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 (Yu et al., 2018).

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 (Zhang, 2019). Among them, polybutadiene e.g. cis-1,4- polybutadiene, trans-1,4-polybutadiene, 1,2-polybutadiene and polybutadiene with different contents can be synthesized through different polymerization conditions and methods. The side chain of high 1,2-polybutadiene contains many double bonds (with a high degree of unsaturation) and is an amorphous structure, mainly used as an adhesive and sealant. Peroxide decomposed at different temperatures can be selected as crosslinking agent to control the curing temperature of crosslinking reaction. The absence of polar groups limits its application range to a certain extent. For low molecular weight non-vulcanized polybutadiene liquid rubber, the characteristic of easy-creeping makes it difficult to form a stable coating film at room temperature. Up to now, researches on polybutadiene utilized in CCL are relatively less. Other dielectric polymers also include polyphenylene ether (PPO), cyanate ester (CE), Poly (arylene ether nitrile) (Tong et al., 2019), parylene, polynaphthalene, polybenzoxazole resin, polynorbornene, Polyaryl ether, polyarylene, SiLK resin, polyquinoline, polyquinoxaline, polybenzocyclobutene, porous polymer materials (Volksen et al., 2010).

%1.2 Principles and examples of molecular structure design.

According to the Clausius-Mossotti formula: ε = 1 + 2   (   p m v m   ) 1 − (   p m v m   ) Where k is dielectric constant, Pm is molar polarizability and Vm is molar free volume fraction. It can be seen from the formula decreasing the molecular molar polarization and increasing the free volume of the polymer are the main ways to reduce the dielectric constant.

The reduction of dielectric constant and dielectric loss by polymer molecular structure design is usually achieved by several key methods. 1) Introducing C-F and C-C chemical bonds composed of chemical elements with low polarizability. 2) Introducing asymmetric, rigid non-planar conjugated structure or flexible structure into the polymer main chain through copolymerization and block polymerization to reduce the molar polarizability. 3) Adding space-occupying groups such as methyl, ethyl and phenyl to the dielectric material to increase the free volume in the polymer structure.

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 (Dhara and Banerjee, 2010). On the other hand, the bond energy of the C-F bond is very strong, and the fluorine-containing group contains a larger free volume.

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 (Qi et al., 2019). Among them, the segments of ODPA-BATMS and ODPA-ODA are randomly distributed along the main chain of the copolymer, but the segments of ODPA-BATMS are alternately composed of disiloxane alkyl and imide rings (Figure 2). The flexible, high-temperature resistant disiloxane segment increases the molecular chain spacing and inhibits the charge transfer complex (CTC). The interaction between polymer chains is reduced, which helps to reduce chain accumulation and charge transfer electron polarization interactions. When the molar ratio of BATMS reaches 75%, the free volume and hydrophobic properties increase, and the molar polarization is significantly reduced. The dielectric constant of the copolymer film PI-75 is as low as 2.48 (1 MHz, 25°C). In addition, there exists the inherent non-polarization of the disiloxane segment and the change of the aggregation state of the copolymer film. The molar volume of the siloxane segment unit increases and the unit molar polarizability of the segment decreases. On the one hand, the introduction of asymmetric monomers or large side groups increases the free volume of PI molecules and disrupts the formation of effective CTCs. On the other hand, this reduces the interaction between polymer chains and the rigid structure of the polymer, which can improve the melt processability of the polymer or the solubility in organic solvents.

Polyhedral oligomeric silsesquioxane (POSS) with empirical formula (RSiO_1.5) 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 (Mohamed and Kuo, 2019), and the covalent connection of the end or side chain, the covalent bonding of the bifunctional POSS nanocomposite material and the polyimide main chain, and the formation of the polyimide thermal crosslinking structure of the multifunctional POSS nanocomposite material. Among them, POSS molecules are usually stable at temperatures as high as 400°C, which is higher than the thermal degradation temperature of most polymers. Incorporating them into some polymers leads to improved thermal stability and mechanical properties (Leu et al., 2003a). Wei et al. synthesized POSS-diamine monomer, and synthesized nano-polyimide composites with an adjustable dielectric constant by controlling the molar ratio of POSS-diamine monomer, PMDA, and ODA (Leu et al., 2003b). At the same time, POSS can be used as a large steric hindrance substituent group, the lowest dielectric constant of this composite material is 2.3, and the mechanical properties are controllable.

Leu et al. used pyromellitic dianhydride (PMDA) react with 4,4'-oxydiphenylamine (ODA) in DMAc under N₂ atmosphere at room temperature to form PAA (Leu et al., 2003a). Then, POSS-NH₂ reacts with PAA in DMAc to obtain PAA/POSS nanocomposite (Figure 3). After the PAA/POSS nanocomposite is thermally imidized at 300°C, its chain end is connected with POSS nanoparticles to obtain PI. Upon addition of 2.5 mol% of POSS units, the dielectric constant of the resulting polyimide POSS nanocomposite decreased from 3.40 to 3.09 for the pure PI without any decrease in mechanical strength. POSS cage-like siloxane structure is currently a better silicon-containing group for modified PI, which can overcome the disadvantages of reduced mechanical properties and high cost due to dependence on fluorine-modified materials.

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 (Chen et al., 2014). On the one hand, PN-PSQ resin with phosphide-phenanthryl group has larger benzene side groups and non-coplanar structure; on the other hand, the–NH₂ group of PN-PSQ resin shortens the distance between curing crosslinking points of BMI resin which to a certain extent increased the crosslinking density of BMI resin, so the orientation and relaxation of dipoles which accompanying the movement of polymer segments is limited, and finally the dielectric properties (the dielectric constant and loss at 1 MHz are 3.01 and 0.008, which are only 88 and 80% of that of BMI resin, respectively) are optimized. While there are polar reactive groups in PN-PSQ, their good symmetry has little effect on the dielectric constant and loss of the 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) (Devaraju et al., 2013). With the increase of HPSiE composition, the flexible polysiloxane bonding arising more porous voids, compared with pure DGEBA resin (the dielectric constant is 4.5), the dielectric constant can reach 3.5 at 15 wt%. Owing to the chemical interaction between HPSiE and DGEBA and the existence of Si-O-Si hydrophobic bond in the composites, the contact angle and water resistance are enhanced. Jie Dong et al. make amino-functionalized hyperbranched polysiloxane (NH₂-HBPSi) graft onto PI chain by in-situ polymerization (Dong et al., 2017). The large bulk of HBPSi enhances the free volume and dielectric confinement effect, the dielectric constant of 16 wt% NH₂-HBPSi/PI can be as low as 2.2, and the dielectric loss is about 0.01 at 10⁸ Hz.

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 (Yao et al., 2015) SiO₂、TiO₂、h-BN (Yang et al., 2017; Ge et al., 2019)、AlN (Wang et al., 2019) and ZrTi₂O₆ to improve the dielectric and mechanical properties. Because of the hydrophilicity of ceramic filler surface and the hydrophobicity of polymer, there is interface incompatibility, which leads to the aggregation of nanoparticles (Mittal et al., 2018). The difference of dielectric constant and conductivity between ceramic filler and polymer does not match at the interface, which leads to the accumulation of charge and local redistribution of electric field, and produces Maxwell-Wagner interface polarization (Murugaraj et al., 2005). Compared with zero-dimensional nano-filler, one-dimensional ceramic filler is easier to permeate, which leads to higher relative dielectric constant and heat conductivity coefficient. At the same time, large dipole moment is beneficial to the dielectric enhancement of polymer composites under low load. The zero-dimensional nano filler is easy to agglomerate because of its high surface area and volume effect, and is difficult to form monodisperse nanoparticles in polymer matrix, so it is necessary to modify the surface of inorganic particles (Kango et al., 2013). In order to obtain the strong interaction between polymer and nano-filler, the dispersibility of nano-filler and its compatibility with polymer are of vital importance. The filler dispersion of polymer/nanofiller composites (PNC) which is prepared by solution intercalation, in-situ intercalation, melt intercalation and other methods can be improved to a varying extent. The surface treatment of nano-filler, among which the surface modification can obtain good dispersibility and/or compatibility with polymer of nano-filler, and improve Tg of composite material.

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 (He et al., 2017; Zhao et al., 2019). It is well known that a sudden increase in the dielectric permittivity with one or several orders of magnitude is presented when the loading of conductive fillers reaches a critical value (Xu et al., 2015), i.e., the percolation threshold. However, when the content of the conductive filler is very close to or above the percolation threshold, conductive networks are generated and, thus, significant dielectric loss caused by leakage conduction loss from the capacitors would occur (Huang and Terentjev, 2010). On this basis, the dielectric behavior of polymeric composites can be tailored by tuning the conductive network structures, such as by compounding appropriate contents of carbon fillers in polymer matrix, as well as controlling the agglomeration situation of conductive nanofillers.

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 (Li et al., 2019). As for the PA11/CNT system, when the content of CNTs reaches up to 0.1 wt%, it shows an obvious increase of the dielectric constant from 6 to 50 at 100 Hz. The AC conductivity of the exceeded loading of 2 wt% CNTs shows a typical frequency-independence within the low frequency range, presenting a typical transition from insulation to conduction. In terms of PA11/PP/CNT composites, a lower percolative loading on 0.05 wt% of CNTs was presented (A9P1C0.05) when the dielectric constant underwent its sharp increase, arising from the segregated conductive structure of CNTs in the PA11/PP blend. For the possible reason, the author proposed the effect of “barrier medium” PP in the segregated CNT network on the dielectric property. They thought the PP long chain can diffuse into the CNT bundles with the incorporation of polar PP-g-MAH, enhancing interfacial interaction between CNTs and PP. As a result, the charge accumulation under electric field would be enhanced via interfacial polarization. The mini-capacitor network with CNTs as electrodes and a very thin PP layer as dielectric can be formed in the composites near the percolative threshold, which is correlated with the increment of dielectric constant. In addition, the PP in between CNTs can hinder the connection of CNTs and the current leakage, finally resulting in the suppressed increase of dielectric loss. (Re. Constructing a segregated carbon nanotube network in polyamide-based composites towards high dielectric constant and low loss).

As is mentioned above, nanofiller is easy to agglomerate because of its high surface area and volume effect. According to the micro-capacitor model (Prateek et al., 2016), a better dispersion of conductive particles in an insulating polymer matrix is capable of generating more micro-capacitors, which is benefit for achieving higher polarization effect. Zhu et al. reported CNT/PVDF dielectric composites dielectric composites, in which CNT was modified by coating with the polydopamine (PDA) (Zhu et al., 2017). In this study, the coating layer was controlled by changing the concentration of dopamine monomers, and this PDA coating, playing roles of constructing the barrier layer and hindering CNTs aggregation, benefiting the dispersion of CNT in the PVDF matrix. It is exhibited that both of the dielectric permittivity and loss factor of the composites gradually decrease accompanied with the thickening of PDA coatings, which can be ascribed to the weakened interfacial polarization between conducting particles and polymeric matrix.

The influence of interface on the dielectric properties of nanocomposites has been widely studied and can be described from two aspects (Zhang et al., 2018). On one hand, the interface structure characteristics (such as inorganic/organic compatibility, polymer/filler interaction, polymer chain configuration and particle surface state) can determine the final dispersion or aggregation of ceramic nanoparticles in polymer matrix. On the other hand, abnormal local interface electrical/dielectric properties (e.g. interface electronic state and effects on traps or scattering points, charge density, charge redistribution and electric field) can adjust the dielectric and breakdown behavior of nanocomposites and the overall performance of polymer and ceramic fillers. Polymer-filler particle interface model, such as dielectric bilayer model (LEWIS, 1994), multi-core model (Tanaka et al., 2005) and interphase power law model (Todd and Shi, 2005), phase field model (Shen et al., 2018) etc., the development of these model can explain the influence of interface effect on electrical and dielectric properties of nanocomposites. The model makes idealized assumptions on the size and shape of micro/nano-particles, and a single theory or model cannot explain the interaction between interfaces. According to the volume fraction, shape and direction of particles in the filler, the ratio of dielectric constant between filler and matrix and the interaction between filler and matrix, the geometric shape, size, concentration and size-to-dielectric constant of composite materials can be predicted through one or more equations in the model.

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 (Anandraj and Joshi, 2018). Theoretically, nanoparticles with larger surface area and higher interface strength can effectively limit the orientation and movement of dipoles in the interface region. Although interfacial polarization increases the dielectric constant to a certain extent, interfacial inhibition plays a major role in affecting the dielectric constant of composites. The bonding between filler and matrix also helps to reduce dipole polarity, the decreasing of mobility of polymer chain and the charge transfers lead the dielectric loss decreases simultaneously. However, in fact, nano-materials have a larger surface area than micron-materials (higher dielectric constant and loss due to higher interfacial polarization), which leads to easy aggregation of materials in the matrix (especially at higher load, aggregation of nanofillers and its poor compatibility with polymers will lead to induced voids and matrix density decreases). Aggregation of nanofillers and poor compatibility with polymers will lead to weak interaction between nanofillers and polymers, it means that there is more free volume at the interface between filler and polymer, which makes it cannot effectively limit the mobility of polymer chain. At the same time, the ceramic filler is hydrophilic in nature, if the particle surface is not coupled with silane coupling agent, its hygroscopicity will increase, resulting in a decrease in dielectric properties. In addition, if silane coupling agent is used for surface modification, the introduction of polar groups may increase the dielectric constant. It should be understood that silane coupling agent improves the interface adhesion.

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 (Wu et al., 2017). Under the lower filling of nanometer materials, the firm interface between the filler and the polymer and the entanglement of EP molecular chains are conducive to restricting the movement of charge carriers, and contributing to the reduction of dielectric constant and dielectric loss. On the one hand, its surface has abundant polar groups (-OH), which produces higher interface strength with the polymer. On the other hand, the one-dimensional filler carbon nanotubes (CNT) with high aspect ratio has a relatively low surface area, which helps to reduce the surface energy, thereby preventing filler agglomeration (Huang et al., 2019a). In addition, the conductive percolation threshold of most CNT/polymer conductive nanocomposites is less than 1 wt%. Therefore, higher interface strength and larger surface area can effectively limit the orientation of the dipole and the movement at the interface. At the same time, the SiO₂ layer can effectively inhibit the electron transfer and accumulation of the composite. However, under the filling of higher nanomaterials, the aggregation of CNTs will reduce the area of the interface region. Compared with epoxy resin, carbon nanotubes have a higher dielectric constant. At this time, the interface polarization is dominant, so the dielectric constant increases instead.

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 in-situ polymerization (Wang et al., 2011). As the GO content increases, the dielectric constant (Dk) decreases to 2.0. Moreover, the uniform dispersion of ODA-GO in the PI-ODA-GO composite film prepared by in-situ polymerization leads to an increase in the dielectric constant. In addition to the introduction of ODA-GO can increase the free volume in the PI matrix, at the same time the partially reduced GO sheet evenly dispersed in the PI can also form many heterojunctions, limiting the movement of electrons in the PI, that is, limiting The electrons in PI are polarized, thereby reducing its dielectric constant. On the one hand, if the GO surface has many polar functional groups, the molecular polarization of PI is increased. On the other hand, many tiny incompatible interfaces are formed between PI and GO, and the interaction force is weak, which is equivalent to introducing a large amount of interface polarization to produce interface polarization.

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 (Yang et al., 2013). The encapsulated shell polymer is chosen to be more insulating to maintain high resistivity and low loss. Using this hybrid nanoparticle as a filler, a solution blending method was used to prepare an energy storage nanocomposite based on polyvinylidene fluoride-hexafluoropropylene copolymer [P(VDF-HFP)] (Figure 4). Among them, the BT-PTFEA hybrid nanoparticles with shorter side groups have a higher graft density, and the interface formed by the close packing limits the mobility of the macromolecular chains of the matrix polymer, resulting in the lowest dielectric loss [from Pure (VDF-HFP) 0.032 is reduced to the lowest 0.016].

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 (Kondo et al., 2016): α = α C + α d

Conductor loss can be divided into scattering loss caused by surface roughness and skin effect loss (Okubo et al., 2013): α C = α H + α S

Among them, α_H is the loss caused by the skin effect, and α_S 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 (Kondo et al., 2016). Compared with the conductor surface that is usually micro-etched or roughened in the past, the film has a smooth surface to reduce the conductor loss.

In addition, the surface of dielectric materials can be treated by plasma treatment, ultraviolet light, γ-ray, ion radiation, and chemical modification (Baklanov et al., 2013). Polar functional groups are introduced on the surface to increase the polarity and polarity of the material surface. Surface tension can avoid the introduction of a large number of polar functional groups in the body, thereby avoiding poor dielectric properties when improving adhesion performance. Oxygen-containing polar functional groups at the interface have strong electronic polarization and atomic polarization, resulting in high dielectric constant and dielectric loss. In particular, while the interface adhesion is improved, when the peel strength of the polymer-metal interface exceeds a certain value, the failure mode of the polymer and copper foil after surface modification is essentially cohesive failure. There is no point in increasing the interface bonding at this point.

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) (Iwamoto, 1995). For the general interfacial interaction energy, this can be qualitatively summarised by the following expression: W ∝ ∑ f ( ε , A d , ρ , ω , R ) m e t a l − d i e l e c t r i c + ∑ f ( μ , R ) d i e l e c t r i c − d i e l e c t r i c

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 (Lee and Du, 2016). By adjusting the thickness of the core layer, it can be used as a bonding sheet in a copper-clad laminate (CCL) It can also be used together with copper foil for the preparation of high frequency single/dual sided substrates. HFBP films offer better performance than LCP films in terms of workability and thickness tolerance.

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.