JPH11274721A - Manufacture of multilayer printed-wiring board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacture of a multilayer printed-wiring board that facilitates formation of via holes by a laser and improves adhesion between the external layer circuits and a thermosetting insulating resin provided between the external layer circuits and the internal layer circuits connected to the external layer circuits when a multilayer printed-wiring board is manufactured. SOLUTION: After a metal hardly soluble in alkali 2 is electrodeposited on the surface of a copper foil 1, a thermosetting resin 3 is applied on the surface and heated until it is half-set. Using the resin side as an adhesion surface, the compound copper foil is placed on one surface or both surfaces of the internal layer resin substrate having internal layer circuits on one surface or both surfaces, heated and laminated. The copper foil 1 on the surface is removed by alkaline etching, leaving selectively the metal layer hardly soluble in alkali 2. Subsequently, after a laser beam is irradiated to pierce the metal layer slightly soluble in alkali 2 and the thermosetting resin layer 3 at the same time, a copper layer is formed to form the external layer circuits connected to the internal layer circuits in this manufacture of a multilayer printed- wiring board.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a multilayer printed wiring board, and more particularly to a method of manufacturing a multilayer printed wiring board, which facilitates formation of a via hole by a laser at the time of manufacturing the multilayer printed wiring board and an outer layer formed of plated copper (copper layer). The present invention relates to a method for manufacturing a multilayer printed wiring board having improved adhesion between a circuit and an insulating resin (thermosetting insulating resin) existing between the outer layer circuit and the inner layer circuit.

[0002]

2. Description of the Related Art In order to meet demands for smaller, lighter, and higher-performance electronic devices, multilayer printed wiring boards are required to have smaller circuit widths and smaller via holes. Diameter 20
Drilling of 0 μm or less is difficult with mechanical drilling, and lasers have recently been widely used.

[0003] Among various lasers, a carbon dioxide gas laser, in particular, is capable of forming holes at high speed in organic substances such as epoxy resin and polyimide resin, and has been used most industrially for printed wiring. Since the laser beam is reflected, it is difficult to make a hole in a thick copper foil. Therefore, as disclosed in Japanese Patent Application Laid-Open No. Hei 4-3676, the copper foil is removed by etching only in the portion of the hole having the same size as the via hole diameter, and then the same position is irradiated with a laser beam. It is necessary to make a hole. The diameter of the laser beam used at this time is larger than the diameter of the via hole.

[0004]

In a method in which an ordinary resin-coated copper foil is laminated on both sides of an inner resin substrate and a multilayer board is opened and etched, and then a laser beam is applied to form a hole, the thickness of the copper foil is increased. Since the plating must be applied on top, the thickness of the outer copper layer is the sum of the original copper foil thickness and the thickness of the plated copper, and it is not possible to obtain a fine circuit by etching for circuit formation. It's not easy.
Further, it is not easy to etch a hole portion of an outer layer circuit in accordance with the position of an inner layer circuit because special high precision is required for alignment.

On the other hand, there is a method in which an insulating resin is coated on both surfaces of an inner insulating resin substrate on which an inner circuit is formed, a hole is formed by a laser, and then a copper surface is directly plated with copper to form an outer copper layer. In this case, only the copper layer is applied. In such a case, in order to obtain the adhesion strength between the copper layer and the insulating resin formed by coating the insulating resin, the surface of the insulating resin must be roughened, and further the surface of the insulating resin is roughened. However, the adhesive strength with the insulating resin is often insufficient.

The present invention solves these problems of the prior art, and facilitates the formation of via holes by laser during the manufacture of a multilayer printed wiring board, and improves the adhesion between an outer layer circuit and an insulating resin. An object of the present invention is to provide a method for manufacturing a wiring board.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on the above-mentioned problems of the prior art, and as a result, they have been found to be soluble in the surface of a copper foil in a normal acidic etching solution but not in an alkaline etching solution. A multi-layer print that solves the above-mentioned problems of the prior art by facilitating electrodeposition of a poorly soluble alkali metal, facilitates formation of via holes by a laser on a multi-layer printed wiring board, and improves adhesion between an outer layer circuit and an insulating resin. They have found that a wiring board can be obtained, and have completed the present invention.

That is, according to the method for producing a multilayer printed wiring board of the present invention, after a copper alkali foil is electrodeposited with an alkali-insoluble metal soluble in an acid etching solution, a thermosetting resin is applied to the surface. Applying the heat-curable resin to a semi-cured state by heating to form a composite copper foil, using the resin side of the composite copper foil as an adhesive surface, one or both surfaces of an inner resin substrate having an inner circuit on one or both surfaces Then, heat-molded and laminated, and the copper foil on the surface was removed by alkali etching to selectively leave the alkali-insoluble metal layer selectively, and then irradiated with a laser beam to form the alkali-insoluble metal layer and the heat. After drilling the curable resin layer at the same time, form the copper layer by electroplating after electroless plating or electroless plating to form the outer layer circuit connected to the inner layer circuit It is intended to.

As the copper foil surface on which the alkali-insoluble metal is electrodeposited, 1) glossy surface, 2) matte surface, 3) roughened glossy surface or 4) roughened matte surface is used. .

According to such a method, a via hole can be easily formed in a multilayer printed wiring board by a laser beam, and an outer layer circuit formed from a copper layer;
The adhesive force between the outer layer circuit and the insulating resin existing between the inner layer circuits can be improved.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for manufacturing a multilayer printed wiring board according to the present invention will be described in more detail with reference to FIGS.

As the copper foil, either an electrolytic copper foil or a rolled copper foil can be used. Hereinafter, the case where the electrolytic copper foil is used will be described.

FIG. 1 is a view showing a process of manufacturing a multilayer printed wiring board by a panel plating method according to the present invention. FIG.
FIG. 3 is a view showing a manufacturing process of a multilayer printed wiring board by a pattern plating method in the present invention. In these FIGS. 1 and 2, 1 is a copper foil, 2 is a poorly soluble alkali metal, 3 is a thermosetting insulating resin, 4 is an inner circuit, 5 is an inner resin layer, 6 is a via hole, 7 is an outer copper layer, Reference numeral 8 denotes an outer layer circuit, 9 denotes a resist pattern, and 10 denotes a pad.

In the production of the multilayer printed wiring board of the present invention, first, the surface of the copper foil 1 (a glossy surface, a matte surface, a roughened glossy surface or a roughened matte surface) is hardly alkali-soluble. Metal 2 is electrodeposited. The roughness (Rz) of the copper foil surface on which the alkali poorly soluble metal 2 is electrodeposited is 0.5 to 1
5 μm, preferably in the range of 2.5 to 15 μm. When the roughness of the copper foil surface is smaller than 0.5 μm, it is not preferable in that the adhesive strength between the alkali-insoluble metal and the thermosetting resin is insufficient. This is not preferable in that a cutting phenomenon easily occurs.

The roughening treatment of the copper foil is performed, for example, by applying a copper sulfate solution of 10 to 20 g / L, 30 to 100 g / L of sulfuric acid, and a temperature of 20 to 40 ° C. on the glossy or matte surface of the copper foil 1. It can be performed by electrodepositing copper on one side at a current density of 30 to 50 A / dm ² for 5 to 20 seconds using the copper foil as a cathode.

The thickness of the copper foil 1 used is preferably in the range of 5 to 100 μm. If the copper foil is too thick, the etching takes a long time, which causes a problem in productivity. on the other hand,
If the copper foil is too thin, the production as a copper foil is difficult.

Next, in the present invention, a poorly alkali-soluble metal 2 which is dissolved in a normal acidic etching solution but not dissolved in an alkaline etching solution is electrodeposited on the surface of the copper foil 1. In the present invention, various metals which are hardly soluble in an alkali and soluble in an acid can be used. Examples of the hardly soluble alkali metal 2 include tin, nickel and cobalt, or a tin-zinc alloy, a zinc-nickel alloy and a tin-copper. Alloys, such as tin, zinc-tin alloy, zinc-nickel alloy, and tin-copper alloy.
Species are preferred, and tin and tin-zinc alloys are most preferred in terms of alkali etching resistance. As a method for forming the alkali poorly soluble metal layer 2, for example, a copper foil is used as a cathode, for example,
The electrolytic treatment is performed in a bath shown below as a tin plating bath composition.

[0018]

[Table 1]

The thickness of the poorly soluble alkali metal layer 2 is as follows:
The range of 0.005 to 3.0 μm is preferred. When the thickness of the alkali-poorly-soluble metal layer 2 is less than 0.005 μm, the adhesion strength between the alkali-poorly-soluble metal layer and the thermosetting resin layer is weak, so that an outer layer formed on the alkali-poorly-soluble metal layer Sufficient adhesion strength between the copper layer 7 and the thermosetting resin layer 3 cannot be obtained. If the thickness is more than 3.0 μm, it becomes difficult to form a hole by a carbon dioxide gas laser.

In the present invention, the alkali-insoluble metal layer 2
When the outside is subjected to a chromate treatment, and the outside thereof is further treated with a silane coupling agent, the adhesive strength between the thermosetting resin 3 and the outer copper layer 7 via the alkali-insoluble metal 2 can be further increased. Further, a rust-preventive treatment such as zinc, tin, nickel, chromate, imidazole, aminotriazole, benzotriazole or the like can be performed on a surface of the copper foil 1 on which the hardly alkali-soluble metal layer 2 is not provided (for example, a shine surface). .

Next, a thermosetting resin varnish is applied to the surface of the poorly alkali-soluble metal layer 2 electrodeposited on the surface of the copper foil to form a thermosetting resin 3. The resin is heated at 140 to 150 ° C. for 5 to 20 minutes to be in a semi-cured state to obtain a composite copper foil. An epoxy resin (Epicoat 1001 manufactured by Yuka Shell Co., Ltd.) or the like can be used as a base resin of the thermosetting resin varnish. As the thermosetting resin varnish for forming the thermosetting resin 3, epoxy resin, dicyandiamide as a curing agent, and 2E4MZ as a curing accelerator (Shikoku Chemicals Co., Ltd.)
And an epoxy resin composition obtained by appropriately mixing them using methyl ethyl ketone as a solvent. As the thermosetting resin layer, a prepreg or a thermosetting resin film in which a thermosetting resin is impregnated and semi-cured into a fiber base material such as glass cloth or aramid paper may be used. The thickness of the thermosetting resin 3 in the semi-cured state is preferably in the range of 20 to 200 μm. If the thickness of the thermosetting resin 3 is less than 20 μm, the interlayer insulating property and sufficient adhesive strength cannot be obtained, and if it is more than 200 μm, it becomes difficult to form small-diameter via holes, which is not preferable.

After the composite copper foil with resin in which the thermosetting resin layer 3 is in a semi-cured state is placed on one or both sides of an inner resin substrate having an inner circuit with the resin side as an adhesive surface, 150 to 200 The laminate is formed by heating and forming at a temperature of ° C. (FIG. 1A).

Next, the copper foil on the surface is removed by selectively etching the thus-laminated multilayer board containing the inner layer circuit by alkali etching while leaving the poorly alkali-soluble metal layer (FIG. 1).
(B)). As such an alkaline etching solution,
For example, NH ₄ OH 200 to 250 g / L, NH ₄ C11
The reaction is performed at a temperature of 40 to 50 ° C. using a solution containing 30 to 160 g / L and 150 to 160 g / L of Cu. Although the surface shape of the alkali-soluble metal 2 differs depending on the copper foil surface on which the alkali-soluble metal is electrodeposited, it is preferable to use a copper foil roughened surface as the copper foil surface. When such an alkali-soluble metal is electrodeposited on the roughened surface of the copper foil, the surface of the hardly-soluble alkali metal 2 becomes a transfer of the roughened surface of the copper foil, so that the surface becomes rich in irregularities where the laser beam is easily absorbed. Drilling becomes easy.

Next, FIG. 1 from which the copper foil layer on the surface has been removed
The multilayer board with the inner layer circuit shown in (b) is irradiated with a laser beam to pierce the alkali-poorly soluble metal layer 2 and the resin layer 3 at the same time to form a via hole 6 to form a perforated multilayer board (a).
Is manufactured (FIG. 1C). The type of laser used in the present invention is not particularly limited, but a carbon dioxide laser is preferably used. After irradiating a laser beam and making a hole, desmearing may be performed as necessary.

A copper pyrophosphate plating solution (OPC-750 electroless copper plating solution manufactured by Okuno Pharmaceutical Co., Ltd.) is applied to the surface of the multilayer board (a) containing the inner layer circuit in which the via holes are formed as described above. Electroless plating is performed at a temperature of 20 to 25 ° C. for 15 to 20 minutes to form a plating layer of about 0.1 μm. This plating layer is also formed on the surface of the insulating resin layer in the via hole. Furthermore, copper 3 is applied to the surface of the electroless copper plating.
Using a solution containing 0 to 100 g / L and sulfuric acid 50 to 200 g / L, a temperature of 30 to 80 ° C. and a cathode current density of 10 to 10
Electroplating is performed at 0 A / dm ² to form a copper plating layer 7 having a thickness of 5 to 35 μm (FIG. 1D). This copper plating layer 7
Is also formed on the surface of the insulating resin layer of the via hole. When the copper plating layer 7 is formed on the hardly-soluble alkali metal 2 having a strong adhesive strength with the insulating resin, the outer copper layer 7 and the hardly-soluble alkali metal layer 2 have a bonding force. As compared with the case where the resin layer 3 is directly plated, a higher adhesive strength can be obtained between the insulating resin layer and the outer copper layer.

According to a standard method, a microposit 2400 (manufactured by Shipley Co., Ltd.) is applied to the surface of the outer copper layer 7 formed as described above in a thickness of about 7 μm and dried. Thus, an exposed portion and a non-exposed portion are formed using a mask on which a predetermined circuit is formed. After exposure, the resist pattern 9 is formed by developing with a KOH 10% solution (FIG. 1E). Subsequently, cupric chloride (CuCl ₂ ) 100 g / L, free hydrochloric acid concentration 10
Using a solution containing 0 g / L, acid etching is performed at a temperature of 50 ° C. to dissolve a part of the poorly alkali-soluble metal layer 2 and a part of the outer copper layer 7 to form an outer layer connected to the pad 10 on the inner circuit 4. The circuit 8 is formed.

Finally, using a 3% NaOH solution at a temperature of 5
Dissolve and remove the photoresist applied to the copper foil surface at 0 ° C,
A multilayer printed wiring board is created (FIG. 1 (f)).

In the case of the present invention, the thickness of the poorly soluble alkali metal layer 2 is very thin as compared with the thickness of the outer copper layer 7, and the total thickness of the metal foil is smaller than that of the usual subtractive method. (Usually 18 μm) and the thickness (up to 15 μm) of the thickness of the alkali poorly soluble metal (up to 3 μm), thereby significantly improving the etching property and facilitating the formation of a fine circuit.

As another method, there is a pattern plating method shown in FIG. 2 (a) to 2 (c) are similar to FIGS. 1 (a) to 1 (c), in which a photoresist is formed on the surface of the multilayer board (a) containing the inner layer circuit in which the via holes 6 are formed. After lamination or coating, exposure and development are performed to form a resist pattern 9, and a circuit and a pad 10 are formed.
Only the portion where the hardly soluble alkali metal is exposed (see FIG. 2)
(D)). Next, electroless plating and electrolytic plating are performed in the same manner as above to form a copper layer 7 having the same arrangement as that of the circuit (FIG. 2E). After the plating is completed, the photoresist is dissolved and removed by a normal method using a 3% NaOH solution, so that an outer layer circuit 8 is formed and a poorly alkali-soluble metal remains between the copper circuits (FIG. 2 (f)).

The alkali-insoluble metal layer is formed on the outer layer circuit 8.
Is extremely thin compared to the thickness of copper, and can be dissolved by treating with a normal acid etching solution such as cupric chloride or ferric chloride for a short time, without protecting the outer layer circuit 8 with tin plating. The alkali-insoluble metal layer between the outer copper circuits can be removed (FIG. 2 (g)). As described above, according to the present invention, a fine pattern can be formed with high precision without causing undercut.

The present invention is also applicable to a multilayer inner resin substrate having three or more layers. Also, by repeating the steps of lamination, laser drilling, plating, and patterning, a layer having a laser via can be formed into multiple layers, and the present invention can be applied to the manufacture of a multilayer printed wiring board having an arbitrary number of layers.

[0032]

EXAMPLES The present invention will be described below more specifically based on examples and comparative examples. Example 1 Glossy Surface Roughness (Rz) 1.9 μm, Roughness Roughness (Rz) 5 μm
m on a glossy surface of an electrolytic copper foil having a nominal thickness of 18 μm, in a copper sulfate solution of 10 g / L of copper, 100 g / L of sulfuric acid and a temperature of 40 ° C. at a current density of 30 A / dm ^{2 using} the copper foil as a cathode. Copper was electrodeposited on one side for 5 seconds for roughening treatment. The roughness (Rz) of the surface thus roughened was 2.9 μm.

Using the thus roughened electrolytic copper foil as a cathode, an electrolytic treatment was carried out at a temperature of 20 ° C. in the following bath, and tin plating (a poorly soluble alkali metal) was performed to form a tin layer. The amount of tin on the treated surface was 1.2 g / m ² (about 0.2 μm).
m).

[0034]

[Table 2]

After washing with water, the tin-plated surface is further subjected to electrolytic chromate treatment at a current density of 0.5 A / dm ² for 5 seconds using an electrolytic solution of 2 g / L of chromic anhydride and a pH of 11.0. A chemically treated copper foil was obtained.

On the roughened surface of this copper foil, 100 parts of an epoxy resin (Epicoat 1001 manufactured by Yuka Shell Co., Ltd.), 2.5 parts of dicyandiamide as a curing agent, and 2E as an accelerator
75 parts of an epoxy resin varnish obtained by mixing 0.2 parts of 4MZ (manufactured by Shikoku Chemicals) in a solvent of methyl ethyl ketone.
A composite copper foil with a resin was applied in a thickness of μm and heated in a semi-cured state by heating at 130 ° C. for 10 minutes.

A 0.5 mm-thick FR-4 substrate having an inner layer circuit formed on both surfaces and subjected to a blackening treatment (Matsushita Electric Works, Ltd.)
R-1766) as a core, the composite copper foil with the resin is stacked on both sides thereof such that the resin surface is on the inner layer circuit side, and molded at 180 ° C. for 60 minutes using a vacuum press at a pressure of 20 kg / cm ^2. Then, a multilayer board containing an inner circuit as shown in FIG. 1 (a) was obtained.

On the copper foil surface of the multilayer board containing the inner layer circuit thus formed, NH ₄ OH 200 g / L, NH ₄ Cl 130
g / L and a solution containing 150 g / L of Cu at a temperature of 5
Alkaline etching was performed at 0 ° C., and the copper foil was removed to expose a tin layer (a poorly soluble alkali metal layer).

Next, a carbon dioxide gas laser (laser output: 60 W) is applied to the surface where the tin layer is exposed by etching as described above.
With a laser diameter of 100 μm, the
Drill a hole of 00 μmφ to form a via hole,
A perforated multilayer board (a) as shown in FIG. 1 (c) was obtained.

The electroless electroless copper plating solution (manufactured by Okuno Pharmaceutical Co., Ltd.) is used on the tin layer including the opening in which the via hole is formed as described above, for 18 minutes at a liquid temperature of 23 ° C. Plating was performed to a plating thickness of about 0.1 micron. Furthermore, 100 g of copper /
L, using a solution containing 150 g / L of sulfuric acid, at a temperature of 25 ° C.
Electrolytic plating at a cathode current density of 5 A / dm ² at 20
An outer copper layer having a thickness of μm was formed.

On the surface of the outer copper layer thus formed,
Microposit 2 is used as a photoresist according to the usual method.
400 (manufactured by Shipley Co., Ltd.) is applied to a thickness of about 7 μm and dried. After that, an exposed portion and a non-exposed portion are formed on the photoresist surface by using an exposure machine using a mask on which a predetermined circuit is formed, and a resist pattern is provided. Was. After exposure, the film was developed with a KOH 10% solution, and cupric chloride (CuCl ₂ ) 100 g /
L, acid etching was performed at a temperature of 50 ° C. using a solution containing a free hydrochloric acid concentration of 100 g / L to form an outer layer circuit. Finally, the photoresist remaining on the outer layer circuit was removed at a temperature of 30 ° C. by using a 5% NaOH solution to prepare a multilayer printed wiring board.

The peel strength (kgf / cm) of the thus obtained outer copper circuit having a thickness of 20 μm from the multilayer printed wiring board was measured by the JIS-C6481 method. Table 3 shows the obtained results. Comparative Example 1 As a copper foil 1, a normal 18 μm copper foil (3E manufactured by Mitsui Kinzoku Mining)
Epoxy resin varnish is applied on a rough surface in the same manner as in Example 1 except that C-III) is not used and electrodeposition of a poorly alkali-soluble metal (tin) is performed, and the coating is semi-cured by heating at 130 ° C. for 10 minutes. A composite copper foil with a resin in a state was prepared.

An inner layer circuit is formed on both sides of this copper foil with resin, and a 0.5 mm thick FR-4 inner layer circuit board which has been subjected to a blackening process is used as a core. They were stacked so that the surface was on the side of the inner layer circuit, and were molded under the same conditions as in Example 1 to obtain a multilayer board containing the inner layer circuit. Thereafter, a multilayer printed wiring board was prepared in the same steps as in Example 1.

The peel strength (k) of the thus obtained outer copper layer having a thickness of 20 μm from the multilayer printed wiring board was measured.
gf / cm) was measured by JIS-C6481 method.
Table 3 shows the obtained results. Comparative Example 2 The same copper foil with resin as in Comparative Example 1 was used and molded in the same manner to form a multilayer board with an inner circuit. Next, a hole having the same diameter was formed in the portion of the copper foil to be drilled by etching before laser processing, and thereafter, processing was performed under the same conditions as in Example 1 to produce a multilayer printed wiring board. However, the outer copper layer was not removed.

The peel strength (kgf) of the outer layer circuit having a thickness of 38 μm of the total copper layer thus obtained was obtained.
/ Cm) was measured by JIS-C6481 method. Table 3 shows the obtained results.

[0046]

[Table 3]

As shown in Table 3, in Example 1, Comparative Example 1
In comparison with Comparative Example 2, the adhesive strength between the inner resin and the outer layer circuit can be increased, and the outer layer circuit finer than Comparative Example 2 can be etched.

[0048]

According to the method for manufacturing a multilayer printed wiring board of the present invention, via holes can be easily formed by laser on the multilayer printed wiring board, as compared with the conventional method.
In addition, the adhesion between the outer layer circuit and the insulating resin is improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a manufacturing process of a multilayer printed wiring board according to the present invention.

FIG. 2 is a diagram showing a manufacturing process of the multilayer printed wiring board of the present invention.

[Explanation of symbols]

 1: Copper foil 2: Alkali sparingly soluble metal 3: Thermosetting resin layer 4: Inner layer circuit 5: Inner layer resin layer 6: Via hole 7: Outer layer copper layer 8: Outer layer circuit 9: Resist pattern 10: Pad

Claims (10)

[Claims] Claims: 1. An electrodepositive solution of a poorly alkali-soluble metal, which is soluble in an acid etching solution but hardly soluble in an alkali etching solution, is applied to the surface of a copper foil, and then a thermosetting resin is applied to the surface thereof. The thermosetting resin was heated to a semi-cured state to form a composite copper foil, and the resin side of the composite copper foil was placed on one or both sides of an inner layer resin substrate having an inner layer circuit on one or both sides as an adhesive surface. After heat molding and laminating, the copper foil on the surface is selectively removed by alkali etching while leaving the alkali-insoluble metal layer selectively, and then the alkali-soluble metal layer and the thermosetting resin are irradiated with a laser beam. Forming a perforated multilayer board (a) by simultaneously perforating layers, forming a copper layer on the perforated multilayer board, and forming an outer layer circuit connected to the inner layer circuit; Plate manufacturing method. 2. A copper layer is formed on the perforated multilayer board (a), a resist is applied, a resist pattern is formed, and then an acid etching is performed to remove the outer layer copper layer and a part of the alkali poorly soluble metal. The method for manufacturing a multilayer printed wiring board according to claim 1, wherein the outer layer circuit is formed by removing. 3. A resist pattern is formed after applying a resist on the perforated outer layer plate (a), a copper layer is formed between the resist patterns and on the surface of the perforated resin, and after removing the resist pattern, an acid is formed. 2. The method according to claim 1, wherein the outer layer circuit connected to the inner layer circuit is formed by removing the hardly soluble alkali metal layer by etching. 4. The method according to claim 1, wherein a roughness Rz of a surface of the copper foil on which the poorly alkali-soluble metal is electrodeposited is 0.5 to 15 μm. 5. The method according to claim 1, wherein the copper foil has a thickness of 5 to 100 μm, and the alkali-refractory metal layer has a thickness of 0.005 to 3.0 μm. Method. 6. The method according to claim 1, wherein the poorly soluble alkali metal is one selected from the group consisting of tin, a zinc-tin alloy, a zinc-nickel alloy, and a tin-copper alloy. The method described in. 7. The method according to claim 1, wherein the copper foil is an electrolytic copper foil or a rolled copper foil. 8. The method according to claim 1, wherein a chromate layer is further provided on the hardly-soluble alkali metal layer of the composite copper foil. 9. A prepreg or a thermosetting resin film in which a thermosetting resin is impregnated into a fiber base material is used instead of the thermosetting resin layer. the method of. 10. A multilayer printed wiring board manufactured by the method according to claim 1.