JP2006036936A - Epoxy resin composition, prepreg, and multilayer printed wiring board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an epoxy resin composition which can ensure the flame retardancy without generating an injurious substance at the time of the burning and can obtain high resistance to the crack owing to the drop impact. <P>SOLUTION: The epoxy resin composition contains an epoxy resin, a hardener and an inorganic filler and contains no halogen element. The epoxy resin used is a bifunctional epoxy resin which has a weight per epoxy equivalent of 400 or more and contains a phosphorus atom. The hardener used is a hardener which has such a reactive group to react with an epoxy group as has an equivalent of less than 50. The inorganic filler used is silica having an average particle diameter of 10 μm or less. The bifunctional epoxy resin has a content, in the whole amount of the epoxy resin, of more than 30% by mass to less than 90% by mass. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an epoxy resin composition, a prepreg produced using the epoxy resin composition as a material, and a multilayer printed wiring board produced using the prepreg as a material.

  Thermosetting resins typified by epoxy resins are widely used as printed wiring board materials and semiconductor sealing materials because of their excellent adhesion, electrical insulation, chemical resistance, and the like. Usually, when used in such applications, in order to ensure safety against fire, flame retardancy is achieved by blending the above materials with a compound containing a halogen element typified by bromine. Thus, it is widely known that when a halogen such as bromine is introduced, a molded product (cured product) exhibiting excellent flame retardancy can be obtained.

  However, such molded articles may produce compounds that adversely affect the human body such as polybrominated dibenzodioxins and furans during combustion. Moreover, the bromine-containing compound decomposes bromine when heated, resulting in poor long-term heat resistance. Therefore, in order to solve such problems, development of epoxy resins and compositions thereof that exhibit excellent flame retardancy without containing a halogen element has been widely performed.

  By the way, downsizing and high performance of electronic devices are remarkable, and many electronic devices typified by mobile phones are generally carried and used all the time. In such a portable device, a sudden drop can always occur, and therefore it is necessary that the device is not damaged when an impact due to the drop is applied. Although there are various causes of damage to electronic devices, printed circuit boards are required not to be disconnected due to the occurrence of cracks or to have components fall off due to breakage of the insulating layer.

  Here, for flame retardancy not containing a halogen element, a system mainly using a phosphorus element has been studied. For example, an appropriate amount of an additive-type phosphorus flame retardant such as triphenyl phosphate (TPP), tricresyl phosphate (TCP), cresyl diphenyl phosphate (CDP), etc., which is a phosphate ester compound, is blended in an epoxy resin composition. By doing so, flame retardancy can be ensured. However, since the additive-type phosphorus flame retardant as described above does not react with the epoxy resin, there arises a problem that the chemical resistance such as solder heat resistance and alkali resistance after the moisture absorption of the molded product is significantly reduced.

In contrast, for example, it has been proposed to employ a phosphorus compound having reactivity with an epoxy resin (see, for example, Patent Documents 1 and 2). However, when a polyfunctional epoxy resin such as a novolac type is used as a main component, the molded article has a high crosslinking density and becomes hard and brittle, so that the crack resistance against a drop impact is deteriorated.
JP-A-11-124489 Japanese Patent Laid-Open No. 2001-151991

  The present invention has been made in view of the above points, and an epoxy resin composition that can ensure flame retardancy without generating harmful substances during combustion and can also have high resistance to cracking caused by a drop impact. An object of the present invention is to provide a prepreg and a multilayer printed wiring board.

  The epoxy resin composition according to claim 1 of the present invention is an epoxy resin composition containing an epoxy resin, a curing agent, an inorganic filler and not containing a halogen element. Using a bifunctional epoxy resin containing a phosphorus atom, using as the curing agent the equivalent of the reactive group that reacts with the epoxy group is less than 50, and using silica with an average particle diameter of 10 μm or less as the inorganic filler, The content of the bifunctional epoxy resin is 30% by mass or more and less than 90% by mass with respect to the total amount of the epoxy resin.

  The invention of claim 2 is obtained by reacting the bifunctional epoxy resin in claim 1 with a phosphorus compound having two phenolic hydroxyl groups and a bifunctional epoxy compound having an epoxy equivalent of 150 or more. It is characterized by this.

  The invention of claim 3 is characterized in that, in claim 2, the phosphorus compound is represented by the following formula (1).

  The invention of claim 4 is characterized in that, in claim 2, the phosphorus compound is represented by the following formula (2).

  The invention of claim 5 is characterized in that, in claim 2, the phosphorus compound is represented by the following formula (3).

  The invention of claim 6 is characterized in that in any one of claims 1 to 5, the curing agent is dicyandiamide.

  Invention of Claim 7 WHEREIN: Content of phosphorus element is 0.8 mass% or more and less than 3.5 mass% in resin solid content whole quantity of an epoxy resin composition in any one of Claim 1 thru | or 6. It is characterized by.

  A prepreg according to an eighth aspect of the present invention is characterized in that a base material is impregnated with the epoxy resin composition according to any one of the first to seventh aspects and heated to a semi-cured state. Is.

  A multilayer printed wiring board according to claim 9 of the present invention is characterized by being formed by laminating the prepreg according to claim 8 on an inner layer substrate having a circuit pattern formed on the surface thereof.

  According to the epoxy resin composition of the first aspect of the present invention, it is possible to ensure flame retardancy without generating harmful substances during combustion and to obtain high crack resistance due to drop impact. .

  According to the invention of claim 2, a bifunctional epoxy resin having an epoxy equivalent of 400 or more and containing a phosphorus atom can be easily prepared.

  According to invention of Claim 3, while the flame retardance of a molded object can further be improved, the heat resistance of a molded object can also be improved.

  According to invention of Claim 4, while the flame retardance of a molded object can further be improved, the heat resistance of a molded object can also be improved.

  According to the fifth aspect of the invention, the flame retardancy of the molded product can be further increased, and the heat resistance of the molded product can also be increased.

  According to the invention of claim 6, the bifunctional epoxy resin is cured to obtain a molded article excellent in toughness, flexibility, adhesive force and stress relaxation during heating in addition to good electrical characteristics. It can be done.

  According to invention of Claim 7, sufficient flame retardance can be ensured even if it does not use a halogen compound.

  The prepreg according to claim 8 of the present invention can ensure flame retardancy without generating harmful substances during combustion, and can also obtain high crack resistance due to drop impact.

  The multilayer printed wiring board according to claim 9 of the present invention can ensure flame retardancy without generating harmful substances at the time of combustion, and can obtain high crack resistance due to drop impact.

  Embodiments of the present invention will be described below.

  The epoxy resin composition according to the present invention contains a bifunctional epoxy resin (epoxy resin having two epoxy groups), a curing agent, and an inorganic filler as essential components and does not contain a halogen element. That is, as the bifunctional epoxy resin, the curing agent, and the inorganic filler, those containing no halogen element are used. Thereby, harmful substances based on halogen elements can be prevented from being generated during combustion.

  In the present invention, the bifunctional epoxy resin is not particularly limited as long as it has an epoxy equivalent of 400 or more (substantially upper limit is 10,000) and contains a phosphorus atom, but it has an extremely long side chain. It is preferable to use those having no large substituent. Thus, when the epoxy equivalent in the bifunctional epoxy resin is 400 or more, the cross-linking density of the molded product is not increased, and the crack resistance due to the drop impact can be increased. By containing, flame retardance can be ensured. When the epoxy equivalent is less than 400, the cross-linked density of the molded product is high, it becomes hard and brittle, and the crack resistance at the time of a drop impact is deteriorated. In addition, although an epoxy equivalent can be calculated | required by various methods, the measuring method by JISK7236 can be mentioned as an example.

  And content of bifunctional epoxy resin is 30 mass% or more and less than 90 mass% with respect to the epoxy resin whole quantity. By curing the bifunctional epoxy resin with a curing agent to be described later within this range, a molded product having flame retardancy and excellent crack resistance at the time of dropping impact can be obtained. However, if the content of the bifunctional epoxy resin is less than 30% by mass, the crack resistance at the time of dropping impact is deteriorated and the flame retardancy is also deteriorated. Conversely, the content of the bifunctional epoxy resin is When it is 90% by mass or more, a sufficient glass transition temperature cannot be obtained.

  Moreover, it is preferable that content of a phosphorus element is 0.8 mass% or more and less than 3.5 mass% with respect to the resin solid content whole quantity of an epoxy resin composition. Thereby, sufficient flame retardance can be ensured without using a halogen compound. However, if the phosphorus element content is less than 0.8% by mass, it may be difficult to obtain sufficient flame retardancy due to the phosphorus element. Conversely, the phosphorus element content is 3.5% by mass. If it is at least%, the moisture absorption rate of the molded product may increase or the heat resistance may decrease.

  Here, the bifunctional epoxy resin as described above can be easily prepared as follows. That is, in the presence or absence of a solvent such as methoxypropanol (MP), a phosphorus compound having two phenolic hydroxyl groups, and a bifunctional epoxy compound having an epoxy equivalent of 150 or more (the practical upper limit is 5000) When the phosphorus compound and the bifunctional epoxy compound are reacted by adding a curing accelerator such as triphenylphosphine and stirring with heating, the above-described bifunctional epoxy resin can be obtained. .

  The phosphorus compound for preparing the bifunctional epoxy resin is not particularly limited as long as it has two phenolic hydroxyl groups, but the phosphorus compound represented by the above formulas (1) to (3) is used. It is preferable to use it. When such a phosphorus compound is used, the flame retardancy of the molded product can be further increased, and the heat resistance of the molded product can also be increased.

  The bifunctional epoxy compound for preparing the bifunctional epoxy resin is not particularly limited as long as the epoxy equivalent is 150 or more, but those having no extremely long side chain or large substituent are used. Is preferred. As such a bifunctional epoxy compound, for example, a linear compound such as a bisphenol type epoxy compound or a biphenyl type epoxy compound can be used, and it is particularly preferable to use a bisphenol type epoxy compound. Specific examples of the bisphenol type epoxy compound include “Epicoat 828” manufactured by Yuka Shell Epoxy Co., Ltd., which is a bisphenol A type bifunctional epoxy resin.

  In the present invention, as the curing agent, one having an equivalent of a reactive group that reacts with an epoxy group is less than 50 (substantially lower limit is 10). Specifically, the equivalent means a theoretically active hydrogen equivalent or a hydroxyl equivalent of the curing agent, and when such an equivalent is less than 50, the crosslinking density around the curing agent is not lowered, A sufficient glass transition temperature can be obtained. However, when the above equivalent is 50 or more, the crosslinking density around the curing agent is lowered, and there is a possibility that a sufficient glass transition temperature cannot be obtained.

  In particular, dicyandiamide (theoretical active hydrogen equivalent 21) is preferably used as the curing agent. As a result, the above-described bifunctional epoxy resin is cured, and in addition to good electrical characteristics, a molded product excellent in toughness, flexibility, adhesive force and stress relaxation during heating can be obtained. .

  In the present invention, silica having an average particle diameter of 10 μm or less is used as the inorganic filler. As a result, the crack resistance at the time of drop impact can be improved, the moisture absorption rate (water absorption rate) of the molded product can be reduced, the strength during high-temperature heating such as soldering can be increased, or the dimensions associated with heating. The rate of change can be reduced. However, when silica having an average particle diameter of more than 10 μm is used, the electrical insulation of the molded product is reduced, the stress reduction effect is not uniform, and the solder heat resistance after moisture absorption is reduced. . In addition, the minimum of the average particle diameter of a silica is 0.05 micrometer, and when an silica with an average particle diameter smaller than this is used, there exists a possibility that an epoxy resin composition may thicken.

  Further, the addition amount of silica having an average particle diameter of 10 μm or less is preferably 15 parts by mass or more and less than 100 parts by mass, and preferably 35 parts by mass or more and less than 100 parts by mass with respect to 100 parts by mass of the resin solid content. More preferred. As a result, the moisture absorption rate of the molded product can be reduced, the strength during high-temperature heating such as soldering can be increased, or the dimensional change rate associated with heating can be reduced. However, if the amount of silica added is less than 15 parts by mass, the moisture absorption rate may increase, the solder heat resistance may decrease, or the rate of dimensional change with heating may increase. If the amount of silica added is 100 parts by mass or more, it may be difficult to uniformly disperse such a filler, or the adhesive strength of the molded product may be reduced.

  In the present invention, if the compound does not contain a halogen element and does not deteriorate the properties of the epoxy resin composition and the prepreg produced using the same, the above-described bifunctional epoxy resin, curing agent, inorganic filling Epoxy resins, inorganic fillers, curing accelerators, modifiers, additives and the like other than the agent can be used as optional components.

  For example, as the epoxy resin other than the bifunctional epoxy resin described above, bifunctional epoxy resins such as bisphenol A type epoxy resin and bisphenol F type epoxy resin, phenol novolac type epoxy resin, cresol novolac type epoxy resin, bisphenol A novolak type epoxy resin Polyfunctional epoxy resins such as resins, bisphenol F novolac type epoxy resins, naphthalene type epoxy resins, biphenyl type epoxy resins, dicyclopentadiene type epoxy resins, etc. may be used alone or in combination. it can. However, the general-purpose novolac type epoxy resin hardens the molded product after curing, and is inferior in terms of toughness. Therefore, when using a polyfunctional epoxy resin in combination, a polyfunctional epoxy that has been provided with toughness in advance. It is preferable to use a resin.

  Moreover, as inorganic fillers other than the inorganic filler (silica) mentioned above, alumina, talc, mica, clay, titanium oxide, silicon nitride, glass beads, glass hollow spheres, aluminum hydroxide, magnesium hydroxide, etc. are used. Can do.

  Moreover, as a hardening accelerator, imidazoles, tertiary amines, etc., such as 2-ethyl-4-methylimidazole, can be used.

  Moreover, as a modifier, rubber components, such as polyvinyl acetal resin, SBR, BR, butyl rubber, butadiene-acrylonitrile copolymer rubber, can be used.

  And the epoxy resin composition which concerns on this invention can be prepared as a varnish by throwing in the essential component mentioned above to a solvent, also adding the arbitrary component mentioned above as needed, and stirring and mixing this. As the solvent, methyl ethyl ketone (MEK), methoxypropanol (MP), dimethylformamide (DMF), or the like can be used.

  Next, after impregnating the base material with the varnish obtained as described above, this is heated in a dryer at 120 to 190 ° C. for about 3 to 15 minutes and dried to be in a semi-cured state (B- A prepreg made into a stage) can be manufactured. As the base material, craft paper, linter paper, natural fiber cloth, organic synthetic fiber cloth and the like can be used in addition to glass fiber cloth such as glass cloth, glass paper, and glass mat.

  Next, a multilayer printed wiring board can be produced by laminating the prepreg obtained as described above on an inner layer substrate having a circuit pattern formed in advance on the surface. Specifically, first, an inner layer substrate is manufactured by forming a circuit pattern on the surface of a metal-clad laminate such as a copper-clad laminate by a subtractive method or the like. The circuit pattern formed here is preferably subjected to chemical treatment such as blackening treatment. Thereby, high adhesiveness with the prepreg can be obtained. Next, a required number of the above prepregs are placed on one or both sides of the inner layer substrate, and a metal foil is placed on the outer side of the prepreg, which is placed at 150 to 180 ° C. and 0.98 to 4.9 MPa. A multilayer laminated board processed into a multilayer printed wiring board can be manufactured by heating and pressing under the conditions described above. As said metal foil, copper foil, silver foil, aluminum foil, stainless steel foil, etc. can be used. If the molding temperature is less than 150 ° C., curing may be insufficient and it may be difficult to obtain desired heat resistance, or adhesion between the prepreg and the metal foil of the inner layer substrate may be insufficient. Conversely, when the molding temperature exceeds 180 ° C., the unevenness on the surface of the metal foil of the inner layer substrate that has been chemically treated disappears, and the adhesion between the prepreg and the metal foil of the inner layer substrate becomes insufficient. There is a fear.

  And a multilayer printed wiring board can be manufactured by forming a circuit pattern by the subtractive method etc. on the surface of the multilayer laminated board obtained as mentioned above. In this multilayer printed wiring board, flame retardance can be ensured without generating harmful substances during combustion, and crack resistance due to drop impact can be enhanced.

  Hereinafter, the present invention will be specifically described by way of examples.

<Raw materials>
First, the used epoxy resin, curing agent, phosphorus compound, inorganic filler, and solvent are shown in this order.

(Epoxy resin)
The following three types of epoxy resins were used.

Epoxy resin 1: Bisphenol A type bifunctional epoxy resin, “Epicoat 828” manufactured by Yuka Shell Epoxy Co., Ltd. (2.0 epoxy groups average, epoxy equivalent 190)
Epoxy resin 2: Bisphenol A type bifunctional epoxy resin, “EPICLON 1055” manufactured by Dainippon Ink and Chemicals, Inc. (epoxy group average 2.0, epoxy equivalent 475)
Epoxy resin 3: phenol novolac type epoxy resin, “EPICLO-N770” manufactured by Dainippon Ink & Chemicals Co., Ltd. (5.0 epoxy groups average, epoxy equivalent 190)
(Curing agent)
The following two types of curing agents were used.

Curing agent 1: Dicyandiamide (molecular weight 84, theoretically active hydrogen equivalent 21)
Curing agent 2: Phenol novolac resin, “PSM6200” manufactured by Gunei Chemical Industry Co., Ltd. (melting point: about 80 ° C., hydroxyl group equivalent: 105)
(Phosphorus compound)
The following three types of phosphorus compounds were used.

Phosphorus compound 1: Compound represented by the above formula (1) having an average of 2.0 phenolic hydroxyl groups, “HCA-HQ” manufactured by Sanko Co., Ltd. (phosphorus content: about 9.6 mass%, hydroxyl equivalent: about 162)
Phosphorus compound 2: Compound represented by the above formula (2) having an average of 2.0 phenolic hydroxyl groups, “HCA-NQ” manufactured by Sanko Co., Ltd. (phosphorus content: about 8.2 mass%, hydroxyl group equivalent: about 188)
Phosphorus compound 3: Compound (diphenylphosphinyl hydroquinone) represented by the above formula (3) having an average of 2.0 phenolic hydroxyl groups, “PPQ” manufactured by Hokuko Chemical Co., Ltd. (phosphorus content about 10.1 mass) %, Hydroxyl equivalent weight of about 155)
(Inorganic filler)
The following three types of inorganic fillers were used.

Inorganic filler 1: Spherical silica, “SO-C3” manufactured by Admatechs Co., Ltd. (average particle diameter of about 1 μm)
Inorganic filler 2: crushed silica, “RD-120” manufactured by Tatsumori Co., Ltd. (average particle size of about 30 μm)
Inorganic filler 3: Aluminum hydroxide, “C302A” manufactured by Sumitomo Chemical Co., Ltd. (average particle size of about 2 μm)
(solvent)
The following three types of solvents were used.

Solvent 1: methyl ethyl ketone (MEK)
Solvent 2: Methoxypropanol (MP)
Solvent 3: Dimethylformamide (DMF)
Next, the following three types of bifunctional epoxy resins A to C were prepared using the above epoxy resin 1 (epoxy equivalent 190) as the bifunctional epoxy compound and the above phosphorus compounds 1 to 3 as the phosphorus compound. .

(Bifunctional epoxy resin A)
Epoxy resin 1 (70 parts by mass) and phosphorus compound 1 (30 parts by mass) were heated and stirred in solvent 2 (20 parts by mass) at 130 ° C., and then 0.2 parts by mass of triphenylphosphine was added. The bifunctional epoxy resin A was obtained by continuing heating and stirring for 3 hours. In this bifunctional epoxy resin A, the epoxy equivalent in the solid content was about 503, the solid content was 83.33% by mass, and the phosphorus content in the solid content was about 3.1% by mass.

(Bifunctional epoxy resin B)
Epoxy resin 1 (70 parts by mass) and phosphorus compound 2 (30 parts by mass) are heated and stirred at 130 ° C. in the absence of a solvent, and then 0.2 parts by mass of triphenylphosphine is added, followed by heating and stirring for about 4 hours. By continuing, the bifunctional epoxy resin B was obtained. In this bifunctional epoxy resin B, the epoxy equivalent was about 528, the melt viscosity at 150 ° C. was about 100 poise, and the phosphorus content was about 2.9% by mass.

(Bifunctional epoxy resin C)
Epoxy resin 1 (70 parts by mass) and phosphorus compound 3 (30 parts by mass) are heated and stirred at 130 ° C. in the absence of a solvent, and then 0.2 parts by mass of triphenylphosphine is added, followed by heating and stirring for about 4 hours. By continuing, the bifunctional epoxy resin C was obtained. In this bifunctional epoxy resin C, the epoxy equivalent was about 496, the melt viscosity at 150 ° C. was about 100 poise, and the phosphorus content was about 3.1 mass%.

<Epoxy resin composition>
Bifunctional epoxy resins A to C, epoxy resins 2 and 3, curing agents 1 and 2, and inorganic fillers 1 to 3 in a predetermined solvent (solvent 1: solvent 2: solvent 3 = 3: 3: 1 (mass ratio)), and this was mixed at about 1000 rpm for about 90 minutes using “Homomixer” manufactured by Tokushu Kika Kogyo Co., Ltd. Further, a curing accelerator (2-ethyl-4-methylimidazole) was added to this mixture, and the mixture was stirred again for about 15 minutes, and then degassed to obtain an epoxy resin composition of about 5 to 10 poise at 25 ° C. Prepared as a varnish.

  And the prepreg was manufactured as follows using the varnish obtained as mentioned above, and the copper clad laminated board and the multilayer laminated board were manufactured using this prepreg.

<Prepreg>
After impregnating the above varnish into a glass cloth ("WEA 7628" manufactured by Nittobo Co., Ltd., thickness 0.18 mm glass cloth), this is heated in a dryer at a temperature of 120 to 190 ° C for about 5 to 10 minutes. A prepreg in a semi-cured state (B-stage) was produced by drying the mixture.

<Copper-clad laminate>
By stacking four prepregs obtained as described above, and further stacking copper foils on both sides of the prepreg, heating and pressurizing them under the conditions of 140 to 180 ° C. and 0.98 to 3.9 MPa, the thickness is increased. A copper clad laminate having a thickness of about 0.75 mm was produced. Here, the heating time was set so that the time when the entire prepreg was 160 ° C. or higher was at least 60 minutes. At this time, the inside of the press was in a reduced pressure state of 133 hPa or less. By doing so, the adsorbed water of the prepreg can be efficiently removed, and voids (voids) can be prevented from remaining after molding. As the copper foil, “GT” (thickness 0.018 mm) manufactured by Furukawa Circuit Foil Co., Ltd. was used.

<Multilayer laminate>
First, a circuit pattern was formed on the copper foil (thickness 35 μm) on the surface of the inner layer substrate (“CR1766” manufactured by Matsushita Electric Works Co., Ltd., thickness 0.2 mm). Next, the circuit pattern was blackened. For the blackening treatment, an aqueous solution composed of sodium chlorite 50 g / l, sodium hydroxide 10 g / l, and trisodium phosphate 10 g / l was used as a treatment solution, and the treatment was performed at 95 ° C. for 60 seconds. Next, prepregs are stacked one by one on both sides of the inner layer substrate after the blackening treatment, and a copper foil is further stacked on the outer side, and heated and pressed under the same molding conditions as those for the copper-clad laminate described above. A multilayer laminate was produced by laminate molding.

<Physical property evaluation>
The molded article (copper-clad laminate and multilayer laminate) obtained as described above was evaluated for physical properties as shown below.

(1) Flame retardancy, average flame extinction time After removing the copper foil of the copper clad laminate by etching, it was cut into a length of 125 mm and a width of 13 mm to obtain a test piece. Then, using this test piece, a combustion behavior test was performed in accordance with “Test for Flammability of Plastic Materials—UL94” of Under Writers Laboratories. In addition, the average time from ignition to extinction was measured in order to see the difference in flame retardant properties.

(2) Crack resistance of multilayer laminate After removing the copper foil of the multilayer laminate by etching, it was cut into a length of 50 mm and a width of 25 mm to obtain a test piece. Next, the test piece was placed on a table with a fulcrum distance of 30 mm and allowed to stand, and then a weight of 200 mg and a weight of a tip of 2.0 mmφ was naturally dropped from the height of 200 mm onto the center of the multilayer laminate. I collided. And the surface of the multilayer laminated board after a weight collided was observed. The observation results were “◯” when no crack was generated, “△” when three or less cracks were generated and all cracks were 2 mm or less, and “×” when more cracks were generated. ".

  The results of the above physical property evaluation are summarized in [Table 1] below.

Claims (9)

  In an epoxy resin composition containing an epoxy resin, a curing agent, an inorganic filler and not containing a halogen element, a bifunctional epoxy resin having an epoxy equivalent of 400 or more and containing a phosphorus atom is used as the epoxy resin, and the curing agent As the inorganic filler, silica having an average particle diameter of 10 μm or less is used, and the content of the bifunctional epoxy resin is based on the total amount of the epoxy resin. 30% by mass or more and less than 90% by mass of an epoxy resin composition.   The epoxy according to claim 1, wherein the bifunctional epoxy resin is obtained by reacting a phosphorus compound having two phenolic hydroxyl groups with a bifunctional epoxy compound having an epoxy equivalent of 150 or more. Resin composition. The epoxy resin composition according to claim 2, wherein the phosphorus compound is represented by the following formula (1).

The epoxy resin composition according to claim 2, wherein the phosphorus compound is represented by the following formula (2).

The epoxy resin composition according to claim 2, wherein the phosphorus compound is represented by the following formula (3).

  6. The epoxy resin composition according to claim 1, wherein the curing agent is dicyandiamide.   The epoxy resin according to any one of claims 1 to 6, wherein the content of the phosphorus element is 0.8% by mass or more and less than 3.5% by mass with respect to the total resin solid content of the epoxy resin composition. Composition.   A prepreg formed by impregnating a base material with the epoxy resin composition according to any one of claims 1 to 7 and heating the base material into a semi-cured state.   A multilayer printed wiring board obtained by laminating and molding the prepreg according to claim 8 on an inner layer substrate having a circuit pattern formed on a surface thereof.