JP2005336324A - Prepreg, metal clad laminate and printed circuit board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a prepreg free from impregnation unevenness of resin and having smooth surface. <P>SOLUTION: The prepreg is produced by dissolving an epoxy resin and a phenolic hardener in a solvent to prepare a phenol-hardenable epoxy resin varnish, impregnating the varnish in a substrate and drying the product. The phenolic hardener has a softening point of ≥100°C and contains ≥2.5 phenolic hydroxy groups in the molecule on an average. The solvent is composed of at least one kind of solvent having a boiling point of ≥150°C, and the substrate has a thickness of ≥150μm. The amount of the substrate is <60 mass% based on the total amount of the prepreg. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a prepreg used as a material for a multilayer printed wiring board, a metal-clad laminate such as a copper-clad laminate produced using the prepreg, and a printed wiring board produced using the metal-clad laminate. is there.

  A prepreg used for the production of printed wiring boards and the like is prepared by using a resin composition mainly composed of a thermosetting resin such as an epoxy resin as a varnish, and impregnating the varnish into a substrate such as a glass cloth. It is manufactured by drying to a semi-cured state (B-stage). At this stage, the resin composition changes from an uncured resin to a semi-cured resin.

  Then, after cutting the prepreg produced as described above to a predetermined size, the required number of sheets are stacked, and a metal foil such as a copper foil is stacked on one or both sides of the prepreg laminate, and this is heated and pressed. A metal-clad laminate used for the production of a printed wiring board can be produced by laminate molding. At this stage, the semi-cured resin of the prepreg is changed to a cured resin, and an insulating layer is formed together with the base material.

  Then, through holes are formed in the metal-clad laminate by drilling and plating, and further, a circuit is formed by processing the metal foil on one or both sides of the metal-clad laminate by performing a subtractive method or the like. Thus, a printed wiring board can be manufactured. Here, when circuits are formed on both surfaces of the metal-clad laminate, these circuits are electrically connected by the through holes.

  In addition, a predetermined number of prepregs cut to a predetermined size are stacked on one or both sides of an inner layer substrate on which a circuit (inner layer circuit) has been formed in advance, and a metal foil such as copper foil is placed on one or both sides of the laminate. A multilayer laminated board used for the production of a multilayer printed wiring board can be produced by stacking and heating and pressurizing the laminate. As described above, at this stage, the semi-cured resin of the prepreg is changed to a cured resin, and an insulating layer is formed together with the base material.

  Then, via holes and the like are formed by drilling and plating the multilayer laminate, and further, a circuit (outer layer circuit) is processed by processing a metal foil on one or both sides of the multilayer laminate by performing a subtractive method or the like. A multilayer printed wiring board can be manufactured by forming. Here, the inner layer circuit and the outer layer circuit are electrically connected by the via hole.

A multilayer printed wiring board is manufactured as described above, and a phenol curable epoxy resin composition is particularly used as a resin composition used for manufacturing a prepreg (see, for example, Patent Documents 1-3). .) This phenol curable epoxy resin composition refers to an epoxy resin composition using a phenolic hydroxyl group-containing compound (phenolic curing agent) as a curing agent. Generation | occurrence | production can be prevented, CAF (Conductive Anodic Filament) resistance of a multilayer printed wiring board can be improved, and heat resistance can be improved.
JP-A-7-16585 JP-A-53-101082 US Pat. No. 4,501,787

  The above phenol curable epoxy resin composition is usually used as a varnish after being dissolved in a solvent such as methyl ethyl ketone.

  However, when a thick base material (for example, 150 μm or more) is used and the amount of resin impregnated into the base material is increased (for example, 40% by weight or more), uneven resin impregnation occurs and the surface is not smooth. A prepreg will be obtained. When a metal-clad laminate is manufactured using such a prepreg, the surface roughness of the resulting metal-clad laminate is increased, which hinders circuit formation. Furthermore, when a printed wiring board is manufactured using such a metal-clad laminate, even if CAF resistance and heat resistance are good, circuit formation is poor and reliability is lowered.

  Moreover, when a novolac type epoxy resin or a dicyclopentadiene type epoxy resin is used as the epoxy resin, the heat resistance of the printed wiring board can be increased and the hygroscopicity can be reduced. As the process proceeds, the viscosity rises rapidly, and as a result, there is a problem that uneven impregnation of the resin is remarkably generated. For this reason, novolak type epoxy resins and dicyclopentadiene type epoxy resins are inevitably used.

  The present invention has been made in view of the above points, and has a prepreg with a smooth surface without uneven resin impregnation, a metal-clad laminate with a small surface roughness and an easy circuit formation, and excellent CAF resistance and heat resistance. An object of the present invention is to provide a highly reliable printed wiring board.

  The prepreg according to claim 1 of the present invention is a prepreg obtained by dissolving an epoxy resin and a phenol-based curing agent in a solvent to prepare a phenol-curable epoxy resin varnish, impregnating it into a substrate and drying it. A phenolic curing agent having a softening point of 100 ° C. or higher and an average of 2.5 or more phenolic hydroxyl groups in the molecule is used, and a solvent having a boiling point of 150 ° C. or higher is used as the solvent. A type having a thickness of 150 μm or more is used as a base material, and the base material content is less than 60% by mass with respect to the total amount of the prepreg.

  The invention of claim 2 is characterized in that, in claim 1, the solvent having a boiling point of 150 ° C. or higher is a solvent containing no nitrogen atom in the molecule.

  The invention of claim 3 is characterized in that in claim 1 or 2, cyclohexanone is used as a solvent having a boiling point of 150 ° C. or higher.

  The invention of claim 4 is characterized in that in any one of claims 1 to 3, cyclohexanone and methyl ethyl ketone are used in combination as the solvent.

  The invention of claim 5 is characterized in that in any one of claims 1 to 4, the liquid epoxy resin is contained in an amount of 5% by mass or more at 20 ° C. with respect to the total amount of the phenol curable epoxy resin varnish. is there.

  A sixth aspect of the invention is characterized in that, in any one of the first to fifth aspects, 15% by mass or more of a novolak type epoxy resin is contained with respect to the total amount of the phenol curable epoxy resin varnish.

  The invention of claim 7 is characterized in that, in any one of claims 1 to 6, the dicyclopentadiene type epoxy resin is contained in an amount of 15% by mass or more based on the total amount of the phenol curable epoxy resin varnish. .

  A metal-clad laminate according to an eighth aspect of the present invention is formed by laminating the prepreg according to any one of the first to seventh aspects and a metal foil.

  A printed wiring board according to a ninth aspect of the present invention is formed by forming a circuit on the metal-clad laminate according to the eighth aspect.

  According to the invention of claim 1, a prepreg having a smooth surface with no unevenness of resin impregnation can be obtained.

  According to the invention of claim 2, it is possible to prevent the heat resistance of the printed wiring board from being lowered.

  According to the invention of claim 3, it is possible to manufacture a metal-clad laminate that has a smaller surface roughness and can form a circuit more easily than when using another solvent having a boiling point of 150 ° C. or higher. It is.

  According to invention of Claim 4, compared with the case where only cyclohexanone is used, the surface roughness is smaller and the metal-clad laminated board which can form a circuit more easily can be manufactured.

  According to the invention of claim 5, a metal-clad laminate having a smaller surface roughness and capable of forming a circuit more easily can be produced.

  According to the invention of claim 6, it is possible to manufacture a metal-clad laminate having a smaller surface roughness and capable of forming a circuit more easily, and when using the metal-clad laminate produced in this way, The heat resistance of the printed wiring board can be further increased and the hygroscopicity can be reduced.

  According to the invention of claim 7, it is possible to manufacture a metal-clad laminate having a smaller surface roughness and capable of forming a circuit more easily, and when using the metal-clad laminate produced in this way, The heat resistance of the printed wiring board can be further increased and the hygroscopicity can be reduced.

  According to the invention of claim 8, a metal-clad laminate having a small surface roughness and capable of forming a circuit easily can be obtained.

  According to the ninth aspect of the present invention, a highly reliable printed wiring board having good CAF resistance and heat resistance can be obtained.

  Embodiments of the present invention will be described below.

  The prepreg according to the present invention can be produced by preparing a phenol-curable epoxy resin varnish by dissolving an epoxy resin and a phenol-based curing agent in a solvent, impregnating the substrate and drying it.

  The epoxy resin is not particularly limited. For example, an epoxy resin that is liquid at 20 ° C., a novolac epoxy resin such as a cresol novolac epoxy resin or a phenol novolac epoxy resin, a dicyclopentadiene epoxy resin, or bromine Brominated epoxy resins such as bisphenol A epoxy resin, bisphenol F epoxy resin, and the like can be used. Of these epoxy resins, only one type may be used, or two or more types may be mixed and used. According to the present invention, it is possible to obtain a prepreg having a smooth surface without unevenness of impregnation regardless of the epoxy resin to be used, so that an epoxy resin such as a novolac type epoxy resin or a dicyclopentadiene type epoxy resin can be obtained. Even so, there is no need to refrain from using them. Rather, it is preferable to use these epoxy resins, whereby the heat resistance of the printed wiring board can be increased and the hygroscopicity can be reduced.

  When an epoxy resin that is liquid at 20 ° C. is used as the epoxy resin, the epoxy resin is preferably contained in an amount of 5% by mass or more (upper limit is 50% by mass) based on the total amount of the phenol curable epoxy resin varnish. This is because a metal-clad laminate having a smaller surface roughness and capable of forming a circuit more easily than when using other epoxy resins can be produced. If the content of the liquid epoxy resin at 20 ° C. is less than 5% by mass, the above effects may not be sufficiently obtained.

  When a novolac type epoxy resin is used as the epoxy resin, the epoxy resin is preferably contained in an amount of 15% by mass or more (upper limit is 85% by mass) with respect to the total amount of the phenol curable epoxy resin varnish. This is because a metal-clad laminate having a smaller surface roughness and capable of forming a circuit more easily than when using other epoxy resins can be produced. In addition, when the metal-clad laminate manufactured in this way is used, the heat resistance of the printed wiring board can be further increased and the hygroscopicity can be reduced. If the content of the novolac-type epoxy resin is less than 15% by mass, the above effects may not be sufficiently obtained.

  When a dicyclopentadiene type epoxy resin is used as the epoxy resin, the epoxy resin is preferably contained in an amount of 15% by mass or more (upper limit is 90% by mass) based on the total amount of the phenol curable epoxy resin varnish. This is because a metal-clad laminate having a smaller surface roughness and capable of forming a circuit more easily than when using other epoxy resins can be produced. In addition, when the metal-clad laminate manufactured in this way is used, the heat resistance of the printed wiring board can be further increased and the hygroscopicity can be reduced. If the content of the dicyclopentadiene type epoxy resin is less than 15% by mass, the above effects may not be sufficiently obtained.

  As the phenol-based curing agent, those having a softening point of 100 ° C. or higher (upper limit is 150 ° C.) and having an average of 2.5 or more phenolic hydroxyl groups in the molecule (upper limit is 15.0) are used. Content of a phenol type hardening | curing agent can be set to 5-30 mass% with respect to phenol hardening type epoxy resin varnish whole quantity. Specific examples of the phenolic curing agent include bisphenol A novolac resin. In particular, when a bisphenol A novolac resin is used, the moldability of the prepreg can be increased. If the softening point of the phenolic curing agent is less than 100 ° C or the phenolic curing agent has less than 2.5 phenolic hydroxyl groups in the molecule, the heat resistance of the finally obtained printed wiring board will decrease. To do.

  As the solvent, at least one solvent having a boiling point of 150 ° C. or higher (upper limit is 200 ° C.) is used. That is, in addition to using only one or two or more solvents having a boiling point of 150 ° C. or higher, a solvent having a boiling point of less than 150 ° C. (for example, methoxypropanol (boiling point 121 ° C.) or the like) is used. A mixture of two or more types can also be used. Specific examples of the solvent having a boiling point of 150 ° C. or higher include cyclohexanone (boiling point 155.7 ° C.) and dimethylformamide (boiling point 153 ° C.). If only a solvent having a boiling point of less than 150 ° C. is used, uneven impregnation of the resin occurs.

  As the solvent having a boiling point of 150 ° C. or higher, a solvent containing no nitrogen atom in the molecule is preferably used. As a specific example, cyclohexanone (boiling point 155.7 ° C.) or the like can be used. If a solvent containing a nitrogen atom in the molecule (for example, dimethylformamide) is used, the heat resistance of the printed wiring board may be lowered. Therefore, it is preferable to use a solvent having a boiling point of 150 ° C. or higher and containing no nitrogen atom in the molecule, thereby preventing a decrease in heat resistance of the printed wiring board. In particular, when cyclohexanone is used, a metal-clad laminate having a smaller surface roughness and easier circuit formation can be produced compared to the case of using another solvent having a boiling point of 150 ° C. or higher.

  As the solvent, cyclohexanone (boiling point: 155.7 ° C.) and methyl ethyl ketone (79.6 ° C.) may be used in combination. This is because it is possible to manufacture a metal-clad laminate that has a smaller surface roughness and can form a circuit more easily than when only cyclohexanone is used. The mixing ratio of cyclohexanone and methyl ethyl ketone is preferably 80:20 to 20:80 by mass ratio.

  As the substrate, glass cloth or the like can be used, but one having a thickness of 150 μm or more (upper limit is 250 μm) is used. When the thickness of the substrate is less than 150 μm, the resulting prepreg becomes thin and handling becomes difficult. In addition, such a thin prepreg requires more sheets for manufacturing a printed wiring board than a thick prepreg, increases the number of stacking steps, and increases the cost.

  In preparing the phenol curable epoxy resin varnish, a curing accelerator can be added and used as necessary. Although it does not specifically limit as a hardening accelerator, For example, imidazoles, such as 2-ethyl-4-methylimidazole (2E4MZ), 2-methylimidazole, 2-phenyl-4-methylimidazole, dimethylbenzylamine , Amines such as triethylenediamine, benzyldimethylamine and triethanolamine, organic phosphines such as triphenylphosphine, diphenylphosphine and phenylphosphine, tetrasubstituted phosphonium / tetrasubstituted borates such as tetraphenylphosphonium / ethyltriphenylborate, 2 Tetraphenylboron salts such as ethyl-4-methylimidazole and tetraphenylborate can be used. In addition, when using a hardening accelerator, content of a hardening accelerator can be set to 0.01-3.0 mass% with respect to the phenol hardening epoxy resin varnish whole quantity.

  The phenol curable epoxy resin varnish is blended with the above-described epoxy resin and phenol-based curing agent, and a curing accelerator if necessary, and after uniformly mixing with a mixer or blender, the mixture is dissolved in the above-described solvent. Can be prepared by diluting. And the prepreg which concerns on this invention can be manufactured by making the base material mentioned above impregnate the phenol curable epoxy resin varnish prepared in this way, and making it dry to a semi-hardened B stage state. In this way, a prepreg having a smooth surface without uneven resin impregnation can be obtained. However, the content of the base material is less than 60% by mass (the lower limit is 30% by mass) with respect to the total amount of the prepreg. The content of the substrate can be adjusted by the amount impregnated with the phenol curable epoxy resin varnish. Specifically, the content of the base material can be adjusted by removing the excess phenol-curing epoxy resin varnish through the base material after impregnation between a pair of rolls with adjusted intervals. The total of the base material content and the resin amount (excluding the solvent) is the total amount of the prepreg. When the content of the base material is 60% by mass or more, the resin impregnation unevenness occurs and a prepreg having a non-smooth surface is obtained.

  The metal-clad laminate according to the present invention can be manufactured by laminating the prepreg and metal foil obtained as described above. Specifically, it can be produced as follows. That is, the required number of prepregs are stacked, and a metal foil such as a copper foil is stacked on one or both sides of the laminate of the prepregs, and this is heated and pressed to form a laminate. Laminates can be manufactured. In this way, it is possible to obtain a metal-clad laminate having a small surface roughness and capable of easily forming a circuit.

  The printed wiring board according to the present invention can be manufactured by forming a circuit on the metal-clad laminate obtained as described above. Specifically, it can be produced as follows. That is, a metal-clad laminate produced as described above is formed by drilling and plating (for example, copper plating) by a known method to form a through hole, etc., and further by performing a subtractive method or the like. A printed wiring board can be produced by processing a metal foil on one or both sides of a stretched laminate to form a circuit. In this way, a highly reliable printed wiring board having good CAF resistance and heat resistance can be obtained. When circuits are formed on both surfaces of the printed wiring board, these circuits are electrically connected through the through holes. Moreover, the printed wiring board manufactured as mentioned above can be used as an inner layer substrate for manufacturing a multilayer printed wiring board described later.

  The multilayer printed wiring board can be manufactured as follows. First, the required number of prepregs are stacked on one or both sides of an inner layer substrate on which a circuit (inner layer circuit) has been formed in advance, and a metal foil such as copper foil is stacked on one or both sides of this laminate, and this is heated and pressed. By multilayer molding, a multilayer laminated board used for producing a multilayer printed wiring board can be produced. At this stage, the semi-cured resin of the prepreg is changed to a cured resin, and an insulating layer is formed together with the base material. In addition, as a board | substrate for inner layers, the printed wiring board mentioned above can be used, and what is marketed (for example, "CR-1766" by Matsushita Electric Works Co., Ltd.) can also be used. Further, it is preferable that the surface of the circuit of the inner layer substrate is previously roughened by roughening before lamination molding. This is because the adhesion strength of the prepreg to the inner layer substrate in the multilayer wiring board can be improved.

  Then, via holes and the like are formed by performing drilling and plating (for example, copper plating) on the multilayer laminate by a known method, and further, a single-sided or double-sided metal of the multilayer laminate is performed by performing a subtractive method or the like. A multilayer printed wiring board can be manufactured by forming a circuit (outer layer circuit) by processing the foil. The inner layer circuit and the outer layer circuit are electrically connected by the via hole.

  As described above, according to the present invention, it is possible to obtain a prepreg having a smooth surface without uneven resin impregnation, and a metal-clad laminate that can be easily formed with a small surface roughness. Even a printed wiring board that has been made can have a high CAF resistance and heat resistance and high reliability.

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

<Phenol curable epoxy resin varnish>
As epoxy resin, brominated epoxy resin (“YDB-500” manufactured by Tohto Kasei Co., Ltd., epoxy equivalent 500, bromine content 21 mass%), cresol novolak type epoxy resin (“N” manufactured by Dainippon Ink & Chemicals, Inc.) -690 ", epoxy equivalent 225), dicyclopentadiene type epoxy resin (manufactured by Dainippon Ink & Chemicals, Inc.," HP-7200H ", epoxy equivalent 283), liquid epoxy resin at 20 ° C (manufactured by Toto Kasei Co., Ltd.) “YD-128”, epoxy equivalent 186) was used.

  As a phenol-based curing agent, bisphenol A novolak resin (softening point 110 ° C., average number of phenolic hydroxyl groups in the molecule 2.5, hydroxyl group equivalent 118), “Tamanol 752” manufactured by Arakawa Chemical Industries, which is a phenol novolak resin. (Softening point 80 ° C., average number of phenolic hydroxyl groups in the molecule: 1.5, hydroxyl group equivalent: 105).

  As a curing accelerator, 2-ethyl-4-methylimidazole (“2E4MZ” manufactured by Shikoku Chemicals Co., Ltd.) was used.

  As the solvent, cyclohexanone (boiling point 155.7 ° C.), methoxypropanol (MP) (boiling point 121 ° C.), and methyl ethyl ketone (MEK) (boiling point 79.6 ° C.) were used.

  And Examples 1-5 and a comparative example by mix | blending said epoxy resin, a phenol type hardening | curing agent, a hardening accelerator, and a solvent with the compounding quantity shown to following [Table 1], and mixing this uniformly with a mixer. 1-4 phenol curable epoxy resin varnishes were prepared.

<Prepreg>
A glass cloth (“7628 type cloth” manufactured by Nitto Boseki Co., Ltd., thickness: 173 μm) was used as the substrate.

  And the said base material was impregnated at room temperature with the phenol curable epoxy resin varnish prepared as mentioned above. Next, the prepregs of Examples 1 to 5 and Comparative Examples 1 to 4 were manufactured by heating and drying to about 130 to 170 ° C. with a non-contact type heating unit to remove the solvent.

  In addition, the resin amount of the prepreg was adjusted to 43% by passing the base material after impregnation between a pair of rolls with adjusted intervals. That is, the contents of the base materials of the prepregs of Examples 1 to 5 and Comparative Examples 1 to 4 are all 57% by mass.

<Test item 1: Uneven prepreg resin>
The surface of each prepreg was visually observed to check for unevenness of resin impregnation. The results are shown in [Table 1] below.

<Test item 2: Copper foil surface roughness after molding>
An inner layer double-sided plate having a thickness of 0.2 mm (solid copper: “CR1766” manufactured by Matsushita Electric Works Co., Ltd .: copper foil thickness: 35 μm) was used as the inner layer substrate. First, the inner layer treatment (blackening treatment) was performed on the copper foils on both sides of the inner layer substrate. Next, the prepregs manufactured as described above are stacked one by one on both surfaces of the inner layer substrate, and a copper foil is stacked on both surfaces of the laminate, and this is heated and pressurized at 170 ° C. and 2.94 MPa for 90 minutes. A multi-layer laminate was produced by laminating and molding.

  And about the multilayer laminated board obtained in this way, the roughness of the surface of copper foil was measured with the surface roughness measuring device. The results are shown in [Table 1] below.

<Test item 3: Glass transition temperature>
Based on JIS C 6481, the glass transition temperature (Tg) of the multilayer laminate produced as described above was measured by the TMA method (Thermo-mechanical analysis). The results are shown in [Table 1] below.

<Test item 3: CAF resistance>
The multilayer laminate produced as described above was placed in an atmosphere of 121 ° C. and 85% humidity, and after applying a voltage of 50 V for 500 hours, the presence or absence of insulation deterioration was examined. As a result, it was confirmed that none of the multilayer laminates had insulation deterioration.

<Test item 4: heat resistance>
Based on JIS C 6481, the multilayer laminate produced as described above was heated in an oven at 280 ° C. for 60 minutes, and then visually checked for the presence of swelling or peeling. As a result, it was confirmed that swelling and peeling did not occur in the multilayer laminates of Examples 1 to 5 and Comparative Examples 1 to 3, and swelling and peeling occurred in the multilayer laminate of Comparative Example 4.

  As seen in the above [Table 1], it was confirmed that the prepregs of Examples 1 to 5 had no uneven resin impregnation.

  Moreover, about the multilayer wiring board of Examples 1-5, the surface roughness of copper foil was all 5 micrometers or less, and it was confirmed that circuit formation is easy.

  On the other hand, since the prepreg of Comparative Example 1 was manufactured using a phenol curable epoxy resin varnish using only methyl ethyl ketone (boiling point 79.6 ° C.) as a solvent, uneven impregnation of the resin occurred. Was confirmed. Moreover, it was confirmed that the multilayer laminate produced using this prepreg had a copper foil surface roughness of 5 μm or more, which hinders circuit formation.

  Further, since the prepregs of Comparative Examples 2 and 3 were produced using only methyl ethyl ketone as a solvent and further using a cresol novolac type epoxy resin or a dicyclopentadiene type epoxy as an epoxy resin, the uneven impregnation of the resin was remarkable. It was confirmed that it occurred. In addition, it was confirmed that the multilayer laminate produced using this prepreg has a further increased copper foil surface roughness, which hinders circuit formation.

  Moreover, although the prepreg of Comparative Example 4 had no uneven resin impregnation, it was confirmed that the heat resistance of the multilayer laminate was lowered because a phenolic curing agent having a softening point of less than 100 ° C. was used.

Claims (9)

  In a prepreg obtained by dissolving an epoxy resin and a phenol-based curing agent in a solvent to prepare a phenol-curable epoxy resin varnish, impregnating the varnish into a base material and drying it, the softening point is 100 as a phenol-based curing agent. A solvent having an average of 2.5 or more phenolic hydroxyl groups in the molecule, a solvent having at least one boiling point of 150 ° C. or more, and a substrate having a thickness of 150 μm or more. A prepreg characterized in that the content of the substrate is less than 60% by mass with respect to the total amount of the prepreg.   2. The prepreg according to claim 1, wherein the solvent having a boiling point of 150 [deg.] C. or higher is a solvent containing no nitrogen atom in the molecule.   The prepreg according to claim 1 or 2, wherein cyclohexanone is used as a solvent having a boiling point of 150 ° C or higher.   The prepreg according to any one of claims 1 to 3, wherein cyclohexanone and methyl ethyl ketone are used in combination as the solvent.   5. The prepreg according to claim 1, wherein the epoxy resin is contained in an amount of 5% by mass or more at 20 ° C. with respect to the total amount of the phenol curable epoxy resin varnish.   The prepreg according to any one of claims 1 to 5, wherein a novolak type epoxy resin is contained in an amount of 15% by mass or more based on the total amount of the phenol curable epoxy resin varnish.   The prepreg according to any one of claims 1 to 6, wherein the cyclohexane diene type epoxy resin is contained in an amount of 15% by mass or more based on the total amount of the phenol curable epoxy resin varnish.   A metal-clad laminate comprising the prepreg according to any one of claims 1 to 7 and a metal foil formed by lamination.   A printed wiring board comprising a circuit formed on the metal-clad laminate according to claim 8.