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The development of information transmission technology towards high-frequency microwaves and highly integrated and multi-functional electronic devices has been the mainstream direction of the current communication technology. During signal transmission, resistance-capacitance time delay, crosstalk, energy consumption increase and impedance mismatch restrict the high density and miniaturization of Printed circuit board (PCB). In order to achieve high fidelity and low delay characteristics of high-frequency signal transmission, the development of interlayer dielectric materials with low dielectric constant (Dk) and low dielectric loss factor (Df) has become the focus of researchers. This review introduces the dielectric loss mechanism of polymer composites and the resin matrix commonly used in several high-frequency copper-clad laminates, and mainly describes how to reduce the dielectric constant and dielectric loss of materials from the level of molecular structure design, as well as the effect of fillers on the dielectric properties of polymer substrates. As a kind of potential functional fillers for dielectric polymeric composites, the carbon nanofillers are used to tailor the dielectric properties of their composites
In recent years, the rapid development of information transmission technology has led to the gradual development of electronic devices in the direction of thin and light, high performance, high frequency and high speed. Owing to the development of high density and miniaturization of PCB, the core component of high frequency microwave communication, microwave signal transmission between integrated circuits and microstrip lines in the system is prone to mutual interference, resulting in the delay of resistance and capacitance (RC), crosstalk and increase of energy consumption. The copper clad laminate (CCL) is the carrier of the printed circuit board and plays a major role in its conductivity, insulation and support, largely determining the performance, quality, manufacturing costs and long-term reliability of PCB. Therefore, the fabrication of copper clad laminates for use at high frequencies has become the focus of researchers.
According to the signal transmission rate formula, the relationship between high frequency signal transmission speed V and signal transmission loss and interlayer dielectric resin can be expressed as follows:
Compared to the inorganic dielectric material silicon dioxide (SiO2), organic polymer materials as interlayer dielectric (ILD) generally have lower dielectric constants due to their smaller material density and lower single-bond polarizability. In addition, they show obvious advantages in the ease of chemical and geometric structure design. Resin matrix, as an important part of CCL, plays a decisive role in the dielectric properties of CCL. In order to ensure no void electroplating deposition during copper metallization, in addition to composition stability at high temperatures, organic polymers must also have very high glass transition temperatures: minimum temperature above 300°C and ideal temperature above 400°C (
The ε′ and tanδ of Representative Polymers.
Polymer | ε′ | Tanδ |
---|---|---|
Polytetrafluoroethylene | 2.1 | 1 × 10–4 |
Polypropylene (isotactic) | 2.2 | 1 × 10–4 |
Cycloolefin polymer | 2.3 | 2 × 10–4 |
Polybutadiene | 2.5–2.8 | 5 × 10–3 |
Poly(methyl methacrylate) | 2.6 | 8 × 10–3 |
Polyphenylene Oxide (PPE) | 3.5 | 2 × 10–3 |
Thermosetting cyanate resin | 3.45–3.55 | 4 × 10−3–5 × 10–3 |
bismaleimide-triazine resin | 4.1–4.3 | 4 × 10−3–8 × 10–3 |
Dielectric loss is caused by the failure of the molecular polarization process to keep pace with the rate of change of the oscillating applied electric field. When the relaxation time (τ) in the polymer is less than or equal to the rate of the oscillating electric field, there is no or minimum loss. However, when the oscillating speed of the electric field is much greater than the relaxation time, the polarization cannot change with the oscillation frequency, resulting in energy absorption and heat dissipation. When the dipole polarization is completely out of sync with the frequency of the applied oscillating electric field, the dielectric loss is also minimal.
The relative permittivity can be expressed by the following complex equation:
It consists of the real part of the dielectric constant (
Among them, dielectric materials have five types of polarization: interface polarization, ion polarization, dipole (orientation) polarization, atomic (vibration) polarization, and electron polarization. Each polarization is related to the dielectric loss at a particular frequency (
Different types of polarization as a function of frequency in polymers. Pelect, electronic polarization; Pat, atomic polarization; Pdip, (dipolar) orientational polarization; Pion, ionic polarization; Pint, interfacial polarization. The top panel shows the molar polarization (or the real part of permittivity), and the bottom panel shows the dissipation factor (the imaginary part of permittivity). Reprinted with permission from
Use of Disiloxane-Linked Diamine BATMS in Preparing Poly(imide siloxane) Films PI-χ.
Preparation of
Schematic illustration for the fluoro-polymer@BT nanoparticles and the interfacial region within the nanocomposites. Reprinted with permission from Yang el al. (2013). Copyright© 2013 American Chemical Society.
Cole-Cole Plot showing the relationship between dielectric constant and dielectric loss.
Simulation geometry in the finite element model.
For polymer materials, resonance includes electron polarization and atomic (vibrational) polarization. Both types of polarization exist in polymers, whether polar or nonpolar, amorphous or crystalline. Because their dielectric losses are in the infrared and optical ranges, they are not seen in power and radiofrequency
The molecular structure of the polymer plays a major role in the dielectric and adhesive properties. However, the requirements for low dielectric constant, dielectric loss and strong adhesion of polymer materials are often contradictory. On the one hand, the strong adhesion to the microstrip conductor requires that the polymer chain contains polar functional groups such as amino, cycloxyl, and isocyanate, which can enhance the interfacial forces between polymer chains and microstrip conductors (usually copper foil) by electrostatic interaction, thereby improving the durability and reliability of integrated circuit boards. On the other hand, the orientation polarization of the intrinsic dipole moments combined with the dipole polarization of polar functional groups decisively causes a large increase in the dielectric loss; so, optimizing the dielectric properties (especially low dielectric loss) requires that polymer chains contain nonpolar functional groups. Besides, the position of the polar functional group is also very important: if the polar group is on the side chain of the polymer, especially the flexible polar group, which has strong mobility, it will have a greater impact on the dielectric properties; Polar groups have little effect if they are in the polymer backbone. The branching, cross-linking and orientation stretching of macromolecules also have an impact on the dielectric properties.
Polytetrafluoroethylene [-(CF2−CF2)n-] is a non-polar linear polymer with highly symmetrical structure, composed of two elements: carbon and fluorine. Due to the lack of active polar groups, high crystallinity, high electronegativity of fluorine atom and high dissociation energy of C-F bond, it has lower surface energy and higher surface hydrophobicity, making it hard affected by frequency, temperature and humidity. Except for compound modification and molecular structure design, the surface of PTFE can be directly treated to improve the adhesion performance of PTFE without reducing the bulk performance. PTFE is an available candidate for the high-frequency application in CCL, but its low inferior adhesion to substrates and poor processability, high coefficient of thermal expansion restrict its practical application. At present, sodium naphthalene, ion irradiation and plasma treatment are mostly used for its surface modification. Particularly, plasma modified the surface of PTFE without affecting the overall physical and chemical properties of PTFE.
Polyimide (PI) is a polymer with rigid chain of imide heterocycle, which is composed of binary amine and binary acid/anhydride with large free volume. Due to its excellent overall properties, such as thermal oxidation stability, unique electrical property and high mechanical strength, it is used as a polymer matrix for aerospace and microelectronics. In order to facilitate the processing of polyimide, many researchers have developed soluble polyimide. The main purpose of improving solubility is to reduce the interaction between polymer chains and the stiffness of polymer structure by introducing non coplanar, flexible and kink elements. For thermosetting polyimide, the molecular weight can be turned by the ratio of anhydride to amine group, and the polymer meeting the performance requirements can be obtained by crosslinking. The introduction of asymmetric dianhydride and diamine can not only limit the stacking of main chain and charge transfer electron polarization interaction, but also make polyimide have high Tg and thermal properties. Therefore, polyimides based on molecular or geometric asymmetry exhibit higher solubility, low melt viscosity and other required properties (
Epoxy resins are used as substrates or adhesives for copper-clad laminates due to its excellent properties, such as good moisture resistance, solvent resistance and chemical resistance, low shrinkage after curing, excellent electrical and mechanical properties, and good adhesion to many substrates. Taking bisphenol A diglycidyl ether epoxy resin as an example, the polar groups in the molecular chain such as ether bond and hydroxyl group are conducive to improve the adhesion between epoxy resin and copper foil. Rigid groups such as benzene ring in the molecular chain are beneficial to improve the rigidity and heat resistance of epoxy resin. The ether bond in the molecular chain endows the epoxy resin with good alkali resistance and flexibility. As a thermosetting resin, epoxy resin has good processability and reactivity, but the application of epoxy resins is limited by its brittleness, high dielectric loss and high dielectric constant. Generally, higher crosslinking degree will make it difficult to orient polar groups, so it will reduce the dielectric constant, but it is unfavorable to the mechanical properties. The crosslinking density and polar group concentration can be reduced by grafting flexible chains or blending with reactive functional groups.
Olefin polymers are carbon chain polymers without any polar groups, which endow dielectric materials with excellent dielectric properties. The common hydrocarbon resins include styrene-butadiene copolymer, styrene-butadiene-divinylbenzene copolymer, butadiene homopolymer and so on, only composed of C and H elements. Due to the small electronic polarizability of C-C and C-H, olefin polymers exhibit low dielectric constant and ultralow dielectric loss over wide frequency and temperature ranges. which are the ideal candidates for high-frequency CCL
%1.2 Principles and examples of molecular structure design.
According to the Clausius-Mossotti formula:
The reduction of dielectric constant and dielectric loss by polymer molecular structure design is usually achieved by several key methods.
First, the chemical elements involved in the preparation of low dielectric constant polymer materials can be appropriately selected to reduce the polarizability of the molecules. Bonding the precursors of atoms with appropriate electronegativity in an appropriate configuration can reduce the molecular polarizability. The incorporation of fluorine atoms is a good way to reduce the dielectric constant of polymers. The C-F bond has the lowest electronic susceptibility and dipole moment, making fluorination a potential candidate for low dielectric constant applications. On the one hand, fluorine atoms possess the strongest electronegativity and can better fix electrons, so the doping of fluorine atoms is particularly effective in reducing the electron and ion polarizability (
Except for C and F, the thermal motion ability and orientation polarization ability of the segment units are different. Compared with aliphatic segment units, silicon is larger than carbon atoms, silicon-oxygen bonds are more flexible than carbon-carbon bonds, and siloxane (Si-O-Si) possesses a low dielectric constant. Therefore, the bulky silicone resin unit has relatively small mobility, which reduces the efficiency of the dipole’s response to changes in polarity at alternating frequencies. Haixia Qi et al. prepared a polyimide siloxane film by copolymerizing the synthesized disiloxane diamine (BATMS) with ODA and ODPA (
Polyhedral oligomeric silsesquioxane (POSS) with empirical formula (RSiO1.5) is the smallest silica nanoparticle (NP), where R can be an organic functional group (such as amine, alkyl, alkylene, epoxide unit, acrylate, hydroxyl) or hydrogen atom. According to the number of R and the preparation method of nanocomposites, Mohamed et al. classified it as a blend of non-functional POSS nanocomposites and polyimide, and a chain of monofunctional POSS nanocomposites and polyimide (
Leu et al. used pyromellitic dianhydride (PMDA) react with 4,4'-oxydiphenylamine (ODA) in DMAc under N2 atmosphere at room temperature to form PAA (
Xiangxiu Chen et al. synthesized trapezoidal multifunctional polysiloxane (PN-PSQ) with a large number of amino groups and phenanthrene phosphide structures so as to improve the heat and fire resistance, dimensional stability and dielectric properties of BMI resin (
Devaraju et al. blended glycidyl terminated hyperbranched polysiloxane (HPSiE) with different percentages into diglycidyl ether bisphenol A (DGEBA) resin, and added curing agent diaminodiphenylmethane (DDM) (
Chen, Jiangbin et al. designed and synthesized a benzoxazine resin with high frequency, low dielectric constant (<3) and very low dielectric loss (tan<0.005) (
Ping Yu et al. used 3,4'-diaminodiphenyl ether (3,4'-ODA), 2,3,3',4'-biphenyl tetracarboxylic dianhydride (a-BPDA) and 2,3,3',4'-Oxydiphthalicanhydride (a-ODPA) as raw materials, synthesizing a new asymmetric bismaleimide oligomers with different molecular weights and dianhydride (
Introducing air holes or nano foam materials, blending inorganic low dielectric materials, forming hyperbranched structures (
The morphology of inorganic particles includes zero-dimensional nanofillers (spherical nanoparticles, nanocubes and nanoparticles with irregular morphology), one-dimensional nanofillers (nanowires, nanofibers, nanotubes, such as carbon nanotubes CNT) and two-dimensional nanofillers (including nanosheets and nanoplates, such as graphene and nanoclay). Among them, it is often used Al2O3 (
Carbon nanotubes (CNTs) and graphene nanoplatelets (GnPs) are respectively typical one-dimensional and two-dimensional carbon nanofillers, which are commonly used in polymeric composites with dielectric properties (
Li et al. studied the dielectric properties of polyamide 11(PA11)/CNT and PA11/Polypropylene (PP)/CNT composites, and the dielectric constant of all systems enhances significantly when the loading of CNTs approaches the conductively percolative threshold (
As is mentioned above, nanofiller is easy to agglomerate because of its high surface area and volume effect. According to the micro-capacitor model (
The influence of interface on the dielectric properties of nanocomposites has been widely studied and can be described from two aspects (
The change of polymer molecular structure (free volume fraction, mobility, crystallinity and polymer chain configuration) from interface to matrix caused by interface will affect the way of charge transport and the dielectric properties of polymer matrix. Generally, it is proved that the dielectric constant of the interface region surrounded by filler is lower than that of the bulk polymer matrix (
Wu et al. used silica (SiO2) to coat multi-walled carbon nanotubes (MWCNTs) to form a core-shell structure (SiO2@MWCNTs) and organically modified montmorillonite (O-MMT) synergistically modified Reinforce epoxy resin (
Graphene oxide acts as an insulator, and its oxygen atoms are randomly attached to the graphene. The sp2−hybridized carbon atoms are converted into the sp3−hybridized carbon in GO, which reduces the conjugation and limits the π electrons, and loses the graphene’s conductivity. In view of the homogeneity of GO is not as high as that of graphene, and the electron mobility is much lower. The dielectric constant of the film is determined by the dielectric constant of the PI matrix and the impedance of GO. Jen-Yu Wang prepared two low dielectric constant and ultra-high strength graphene oxide (GO)/polyimide composite films (PI-GO and PI-ODA-GO films) through solution blending and
Yang et al. synthesized core-shell fluoropolymer@BaTiO3 hybrid nanoparticles with different shell thicknesses or different shell structures through reversible addition-fragmentation chain transfer (RAFT) polymerization (
Among them, the dielectric loss of dielectric materials usually comes from three different factors: direct current (DC) conduction, space charge transfer (interface polarization) and the movement of molecular dipoles (dipole loss). Generally speaking, a decrease in polymer content will result in a decrease in molecular dipoles, which means a decrease in dipole loss; BaTiO3 nanoparticles are covered by a stable and dense fluoropolymer shell, the formation of an insulating layer outside the dielectric nanofiller limits the transfer and accumulation of space charges in the nanocomposite material. Surface modification by fluoropolymer improves the dispersibility of BT nanoparticles and enhances the interfacial adhesion of nanocomposites, which may further limit the movement of molecular dipoles.
The interface not only exists between polymer-filler particles, polymer-polymer, and filler particles-filler particles. For copper-clad laminates, the interface between polymer composites and metal microstrip wires has an impact on the performance of the final product. important influence. In order to manufacture very fine copper circuits with line widths and pitches less than 1 micron, it is necessary to maintain the dielectric properties of the printed circuit board while improving the adhesion between the copper and the dielectric substrate.
In high-speed signal transmission, there are two factors that cause transmission loss due to signal propagation loss on printed circuit boards: conductor loss and dielectric (insulating material) loss (
Conductor loss can be divided into scattering loss caused by surface roughness and skin effect loss (
Among them, αH is the loss caused by the skin effect, and αS is the scattering loss. The skin effect is a phenomenon called the skin effect that AC signals mainly flow near the surface of the conductor, rather than the entire cross-section of the conductor. The general way to reduce conductor loss is to make the surface of the circuit smooth. However, smoothing the surface of copper circuits will weaken the adhesion between them and insulating materials. Therefore, it is difficult to manufacture a copper circuit with a smooth surface and good adhesion to insulating materials. Silica nanofillers include spherical, amorphous, fused, crystalline, and synthetic silica. Hisao Kondo et al. used uniform spherical SiO2 as a filler to make the insulating material and the substrate have a similar coefficient of thermal expansion (
In addition, the surface of dielectric materials can be treated by plasma treatment, ultraviolet light, γ-ray, ion radiation, and chemical modification (
In dielectric bonded films, the adhesive force is related to the dielectric properties of the substances involved, so that the dielectric constant, conductivity, dipole moment and refractive index can also be used to determine the general adhesive tendency. IWAMOTO, NE et al. state that the adhesive force is the sum of the forces acting between metal-polymer, polymer-polymer and any intermediate layers (e.g. metal-oxide and oxide-polymer) (
The metal-polymer interface is analogous to a capacitive circuit, where A is the surface area of the capacitor; d is the distance between the capacitor poles. ε is the dielectric constant, ρ is the resistivity, ω is the frequency, R is the electric field induced by the dipole in the interaction and μ is the dipole moment. However, it cannot be ignored that the structure and geometry of the dielectric bonding film interface, plastic deformation has an influence on the measured structure of the experiment, but this complicates the model.
Copper-clad laminates (CCL) are the dominant base material for the manufacture of PCBs. Copper-clad laminates include double-layer flexible copper-clad laminates (FCCL) made of insulating film and copper or triple-layer flexible copper-clad laminates (3-FCCL) with additional adhesive. Most of the existing commonly used high temperature resistant adhesives are epoxy resin adhesives, which are widely used as adhesives due to their strong bonding properties, chemical resistance, good insulating properties, low curing shrinkage and other excellent properties. The different molecular weights of epoxy resins affect their bonding properties because the number of epoxy groups in the molecule decreases as the average molecular weight increases; the distance between the crosslinking points then increases after curing, which leads to a reduction in crosslink density. Due to the severe stress concentration, the mechanical properties of the cured product will decrease as the crosslinking density decreases. This coupled with the fact that internal stresses are difficult to relieve as the crosslink density increases, will result in a non-uniform network structure and reduced mechanical properties. To solve this problem, some researchers have tried to blend two or more epoxy resins with different molecular weights to obtain products with medium crosslinking density.
Many add rubber elastomers to epoxy resins to overcome their brittleness, and phenolic resins and modified polyimides are receiving increasing attention as an ideal class of high temperature resistant adhesives. Polyimides can be divided into two categories, thermosetting and thermoplastic. In recent years, thermosetting PI resins have been valued and developed significantly for their excellent heat resistance. The curing process requires appropriate pressurisation to remove air bubbles due to moisture and solvent evaporation.
Lee et al. designed a three-layer composite structure of polyimide (PI) film as a core layer with low dielectric bonding layers on both sides for a high frequency bonding layer (HFBP) film with a Dk of about 2.80 and a Df of about 0.006 at 10 GHz and a peel strength of more than 1.2 kgf/cm (
Takashi Tasaki et al. developed solvent soluble polyimide with good heat resistance and low dielectric constant (Dk)/dissipation factor (Df) characteristics by optimising the composition ratio of aliphatic, alicyclic and aromatic groups present in the polyimide backbone (Tasaki, 2018). Using this PI binder, a low-profile copper foil and a plain PI film, a three-layer FCCL was developed which showed similar transmission losses to the LCP FCCL at frequencies less than 20 GHz.
In addition to properties such as low dielectric constant, low hygroscopicity and high Tg, materials used as insulating layers in microelectronic applications are expected to exhibit high adhesive adhesion to metals used as interconnecting wires. Bond damage can occur either within the bonding layer or at the interface between the bonding layer and the surface to be bonded. The former refers to cohesive failure, where the adhesive ends up on both surfaces. The latter refers to interfacial bond failure, where the damage occurs at the interface between the bonding layer and the surface to be bonded (
Before introducing the mechanism of bond breakage, we first introduce four mechanisms that explain adhesion.
Spherical particles have a higher packing density and uniform stress distribution than laminated particles, which when filled into the resin increases fluidity, resulting in better mechanical and adhesive properties of CCL.
According to the Debye relaxation model, the real and imaginary parts of the dielectric constant can be expressed by the following equation:
The model relates the dielectric properties to the relaxation time. The relationship between
This is a form of the hemispherical diagram, often referred to as the Cole-Cole diagram (
Todd et al. used the dielectric constants of the filler component, matrix component and interphase region and their respective volume fractions to determine the effective dielectric constants of the composite system (
The IPL model is applicable to any random, homogeneous dispersion of filler particles in the matrix. The model takes into account interphase permittivity, interphase volume fraction, interphase overlap and filler shape/orientation effects.
On the one hand, experiments provide an intuitive understanding of physical and chemical processes from a qualitative or quantitative perspective, and the simulation method can even further quantitatively evaluate the microstructure and properties of polymeric materials systematically (
On the basis of MD, Lina Si et al. analyzed the nanoporous amorphous silica (n-a-SiO2) with different porosities and mechanical properties of the films, as well as the interfacial adhesion strength of silica (
If the interaction energy is negative, there is an attractive force between the objects, such as adhesion; if it is positive, there is a repulsive force. The interfacial strength of n-a-SiO2/SiO2 decreases with the increase of the porosity, and the introduction of -CH2 groups in SiOCH improves its bonding strength.
Tao Pang et al. used furfurylamine (FU), aniline (AN) and bisphenol A to prepare furfural-based thermosetting benzoxazine resins by using toluene as a non-polar solvent (
The elastic modulus of the crosslink density (ρ) of the thermoset material in the rubber region can be estimated by the following equation (
In addition, the concentration of cross-linking per unit volume (Xdensity) can be used to define the cross-linking density, using the following semi-empirical equation (
In the formula, G' is the storage modulus of cured resins in the rubber platform area where the temperature is higher than glass transition temperature (Tg). In this work, G' is the modulus at absolute temperature T, which is 40°C higher than Tg. The possible cause of this phenomenon can be analyzed through the following Debye formula (
In the signal transmission, the chemical structure of furan without benzene ring is conducive to the reduction of dielectric constant and dielectric loss factor. Firstly, after AN with the phenyl in high polarizability being replaced by FU, a reduction in polar functional groups occurred in polybenzoxazines, leading to the reduction of N and
Yanhui (
Due to the inherent structural heterogeneity of polymers, it can be described by the superposition of Debye functions with different relaxation time:
α and β represent the relaxations of different structures in the interface region. α relaxation is the micro-Brownian motion of entire chain segment, and β relaxation is the rotation of polar groups. τi is higher than τ0 when α is relaxed, and τi is lower τ0 when β is relaxed. M, S and c respectively are tuning parameters under the corresponding relaxation obtained by matching the simulation output and the experimental data, the relaxation time and effects of dipole, ion and electronic polarization. The increase in dielectric loss at low frequencies is attributed to the α relaxation caused by the movement of polymer main chains, while the peak around 104 Hz is attributed to β relaxation caused by the movement of polar groups on side chains. It shows that the horizontal moving of Sα and Sβ in the curve reflect the change of the relaxation time of α and β in interface area, and the vertical displacement of Mα and Mβ in the curve reflect the change of the degree of polarization. The tuning parameter c reflects the change of dielectric constant over 106 Hz under effects of dipolarization, ion polarization and electronic polarization. For the tested system, S is usually less than 1 and M is greater than 1, which indicates that the chain segment at the interface has greater mobility and polarizability than that in the matrix. At the same time, the interface area of the composite may have stronger chain mobility and polarization in the frequency range measured. The change in polarization at higher frequencies may be attributed to short functional groups such as short-chain molecules (i.e. p-thiophene and ferrocene). It should be noted that only unimodal PGMA nano-dielectric material (Mα = 4) cannot reflect the change of polarization degree very well, which is caused by relatively poor fitting of its loss curve.
Furthermore, Yanhui Huang et al. used three-dimensional finite element analysis again to analyze a series of core@single-shell/double-shell nanocomposites (NP) (
For BT@PMMA, at low BT content (16 vol% BT), the simulated dielectric spectrum of the longer graft chain matches the experimental data very well. At high BT content (40 vol% BT), the short graft chain would induce the elimination of the volume effect, and the polymer chains near the particle surface are more easily stretched and tightly packed. At this time, the filler/matrix interface interaction cannot be ignored. Compared to free polymer, this produces an interfacial region with a slower relaxation time and reduced strength. Since the simulation is assuming that the grafted polymer has the same relaxation behavior as the free polymer, the difference between the simulated dielectric spectrum and the experimental dielectric spectrum may be related to the grafted polymer having an interface area with the free polymer. As the geometry and interface are considered in the simulation, the interface relaxation behavior is clearly included in the simulated dielectric spectrum, which is different from the two-dimensional finite element analysis.
Since the grafted polymer and the free polymer have different relaxation behaviors, the dielectric spectrum of the free polymer initiated without barium titanate is decomposed into a superposition of 20 Debye relaxation functions, and the tuning parameters M, S is used as the offset factor of relaxation strength and relaxation time to inversely calculate the interface relaxation:
Similarly,
The strong adhesion and excellent dielectric properties of polymer materials are often contradictory. It has been proved difficult to accurately predict the dielectric properties. Understanding the mechanism of interface adhesion failure and the mechanism of high-frequency dielectric response is an important step in the design to meet the requirements of high-speed and high-frequency signal transmission. Due to the inherent multi-variable problem of adhesion, the morphological factors of different chemical structures and the bonding strength of elements to copper will affect the results of the peeling test between copper and polymer. The method of surface modification improves the bonding strength (addition of functional groups that promote adhesion into the polymer/metal interface), which can avoid the introduction of a large number of polar groups in the body of the adhesive film, but this lacks the specificity of the groups to improve the adhesion. At the same time, the surface treatment conditions, such as the pre-curing of the polymer and the surface treatment, affect the interface roughness and mechanical strength, and ultimately determine the adhesion and dielectric properties.
As a non-polar liquid rubber, polybutadiene has a low dielectric constant and loss due to its extremely weak polarity, making it very suitable for high insulation materials. There have been studies on the modification of polybutadiene with functional groups and epoxy resins, for example, HTPB (hydroxy-terminated polybutadiene), EHTPB (epoxidized hydroxy-terminated polybutadiene), CTPB (carboxyl-terminated polybutadiene) Polybutadiene), NCOTPB (isocyanate terminated polybutadiene) to modify and toughen epoxy resin. However, the interface bonding strength between non-polar polybutadiene and copper foil is poor, and it is very challenging to improve the interface bonding strength as much as possible without damaging the dielectric properties. In terms of low dielectric constant, since the dielectric constant of the filler particles is much higher than the dielectric constant of the polymer, the filler particles are the main factor that affects the dielectric constant of the dielectric bonding film material. How to control the dielectric constant of the filler particles to achieve the low thermal expansion coefficient and low dielectric constant of the dielectric bonding film material has important theoretical significance and engineering application value.
Ceramic particles are often used to reduce the thermal expansion coefficient of the polymer matrix material and adjust the dielectric constant of the adhesive film material. Compared with layered particles, spherical particles have higher bulk density and uniform stress distribution, and can increase fluidity when filled into resin, so that CCL has better mechanical properties and adhesion properties. SiO2 is the most common surface modifier and filler used in CCL. Due to its excellent performance, many literature reports that the spherical SiO2 used in electronic equipment can improve dielectric properties, increase heat resistance, improve drilling processability and reduce costs. In addition to the molecular structure design of the polymer, attention should be paid to the influence of nano-filler particles on the dielectric properties of the composite material. The higher interface area of the nano-composite material effectively improves the polarization effect. The improvement of the chemical or physical bonding between the polymer matrix and the filler helps to reduce the conduction loss and the dielectric loss caused by the interface polarization. Therefore, the surface modification of nano-filler particles such as SiO2 can be carried out. For example, functional groups (such as γ-APS, γ-aminopropyltriethoxysilane) are introduced after treatment with the coupling agent and take advantage of the synergistic effect between different types of filler particles (such as multi-component polydisperse fillers and core-shell fillers) to give composite materials more excellent comprehensive properties. At the same time, a theoretical model of the relationship between the structure of the functional filler and the dielectric properties is established to provide new ideas for the development of high-performance dielectric materials. In addition to directly introducing nano-filler particles SiO2 (or POSS-NH2 with functional groups), tetraethoxysilane can be used to prepare multiple composite films through a sol-gel process. Through the modification and adjustment of the composite system, the performance of the hybrid multi-element composite material and its relationship with the composition could be studied, and the thermal expansion coefficient of the composite material would be optimized. Thus, its complementary effect would improve the weakness of the single or binary system.
LW: article framework design and writing; JY: literature review and summary of relevant conclusions; WC: picture editing and related copyright acquisition; JZ: form editing and correction; DZ: Reference editing and correction.
Authors LW, JY, WC, JZ, and DZ were employed by the company China Electronics Technology Group Corporation No. 38 Research Institute.
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