WIRING BOARD

A wiring board that includes: a wiring conductor; a first dielectric layer around the wiring conductor and containing a first glass and a first ceramic filler; and a second dielectric layer interposed between the wiring conductor and the first dielectric layer, the second dielectric layer being in contact with the wiring conductor and the first dielectric layer, and the second dielectric layer containing a second glass and a second ceramic filler. A sintering temperature of the second glass contained in the second dielectric layer is higher than a sintering temperature of the wiring conductor, and a grain size of the second glass contained in the second dielectric layer is smaller than a grain size of the first glass contained in the first dielectric layer.

TECHNICAL FIELD

The present disclosure relates to a wiring board including a wiring conductor and a dielectric layer adjacent to the wiring conductor and containing a glass and a ceramic filler.

BACKGROUND ART

Glass-ceramics contain a crystallized glass and a ceramic filler. A wiring board in which glass-ceramics are used for a dielectric layer has high strength and excellent heat resistance because of the crystallized glass.

On the other hand, in a wiring board in which glass-ceramics are used for a dielectric layer, crystals are precipitated from the glass, and thus, irregularities are generated at an interface between the dielectric layer and a wiring conductor, resulting in roughness. When an electric signal passing through the wiring conductor has a high frequency, the electric signal passing through the wiring conductor concentrates near the interface with the dielectric layer because of a skin effect. Thus, when the interface between the dielectric layer and the wiring conductor is rough, the conductivity of the wiring conductor decreases, and transmission loss may occur.

In the wiring board of Patent Document 1, an intermediate layer made of a non-crystallized glass is interposed between a ceramic insulating layer corresponding to the dielectric layer and a wiring conductor. Thus, in the wiring board of Patent Document 1, crystals are not deposited from glass in the intermediate layer. Therefore, the roughness of the interface between the wiring conductor and the adjacent material can be reduced as compared with a structure without the intermediate layer.Patent Document 1: JP-B2-4703212

SUMMARY OF THE DISCLOSURE

However, the wiring board of Patent Document 1 reduces the roughness of the interface with the wiring conductor by not depositing crystals from glass. Thus, in the wiring board of Patent Document 1, only a structure in which a non-crystallized glass is disposed at the interface with the wiring conductor can be adopted, and thus there is room for further improvement.

An object of the present disclosure is to solve the above problems and to provide a wiring board capable of reducing the roughness of an interface between a dielectric layer and a wiring conductor as compared with the conventional technology while reducing the limitation of the structure.

To achieve the above object, the present disclosure is configured as follows. A wiring board according to one aspect of the present disclosure includes: a wiring conductor; a first dielectric layer around the wiring conductor and containing a first glass and a first ceramic filler; and a second dielectric layer interposed between the wiring conductor and the first dielectric layer, the second dielectric layer being in contact with the wiring conductor and the first dielectric layer, and the second dielectric layer containing a second glass and a second ceramic filler, wherein a sintering temperature of the second glass contained in the second dielectric layer is higher than a sintering temperature of the wiring conductor, and a grain size of the second glass contained in the second dielectric layer is smaller than a grain size of the first glass contained in the first dielectric layer.

According to the present disclosure, it is possible to reduce the roughness of the interface between the dielectric layer and the wiring conductor as compared with the conventional technology while reducing the limitation of the structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A wiring board according to one aspect of the present disclosure includes: a wiring conductor; a first dielectric layer around the wiring conductor and containing a first glass and a first ceramic filler; and a second dielectric layer interposed between the wiring conductor and the first dielectric layer, the second dielectric layer being in contact with the wiring conductor and the first dielectric layer, and the second dielectric layer containing a second glass and a second ceramic filler, wherein a sintering temperature of the second glass contained in the second dielectric layer is higher than a sintering temperature of the wiring conductor, and a grain size of the second glass contained in the second dielectric layer is smaller than a grain size of the first glass contained in the first dielectric layer.

According to this configuration, the grain size of the second glass contained in the second dielectric layer is smaller than the grain size of the first glass contained in the first dielectric layer. Thus, the roughness of the interface between the second dielectric layer and the wiring conductor can be reduced as compared with a configuration in which the second dielectric layer is not provided and the first dielectric layer is in contact with the wiring conductor. As a result, transmission loss in the wiring conductor can be reduced.

According to this configuration, the sintering temperature of the second glass contained in the second dielectric layer is higher than the sintering temperature of the wiring conductor. Thus, during sintering of the wiring board, the wiring conductor is sintered before the second glass contained in the second dielectric layer is sintered in the process of increasing the temperature. This causes the second glass contained in the second dielectric layer to maintain the original grain shape during sintering of the wiring conductor. As a result, the roughness of the interface between the wiring conductor and the second dielectric layer can depend on the grain size of the second glass contained in the second dielectric layer.

If the grain size of the first glass contained in the first dielectric layer is as small as the grain size of the second glass contained in the second dielectric layer, that is, if the glass of all the dielectric layers is atomized, the degreasing property at the time of sintering the wiring board deteriorates. As a result, the gas generated by the combustion of a binder is less likely to escape, and the possibility of occurrence of structural defects such as peeling of the dielectric layer from the wiring conductor increases. According to this configuration, the grain size of the second glass contained in the second dielectric layer is smaller than the grain size of the first glass contained in the first dielectric layer. That is, only the glass of some dielectric layers is atomized. This can reduce occurrence of structural defects such as peeling of the dielectric layer from other layers as compared with a configuration in which the glass of all the dielectric layers is atomized.

According to this configuration, the first glass contained in the first dielectric layer and the second glass contained in the second dielectric layer may be a crystallized glass or a non-crystallized glass. That is, according to this configuration, the restriction of the structure of the wiring board can be reduced.

In the wiring board, the grain size of the second ceramic filler contained in the second dielectric layer may be smaller than the grain size of the first ceramic filler contained in the first dielectric layer.

According to this configuration, the grain size of the second ceramic filler contained in the second dielectric layer is smaller than the grain size of the first ceramic filler contained in the first dielectric layer. Thus, the roughness of the interface between the dielectric layer and the wiring conductor can be reduced as compared with a configuration in which the second dielectric layer is not provided and the first dielectric layer is in contact with the wiring conductor.

In the wiring board, a through via that penetrates the first dielectric layer and the second dielectric layer and is electrically connected to the wiring conductor may be formed in the first dielectric layer and the second dielectric layer.

According to this configuration, in the vicinity of the boundary between the through via and the wiring conductor, a decrease in conductivity of the wiring conductor is prevented because of a reduction in roughness of the interface between the wiring conductor and the second dielectric layer. Thus, since the roughness of the interface between a part of the through via and the second dielectric layer is reduced, transmission loss between the wiring conductor and the through via can be reduced as compared with a configuration in which only the first dielectric layer is provided.

In the wiring board, the through via may have a tapered shape that reduces its diameter in a direction toward the wiring conductor.

Normally, in a through via having a tapered shape, transmission loss is likely to occur in a part having a smaller diameter than a part having a larger diameter. According to this configuration, the part having a smaller diameter of the through via where transmission loss is likely to occur is positioned near the boundary with the wiring conductor. In the vicinity of the boundary, a decrease in conductivity of the wiring conductor is prevented because of a reduction in roughness of the interface between the wiring conductor and the second dielectric layer. Thus, transmission loss between the wiring conductor and the through via can be reduced.

First Embodiment

FIG.1is a plan view of a wiring board according to a first embodiment of the present disclosure.FIG.2is a longitudinal sectional view showing a section taken along the line A-A inFIG.1. In the wiring board, a wiring conductor is formed adjacent to a dielectric layer containing a crystallized glass and a ceramic filler. In the wiring board according to the first embodiment, a wiring conductor is formed inside.

As shown inFIGS.1and2, a wiring board10has a rectangular parallelepiped shape. The shape of the wiring board10is not limited to a rectangular parallelepiped shape.

As shown inFIG.2, the wiring board10includes a wiring conductor20and dielectric layers30and40. The dielectric layer30is an example of the second dielectric layer. The dielectric layer40is an example of the first dielectric layer.

The wiring conductor20functions as a wiring that electrically connects a plurality of electronic components (not shown) to be mounted on the wiring board10to each other. The wiring conductor20can also function as an electrode to which an electronic component to be mounted on the wiring board10is electrically connected via solder or the like.

In the first embodiment, the wiring conductor20is covered with the dielectric layer30. The wiring conductor20is formed inside the wiring board10.

The wiring conductor20is obtained by printing a conductive paste on a surface of the dielectric layer30. The conductive paste is made of, for example, copper.

The dielectric layer30covers the periphery of the wiring conductor20. The dielectric layer30is in contact with the wiring conductor20.

The dielectric layer30is made of a glass-ceramic. The dielectric layer30contains a crystallized glass and a ceramic filler. In the first embodiment, the dielectric layer is made of a crystallized glass and alumina and zirconia constituting the ceramic filler. In the first embodiment, the proportion of each material contained in the dielectric layer30is as follows. That is, the crystallized glass contained in the dielectric layer30is 98 (vol %) of the dielectric layer30. The alumina contained in the dielectric layer30is 1.5 (vol %) of the dielectric layer30. The zirconia contained in the dielectric layer30is 0.5 (vol %) of the dielectric layer30.

The dielectric layer40covers the periphery of the dielectric layer30. The dielectric layer40is in contact with the dielectric layer30. On the other hand, in the first embodiment, the dielectric layer40is not in contact with the wiring conductor20. That is, the dielectric layer40is provided around the wiring conductor20, and the dielectric layer30is interposed between the wiring conductor20and the dielectric layer40.

The dielectric layer40is made of a glass-ceramic. The dielectric layer40contains a crystallized glass and a ceramic filler. In the first embodiment, the dielectric layer contains the same materials as the dielectric layer30at the same proportions as in the dielectric layer30. That is, dielectric layer40is made of a crystallized glass and alumina and zirconia constituting the ceramic filler. In the first embodiment, the crystallized glass contained in the dielectric layer40is 98 (vol %) of the dielectric layer30. The alumina contained in the dielectric layer40is 1.5 (vol %) of the dielectric layer30. The zirconia contained in the dielectric layer40is 0.5 (vol %) of the dielectric layer30.

The grain size of the crystallized glass (specifically, the glass powder constituting the crystallized glass) contained in the dielectric layer30is smaller than the grain size of the crystallized glass contained in the dielectric layer40. In the first embodiment, the grain size of each material (crystallized glass, alumina, and zirconia) contained in the dielectric layer30is the average grain size in the whole of the dielectric layer30or in any part of the dielectric layer30when the part is cut out. In the same manner, the grain size of each material (crystallized glass, alumina, and zirconia) contained in the dielectric layer40is the average grain size in the whole of the dielectric layer40or in any part of the dielectric layer40when the part is cut out.

The grain size of the crystallized glass contained in the dielectric layer30is smaller than the grain size of the crystallized glass contained in the dielectric layer40. In the first embodiment, the grain size of the crystallized glass contained in the dielectric layer is 2.0 (μm) or less. In the first embodiment, the grain size of the crystallized glass contained in the dielectric layer30is 1.0 (μm) or less.

The grain size of each ceramic filler (specifically, alumina and zirconia) contained in the dielectric layer30is equal to or smaller than the grain size of each ceramic filler (specifically, alumina and zirconia) contained in the dielectric layer40.

In the first embodiment, the grain size of the alumina contained in the dielectric layer40is 2.5 (μm). The grain size of the alumina contained in the dielectric layer30is 2.5 (μm) or less.

In the first embodiment, the grain size of the zirconia contained in the dielectric layer40is 1.3 (μm). The grain size of the zirconia contained in the dielectric layer30is 1.3 (μm) or less.

The sintering temperature of the crystallized glass contained in the dielectric layer is higher than the sintering temperature of the wiring conductor20. In the first embodiment, the sintering temperature of the crystallized glass contained in the dielectric layer30is 900 to 950 (° C.). The sintering temperature of the wiring conductor20is 850 to 900 (° C.).

In the first embodiment, a thickness T1of the wiring conductor20is larger than a thickness T2of the dielectric layer30. In the first embodiment, the thickness T1of the wiring conductor20is 5 to 30 (μm), and the thickness T2of the dielectric layer30is 5 to 20 (μm). The thickness T1may be equal to or less than the thickness T2.

Hereinafter, an example of a method for producing the wiring board10according to the first embodiment will be described.FIG.3is a longitudinal sectional view of a green sheet on which a ceramic paste is stacked.FIG.4is a longitudinal sectional view of a green sheet in which a conductor paste is stacked on the ceramic paste ofFIG.3.FIG.5is a longitudinal sectional view of a green sheet in which a ceramic paste is stacked on the ceramic paste and the conductor paste ofFIG.4.FIG.6is a longitudinal sectional view showing stacking of another green sheet on the green sheet ofFIG.5.

First, a green sheet41constituting a part of the dielectric layer40is produced by a known process (seeFIG.3). In the first embodiment, as described above, the materials of the green sheet41are 98 (vol %) of crystallized glass, 1.5 (vol %) of alumina, and 0.5 (vol %) of zirconia.

Next, as shown inFIG.3, a ceramic paste31constituting a part of the dielectric layer30is printed on a principal surface41A of the green sheet41by a known process. In the first embodiment, the materials of the ceramic paste31are the same as that of the green sheet41. However, the grain size of the crystallized glass contained in the ceramic paste31is smaller than the grain size of the crystallized glass contained in the green sheet41. The grain size of each of the alumina and zirconia contained in the ceramic paste31is equal to or smaller than the grain size of each of the alumina and zirconia contained in the green sheet41.

Next, as shown inFIG.4, a conductive paste21constituting the wiring conductor is printed on a principal surface31A of the ceramic paste31by a known process. In the first embodiment, the material of the conductive paste21is copper.

Next, as shown inFIG.5, a ceramic paste32constituting the rest of the dielectric layer30is printed on the principal surface31A of the ceramic paste31by a known process so as to cover the conductive paste21. This causes the conductive paste21to be buried in the dielectric layer30constituted by the ceramic pastes31and32. The material of the ceramic paste32is the same as that of the ceramic paste32.

Next, as shown inFIG.6, a green sheet42constituting the rest of the dielectric layer40is stacked on the principal surface41A of the green sheet41so as to cover the dielectric layer30by a known process. This causes the dielectric layer30to be buried in the dielectric layer40constituted by the green sheets41and42. The material of the green sheet42is the same as that of the green sheet41.

Next, a stack formed by the step shown inFIG.6is sintered. The wiring board shown inFIG.2is thus completed.

Hereinafter, results of a performance evaluation test performed on the wiring boards according to examples 1 to 6 and the wiring boards according to comparative examples 1 and 2 will be described.FIG.7is a table showing the results of the performance evaluation test.

In each of the wiring boards10according to examples 1 to 3, the grain size of the crystallized glass contained in the dielectric layer30is smaller than the grain size of the crystallized glass contained in the dielectric layer40. On the other hand, in each of the wiring boards10according to examples 1 to 3, the grain size of each of the alumina and zirconia contained in the dielectric layer30is the same as the grain size of each of the alumina and zirconia contained in the dielectric layer40.

In each of the wiring boards10according to examples 4 to 6, the grain size of the crystallized glass contained in the dielectric layer30is smaller than the grain size of the crystallized glass contained in the dielectric layer40. In each of the wiring boards10according to examples 4 to 6, the grain size of each of the alumina and zirconia contained in the dielectric layer30is smaller than the grain size of each of the alumina and zirconia contained in the dielectric layer40.

Each of the wiring boards according to comparative examples 1 and 2 includes the wiring conductor20and the dielectric layers30and40, as in each of the wiring boards10according to examples 1 to 6. However, in each of the wiring boards according to comparative examples 1 and 2, the grain size of the crystallized glass contained in the dielectric layer30is the same as the grain size of the crystallized glass contained in the dielectric layer40. In each of the wiring boards according to comparative examples 1 and 2, the grain size of each of the alumina and zirconia contained in the dielectric layer30is the same as the grain size of each of the alumina and zirconia contained in the dielectric layer40.

As the performance evaluation, three items of Rmax, presence or absence of transmission loss, and presence or absence of delamination were evaluated.

Rmax is the maximum height of the irregularities of the interface between the wiring conductor20and the dielectric layer30, and the unit is micrometers (μm).

A tri-plate line having a width of 50 (μm) and a thickness of 7 (μm) of the wiring conductor20was produced, and the presence or absence of transmission loss was evaluated. When transmission is improved by 0.01 (dB/mm) or more at 60 (GHz), “∘” is given inFIG.7as an evaluation of transmission loss. When transmission is improved by 0.02 (dB/mm) or more at 60 (GHz), double circles is given inFIG.7as an evaluation of transmission loss. When the improvement in transmission is less than 0.01 (dB/mm), it is assumed that transmission loss has occurred, and “x” is given inFIG.7.

The delamination is peeling of the dielectric layer30from the wiring conductor When a section of the wiring board10is observed, and there is even one gap of 5 (μm) or more continuing 1 (mm) or more in a length direction in the section, it is assumed that delamination has occurred, and “x” is given inFIG.7. In the other cases than the above, it is assumed that delamination has not occurred, and “∘” is given inFIG.7.

As can be seen fromFIG.7, in the wiring board10according to example 3, the value of Rmax is smaller than that of the wiring boards according to comparative examples 1 and 2. That is, it can be seen that, in the wiring board10according to example 3, the irregularities of the interface between the wiring conductor20and the dielectric layer30is reduced as compared with the wiring boards according to comparative examples 1 and 2.

As can be seen fromFIG.7, in the wiring boards10according to examples 1 to 3, transmission loss is practically acceptable, and delamination does not occur. On the other hand, transmission loss has occurred in the wiring board according to comparative example 1, and delamination has occurred in the wiring board according to comparative example 2.

As can be seen fromFIG.7, in the wiring boards10according to examples 4 to 6, the value of Rmax is smaller than that of the wiring boards according to examples 1 to 3. That is, it can be seen that, in the wiring boards10according to examples 4 to 6, the irregularities of the interface between the wiring conductor20and the dielectric layer30is reduced as compared with the wiring boards according to examples 1 to 3.

As can be seen fromFIG.7, in the wiring boards10according to examples 4 to 6, transmission loss is reduced as compared with the wiring boards10according to example 1 to 3. In the wiring boards10according to examples 4 to 6, delamination does not occur as in the wiring boards10according to examples 1 to 3.

According to the first embodiment, the grain size of the crystallized glass contained in the dielectric layer30is smaller than the grain size of the crystallized glass contained in the dielectric layer40. Thus, the roughness of the interface between the dielectric layer30and the wiring conductor20can be reduced as compared with a configuration in which the dielectric layer30is not provided and the dielectric layer40is in contact with the wiring conductor20. As a result, transmission loss in the wiring conductor20can be reduced. In addition, peeling of the wiring conductor20from the dielectric layer can be reduced as compared with the conventional technology.

According to the first embodiment, the sintering temperature of the crystallized glass contained in the dielectric layer30is higher than the sintering temperature of the wiring conductor20. Thus, during sintering of the wiring board10, the wiring conductor20is sintered before the crystallized glass contained in the dielectric layer30is sintered in the process of increasing the temperature. This causes the crystallized glass contained in the dielectric layer30to maintain the original grain shape during sintering of the wiring conductor20. As a result, the roughness of the interface between the wiring conductor20and the dielectric layer30can depend on the grain size of the crystallized glass contained in the dielectric layer30.

If the grain size of the crystallized glass contained in the dielectric layer40is as small as the grain size of the crystallized glass contained in the dielectric layer30, that is, if the glass of all the dielectric layers (dielectric layers30and40) is atomized, the degreasing property at the time of sintering the wiring board10deteriorates. As a result, the gas generated by the combustion of a binder is less likely to escape, and the possibility of occurrence of structural defects such as peeling of the dielectric layer from the wiring conductor increases. According to the first embodiment, the grain size of the crystallized glass contained in the dielectric layer30is smaller than the grain size of the crystallized glass contained in the dielectric layer40. That is, only the crystallized glass of some dielectric layers (dielectric layers30) is atomized. This can reduce occurrence of structural defects such as peeling of the dielectric layer from other layers as compared with a configuration in which the crystallized glass of all the dielectric layers (dielectric layers30and40) is atomized.

According to the first embodiment, both the dielectric layers30and40contain a crystallized glass. Thus, the strength and heat resistance of the wiring board10can be favorably maintained as compared with a configuration in which a non-crystallized glass is used instead of a crystallized glass for at least one of the dielectric layers30and40.

According to the first embodiment, the grain size of the ceramic filler contained in the dielectric layer30is smaller than the grain size of the ceramic filler contained in the dielectric layer40. Thus, the roughness of the interface between the dielectric layer30and the wiring conductor20can be reduced as compared with a configuration in which the dielectric layer30is not provided and the dielectric layer40is in contact with the wiring conductor20.

In the first embodiment, the grain size of each ceramic filler (specifically, alumina and zirconia) contained in the dielectric layer30is equal to or smaller than the grain size of each ceramic filler (specifically, alumina and zirconia) contained in the dielectric layer40. However, the grain size of each ceramic filler contained in the dielectric layer30may be larger than the grain size of each ceramic filler contained in the dielectric layer40.

The proportion of each material contained in the dielectric layers30and40is not limited to the proportion described above. However, the proportion of the crystallized glass contained in the dielectric layers30and40is desirably 80 (vol %) or more.

In the dielectric layers30and40, the ceramic filler is not limited to alumina and zirconia. For example, the ceramic filler may contain quartz, titania, or the like instead of at least one of alumina or zirconia, or in addition to at least one of alumina or zirconia.

The proportion of each of the crystallized glass, alumina, and zirconia contained in the dielectric layer30may be different from the proportion of each of the crystallized glass, alumina, and zirconia contained in the dielectric layer40.

In the first embodiment, the wiring conductor is formed inside the wiring board. However, the wiring conductor may be formed on an outer surface of the wiring board.

In the first embodiment, each of the dielectric layers30and40is made of a glass-ceramic and contains a crystallized glass. However, the glass contained in the dielectric layers30and40is not limited to a crystallized glass, and may be a non-crystallized glass. For example, both of the dielectric layers30and40may contain a non-crystallized glass. For example, the dielectric layer30may contain a crystallized glass, and the dielectric layer40may contain a non-crystallized glass. For example, the dielectric layer40may contain a crystallized glass, and the dielectric layer30may contain a non-crystallized glass. In this manner, according to the first embodiment, since the glass contained in the dielectric layers30and40is not limited to either a crystallized glass or a non-crystallized glass, the restriction of the structure of the wiring board10can be reduced.

Second Embodiment

FIG.8is a longitudinal sectional view of a wiring board according to a second embodiment of the present disclosure, corresponding to the section taken along the line A-A inFIG.1. A wiring board10A according to the second embodiment is different from the wiring board10according to the first embodiment in that a through via50penetrating the dielectric layers30and40is further provided. Hereinafter, differences from the first embodiment will be described. Common points with the wiring board10of the first embodiment are denoted by the same reference numerals, and description thereof will be omitted in principle and described as necessary.

As shown inFIG.8, the through via50is formed in the dielectric layers30and40. The through via50is formed by filling a through hole (via)50A penetrating the dielectric layers30and40with a conductive paste50B and co-sintering with the ceramic. The conductive paste50B is made of, for example, copper. The through via50is exposed to the outside of the wiring board10A via a principal surface40A of the dielectric layer40. The through via50is exposed to the wiring conductor20. This causes the through via50to be electrically connected to the wiring conductor20. In the second embodiment, one end of the through via50in a penetrating direction is electrically connected to the wiring conductor20, and the other end of the through via50in the penetrating direction is exposed to the outside of the wiring board10A.

A surface constituting the through hole50A is inclined with respect to a thickness direction TD of the wiring board10A. The diameter of the through hole50A decreases from the principal surface40A of the dielectric layer40toward the wiring conductor20. That is, the through via50has a tapered shape that reduces its diameter toward the wiring conductor20. In other words, the through via50has a truncated cone shape.

According to the second embodiment, in the vicinity of the boundary between the through via50and the wiring conductor20, a decrease in conductivity of the wiring conductor20is prevented because of a reduction in roughness of the interface between the wiring conductor20and the dielectric layer30. Thus, since the roughness of the interface between a part of the through via50and the dielectric layer30is reduced, transmission loss between the wiring conductor20and the through via50can be reduced as compared with a configuration in which only the dielectric layer40is provided.

Normally, in the through via50having a tapered shape, transmission loss is likely to occur in a part having a smaller diameter than a part having a larger diameter. According to the second embodiment, the part having a smaller diameter of the through via50where transmission loss is likely to occur is positioned near the boundary with the wiring conductor20. In the vicinity of the boundary, a decrease in conductivity of the wiring conductor20is prevented because of a reduction in roughness of the interface between the wiring conductor20and the dielectric layer30. Thus, transmission loss between the wiring conductor20and the through via50can be reduced.

In the second embodiment, the through via50has a tapered shape that reduces its diameter toward the wiring conductor20, but the through via is not limited to such a shape. For example, the diameter of the through via50may be constant. That is, the through via50may have a cylindrical shape.

In the second embodiment, the through via50is exposed to the outside of the wiring board10A. However, the through via50does not have to be exposed to the outside of the wiring board10A. For example, when the wiring board10A is a multilayer board, the through via50may be formed in a board constituting an inner layer of the wiring board10A.

In the second embodiment, one through via50is formed in the wiring board10A, but the number of through via50is not limited to one and may be plural. In the second embodiment, the through via50is formed on the principal surface40A of the dielectric layer40, but the through via50may be formed on a surface other than the principal surface40A, for example, a principal surface40B of the dielectric layer40. The principal surface40B is the surface opposite to the principal surface40A. The through via50may be formed on a plurality of surfaces. For example, the through via50may be formed on both the principal surfaces40A and40B.

By appropriately combining any embodiments among the various embodiments described above, the effects of the respective embodiments can be achieved.

Although the present disclosure has been sufficiently described in connection with preferable embodiments with reference to the drawings as appropriate, various modifications and corrections are apparent to those skilled in the art. Such modifications and corrections should be understood to be included within the scope of the present disclosure according to the appended claims as long as they do not depart from the scope of the present disclosure.

EXPLANATION OF REFERENCE NUMBERS