JP2000252152A - Laminated ceramic capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve breakdown voltage and reliability by adjusting the thickness of a dielectric and laminating it, so that the breakdown voltages of respective dielectric are substantially uniform. SOLUTION: A laminated body 2 is comprised of three kinds of dielectrics 6 (A, B, C), which are made of various dielectric materials (A', B', C') differing in the temperature properties of permittivities and inner electrodes 7 (7a-7d) formed respectively on one surfaces of the dielectrics A, B and C. The dielectric material A' of the dielectric A, for example, is 0.35PNN-0.65PMN, the dielectric material B' of the dielectric B is 0.95PMN-0.05PT, and the dielectric material C' of the dielectric C is 0.85PMN-0.15PT. (PNN: is Pb(Ni1/3Nb2/3)O3; PMN is Pb(Mg1/3Nb2/3)O3, and PT is PbTiO3). The film thickness of the respective dielectrics A, B and C after baking is about 20 to 40 μm per layer, and it is adjusted so that the breakdown voltages are substantially uniform.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multilayer ceramic capacitor.

[0002]

2. Description of the Related Art Multilayer ceramic capacitors include those having a ferroelectric material such as barium titanate (BaTiO ₃ ) as a main component as a dielectric material and those using a dielectric material containing lead (Pb). When an additive or the like is added to these dielectric materials, a change in capacitance due to a change in temperature can be reduced, but the dielectric constant (ε) decreases. For example, when the main component is a barium titanate-based material, JIS temperature characteristic standard B
In terms of characteristics, when the temperature range is −25 ° C. to 85 ° C. and the rate of change in capacitance is ± 10% in a state where no voltage is applied, the dielectric constant ε is about 3000. Further, when the temperature characteristics of the dielectrics are made different, the breakdown voltage generally decreases.
The breakdown voltage decreases because the dielectric loss factor (dielectric constant ε
× tan δ) also changes, and the dielectric loss factor of each dielectric becomes non-uniform.

Therefore, in order to improve the temperature characteristics of the dielectric constant, for example, a multilayer ceramic capacitor (hereinafter referred to as prior art 1) described in Japanese Patent Application Laid-Open No. 48-65446 has a dielectric loss factor similar to each other. Are laminated and fired with high dielectric ceramic thin plates having different temperature characteristics. Also, in this capacitor, the breakdown voltage is increased by selecting the dielectric loss factor of each ceramic thin plate to be substantially the same.

A multilayer capacitor described in Japanese Utility Model Laid-Open Publication No. 55-156425 (hereinafter referred to as Prior Art 2) is a set of two or more ferroelectric layers having different Curie points sandwiching a thin-film conductor layer. By stacking them together, the temperature characteristics are improved.

[0005]

However, the above prior arts 1 and 2 are all different dielectric materials, such as BaO, TiO ₂ , SiO ₂ , and AlO in the prior art 1.
_{Since the} dielectric ceramic thin plate was formed in ₃ , the mechanical and electrical properties of each material were different, and as a result, it was found that the load was concentrated on the weakest material. For example,
If the electric field strength of one of the constituent materials is lower than that of the other materials, a particularly large electric field is applied between the opposing electrodes of a certain material when a voltage is applied. As the breakdown voltage.

The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide a highly reliable multilayer ceramic capacitor having a high breakdown voltage.

[0007]

In order to achieve the above object, the present invention provides a multilayer ceramic capacitor comprising a plurality of different types of dielectrics having different dielectric constants having different temperature characteristics. It is characterized in that the dielectrics are laminated while adjusting the thickness so that the breakdown voltage becomes substantially uniform.

In the present invention, when a plurality of dielectrics on which internal electrodes are formed are laminated, and external electrodes are formed on the edges, the respective capacitors are connected in parallel in an electric circuit. A small and large capacity capacitor can be obtained. When dielectrics are formed of the same dielectric material, the same voltage is applied to each dielectric. On the other hand, when the dielectric is made of a different kind of dielectric material, a large electric field is applied between the opposite electrodes of the dielectric having a low electric field strength, and these opposite electrodes are destroyed. Therefore, by adjusting the thickness of the dielectrics so that the breakdown voltage of each dielectric becomes substantially equal, specifically, when a dielectric made of a material having a low electric field strength is formed thicker than a dielectric made of a material having a high electric field strength, The voltage distribution can be made uniform. Therefore, even if a dielectric is formed of different kinds of dielectric materials, a large voltage is not applied between the opposing electrodes of some of the dielectrics, and the breakdown voltage and reliability of the capacitor can be improved.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on an embodiment shown in the drawings. FIG. 1 is a sectional view showing an embodiment of the multilayer ceramic capacitor according to the present invention. In the figure, a multilayer ceramic capacitor 1 is
A laminate 2, a pair of external electrodes 3 provided on both end edges in the surface direction of the laminate 2, that is, the internal electrode extraction surfaces 4 a and 4 b, and an exterior body 5 for protecting the front and back surfaces of the laminate 2, respectively. It is composed of

The laminate 2 includes three types of dielectrics 6 (A, B, C) formed of different types of dielectric materials (A ′, B ′, C ′) having different dielectric constant temperature characteristics. , And internal electrodes 7 (7a to 7d) formed on one surface of these dielectrics A, B, and C, respectively. As the dielectric material A 'of the dielectric A, for example, 0.35 PNN-
0.65 PMN is used, and the dielectric material B ′ of the dielectric B is used.
For example, 0.95 PMN-0.05PT is used, and as the dielectric material C ′ of the dielectric C, for example, 0.
85 PMN-0.15PT is used. PNN is Pb
_{(Ni 1/3 Nb 2/3)} O 3, PMN is Pb (Mg _1/3 Nb
_2/3 ) O ₃ and PT are PbTiO ₃ . The thickness of each of the dielectrics A, B, and C after firing is 20 to 40 μm.
With m, the thickness is adjusted so that the breakdown voltage becomes substantially uniform. Specifically, the thickness is set to be thicker for a dielectric formed of a material having a lower breakdown voltage.

The internal electrode 7 is a conductive film made of a noble metal such as palladium and silver-palladium alloy, and is formed to a thickness of about 1 to 3 μm. In addition, each of the internal electrodes 7a to 7d
Are alternately arranged with respect to the left and right external electrodes 3, that is, one end edges of the odd-numbered internal electrodes 7a and 7c are exposed on one internal electrode extraction surface 4a of the laminate 2 and the even-numbered internal electrodes 7b and One end of 7d is exposed on the other internal electrode extraction surface 4b, and is electrically connected to the left and right external electrodes 3 respectively.

The external electrode 3 is formed by a conductor film layer. Such a conductor film layer is formed of silver or a silver-palladium alloy and glass particles.

The package 5 is made of the same dielectric material (A '), (B') or (C ') as the dielectrics A, B or C or a dielectric material completely different from these. I have.

In such a multilayer ceramic capacitor 1, the thickness of each of the dielectrics A, B, C formed of different dielectric materials (A ', B', C ') having different temperature characteristics of the dielectric constant. Is adjusted so that a substantially uniform breakdown voltage is applied, so that a large voltage is not applied between the opposing electrodes of some dielectrics having low electric field strength, and the voltage distribution can be made uniform. it can. Therefore, there is no possibility of the dielectric counter electrode having a low electric field strength being broken,
The breakdown voltage and reliability of the capacitor can be improved.

(Embodiment) Next, as an embodiment, a required number of dielectrics A, B and C formed by three kinds of dielectric materials (A ', B' and C ') having different temperature characteristics are laminated. A method for manufacturing a multilayer ceramic capacitor will be briefly described.
FIG. 2 shows the temperature characteristics of the dielectrics A, B, and C, and Table 1 shows the breakdown voltage (electric field strength).

[0016]

[Table 1]

First, a polyvinyl alcohol resin 10, toluene 15 and isopropyl alcohol 15 are added to the dielectric material 100 for each dielectric material (A ', B', C ') of each of the dielectrics A, B, C. After kneading with a ball mill for 24 hours, the mixture is applied on a PET film that has been subjected to silicon processing and dried to form a ceramic sheet. At this time, the thickness of each ceramic sheet is determined by each of the dielectrics A, B, and C.
And the thickness described in Table 1 calculated from the electric field strength of In addition, in order to set the JIS temperature characteristic standard of the capacitor made of such a ceramic sheet to the B characteristic, in this embodiment, the number of stacked ceramic sheets of different materials is set to the number shown in Table 1.

Next, three dielectric layers which do not include the internal electrodes serving as the outer package 5 are stacked. In this case, in the present embodiment, the dielectric layer is formed of the dielectric material (A '), but is formed of the dielectric material (B') or (C ') or a completely different dielectric material. You may.

Next, a 60 μm-thick green sheet made of a dielectric material A ′ in which a conductor film serving as the internal electrode 7 is formed by screen printing on a dielectric layer serving as the outer package 5 is provided. Twenty layers are stacked alternately.
Next, the internal electrodes 7 were placed on the laminated green sheets.
A green sheet having a thickness of 30 μm and made of a dielectric material B ′ in which a conductor film to be formed is formed by screen printing is laminated in ten layers so that the conductor films are alternately formed. further,
On top of this, 10 layers of 30 μm thick green sheets made of a dielectric material C ′ in which a conductor film serving as the internal electrode 7 is formed by screen printing are laminated such that the conductor films are alternately arranged. Then, three dielectric layers that do not include the internal electrodes serving as the exterior body 5 are superimposed thereon, and are integrated by thermocompression bonding. This is cut into a predetermined size to produce a green chip. Then, the green chip is fired at a predetermined temperature and integrated to form a laminate 2, and then the external electrodes 3
Was formed to produce a multilayer ceramic capacitor.

On the other hand, a capacitor in which each of the dielectrics A, B and C had a thickness of 30 μm and was laminated in ten layers was prepared as a comparative example in the same manner as in the example. The temperature characteristics of both the example and the comparative example satisfied the B characteristic of the temperature characteristic standard of JIS. This is reflow-soldered to a substrate made of glass epoxy resin, and 50 ° C at 60 ° C. and 90% RH.
A voltage of V was applied to perform a moisture resistance load test. The results are shown in FIGS. 3 (a) and 3 (b). As is apparent from this figure, the insulation of the capacitor of the comparative example decreased with time, whereas the insulation of the capacitor of the example did not show any abnormality.

In the embodiment described above, an example is shown in which the external electrode 3 is formed in a one-layer structure. However, the present invention is not limited to this, and the external electrode 3 may have two or more layers.

[0022]

As described above, according to the multilayer ceramic capacitor of the present invention, it is possible to make the voltage distribution uniform without applying a large voltage between the opposing electrodes of some dielectrics. . Therefore, the breakdown voltage and reliability of the capacitor can be improved.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an embodiment of a multilayer ceramic capacitor according to the present invention.

FIG. 2 is a diagram illustrating temperature characteristics of capacitance.

FIGS. 3A and 3B are diagrams showing a change in insulation resistance value in a moisture resistance load test.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Laminated ceramic capacitor, 2 ... laminated body, 3 ... external electrode, 5 ... exterior body, 6, ... dielectric, 7, 7a-7d ... internal electrode, A, B, C ... dielectric.

Claims (1)

[Claims] 1. A multilayer ceramic capacitor comprising a plurality of different types of dielectrics having different dielectric constants having different temperature characteristics, wherein the thickness of the dielectrics is adjusted so that the breakdown voltage of each of the dielectrics becomes substantially equal. A multilayer ceramic capacitor characterized by being adjusted and laminated.