JP2024061741A - Manufacturing method of resin film and manufacturing method of metal-clad laminate - Google Patents

Abstract

[Problem] To provide a resin film capable of handling higher frequencies in electronic devices by further improving the dielectric properties of a resin film made from polyimide. [Solution] A method for producing a resin film having at least one crystalline silica-containing polyimide layer containing a solid content of an organic compound containing polyimide and crystalline silica particles, comprising the steps of: directly or indirectly applying a resin composition containing a polyamic acid, which is a precursor of polyimide, and crystalline silica particles onto a supporting substrate to laminate a coating film; and heat treating the coating film to imide the polyamic acid and form a crystalline silica-containing polyimide layer. The crystalline silica particles as a whole have a ratio of the total area of peaks derived from the cristobalite crystal phase and the quartz crystal phase to the total area of peaks derived from SiO2 in the range of 2θ=10° to 90° in an X-ray diffraction analysis spectrum using CuKα radiation of 20% by weight or more. [Selected Figure] None

Description

The present invention relates to a resin composition, a resin film, and a metal-clad laminate that are useful, for example, as circuit board materials.

In recent years, the use of thin and lightweight flexible printed circuit boards (FPCs) has increased as electronic devices become smaller, lighter, and more space-saving. FPCs allow for three-dimensional, high-density mounting even in a limited space, so their applications are expanding to wiring the moving parts of electronic devices, cables, connectors, and other components. In addition, as communication devices become faster and more sophisticated, there is a need for FPCs that are compatible with high-frequency transmission signals to achieve high transmission speeds.

One of the issues when transmitting high-frequency signals is the transmission loss in the signal transmission path. If the transmission loss is large, it causes inconveniences such as loss of electrical signals and signal delays. In order for FPCs to handle higher frequencies, it is important to reduce transmission loss by lowering the dielectric constant and dielectric dissipation factor of the insulating resin layer. For this reason, liquid crystal polymers and fluororesins with low dielectric constants and dielectric dissipation factors are used as insulating resin layers in FPCs for high-frequency signals. However, while liquid crystal polymers have excellent dielectric properties, there is room for improvement in their heat resistance and adhesion to metal foils, and fluororesins have a high thermal expansion coefficient and are therefore difficult to handle.

Polyimide is also used as an insulating resin with excellent heat resistance and adhesion to metal foil. However, polyimide generally has inferior dielectric properties compared to liquid crystal polymers and fluororesins. One method for reducing the dielectric constant of aromatic polyimides is to incorporate an aliphatic structure into the resin skeleton. However, the advantage of incorporating an aliphatic structure into the resin skeleton is often in a trade-off with the required properties of FPCs, such as heat resistance and dimensional stability. Other methods include using special monomers or expensive fluorine-based monomers, but there are various hurdles in terms of cost and productivity to achieve low dielectric constant while maintaining excellent heat resistance and the required properties of FPCs using only the polyimide skeleton.

Therefore, a method for improving the properties by compounding a resin with a different material is known. For example, Patent Document 1 reports a polyimide composition with excellent thermal conductivity obtained by compounding aluminum nitride or silicon nitride powder with a specific polyimide.
In addition, in Patent Document 2, cristobalite silica is added to an epoxy resin to suppress the occurrence of warping and cracks in a semiconductor package.
Combining is also effective in improving dielectric properties, and Patent Document 3 proposes a resin film with low dielectric tangent and low thermal expansion by combining a thermosetting polyimide resin synthesized from amorphous silica and bismaleimide with an elastomer. Patent Document 4 also presents a method for producing a low dielectric polyimide film by combining polyimide and fluororesin particles.

Patent No. 3551687 JP 2019-1937 A JP 2018-12747 A Patent No. 6387181

As described above, there has been a demand for the development of a polyimide with low dielectric constant while maintaining properties such as high heat resistance and excellent adhesiveness. However, in Patent Document 3, the polyimide is limited to a thermosetting resin, and there is a problem with the adhesive strength when multi-layered. In addition, in Patent Document 4, a relatively expensive fluororesin filler is used in combination, and there is a problem with a high thermal expansion coefficient.
Furthermore, in the prior art composite technology, emphasis is placed on the composition of the filler, and there is little mention of its crystal structure.

The object of the present invention is therefore to provide a resin composition and a resin film that can accommodate the increasing frequency of electronic devices by further improving the dielectric properties of polyimide.

The inventors discovered that when polyimide resin and silica particles are combined, the dielectric properties are improved when silica particles with a high proportion of cristobalite phase are used compared to amorphous silica, and thus the present invention was completed.

That is, the resin composition of the present invention contains a solid content of an organic compound containing a polyamic acid or a polyimide, and silica particles,
the content of the silica particles is within a range of 10 to 85% by weight (wherein the polyamic acid is converted into an imidized polyimide) based on the total amount of the solid content and the silica particles;
As a whole, the silica particles have a ratio of the total area of peaks derived from the cristobalite crystal phase and the quartz crystal phase to the total peak area derived from _SiO2 in the range of 2θ = 10 ° to 90 ° in the X-ray diffraction analysis spectrum using CuKα rays of 20 wt % or more.

In the resin composition of the present invention, the median diameter D ₅₀ in a frequency distribution curve of the silica particles obtained by volume-based particle size distribution measurement using a laser diffraction scattering method may be within a range of 6 to 20 μm.

In the resin composition of the present invention, 90% by weight or more of the silica particles having a particle diameter of 3 μm or more may have a circularity of 0.7 or more.

In the resin composition of the present invention, 90% by weight or more of the silica particles may have a true specific gravity of 2.3 or more.

The resin film of the present invention is a resin film having a single or multiple polyimide layers, at least one of which is a crystalline silica-containing polyimide layer formed from any of the above resin compositions, and the thickness of the crystalline silica-containing polyimide layer is within the range of 15 μm to 200 μm.

The resin film of the present invention may have a thickness ratio of the crystalline silica-containing polyimide layer to the total thickness of the resin film of 50% or more.

The resin film of the present invention may have a relative dielectric constant of 3.1 or less at a frequency of 3 to 20 GHz when measured using a split post dielectric resonator (SPDR).

The resin film of the present invention may have a dielectric loss tangent of 0.005 or less at a frequency of 3 to 20 GHz when measured using a split post dielectric resonator (SPDR).

The resin film of the present invention may have a thermal expansion coefficient of 50 ppm/K or less.

The metal-clad laminate of the present invention comprises an insulating resin layer and a metal layer laminated on at least one surface of the insulating resin layer, and at least one layer of the insulating resin layer is made of any of the resin films described above.

The resin composition of the present invention has excellent dielectric properties because it contains silica particles with a specific crystal structure in addition to the inherent heat resistance of polyimide. Therefore, the resin composition and resin film of the present invention can be particularly suitably used as a circuit board material such as FPC in electronic devices that require high-speed signal transmission. Specifically, it is suitable for applications such as insulating resin layers in FPCs for mobile devices such as smartphones that handle high-frequency signals, and insulating resin layers in FPCs for vehicles that require heat resistance.

The following describes an embodiment of the present invention.

[Resin composition]
A resin composition according to one embodiment of the present invention is a resin composition containing a solid content of an organic compound containing polyamic acid or polyimide and silica particles,
the content of the silica particles is within a range of 10 to 85% by weight based on the total amount of the solid content and the silica particles;
As a whole, the silica particles have a ratio of the total area of peaks derived from the cristobalite crystal phase and the quartz crystal phase to the total peak area derived from _SiO2 in the range of 2θ = 10 ° to 90 ° in the X-ray diffraction analysis spectrum using CuKα radiation of 20 wt % or more. The resin composition may be a varnish (resin solution) containing a polyamic acid, or a polyimide solution containing a solvent-soluble polyimide.

[Solid content of organic compounds]
The solid content of the organic compound means the remaining solid content after removing the solvent and inorganic solid content from the resin composition. That is, the solid content of the organic compound contains polyamic acid or polyimide, and may contain non-volatile organic compounds such as resins other than polyimide, crosslinking agents, organic fillers, plasticizers, curing accelerators, coupling agents, organic pigments, organic flame retardants, and fillers as optional components. Here, examples of resins other than polyimide include epoxy resins, fluororesins, olefin resins, polyether resins, and polyester resins. Examples of crosslinking agents include amino compounds (amino compounds for crosslinking) having at least two primary amino groups as functional groups, which will be described later. Examples of organic fillers include liquid crystal polymer particles, fluorine-based polymer particles, polyether resin particles, and olefin resin particles.

The content of polyamic acid or polyimide in the solid content of the organic compound is preferably 60% by weight or more, more preferably 70% by weight or more, and most preferably 80% by weight or more. If the content of polyamic acid or polyimide is less than 60% by weight, the toughness of the resin decreases, and the film retention when a resin film is formed decreases. In the case of polyamic acid, the weight ratio is calculated by converting it into imidized polyimide.

[Polyamic acid or polyimide]
Polyimides are generally represented by the following general formula (1). Such polyimides can be produced by a known method in which substantially equimolar amounts of a diamine component and an acid dianhydride component are used and polymerized in an organic polar solvent. In this case, the molar ratio of the acid dianhydride component to the diamine component may be adjusted to set the viscosity in a desired range, and the range is preferably, for example, within a molar ratio range of 0.98 to 1.03.

Here, _Ar1 is a tetravalent organic group having one or more aromatic rings, and _Ar2 is a divalent organic group having one or more aromatic rings. _Ar1 can be said to be a residue of an acid dianhydride, and _Ar2 can be said to be a residue of a diamine. Furthermore, n represents the number of repetitions of the structural unit of general formula (1) and is a number of 200 or more, preferably 300 to 1000.

As the acid dianhydride, for example, an aromatic tetracarboxylic acid dianhydride represented by O(OC) ₂ --Ar ₁ --(CO) ₂ O is preferable, and examples thereof include those which give the following aromatic acid anhydride residue as Ar ₁ .

The acid dianhydrides can be used alone or in combination of two or more. Among these, it is preferable to use one selected from pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride (DSDA), and 4,4'-oxydiphthalic dianhydride (ODPA).

As the diamine, for example, an aromatic diamine represented by H ₂ N-Ar ₂ -NH ₂ is preferable, and an aromatic diamine giving the following aromatic diamine residue as Ar ₂ is exemplified.

Among these diamines, preferred examples include diaminodiphenyl ether (DAPE), 2,2'-dimethyl-4,4'-diaminobiphenyl (m-TB), paraphenylenediamine (p-PDA), 1,3-bis(4-aminophenoxy)benzene (TPE-R), 1,3-bis(3-aminophenoxy)benzene (APB), 1,4-bis(4-aminophenoxy)benzene (TPE-Q), 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), and 2,2-bis(trifluoromethyl)benzidine (TFMB).

Polyimide can be produced by reacting an acid dianhydride with a diamine compound in a solvent to produce a precursor polyamic acid, which is then heated to close the ring (imidization). For example, polyamic acid can be obtained by dissolving an acid dianhydride and a diamine compound in an organic solvent in approximately equal moles, and stirring the mixture at a temperature in the range of 0 to 100°C for 30 minutes to 72 hours to polymerize the mixture. In the reaction, the reaction components are dissolved so that the precursor produced is in the range of 5 to 30% by weight, preferably 10 to 20% by weight, in the organic solvent. Examples of organic solvents used in the polymerization reaction include N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N,N-diethylacetamide, N-methyl-2-pyrrolidone (NMP), 2-butanone, dimethylsulfoxide (DMSO), hexamethylphosphoramide, N-methylcaprolactam, dimethyl sulfate, cyclohexanone, dioxane, tetrahydrofuran, diglyme, triglyme, and cresol. Two or more of these solvents can be used in combination, and aromatic hydrocarbons such as xylene and toluene can also be used in combination. There are no particular limitations on the amount of organic solvent used, but it is preferable to adjust the amount used so that the concentration of the polyamic acid solution obtained by the polymerization reaction is about 5 to 30% by weight.

The synthesized polyamic acid is usually advantageously used as a reaction solvent solution, but if necessary, it can be concentrated, diluted, or replaced with another organic solvent to form a resin composition. There are no particular limitations on the method for imidizing the polyamic acid, and a suitable method is, for example, heat treatment in the solvent at a temperature in the range of 80 to 400°C for 1 to 24 hours.

The polyimide may be a thermoplastic polyimide or a thermosetting polyimide. The term "thermoplastic polyimide" generally refers to a polyimide whose glass transition temperature (Tg) can be clearly confirmed, but in the present invention, it refers to a polyimide whose storage modulus at 30° C. is 1.0×10 ⁸ Pa or more and whose storage modulus at 300° C. is less than 3.0×10 ⁷ Pa, as measured using a dynamic viscoelasticity measuring device (DMA). The term "non-thermoplastic polyimide" generally refers to a polyimide that does not soften or show adhesiveness even when heated, but in the present invention, it refers to a polyimide whose storage modulus at 30° C. is 1.0×10 ⁹ Pa or more and whose storage modulus at 300° C. is 3.0×10 ⁸ Pa or more, as measured using a dynamic viscoelasticity measuring device (DMA).

[Silica particles]
The silica particles may contain crystalline silica particles. In addition, amorphous silica particles may be contained. By blending crystalline silica particles in the resin composition, the dielectric loss tangent when the resin film is formed can be reduced. From the viewpoint of achieving a low dielectric loss tangent when the resin film is formed, it is particularly preferable to use silica particles having a cristobalite crystal phase as the crystalline silica particles. Silica particles having a cristobalite crystal phase have very excellent dielectric properties compared to general silica particles (for example, silica particles containing 90% by weight or more of the cristobalite crystal phase have a dielectric loss tangent of about 0.0008 at 10 GHz by itself), and can greatly contribute to the low dielectric loss tangent of the resin film.

The silica particles have a median diameter _D50 in a frequency distribution curve obtained by a volume-based particle size distribution by a laser diffraction scattering method in the range of 6 to 20 μm, preferably in the range of 8 to 15 μm. If it is within this range, the dielectric properties are effectively increased, and a low-dielectric resin film can be obtained without deteriorating the surface smoothness when blended into a resin film. If the median diameter _D50 is below the range, the surface area of the silica particles increases, and water adsorbed on the silica particle surface may affect the dielectric properties. If the median diameter _D50 exceeds the range, it may appear as unevenness on the film surface and degrade the smoothness of the film surface.

As the silica particles, it is preferable to use spherical silica particles. Spherical silica particles are silica particles whose shape is close to a perfect sphere, and whose ratio of the average major axis to the average minor axis is 1 or close to 1. In order to enhance the dispersibility of the silica particles and the improvement of the dielectric properties, it is preferable that 90% by weight or more of the silica particles whose particle diameter is 3 μm or more have a circularity of 0.7 or more, and more preferably 0.9 or more. The circularity of the silica particles can be determined by an image analysis method, assuming a circle having the same projected area as the photographed particle, and calculating the ratio of the perimeter of the circle to the perimeter of the particle. If the circularity is less than 0.7, the surface area increases, which may adversely affect the dielectric properties, and the viscosity increases significantly when the particles are mixed into a resin solution, making handling difficult. In addition, it is preferable that the sphericity determined three-dimensionally also has a value that substantially corresponds to the circularity value.

In addition, it is preferable that the silica particles have a true specific gravity of 2.3 or more in order to enhance the dispersibility of the silica particles and improve the dielectric properties. If the true specific gravity is less than 2.3, it indicates that the crystallinity of the silica particles is low, and the effect of improving the dielectric properties is reduced.

The silica particles may be surface-treated. Known techniques can be used for the surface treatment, and preferable examples include modification by corona treatment, plasma treatment, UV treatment, and functionalization using a silane coupling agent. By surface-treating the silica particles, alkyl groups, amino groups, alkoxy groups, and the like are imparted to the particle surface, improving the affinity with the solvent or polyamic acid, or improving the repulsive force between the particles, thereby improving the dispersibility of the silica particles and the long-term stability of the varnish.

From the viewpoint of achieving a low dielectric tangent when a resin film is formed, the ratio of the total area of the peaks derived from the cristobalite crystal phase and the quartz crystal phase to the total area of all peaks derived from SiO ₂ in the range of 2θ = 10 ° to 90 ° in the X-ray diffraction analysis spectrum by CuKα rays is preferably 20 wt% or more, more preferably 40 wt% or more, and desirably 80 wt% or more for the entire aggregate of silica particles used. By increasing the ratio of the cristobalite crystal phase and / or quartz crystal phase in the entire silica particles, it is possible to further reduce the dielectric tangent of the polyimide. If the ratio of the area of the peaks derived from the cristobalite crystal phase and the quartz crystal phase in the entire silica particles is less than 20 wt%, the effect of improving the dielectric properties becomes unclear. The proportion of the cristobalite crystalline phase is determined by the ratio of the total area of the diffraction peaks derived from the cristobalite crystalline phase and the quartz crystalline phase measured in the range of 10°≦2θ≦90° by X-ray diffraction (XRD) using α radiation for all silica particles in a uniformly mixed state to the total area of the diffraction peaks derived from all silica particles within the range. When the target peak in the X-ray diffraction analysis spectrum is difficult to separate from the broad amorphous peak or overlaps with other crystalline phase peaks, various known analytical methods, such as the internal standard method and the PONKCS method, can be used.

As a preferred embodiment, an aggregate of silica particles having a cristobalite crystalline phase of 50% by weight or more can be used. As an even more preferred embodiment, a portion (e.g., 80% by weight or less of the total) of an aggregate of silica particles having a cristobalite crystalline phase of 90% by weight or more can be replaced with amorphous silica particles. The use of amorphous silica particles as a replacement can be performed as necessary, and has the advantages of reducing costs, enabling high-density packing and adjustment of solution viscosity, and allowing the preparation of a resin composition with excellent handleability.

The silica particles can be appropriately selected from commercially available products. For example, spherical cristobalite silica powder (manufactured by Nippon Steel Chemical & Material Co., Ltd., product name: CR10-20) can be preferably used as the crystalline silica particles. In addition, spherical amorphous silica powder (manufactured by Nippon Steel Chemical & Material Co., Ltd., product name: SC70-2, product name: SP40-10) can be used as the amorphous silica particles that can be combined with the crystalline silica particles. Furthermore, two or more different types of silica particles can be used in combination.

[Composition]
The content of silica particles in the resin composition is within the range of 10 to 85% by weight (wherein polyamic acid is converted to imidized polyimide) based on the total amount of the solid content of the organic compound in the resin composition and the silica particles, preferably within the range of 10 to 80% by weight, more preferably within the range of 15 to 75% by weight. If the content of silica particles is less than 10% by weight, the effect of lowering the dielectric tangent cannot be sufficiently obtained. If the content of silica particles exceeds 85% by weight, the properties such as the adhesiveness of the polyimide are reduced, and especially if the content of silica particles exceeds 80% by weight, the resin film becomes brittle when formed, the bending property is reduced, and when a resin film is to be formed, the viscosity of the resin composition increases and the workability is reduced.

[Optional ingredients]
The resin composition of the present embodiment may contain an organic solvent. Examples of the organic solvent include N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N,N-diethylacetamide, N-methyl-2-pyrrolidone (NMP), 2-butanone, dimethylsulfoxide (DMSO), hexamethylphosphoramide, N-methylcaprolactam, dimethyl sulfate, cyclohexanone, dioxane, tetrahydrofuran, diglyme, triglyme, and cresol. Two or more of these solvents may be used in combination, and aromatic hydrocarbons such as xylene and toluene may also be used in combination. The content of the organic solvent is not particularly limited, but it is preferable to adjust the amount of use so that the concentration of the polyamic acid or polyimide is about 5 to 30% by weight.

The resin composition of the present invention may contain optional components such as inorganic fillers other than silica, inorganic pigments, inorganic flame retardants, and inorganic heat dissipation agents, as necessary, within the scope of not impairing the effects of the invention. Examples of inorganic fillers other than silica particles include aluminum oxide, magnesium oxide, beryllium oxide, niobium oxide, titanium oxide, boron nitride, aluminum nitride, silicon nitride, aluminum fluoride, calcium fluoride, magnesium fluoride, potassium silicofluoride, and metal phosphinates. These may be used alone or in combination of two or more.

[viscosity]
The viscosity of the resin composition is preferably within a range of, for example, 3000 cps to 100000 cps, more preferably 5000 cps to 50000 cps, so as to improve the handling property when coating the resin composition particles and to easily form a coating film of uniform thickness. If the viscosity is outside the above range, defects such as uneven thickness and streaks are likely to occur in the film during coating operation using a coater or the like.

[Preparation of resin composition]
In preparing the resin composition, for example, silica particles may be directly mixed into a resin solution of polyamic acid prepared using any solvent. Alternatively, taking into consideration the dispersibility of silica particles, silica particles may be mixed in advance into a reaction solvent containing either an acid dianhydride component or a diamine component, which are raw materials for polyamic acid, and then the other raw material may be added under stirring to proceed with polymerization. In either method, the entire amount of silica particles may be added at once, or may be added little by little in several portions. In addition, the raw materials may be added all at once, or may be mixed little by little in several portions.

[Resin film]
The resin film of the present embodiment is a resin film having a single or multiple polyimide layers, and at least one, preferably all, of the polyimide layers may be a crystalline silica-containing polyimide layer formed from the above-mentioned resin composition.

In the resin film, the thickness of the crystalline silica-containing polyimide layer formed by the resin composition is preferably within the range of 15 to 200 μm, and more preferably within the range of 20 to 150 μm. If the thickness of the crystalline silica-containing polyimide layer is less than 15 μm, the silica particles will protrude to the surface of the resin film, resulting in poor surface smoothness. Conversely, if the thickness of the crystalline silica-containing polyimide layer exceeds 200 μm, there is a tendency for this to be disadvantageous in that the foldability of the resin film decreases.

The overall thickness of the resin film is, for example, in the range of 15 to 200 μm, preferably in the range of 20 to 200 μm, and more preferably in the range of 25 to 200 μm. If the thickness of the resin film is less than 15 μm, problems such as wrinkles in the metal foil and tears in the resin film are likely to occur during the transport process in the manufacture of the metal-clad laminate. Conversely, if the thickness of the resin film exceeds 200 μm, there is a tendency for problems such as reduced foldability of the resin film to occur.

The ratio of the thickness of the crystalline silica-containing polyimide layer to the total thickness of the resin film is preferably 50% or more. If the ratio of the thickness of the crystalline silica-containing polyimide layer to the total thickness of the resin film is less than 50%, the effect of improving the dielectric properties is not sufficiently obtained.

The method for forming the crystalline silica-containing polyimide layer is not particularly limited, and any known method can be used. Here, the most typical example will be described.
First, the resin composition is directly cast onto any supporting substrate to form a coating film. Next, the coating film is dried to remove the solvent to a certain extent at a temperature of 150° C. or less. If the resin composition contains polyamic acid, the coating film is then further heat-treated at a temperature range of 100 to 400° C., preferably 130 to 360° C., for about 5 to 30 minutes for imidization. In this manner, a crystalline silica-containing polyimide layer can be formed on the supporting substrate.
When forming two or more polyimide layers, a first polyamic acid resin solution is applied and dried, and then a second polyamic acid resin solution is applied and dried. Thereafter, a third polyamic acid resin solution, then a fourth polyamic acid resin solution, ..., and so on, are applied and dried in the same manner as many times as necessary. Then, it is preferable to perform a heat treatment at a temperature range of 100 to 400°C for about 5 to 30 minutes to perform imidization. If the imidization temperature is lower than 100°C, the dehydration ring-closing reaction of the polyimide does not proceed sufficiently, and conversely, if it exceeds 400°C, the polyimide layer may deteriorate.
In addition, by subjecting any imidized polyimide layer to an appropriate surface treatment, etc., it is possible to further overlay a new layer through the steps of applying a resin solution, drying, and imidization. In this case, it is not necessary to complete the imidization in the middle steps, and the imidization can be completed all at once in the final step.
Furthermore, any imidized polyimide layer can be bonded by heating and pressing to a separately formed resin film.
The resin film may be attached to a supporting substrate.

Another example of forming a crystalline silica-containing polyimide layer will be described.
First, the resin composition is cast onto an arbitrary support substrate to form a film. This film-shaped molded product is heated and dried on the support substrate to form a gel film having self-supporting properties. After peeling the gel film from the support substrate, if the resin composition contains polyamic acid, it is further heat-treated at a high temperature to imidize the resin composition to form a polyimide resin film.

The support substrate used in forming the crystalline silica-containing polyimide layer is not particularly limited, and a substrate of any material can be used. In addition, when forming the resin film, it is not necessary to form a resin film on the substrate that has been completely imidized. For example, a resin film in a semi-cured polyimide precursor state can be separated from the support substrate by a means such as peeling, and after separation, the imidization can be completed to form a resin film.

The resin film may be composed of only a polyimide layer containing inorganic filler (including the above-mentioned crystalline silica-containing polyimide layer), or may have a polyimide layer that does not contain inorganic filler. When the resin film has a laminated structure of multiple layers, it is preferable to have all layers contain inorganic filler in consideration of improving the dielectric properties. However, by making the layer adjacent to the polyimide layer containing inorganic filler a layer that does not contain inorganic filler or a layer with a low content of inorganic filler, it is possible to obtain an advantageous effect of preventing the inorganic filler from slipping off during processing, etc. When a polyimide layer that does not contain inorganic filler is included, the thickness of the layer is preferably within a range of 1/100 to 1/2, preferably 1/20 to 1/3, of the polyimide layer containing inorganic filler. When a polyimide layer that does not contain inorganic filler is included, the adhesion between the metal layer and the insulating layer is improved if the polyimide layer is in contact with the metal layer.

The thermal expansion coefficient (CTE) of the resin film is not particularly limited, but is preferably within the range of 10×10 ⁻⁶ to 50×10 ⁻⁶ /K (10 to 50 ppm/K), and more preferably within the range of 15×10 ⁻⁶ to 40×10 ⁻⁶ /K (15 to 40 ppm/K). If the thermal expansion coefficient of the resin film is smaller than 10×10 ⁻⁶ /K, curling is likely to occur after being made into a metal-clad laminate, resulting in poor handling properties. On the other hand, if the thermal expansion coefficient of the resin film exceeds 50×10 ⁻⁶ /K, the dimensional stability as an electronic material such as a flexible substrate is poor, and the heat resistance also tends to decrease.

[Dielectric tangent]
When the resin film is applied, for example, as an insulating resin layer of a circuit board, in order to reduce the dielectric loss during the transmission of a high-frequency signal, the dielectric loss tangent (Tan δ) of the entire film at 3 to 20 GHz measured by a split post dielectric resonator (SPDR) is preferably 0.005 or less, more preferably 0.004 or less. In order to improve the transmission loss of the circuit board, it is particularly important to control the dielectric loss tangent of the insulating resin layer, and by setting the dielectric loss tangent within the above range, the effect of reducing the transmission loss is increased. Therefore, when the resin film is applied, for example, as an insulating resin layer of a high-frequency circuit board, the transmission loss can be efficiently reduced. If the dielectric loss tangent at 3 to 20 GHz exceeds 0.005, when the resin film is applied as an insulating resin layer of a circuit board, inconveniences such as increased loss of electrical signals on the transmission path of high-frequency signals are likely to occur. Although the lower limit of the dielectric loss tangent at 3 to 20 GHz is not particularly limited, it is necessary to consider the control of physical properties when the resin film is applied as an insulating resin layer of a circuit board.

[Dielectric constant]
In order to ensure impedance matching when the resin film is used as an insulating resin layer of a circuit board, for example, the film as a whole preferably has a relative dielectric constant of 3.1 or less at 3 to 20 GHz as measured by a split post dielectric resonator (SPDR). If the relative dielectric constant at 3 to 20 GHz exceeds 3.1, when the resin film is used as an insulating resin layer of a circuit board, this leads to deterioration of dielectric loss, and is likely to cause inconveniences such as increased loss of electrical signals on the transmission path of high-frequency signals.

[Metal-clad laminate]
The metal-clad laminate of the present embodiment is a metal-clad laminate including an insulating resin layer and a metal layer laminated on at least one surface of the insulating resin layer, at least one of the insulating resin layers being made of the above-mentioned resin film. The metal-clad laminate may be a single-sided metal-clad laminate having a metal layer on only one surface of the insulating resin layer, or a double-sided metal-clad laminate having metal layers on both surfaces of the insulating resin layer.

[Insulating resin layer]
The insulating resin layer is composed of a single layer or multiple layers, and includes a layer made of the resin film. For example, the resin film may form a non-thermoplastic polyimide layer as a main layer of the insulating resin layer to ensure mechanical properties and thermal properties. The resin film may also form a thermoplastic polyimide layer as an adhesive layer that provides adhesive strength to a metal layer such as a copper foil. The "main layer" refers to a layer that occupies 50% or more of the total thickness of the insulating resin layer.
In addition, the metal-clad laminate of the present embodiment does not exclude the use of an adhesive for bonding the inorganic filler-containing polyimide layer and the metal foil. However, when an adhesive layer is interposed in a double-sided metal-clad laminate having metal layers on both sides of an insulating resin layer, the thickness of the adhesive layer is preferably less than 30% of the thickness of the entire insulating resin layer so as not to impair the dielectric properties, and more preferably less than 20%. When an adhesive layer is interposed in a single-sided metal-clad laminate having a metal layer on only one side of an insulating resin layer, the thickness of the adhesive layer is preferably less than 15% of the thickness of the entire insulating resin layer so as not to impair the dielectric properties, and more preferably less than 10%. In addition, since the adhesive layer constitutes a part of the insulating resin layer, it is preferable that it is a polyimide layer. When the crystalline silica-containing polyimide layer constitutes the main layer of the insulating resin layer, the glass transition temperature of the crystalline silica-containing polyimide layer is preferably 300° C. or higher from the viewpoint of imparting heat resistance. In order to make the glass transition temperature 300° C. or higher, it is possible to appropriately select the above-mentioned acid dianhydride and diamine components constituting the polyimide.

Methods for manufacturing a metal-clad laminate with a resin film as an insulating resin layer include, for example, a method of heat-pressing a metal foil directly or via an arbitrary adhesive to a resin film, and a method of forming a metal layer on a resin film by a technique such as metal vapor deposition. Note that a double-sided metal-clad laminate can be obtained, for example, by forming a single-sided metal-clad laminate, and then arranging the polyimide layers to face each other and pressing them together by a heat press, or by pressing a metal foil onto the polyimide layer of a single-sided metal-clad laminate.

[Metal Layer]
The material of the metal layer is not particularly limited, and examples thereof include copper, stainless steel, iron, nickel, beryllium, aluminum, zinc, indium, silver, gold, tin, zirconium, tantalum, titanium, lead, magnesium, manganese, and alloys thereof. Among these, copper or a copper alloy is particularly preferred. The metal layer may be made of a metal foil, or may be a film on which metal has been deposited, or a paste or the like has been printed. In addition, since the resin composition can be directly applied, either a metal foil or a metal plate can be used, and copper foil or a copper plate is preferred.

The thickness of the metal layer is not particularly limited as it is set appropriately depending on the intended use of the metal-clad laminate, but is preferably in the range of 5 μm to 3 mm, and more preferably in the range of 12 μm to 1 mm. If the metal layer is less than 5 μm thick, problems such as wrinkles may occur during transportation in the manufacture of the metal-clad laminate. Conversely, if the metal layer is more than 3 mm thick, it will be hard and difficult to process. Generally, a thick metal layer is suitable for applications such as in-vehicle circuit boards, while a thin metal layer is suitable for applications such as LED circuit boards.

The following examples are provided to more specifically explain the features of the present invention. However, the scope of the present invention is not limited to the examples. In the following examples, the various measurements and evaluations are as follows, unless otherwise noted.

[Viscosity measurement]
The viscosity of the resin solution was measured at 25° C. using an E-type viscometer (manufactured by Brookfield, product name: DV-II+Pro). The rotation speed was set so that the torque was 10% to 90%, and the value was read when the viscosity stabilized 1 minute after the start of the measurement.

[Measurement of relative dielectric constant and dielectric loss tangent]
<Silica particles>
A vector network analyzer (Keysight Technologies, product name: Vector Network Analyzer E8363C) and a dielectric constant measuring device manufactured by Kanto Electronics Application Development Co., Ltd. using a cavity resonator perturbation method were used, and the dielectric constant (ε1) and dielectric loss tangent (Tanδ1) of the silica particles at a frequency of 10 GHz were measured using a dielectric constant measurement mode of TM020. The silica particles were in powder form and filled into a sample tube (inner diameter: 1.68 mm, outer diameter: 2.28 mm, height: 8 cm) for measurement.
<Resin film>
Using a vector network analyzer (manufactured by Keysight Technologies, Inc., product name: Vector Network Analyzer E8363C) and an SPDR resonator, the relative dielectric constant (ε1) and dielectric loss tangent (Tan δ1) of the resin film at a frequency of 10 GHz were measured. The resin film used for the measurement was left for 24 hours under the conditions of temperature: 24 to 26°C and humidity: 45 to 55%.

[Measurement of coefficient of thermal expansion (CTE)]
A resin film measuring 3 mm x 20 mm was heated from 30°C to 265°C at a constant heating rate while applying a load of 5.0 g using a thermomechanical analyzer (manufactured by Bruker, product name: 4000SA), and then held at that temperature for 10 minutes. The film was then cooled at a rate of 5°C/min to determine the average thermal expansion coefficient (coefficient of thermal expansion, CTE) from 200°C to 100°C.

[Measurement of particle size]
Using a laser diffraction particle size distribution measuring device (Malvern Instruments, trade name: Master Sizer 3000), the particle size of the silica particles was measured by a laser diffraction/scattering measuring method under conditions of water as a dispersion medium and a particle refractive index of 1.54.

[Measurement of true specific gravity]
The true specific gravity of the silica particles was measured by the pycnometer method (liquid phase displacement method) using a continuous automatic powder true density measuring device (manufactured by Seishin Enterprise Co., Ltd., product name: AUTO TRUE DENSERMAT-7000).

[Measurement of cristobalite crystal phase]
Using an X-ray diffraction measuring device (manufactured by Bruker, product name: D2PHASER), the total area of all peaks derived from SiO ₂ is calculated from all diffraction patterns (peak positions, peak widths and peak intensities) derived from SiO ₂ in the range of diffraction angles (Cu, Kα) 2θ = 10 ° to 90 °. Next, the peak positions derived from the cristobalite crystal phase were identified, the total area of all peaks of the cristobalite crystal phase was calculated, and the ratio (wt%) to the total area of all peaks derived from SiO ₂ was obtained. The attribution of each peak was made by referring to the database of the International Centre for Diffraction Data (ICDD).

[Measurement of circularity]
The average circularity of the silica particles was measured by dynamic flow particle image analysis using a wet flow type particle size/shape analyzer (manufactured by Sysmex Corporation, product name: FPIA-3000).

[Evaluation of bending properties]
In accordance with JIS K5600-5-1, a resin film measuring 5 cm x 10 cm was uniformly bent at the center of the long side around a metal rod with a diameter of 5 mm over 1 to 2 seconds in such a way that the film was wrapped around the metal rod. If the resin film did not break or crack even when bent at 180°, it was rated as "good", and if breaks or cracks occurred, it was rated as "unacceptable".

The abbreviations used in the Synthesis Examples, Comparative Examples, and Examples represent the following compounds.
PMDA: Pyromellitic dianhydride BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride BAPP: 2,2-bis[4-(4-aminophenoxy)phenyl]propane m-TB: 2,2'-dimethyl-4,4'-diaminobiphenyl DMAc: N,N-dimethylacetamide
Filler 1: Manufactured by Nippon Steel Chemical & Material Co., Ltd. Product name: CR10-20 (spherical cristobalite silica powder, circularity: 0.98, cristobalite crystal phase: 98% by weight, true specific gravity: 2.33, D ₅₀ : 10.8 μm, relative dielectric constant at 10 GHz: 3.16, dielectric tangent at 10 GHz: 0.0008)
Filler 2: Manufactured by Nippon Steel Chemical & Material Co., Ltd. Product name: SC70-2 (spherical amorphous silica powder, circularity: 0.98, true specific gravity: 2.21, _D50 : 11.7 μm, relative dielectric constant at 10 GHz: 3.08, dielectric tangent at 10 GHz: 0.0015)
Filler 3: Nippon Steel Chemical & Material Co., Ltd., product name: SP40-10 (spherical amorphous silica powder, true sphere, true specific gravity: 2.21, D ₅₀ : 2.5 μm, relative dielectric constant at 10 GHz: 2.78, dielectric tangent at 10 GHz: 0.0030)

(Synthesis Example 1)
24 g of BAPP (60 mmol) and 230 g of DMAc were added to a 300 ml separable flask and stirred at room temperature under a nitrogen stream. After complete dissolution, 6.5 g of PMDA (30 mmol) and 8.7 g of BPDA (30 mmol) were added and stirred at room temperature for 18 hours to obtain polyamic acid solution A. The viscosity of the obtained polyamic acid solution A was 21,074 cps.

(Synthesis Example 2)
19 g of m-TB (90 mmol) and 230 g of DMAc were placed in a 300 ml separable flask and stirred at room temperature under a nitrogen stream. After complete dissolution, 16 g of PMDA (72 mmol) and 5.3 g of BPDA (18 mmol) were added and stirred at room temperature for 18 hours to obtain polyamic acid solution B. The viscosity of the obtained polyamic acid solution B was 22,400 cps.

[Example 1]
58.7 g of the polyamic acid solution A and 6.1 g of the filler 1 were mixed and stirred until a visually homogeneous solution was obtained, to prepare a polyamic acid composition 1a (viscosity: 23,000 cps).
A polyamic acid composition 1a was applied onto copper foil 1 (electrolytic copper foil, thickness: 12 μm) and dried at 130 ° C for 3 minutes. Then, stepwise heat treatment was performed from 155 ° C to 360 ° C to imidize, and a metal-clad laminate 1b was prepared. The copper foil of the metal-clad laminate 1b was removed by etching, and a resin film 1c was prepared. The resin film 1c (thickness: 51.6 μm) had a relative dielectric constant of 3.04, a dielectric loss tangent of 0.0044, and a bending property of "good".

[Example 2]
70.0 g of the polyamic acid solution B and 7.8 g of the filler 1 were mixed and stirred until a visually homogeneous solution was obtained, thereby preparing a polyamic acid composition 2a (viscosity: 24,800 cps).
A metal-clad laminate 2b and a resin film 2c were prepared in the same manner as in Example 1. The resin film 2c (thickness: 46.1 μm) had a relative dielectric constant of 2.78, a dielectric loss tangent of 0.0037, a CTE of 34 ppm/K, and a bending property of "good."

[Example 3]
60.0 g of the polyamic acid solution B and 20.0 g of the filler 1 were mixed and stirred until a visually homogeneous solution was obtained, to prepare a polyamic acid composition 3a (viscosity: 28,400 cps).
A metal-clad laminate 3b and a resin film 3c were prepared in the same manner as in Example 1. The resin film 3c (thickness: 78.1 μm) had a relative dielectric constant of 2.71, a dielectric loss tangent of 0.0028, a CTE of 34 ppm/K, and a bending property of "good."

[Example 4]
55.0 g of the polyamic acid solution B and 36.6 g of the filler 1 were mixed and stirred until a visually homogeneous solution was obtained, to prepare a polyamic acid composition 4a (viscosity: 29,900 cps).
A metal-clad laminate 4b was prepared, and then a resin film 4c was prepared in the same manner as in Example 1. The resin film 4c (thickness: 117.8 μm) had a relative dielectric constant of 2.56, a dielectric loss tangent of 0.0015, a CTE of 31 ppm/K, and a bending property of "unacceptable."

[Example 5]
50.0 g of the polyamic acid solution B, 6.6 g of the filler 1, and 25.3 g of the filler 2 were mixed and stirred until a visually homogeneous solution was obtained, to prepare a polyamic acid composition 5a (viscosity: 31,000 cps).
Metal-clad laminate 5b was prepared, and then resin film 5c was prepared in the same manner as in Example 1. Resin film 5c (thickness: 115.5 μm) had a relative dielectric constant of 2.71, a dielectric loss tangent of 0.0017, a CTE of 27 ppm/K, and a bending property of "unacceptable."

(Comparative Example 1)
Polyamic acid solution A was applied onto copper foil 1, and dried at 90° C. for 1 minute and at 130° C. for 5 minutes. Then, the solution was heat-treated stepwise from 155° C. to 360° C. to imidize the solution, thereby preparing metal-clad laminate A1.
Resin film A1 was prepared by etching away the copper foil of metal-clad laminate A1 in the same manner as in Example 1. Resin film A1 (thickness: 41.4 μm) had a relative dielectric constant of 3.16, a dielectric dissipation factor of 0.0062, a CTE of 51 ppm/K, and a bending property of "good."

(Comparative Example 2)
56.2 g of the polyamic acid solution A and 5.5 g of the filler 2 were mixed and stirred until a visually homogeneous solution was obtained, to prepare a polyamic acid composition A2 (viscosity: 23,600 cps).
A metal-clad laminate A2 and a resin film A2 were prepared in the same manner as in Example 1. The resin film A2 (thickness: 50.7 μm) had a relative dielectric constant of 3.04, a dielectric dissipation factor of 0.0047, and a bending property of "good."

(Comparative Example 3)
56.2 g of the polyamic acid solution A and 5.5 g of the filler 3 were mixed and stirred until a visually homogeneous solution was obtained, to prepare a polyamic acid composition A3 (viscosity: 30,000 cps).
A metal-clad laminate A3 was prepared, and then a resin film A3 was prepared in the same manner as in Example 1. The resin film A3 (thickness: 45.6 μm) had a relative dielectric constant of 3.25, a dielectric dissipation factor of 0.0052, a CTE of 41, and a bending property of "good".

The above results are summarized in Tables 1 and 2.
The crystalline phase in Table 1 means the proportion of the cristobalite crystalline phase, and the filler proportion means the proportion (weight ratio) of the filler relative to the entire polyamic acid solution (varnish) or the total of the solid content of the organic compound and the silica particles (however, the weight of the polyamic acid was calculated in terms of the polyimide after imidization). Also, the filler content in Table 2 means the proportion (weight ratio) of the filler relative to the resin film.

It was confirmed that the polyamic acid composition 1a of Example 1 had a lower viscosity than the polyamic acid compositions A2 and A3 of Comparative Examples 2 and 3, respectively. In addition, the resin film 1c of Example 1 had a lower dielectric constant and dielectric loss tangent than the resin films A2 to A3 of Comparative Examples 2 to 3, and it was confirmed that the higher the proportion of the cristobalite crystal phase, the greater the effect of reducing the dielectric constant.

These results confirm that the resin film according to this embodiment can be suitably used as a material for high-frequency flexible printed circuit boards.

Although the embodiment of the present invention has been described in detail above for the purpose of illustration, the present invention is not limited to the above embodiment, and various modifications are possible.

Claims (8)

A method for producing a resin film having at least one crystalline silica-containing polyimide layer containing a solid content of an organic compound containing polyimide and crystalline silica particles, comprising the steps of:
The crystalline silica particles, as a whole, have a ratio of a total area of peaks derived from a cristobalite crystal phase and a quartz crystal phase to a total peak area derived from _SiO2 in a range of 2θ = 10 ° to 90 ° in an X-ray diffraction analysis spectrum using CuKα radiation of 20 wt % or more;
A method for producing a resin film, comprising: a step of directly or indirectly applying a resin composition containing a polyamic acid, which is a precursor of the polyimide, and the crystalline silica particles onto a supporting substrate to form a coating film; and a step of heat-treating the coating film to imide the polyamic acid and form a crystalline silica-containing polyimide layer. The method for producing a resin film according to claim 1, wherein the crystalline silica particles have a median diameter _D50 in a frequency distribution curve obtained by measuring a volume-based particle size distribution by a laser diffraction scattering method, within a range of 6 to 20 μm. The method for producing a resin film according to claim 1, wherein 90% by weight or more of the crystalline silica particles having a particle diameter of 3 μm or more have a circularity of 0.7 or more. The method for producing a resin film according to claim 1, wherein 90% by weight or more of the crystalline silica particles have a true specific gravity of 2.3 or more. The method for producing a resin film according to claim 1, characterized in that the content of the crystalline silica particles is within the range of 10 to 85% by weight based on the total amount of the solid content and the crystalline silica particles. The method for producing a resin film according to claim 1, characterized in that the thickness of the crystalline silica-containing polyimide layer is within the range of 15 μm to 200 μm. The method for producing a resin film according to claim 6, wherein the ratio of the thickness of the crystalline silica-containing polyimide layer to the total thickness of the resin film is 50% or more. 2. The method for producing a metal-clad laminate according to claim 1, wherein a metal foil is used as the supporting substrate to form a metal-clad laminate in which a metal layer and the resin film are laminated together.