Wiring board

Abstract

This wiring board comprises a ceramic insulating base and internal wiring located inside the insulating base. The internal wiring includes a sintered body of a plurality of crystal grains mainly composed of copper, and a plurality of glass particles. In a cross section orthogonal to the longitudinal direction of the internal wiring, the internal wiring includes a central region, an intermediate region that surrounds the central region and has more glass particles distributed than the central region, and an outer peripheral region that is a region between the intermediate region and the outer periphery of the internal wiring and in which the distribution of the glass particles is smaller than in the intermediate region. The area density of the glass particles in the outer peripheral region is lower than in the central region and also lower than in the intermediate region.

Description

wiring board

[0001] The present disclosure relates to a wiring substrate.

[0002] Japanese Patent Application Laid-Open No. 2021-011411 describes a wiring board using a ceramic sintered body. The wiring board is provided with internal wiring.

[0003] The wiring board according to the present disclosure comprises an insulating base made of ceramics and internal wiring located inside the insulating base, wherein the internal wiring includes a sintered body of a plurality of crystal grains whose main component is copper and a plurality of glass particles, and in a cross section perpendicular to the longitudinal direction of the internal wiring, the internal wiring has a central region, an intermediate region surrounding the central region and having a greater distribution of the glass particles than the central region, and an outer periphery region between the intermediate region and the outer periphery of the internal wiring and having a smaller distribution of the glass particles than the intermediate region, and the area density of the glass particles is lower in the outer periphery region than in the central region and also lower than in the intermediate region.

[0004] Fig. 1 is a perspective view showing a wiring board according to an embodiment of the present disclosure; Fig. 2 is a cross-sectional view of the wiring board including a cross-section of the internal wiring of Fig. 1; Fig. 3 is a diagram illustrating components of the internal wiring; Fig. 4 is a diagram illustrating a cross-section of the internal wiring of embodiment 1; Fig. 5 is a diagram illustrating a cross-section of the internal wiring of embodiment 2; Fig. 6 is a diagram illustrating a cross-section of the internal wiring of embodiment 3; Fig. 7 is a diagram illustrating a cross-section of the internal wiring of embodiment 4; and Fig. 8 is a diagram illustrating a cross-section of the internal wiring of embodiment 5.

[0005] Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the drawings. In this embodiment, a cross section that intersects the internal wiring 21 and is perpendicular to the longitudinal direction (i.e., the extending direction) of the internal wiring 21 will be simply referred to as a cross section of the internal wiring 21.

[0006] 1 is a perspective view showing a wiring board 1 according to an embodiment of the present disclosure. The wiring board 1 of this embodiment includes an insulating base 10 made of ceramic and wiring 20 located on the insulating base 10.

[0007] The insulating base 10 is a fired product having insulating properties, such as ceramics. The insulating base 10 may be a glass ceramic material containing glass particles in a powder material before sintering. The insulating base 10 may have a plate-like configuration. The insulating base may be configured by stacking and firing a plurality of green sheets made of a ceramic material.

[0008] The wiring 20 may include surface wiring 22 located on the surface of the insulating base 10 and internal wiring 21 located inside the insulating base 10. The internal wiring 21 may include a film conductor extending in a direction along the plate surface of the insulating base 10 and a via conductor extending in a direction perpendicular to the plate surface of the insulating base 10. Below, the components of the internal wiring 21 will be described using a film conductor as an example of the internal wiring 21, but components not explicitly stated as being part of the film conductor are also present in the via conductor. The film conductor and the via conductor differ mainly in the cross-sectional shape of the internal wiring 21; the film conductor has a flattened shape, while the via conductor may have a circular shape. The circular shape is a concept that includes not only a strictly circular shape but also shapes that are distorted from a strictly circular shape.

[0009] 2 is a diagram showing a cross section of the internal wiring 21. In the cross section of the internal wiring 21, hatching of the crystal grains 33 that make up the internal wiring 21 is omitted. The same applies to FIGS. 3 to 8.

[0010] The internal wiring 21 may include a sintered body of a plurality of conductive crystal grains 33. The sintered body of the plurality of crystal grains 33 is located in the white portion of the internal wiring 21 in FIG. 2. The plurality of crystal grains 33 may be primarily composed of copper. "Mainly composed" means that it accounts for 70% or more by volume. In addition to copper as the primary component, the crystal grains 33 may also include metal materials such as silver, palladium, gold, platinum-tungsten, molybdenum, or manganese, or alloy materials or mixed materials containing these metal materials. The volume percentage of copper in the crystal grains 33 may be 80% or more, or 90% or more. The upper limit of the volume percentage of copper in the crystal grains 33 may be 99%.

[0011] The internal wiring 21 may further include a plurality of glass particles 35. The main component of the glass particles 35 may be silica. Including the glass particles 35 may mean that the material of the internal wiring 21 before firing contains the glass particles 35. When the material of the insulating base 10 and the material of the internal wiring 21 are fired simultaneously, the nano-sized silica associated with the glass particles 35 migrates to the boundary between the insulating base 10 and the internal wiring 21, thereby improving the adhesion between the insulating base 10 and the internal wiring 21. This effect of improving adhesion is particularly useful when the insulating base 10 is a glass ceramic that is fired at a low temperature.

[0012] Next, a cross section of the internal wiring 21 will be described in detail. The cross section can be observed in a photograph (hereinafter referred to as a SEM (Scanning Electron Microscope) photograph) taken by mirror-polishing the cross section. The cross section of the internal wiring 21 includes a plurality of crystal grains 33 composed mainly of copper, grain boundaries 34 located at the boundaries between pairs of adjacent crystal grains 33, and glass particles 35. The grain boundaries 34 are linear in the cross section. The grain boundaries 34 may also include silica.

[0013] The boundaries between adjacent crystal grains 33 may not be visible in SEM photographs because the multiple crystal grains 33 are tightly adhered to each other due to densification during firing. The boundaries between adjacent glass particles 35 may also not be visible when the glass particles 35 are in close contact with each other. Even in such cases, the individual crystal grains 33, grain boundaries 34, and individual glass particles 35 can be distinguished as follows.

[0014] <Example of a method for distinguishing individual crystal grains, grain boundaries, and individual glass particles> Figure 3 is a diagram illustrating the components of the internal wiring 21. Figure 3 shows the boundaries of some of the distinguished crystal grains 33 and the boundaries of some of the glass particles 35. The boundaries were distinguished by the method described below, and are represented by two-dot chain lines in Figure 3. The cross section of Figure 3 shows the boundaries of seven crystal grains 33 located at the upper center of the internal wiring 21 and the boundaries of two glass particles 35 located at point C1 in the upper left.

[0015] Each crystal grain 33 is a crystal grain in the material before firing that has been densified by firing. Due to densification with neighboring crystal grains 33, each crystal grain 33 has a shape resembling an oblate spheroid (specifically, a spheroid) crushed from multiple directions with flat or curved surfaces. Meanwhile, in the outer periphery of the internal wiring 21, the crystal grains 33 are adjacent to particles of the insulating base 10, leaving the outer shape of the crystal grains 33 largely rounded. Furthermore, in the outer periphery of the internal wiring 21, convex curved portions A, which are the belly portions of each of the multiple crystal grains 33 aligned along the periphery, and valley portions B, which are the contact points between pairs of adjacent crystal grains 33, are likely to appear. Furthermore, grain boundaries 34 appear at the boundaries between pairs of adjacent crystal grains 33, and glass particles 35 are located between the multiple crystal grains 33.

[0016] Therefore, the boundaries of the individual crystal grains 33, i.e., the size and shape of the individual crystal grains 33, can be estimated from the convex curves A and valleys B that appear in the outline of the cross section of the internal wiring 21, the grain boundaries 34 that can be observed here and there, and the positions of the glass particles 35. Figure 3 shows the boundaries of seven crystal grains 33 estimated as described above, at the upper center of the internal wiring 21.

[0017] The grain boundaries 34 and the glass particles 35 are displayed with a contrast different from that of the plurality of crystal grains 33 in the SEM photograph of the cross section of the internal wiring 21. Therefore, by dividing each pixel in the SEM photograph into two types using a contrast threshold, it can be determined that one pixel is included in the plurality of crystal grains 33 and the other pixel is included in the grain boundaries 34 or the glass particles 35.

[0018] The grain boundaries 34 and the glass particles 35 can be distinguished by their shapes. The grain boundaries 34 are linear, and the glass particles 35 have a two-dimensional width that is distinguishable from the grain boundaries 34. Furthermore, the individual glass particles 35 can be distinguished by their rounded outlines. As shown in area C1 in the upper left of Figure 3, if the outline contains multiple rounds, with a continuous roundness considered to be one round, and there is a valley between the individual rounds, it can be determined that two glass particles 35 are combined at the valley. The boundary line estimated by the above-mentioned discrimination is shown between the two glass particles 35 at area C1 in Figure 3.

[0019] By the above method, it is possible to distinguish the individual crystal grains 33, the grain boundaries 34, and the individual glass particles 35 in the cross section of the internal wiring 21.

[0020] (Internal Wiring of First Embodiment) Figure 4 is a cross-sectional view showing an internal wiring 21 according to a first embodiment of the present disclosure. The internal wiring 21 according to the first embodiment may include a central region 25, an intermediate region 26, and an outer periphery region 27 in the cross section of the internal wiring 21. In Figure 4, the outer periphery of the central region 25 and the outer periphery of the intermediate region 26 are indicated by dashed dotted lines. The intermediate region 26 is a region that surrounds the central region 25 and has a higher distribution of glass particles 35 than the central region 25. The outer periphery region 27 is a region between the intermediate region 26 and the outer periphery of the internal wiring 21, and has a lower distribution of glass particles 35 than the intermediate region 26. The area proportion occupied by the glass particles 35 may be lower in the outer periphery region 27 than in the intermediate region 26, and may also be lower in the outer periphery region 27 than in the central region 25.

[0021] The radial widths of the central region 25, intermediate region 26, and outer peripheral region 27 may be derived from the distribution of glass particles 35. That is, when the distribution of glass particles 35 in the cross section of the internal wiring 21 is measured from the center to the outer edge of the internal wiring 21, the distribution is arranged as a region with little distribution, a region with a lot, and a region with little distribution. Therefore, the middle region with a lot of distribution can be set as the intermediate region 26, and the inside and outside of that can be set as the central region 25 and the outer peripheral region 27, respectively.

[0022] The outer peripheral region 27 may have zero distribution of glass particles 35 .

[0023] When transmitting a signal through the internal wiring 21, the higher the frequency band of the signal, the more the skin effect appears in the signal current. The skin effect is a phenomenon in which the current density of a high-frequency signal increases the closer it is to the conductor surface and decreases the further it is from the conductor surface. According to the internal wiring 21 of the first embodiment, the distribution of glass particles 35 is small in the outer circumferential region 27, which reduces the resistance of the high-frequency signal current caused by the glass particles 35 due to the skin effect, thereby improving the transmission characteristics.

[0024] EXAMPLES Next, a specific wiring board 1 was created, and the results of evaluating the transmission characteristics of high-frequency signals of the wiring board 1 will be described.

[0025] First, a mixture of 40 wt % alumina particles and 60 wt % borosilicate glass was prepared as the material for the insulating base 10. This mixture is a glass ceramic raw material with a firing temperature of 900°C to 1000°C. 20 parts by mass of isobutyl methacrylate resin and dibutyl phthalate were used as the organic binder for 100 parts by mass of the glass ceramic raw material, and multiple green sheets measuring 90 mm x 80 mm and 50 μm and 75 μm in thickness were produced by doctor blade molding.

[0026] Copper powder with a particle size of 1 μm to 5 μm and an average particle size of 2 μm and silica particles with a particle size of 20 nm to 50 nm and an average particle size of 30 nm were prepared as raw materials for the internal wiring 21. The amount of silica particles added was 1 part by mass per 100 parts by mass of copper powder. Furthermore, isobutyl methacrylate resin and a mixed solvent of butyl carbitol acetate and dibutyl phthalate were used as the organic binder. A conductor paste containing copper powder and silica particles was prepared by adding 5 parts by mass of isobutyl methacrylate resin per 100 parts by mass of copper powder and further adding a mixed solvent of butyl carbitol acetate and dibutyl phthalate.

[0027] The above-mentioned conductor paste was printed in a predetermined arrangement on both surfaces of the prepared green sheet, and through holes were formed between the two surfaces at predetermined positions of the green sheet and filled with the conductor paste. The conductor paste printed on the surface of the green sheet constituted the pre-fired film conductor of the internal wiring 21, and the conductor paste filled in the through holes constituted the pre-fired via conductor of the internal wiring 21. Furthermore, multiple green sheets containing the conductor paste were stacked to prepare a pre-fired material. The thickness of the pre-fired material was 2 mm.

[0028] The prepared pre-fired material was fired in a reducing atmosphere using a hydrogen-nitrogen mixed gas at a maximum temperature of 930° C. for a holding time of 2 hours.

[0029] When SEM photographs of the cross sections of the internal wiring 21 at multiple locations on the wiring board 1 fired as described above were observed, it was confirmed that there were differences in the distribution of the glass particles 35 among the central region 25, the intermediate region 26, and the peripheral region 27. Furthermore, it was confirmed that the glass particles 35 were not located in the peripheral region 27.

[0030] The frequency characteristics of the wiring included in the fabricated wiring board 1 were measured and compared with a wiring board containing glass particles in the peripheral region and otherwise substantially identical in configuration. As a result, it was confirmed that the insertion loss of high-frequency signals improved by about 0.1 dB in the 8 GHz to 15 GHz band, and by 0.1 to 0.25 dB in the 20 GHz to 70 GHz band.

[0031] The above-described manufacturing method is merely one example of a method for manufacturing the wiring board 1 of this embodiment, and the manufacturing parameters for manufacturing the wiring board 1 of this embodiment can be selected from a relatively wide range. The manufacturing parameters that cause the distribution of glass particles 35 contained in the internal wiring 21 to be high in the intermediate region 26 and almost zero in the peripheral region 27 can be easily found by a moderate number of trials.

[0032] 5 shows a cross-sectional view of a wiring board 1A according to a second embodiment of the present disclosure. The second embodiment differs from the first embodiment in that the outer periphery region 27 is defined by crystal grains 33, but the other components may be the same as those of the first embodiment.

[0033] As shown in Figure 5, the outer peripheral region 27 may be a region in which a plurality of crystal grains 33 in contact with the outer periphery of the internal wiring 21 are connected in the circumferential direction. The crystal grains 33 may have a grain size of 1 μm to 5 μm. The grain size refers to the average value of the major axis and the minor axis. This grain size can be achieved by adjusting the grain size of the crystal grains in the material stage. The distribution of glass particles 35 in the outer peripheral region 27 may be zero.

[0034] This configuration means that the proportion of glass particles 35 present at the boundary between each of the multiple crystal grains 33 located at the outermost periphery of the internal wiring 21 and another circumferentially adjacent crystal grain 33 is low or zero. Furthermore, the radial width of the outer periphery region 27 corresponds to the grain size of the crystal grains 33.

[0035] According to the wiring board 1A of the second embodiment, by increasing the density between the plurality of crystal grains 33 located at the outermost periphery of the internal wiring 21, it is possible to easily reduce or eliminate the area ratio of the glass particles 35 in the outer peripheral region 27. Furthermore, the radial width of the outer peripheral region 27, where the area ratio of the glass particles 35 is reduced or eliminated, can be controlled at the material selection stage by adjusting the particle size of the crystal grains 33. The outer peripheral region 27 reduces the resistance of the glass particles 35 to the current of high-frequency signals due to the skin effect, thereby improving the transmission characteristics in the high-frequency band.

[0036] Furthermore, because the grain size of the crystal grains 33 is 1 μm to 5 μm, the area ratio occupied by the glass particles 35 is low or zero in the range of 1 μm to 5 μm wide along the periphery of the internal wiring 21, forming the peripheral region 27. Therefore, the transmission characteristics can be improved from a frequency band of about 8 GHz, where the width of the surface layer where the current density becomes high due to the skin effect is 1 μm to 5 μm wide.

[0037] The wiring board 1A having the configuration of the second embodiment can be manufactured by the manufacturing method shown in the example of the first embodiment.

[0038] 6 shows a cross-sectional view of a wiring board 1B according to a third embodiment of the present disclosure. The third embodiment differs in that the distribution pattern of the glass particles 35 contained in the internal wiring 21 is specified, but the other components may be the same as those of either the first or second embodiment.

[0039] As shown in Figure 6, in the cross section of the internal wiring 21, the multiple glass particles 35 may include first glass particles 35A with a particle size of 2 μm to 4 μm and second glass particles 35B with a particle size of less than 2 μm. This configuration can be achieved by adjusting the particle size of the glass particles in the material stage. The first glass particles 35A may also be called large glass particles, and the second glass particles 35B may also be called small glass particles.

[0040] Furthermore, in the cross section of the internal wiring 21, the second glass particles 35B may be scattered around the first glass particles 35A. Furthermore, the plurality of glass particles 35 may include single particles 35C located singly. Furthermore, the plurality of glass particles 35 may include composite particles 35D that are linked together so as not to form rings. According to the distribution mode of the glass particles 35, the glass particles 35 are dispersed in the central region 25 and the intermediate region 26, so that it is possible to reduce or eliminate the possibility of the glass particles 35 gathering in one place and becoming large, or linking together to form rings or linking together in a lattice pattern.

[0041] Furthermore, in the cross section of the internal wiring 21, the first glass particles 35A may be distributed more in the intermediate region 26 than in the central region 25. According to this distribution of the first glass particles 35A, the first glass particles 35A are more distributed in the intermediate region 26, which is wider than the central region 25, and therefore the first glass particles 35A can be easily dispersed. Therefore, it is possible to reduce the possibility of the glass particles 35A gathering in one place and becoming large.

[0042] Glass particles 35 that gather in large numbers in one place increase the conductor loss in internal wiring 21, and glass particles 35 that are connected in a ring or lattice pattern increase the conductor loss in internal wiring 21. Therefore, wiring board 1B of embodiment 3 can provide internal wiring 21 with low conductor loss.

[0043] The wiring board 1B having the configuration of the third embodiment can be manufactured by the manufacturing method shown in the example of the first embodiment.

[0044] 7 shows a cross-sectional view of a wiring board 1C according to a fourth embodiment of the present disclosure. The fourth embodiment differs in that the distribution pattern of the grain boundaries 34 included in the internal wiring 21 is specified, but the other components may be the same as those of any of the first to third embodiments.

[0045] 7 , in the cross section of the internal wiring 21, the total length of the grain boundaries 34 may be longer in the intermediate region 26 than in the peripheral region 27. The distribution of the grain boundaries 34 in the peripheral region 27 may be zero. Furthermore, in the cross section of the internal wiring 21, the plurality of glass particles 35 may include connected particles 35E connected to the grain boundaries 34, and the peripheral region 27 may not include connected particles 35E.

[0046] The grain boundaries 34 are configured to extend in a planar manner in three-dimensional space and may extend obliquely along the longitudinal direction of the internal wiring 21. Furthermore, the grain boundaries 34 cause current resistance. Therefore, if there are many grain boundaries 34 in the peripheral region 27, the skin effect will increase conductor loss in areas where current density is high. Furthermore, the interconnected particles 35E generate resistance over a wide range within the internal wiring 21. Therefore, if the interconnected particles 35E are present in the peripheral region 27, the skin effect will increase conductor loss in areas where current density is high. On the other hand, the distribution of the grain boundaries 34 and the interconnected particles 35E in the wiring board 1C of embodiment 4 described above can reduce conductor loss in areas where current density is high due to the skin effect of the internal wiring 21. Therefore, the signal transmission characteristics of the wiring board 1C at high frequencies can be further improved.

[0047] The wiring board 1C having the configuration of the fourth embodiment can be manufactured by the manufacturing method shown in the example of the first embodiment.

[0048] 8 shows a cross-sectional view of a wiring board 1D according to a fifth embodiment of the present disclosure. The fifth embodiment differs in that a part of the cross-sectional outline of the internal wiring 21 serving as a film conductor is specified, but the other components may be the same as those of any of the first to fourth embodiments.

[0049] The internal wiring 21, which is a film-like conductor, has a shape in which the horizontal dimension is longer than the vertical dimension in the cross section of the internal wiring 21, and the outline of the end E21 in the horizontal direction may be a convex curve. That is, the end may be rounded without having sharp corners. Specifically, one end in the horizontal direction may be occupied by one crystal grain 33D.

[0050] This configuration can reduce the concentration of the electric field generated by the signal at the lateral end when the signal is transmitted through the internal wiring 21, which is a film-like conductor, and therefore reduce the conductor loss of the internal wiring 21.

[0051] The wiring board 1D having the configuration of the fifth embodiment can be manufactured by the manufacturing method shown in the example of the first embodiment.

[0052] Although the embodiments of the present disclosure have been described above, the wiring board of the present disclosure is not limited to the above-described embodiments, and the details shown in the embodiments can be appropriately modified without departing from the spirit of the invention.

[0053] An embodiment of the present disclosure is described below. In one embodiment, (1) a wiring board includes an insulating base made of ceramics and an internal wiring located inside the insulating base, the internal wiring including a sintered body of a plurality of crystal grains whose main component is copper and a plurality of glass particles, in a cross section perpendicular to the longitudinal direction of the internal wiring, the internal wiring has a central region, an intermediate region surrounding the central region and having a larger distribution of the glass particles than the central region, and an outer periphery region between the intermediate region and the outer periphery of the internal wiring and having a smaller distribution of the glass particles than the intermediate region, and the areal density of the glass particles is lower in the outer periphery region than in the central region and also lower than in the intermediate region.

[0054] (2) In the wiring substrate of (1) above, the glass particles are not distributed in the peripheral region.

[0055] (3) In the wiring board of (1) or (2) above, in a cross section perpendicular to the longitudinal direction of the internal wiring, the outer peripheral region is a region in which a plurality of the crystal grains in contact with the outer periphery of the internal wiring are connected in the circumferential direction.

[0056] (4) In the wiring substrate according to any one of (1) to (3) above, the crystal grains have a grain size of 1 μm to 5 μm.

[0057] (5) In the wiring board according to any one of (1) to (4) above, the plurality of glass particles include first glass particles having a particle size of 2 μm to 4 μm and second glass particles having a particle size of less than 2 μm.

[0058] (6) In the wiring board of (5), the second glass particles are scattered around the first glass particles.

[0059] (7) In the wiring board of (5) or (6) above, the first glass particles are distributed more in the intermediate region than in the central region.

[0060] (8) In the wiring board according to any one of (1) to (7), the plurality of glass particles include a single particle positioned singly.

[0061] (9) In the wiring board according to any one of (1) to (8), the plurality of glass particles include composite particles in which two or more of the glass particles are linked together so as not to form a ring.

[0062] (10) In the wiring board of any one of (1) to (9) above, the internal wiring includes grain boundaries that are located between a plurality of the crystal grains and contain silica, and in a cross section perpendicular to the longitudinal direction of the internal wiring, the total length of the grain boundaries is longer in the intermediate region than in the peripheral region.

[0063] (11) In the wiring substrate of (10), the outer peripheral region does not include the grain boundary.

[0064] (12) In the wiring substrate of (10) or (11), the plurality of glass particles include connected particles connected to the grain boundaries, and the peripheral region does not include the connected particles.

[0065] (13) In the wiring board of any one of (1) to (12) above, in a cross section perpendicular to the longitudinal direction of the internal wiring, the internal wiring has a shape in which the horizontal dimension is longer than the vertical dimension, and the outline of the end in the horizontal direction is a convex curve.

[0066] (14) In the wiring board of (13), one of the crystal grains occupies one end in the lateral direction.

[0067] The present disclosure can be used for wiring boards.

[0068] REFERENCE SIGNS LIST 1, 1A to 1D Wiring substrate 10 Insulating base 20 Wiring 21 Internal wiring 25 Central region 26 Intermediate region 27 Peripheral region 33, 33D Crystal grain 34 Grain boundary 35 Glass particle 35A First glass particle 35B Second glass particle 35C Single particle 35D Composite particle 35E Connected particle E21 End

Claims

1. A wiring board comprising an insulating substrate made of ceramics and internal wiring located inside the insulating substrate, wherein the internal wiring includes a sintered body of a plurality of crystal grains mainly composed of copper and a plurality of glass particles, and in a cross section orthogonal to the longitudinal direction of the internal wiring, the internal wiring has a central region, an intermediate region surrounding the central region and having a higher distribution of the glass particles than the central region, and an outer peripheral region between the intermediate region and the outer periphery of the internal wiring and having a lower distribution of the glass particles than the intermediate region, and the area density of the glass particles is lower in the outer peripheral region than in the central region and lower than in the intermediate region.

2. The wiring board according to claim 1, wherein no glass particles are distributed in the outer peripheral region.

3. The wiring board according to claim 1 or 2, wherein in a cross section orthogonal to the longitudinal direction of the internal wiring, the outer peripheral region is a region where a plurality of the crystal grains in contact with the outer periphery of the internal wiring are connected in the circumferential direction.

4. The wiring board according to any one of claims 1 to 3, wherein the crystal grains have a particle size of 1 μm to 5 μm.

5. The wiring board according to any one of claims 1 to 4, wherein the plurality of glass particles include first glass particles having a particle size of 2 μm to 4 μm and second glass particles having a particle size of less than 2 μm.

6. The wiring board according to claim 5, wherein the second glass particles are scattered around the first glass particles.

7. The wiring board according to claim 5 or 6, wherein the first glass particles are more distributed in the intermediate region than in the central region.

8. The wiring board according to any one of claims 1 to 7, wherein the plurality of glass particles include single particles located alone.

9. The wiring board according to any one of claims 1 to 8, wherein the plurality of glass particles include composite particles in which two or more of the glass particles are connected without forming a ring.

10. The internal wiring includes grain boundaries located between the plurality of crystal grains and containing silica, and in a cross section orthogonal to the longitudinal direction of the internal wiring, the intermediate region has a longer total length of the grain boundaries than the outer peripheral region. The wiring board according to any one of claims 1 to 9.

11. The wiring board according to claim 10, wherein the outer peripheral region does not include the grain boundaries.

12. The plurality of glass particles include connecting particles connected to the grain boundaries, and the outer peripheral region does not include the connecting particles. The wiring board according to claim 10 or claim 11.

13. In a cross section orthogonal to the longitudinal direction of the internal wiring, the internal wiring has a shape with a longer horizontal dimension than the vertical dimension, and the contour of the end portion in the horizontal direction is a convex curve. The wiring board according to any one of claims 1 to 12.

14. One of the end portions in the horizontal direction is occupied by one of the crystal grains. The wiring board according to claim 13.

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