a vacuum of ≤ -0.095 MPa to eliminate air bubbles introduced during mixing. During degassing, the vacuum should be reduced slowly (in two stages) to avoid foam splashing caused by sudden decompression.

[0056] The final viscosity of the mixture is 8500 mPa·s (shear rate 10 s⁻¹). -1 The composite exhibited significant shear-thinning behavior (Power-law index n≈0.5), a typical rheological characteristic of high-filler composites. The dispersion index (DI value) was determined by a particle size analyzer, and the result was ≥0.95, indicating that the filler agglomerates were sufficiently dispersed.

[0057] (3) Coating process based on doctor blade leveling simulation: After the mixture obtained in step (2) was allowed to stand under vacuum for 2 hours to defoam, it was coated using a doctor blade coater. According to the film leveling model, the optimal doctor blade gap was set to 50 μm, the coating speed was set to 10 m / min, and a 25 μm thick polyimide film was selected as the substrate.

[0058] During the coating process, a constant magnetic field (intensity 0.5 T, using NdFeB N52 grade neodymium iron boron permanent magnets with remanence Br ≥ 1.45 T) perpendicular to the coating direction is simultaneously applied. The soft magnetism of carbonyl iron is utilized to achieve its oriented alignment within the wet film plane. Under the influence of the magnetic field, the carbonyl iron particles form chain-like aggregates along the magnetic field direction. These chain-like structures are fixed after the wet film cures, forming an ordered microstructure. When electromagnetic waves are obliquely incident on the film surface, the ordered magnetic chain-like structures form a specific angle with the polarization direction of the electromagnetic wave, which is beneficial for impedance matching and effective absorption of electromagnetic waves.

[0059] After coating, the film was leveled at room temperature for 10 minutes to further smooth the wet film surface under the action of capillary force and gravity. Simulation results of the leveling model showed that the leveling time of 10 minutes could reduce the waviness Ra of the wet film surface from 2.5 μm to 0.4 μm.

[0060] (4) Gradient curing: The gradient heating program designed based on the heat conduction model is as follows: First stage: room temperature (25℃) to 120℃, heating rate 5℃ / min. In this temperature range, the viscosity of silicone rubber decreases rapidly, and the filler may settle under gravity. Therefore, the holding time is set to 15min to make the temperature field fully uniform and avoid excessive holding time that may cause filler to settle. The second stage: 120℃ to 200℃, with a heating rate of 2℃ / min. In this temperature range, the addition curing reaction of the addition-curing silicone rubber begins (the initial temperature of the curing reaction is about 80℃, and the peak temperature is about 150℃). The slow heating rate of 2℃ / min can avoid uncontrolled temperature rise caused by the exothermic curing reaction. Third stage: 200℃ to 250℃ (curing temperature), heating rate 1.5℃ / min, and holding at this temperature for 30 minutes to ensure complete curing reaction.

[0061] The entire curing process is carried out under a nitrogen protective atmosphere (oxygen content <100 ppm) to prevent oxidative crosslinking or thermo-oxidative aging of the silicone rubber during high-temperature curing. After curing, the film is allowed to cool naturally to room temperature before being removed for performance testing.

[0062] Performance test results of the obtained composite film: Thermal conductivity (Hot Disk method, TPS 2200 thermal constant analyzer, 25℃): 18.6 W / m·K; Radar wave return loss (bow method, 2~18 GHz): minimum -38.2 dB@11.5 GHz, average ≤ -25 dB (covering the entire 8~18 GHz frequency band); Vertical flammability rating (UL94 V-0, 3.0 mm thick sample, self-extinguishing within 10 seconds, no dripping); Tensile strength (ASTM D412, dumbbell-shaped specimen, 25 °C): 4.2 MPa; Elongation at break (ASTM D412): 180%; Thickness: 50±3 μm (measured by cross-sectional method of optical microscopy); Thickness uniformity (measured at one point every 10 cm, n=10): ≤ ±4%; Surface roughness Ra (white light interferometer): 0.38 μm; Volume resistivity: 1.2 × 10⁻⁶ 14 Ω·cm (good insulation performance); Dielectric constant (10 GHz): 3.2 + j0.15 (low dielectric loss); Limiting oxygen index (LOI): 32%.

[0063] Example 2: Preparation of polyimide-based composite films This embodiment provides a method for preparing a microwave-absorbing, thermally conductive, and flame-retardant composite film using polyimide as the matrix and hexagonal boron nitride nanosheets (BNNS) and nickel-zinc ferrite as fillers. The preparation method includes the following steps: (1) Nanoscale interface modification: Methacryloxypropyltrimethoxysilane (KH-570) was mixed with anhydrous ethanol and deionized water in a volume ratio of 1:15:5. The pH was adjusted to 4.5 with acetic acid, and the mixture was stirred and hydrolyzed at 50°C for 6 hours to form a sol. Compared with KH-560, KH-570 has the advantage that the methacryloyloxy group (CH2=C(CH3)COO-) in its molecule can participate in the condensation reaction of the polyimide precursor to form covalent bonds, resulting in stronger interfacial bonding.

[0064] The average particle size was 3 μm, the thickness was 0.5 μm, and the specific surface area was 25 m². 2 40 g of boron nitride nanosheets (BNNS) were added to a sol and stirred and refluxed at 70 °C for 4 h. After filtration and washing, the mixture was vacuum dried at 130 °C for 6 h to obtain surface-modified BNNS filler. The high specific surface area of ​​BNNS means a larger interfacial contact area and more silane coupling agent anchoring sites, resulting in a more uniform shell distribution after modification.

[0065] Simultaneously, nickel-zinc ferrite powder with a particle size distribution of 0.5~3 μm (Ni 0.5 Zn 0.5 25 g of Fe2O4 (saturation magnetization Ms≈65 emu / g, coercivity Hc≈20 Oe) was surface-modified using in-situ polymerization. The specific steps were as follows: Nickel-zinc ferrite powder was dispersed in a polyamic acid precursor solution (solid content 15 wt.%, PAA viscosity approximately 5000 mPa·s), and in-situ silanization was performed at 150℃ for 2 h. Hydrogen bonding and partial esterification reactions occurred between the -OH functional groups on the ferrite powder surface and the -COOH and -NH2 functional groups at the ends of the PAA molecular chains. After thermal imidization (300℃), PAA lost water molecules and transformed into polyimide. Simultaneously, chemical bonds formed between the ferrite and the PI matrix, achieving in-situ hybridization enhancement.

[0066] (2) Dispersion process based on rheological simulation: A polyamic acid (PAA) solution with a solid content of 15 wt.% was prepared by polycondensation of 3,4,3',4'-biphenyltetracarboxylic dianhydride (BPDA) and 4,4'-diaminodiphenyl ether (ODA) in N,N'-dimethylacetamide (DMAc) at -15°C for 12 h. The BPDA-ODA system is one of the most well-balanced polyimide systems, exhibiting a high Tg (approximately 270°C), good mechanical properties (tensile strength > 100 MPa), and excellent solvent resistance.

[0067] 25 parts of surface-modified BNNS filler and 15 parts of in-situ polymerized nickel-zinc ferrite filler were added to the PAA matrix. Due to the high viscosity of the PAA solution (5000 mPa·s), uniform dispersion of the filler was difficult to achieve using conventional stirring. In this embodiment, a planetary mixer was used to disperse the filler at a high speed of 2000 rpm for 20 minutes. During dispersion, the mixing temperature was controlled at ≤30℃ using external circulating cooling water to prevent PAA degradation at high temperatures (PAA may undergo partial imidization at temperatures exceeding 50℃, affecting subsequent coating performance).

[0068] Based on rheological simulations, a three-bladed impeller with a blade inclination angle of 45° was selected, which can generate sufficient shear force in high-viscosity systems. Simulation results show that at a rotation speed of 2000 rpm, the average shear rate in the stirred zone is approximately 3000 s⁻¹. -1 With a viscosity of approximately 2000 mPa·s (after shear thinning), effective dispersion of the filler can be achieved. After dispersion, vacuum degassing is performed for 15 min (vacuum degree ≤ -0.095 MPa).

[0069] (3) Coating process based on doctor blade leveling simulation: A PAA mixture solution was uniformly coated onto a glass substrate using a casting coating method. Based on leveling model calculations, the optimal squeegee gap was set to 80 μm, and the coating speed was set to 8 m / min. Compared to silicone rubber systems, PAA solutions have higher viscosity, therefore the squeegee gap needs to be increased accordingly, while the coating speed needs to be reduced to avoid defects such as missed coatings or coating streaks during the coating process.

[0070] A vertical magnetic field of 1T (provided by an electromagnet, allowing for precise control of the magnetic field strength) is applied simultaneously with coating, utilizing the magnetocrystalline anisotropy of nickel-zinc ferrite to achieve its oriented alignment within the thin film plane. A magnetic field strength of 1T can completely magnetize nickel-zinc ferrite particles with a diameter of 1~3μm and orient them along the magnetic field direction, achieving an orientation rate of over 90%. After coating, the film is pre-dried at 80℃ for 10 min to remove most of the solvent (DMAc has a boiling point of 165℃; approximately 80% of the solvent can be removed at 80℃). At this point, the wet film has preliminarily cured and can be peeled off from the glass substrate for subsequent imidization treatment.

[0071] (4) Gradient curing and imidization: The pre-dried wet film is subjected to gradient-temperature imidization. The imidization process includes two key steps: first, chemical imidization (at 150-200°C, the -COOH and -NH2 groups in the PAA molecular chain undergo dehydration condensation to form an imide ring), and then thermal imidization (at 280-300°C, the remaining PAA is completely converted to PI). The gradient temperature program in this embodiment is as follows: First stage: From room temperature to 150℃, heating rate 3℃ / min, hold for 30 min to remove residual solvent; Second stage: 150℃ to 280℃, heating rate 2℃ / min, hold for 60 min, to complete the chemical imidization reaction; The third stage: 280℃ to 300℃, heating rate 1℃ / min, holding for 30 min, to complete the thermal imidization reaction and eliminate internal stress.

[0072] The imidization process was carried out entirely under nitrogen protection. The fully imidized PI film was golden yellow and transparent, with a 1780 cm⁻¹ FTIR spectrum. -1 (Imide C=O antisymmetric stretching vibration), 1720 cm -1 (Imide C=O symmetric stretching vibration) and 725cm -1 The characteristic absorption peak at the (imide ring bending vibration) is clearly visible, indicating that the imidization reaction is complete.

[0073] Performance test results of the obtained composite film: Thermal conductivity (Hot Disk method, 25 °C): 16.3 W / m·K; Radar wave return loss (bow method, 2~18 GHz): minimum -34.5 dB @ 13.2 GHz, average ≤ -22 dB; Vertical flammability rating (UL94): V-0 (0.5 mm thick sample); Glass transition temperature (Tg, DSC method, N2 atmosphere, heating rate 20℃ / min): >320℃; Tensile strength (ASTM D882, thin film specimen): 85 MPa; Elongation at break: 45%; Thickness: 80±5μm; Coefficient of thermal expansion (CTE, TMA method, 50-200℃): 18 ppm / ℃; Water absorption rate (23℃×24h, distilled water): 0.8 wt.% (lower for fluorine-containing PI samples, only 0.3 wt.%). Dielectric constant (10 GHz): 3.0 + j0.08 (2.7 + j0.05 for fluorine-containing PI sample).

[0074] Example 3: In contrast, the same materials and formulation as in Example 1 were used, but the eddy current stirring step was replaced with a conventional planetary ball milling dispersion process (planetary ball mill, speed 500 rpm, time 4 h, ball-to-material ratio 10:1), the magnetic field-assisted orientation in the doctor blade coating step was removed, and the curing process was simplified to one-step curing (directly heating to 250°C and holding for 60 min).

[0075] Comparison of sample performance test results: Thermal conductivity: 8.2 W / m·K (only 44% of that in Example 1, failing to meet the technical requirement of ≥15 W / m·K); Radar wave return loss: minimum -18.5dB (not meeting the technical requirement of ≤-30dB); Vertical flammability rating (UL94): V-1 (does not meet V-0 requirements); Tensile strength: 2.8 MPa (only 67% of that in Example 1); Thickness uniformity: ±12% (3 times that of Example 1).

[0076] Experimental results show that the thermally conductive filler in the comparative samples is randomly distributed and does not form a continuous three-dimensional network structure. The filler particles are isolated and dispersed, and there are a large number of discontinuities in the thermal conduction path. The microwave absorbing filler shows obvious agglomeration, with agglomerate particle size of 50~100μm, which is 25~50 times the original particle size. The strong magnetic interaction between the microwave absorbing filler inside the agglomerates further exacerbates the irreversibility of agglomeration. The interface between the filler and the matrix is ​​clear, with a large number of microcracks (width 50~200 nm). These microcracks become channels for the escape of pyrolysis products during combustion, reducing the flame retardant performance.

[0077] The above comparison results fully demonstrate the significant advantages of the co-design strategy for macro-micro-nano structures and the fabrication process guided by the theoretical model of this invention.

[0078] Example 4: Experiment on Optimization of Particle Size Distribution of Microwave Absorbing Filler Based on the matrix and thermally conductive filler formulations of Example 1, five sets of microwave absorbing filler particle size distribution schemes were designed to study the influence of particle size distribution on microwave absorption performance and determine the optimal multi-scale distribution design: (1) Scheme A (single-grade, micron-scale only): only 20 parts of carbonyl iron with a particle size of 2~3 μm are used; (2) Scheme B (two-stage, micron + submicron): 12 parts of particles with a diameter of 2~3 μm and 8 parts of particles with a diameter of 0.3~0.5 μm; (3) Scheme C (three-level distribution, preferred in this invention, micron + submicron + nano): 8 parts of particles with a diameter of 2~3 μm, 7 parts of particles with a diameter of 0.3~0.5 μm, and 5 parts of particles with a diameter of 50~100 nm; (4) Scheme D (two-stage, micron + nano): 12 parts of particles with a diameter of 2~3 μm and 8 parts of particles with a diameter of 50~100 nm; (5) Scheme E (nanoscale only): Use only 20 parts of carbonyl iron with a particle size of 50~100 nm.

[0079] Comparison of absorption performance (8~300 GHz wideband test, using the bow-shaped method): Option A: Effective absorption bandwidth (RL < -10 dB) is approximately 4 GHz (mainly in the 8~12 GHz range), with a peak absorption of -22.3 dB @ 10.2 GHz, and absorption in the high-frequency band (>40 GHz) is almost zero; Option B: The effective absorption bandwidth is about 8 GHz (mainly in the range of 8~18 GHz), with a peak absorption of -28.6 dB @12.5 GHz, but the absorption in the 60~300 GHz band is weak (average -10 dB). Option C: The effective absorption bandwidth reaches more than 18 GHz (covering the 8~26 GHz main frequency band, and also taking into account 40-100 GHz), the absorption peak is -42.1 dB @ 14.8 GHz, and the average return loss of 60~300 GHz frequency band is -18.5 dB. It is the only option that meets the technical requirements of 6G full frequency band. Option D: The effective absorption bandwidth is about 12 GHz (mainly 8~20 GHz), but the high surface energy of the nanoscale filler causes it to settle and agglomerate significantly in silicone rubber, resulting in unstable absorption in the high-frequency band. Option E: Nanoscale carbonyl iron has a low initial permeability (Snoek limit), poor absorption in the low-frequency range (average -15 dB in the 8~18 GHz range), and although its performance in the high-frequency range is acceptable, it does not meet the overall technical specifications.

[0080] Experimental results show that a multi-level particle size distribution can effectively broaden the absorption bandwidth through the synergistic effect of fillers of different sizes. Micrometer-sized fillers provide sufficient mass to achieve strong absorption in the low-frequency range; submicrometer-sized fillers fill the gaps between micrometer-sized fillers, broadening the absorption in the mid-frequency range; and nanometer-sized fillers utilize their high specific surface area and quantum size effect to enhance the high-frequency electromagnetic response. The resonant frequencies of the three-level fillers are staggered, forming a continuous absorption peak cluster, which is a key design element for achieving broadband absorption.

[0081] Example 5: Effects of different silane coupling agents on interfacial thermal resistance and thermal conductivity Using the matrix and filler formulation of Example 1, four different silane coupling agents were used for surface modification to investigate the effect of coupling agent type on the thermal conductivity and interfacial adhesion of the composite film. (1) Sample E1: γ-aminopropyltriethoxysilane (KH-550, aminosilane) was used, with a shell thickness of approximately 3 nm. The -NH2 functional group in the aminosilane can coordinate with the platinum catalyst in the addition-type silicone rubber, and -NH2 can also participate in partial addition reactions. The interfacial thermal resistance is 0.8 × 10⁻⁶. -8 m 2 ·K / W, thermal conductivity 17.2 W / m·K.

[0082] (2) Sample E2: γ-glycidoxypropyltrimethoxysilane (KH-560, epoxy silane) was used, with a shell thickness of approximately 5 nm. The epoxy groups are stable at room temperature, but can undergo ring-opening addition reactions with hydroxyl or amino groups in silicone rubber under heating conditions. The interfacial thermal resistance is 0.6 × 10⁻⁶. -8 m 2 ·K / W, thermal conductivity 18.6 W / m·K (highest among all samples).

[0083] (3) Sample E3: Methacryloxypropyltrimethoxysilane (KH-570, methacryloyloxysilane) was used, with a shell thickness of approximately 4 nm. The methacryloyloxy group can participate in the vinyl addition curing reaction of addition-type silicone rubber to form covalent bonds. The interfacial thermal resistance is 0.7 × 10⁻⁶. -8 m 2 ·K / W, thermal conductivity 17.8 W / m·K.

[0084] (4) Sample E4 (Control): No silane coupling agent was used; unmodified filler was used directly. The interfacial thermal resistance was as high as 2.5 × 10⁻⁶. -8 m 2 With a thermal conductivity of only 9.8 W / m·K, the interfacial adhesion between the filler and the matrix is ​​extremely weak, and the filler is easily peeled off from the matrix when the film is bent.

[0085] Quantitative determination of interfacial bonding strength (lap shear strength test, ASTM D905) results: E1 1.8 MPa, E2 2.4 MPa, E3 2.1 MPa, E4 0.5 MPa. Considering both thermal conductivity and interfacial bonding strength, KH-560 (epoxy silane) is the most preferred silane coupling agent in this invention. Its epoxy groups have the best reaction compatibility with the addition-type silicone rubber curing system, forming a tough interfacial transition layer after curing. Furthermore, the hydrolytic stability of the epoxy groups is superior to that of amino silanes, resulting in less performance degradation in humid environments.

[0086] The following performance tests were performed on the microstructure-coated cured PMMA film prepared in this embodiment: Light transmittance test: The average transmittance of visible light is 94.8%. Due to the focusing effect of the lens array, the light utilization rate is significantly improved.

[0087] Hardness test: The surface hardness remains 6H.

[0088] Optical performance testing: The lens array can focus the incident light to form a uniform light spot with a diameter of about 80μm and a uniformity of >95%, which is suitable for the optical inspection system of 3D printers.

[0089] Example 6: Effect of different thermally conductive filler contents on overall performance Based on the matrix and microwave absorbing filler formulation (20 parts carbonyl iron) of Example 1, the amount of h-BN filler added was changed to study the effect of the volume fraction of thermally conductive filler on the overall performance and determine the optimal filler ratio: (1) F1 (h-BN 10 vol.%): thermal conductivity 10.2 W / m·K, minimum return loss -32.5dB, tensile strength 3.8 MPa, elongation at break 210%, viscosity 4500 mPa·s (good coating performance). (2) F2 (h-BN 20 vol.%): thermal conductivity 15.8 W / m·K, minimum return loss -35.8 dB, tensile strength 4.0 MPa, elongation at break 195%, viscosity 6200 mPa·s (good coating performance). (3) F3 (h-BN 25 vol.%, the optimal ratio of the present invention): thermal conductivity 18.6 W / m·K, minimum return loss -38.2 dB, tensile strength 4.2 MPa, elongation at break 180%, viscosity 8500 mPa·s (coating performance is acceptable). (4) F4 (h-BN 30 vol.%): thermal conductivity 22.1 W / m·K, minimum return loss -30.5 dB, tensile strength 3.5 MPa, elongation at break 145%, viscosity 15000 mPa·s (difficult to coat, requires the addition of diluent). (5) F5 (h-BN 35 vol.%): thermal conductivity 24.8 W / m·K, minimum return loss -26.2 dB, tensile strength 2.8 MPa, elongation at break 95%, viscosity 35000 mPa·s (cannot be coated into a thin film, brittleness is significantly increased). (6) F6 (h-BN 40 vol.%): thermal conductivity 26.5 W / m·K, minimum return loss -22.1 dB, tensile strength 1.9 MPa, elongation at break 40%, brittle fracture, cannot be used as a thin film.

[0090] Experimental results show that when the h-BN filling amount exceeds 30 vol.%, the microwave absorption performance begins to decline significantly and the mechanical properties deteriorate sharply. This is because at high filling amounts, the carbonyl iron is physically "diluted" by the h-BN network—the carbonyl iron particles are isolated by h-BN nanosheets, the magnetic dipole interaction between the particles is weakened, and the magnetic loss absorption capacity decreases; at the same time, excessive inorganic fillers disrupt the continuity of the silicone rubber matrix, and the flexibility and mechanical load-bearing capacity of the matrix are greatly reduced. Considering thermal conductivity, microwave absorption, and mechanical properties, this invention recommends an optimal thermally conductive filler volume fraction of 20~25 vol.%.

[0091] Example 7: Wave absorption performance test in the 6G millimeter-wave to terahertz band (26~300 GHz) The composite film prepared in Example 1 (F3 ratio: h-BN 25 vol.% + CIP 15 vol.%) underwent a systematic bow-shaped reflectivity test across the entire 6 GHz band. (1) Sub-6G band (3-6 GHz): average reflectivity -18.2 dB, peak absorption -24.5 dB @ 4.5 GHz. Although this frequency band is not the main design target of the absorbing material, the composite film of the present invention still exhibits considerable absorption capacity in this frequency band, meeting general electromagnetic compatibility requirements.

[0092] (2) 5G millimeter wave low frequency band (24-40 GHz, n257 / n258 band): average reflectivity -26.8 dB, peak absorption -33.2 dB @ 33 GHz. The micron-sized carbonyl iron component exhibits good magnetic loss absorption in this frequency band.

[0093] (3) 5G / 6G millimeter wave mid-frequency band (37-52 GHz, n259 / n260 band): average reflectivity -24.5 dB, peak absorption -29.8 dB @ 43 GHz. Submicron carbonyl iron contributes the main absorption effect in this band.

[0094] (4) 6G low-frequency millimeter wave (57-71 GHz, W-band low frequency): average reflectivity -21.3 dB, peak absorption -27.6 dB @ 64 GHz. The contribution of nanoscale carbonyl iron begins to appear.

[0095] (5) 6G high-frequency millimeter wave (71-110 GHz, W-band high frequency): average reflectivity -22.8 dB, peak absorption -30.5 dB @ 94 GHz. The interfacial polarization effect between nanoscale fillers and BNNS contributes to high-frequency absorption.

[0096] (6) 6G terahertz low frequency (110-170 GHz, D band): average reflectivity -19.2 dB, peak absorption -24.5 dB @ 140 GHz. Although the absorption capability is reduced, it still meets the basic requirements of 6G system for electromagnetic compatibility in millimeter wave band (≤-15 dB).

[0097] (7) 6G terahertz intermediate frequency (170-220 GHz, G band): average reflectivity -16.5 dB, peak absorption -21.2 dB @ 195 GHz.

[0098] (8) 6G terahertz high frequency (220-300 GHz): average reflectivity -13.8 dB, peak absorption -17.5 dB @260 GHz. The absorption capacity is further weakened because at such high frequencies, the wavelength of electromagnetic waves is comparable to the microstructure size of the material (filler spacing, shell thickness, etc.), and complex multiple reflections and interference effects occur inside the material.

[0099] The above results show that the composite film of the present invention has a return loss of ≤-13 dB in the entire frequency band of 8-300 GHz (covering Sub-6G to terahertz low frequency), which is better than 95% of the incident electromagnetic energy being absorbed, and can meet the basic technical requirements of electromagnetic compatibility of 6G base stations and terminal equipment in the millimeter wave to terahertz frequency band.

[0100] Example 8: Aging performance and reliability testing The composite film prepared in Example 1 (F3 ratio) was subjected to comprehensive accelerated aging tests to examine its long-term performance stability under simulated real-world usage conditions: (1) Thermal aging test (200℃, in air, 1000h): Appearance changes: The surface color of the film changed from light gray to light yellowish brown, but there were no cracks or delamination; Thermal conductivity: Change rate +3.2% (18.6→19.2 W / m·K, a slight increase, due to partial cross-linking of silicone rubber and filler rearrangement at high temperatures); Return loss: rate of change +2.8 dB (minimum value changes from -38.2 dB to -35.4 dB, still meeting the ≤-30 dB requirement); Tensile strength: Change rate -8.5% (4.2→3.84 MPa); Weight loss rate: 0.8 wt.% (mainly due to the escape of unreacted small molecules).

[0101] (2) Damp heat aging test (85℃, 85% RH, 1000h): Appearance changes: The film surface is slightly whitish, but there is no bubbling or delamination; Thermal conductivity: Change rate +5.6% (18.6→19.6 W / m·K, a slight increase, due to the enhanced interfacial heat transfer caused by the penetration of water molecules). Return loss: rate of change +3.9 dB (minimum value changes from -38.2 dB to -34.3 dB); Water absorption rate: 1.2 wt.% (the hydrophobicity of the modified silicone rubber effectively inhibits water absorption); Flame retardant rating: still UL94 V-0 (the whitening on the surface is a trace left by moisture evaporation and does not affect the flame retardant performance).

[0102] (3) Thermal cycling shock test (-55℃↔+125℃, 1000 cycles): Appearance changes: No cracks, no delamination, no discoloration; Thermal conductivity: Change rate +1.2% (18.6→18.8 W / m·K); Return loss: rate of change +1.5 dB (minimum value changes from -38.2 dB to -36.7 dB); Peel strength (with copper foil): Change rate -3.1% (interfacial bonding remains good).

[0103] (4) Evaluation of high-temperature accelerated aging using the Arrhenius model: Based on the Arrhenius model, the lifespan of the chip within its operating temperature range (maximum 125℃) was estimated using accelerated thermal aging test data at three temperature points: 200℃, 185℃, and 170℃. The results show that at an operating temperature of 125℃, a 5% decrease in thermal conductivity results in a characteristic lifespan of approximately 150,000 hours (about 17 years), meeting the design lifespan requirements (>10 years) for 6G communication equipment.

[0104] Example 9: Comparative Experiment of Nanofiller Surface Modification Processes This embodiment compares two different nanofiller surface modification processes—sol-gel method and in-situ polymerization method—to examine their impact on the performance of the final composite film.

[0105] The matrix and filler formulation of Example 1 (h-BN 25 vol.% + CIP 15 vol.%) was used, wherein h-BN was modified by the sol-gel method (KH-560), and CIP was modified by both the sol-gel method and in-situ polymerization. (1) Sol-gel modification of CIP (sample G1): The same KH-560 sol-gel method as in Example 1. Interfacial thermal resistance 0.6 × 10⁻⁶ -8 m 2 ·K / W, thermal conductivity 18.6 W / m·K, minimum return loss -38.2 dB.

[0106] (2) In-situ polymerization modification of CIP (sample G2): Nanoscale carbonyl iron-silane hybrid particles are generated in situ during the curing process of silicone rubber. Specifically, CIP powder (original particle size 5 μm) is added to addition-type silicone rubber and ball-milled at 3000 rpm for 2 hours to prepare a nanoscale CIP dispersion with a particle size of 50~200 nm. Then, KH-560 coupling agent is added, and surface modification is performed at 80℃. The interfacial thermal resistance is 0.45 × 10⁻⁶. -8 m 2 ·K / W, thermal conductivity 20.1 W / m·K, minimum return loss -35.8 dB.

[0107] Although the in-situ polymerization-modified sample G2 had a higher thermal conductivity (20.1 W / m·K), its microwave absorption performance decreased slightly (-35.8 dB). This may be because the high specific surface area of ​​the nanoscale CIP weakened the magnetic dipole interaction. Overall, the sol-gel method is the preferred method for modifying microwave-absorbing fillers, while in-situ polymerization can be considered as an alternative to further improve thermal conductivity.

[0108] To facilitate understanding and implementation of this invention by those skilled in the art, the core theoretical model is described in detail below: (1) Fluid dynamics dispersion model: This invention employs computational fluid dynamics (CFD) to establish a fluid dynamics model of the eddy current dispersion process. The basic assumptions of the model include: the fluid is a Newtonian fluid or a pseudoplastic fluid (its rheological behavior is described using the Power-law model); the filler particles are spherical or plate-shaped (characterized by equivalent volume diameter); the filler volume fraction is low (<40 vol.%), and the direct interaction between particles is negligible.

[0109] The solution domain of the model is the cross-section of the cylindrical container. The velocity and pressure fields are obtained by solving the Navier-Stokes equations (with time-averaged inertia terms and a closed k-ε turbulence model for Reynolds stress). Based on this, a particle swarm equilibrium (PBM) model is used to track the motion trajectory of the packing particles in the flow field and the dynamic equilibrium of aggregation and breakup. The breakup model adopts the Turek criterion: particles break up when the shear force exceeds their critical breakup stress. The aggregation model employs both Brownian aggregation and shear-induced aggregation mechanisms.

[0110] The key output parameters of the model include: (a) the dispersion index DI = (d 90 ,initial-d 90 ,final) / d 90 (a) Initially, the closer the DI value is to 1, the better the dispersion effect; (b) Characteristic particle size d of the filler agglomerates32 (a) Volume-area average diameter; (b) Dispersion uniformity, assessed by measuring the coefficient of variation (CV) of the packing concentration at different locations in the container.

[0111] For addition-cure silicone rubber systems (viscosity 3000 mPa·s, density 1.05 g / cm³), 3 The process parameters optimized by CFD simulation for the BNNS+CIP packing system (total packing volume fraction 30 vol.%) are: inclined blade impeller, blade inclination angle 45°, rotation speed 1500 rpm, stirring time 15 min. At this time, the DI value reaches 0.97, CV<5%, and energy consumption is about 0.8 kWh / kg.

[0112] (2) Thin film leveling model: During the doctor blade coating process, the wet film surface gradually levels under the influence of capillary force and gravity. The leveling dynamics are described by the lubrication approximation theory, and the amplitude A(t) of the surface ripples decays exponentially with time t: A(t) = A0 exp[[A(t)] / [A0]] The leveling time constant τ = 3η / (γh) 3 k 2 ), where η is the liquid viscosity, γ is the surface tension, h is the wet film thickness, and k is the wave number of the ripples.

[0113] The model considers the following key factors: (a) non-Newtonian fluid behavior of viscosity as a function of shear rate (described using the Cross or Carreau model); (b) the effect of filler on rheological properties—at high filler levels, the mixture exhibits shear-thinning behavior, with much lower viscosity in the doctor blade region (high shear rate) than in the leveling region (low shear rate), which is beneficial for coating and leveling; (c) the driving force of capillary force and gravity on leveling—capillary force is proportional to surface tension γ, and gravity is proportional to the cube of thickness h. For thin wet films (h < 100 μm), capillary force is the dominant driving force for leveling; (d) the effect of filler settling on leveling—filler settling caused by density differences creates a concentration gradient in the wet film, which in turn affects local viscosity and leveling behavior.

[0114] The optimal process window output by the model is: doctor blade gap 40~80 μm, coating speed 8~15 m / min, and wet film viscosity (shear rate 100 s⁻¹). -1 5000~12000 mPa·s. Within this process window, the wet film thickness uniformity σ<1 μm and the surface waviness Ra<0.5 μm.

[0115] Specific embodiments have been used to illustrate the principles and implementation methods of this invention. The descriptions of the embodiments above are only for the purpose of helping to understand the method and core ideas of this invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this invention. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A microwave-absorbing, thermally conductive, and flame-retardant composite thin film with synergistic macro-micro-nano structures for 6G chips, characterized in that: The composite film includes a matrix and a filler system filled in the matrix; The packing system is selected from thermally conductive packing and / or microwave absorbing packing; the surface of the packing is modified with a silane coupling agent to form a nanoscale low interfacial thermal resistance transition layer.

2. The microwave-absorbing, thermally conductive, and flame-retardant composite film according to claim 1, characterized in that: The matrix is ​​selected from modified silicone rubber matrix and / or polyimide matrix; Preferably, the modified silicone rubber matrix is ​​a product of addition-type silicone rubber or condensation-type silicone rubber modified with methacryloxysilane or epoxysilane; Preferably, the polyimide matrix is ​​selected from polyimides prepared by thermal imidization or chemical imidization of 3,4,3',4'-biphenyltetracarboxylic acid dianhydride and 4,4'-diaminodiphenyl ether, or is a fluorinated polyimide; Preferably, the silane coupling agent is selected from one or more combinations of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, methacryloyloxypropyltrimethoxysilane, and phenyltrimethoxysilane.

3. The microwave-absorbing, thermally conductive, and flame-retardant composite film according to claim 1, characterized in that: The thermally conductive filler is selected from boron nitride and / or aluminum oxide, and the microwave absorbing filler is selected from carbonyl iron and / or nickel-zinc ferrite. Preferably, the mass ratio of boron nitride to alumina in the thermally conductive filler is 1:0 to 0:1; the volume fraction of the thermally conductive filler in the matrix is ​​5% to 40%. Preferably, the mass ratio of carbonyl iron to nickel-zinc ferrite in the microwave absorbing filler is 1:0 to 0:1; and the volume fraction of the microwave absorbing filler in the matrix is ​​5% to 30%.

4. The microwave-absorbing, thermally conductive, and flame-retardant composite film according to claim 1 or 3, characterized in that: The microwave absorbing filler has a three-level particle size distribution: micron-sized particles with a diameter of 1~10 μm account for 20%~40% of the total microwave absorbing filler mass; submicron-sized particles with a diameter of 0.1~1 μm account for 30%~50% of the total microwave absorbing filler mass; and nano-sized particles with a diameter of 10~100 nm account for 20%~40% of the total microwave absorbing filler mass.

5. The microwave-absorbing, thermally conductive, and flame-retardant composite film according to claim 1 or 3, characterized in that: The thermally conductive filler is a hexagonal boron nitride nanosheet with a diameter of 0.1~10 μm and a thickness of 10~200 nm, arranged in a two-dimensional layered structure.

6. The microwave-absorbing, thermally conductive, and flame-retardant composite film according to claim 1, characterized in that: The composite film has a thickness of 10~200μm, a thickness uniformity of ≤±5%, a thermal conductivity of ≥15 W / m·K, a radar wave return loss of ≤-30 dB, and a vertical combustion rating of V-0.

7. The method for preparing the microwave-absorbing, thermally conductive, and flame-retardant composite thin film according to any one of claims 1 to 6, characterized in that, The preparation method includes the following steps: Step 1, Nano-interface modification: The surface of thermally conductive fillers and / or microwave absorbing fillers is modified by sol-gel method or in-situ polymerization to construct a core-shell structure on the filler surface, wherein the shell is an organic-inorganic hybrid transition layer formed by silane coupling agent, and the shell thickness is 1~10 nm. Step 2, Dispersion process based on rheological simulation: The surface-modified filler is mixed with the matrix and dispersed using eddy current stirring technology. The shape of the stirring blade and process parameters are controlled according to the fluid dynamics simulation results. The stirring speed is 1000~3000 rpm and the stirring time is 5~30 min, so that the filler achieves a homogeneous dispersion state without agglomeration in the matrix. Step 3, coating process based on doctor blade leveling simulation: The mixture obtained in Step 2 is coated by doctor blade coating. The doctor blade gap and coating speed are calculated according to the film leveling model. The doctor blade gap is 10~100 μm and the coating speed is 5~20 m / min. With magnetic field-assisted orientation, a wet film with specific thickness and flatness is formed. Step 4, gradient curing: Set a gradient heating program according to the heat conduction model to cure the wet film. The curing temperature is 120~300℃ and the heating rate is 1~10℃ / min. Eliminate internal stress and obtain the composite film.

8. The preparation method according to claim 7, characterized in that: In step one, when surface modification is performed using the sol-gel method, silane coupling agent is mixed with anhydrous ethanol and deionized water at a volume ratio of 1:(10~50):(1~10), the pH is adjusted to 4~6 with acetic acid, and the mixture is stirred and hydrolyzed at 25~80℃ for 2~24 h to form a sol; then thermally conductive filler and / or microwave absorbing filler are added to the sol, and the mixture is stirred and refluxed at 40~120℃ for 1~6 h, and the surface-modified filler is obtained after filtration, washing and drying.

9. The preparation method according to claim 7, characterized in that: In step two, after dispersion is completed, vacuum degassing is performed, with a vacuum degree ≤ -0.09 MPa and a time of 5~30 min, to eliminate air bubbles introduced during the mixing process; Preferably, in step three, the magnetic field-assisted orientation uses a permanent magnet or electromagnet to provide a stable magnetic field with an intensity of 0.1~2 T, and the direction of the magnetic field is perpendicular or parallel to the coating direction, so as to achieve the orientation and arrangement of the microwave absorbing filler in the wet film. Preferably, in step four, the gradient heating program includes: heating from room temperature to 120°C at a rate of 3-8°C / min and holding for 10-30 min; heating from 120°C to 200°C at a rate of 1-5°C / min and holding for 10-30 min; and heating from 200°C to the curing temperature at a rate of 1-3°C / min and holding for 10-60 min.

10. The microwave absorbing, thermally conductive, and flame-retardant composite film according to claims 1 to 6 or the microwave absorbing, thermally conductive, and flame-retardant composite film prepared by the preparation method according to any one of claims 7 to 9 is used as an encapsulation material in high-frequency electronic components of 6G communication base station radio frequency front-end modules, power amplifier packaging, millimeter-wave antenna arrays, and terminal equipment.