1. Field of the Invention
The present invention relates to a multilayer chip varistor.
2. Related Background Art
There is a known multilayer chip varistor having a varistor layer provided through firing to be integrated on a ceramic insulating substrate, a plurality of internal electrodes provided as opposed to each other with a part of the varistor layer in between, external electrodes provided on the exterior surface of the varistor layer, and through-hole conductors electrically connecting the external electrodes and the plurality of internal electrodes (e.g., cf. Japanese Patent Application Laid-open No. 2006-269876).
For producing the above-described multilayer chip varistor, through holes are formed in ceramic green sheets containing a ceramic powder whose major component is ZnO, and a conductor paste containing a metal (e.g., Ag or the like) as a major component is used to form conductor patterns for forming the internal electrodes, on the ceramic green sheets after the formation of the through holes. In the production process of the multilayer chip varistor, the through holes are filled with the conductor paste, and the ceramic green sheets are stacked in order and subjected to firing, whereby the ceramic green sheets become integrated. In this case, the varistor layer is made of the ceramic as a major component, whereas the internal electrodes and through-hole conductors are made of the metal. Therefore, a contraction percentage of the varistor layer in the firing is larger than that of the internal electrodes and through-hole conductors in the firing. Furthermore, the internal electrodes are formed so as to spread in a planar shape in the varistor layer; therefore, the internal electrodes contract in in-plane directions in the firing, whereas the through-hole conductors contract in directions of penetration of the through holes, i.e., in directions perpendicular to the contraction directions of the internal electrodes.
In the foregoing multilayer chip varistor, therefore, the internal electrodes and the through-hole conductors with the different contraction directions are connected near connection portions between the internal electrodes and the through-hole conductors. Furthermore, the internal electrodes and the through-hole conductors are so configured as to be surrounded by the varistor layer made of the component with the different contraction percentage from that of the internal electrodes and through-hole conductors. Therefore, the aforementioned multilayer chip varistor had the configuration wherein stress was readily generated in the firing, near the connection portions between the internal electrodes and the through-hole conductors. For this reason, the foregoing multilayer chip varistor may suffer cracks originating in the connection portions and the cracks could cause a connection failure between the internal electrodes and the through-hole conductors.
An object of the present invention is to provide a multilayer chip varistor capable of suppressing the production of cracks and thereby preventing the connection failure between the internal electrodes and the through-hole conductors.
A multilayer chip varistor according to the present invention is a multilayer chip varistor comprising: a varistor layer to exhibit a nonlinear voltage-current characteristic; a plurality of internal electrodes arranged as opposed to each other with the varistor layer in between; and a through-hole conductor formed in a through hole penetrating the varistor layer and the plurality of internal electrodes, the through-hole conductor electrically connecting the plurality of internal electrodes, wherein at least one of the plurality of internal electrodes is curved toward a direction of penetration of the through hole in a connection portion thereof to the through-hole conductor.
In the multilayer chip varistor according to the present invention, at least one of the plurality of internal electrodes is so configured as to be curved toward the direction of penetration of the through hole in the connection portion to the through-hole conductor. When the connection portion is curved toward the direction of penetration of the through hole, a region sandwiched between a curved surface of the connection portion and the through-hole conductor is formed in the varistor layer near the connection portion, on a one-side surface of the internal electrode. This region is sandwiched between the curved surface of the internal electrode and the through-hole conductor. Therefore, metal diffuses into the varistor layer to increase a metal concentration in this region, whereby the contraction percentage of this region in the firing becomes larger than that of the internal electrodes and the through-hole conductor and smaller than that of the varistor layer. Because of this effect, the region with the high metal concentration in the varistor layer acts to relax stress generated in the firing, which suppresses production of cracks originating in the connection portion. Furthermore, the suppression of production of cracks leads eventually to preventing a connection failure between the internal electrode and the through-hole conductor.
The multilayer chip varistor according to the present invention is preferably so configured that in the internal electrode curved in the connection portion, a thickness in a contact portion with the through-hole conductor is larger than a thickness in a portion other than the connection portion. This configuration increases the area of the contact portion of the internal electrode to the through-hole conductor and thereby further prevents the connection failure between the internal electrode and the through-hole conductor.
Another multilayer chip varistor according to the present invention is a multilayer chip varistor comprising: a varistor layer to exhibit a nonlinear voltage-current characteristic; a plurality of internal electrodes arranged as opposed to each other with the varistor layer in between; and a through-hole conductor formed in a through hole penetrating the varistor layer and the plurality of internal electrodes, the through-hole conductor electrically connecting the plurality of internal electrodes, wherein at least one of the plurality of internal electrodes is so configured in a connection portion thereof to the through-hole conductor that the varistor layer is sandwiched between the internal electrode and the through-hole conductor in a direction perpendicular to a direction of penetration of the through hole.
In the multilayer chip varistor according to the present invention, at least one of the plurality of internal electrodes is so configured in the connection portion to the through-hole conductor that the varistor layer is sandwiched between the internal electrode and the through-hole conductor in the direction perpendicular to the direction of penetration of the through hole. In the region of the varistor layer sandwiched between the connection portion of the internal electrode and the through-hole conductor, metal diffuses into the varistor layer to increase a metal concentration. Therefore, the contraction percentage of this region in the firing becomes larger than that of the internal electrodes and the through-hole conductor and smaller than that of the varistor layer. Because of this effect, the region with the high metal concentration in the varistor layer acts to relax stress generated in the firing, which suppresses production of cracks originating in the connection portion. Furthermore, the suppression of production of cracks leads eventually to preventing a connection failure between the internal electrode and the through-hole conductor.
The multilayer chip varistor according to the present invention is preferably so configured that at least one of the internal electrodes is depressed in the direction of penetration of the through hole so as to be tapered in the connection portion, whereby the varistor layer is sandwiched between the internal electrode and the through-hole conductor in the direction perpendicular to the direction of penetration of the through hole. In the simple configuration wherein the internal electrode is depressed so as to be tapered in the connection portion, the varistor layer is sandwiched between the internal electrode and the through-hole conductor in the direction perpendicular to the direction of penetration of the through hole.
The preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the description, the same elements or elements with the same functionality will be denoted by the same reference symbols, without redundant description.
A configuration of a multilayer chip varistor V1 according to the present embodiment will be described with reference to
As shown in
The external electrode 5 and the external electrode 6 are arranged with a predetermined space between them and on both longitudinal end sides of the principal face 2, on the rectangular principal face 2 of the varistor element body 1. The external electrodes 5, 6 are of a rectangular shape extending along the width direction of the principal face 2. The dimensions of the external electrodes 5, 6 are set, for example, as follows: a length of each long side approximately 600 μm; a length of each short side approximately 300 μm; a thickness approximately 2 μm.
The external electrode 7 and the external electrode 8 are arranged with a predetermined space between them and on both longitudinal end sides of the principal face 3, on the rectangular principal face 3 of the varistor element body 1. The external electrodes 7, 8 are of a rectangular shape extending along the width direction of the principal face 3. The dimensions of the external electrodes 7, 8 are set, for example, as follows: a length of each long side approximately 600 μm; a length of each short side approximately 300 μm; a thickness approximately 2 μm.
The external electrodes 5, 6 and the external electrodes 7, 8 are formed by transferring an electrode paste containing Ag or the like as a major component, onto the exterior surface of the varistor element body 1, firing it at a predetermined temperature (e.g., about 700° C.), and electroplating the fired electrode paste. The electroplating can be carried out using Ni/Au or the like.
The varistor element body 1, as shown in
The varistor layers 9 are made of a substance containing ZnO (zinc oxide) as a major component and also containing as minor components, simple metals such as rare-earth metals, Co, IIIb elements (B, Al, Ga, In), Si, Cr, Mo, alkali metals (K, Rb, Cs), and alkali earth metals (Mg, Ca, Sr, Ba), and/or oxides thereof. In the present embodiment, the varistor layers 9 may contain Pr, Co, Cr, Ca, Si, K, Al, and so on as minor components. Co and Pr serve as materials to make the varistor layers 9 exhibit the varistor characteristic. There are no particular restrictions on a content of ZnO in the varistor layers 9, but the content is preferably in the range of 69.0 to 99.8% by mass, where the total content of materials forming the varistor layers 9 is 100% by mass. In the present embodiment, the content of ZnO is particularly preferably not less than 95% by mass. The thickness of each varistor layer 9 is, for example, approximately in the range of 20 to 30 μm.
Each of the internal electrodes 11, as shown in
The electrode portions 15, as shown in
Each of the internal electrodes 21, as shown in
The electrode portions 25, as shown in
The internal electrodes 11, 21 contain an electroconductive material. There are no particular restrictions on the electroconductive material in the internal electrodes 11, 21, but it is preferably a material consisting of Ag or an Ag—Pd alloy. When the internal electrodes 11, 21 contain Ag, Ag in them can readily diffuse into ZnO in the varistor layers 9. The thickness of each internal electrode 11, 21 is, for example, approximately in the range of 1 to 5 μm and in the present embodiment the thickness is particularly preferably not less than 2 μm. This thickness range enables the internal electrodes 11, 21 to be well connected to the through-hole conductors 17, 27 and can keep the thickness of the internal electrodes 11, 21 enough even with the curvature of the connection portions between the internal electrodes 11, 21 and the through-hole conductors 17, 27 so as to prevent breakage of the internal electrodes (the details of which will be described later). The internal electrodes 11, 21 are constructed as sintered bodies of an electroconductive paste containing the aforementioned electroconductive material.
The through-hole conductors 17, 27 contain an electroconductive material. The electroconductive material in the through-hole conductors 17, 27 is preferably at least one metal selected from the group consisting of Pd, Ag, Cu, W, Mo, Sn, and Ni, or an alloy containing at least one of the foregoing metals. In the present embodiment, the electroconductive material contains Ag. When the through-hole conductors 17, 27 contain Ag, Ag in them can readily diffuse into ZnO in the varistor layers 9. The diameter of the through-hole conductors 17, 27 is, for example, approximately in the range of 10 to 500 μm.
The through-hole conductors 17, 27 are made by forming the through holes 10 penetrating each of the varistor layers 9 in the lamination direction, as shown in
The electrode portions 13 of the internal electrodes 11 and the electrode portions 23 of the internal electrodes 21 overlap with each other as described above. Therefore, overlap regions of the varistor layers 9 with the electrode portions 13 and the electrode portions 23 function as regions to exhibit the varistor characteristic. In the multilayer chip varistor V1 having the above-described configuration, one varistor section is constituted by the electrode portions 13, the electrode portions 23, and the overlap regions of the varistor layers 9 with the electrode portions 13 and electrode portions 23.
The below will detail the configuration near the connection portions of the internal electrodes with the through-hole conductors, with reference to
As shown in
As the curved connection portion 28 is so formed, a region T is defined as a region sandwiched between the internal electrode 21 and the through-hole conductor 27 in a direction perpendicular to the penetration direction of the through hole 10, in part of the varistor layer 9 near the connection portion 28 (which is a region indicated by a pear-skin pattern in
In the internal electrode 21, the thickness of a contact portion 28c with the through-hole conductor 27 is larger than the thickness of the portion other than the connection portion 28. The contact portion 28c of the internal electrode 21 is an inner peripheral surface of the through hole 10 in the internal electrode 21 and is a portion in surface contact throughout the entire circumference with the outer peripheral surface 27a of the through-hole conductor 27. In
The connection portion 28 curved in this manner is formed as follows: on the occasion of forming the through hole 10 in the unfired varistor layer 9 by punching or drilling, the edge of the varistor layer 9 around the through hole 10 is curved toward the penetration direction; on the occasion of applying the Ag paste for formation of the internal electrode 21, onto the varistor layer 9, the Ag paste is applied along the curvature of the edge around the through hole 10; it is fired simultaneously with the varistor layers 9 and the through-hole conductors 17, 27.
The below will describe the action and effect of the multilayer chip varistor V1 of the present embodiment with reference to
First, the conventional multilayer chip varistor will be explained for a comparison's sake. The conventional multilayer chip varistor is constructed by laminating a plurality of varistor layers 9 with internal electrodes thereon and physically and electrically connecting the internal electrodes through through-hole conductors, as the multilayer chip varistor V1 of the present embodiment is. However, the conventional multilayer chip varistor is different from the multilayer chip varistor V1 of the present embodiment in that the connection portions of the internal electrodes to the through-hole conductors are not curved. Specifically, as shown in
The conventional multilayer chip varistor as described above has such a configuration that the internal electrode 41, the through-hole conductor 47, and the varistor layers 9 are congested near the connection portion 48 of the internal electrode 41 to the through-hole conductor 47. The varistor layers 9 are made of the material containing ZnO as a major component, whereas the internal electrode 41 and through-hole conductor 47 are made of the metal containing Ag as a major component. A contraction percentage of the varistor layers 9 in the firing is different from a contraction percentage of the internal electrode 41 and through-hole conductor 47 in the firing.
The internal electrode 41 and the through-hole conductor 47 start contracting at a temperature lower than the varistor layers 9, and a contraction amount thereof at a temperature at the time of completion of the firing is smaller than that of the varistor layers 9. Namely, the varistor layers 9 have the contraction percentage in the firing larger than that of the internal electrode 41 and the through-hole conductor 47.
Furthermore, in the conventional multilayer chip varistor the internal electrode 41 is formed on a top surface of one varistor layer 9 and is formed so as to spread in a planar shape in directions perpendicular to the lamination direction inside the varistor element body. Therefore, the internal electrode 41 contracts in in-plane directions perpendicular to the lamination direction in the firing. On the other hand, the through-hole conductor 47 is formed so as to extend in the penetration direction of the through hole, i.e., in a direction coincident with the lamination direction. Therefore, the through-hole conductor 47 contracts in the penetration direction (lamination direction) of the through hole in the firing. As described above, the internal electrode 41 and the through-hole conductor 47 are configured so as to contract in their respective directions perpendicular to each other, in the firing.
In the conventional multilayer chip varistor, as described above, the internal electrode 41 and the through-hole conductor 47 to contract in the different directions are connected near the connection portion 48 of the internal electrode 41 to the through-hole conductor 47 and, the internal electrode 41 and through-hole conductor 47 are surrounded by the varistor layers 9 made of the component with the different contraction percentage; therefore, it is the configuration to easily generate stress in the firing. This may produce cracks originating in the connection portion 48 and the cracks could cause a connection failure between the internal electrode 41 and the through-hole conductor 47.
In contrast to it, the multilayer chip varistor V1 of the present embodiment has the configuration wherein the internal electrode 21 is curved toward the penetration direction of the through hole 10 in the connection portion 28 to the through-hole conductor 27, as shown in
The region T of the varistor layer 9 starts contracting at a temperature higher than the internal electrode 21 and through-hole conductor 27 and lower than the other region of the varistor layer 9. In the region T of the varistor layer 9, a contraction amount thereof at the temperature at the time of completion of the firing is larger than that of the internal electrode 21 and through-hole conductor 27 and smaller than that of the other region of the varistor layer 9. Namely, the region T of the varistor layer 9 has an intermediate contraction characteristic between the contraction characteristic of the internal electrode 21 and through-hole conductor 27 and the contraction characteristic of the other region of the varistor layer 9 in the firing.
As described above, the multilayer chip varistor V1 has the configuration wherein the region T having the intermediate contraction percentage between that of the conductor metal and that of the varistor layers 9 is arranged near the connection portions 28 where the internal electrode 21, through-hole conductor 27, and varistor layers 9 are congested to readily cause cracks. This permits the region T to act to relax the stress generated near the connection portions 28 in the firing, which suppresses production of cracks originating at the connection portion 28. Furthermore, the suppression of production of cracks leads eventually to also preventing a connection failure between the internal electrode 21 and the through-hole conductor 27.
In the multilayer chip varistor V1 of the present embodiment, the thickness of the internal electrode 21 in the contact portion 28c with the through-hole conductor 27 is larger than the thickness in the portion other than the connection portion 28. Therefore, the area of the contact portion 28c of the internal electrode 21 to the through-hole conductor 27 becomes larger, which further prevents the connection failure between the internal electrode 21 and the through-hole conductor 27.
In the multilayer chip varistor V1 of the present embodiment, the internal electrode 21 is depressed in the penetration direction of the through hole 10 so as to be tapered in the connection portion 28, whereby the varistor layer 9 is sandwiched between the internal electrode 21 and the through-hole conductor 27. This structure permits the varistor layer 9 to be sandwiched between the internal electrode 21 and the through-hole conductor 27 in the simple configuration wherein the internal electrode 21 is tapered in the connection portion 28.
The above described only the connection portion 28 of the internal electrode 21 with the through-hole conductor 27, but the same action and effect can also be achieved as to the connection portion of the internal electrode 11 with the through-hole conductor 17.
The above described the preferred embodiment of the present invention, but it should be noted that the present invention is by no means limited to the above-described embodiment and that the present invention can be modified in various ways without departing from the spirit and scope of the invention.
For example, the present embodiment showed the configuration wherein the connection portions 28 were curved toward the penetration direction from the principal face 3 to the principal face 2, but the connection portions may be curved toward the opposite direction. Namely, the connection portions 28 may be curved toward the penetration direction from the principal face 2 to the principal face 3.
The present embodiment showed the configuration wherein all the internal electrodes were curved in their connection portions with the through-hole conductors, but it is sufficient that at least one internal electrode be curved in its connection portion; for example, it is possible to adopt a configuration wherein only an internal electrode is curved at a position where cracks are particularly easily produced in the varistor element body.
The present embodiment showed the configuration wherein the region T of the varistor layer was sandwiched by curving the connection portion, but instead thereof, the varistor layer may be sandwiched by bending the internal electrode toward the penetration direction of the through hole in the connection portion. Namely, there are no restrictions on the shape of the connection portion, and any shape can be adopted as long as a part of the varistor layer can be sandwiched between the internal electrode and the through-hole conductor.
The present embodiment showed the configuration wherein the internal electrode 11 had the electrode portions 13, 15 and wherein the internal electrode 21 had the electrode portions 23, 25, but there are no particular restrictions on the shape of the internal electrodes; for example, it is possible to adopt internal electrodes of a rectangular shape, as shown in
Number | Date | Country | Kind |
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2008-221764 | Aug 2008 | JP | national |