Printed wiring board

A printed wiring board includes an insulative resin substrate having a penetrating hole, a first conductive layer formed on first surface of the substrate, a second conductive layer formed on second surface of the substrate, and a through-hole conductor formed in the hole such that the conductor is connecting the first and second conductive layers. The conductor has a seed layer on inner wall of the hole, a laminated plated layer on the seed layer and a filled plated layer on the laminated layer, the laminated layer is formed such that the laminated layer is closing center portion of the hole and forming recess at end of the hole, the filled layer is formed such that the filled layer is filling the recess, and the laminated layer includes multiple electrolytic plated films laminated along the seed layer and each having thickness which is less at edge than at center.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based upon and claims the benefit of priority to Japanese Patent Application No. 2013-103299, filed May 15, 2013, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a printed wiring board with a through-hole conductor.

Description of Background Art

JP 2005-093934 A describes forming a cylindrical through hole in a resin substrate, forming a thin metal layer on the inner wall surface of the through hole, and filling plating in the through hole with electrolytic plating. The metal deposition speed of the electrolytic plating solution in JP 2005-093934 A is faster in the through hole than on a resin substrate surface. In JP 2005-093934 A, a predetermined portion inside the through hole is closed by the plating. The entire contents of this publication are incorporated herein by reference.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a printed wiring board includes an insulative resin substrate having a penetrating hole, a first conductive layer formed on a first surface of the insulative resin substrate, a second conductive layer formed on a second surface of the insulative resin substrate on the opposite side with respect to the first surface of the insulative resin substrate, and a through-hole conductor formed in the penetrating hole of the insulative resin substrate such that the through-hole conductor is connecting the first conductive layer and the second conductive layer. The through-hole conductor has a seed layer formed on an inner wall of the penetrating hole, a laminated electrolytic plated layer formed on the seed layer and a filled electrolytic plated layer formed on the laminated electrolytic plated layer, the laminated electrolytic plated layer is formed in the penetrating hole such that the laminated electrolytic plated layer is closing a center portion of the penetrating hole and forming a recessed portion at an end portion of the penetrating hole, the filled electrolytic plated layer is formed in the penetrating hole such that the filled electrolytic plated layer is filling the recessed portion of the penetrating hole, and the laminated electrolytic plated layer includes multiple electrolytic plated films such that the electrolytic plated films are laminated along the seed layer and that each of the electrolytic plated films has a thickness which is less at an edge portion than at a center portion.

According to another aspect of the present invention, a method for manufacturing a printed wiring board includes forming a penetrating hole through an insulative resin substrate, forming a first conductive layer on a first surface of the insulative resin substrate, forming a second conductive layer on a second surface of the insulative resin substrate on the opposite side with respect to the first surface of the insulative resin substrate, and forming a through-hole conductor in the penetrating hole of the insulative resin substrate such that the through-hole conductor connects the first conductive layer and the second conductive layer. The forming of the through-hole conductor includes forming a seed layer on an inner wall of the penetrating hole, forming a laminated electrolytic plated layer on the seed layer and forming a filled electrolytic plated layer on the laminated electrolytic plated layer, the laminated electrolytic plated layer is formed in the penetrating hole such that the laminated electrolytic plated layer is closing a center portion of the penetrating hole and forming a recessed portion at an end portion of the penetrating hole, the filled electrolytic plated layer is formed in the penetrating hole such that the filled electrolytic plated layer is filling the recessed portion of the penetrating hole, the forming of the laminated electrolytic plated layer includes forming multiple electrolytic plated films such that the electrolytic plated films are laminated along the seed layer and that each of the electrolytic plated films has a thickness which is less at an edge portion than at a center portion.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1is an image taken by an electron microscope showing an enlarged cross-sectional view of a through-hole conductor of a printed wiring board according to the present embodiment.FIG. 2is a view schematically illustrating a cross-sectional view of the through-hole conductor of a printed wiring board according to the present embodiment.

As shown inFIG. 2, a printed wiring board of the present embodiment has insulative resin substrate1having first surface (F) and second surface (S) opposite the first surface; first conductive layer2formed on first surface (F) of insulative resin substrate1; second conductive layer3formed on second surface (S) of insulative resin substrate1; and through-hole conductor5penetrating through insulative resin substrate1and connecting first conductive layer2and second conductive layer3. Through-hole conductor5is formed in penetrating hole4in insulative resin substrate1.

Insulative resin substrate1is made of cured resin and reinforcing material such as glass cloth. First and second conductive layers (2,3) each include multiple conductive circuits, and are each made up of metal foil (copper foil)6laminated on insulative resin substrate1, electroless plated layer (electroless copper-plated layer)7formed on metal foil6, laminated electrolytic plated layer10formed on electroless plated layer7, and filled electrolytic plated layers (11,12) formed on laminated electrolytic plated layer10.

Penetrating hole4(penetrating hole for a through-hole conductor) in insulative resin substrate1is formed as a straight hole. Through-hole conductor5is made up of electroless plated layer (electroless copper-plated layer)7formed on the inner wall surface of penetrating hole4, and electrolytic plated layer (electrolytic copper-plated layer)8formed on electroless plated layer7. Penetrating hole4is filled with the electrolytic plated layer.

Electrolytic plated layer8of through-hole conductor5has multiple layers of electrolytic plated film9that are laminated on electroless plated layer7layer by layer along electroless plated layer7. Multiple layers of the electrolytic plated film are formed along the inner wall of penetrating hole4. Laminated electrolytic plated layer10is made up of multiple layers of electrolytic plated film9.FIG. 2shows 14 layers of electrolytic plated film9.

The thickness of each layer in electrolytic plated film9is greater at the center portion in a thickness direction of insulative resin substrate1(center portion (C) of penetrating hole4) than near the entrance portions of penetrating hole4(edges (E) of penetrating hole4). Thus, as shown inFIGS. 1 and 2, layers of electrolytic plated film9that form the laminated electrolytic plated layer make contact with each other at a predetermined portion near the center in a cross-sectional direction (center portion (C) of penetrating hole4). The portions of electrolytic plated film9touch each other. The approximate center portion (C) of penetrating hole4is closed by laminated electrolytic plated layer10. Accordingly, in penetrating hole4, first recessed portion (R1) is formed on the first-surface (F) side and second recessed portion (R2) is formed on the second-surface (S) side.

Furthermore, electrolytic plated layer8which forms through-hole conductor5has first filled electrolytic plated layer11filled in first recessed portion (R1) and second filled electrolytic plated layer12filled in second recessed portion (R2). First recessed portion (R1) and second recessed portion (R2) are not yet fully filled inFIG. 1. The vicinity of the entrance portion of penetrating hole4is closer to first surface (F) or second surface (S) of the insulative resin substrate, and the center portion in a thickness direction is farther from first surface (F) or second surface (S) of the insulative resin substrate.

In a printed wiring board of the present embodiment, the laminated electrolytic plated layer that closes the penetrating hole is made up of multiple layers of electrolytic plated film9. The tips of laminated electrolytic plated layer10, which grows from the inner wall of the penetrating hole toward the center axis of the penetrating hole, make contact with each other, or the tips of laminated electrolytic plated layer10touch each other. The contact portion or touching portion is junction (A) (seeFIG. 2). In the present embodiment, laminated electrolytic plated layer10is formed by layers of electrolytic plated film9. Thus, even when stress is concentrated in junction (A) of the laminated electrolytic plated layer, it is thought that the stress is unlikely to progress straight to the inner wall of the penetrating hole. Namely, the stress is thought to be mitigated along the interfaces of the layers of electrolytic plated film9as shown inFIG. 3.FIG. 3shows assumed progressive direction (B) of stress. Therefore, the connection reliability of a through-hole conductor is high according to the embodiment.

FIG. 4shows a reference example. When electrolytic plated film9is double layered and greater stress is exerted, foreseeable progressive direction (B) is shown inFIG. 4. Even if there are only two electrolytic plated films, it is thought that stress is unlikely to progress straight to the inner wall of the penetrating hole. Thus, cracking is less likely to occur in the through-hole conductor. However, to avoid defects, the number of layers of electrolytic plated film9is preferred to be seven or greater. When the number of layers is seven or greater, stress is less likely to reach the inner wall of the penetrating hole. Thus, cracking is less likely to occur in the through-hole conductor. If the number of layers is 21 or greater, significant change in the effect is not observed. For example, the effect can be verified by comparing the resistance values of the through-hole conductor before and after a heat cycle.

Also, the through-hole conductor is made up of laminated electrolytic plated layer10and first and second filled electrolytic plated layers (11,12). Especially, since laminated electrolytic plated layer10is formed by laminating multiple layers of electrolytic plated film9, a void is unlikely to occur in the through-hole conductor. Therefore, even in a usage state where a significant amount of voltage or current is applied onto through-hole conductor5, problems caused by an increase in current density will be prevented.

The printed wiring board of the present embodiment can be used as the core substrate of a buildup wiring board shown in JP 2000-101243 A. A method for manufacturing the buildup wiring board is described in JP 2000-101243 A. The entire contents of this publication are incorporated herein by reference.

Steps for manufacturing a printed wiring board of the present embodiment are described below.

(1) A commercially available double-sided copper-clad laminate is prepared as a starting material. The double-sided copper-clad laminate prepared as insulative resin substrate1has an approximate thickness of 100 μm. Penetrating hole4is formed by a drill in the double-sided copper-clad laminate. The diameter of the penetrating hole is 80 μm.

(2) Electroless plated layer7as a seed layer is formed by performing electroless copper plating on copper foil6on both surfaces of the double-sided copper-clad laminate and on the inner wall of penetrating hole4. The thickness of electroless plated layer7is 0.3 to 3.0 μm.

An example of the electroless plating solution and plating conditions are shown below.

Electroless Copper Plating Solution

(3) Next, the first layer of electrolytic plated film9(first electrolytic plated layer) is formed. The thickness of the first-layer electrolytic plated film9at the edge (E) of penetrating hole4is less than the thickness of the first-layer electrolytic plated film9at the center portion (C) of penetrating hole4. When the first layer of electrolytic plated film9is formed, the substrate with the first layer of electrolytic plated film9is transferred from the electrolytic plating solution to a tank for washing with water. Next, the substrate is washed with acid and immersed in the electrolytic plating solution again. Then, a second layer of electrolytic plated film9(second electrolytic plated layer) is formed on the first layer of electrolytic plated film9.

The above process is repeated multiple times (N times, N is a whole number). An Nth electrolytic plated film9(Nth electrolytic plated layer) is formed on the (N−1)th electrolytic plated film9((N−1)th electrolytic plated layer). In the present embodiment, the above process is repeated 14 times. As a result, 14 layers of electrolytic plated film9are formed on the seed layer. As shown inFIG. 2, the thickness of laminated electrolytic plated layer10is less at edge (E) of penetrating hole4than that of electrolytic plated layer10at center portion (C) of penetrating hole4. The tips of the 14th layer of electrolytic plated film9touch each other at junction (A). Accordingly, penetrating hole4is closed. First recessed portion (R1) is formed on the first-surface (F) side of insulative resin substrate1, and second recessed portion (R2) is formed on the second-surface (S) side of insulative resin substrate1.

As shown inFIGS. 1 and 2, the thickness of the center portion of the Nth layer of electrolytic plated film9(electrolytic plated layer) is greater than the thickness of the center portion of the (N−1)th layer of electrolytic plated film9(electrolytic plated layer). The thickness of the center portion of the uppermost layer of electrolytic plated film9(electrolytic plated layer) is greater than the thickness of the center portion of any other layer of electrolytic plated film9(electrolytic plated layer).

Penetrating hole4is closed by the uppermost layer of electrolytic plated film9(electrolytic plated layer). As shown inFIG. 3, when the number N is 7, the uppermost layer of electrolytic plated film9is the 7th layer of electrolytic plated film9. When the thickness of the plating at the center is great, the tip of the uppermost layer of electrolytic plated film9tends to be pointing. Thus, two tips touch each other, thereby closing penetrating hole4. If the number N is greater, the slope near the tip of the uppermost layer of electrolytic plated film9tends to be gradual. Accordingly, the reliability of the through-hole conductor according to the present embodiment is high.

Also, when the Nth layer of electrolytic plated film9is the uppermost layer of the electrolytic plated film, the difference is the maximum between the thickness of the center portion of the Nth layer of electrolytic plated film9and the thickness of the center portion of the (N−1)th layer of electrolytic plated film9. By reducing such difference, the slope is made gradual. By increasing the value of N, the difference is made smaller. When the value of N is 10 or greater, (that is, the number of layers of electrolytic plated film9is 10 or greater), the reliability of the through-hole conductor is likely to increase.

To form electrolytic plated film9, examples of the composition of an electrolytic copper plating solution and the temperature for plating are respectively shown below.

In addition, since laminated electrolytic plated layer10has multiple layers of electrolytic plated film9, the diameter of crystalline particles of laminated electrolytic plated layer10is finer than those in first and second filled electrolytic plated layers (11,12) (seeFIG. 1). Because of the finer particle diameter, cracking tends not to occur in laminated electrolytic plated layer10.

Laminated electrolytic plated layer10is also formed on metal foil6.

Electrolytic Copper Plating Solution

(4) Next, first and second filled electrolytic plated layers (11,12) are respectively formed in first recessed portion (R1) and second recessed portion (R2). First and second recessed portions (R1, R2) are filled with electrolytic copper plating.

Examples of the composition of an electrolytic copper plating solution and conditions are respectively shown below.

Electrolytic Copper Plating Solution

First and second filled electrolytic plated layers (11,12) are formed continuously under constant conditions. In addition, since the distances from the edges of penetrating hole4to the bottoms of first and second recessed portions (R1, R2) are each short, the plating solution is fully supplied. Thus, the diameter of crystalline particles in first and second filled electrolytic plated layers (11,12) tends to be greater than the diameter of crystalline particles in each layer of electrolytic plated film9(electrolytic plated layer) that makes up laminated electrolytic plated layer10(seeFIG. 1). The deposition speed of first and second filled electrolytic plated layers (11,12) is faster than the deposition speed of electrolytic plated film9. First and second filled electrolytic plated layers (11,12) are also formed on laminated electrolytic plated layer10on insulative resin substrate1.

(5) Etching resist is formed on first and second filled electrolytic plated layers (11,12).

(6) First and second filled electrolytic plated layers (11,12), laminated electrolytic plated layer10, electroless plated layer7and metal foil6, which are exposed from the etching resist, are removed by etching.

First and second filled electrolytic plated layers (11,12) are formed using direct-current plating. However, first and second filled electrolytic plated layers (11,12) may also be formed by pulse-current plating. The heights of first and second filled electrolytic plated layers (11,12) formed in first and second recessed portions (R1, R2) are preferred to be set at the same heights of first and second conductive layers (2,3) respectively. When the top surfaces (opposite the bottom surfaces) of the plated layers in first and second filled plated layers (11,12) reach substantially the same level as first surface (F) and second surface (S) respectively, it indicates first and second recessed portions (R1, R2) are filled. A buildup layer can be formed on the core substrate.

To reduce the resistance of a through-hole conductor and to decrease the diameter of the penetrating hole for a through-hole conductor, the penetrating hole for a through-hole conductor may be filled with plating.

When electrolytic plating is deposited from a side wall of a penetrating hole for a through-hole conductor (through hole) toward the center axis of the through hole, and a predetermined portion of the through hole is closed by plating, the tips of plated film growing from the side wall of the through hole may touch each other at the predetermined position. When stress is exerted on the through hole, it is thought that the stress tends to concentrate on the touching portion. Especially, when protruding plated films or the tip portions of the plated film touch each other, it is thought that the stress at such a portion is likely to grow. Therefore, the reliability of such a through-hole conductor is likely to decrease. For example, stress is thought to cause cracking in the through-hole conductor.

It is difficult to fill a penetrating hole for a through-hole conductor without experiencing any defects. Thus, reliability tends to be lowered in a through-hole conductor formed by filling a penetrating hole with plating.

A printed wiring board according to an embodiment of the present invention has the following: an insulative resin substrate having a first surface and a second surface opposite the first surface as well as a penetrating hole for a through-hole conductor; a first conductive layer formed on the first surface of the insulative resin substrate; a second conductive layer formed on the second surface of the insulative resin substrate; and a through-hole conductor which is formed in the penetrating hole and connects the first conductive layer and the second conductive layer, and which is made up of a seed layer formed on the inner wall of the penetrating hole, a laminated electrolytic plated layer formed on the seed layer and closing the penetrating hole at approximately the center portion, and a filled electrolytic plated layer formed on the laminated electrolytic plated layer and filling a recessed portion formed by the laminated electrolytic plated layer.

The laminated electrolytic plated layer is made up of multiple layers of electrolytic plated film, the electrolytic plated film is applied layer by layer along the electroless plated layer, and the thickness of the electrolytic plated film at the edge portion is less than the thickness of the electrolytic plated film at approximately the center portion.