Process for producing printed circuit boards

Abstract: A process for producing a printed circuit board is disclosed, in which the following steps are sequentially conducted: (1) a step for dispersing in at least one of the mediums of water and an organic solvent (A) 95 to 20% by weight of a styrene polymer having a syndiotactic structure, (B) 5 to 80% by weight of a fibrous filler having a fiber length of 1 to 50 mm, and (C) 0.1 to 30 parts by weight of at least one of a binder and a binding fiber in proportion to 100 parts by weight of the total amount of said components (A) and (B), to make a slurry having a concentration of 0.5 to 100 g/l, (2) a sedimentation step of the dispersed solids in said slurry, (3) a step for removing at least one of the mediums of water and an organic solvent in the slurry by filtration and drying and molding the residue, (4) a step for melting with heating and pressure forming the molded product, and (5) a step for providing a metal layer on the molded article. The printed circuit board obtained by the process is excellent in impact resistance, heat resistance, mechanical strength and size stability as well as dielectric properties.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for producing a printed circuit board. Particularly, it relates to a process for efficiently producing a printed circuit board excellent in dielectric properties, dimensional stability and flexing properties. 2. Description of the Related Arts Hitherto, composite sheets which are the combinations of thermosetting resins such as an epoxy resin, a phenol resin or an unsaturated polyester resin with materials such as paper, a glass fiber or a synthetic fiber have been widely used as insulating materials for printed circuit boards. These composite base materials are excellent in electrical insulating properties and are in increasing demand in the electric and electronic fields. The composite base materials comprising a thermosetting resin, however, is molded with use of a solvent such as a styrene monomer and has a bad environment of molding operation, so that they are not suitable for a multi-layer printed circuit board used in the fields such as computer industry where high-speed operation is required. As the multi-layer printed circuit board used in the fields such as computer industry where high-speed operation is required, printed circuit boards which comprise a fluorinated polyethylene resin as an insulating base material have been recently developed with great strides. However, fluorinated polyethylene resins are very difficult to mold, so that the development molded articles of usual melt molding, specifically injection molding or extrusion molding has been investigated. Printed circuit boards comprising as a base material polyphenylene sulfide (PPS) which is a thermoplastic resin have also been researched for improving the production efficiency, but they have a problem that they have too high dielectric constant to use them as a multi-layer printed circuit board for the high-speed operation. From EP-A-0 307 488 a molded product is known, which is obtained from a styrene polymer composition comprising e. g. 95 parts by weight of syndiotactic polystyrene, 5 parts by weight of a thermoplastic resin and 5 parts by weight of an inorganic filler per 95 parts by weight of the total amount of the syndiotactic polystyrene and the thermoplastic resin. Thus, the present inventor has earnestly conducted researches for the development of a process for efficiently producing a printed circuit board which are excellent in dielectric properties, dimensional stability and flexing properties under a good operation environment. As a result, the present inventors have found that an aimed printed circuit board having the aforementioned properties can be obtained by using a composition of a styrene polymer having a syndiotactic structure (hereinafter referred to as SPS), a fibrous filler, a binder and/or a binding fiber at a predetermined ratio, molding the composition according to the principle of papermaking and finally providing a metal layer on the molded product thus obtained. The present invention has been accomplished on the basis of the finding. SUMMARY OF THE INVENTION That is, the present invention provides a process for producing a printed circuit board characterized in that the following steps are sequentially conducted: (1) a step for dispersing in at least one of the mediums of water and an organic solvent (A) 95 to 20% by weight of a styrene polymer having a racemic pentad having 30% or more syndiotacticity. (B) 5 to 80% by weight of a fibrous filler having a fiber length of 1 to 50 mm, and (C) 0.1 to 30 parts by weight of a binder and/or a binding fiber in proportion to 100 parts by weight of the total amount of the aforementioned components (A) and (B), to make a slurry having a concentration of the total of (A), (B) and (C) of 0.5 to 100 g/l, (2) a sedimentation step of the solids dispersed in the slurry, (3) a step for removing the at least one of the mediums of water and an organic solvent from the slurry by filtration and drying and molding the residue, (4) a step for melting with heating and pressure forming the molded product obtained in step (3), and (5) a step for providing a metal layer on the molded product obtained in step (4). The present invention also provides a process for conducting (4′) a step for impregnating the molded product obtained in step (3) with a thermosetting resin and curing the product in place of (4) a step for melting with heating and pressure forming the molded product. DESCRIPTION OF PREFERRED EMBODIMENTS First of all, the aforementioned components (A), (B) and (C) are dispersed into water and/or an organic solvent at a predetermined ratio to make a slurry. In this connection, the styrene polymer (SPS) as the component (A) has a syndiotactic structure. The sydniotactic structure means that the polymer has a stereostructure with a syndiotactic configuration, that is the stereostructure in which phenyl groups or substituted phenyl groups as side chains are located alternately at opposite directions relative to the main chain consisting of carbon-carbon bonds. The tacticity is quantitatively determined by the nuclear magnetic resonance method using carbon isotope (¹³C-NMR) method. The tacticity as determined by the ¹³C-NMR method can be indicated in terms of proportions of structural units continuously connected to each other, i.e., a diad in which two structural units are connected to each other, a triad in which three structural units are connected to each other, or pentad in which five structural units are connected to each other. Styrene polymers having a high degree of the syndiotactic configuration of the present invention include polystyrene, poly(alkylstyrene), poly(halogenated styrene), poly(alkoxystyrene), poly(vinyl benzoate) and the hydrogenated polymers thereof and mixtures thereof, and copolymers containing the above polymers as the main component, having such a syndiotacticity that the proportion of racemic diad is at least 75% and preferably at least 85%, or the proportion of racemic pentad is at least 30% and preferably at least 50%. The aforementioned poly(alkylstyrene) includes poly(methylstyrene), poly(ethylstyrene), poly(isopropylstyrene), poly(tert-butylstyrene), poly(phenylstyrene), poly(vinylnaphthalene), poly(vinylstyrene) and the like. The poly(halogenated styrene) includes poly(chlorostyrene), poly(bromostyrene), poly(fluorostyrene) and the like. The poly(halogenated alkylstyrene) includes poly(chloromethylstyrene) and the like, and the poly(alkoxystyrene) includes (methoxystyrene) and the like. The poly(alkoxystyrene) includes poly(methoxystyrene), poly(ethoxystyrene) and the like. Furthermore, the comonomer components of the copolymer containing these structural units include, in addition to the monomers of the above-described styrene polymers, olefinic monomers such as ethylene, propylene, butene, hexene, octene and the like; diene monomers such as butadiene, isoprene and the like; cycloolefin monomers, cyclodiene monomers, and polar vinyl monomers such as methyl methacrylate, maleic anhydride, acrylonitrile and the like. Among these styrene polymers, the particularly preferred ones include polystyrene, poly(alkylstyrene), poly(halogenated styrene), hydrogenated polystyrene and copolymers containing these structural units. The styrene polymers having such syndiotactic structures can be produced by, for example, polymerizing a styrene monomer (a monomer corresponding to the above styrene polymer) using a titanium compound and a condensate product of water and trialkylaluminum as a catalyst in an inert hydrocarbon or in the absence of a solvent (Japanese Patent Application Laid-Open No. 187708/1987). Further, the poly(halogenated alkylstyrene) can be prepared by the method disclosed in Japanese Patent Application Laid-Open No. 46912/1989, and the hydrogenated polymers thereof can be prepared by the method disclosed in Japanese Patent Application Laid-Open No. 178505/1989. The styrene polymers have no particular restriction for the molecular weight and includes the ones having the weight average molecular weight of 2,000 or more, preferably 10,000 or more, particularly 50,000 or more. The molecular weight distribution is not restricted for the width, and it can be possible to apply various widths. According to the process of the present invention, the amount of SPS as the above component (A) to be formulated is in the range of 95 to 20% by weight, preferably 90 to 30% by weight, more preferably 90 to 40% by weight in proportion to the total amount of the components (A) and (B). If the amount of SPS to be formulated is less than 20% by weight, the fibrous filler as the component (B) is coated only insufficiently for the expression of the anticipated physical properties such as heat resistance, mechanical strength or surface smoothness. Further, the composition has bad fluidity on molding, and the upper half of a mold will not fit to the lower half due to the high stiffness of the composition, so that it is apprehended that the mold is broken. On the other hand, if the amount of the component (A) exceeds 95% by weight, heat resistance and mechanical strength will not improved. According to the process of the present invention, SPS as the above-described component (A) is not restricted particularly for its form, and a variety of forms can be used. The component (A) is preferably used in the form of powder having a bulk density of 0.05 to 1.0 g/cm³, more preferably 0.10 to 0.95 g/cm³, and/or in the form of a fibrous molded product having a fiber diameter of 0.01 to 500 µm. In this connection, when the SPS is used in the form of powder having a bulk density in the range of less than 0.05 g/cm³ or more than 1.0 g/cm³, a homogeneous slurry dispersion will not sometimes be obtained on the production of a molded product according to the process of the present invention. Next the fibrous filler used as the component (B) in the process of the present invention includes various fillers, preferably such as ceramic fibers such as glass fiber, asbestos fiber, carbon fiber, gypsum, potassium titanate, magnesium sulfate, magnesium oxide and the like; metal fibers such as those made of copper, aluminum, steel and the like; whiskers such as those made of boron, alumina, silica, silicon carbide and the like; organic synthetic fibers such as an all aromatic polyamide fiber, a polyimide fiber, and aramid fiber and the like; and natural plant fibers such as silk, flax and the like. These fibers are used alone or in combination of the two or more. According to the process of the present invention, the amount of the fibrous filler as the aforementioned component (B) to be formulated is in the range of 5 to 80% by weight, preferably 10 to 70% by weight, more preferably 10 to 60% by weight in proportion to the total amount of the components (A) and (B). The fibrous fillers having a fiber length in the range of 1 to 50 mm, preferably 5 to 30 mm are used. If the fibrous filler has a fiber length less than 1 mm, it is not so improved in mechanical strength. On the other hand, if it has a fiber length exceeding 50 mm, it is dispersed only insufficiently in a slurry on the production of a molded product according to the process of the present invention. The component (C) used in the process of the present invention is a binder and a binding fiber, and the either or both of the two are used. The binder includes various binders such as natural starch (particularly natural maize starch), a phosphate ester of starch (RETAMIL AP or RETAPOND AP type), a carboxymethylated starch, a starch oxide, an enzyme treated starch (for example, a linear polymeric α-amylose having a glycol unit distribution in the range of 50 to 3,000), a hydroxymethylated starch, a carboxymethylcellulose for industrial use (sodium chloride: 5 to 30%; substitution degree: 0.7 to 0.8), an aqueous dispersion having a concentration of 40 to 55% of a polymer comprising 87 to 90 parts by weight of an ethyl acrylate unit, 1 to 8 parts by weight of an acrylonitrile unit, 1 to 6 parts by weight of an N-methylolacrylamide unit and 1 to 6 parts by weight of an acrylic acid unit, an aqueous dispersion of a concentration of 40 to 55% of a polymer comprising 60 to 75 parts by weight of an ethyl acrylate unit, 5 to 15 parts by weight of an acrylonitrile unit, 10 to 20 parts by weight of a butyl acrylate unit and 1 to 6 parts by weight of an N-methylolacrylamide unit, an aqueous dispersion having a concentration of 40 to 55% of a polymer comprising 60 to 65 parts by weight of a butadiene unit, 35 to 40 parts by weight of an acrylonitrile unit and 1 to 7 parts by weight of a methacrylic acid unit, a polymer comprising 38 to 50 parts by weight of a styrene unit, 47 to 59 parts by weight of a butadiene unit and 1 to 6 parts by weight of a methacrylamide unit, an aqueous dispersion having a concentration of 40 to 55% of a polymer comprising 53 to 65 parts by weight of a styrene unit, 32 to 44 parts by weight of a butadiene unit and 1 to 6 parts by weight of a methacrylamide unit, polyvinyl alcohol, casein, carboxymethylcellulose, gelatin, methylethylcellulose, an aqueous dispersion having a concentration of 40 to 55% of a carboxylated styrene-butadiene latex, an alginic acid salt, dextrin, an aqueous dispersion having a concentration of 40 to 55% of a copolymer comprising vinylidene chloride as a base, an ethylene-vinyl acetate copolymer and the like. These binders are used alone or in combination of the two or more appropriately. Among them, latexes (an acrylic type or a styrene-butadiene type), starches, particularly a natural starch (obtained from potato) and a starch having a linear polymer component such as maize natural starch (having 100 to 6,000 anhydroglucose units per molecule in the linear polymer), that is a starch comprising 50 to 6,000 anhydroglucose units per amylose medium-molecule, or a chemically or enzymatically treated starch having 50 to 3,000 anhydroglucose units per molecule are preferred. The binder ensures the bond of the components such as thermoplastic plastic sheets and strengthens the physical properties. The binding fibers include various fibers. The term binding fiber means a fiber such as a cellulose fiber or a polyolefin pulp fibril (see US Patent No. 2,481,707) which can appropriately bonding the other components of the composition. Specific examples of the binding fibers include a bleached softwood kraft pulp, a half bleached softwood kraft pulp, a non-bleached softweed kraft pulp, a bleached softwood sulfite pulp, a non-bleached softwood sulfite pulp, a bleached hardwood kraft pulp, a half bleached hardwood kraft pulp, a non-bleached mechanical pulp, a bleached mechanical pulp, a polyvinyl alcohol fiber, a rayon fiber, a recovered fiber, a polyolefin pulp fibril, a cellulose fiber and the like. In the process of the present invention, the aforementioned binder and/or binding fiber is used as the component (C) and is formulated in an amount of 0.1 to 30 parts by weight per 100 parts by weight of the total amount of the components (A) and (B). If the component (C) is formulated in an amount less than 0.1 part by weight, the composition molded into sheet does not exhibit a safisfactory mechanical strength for stretching it with a papermaking machine. If the amount of the component (C) exceeds 30 parts by weight, the heat resistance is undesirably decreased. The process for dispersing the components to make a slurry is conducted by adding the aforementioned components (A), (B) and (C) to water, an organic solvent (specifically an alcohol, a hydrocarbon, a ketone and the like) or a mixture thereof in a pre-determined ratio and adjusting the concentrations to 0.5 to 100 g/l. A dispersing agent and a sizing agent may be added, if necessary, in order to improve the dispersibility and to improve the adhesive properties of the fibrous filler and the SPS, respectively. A flame-retardant and a flame-retardant aid are also added according to necessity. Particularly, the flame-retardant aid is added so that the dielectric constant will not be increased. It is desirable that these additives be added in the amounts not more than 40 parts by weight for the flame-retardant and 15 parts by weight for the flame-retardant aid per 100 parts by weight of the total weight of the components (A) and (B). Further, usual additives such as an antioxidant, a nucleating agent, an antistatic agent, a thermoplastic resin, an elastomer and the like. The SPS as the component (A) used is not particularly restricted in form and may have a form of powder, fiber or beads which can be suspended into water or an organic solvent. The SPS having various forms may be used in combination thereof. If the SPS molded product in the form of fiber, it has preferably a fiber length of 0.1 mm to 30 mm and a fiber diameter of 0.1 µm to 500 µm. The fibrous molded product can be obtained by a dry spinning method, a wet spinning method, a melt spinning method, a melt blow spinning method, a flash spinning method or the like. The product may be the one which has been heated and stretched again on or after spinning. Likewise, it is sufficient that the component (C), the dispersion agent and the sizing agent can be suspended into water or an organic solvent. The sequence of the addition of these components is not critical, but it is desirable to add first a sizing agent or a surfactant and a dispersing agent and then the components (A), (B) and (C) in this sequence. In addition, the aforementioned dispersing agent, the sizing agent and the thermoplastic resin specifically include the followings. First of all, the dispersing agent includes the following surfactants (1) to (4): (1) Anionic surfactant A sodium fatty acid soap, a potassium fatty acid soap; an alkyl sulfate salt such as a sodium alkylsulfate salt, a sodium alkylethersulfate salt and the like; a sodium salt of an ethylhexylalkylsulfate; a sodium salt of dodecylbenzene sulfonate, dodecylbenzene sulfonic acid, a sodium acylmethyltaurate, sodium lauroylmethyltaurate, and sodium dioctylsulfonesuccinate. (2) Cationic surfactant Amine type Octadecylamine acetate, tetradecylamine acetate, a tallow alkylpropylenediamine acetate. Methyl type Octadecyltrimethylammonium chloride, an alkyltrimethylammonium chloride, dodecyltrimethylammonium chloride, hexadecyltrimethylammonium chloride, behenyltrimethylammonium chloride, an alkylimidazoline quaternary salt. Benzyl type An alkyldimethylbenzylammonium chloride, tetradecyldimethylbenzylammonium chloride. Other types Dioleyldimethylammonium chloride, a polyoxyethylenedodecylmonomethylammonium chloride, oxyethylenedodecylamine, a polyoxyethylenedodecylamine, a polyoxyethylenealkylamine, a polyoxyethyleneoctadecylamine, a polyoxyethylenealkylpropylenediamine, a 1-hydroxyethyl-2-alkylimidazoline quaternary salt, an alkylisoquinolinium bromide, a polymeric amine. (3) Nonionic surfactant Ether type A polyoxyethylene oleylether, a polyoxyethylene cetylether, a polyoxyethylene stearylether, a polyoxyethylene laurylether, a polyoxyethylene tridecylether, a polyoxyethylene alkylether. Alkylphenol type A polyoxyethylene nonylphenylether, a polyoxyethylene octylphenylether. Ester type A polyoxyethylene monolaurate, a polyoxyethylene monostearate, a polyoxyethylene monooleate. Sorbitan ester type Sorbitan monolaurate, sorbitan monostearate, sorbitan monopalmitate, sorbitan monooleate, sorbitan sesqui-oleate, sorbitan trioleate. Sorbitan ester ether type A polyoxyethylene sorbitan monolaurate, a polyoxyethylene sorbitan monopalmitate, a polyoxyethylene sorbitan monostearate, a polyoxyethylene sorbitan monooleate. Other types An oxyethylene-oxypropylene block polymer, an alkylalkylolamide, lauric acid diethanolamide, coconut oil fatty acid diethanolamide, oleic acid diethanolamide, tallow fatty acid diethanolamide, glyceryl monostearate, a polyglycerol fatty acid ester, a polyoxyethylene distearate. (4) Amphoteric surfactant A dimethylalkyl betaine, a dimethylalkyllauryl betaine, an alkylglycine. Specific examples of the sizing agent include the following coupling agents (1) and (2): (1) Silane coupling agent There are mentioned triethoxysilane, vinyl tris(β-methoxyethoxy)silane, γ-methacryloxypropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, β-(1,1-epoxycyclohexyl)-ethyltriethoxysilane, N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, N-β-(aminoethyl)-γ-aminopropylmethyldimethoxysilane, γ-aminopropyltriethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-chloropropyltrimethoxysilane, γ-aminopropyltrimethoxysilane, γ-aminopropyl-tris(2-methoxyethoxy)silane, N-methyl-γ-aminopropyltrimethoxysilane, N-vinylbenzyl-γ-aminopropyltriethoxysilane, triaminopropyltrimethoxysilane, 3-ureidopropyltrimethoxysilane, 3-(4,5-dihydroimidazole)propyltriethoxysilane, hexamethyldisilazane, N,O-(bistrimethylsilyl)amide, N,N-bis(trimethylsilyl)urea and the like. Among them, aminosilanes such as γ-aminopropyltriethoxysilane, N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane and the like, and epoxysilanes are preferred. (2) Titanate coupling agent There are mentioned isopropyltriisostearoyl titanate, isopropyltridodecylbenzenesulfonyl titanate, isopropyltris(dioctylpyrophosphate) titanate, tetraisopropylbis(dioctylphosphite) titanate, tetraoctylbis(ditridecylphosphite) titanate, tetra(1,1-diallyloxymethyl-1-butyl)bis(ditridecyl)phosphite titanate, bis(dioctylpyrophosphate)oxyacetate titanate, bis(dioctylpyrophosphate)ethylene titanate, isopropyltrioctanoyl titanate, isopropyldimethacrylisostearoyl titanate, isopropylisostearoyldiacryl titanate, isopropyltri(dioctylphosphate) titanate, isopropyltricumylphenyl titanate, isopropyltri(N-amidoethyl, aminoethyl) titanate, dicumylphenyloxyacetate titanate, diisostearoylethylene titanate and the like. Among them, isopropyltri(N-amidoethyl, aminoethyl) titanate are preferred. As the flame-retardant, there are mentioned various types of flame-retardants, among which halogen flame-retardants and phosphate flame-retardants are particularly preferred. The halogen flame-retardants include, for example, tetrabromobisphenol A, tetrabromophthalic anhydride, hexabromobenzene, tribromophenyl allyl ether, pentabromotoluene, pentabromophenol, tribromophenyl 2,3-dibromopropyl ether, tris(2,3-dibromopropyl)phosphate, tris(2-chloro-3-bromopropyl)phosphate, octabromodiphenyl ether, decabromodiphenyl ether, octabromobiphenyl, pentachloropentacyclodecane, hexabromocyclododecane, hexachlorobenzene, pentachlorortoluene, hexabromobiphenyl, decabromobiphenyl, decabromobiphenyl oxide, tetrabromobutane, decabromodiphenyl ether, hexabromodiphenyl ether, ethylene-bis(tetrabromophthalimide), tetrachlorobisphenol A, tetrabromobisphenol A, tetrachlorobisphenol A or an oligomer of tetrabromobisphenol A, a halogenated polycarbonate oligomer such as brominated polycarbonate oligomer or the like, a halogenated epoxy compound, polychlorostyrene, a halogenated polystyrene such as polytribromostyrene or the like, poly(dibromophenylene oxide), bis(tribromophenoxy)ethane or the like. On the other hand, the phosphate flame-retardants include, for example, ammonium phosphate, tricresyl phosphate, triethyl phosphate, an acid phosphate, triphenylphosphine oxide and the like. Among them, a polytribromostyrene, a poly(dibromophenylene oxide), decabromodiphenyl ether, bis(tribromophenoxy)ethane, ethylene-bis(tetrabromophthalimide), tetrabromobisphenol A and the brominated polycarbonate oligomer are particularly preferred as the flame-retardant. The flame-retardant aids which can be used together with the flame-retardants include various types of flame-retardant aids, for example antimony flame-retardant aids such as antimony trioxide, antimony pentoxide, sodium antimonate, antimony metal, antimony trichloride, antimony pentachloride, antimony trisulfide, antimony pentasulfide and the like. There can be mentioned in addition to these flame-retardant aids zinc borate, barium metaborate, zirconium oxide and the like. Among them, antimony trioxide is particularly preferred. Further, in the present invention a tetrafluoroethylene polymer can be incorporated in order to prevent the melt dropping. Specific examples of the tetrafluoroethylene polymer include in addition to the tetrafluoroethylene homopolymer (polytetrafluoroethylene) a copolymer of tetrafluoroethylene and hexafluoropropylene, a tetrafluoroethylene copolymer containing a small amount of a copolymerizable ethylenically unsaturated monomer and the like. The tetrafluoroethylene polymer having a fluorine content of 65 to 76% by weight, preferably 70 to 76% by weight is used. Further, as the thermoplastic resin, there is mentioned the one which is compatible or non-compatible with the SPS as the component (A). For example, a styrene polymer having an atactic structure, a styrene polymer having an isotactic structure, a polyphenyl ether and the like are easily compatible with SPS, and thus a molded product having an excellent mechanical properties. Particularly, a polyphenyl ether having a polar group such as maleic anhydride has a remarkable effect for improving the adhesiveness between the fibrous filler and the SPS and is preferred. The non-compatible resins include, for example, a polyolefin such as polyethylene, polypropylene, polybutene, polypentene and the like; a polyester such as polybutylene terephthalate, polyethylene naphthalate and the like; a polyamide such as nylon 6, nylon 6,6 and the like; a polythioether such as a polyphenylene sulfide and the like; a polycarbonate, a polyallylate, a polysulfone, a polyether ether ketone, a polyether sulfone, a polyimide, a halogenated vinyl polymer, an acrylic polymer such as polymethyl methacrylate and the like, polyvinyl alcohol, a styrene-maleic anhydride copolymer (SMA) and all but the aforementioned compatible resins. There are also mentioned crosslinked resins containing the aforementioned compatible resins. The sedimentation step of the process of the present invention is a step for adding, if necessary, an agglomerating agent to the suspension which has been subjected to dispersion to make a slurry in the above-described step (1), agglomerating particles and sedimenting the resultant solid. The agglomerated particles (solid) sediment by its dead load. The agglomerating agent is usually added in an amount of 0.1 to 0.5 g/l, but it is not always necessary if the particles in the suspension have a large size and sediment by their dead load. In this connection, the agglomerating agent to be used in this step includes aluminum sulfate; a polyaluminum chloride (aluminum hydroxychloride); sodium and potassium aluminate; a blend of a polyacrylic acid and a polyacrylamide in the form of a 5 to 30% (weight/volume) aqueous solution; a polyethylenimine in the form of a 2 to 50% (weight/volume) aqueous solution; a copolymer of acrylamide and β-methacryloxyethyltrimethylammonium methylsulfate; polyamine-epichlorohydrin and diamine-propylmethylamine resins in the form of a 2 to 50% aqueous solution; a polyamide-epichlorohydrin resin which is in the form of a 2 to 50% aqueous solution and is made of epichlorohydrin, adipic acid, caprolactam, diethylenetriamine and/or ethylenediamine; a polyamide-polyamine-epichlorohydrin resin which is in the form of a 2 to 50% aqueous solution and is made of epichlorohydrin, a dimethyl ester, adipic acid and diethylenetriamine; a polyamide-epichlorohydrin resin which is made of epichlorohydrin, diethylenetriamine, adipic acid and ethylenediamine; a polyamide-epichlorohydrin which is in the form of a 2 to 50% aqueous solution and is made of adipic acid, diethylenetriamine and a blend of epichlorohydrin and dimethylamine; a cationic polyamide-polyamine resin which is made of triethylenetriamine; a condensation product of an aromatic sulfonic acid and formaldehyde; aluminum acetate; aluminum formate; a blend of aluminum acetate, sulfate and formate; aluminum chloride; and a cationic starch. Next, the filtration, dehydration and drying step (3) in the process of the present invention is a step for removing a liquid phase for the purpose of obtaining only the solid sedimented. The operation in this step is conducted by passing the suspension through a filter fabric or a porous screen or a screen having a network. In this connection, mechanical operations such as compression or suction or a thermal action such as blowing of hot air may be conducted in combination, and most of the liquid phase can be removed efficiently by the operation. Other drying methods such as the irradiation of infrared or high frequency waves may be conducted. If the liquid phase can be removed only by filtration, the aforementioned drying is not always required. The molded product in the shape of sheet or the like is obtained by sufficiently removing the liquid phase. According to the process of the present invention, the melting and molding step (4) or the impregnation and curing step (4′) is conducted after the aforementioned steps (1) - (3). The melting and molding step is a step for melting with heating a molded product in the shape of sheet which has been dehydrated and dried at a temperature from the melting point of SPS to the melting point + 80°C or at a temperature not more than 350°C and next pressure forming the molded product. If the step is completed, the SPS resin in the molded product in the shape of sheet is melted, so that the bond of the SPS resin and the fibrous filler is strengthened for obtaining a formed product having a larger mechanical strength. The cooling after melting may be conducted at a temperature from room temperature to the melting point of the SPS, preferably 200°C or less. Pressure forming may be conducted by a usual method, for example at a pressure of 10 to 200 kg/cm², preferably 30 to 150 kg/cm² with a press or a roll. On the other hand, the impregnation and curing step (4′) which can be conducted in place of the above-described melting and forming step (4) is a step for impregnating the molded product (sheet) obtained in the step (3) with the thermosetting resin and then curing the resin to improve the strength of the sheet. The curing may be conducted under pressure. The impregnation of the sheet with the thermosetting resin is not critical and is usually conducted by some methods such as a method of applying the resin to the sheet with a roller, a brush or the like, a method of dipping the sheet into a varnish prepared by incorporating the thermosetting resin with a curing agent, additives and a solvent and then crying the sheet, a method of applying a varnish that a curing agent has been incorporated into a solvent-free liquid thermosetting resin to the sheet or dipping the sheet into the varnish or the like. The amount of the thermosetting resin to be impregnated may be appropriately determined, and it is preferably in the range of 0.1 to 50 parts by weight per 100 parts by weight of the total amount of the components (A) and (B). As the above-described thermosetting resin, there can be used a variety of resins such as an unsaturated polyester resin (such as an ortho-phthalic acid, isophthalic acid, terephthalic acid, alicyclic unsaturated fatty acid, halogen-containing acid, aliphatic saturated acid, bisphenol, halogen-containing bisphenol, molecular terminal (meth)acrylic acid or vinyl ester type), an epoxy resin (such as a phenolic glycidyl ether, alcoholic glycidyl ether, glycidyl ester, glycidylamine or mixed type or an alicyclic type), a phenol resin, a urea resin, a melamine resin, a diallyl phthalate resin and the like. Among them, the epoxy resin is preferred. When the epoxy resin is used, an aliphatic polyamine, an amidamine, a polyamide, an aromatic polyamine, an acid anhydride, a catalytic curing agent, a polymercaptan, a polysulfide and the like can be used in combination. The process of the present invention conducts the step of providing a metal layer on the molded article (5) after the above-described step (4) or (4′). The metal layer can be provided on the molded article by the methods such as etching a laminated metal foil (subtractive method), directly metal plating a circuit (additive method), laminating and foil stamping a circuit foil, vacuum deposition or sputtering. A circuit foil may be placed in a mold used for molding and bonded to the formed product at the same time as the molding. The metal layer has a thickness in the order from 0.1 µm to 1 mm, which may be appropriately selected according to the necessity. The thickness of the insulating base material may be selected appropriately according to the necessity with a preferred one described in JIS standard. The metal constituting the metal layer is preferably selected from copper, nickel, aluminum, tin, gold, silver, zinc and the like. When the metal foil is bonded to the formed product, an adhesive, preferably at thermosetting adhesive having heat resistance such as an epoxy resin is used. The process of the present invention proceeds by conducting sequentially the aforementioned steps, and the steps may be conducted continuously or batchwise. A printed circuit board obtained by the process of the present invention has a thickness of 0.01 to 5 mm, and a dielectric constant of preferably 3.5 or less. As described above, the printed circuit board obtained by the process of the present invention is excellent in impact resistance, heat resistance and mechanical strength and has a good size stability and flexibility as well as an excellent surface smoothness. Particularly, the printed circuit board is most suitable for a multi-layer printed circuit board used in fields where high-speed operation is required because of its excellent dielectric properties. It also has an advantage of reducing the production cost because of its excellent molding properties. Thus, the printed circuit board of the present invention is expected to have a wide and effective utility in electric and electronic fields. The present invention is further explained in detail with reference to Examples and Comparative Examples. Production Example 1 (1) Preparation of Contact Product of Trimethylaluminum and Water To 200 ml of toluene was 47.4 ml (492 mmole) of trimethylaluminum followed by 35.5 g (142 mmole) of copper sulfate pentahydrate (CuSO₄·5H₂O), and the mixture was reacted at 20°C for 24 hours. After the reaction was completed, toluene as the solvent was removed by filtration to give 12.4 g of methylaluminoxane as a contact product. (2) Production of SPS In a reaction vessel having an internal volume of 2 liters were charged 1 L (L=liter) of a purified styrene, 7.5 mmole calculated as an aluminum atom of the contact product obtained in the above (1), 7.5 mmole of triisobutylaluminum, 0.038 mmole of pentamethylcyclopentadienyl-titanium trimethoxide, and the mixture was polymerized at 90°C for 5 hours. After the reaction was completed, a methanolic solution of sodium hydroxide was added to the reaction mixture to decompose and remove the catalyst components. The residue was washed repeatedly with methanol and dried to give 466 g of a powder of a styrene polymer (polystyrene). When the polymer was evaluated by gel permeation chromatography with 1,2,4-trichlorobenzene as a solvent at 135°C, it had a weight average molecular weilght of 290,000 and a ratio of weight average molecular weight/number average molecular weight of 2.72. The polymer was confilrmed to be a polystyrene having a syndiotactic structure by the measurements of melting point and ¹³C-NMR. Example 1 To 10 L of water containing 2 g of an amine type cationic dispersing agent and 10 g of γ-aminopropyltriethoxysilane was added with vigorous stirring 300 g of a glass fiber having a length of 6 mm and a fiber diameter of 9 µm. Next, 100 g of a synthetic polyethylene pulp was added with stirring to the mixture. When the glass fiber and the polyethylene pulp had been dispersed properly, 600 g of a SPS powder prepared in Production Example 1 and having a bulk density of 0.2 g/cm³ was added and 100 g of a styrenebutadiene binder was successively added. After 20 g of anhydrous aluminum sulfate as an agglomerating agent was further added to the mixture with continuous stirring, the mixture was diluted to a total amount of a solution of 100 L to ensure that the solution has a solid content of about 10 g/l. The solution was passed through a screen having a network to remove water by filtration, and the residue was dried to give a sheet having an apparent amount of 600 g/m². The above-described sheet was pre-heated in an infrared heater at 300°C for 3 minutes, then placed in a mold at a temperature of 180°C and pressure molded at a pressure of 80 kg/cm² and a dwell for 3 minutes to give a molded product having a thickness of 1.2 mm. The appearance of the molded product thus obtained was evaluated visually for the quality of the embossing of the glass fiber. Next, a copper foil for a printed circuit board having a thickness of 70 µm to which an epoxy adhesive had been applied was laminated to make a copper foil laminated printed circuit board. The dielectric properties were measured with the copper foil laminated printed circuit board according to JIS-C-6481. The coefficient of size variation was measured after the above-described molded product had been further heated in an oven at 150°C for 30 minutes. The coefficient of size variation was calculated as follows: First, the variation of the size of the above-described molded product (plate) before and after thermal treatment was measured by the following procedures. A point a is provided on the plate. A line is drawn thorough the point a parallel to one side of the plate, and a point b is provided on the line at a distance of 50 mm from the point a . Another line is drawn through the point a parallel to another side of the plate, and a point c is provided on the line at a distance of 50 mm from the point a . In this connection, the lines ab and the line ac are rectangular. The size variations at the lines ab and ac and the point a before and after thermal treatment was calculated from the following equations: Variation of distance between a and b (%) = |b₂-b₁|/b₁x100, Variation of distance between a and c (%) = |c₂-c₁|/c₁x100, Variation of thickness at a = |h₂-h₁|/h₁x100 wherein b₁: the distance between a and b before thermal treatment, c₁: the distance between a and c before thermal treatment, h₁: the thickness at the point a before thermal treatment, b₂: the distance between a and b after thermal treatment, c₂: the distance between a and c after thermal treatment, h₂: the thickness at the point a after thermal treatment. The formulations of components are shown in Table 1, and the results of the measurements are shown in Table 2. Production Example 2 (Production of SPS) In a reaction vessel having an internal volume of 2 liters were charged 1 L of a purified styrene, 7.5 mmole calculated as an aluminum atom of the contact product obtained in the above (1), 7.5 mmole of triisobutylaluminum and 0.038 mmole of pentamethylcyclopentadienyltitanium trimethoxide, and the mixture was polymerized at 70°C for 3 hours. After the reaction was completed, a methanolic solution of sodium hydroxide was added to the reaction mixture to decompose and remove the catalyst components. The residue was washed repeatedly with methanol and dried to give 580 g of a polymer. When the polymer was evaluated by gel permeation chromatography with 1,2,4-trichlorobenzene as a solvent at 130°C, it had a weight average molecular weight of 592,000 and a ratio of weight average molecular weight/number average molecular weight of 2.81. The polymer was confirmed to be a polystyrene having a syndiotactic structure (SPS) by the measurements of melting point and ¹³C-NMR. Production Example 3 (Production of SPS) In a reaction vessel having an internal volume of 2 liters were charged 1 L of a purified styrene, 5 mmole calculated as an aluminum atom of the contact product obtained in the above (1), 5 mmole of triisobutylaluminum and 0.025 mmole of pentamethylcyclopentadienyltitanium trimethoxide, and the mixture was polymerized at 70°C for 5 hours. After the reaction was completed, a methanolic solution of sodium hydroxide was added to the reaction mixture to decompose and remove the catalyst components. The residue was washed repeatedly with methanol and dried to give 477 g of a polymer. when the polymer was evaluated by gel permeation chromatography with 1,2,4-trichlorobenzene as a solvent at 130°C, it had a weight average molecular weight of 802,000 and a ratio of weight average molecular weight/number average molecular weight of 2.24. The polymer was confirmed to be a polystyrene having a syndiotactic structure (SPS) by the measurements of melting point and ¹³C-NMR. Examples 2 to 7, 11, 12 and Comparative Examples 1 to 6 The procedure of Example 1 was repeated with the exception that the components shown in Table 1 were formulated. The results are shown in Table 2. In this connection, the SPS's of Production Examples 2 and 3 are used in Examples 6 and 7, respectively. Production Example 4 To 100 parts by weight of SPS obtained in Production Example 1 were added 0.7 part by weight of bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite and 0.1 part by weight of 2,6-di-t-butyl-4-methylphenol, and the mixture was blended with a Henschel mixer. The mixture was subjected to melt spinning with a single screw extruder at a cylinder temperature of 300°C and then to cutting to give staples having a fiber length/fiber diameter ratio of 5 mm/10 µm, 10 mm/1 µm and 30 mm/400 µm, respectively. Examples 8 to 10 The procedure of Example 1 was repeated with the exception that the SPS staples obtained in Production Example 4 were used. The results are shown in Table 2. Examples 13 to 15 and Comparative Example 7 To 10 L of water containing 2 g of an amine type cationic dispersing agent and 10 g of γ-aminopropyltriethoxy silane was added with vigorous stirring 300 g of a glass fiber having a length of 6 mm and a fiber diameter of 10 µm. Next, 100 g of a synthetic polyethylene pulp was added with stirring to the mixture. When the glass fiber and the polyethylene pulp had been dispersed properly, 600 g of one of the SPS staples prepared in Production Example 4 was added and 100 g of a styrene-butadiene binder was successively added. After 20 g of anhydrous aluminum sulfate as an agglomerating agent was further added to the mixture with continuous stirring, the mixture was diluted to a total amount of a solution of 100 L to ensure that the solution has a solid content of about 10 g/l. The solution was passed through a screen having a network to remove water by filtration, and the residue was dried to give a sheet having an apparent amount of 600 g/m². The above-described sheet was dipped in an alcoholic glycidyl epoxy resin and dried to impregnate 15 parts by weight (based on 100 parts by weight of the sheet) of the resin into the sheet. The sheet was further cured by heating to give a base material for a printed circuit board. The folding test of the base material was conducted by hand. A copper foil for a printed circuit board having a thickness of 70 µm to which an epoxy adhesive had been applied was laminated on the above molded product to make a copper foil laminated printed circuit board. The dielectric properties were measured with the copper foil laminated printed circuit board according to JIS-C-6481. The staples used are shown in Table 3, and the results of the measurement are shown in Table 4. Example 16 The procedure of Example 1 was repeated to give a molded product having a thickness of 5.0 mm instead of a thickness of 1.2 mm. The test pieces for the flexural test and the Izod test were cut out, the flexural strength the heat distortion temperature and the Izod impact were measured according to JIS-K-7203, JIS-K-7207 and JIS-K-7110, respectively. The appearance of the molded product was evaluated visually for the quality of the embossing of the glass fiber. The result are shown in Table 6. Examples 17 to 22 and Comparative Examples 8 to 12 The procedure of Example 16 was repeated with the exception that the components were formulated as shown in Table 5. The results are shown in Table 6. In this connection, the SPS of Production Examples 2 or 3 was used in Example 21 or 22, respectively Examples 23 to 25 The procedure of Example 16 was repeated with exception that the SPS staple obtained in Production Example 4 was used. The results are shown in Table 6. Examples 26 and 27 and Comparative Examples 13 and 14 The sheet obtained in Example 16 was melt pressed with a pressing machine at a temperature of 300°C and then cold pressed at a cold press at a temperature of 80°C to give a sheet. The above-described sheet was pre-heated in an infrared heater at 300°C for 3 minutes, then placed in a mold at a temperature of 180°C and pressure molded at a pressure of 80 kg/cm² and a dwell for 3 minutes to give a molded product having a thickness of 5.0 mm. The test pieces for the flexural test and the Izod test were cut out, the flexural strength, the heat distortion temperature and the Izod impact were measured according to JIS-K-7203, JIS-K-7207 and JIS-k-7110, respectively. The appearance of the molded product was evaluated visually for the quality of the embossing of the glass fiber. The results are shown in Table 6. Examples 28 and 29 To 10 L of water containing 2 g of an amine type cationic dispersing agent and 10 g of γ-aminopropyltriethoxysilane was added with vigorous stirring 300 g of a glass fiber having a length of 6 mm and a fiber diameter of 9 µm. Next, 100 g of a synthetic polyethylene pulp was added with stirring to the mixture. When the glass fiber and the polyethylene pulp had been dispersed properly, 600 g of a SPS powder prepared in Production Example 1 and having a bulk density of 0.2 g/cm³, 138 grams of flame retardant shown in Table 7 and 36 grams of antimony trioxide were added and 100 g of a styrene-butadiene binder was successively added. After 20 g of anhydrous aluminum sulfate as an agglomerating agent was further added to the mixture with continuous stirring, the mixture was diluted to a total amount of a solution of 100 L to ensure that the solution has a solid content of about 10 g/l. The solution was passed through a screen having a network to remove water by filtration, and the residue was dried to give a sheet having an apparent amount of 600 g/m². The above-described sheet was pre-heated in an infrared heater at 300°C for 3 minutes, then placed in a mold at a temperature of 180°C and pressure molded at a pressure of 80 kg/cm² and a dwell for 3 minutes to give a molded product having a thickness of 1.2 mm. The appearance of the molded product thus obtained was evaluated visually for the quality of the embossing of the glass fiber. Next, a copper foil for a printed circuit board having a thickness of 70 µm to which an epoxy adhesive had been applied was laminated to make a copper foil laminated printed circuit board. The same measurement as in Example 1 was carried out on the copper foil laminated printed circuit board. The formulations of components are shown in Table 7, and the results of the measurements are shown in Table 8.

Claims: A process for producing a printed circuit board characterized in that the following steps are sequentially conducted: (1) a step for dispersing in at least one of the mediums of water and an organic solvent (A) 95 to 20% by weight of a styrene polymer having a racemic pentad having 30% or more syndiotacticity, (B) 5 to 80% by weight of a fibrous filler having a fiber length of 1 to 50 mm, and (C) 0.1 to 30 parts by weight of at least one of a binder and a binding fiber in proportion to 100 parts by weight of the total amount of said components (A) and (B), to make a slurry having a concentration of the total of (A), (B) and (C) of 0.5 to 100 g/l, (2) a sedimentation step of the solids dispersed in said slurry, (3) a step for removing the at least one of the mediums of water and an organic solvent from the slurry by filtration and drying and molding the residue, (4) a step for melting with heating and pressure forming the molded product obtained in step (3), and (5) a step for providing a metal layer on the molded product obtained in step (4). A production process according to claim 1, wherein in the dispersion and slurry forming step (1) at least one of a sizing agent and a dispersing agent are added to the medium. A production process according to claim 1, wherein in the dispersion and slurry forming step (1) at least one of a sizing agent and a dispersing agent are added to the medium and the components (B), (A) and (C) are added in this sequence. A production process according to claim 1, wherein in the sedimentation step, an agglomerating agent is added to said slurry. A process for producing a printed circuit board characterized in that the following steps are sequentially conducted: (1) a step for dispersing in at least one of the mediums of water and an organic solvent (A) 95 to 20% by weight of a styrene polymer having a racemic pentad having 30% or more syndiotacticity, (B) 5 to 80% by weight of a fibrous filler having a fiber length of 1 to 50 mm, and (C) 0.1 to 30 parts by weight of at least one of a binder and a binding fiber in proportion to 100 parts by weight of the total amount of said components (A) and (B), to make a slurry having a concentration of the total of (A), (B) and (C) of 0.5 to 100 g/l, (2) a sedimentation step of the solids dispersed in said slurry, (3) a step for removing the at least one of the mediums of water and an organic solvent from the slurry by filtration and drying and molding the residue, (4′) a step for impregnating the molded product obtained in step (3) with a thermosetting resin and curing it, and (5) a step for providing a metal layer on the molded product obtained in step (4′). A production process according to claim 5, wherein the amount of said thermosetting resin to be impregnated is in a proportion of 0.1 to 50 parts by weight to 100 parts by weight of the total amount of the components (A) and (B).

Inventors: NAKANO AKIKAZU, NAKANO, AKIKAZU
Applicants: IDEMITSU KOSAN CO, IDEMITSU KOSAN COMPANY LIMITED
Application year: 1990
Main classification: H05K1
IPC codes: H05K3, H05K1