JP2016204522A - Prepreg and manufacturing method therefor, printed wire board and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a prepreg having high rigidity and high insulation property without insulation deterioration by getting long term reliability and a manufacturing method therefor, a printed wire board using the prepreg and a manufacturing method therefor.SOLUTION: There is provided a printed wire board 100 that comprises a first prepreg 10 forming a filler-containing resin layer 3 in a semi-cured condition by impregnating an insulation cloth material 1 with varnish containing no filler and applying varnish containing the filler to both surfaces of a dried prepreg intermediate 2 and drying it, a circuit part containing a via hole 6 formed on at least one surface of the first prepreg 10, and a second prepreg 11 manufactured by applying varnish containing the filler to one surface of the prepreg intermediate 2 and drying it to form the filler-containing resin layer 3 in the semi-cured condition and applying varnish containing no filler to another surface and drying it to form a filler-non-containing resin layer 4 in the semi-cured condition, where the surface of the filler-non-containing resin layer 4 of the second prepreg 11 is laid so as to contact a blacking processing surface 61 of the circuit part of a core layer 10'.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a prepreg and a manufacturing method thereof, a printed wiring board and a manufacturing method thereof.

Conventionally, with the miniaturization and thinning of electronic devices, the thickness of the interlayers (20 μm to 40 μm, etc.) of mobile terminal printed wiring boards used for this has progressed. However, there are problems such as manufacturing defects (breaking, clogging, etc. in the manufacturing line) and component mounting after completion of the printed wiring board.
In order to solve this problem, a method in which a filler (filler) is mixed in a resin layer of a printed wiring board has been adopted by material manufacturers.
For example, in Cited Document 1, a resin layer of a printed wiring board (copper-clad laminate) having a property of adhering well to a plated copper layer, or containing a polyimide resin is used as a filler ( The addition of various organic fillers and inorganic fillers) is described.
Further, in Cited Document 2, a prepreg impregnated and dried with a thermosetting resin composition containing 10 to 79% by weight of an insulating inorganic filler (inorganic filler) using a fiber cloth as a base material. On both sides, a thermosetting resin composition in which an insulating inorganic filler is blended in a thermosetting resin of 80 to 99% by weight is attached to one side of a thermoplastic film, which is disposed on both sides of the prepreg and heated. -The prepreg made by adhering is described.

However, these printed wiring boards have problems in that long-term reliability similar to that of the prior art cannot be obtained and insulation deterioration is caused by the influence of the filler contained in the resin layer.
The reason why long-term reliability cannot be obtained is that the filler becomes an obstacle to the flow of the molten resin during the lamination molding in the production of the laminated board. This is because the filler contained in the resin between the insulating cloth materials forming the prepreg inhibits the impregnation of the resin with the insulating cloth material, and voids are generated between the resin and the insulating cloth material. This is because short circuit is likely to occur due to moisture.

  The reason for the deterioration of insulation is that the blackened surface, which is a process that improves the adhesion between the resin layer and the copper circuit, is scraped by the filler when the laminate is created, and the copper oxide on the blackened surface is the resin. This is because the flow diffuses in the prepreg and in a portion where there is no copper circuit on the same plane as the copper circuit.

JP 2006-324654 A JP 2002-265644 A

An object of the present invention is to provide a prepreg having a high rigidity and a high insulating property that has long-term reliability and does not deteriorate in insulation, and a method for manufacturing the same.
Moreover, another subject of this invention is providing the printed wiring board using the prepreg which has high rigidity and high insulation, and its manufacturing method.

The present invention has been completed in order to solve the above problems, and has the following configuration.
(1) A prepreg characterized by applying a varnish containing a filler to at least one surface of a prepreg intermediate obtained by impregnating an insulating cloth material with a varnish not containing a filler and drying it.
(2) An insulating cloth material impregnated with a varnish that does not contain a filler is dried by applying a varnish containing a filler to one side of a dried prepreg intermediate, and a varnish containing no filler is applied to the other side and dried. A prepreg characterized by that.
(3) The prepreg according to (1) or (2), wherein the filler is silica or aluminum hydroxide.
(4) The prepreg according to any one of (1) to (3), wherein the insulating cloth material is glass fiber.
(5) Printed wiring including a core layer including a first prepreg and a circuit portion formed on at least one surface of the first prepreg, and a second prepreg laminated on the circuit portion of the core layer. The first prepreg is a plate formed by impregnating a varnish containing no filler and applying and drying a varnish containing a filler on both sides of a first prepreg intermediate obtained by drying. The second prepreg is impregnated with a varnish that does not contain a filler, and dried by applying a varnish containing a filler to one side of the second prepreg intermediate obtained by drying, and containing the filler on the other side. The second prepreg was laminated so that the varnish-coated surface containing no filler was in contact with the blackened surface of the circuit portion of the core layer. Printed wiring board characterized.
(6) The printed wiring board according to (5), wherein a via hole that is electrically connected to the circuit unit is provided in the second prepreg.
(7) The printed wiring board according to (5) or (6), wherein a through hole penetrating the core layer and the second prepreg is provided.
(8) A step of applying or impregnating a varnish containing no filler to an insulating cloth material and drying it to form a prepreg intermediate, and applying and drying a varnish containing a filler on at least one surface of the prepreg intermediate. A process for producing a prepreg comprising the steps of:
(9) A step of applying or impregnating a varnish containing no filler to an insulating cloth material and drying it to form a prepreg intermediate, and a step of applying and drying a varnish containing a filler on one surface of the prepreg intermediate And a step of applying a varnish that does not contain a filler to the other surface of the prepreg intermediate and drying the prepreg intermediate.
(10) A step of applying or impregnating a varnish not containing a filler to an insulating cloth material and drying to form a prepreg intermediate, and applying a varnish containing a filler to both sides of the prepreg intermediate and drying A step of forming a core material composed of a first prepreg in which a filler-containing resin layer in a semi-cured state is formed, and a copper foil provided on at least one surface of the first prepreg is plated and then etched Forming a core layer as a circuit part, applying a varnish containing a filler to one surface of the prepreg intermediate, drying to form a cured filler-containing resin layer, and applying a filler to the other surface A step of forming a second prepreg in which a varnish not included is applied and dried to form a semi-cured filler-free resin layer, and the second prepreg is used as a filler Surface of the containing resin layer, a method of manufacturing a printed wiring board which comprises a laminating in contact with the blackened surface of the circuit portion of the core layer.

According to the present invention, since a prepreg obtained by impregnating a glass fiber with a varnish not containing a filler and drying is used, the varnish is sufficiently impregnated between the insulating cloth materials. The flow can be improved, and a substrate having long-term reliability can be manufactured.
In addition, it is possible to prevent copper oxide from diffusing on the blackened surface by laminating the surface of the prepreg with a resin-free surface applied to the blackened surface, thereby preventing deterioration of insulation. it can.

(A) is explanatory drawing which shows one Embodiment of the prepreg of this invention, (b) is explanatory drawing which shows another embodiment. It is explanatory drawing which shows one Embodiment of the printed wiring board which concerns on this invention. (A)-(i) is process explanatory drawing which shows one Embodiment of the manufacturing method of the printed wiring board concerning this invention.

As shown in FIG. 1A, a first prepreg 10 according to an embodiment of the present invention is obtained by applying a varnish containing no filler to an insulating cloth material 1 and impregnating and drying the prepreg intermediate 2. A varnish containing a filler is applied to both sides of the resin and dried to form a semi-cured filler-containing resin layer 3.
Moreover, the 2nd prepreg 11 which is another embodiment of this invention is, as shown in FIG.1 (b), the varnish containing a filler on one surface of the prepreg intermediate body 2 obtained by carrying out similarly to the above. It is applied and dried to form a semi-cured filler-containing resin layer 3, and the other surface is coated with a filler-free varnish and dried to form a semi-cured filler-free resin layer 4. is there.

  The insulating cloth material 1 is a material having insulating properties, and preferably has no gap so that a conductor material (plating) forming a circuit portion (conductor) described later does not pass through. As a material having such an insulating property, for example, glass fiber or glass nonwoven fabric is preferable. Examples of the glass fiber include a high density glass cloth and a high weaving cloth made from glass fiber.

The prepreg intermediate 2 is obtained by applying a varnish containing no filler to the insulating cloth material 1, impregnating it, and drying it. This varnish contains a resin and a solvent, and the rigidity of the prepreg intermediate body 2 can be ensured by impregnating or coating the insulating cloth material 1 and drying it.
It is preferable that the varnish is applied with a thickness equal to or less than that of the insulating cloth material 1.

  The filler-containing resin layer 3 is a semi-cured resin layer obtained by applying a varnish containing a filler to the prepreg intermediate 2 and drying it. The filler-containing resin layer 3 is formed on both surfaces of the prepreg intermediate 2. Then, the first prepreg 10 is obtained.

  The filler-free resin layer 4 is a semi-cured resin layer obtained by applying a varnish containing no filler to the prepreg intermediate 2 and drying it. When the filler-free resin layer 4 is formed on one surface of the prepreg intermediate 2 and the filler-containing resin layer 3 is formed on the other surface of the prepreg intermediate 2, a second prepreg 11 is obtained.

The filler contained in the varnish has an effect of ensuring rigidity and may be either an organic filler or an inorganic filler, but is preferably an inorganic filler. Examples of the inorganic filler include silica, alumina, barium sulfate, talc, clay, mica powder, aluminum hydroxide, magnesium hydroxide, calcium carbonate, magnesium carbonate, magnesium oxide, boron nitride, aluminum borate, barium titanate, Examples include strontium titanate, calcium titanate, magnesium titanate, bismuth titanate, titanium oxide, barium zirconate, and calcium zirconate. Among these, fused spherical silica or aluminum hydroxide is particularly preferable. Moreover, 1 type, or 2 or more types can be used for an inorganic filler.
The filler content relative to the resin is, for example, higher than 0 and up to 85 vol%. In particular, it is not necessary to use a plurality of fillers having optimized particle diameters, and it is preferable to use up to 74 vol% that can be realized with one kind of filler having a particle diameter.
Further, the varnish used for the prepreg intermediate 2, the filler-containing resin layer 3 and the filler-free resin layer 4 may be the same material except for the presence or absence of filler, and examples of the resin contained in the varnish include: Examples include thermosetting resins such as phenol resins and epoxy resins, unsaturated polyester resins, melamine resins, and polyimide resins. Examples of solvents include alcohols such as methanol and ethanol, esters such as ethylene acetate, and acetone. And ketones such as hexane, and hydrocarbons such as hexane.

  As shown in FIG. 2, the printed wiring board 100 includes a first prepreg 10, a circuit unit including a via hole 6 formed on at least one surface of the first prepreg 10, and a circuit unit including the via hole 6. A laminated plate made of the laminated second prepreg 11, and the filler-free resin layer 4 of the second prepreg 11 is in contact with the blackened surface 61 of the circuit portion including the via hole 6 of the core layer 10 ′. Thus, they are laminated. Further, the second prepreg 11 is used so that the blackened surface 61 of the circuit portion including the via hole 6 of the core layer 10 ′ and the filler-free resin layer 4 are in contact with each other at the time of lamination.

  Since the prepreg constituting the core layer 10 ′ does not come into contact with the blackened surface of the circuit portion when the core layer 10 ′ is formed, the first prepreg in which the filler-containing resin layer 3 is formed on both surfaces of the prepreg intermediate 2. A prepreg 10 is used.

  The printed wiring board 100 includes a conductor layer 7 that forms a surface layer together with the conductive metal foil 71 on the second prepreg 11 laminated on both surfaces of the core layer 10 ′, and the conductor layer 7 is provided on the wall surface. Further, the via hole 5 or the through hole 8 penetrating the core layer 10 ′ and the second prepreg 11 may be provided.

The via hole 6 is formed by forming a via hole lower hole 6a in the core layer 10 'by drilling or laser processing, and plating the via hole lower hole 6a to provide a conductor. Examples of laser light used in this laser processing include a CO ₂ laser and a UV-YAG laser. Also, a conductor layer 7 is provided immediately above the via hole 6 and on the filler-containing resin layer 3 on the surface of the printed wiring board, and on the core material 10 ′ in contact with the filler-free resin layer 4 of the second prepreg 11. The surface of the circuit portion including the via hole 6 is a blackened surface 61.
This blackened surface 61 is formed by oxidizing the copper foil surface to form a copper oxide film as a pretreatment for improving adhesion when the second prepreg 11 is laminated on the circuit portion including the via hole 6. The blackening process is performed.

  The conductor material formed in the via hole 6 is not particularly limited as long as the via hole 6 is electrically connected to the via hole 5 and other conductor layers described later, and examples thereof include conductive resin and metal plating. Particularly preferred is plating. In addition, the via hole 6 is provided with the conductor layer 60 on the inner wall surface of the via hole lower hole 6a in the same manner as the conductor layer 7 of the via hole 5 described later. However, the present invention is not limited to this, and for example, the conductor layer 60 is placed in the via hole lower hole 6a. It may be filled.

  On the surface of the core layer 10 ′ and the filler-containing resin layer 3 of the second prepreg 11, a conductive metal foil 71 is laminated as a seed layer for electrolytic plating. The conductive metal foil 71 is not particularly limited as long as it is electrically connected, but is preferably a copper foil or a thin copper foil, for example.

  A via hole 5 may be provided in the second prepreg 11. This via hole 5 is formed by forming a conductor made of metal plating (for example, copper plating) on an inner wall surface of a via hole prepared hole 5a described later, and through a conductor layer 7 provided on the upper and lower surfaces of the second prepreg 11, It is electrically connected to the via hole 6 of the core layer 10 ′.

  Further, the printed wiring board 100 may be provided with a through hole 8 penetrating the core layer 10 ′ and the second prepreg 11 on both sides. The through-hole 8 is formed by forming a conductor made of metal plating (for example, copper plating) on the inner wall surface of a through-hole lower hole 8a, which will be described later, and a wiring pattern (not shown) on the front and back surfaces of the insulating resin plate 100. Connected.

  In the printed wiring board 100 shown in FIG. 2, the second prepreg 11 is laminated one by one on the upper and lower surfaces of the core layer 10 ′, but is not limited to one layer. For example, a plurality of second prepregs 11 are alternately arranged. It is good also as a multilayer buildup layer by laminating | stacking. In this case, the via hole 5 is formed in the stacked second prepreg 11 and is electrically connected. Furthermore, it goes without saying that the present invention can be applied not only to build-up printed wiring boards but also to ordinary multilayer printed wiring boards, bonded multilayer printed wiring boards, multiple multilayer printed wiring boards, and the like.

A manufacturing method of one embodiment of the first and second prepregs according to the present invention will be described. The manufacturing method of the first prepreg includes the following steps (i) to (ii).
(I) A step of applying or impregnating a varnish containing no filler to an insulating cloth material and drying it to obtain a prepreg intermediate (ii) Applying a varnish containing a filler to both sides of the prepreg intermediate and drying it The process of obtaining the 1st prepreg which formed the filler containing resin layer of the hardening state Moreover, the manufacturing method of one Embodiment of the 2nd prepreg which concerns on this invention changes into the manufacturing process (ii) of a 1st prepreg, The following step (iii) is added.
(Iii) A varnish containing a filler is applied to one side of the prepreg intermediate and dried to form a semi-cured filler-containing resin layer, and a varnish containing no filler is applied to the other side and dried. Step of obtaining a second prepreg in which a semi-cured filler-free resin layer is formed

  An embodiment of the prepreg manufacturing method according to the present invention will be described with reference to FIGS. 3 (a), (b-1) and (b-2).

First, as shown to Fig.3 (a), the varnish which does not contain a filler is apply | coated or impregnated to the insulating cloth material (glass fiber) 1, and it is made to dry, and the prepreg intermediate body 2 is created.
At this time, the thickness of the varnish to be applied or applied is preferably equal to or less than the thickness of the insulating cloth material 1.

Next, as shown in FIG. 3 (b-1), a varnish containing a filler was applied to at least one surface of the prepreg intermediate 2 and dried to form a semi-cured filler-containing resin layer 3. 1 prepreg 10 is obtained.
In addition, when using as core material 10 'of the printed wiring board 100 mentioned later, it is good to form the said filler containing resin layer 3 on both surfaces of the prepreg intermediate body 2. FIG.

  As shown in FIG. 3 (b-2), the second prepreg 11 is coated with a varnish containing a filler on one surface of the prepreg intermediate 2 and dried to make a semi-cured filler-containing resin layer 3. Can be obtained by applying a varnish containing no filler to the other surface and drying to form a semi-cured filler-free resin layer 4.

  Thus, when forming the prepreg intermediate body 2, the insulating cloth material 1 is applied to the insulating cloth material 1 by applying a varnish that does not contain a filler in a thickness equal to or less than the thickness of the insulating cloth material 1. The varnish (resin) is sufficiently impregnated in between, and the resin flow with respect to the insulating cloth material can be improved in the laminated molding, and no void is generated between the resin and the insulating cloth material 1, and the moisture in the resin In short, no short circuit occurs.

Next, the manufacturing method of one Embodiment of the printed wiring board concerning this invention is demonstrated. The method for manufacturing a printed wiring board according to the present invention includes the following steps (I) to (IX).
(I) Step of applying or impregnating a varnish containing no filler to an insulating cloth material and drying it to obtain a prepreg intermediate (II) Applying a varnish containing a filler to both sides of the prepreg intermediate, drying and semi-curing Step (III) of Forming First Prepreg Forming Filler-Containing Resin Layer in State (III) Conductive metal foil (copper foil) was laminated on the surface of the first prepreg, and press treatment was performed for heating and pressurization. Thereafter, a step of forming a via-hole prepared hole for forming a via hole by laser processing or drilling (IV) After forming a conductor layer with a conductive material in the via-hole prepared hole (plating treatment), a dry film is pasted on the conductor, Step of peeling dry film other than the place where the circuit part is to be formed by exposure and development (V) Etching the conductive metal foil and conductor layer on the surface of the core material to Step to form a circuit portion, peeling the dry film location on the basis of the circuit portion including Le.
(VI) A varnish containing a filler is applied to one surface of the prepreg intermediate, dried to form a semi-cured filler-containing resin layer, and a varnish not containing a filler is applied to the other surface to be semi-cured. After obtaining the second prepreg in which the filler-free resin layer in the state is formed, the second prepreg is laminated so that the surface of the filler-free resin layer of the second prepreg is in contact with the circuit portion of the core material. A process of laminating a conductive metal foil (copper foil) on the surface of the prepreg, and performing a pressing process of heating and pressing to obtain a laminated board (VII) A circuit part of the core layer is reached by laser processing or drilling Step of forming via hole pilot holes and through hole pilot holes penetrating the laminate (VIII) After forming a conductor layer (plating process) with a conductive material in via hole pilot holes and through hole pilot holes, A process of removing the dry film other than the place where the circuit part is to be formed by exposing and developing the film (IX) Etching to form a circuit part including a via hole and a through hole, and drying the place where the circuit part is formed The process of peeling a film.

  One embodiment of the method for producing a printed wiring board according to the present invention will be described with reference to FIGS.

FIG. 3A shows a prepreg intermediate 2 in which the insulating cloth material 1 is coated with a varnish that does not contain a filler, impregnated, and dried.
The prepreg intermediate 2 becomes a first prepreg 10 and a second prepreg 11, respectively, as shown in FIGS. 3 (b-1) and 3 (b-2). Since each manufacturing method has been described above, the description thereof will be omitted.

Next, as shown in FIG.3 (c), after electrically conductive metal foil (copper foil) 71 is laminated | stacked on the front and back of the 1st prepreg 10, and pressurizing and heating with a press, after obtaining the core material 10a, laser The core material 10a is penetrated by processing or drilling, and the via hole prepared hole 6a for forming the via hole 6 is formed. Examples of laser light used in this laser processing include a CO ₂ laser and a UV-YAG laser.

Next, as shown in FIG.3 (d), the conductor layer 60 is formed with a conductor material on the inner wall face in the hole of the via hole lower hole 6a, and both surfaces of the core material 10a (plating process). The conductor material of the conductor layer 60 is, for example, copper plating, and the copper plating may be chemical copper plating (electroless copper plating) or electrolytic copper plating. Further, the via hole prepared hole 6a may be filled with copper plating.
After plating the via hole lower hole 6a, a dry film 9 is pasted on the surface of the core material 10a by a known method, and the dry film 9 other than the place where the circuit portion is to be formed is peeled off by exposure and development. When the conductive metal foil 71 and the conductor layer 60 on the surface of the material 10a are etched, as shown in FIG. 3E, a circuit part including the via hole 6 is formed in the core material 10a, and the dry film at the place where the circuit part is formed When 9 is peeled, the core layer 10 'is obtained.

Next, the second prepreg 11 shown in FIG. 3 (b-2) is laminated on the core layer 10 ′, and further a conductive metal foil (copper foil) 71 is laminated on the second prepreg. 11 is used to perform a pressing process in which heating and pressurization are performed simultaneously until the filler-free resin layer 4 and the core layer 10 ′ are completely cured, thereby producing a laminate 100a as shown in FIG.
At this time, the second prepreg 11 is laminated so that the surface of the filler-free resin layer 4 is in contact with the circuit portion including the via hole 6 of the core layer 10 ′, and the front and back surfaces of the laminate 100 a are filled with the filler-containing resin layer 3. Laminate so that
Further, when the second prepreg 11 is laminated on the circuit portion including the via hole 6, as a pretreatment for improving the adhesion, a blackening process for forming a copper oxide film on the surface of the circuit portion including the via hole 6 is performed. May be performed. The surface of the circuit part that has been blackened is a blackened surface 61.

  Next, as shown in FIG. 3 (g), after laminating a conductive metal foil (copper foil) 71 on the front and back surfaces of the laminate 100a, the via hole 6 of the core layer 10 'is included by laser processing or drilling. A via hole pilot hole 5a reaching the circuit part and a through hole pilot hole 8a penetrating the laminated plate 100a are formed.

Next, as shown in FIG. 3 (h), the conductor layer 7 is made of a conductor material on both the inner wall surfaces of the via hole lower hole 5a and the through hole lower hole 8a and both surfaces of the laminated plate 100a. Form (plating process). The conductor material is preferably the same as that of the conductor layer 60. For example, copper plating is good, and the copper plating may be chemical copper plating (electroless copper plating) or electrolytic copper plating.
After plating the via-hole prepared hole 5a and the through-hole prepared hole 8a, a dry film 9 is applied to the formation position of the via-hole prepared hole 5a and the through-hole prepared hole 8a on the surface of the laminate 100a by a known method, When the dry film 9 other than the place where the circuit part is to be formed is peeled off by exposure and development, the conductor layer 7 is etched, and the dry film 9 is peeled off at the place where the circuit part is formed, as shown in FIG. The printed wiring board 100 in which the via hole 5 and the through hole 8 are formed can be obtained. Finally, a solder resist (not shown) may be formed at a predetermined position on the surface of the printed wiring board.

DESCRIPTION OF SYMBOLS 1 Insulating cloth material 2 Prepreg intermediate 3 Filler containing resin layer 4 Filler non-containing resin layer 5 Via hole 5a Via hole pilot hole 6 Via hole 60 Conductor layer 61 Blackening surface 6a Via hole pilot hole 7 Conductor layer 71 Conductive metal foil 8 Through Hole 8a Through-hole pilot hole 9 Dry film 10 First prepreg 10a Core material 10 'Core layer 11 Second prepreg 100a Laminated board 100 Printed wiring board

Claims (10)

  A prepreg characterized by applying a varnish containing a filler to at least one surface of a prepreg intermediate obtained by impregnating an insulating cloth material with a varnish not containing a filler and drying it.   Applying a varnish containing a filler to one side of a prepreg intermediate impregnated with a varnish containing no filler and drying the prepreg intermediate and drying, applying a varnish containing no filler to the other side and drying Characteristic prepreg.   The prepreg according to claim 1 or 2, wherein the filler is silica or aluminum hydroxide.   The prepreg according to any one of claims 1 to 3, wherein the insulating cloth material is glass fiber. A printed wiring board comprising a core layer comprising a first prepreg, a circuit portion formed on at least one surface of the first prepreg, and a second prepreg laminated on the circuit portion of the core layer. And
The first prepreg is formed by impregnating a varnish containing no filler and applying and drying a varnish containing a filler on both sides of a first prepreg intermediate obtained by drying,
The second prepreg is impregnated with a varnish that does not contain a filler, and dried by applying a varnish containing a filler to one side of a second prepreg intermediate obtained by drying, and the other side does not contain a filler. It is formed by applying varnish and drying,
The printed wiring board, wherein the second prepreg is laminated so that a varnish-coated surface not containing a filler is in contact with a blackened surface of a circuit portion of the core layer.   The printed wiring board according to claim 5, wherein a via hole that is electrically connected to the circuit unit is provided in the second prepreg.   The printed wiring board according to claim 5, wherein a through-hole penetrating the core layer and the second prepreg is provided. Applying or impregnating varnish containing no filler to the insulating cloth material and drying to form a prepreg intermediate; and
Applying a varnish containing a filler to at least one surface of the prepreg intermediate, and drying the prepreg intermediate. Applying or impregnating varnish containing no filler to the insulating cloth material and drying to form a prepreg intermediate; and
Applying a varnish containing a filler to one surface of the prepreg intermediate, and drying;
Applying the varnish containing no filler to the other surface of the prepreg intermediate and drying the prepreg intermediate. Applying or impregnating a varnish containing no filler to the insulating cloth material and drying to form a prepreg intermediate; and
Applying a varnish containing a filler to both surfaces of the prepreg intermediate, and forming a core material composed of a first prepreg in which a filler-containing resin layer in a semi-cured state is formed by drying;
Forming a core layer as a circuit part by etching after plating the copper foil provided on at least one surface of the first prepreg; and
A varnish containing a filler is applied to one side of the prepreg intermediate and dried to form a cured filler-containing resin layer, and a varnish containing no filler is applied to the other side and dried to a semi-cured state. Forming a second prepreg having the filler-free resin layer formed thereon,
Laminating the second prepreg so that the surface of the filler-free resin layer is in contact with the blackened surface of the circuit portion of the core layer.

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