JP4784107B2 - Manufacturing method of laminated part and laminated part - Google Patents

Description

  The present invention is a method of manufacturing a laminated component such as an inductor element built-in ceramic wiring board obtained by firing a laminate in which a plurality of green sheets each having a wiring pattern and a plain green sheet are laminated, and an inductor element, The present invention relates to a green sheet for laminated parts used in this method and a laminated part produced by this method.

  2. Description of the Related Art In recent years, with the improvement in performance of communication devices, various types of electronic components having various functions such as white diodes, flash memories, and image processing ICs are mounted on mobile phones. Since each of these electronic components is driven with a different voltage, an inductive step-up or step-down circuit using an inductor is employed for the power supply circuit. Each of these electronic components is composed of an IC, a diode, and an inductor. However, the circuit space is required to be narrowed, and in particular, a relatively large inductor is desired to be downsized. In order to increase the current flowing through the inductor, it is essential to reduce the wiring resistance in order to reduce the conductor loss.

In response to such demands, composite electronic components have been proposed in which electronic components such as ICs and diodes are mounted on the surface of a multilayer ceramic substrate with a built-in inductor (see, for example, Patent Document 1). Further, in order to realize low resistance, a method of forming a recess by press molding or the like in a region where a wiring pattern of a sheet is formed, and providing a conductor line in the recess (for example, Patent Document 2). And a method of equivalently increasing the cross-sectional area of the wiring by connecting a plurality of sheets having the same conductor pattern (see, for example, Patent Documents 3, 4, and 5).
Japanese Patent No. 2571389 JP-A-3-219605 Japanese Patent Laid-Open No. 10-144526 Japanese Patent Laid-Open No. 10-241982 Japanese Patent Laid-Open No. 11-273554

  In the method proposed in Patent Document 2, since a concave portion is formed in the sheet by press molding or the like, when a thin sheet having a thickness of about 40 μm is used, damage is applied and the density distribution inside the element is uneven. There is a problem. On the other hand, in the method of connecting a plurality of sheets formed with the same conductor pattern as proposed in Patent Documents 3, 4, and 5, the number of sheets on which the wiring pattern is printed increases, so that the lamination of the sheets In a ceramic wiring board with a thickness of 1 mm or less where the number of sheets is limited, the number of plain sheets above and below the coil decreases, the magnetic path cross-sectional area decreases, and a desired inductance cannot be obtained. There is a problem that it is lowered by magnetic saturation.

  It is conceivable to reduce the resistance by increasing the cross-sectional area of the conductor by printing a thick wiring pattern on the green sheet. However, in this method, the adhesion between the wiring pattern and the green sheet can be reduced during press molding. There is a problem that it is lowered and sufficient adhesion cannot be obtained and peeling occurs. When the pressing pressure is increased, the adhesion is improved by deformation of the green sheet directly under the wiring pattern, but the portion directly under the coil is pushed in, resulting in a density difference from the area other than the coil, and shrinkage during firing. Due to the difference in characteristics, there are problems such as deformation such as warpage and cracks around the coil. Further, since the characteristics of the magnetic material are sensitive to stress, there is a problem that core loss increases due to internal stress due to strain, and this core loss becomes a loss at the time of voltage conversion, resulting in a decrease in conversion efficiency.

The present invention has been made in view of such circumstances, and the deformation or generation of cracks during firing can be suppressed, it Ho及 beauty of production of a multilayer component can be suppressed the increase of the core loss by reducing the internal strain An object of the present invention is to provide a laminated part manufactured by this method.

A method for manufacturing a laminated component according to the present invention is a method of firing a laminated body in which a green sheet on which a wiring pattern is formed and a plain green sheet are laminated, and manufacturing a laminated component in which a coil is formed by the wiring pattern . The step of producing the green sheet on which the wiring pattern is formed includes a step of printing the wiring pattern on the green sheet, and a step of printing a paste-like ceramic material on at least a peripheral region of the wiring pattern on which the green sheet is printed. possess the door, in the step of printing the paste-like ceramic member, using a screen mask provided one or more rib-like masking pattern, the rib-like masking pattern, a squeegee direction of the screen printing It is characterized by being long .

A method for manufacturing a laminated component according to the present invention is a method of firing a laminated body in which a green sheet on which a wiring pattern is formed and a plain green sheet are laminated, and manufacturing a laminated component in which a coil is formed by the wiring pattern . The step of producing the green sheet on which the wiring pattern is formed includes a step of printing a paste-like ceramic material in a region other than the wiring pattern forming region of the green sheet, and a wiring pattern in the wiring pattern forming region of the green sheet. possess and printing the, in the step of printing the paste-like ceramic member, using a screen mask provided one or more rib-like masking pattern, the rib-like masking pattern, the screen It is long in the printing squeegee direction .

In the method for manufacturing a laminated part according to the present invention, after printing a wiring pattern on a plain green sheet, a paste-like ceramic material is provided at least in the peripheral region of the wiring pattern so as to eliminate the step between the wiring pattern and the green sheet. To print. Alternatively, a paste-like ceramic material is printed in advance in a region other than the wiring pattern formation region of the green sheet so that there is no step between the wiring pattern and the green sheet, and then the wiring pattern is printed. Therefore, there is no step between the wiring pattern and the green sheet, the deformation of the green sheet immediately below the wiring pattern is suppressed at the time of lamination pressing, and the density inside the laminated body becomes uniform. Accordingly, even when a thick wiring pattern is formed, the green sheet is not deformed at the time of press molding, and warping and cracks at the time of firing due to the non-uniform density distribution inside the laminate do not occur. Further, when printing a paste-like ceramic material, a screen mask provided with one or more rib-like masking patterns that are long in the screen printing squeegee direction is used. Therefore, the paste scraping phenomenon peculiar when printing a wide area is suppressed, and the paste-like ceramic material is printed with a thickness equivalent to the thickness of the wiring pattern.

The method for manufacturing a laminated part according to the present invention is characterized in that the rib-shaped masking pattern is formed in an inner region of a wiring pattern formed in a U-shape .

In the method for manufacturing a laminated part according to the present invention, the masking pattern of the screen mask to be used is formed in the inner region of the wiring pattern formed in a U-shape.

  The laminated component according to the present invention is manufactured by any one of the methods described above.

  In the multilayer component of the present invention, since internal strain is reduced, magnetic loss (decrease in inductance and increase in core loss) associated with internal stress is suppressed, and power loss is reduced.

Multilayer component according to the present invention, the thickness of the post-baked formation is at 1mm or less, the thickness of the wiring pattern after firing, characterized in that it comprises a coil is 20μm or more.

  The laminated component of the present invention has a coil having a thickness of 1 mm or less after firing and a thickness of the wiring pattern after firing of 20 μm or more. A thick wiring pattern can be obtained, the resistance can be reduced, and it can be used as a multilayer ceramic wiring substrate for a power supply through which a large current flows.

  In the method for manufacturing a laminated part according to the present invention, since the paste-like ceramic material is printed so as to eliminate the step between the wiring pattern and the green sheet in the peripheral region of the wiring pattern printing region, the wiring pattern is thickened. Even when the lamination press is performed, deformation of the green sheet around the wiring pattern can be suppressed, and the density inside the laminate can be made uniform. As a result, it is possible to suppress the occurrence of warpage and cracks during firing due to non-uniform density distribution inside the laminate.

In the method for manufacturing a laminated part according to the present invention, a paste-like ceramic material is printed using a screen mask having one or more rib-like masking patterns that are long in the screen printing squeegee direction . The paste scraping phenomenon can be suppressed, the paste-like ceramic material can be printed thickly over a wide area, and a thickness equivalent to the thickness of the wiring pattern can be easily ensured.

  In the multilayer component of the present invention, since internal strain is reduced, magnetic loss (decrease in inductance, increase in core loss) associated with internal stress can be suppressed, power loss can be reduced, and power conversion efficiency can be improved. In addition, when used as a multilayer ceramic wiring board for composite electronic components, the surface is flattened without deformation, so that an electrode for mounting electronic components and external electrode terminals can be manufactured with high dimensional accuracy. As a result, it is possible to manufacture composite electronic components with high yield and low cost.

  Hereinafter, the present invention will be described in detail with reference to the drawings showing embodiments thereof. First, a process for producing a green sheet for laminated parts, which is a characteristic part of the method for producing a laminated part of the present invention, that is, a step of printing a wiring pattern on a predetermined area of a plain green sheet, and at least a peripheral area of the wiring pattern The step of printing the paste-like ceramic material will be described with reference to some embodiments.

(First embodiment)
FIG. 1 is a cross-sectional view showing the manufacturing process of the first embodiment. A plain green sheet 1 formed by slurrying ferrite powder into a sheet is placed on a mold 10 and a wiring pattern 2 is printed on the green sheet 1 in a predetermined region using a silver paste (FIG. 1A )). Next, a screen mask 3 provided with a masking pattern 3a corresponding to the area where the wiring pattern 2 is printed is put on (FIG. 1B), and a paste-like ceramic is formed around the printed area of the wiring pattern 2. The ceramic layer 4 is formed by screen printing the material (FIG. 1C). In this way, a green sheet 20 for a laminated part having no step between the wiring pattern 2 and the green sheet 1 on which the ceramic layer 4 is printed is obtained.

(Second Embodiment)
FIG. 2 is a cross-sectional view showing the manufacturing process of the second embodiment. A plain green sheet 1 formed by slurrying ferrite powder into a sheet is placed on a mold 10 and a wiring pattern 2 is printed on the green sheet 1 in a predetermined region using a silver paste (FIG. 2A )). Next, a screen mask 3 for screen printing is applied (FIG. 2B), and a ceramic layer 4 is formed by screen printing a paste-like ceramic material over the entire area of the green sheet 1 including the printed area of the wiring pattern 2. (FIG. 2C). In this way, a green sheet 20 for a laminated part having no step between the wiring pattern 2 and the green sheet 1 on which the ceramic layer 4 is printed is obtained.

(Third embodiment)
FIG. 3 is a cross-sectional view showing the manufacturing process of the third embodiment. A plain green sheet 1 formed by slurrying ferrite powder into a sheet shape is placed on a mold 10 and covered with a screen mask 3 provided with a masking pattern 3a corresponding to a wiring pattern formation region (FIG. 3 ( a)) A ceramic layer 4 is formed by screen-printing a paste-like ceramic material in the peripheral region of the wiring pattern forming region on the green sheet 1 (FIG. 3B). Next, a screen mask 5 provided with a masking pattern 5a corresponding to an area other than the wiring pattern forming area is covered (FIG. 3C), and the wiring pattern 2 is printed using silver paste (FIG. 3D). ). In this way, a green sheet 20 for a laminated part having no step between the wiring pattern 2 and the green sheet 1 on which the ceramic layer 4 is printed is obtained.

(Fourth embodiment)
FIG. 4 is a cross-sectional view showing the manufacturing process of the fourth embodiment. A plain green sheet 1 formed by slurrying ferrite powder into a sheet is placed on a mold 10 and a wiring pattern 2 is printed on the green sheet 1 in a predetermined region using a silver paste (FIG. 4A )). Next, a masking pattern 3a is provided corresponding to a region where the wiring pattern 2 is printed, and a screen mask 3 provided with a rib-like masking pattern 3b at the center of a solid region outside the printed region of the wiring pattern 2 is covered. (FIG. 4B).

  FIG. 5 is a plan view of the screen mask 3, which is provided with a masking pattern 3 a that matches the shape of the wiring pattern 2 (U-shape) and has a rib shape that is long in the squeegee direction of screen printing. The masking pattern 3b is provided at a position substantially equidistant from the masking pattern 3a.

  Then, using such a screen mask 3, a ceramic layer 4 is formed by screen-printing a paste-like ceramic material in the peripheral area of the printed area of the wiring pattern 2 (FIG. 4C). In this way, a green sheet 20 for a laminated part having no step between the wiring pattern 2 and the green sheet 1 on which the ceramic layer 4 is printed is obtained.

(Fifth embodiment)
FIG. 6 is a cross-sectional view showing the manufacturing process of the fifth embodiment. A plain green sheet 1 formed by slurrying ferrite powder into a sheet is placed on a mold 10 and a wiring pattern 2 is printed on the green sheet 1 in a predetermined region using a silver paste (FIG. 6A )). Next, the screen mask 3 provided with a rib-like masking pattern 3b is put on the center of the solid area outside the printing area of the wiring pattern 2 (FIG. 6B), and the green sheet 1 including the printing area of the wiring pattern 2 is also covered. A ceramic layer 4 is formed by screen printing a paste-like ceramic material over the entire area (FIG. 6C). In this way, a green sheet 20 for a laminated part having no step between the wiring pattern 2 and the green sheet 1 on which the ceramic layer 4 is printed is obtained.

(Sixth embodiment)
FIG. 7 is a cross-sectional view showing the manufacturing process of the sixth embodiment. A plain green sheet 1 formed by slurrying ferrite powder into a sheet shape is placed on a mold 10 and the above-described masking pattern 3a according to the shape of the wiring pattern and the above-described rib-shaped masking pattern A screen mask 3 provided with 3b is placed on the green sheet 1 (FIG. 7A), and a ceramic layer 4 is formed by screen printing a paste-like ceramic material in the peripheral region of the wiring pattern formation region (FIG. 7). 7 (b)). Next, a screen mask 5 provided with a masking pattern 5a corresponding to a region other than the region where the wiring pattern is formed is covered (FIG. 7C), and the wiring pattern 2 is printed using silver paste (FIG. 7D). ). In this way, a green sheet 20 for a laminated part having no step between the wiring pattern 2 and the green sheet 1 on which the ceramic layer 4 is printed is obtained.

  The rib-shaped masking pattern 3b in the fourth to sixth embodiments is not limited to one, but corresponds to FIG. 8 (corresponding to the fourth embodiment) and FIG. 9 (corresponding to the fifth embodiment). ), A plurality of masking patterns 3b may be provided. The number of masking patterns 3b may be appropriately set according to the area of the solid region outside the printed region of the wiring pattern 2, the thickness of the wiring pattern 2, and the like.

  In the fourth to sixth embodiments, since the screen mask 3 provided with the rib-shaped masking pattern 3b is used, it is possible to suppress the paste scraping phenomenon that is peculiar when screen-printing the paste material over a wide area. As a result, in the peripheral region of the printed region of the wiring pattern 2, the ceramic layer 4 can be printed more uniformly and thicker than the first to third embodiments, and the thickness equivalent to the thickness of the wiring pattern 2 can be obtained. Can be easily secured.

  Specific examples will be described below. FIG. 10 is a flowchart showing the steps of the method for manufacturing a laminated part.

  First, the ferrite material is slurried (S1 in FIG. 10). For example, what knead | mixed ferrite powder, PVB, and a solvent is used as a ferrite material. This ferrite material is molded into a sheet having a thickness of, for example, 40 μm by the doctor blade method, and then cut into a size of, for example, 150 mm × 150 mm (S2 in FIG. 10). An through hole for alignment, a through hole for connecting a coil, and a through hole for a side electrode used in the laminating press process are formed on the obtained green sheet with an NC punch machine (S3 in FIG. 10).

  For example, a wiring pattern having a printing thickness of 20 to 40 μm and a printing width of 408 μm and a side electrode pattern are printed on the produced green sheet using a silver paste so as to obtain a predetermined DC resistance (S4 in FIG. 10). . The thickness and width of the wiring pattern after sintering are about 16 to 33 μm and 340 μm, respectively.

  After the wiring pattern is dried, a ferrite paste having a thickness of 20 to 40 μm, for example, is printed on the inner and outer peripheral portions of the wiring pattern by screen printing so as to eliminate the step between the wiring pattern and the green sheet (S5 in FIG. 10). . In place of the ferrite sheet, a non-magnetic ceramic powder (for example, a material having a thermal expansion coefficient of stabilized zirconia or the like matched with the base material ferrite) serving as a magnetic gap may be printed. In the printing process using the screen mask, since the variation in the printing position has an accuracy of about ± 30 μm, the pattern dimensions are determined so that the ferrite paste printing pattern and the wiring pattern coincide with each other with high accuracy. For example, the printing process may be performed so that these two patterns overlap each other by about 50 μm. When the ferrite paste is printed so as to cover the wiring pattern as in the second and fifth embodiments, the screen is provided with masking so as to exclude the connection portion with the upper and lower coil patterns and the side electrode portion. Use a mask.

  In the above example, the ferrite sheet is printed after the wiring pattern is printed on the green sheet. However, as in the third and sixth embodiments, the ferrite paste is printed on the green sheet first. Thereafter, the wiring pattern may be printed.

  In addition, when screen printing is performed on a large area, a phenomenon occurs in which the squeegee pushes the mesh and scrapes off the paste material, so that the printed thickness equivalent to the wiring pattern intended by the present invention cannot be obtained. Can happen. In such a case, a ferrite having a desired thickness can be obtained by using a screen mask provided with a rib-like masking pattern for suppressing the push of the squeegee as in the fourth to sixth embodiments. The paste can be printed. For example, as an example, a screen mask in which two strip-shaped masking patterns having a width of 45 μm and a length of 100 μm were provided in parallel at the center of the coil pattern was used.

  The green sheet on which the wiring pattern and the ferrite paste are printed and the plain green sheet on which the wiring pattern is not printed are laminated and pressed by a predetermined number of times to obtain a laminate (FIG. 10). S6). The number of laminated green sheets may be set so that the thickness of the laminated parts after firing is 1 mm or less and considering the thickness of the green sheets, wiring patterns, and ferrite paste.

The obtained laminate was divided so that the size after firing was 3.2 × 2.5 mm (area 8 mm ² ) (S7 in FIG. 10), and then fired at a temperature of 900 to 950 ° C. (FIG. 10). S8). Finally, the electrode terminal is plated with Au with Ni as a base (S9 in FIG. 10).

  Next, a comparison of characteristics between the multilayer component of the conventional example and the multilayer component of the present invention will be described. A laminated part of the present invention was manufactured by laminating 25 layers of plain green sheets on which a wiring pattern and a ferrite paste having a thickness of 30 μm were printed on a green sheet having a thickness of 40 μm and not printing them. In addition, when producing this green sheet for laminated parts, a case where a screen mask not provided with a rib-like masking pattern is used (Example 1), and a case where a screen mask provided with a rib-like masking pattern is used ( This was divided into Example 2). In the conventional example, a green sheet having a thickness of 40 μm printed with only a wiring pattern having a thickness of 30 μm is laminated.

  Table 1 below shows the measurement results of the characteristics of the laminated parts of these conventional examples, Example 1 and Example 2. In addition, the curvature amount of Table 1 measured the difference (AB) (micrometer) of the height A and the thickness B of the laminated component 30, as shown in FIG.

  Compared with the conventional example in which only the wiring pattern is printed, in Examples 1 and 2, there is no step between the wiring pattern and the green sheet, so that there is no deformation of the green sheet on the lower side of the press die during the lamination press. It was possible to press-mold at low pressure, and the adhesion was improved. As a result, the density of the portion directly under the coil and the peripheral portion of the coil became uniform, and surface deformation, warpage, cracking, and the like during firing could be suppressed, and internal stress due to strain could be reduced. Furthermore, when ferrite having a characteristic sensitive to stress is used, magnetic loss (core loss) can be reduced, and voltage conversion efficiency can be improved.

FIG. 12 is a cross-sectional view showing the laminated component of the present invention. The laminated component 30 has a configuration in which a predetermined number of laminated green sheets 20 on which the wiring pattern 2 and the ceramic layer 4 are printed and a plain green sheet 21 on which these are not printed are laminated. The cross-sectional area of the laminated component 30 is 8 mm ² or more, the thickness is 1 mm or less, and the thickness and width of the coil pattern (wiring pattern) are 20 μm or more and 330 μm or more, respectively.

  FIG. 13 is a perspective view showing a composite electronic component using such a multilayer component 30 as a multilayer ceramic wiring board. A surface electrode 31 is formed on the surface of the laminated component (laminated ceramic wiring substrate) 30 of the present invention, a bottom electrode 32 is formed on the bottom surface, and a side electrode 33 is formed on the side surface. The surface electrode 31 is an electrode for mounting an electronic component 35 such as a control IC, a diode, a capacitor, and a resistor. The electronic component 35 is connected to the surface electrode 31 via a wiring 34 or by direct bonding. It is mounted and molded with resin 36.

  In the above-described example, the case where the multilayer component is used as the multilayer ceramic wiring board has been described. However, the multilayer component of the present invention can also be used as an inductance element.

  As described above, in the present invention, since the paste-like ceramic material is printed so as to eliminate the step between the wiring pattern and the green sheet, the following effects can be obtained. Since there is no level difference, pressure is applied evenly during laminating press, and press molding can be performed at low pressure. Since the deformation of the green sheet directly under the coil can be suppressed, the density distribution in the laminate can be made uniform. As a result, the shrinkage characteristics at the time of firing become uniform, and warpage can be suppressed. Since the shrinkage characteristics around the coil are uniform, the generation of cracks can be suppressed and the quality is stabilized. Since the thick film printing of the wiring pattern by the sheet method can be realized and the resistance can be reduced to the same level as that of the winding inductor, the multilayer component can be applied as a wiring board for a power source through which a large current flows. In addition, a ceramic wiring board with a built-in inductor for low profile can be provided. Internal stress is reduced because internal strain is reduced. As a result, magnetic loss (core loss) can be reduced and voltage conversion efficiency can be improved. Since the influence of the internal stress on the magnetic characteristics can be reduced, the inductance design is facilitated. By reducing the internal stress, it is possible to suppress a decrease in inductance when DC is superimposed. Since the warpage can be reduced, the appearance inspection can be simplified. Thick film printing by the sheet method suitable for mass production becomes possible, so the cost can be reduced. Since there is no deformation, an electrode pattern for mounting an electronic component and an external electrode terminal can be produced with good dimensional accuracy. Composite electronic parts integrated with various electronic parts can be manufactured at a low cost with good yield.

  Further, since the screen mask provided with the rib-like masking pattern is used when printing the pasted ceramic material, the following effects can be obtained. The paste scraping phenomenon can be suppressed, the same thickness as the wiring pattern can be secured, and the thickness of the inner and outer peripheral portions of the coil can be made uniform. Since the thickness can be made uniform, the pressure distribution in the sheet surface can be equalized during the laminating press, and the warpage can be further reduced. Such a rib-shaped masking pattern is particularly effective when the area of the printing region is large.

It is sectional drawing which shows the manufacturing process of 1st Embodiment. It is sectional drawing which shows the manufacturing process of 2nd Embodiment. It is sectional drawing which shows the manufacturing process of 3rd Embodiment. It is sectional drawing which shows the manufacturing process of 4th Embodiment. It is a top view of a screen mask. It is sectional drawing which shows the manufacturing process of 5th Embodiment. It is sectional drawing which shows the manufacturing process of 6th Embodiment. It is sectional drawing which shows the modification of 4th Embodiment. It is sectional drawing which shows the modification of 5th Embodiment. It is a flowchart which shows the process of the manufacturing method of a laminated component. It is a figure for demonstrating the amount of curvature. It is sectional drawing which shows the laminated component of this invention. It is a perspective view which shows the composite electronic component using the laminated component of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Green sheet 2 Wiring pattern 3, 5 Screen mask 3a, 3b, 5a Masking pattern 4 Ceramics layer 20 Green sheet for laminated parts 30 Laminated parts

Claims (5)

In a method for producing a laminated component in which a coil is formed by the wiring pattern by firing a laminate in which a green sheet formed with a wiring pattern and a plain green sheet are laminated,
The step of producing the green sheet on which the wiring pattern is formed includes a step of printing the wiring pattern on the green sheet, and a step of printing a paste-like ceramic material on at least a peripheral region of the wiring pattern on which the green sheet is printed. It has a door,
In the step of printing the paste-like ceramic material, a screen mask provided with one or more rib-like masking patterns is used, and the rib-like masking patterns are long in the squeegee direction of screen printing. A method for manufacturing a laminated part, comprising: In a method for producing a laminated component in which a coil is formed by the wiring pattern by firing a laminate in which a green sheet formed with a wiring pattern and a plain green sheet are laminated,
The step of producing the green sheet on which the wiring pattern is formed includes a step of printing a paste-like ceramic material in a region other than the wiring pattern forming region of the green sheet, and a wiring pattern in the wiring pattern forming region of the green sheet. It possesses a step of printing a,
In the step of printing the paste-like ceramic material, a screen mask provided with one or more rib-like masking patterns is used, and the rib-like masking patterns are long in the squeegee direction of screen printing. A method for manufacturing a laminated part, comprising: 3. The method of manufacturing a laminated part according to claim 1, wherein the rib-shaped masking pattern is formed in an inner region of a wiring pattern formed in a U-shape .   A laminated part manufactured by the manufacturing method according to claim 1. Thickness after baked formation is at 1mm or less, the laminated component according to claim 4, the thickness of the wiring pattern after firing, characterized in that it comprises a coil is 20μm or more.

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