WO2002071825A2

WO2002071825A2 - Electrical component

Info

Publication number: WO2002071825A2
Application number: PCT/DE2002/000756
Authority: WO
Inventors: Axel Pecina
Original assignee: Epcos Ag
Priority date: 2001-03-06
Filing date: 2002-03-01
Publication date: 2002-09-12
Also published as: WO2002071825A3; DE10110680A1

Abstract

The invention relates to an electrical component comprising a base body (1), which contains a ceramic material, and comprises at least two contact layers (2, 3), which are provided on the surface of the base body (1). The portion of the surface of the base body (1) not covered by the contact layers (2, 3) is at least partially covered by a protective layer (4) that contains a spinel ceramic of the composition AB2O4. The inventive component provided with the protective layer made of a spinel ceramic is advantageous in that the electrical properties of the ceramic material, in particular, of a varistor ceramic material are not negatively influenced.

Description

description

Electrical component

The invention relates to an electrical component with a base body that contains a ceramic material and with at least two contact layers provided on the surface of the base body, in which the part of the surface of the base body that is free of contact layers is at least partially covered by a protective layer.

Components of the type mentioned at the outset are known which are used as varistors, which have high thermal stability and whose base body contains a mixture of metal oxides. Components of this type are usually soldered on boards in surface mounting and serve to protect various electronic components from voltage in electrical devices and electrical engineering devices.

The possibility of surface mounting (SMD capability) of the components is achieved by contacting the base body by baking a silver paste on the base body and then galvanizing the silver paste in a nickel and tin bath. The outermost tin contact layer guarantees the SMD capability. The contact layers are applied in usually acid baths, which can chemically attack the ceramic component. A protective layer prevents acid attack on parts of the surface of the ceramic component that are not to be coated with the contact layer. The protective layer also serves to reduce other external influences on the component. In particular, the protective layer prevents the plating of the surfaces of the component that are not covered by the burned-in silver paste.

Ceramic materials based on the perovskite structure type and lead are known as protective layers contain. Examples include Pb (Zr, Ti) 0 ₃ (PZT ceramic), (Pb, La) (Zr, Ti) 0 ₃ (PLZT ceramic) or (Pb, La) (Zr, Sn, Ti) 0 ₃ ( PLZST ceramics). These ceramics are used as protective layers for ceramic components, on the surface of which contact layers are to be applied galvanically.

The known protective layers have the disadvantage that they have a high lead content. The lead can easily penetrate into the interior of the ceramic base body and adversely affect the electrical characteristics of the component. In addition, the non-environmentally friendly and toxic lead accumulates in the sintering furnace necessary for the production of ceramic components.

Electrical components that contain a protective layer made from a PLZT ceramic also have the disadvantage that the protective layer can negatively influence the electrical properties of the component due to chemical reactions that take place between the protective layer and the base body of the component.

There are also known from the publication DE 196 34 498 C2 electrical components whose protective coatings contain barium titanate, silicon oxide or aluminum oxide. These protective casings have the disadvantage that they cannot be sintered together with ceramics, which are used, for example, for varistors, and the manufacture of such components is therefore complex.

The aim of the present invention is therefore to provide a component which has a protective layer which protects the component from external influences and which has a low lead content.

This goal is achieved according to the invention by an electrical component according to claim 1. Advantageous Events of the invention can be found in the further claims.

The invention relates to an electrical component with a basic body, which contains a ceramic material. The ceramic material is generally decisive for the function of the electrical component. At least two contact layers are provided on the surface of the base body, which serve to contact the component. The part of the surface of the base body which is free from the contact layers is at least partially covered by a protective layer which contains a spinel ceramic of the composition AB ₂ 0 ₄ .

The component according to the invention has the advantage that its protective layer consists of a material which can effectively protect the base body of the component from electroplating. Furthermore, the component according to the invention has the advantage that its protective layer contains no lead. A spinel ceramic is to be understood as a ceramic in which the A and B positions can be occupied by the elements magnesium, manganese, selenium, cobalt, nickel, zinc, aluminum or iron.

The component according to the invention also has the advantage that, due to the low lead content of the protective layer, the electrical properties of the ceramic component are not changed by lead penetrating into the base body. Due to the low lead content, the component according to the invention is also particularly environmentally friendly and not very toxic.

A varistor ceramic is particularly suitable as the ceramic material. The use of a varistor ceramic in the component according to the invention has the advantage that a spinel ceramic of a suitable chemical composition does not adversely affect the electrical properties of the component according to the invention. It is furthermore advantageous if the composition of the spinel ceramic is selected such that the spinel ceramic can be sintered together with the ceramic material of the base body. As a result, the production of the component according to the invention can be considerably simplified since the sintering and the production of the protective layer can take place in a single standard step. In the case of the joint sinterability of spinel ceramic and ceramic material of the base body, particular attention must be paid to the sintering temperature, the sintering curve (that is to say the time profile of the temperature during sintering), the sintering shrinkage and the coefficient of expansion.

It is also advantageous if the composition of the spinel ceramic is chosen so that it has a specific electrical resistance greater than 1 MΩcm. This can ensure effective insulation between the contact layers, so that short circuits in the protective layer can be avoided. In addition, a high electrical resistance of the spinel ceramic has the advantage that the galvanic deposition of metals on surfaces other than those provided for the contact layers of the base body can be avoided.

In connection with a varistor ceramic based on ZnO, it is particularly advantageous to use a ceramic with the composition ZnMn ₂ 0 ₄ as spinel ceramic. This spinel ceramic can be sintered together with the varistor ceramic mentioned, does not adversely affect the varistor ceramic chemically and offers sufficient mechanical stability, as is necessary, for example, for the galvanization of several electrical components in the bulk material due to the friction occurring there. In addition, a spinel ceramic of the abovementioned composition can be produced in a sufficiently dense manner, that is to say sufficiently free of pores, so that it effectively inhibits the access of components of an electroplating bath to parts of the surface of the base body covered by the spinel ceramic. Furthermore, a component that is produced by sintering a stack of superimposed ceramic green foils and electrically conductive electrode layers is particularly advantageous, the top and bottom green foils of the stack containing a spinel ceramic of the composition AB ₂ 0 ₄ . Such a component is particularly easy to manufacture by the standard process of sintering a stack of films. The production of the protective layer can be integrated into the production of the base body, which means that further production steps can be dispensed with. The prerequisite for this is that the material used as spinel ceramic can be processed into ceramic green foils (film drawability). The aforementioned spinel ceramic with the composition ZnMn ₂ 0 ₄ is particularly well suited for this.

In a further advantageous embodiment of the invention, a side surface of the stack is covered by a further protective layer which contains a ceramic of the composition (Pb, La) (Zr, Ti) 0 ₃ . The use of such a PLZT ceramic on one side surface of the layer stack has the advantage that it can be applied very simply by screen printing. Since the protective effect of a PLZT ceramic is achieved with smaller layer thicknesses than the protective effect of a spinel ceramic, the production of the further protective layer using a screen printing process is particularly advantageous. Larger layer thicknesses as would be necessary when using a spinel ceramic are no longer so easy to produce using the screen printing process, since there is a risk of the layer being detached.

It is also advantageous if the protective layer is designed in such a way that it inhibits the access of components of an electroplating bath to the parts of the surface of the base body which it covers. This ensures effective protection of the base body of the component against components of the electroplating, in particular the caustic acid baths. The 1 1 _d 1 ω 1 4J

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this enables the assembly of boards with large quantities and little effort.

The surface mountability of the component according to the invention can easily be achieved due to the protective layer.

The invention is explained in more detail below on the basis of an exemplary embodiment and the associated figures.

Figure 1 shows an example of a component according to the invention in a schematic perspective view.

FIG. 2A shows the component from FIG. 1 in a schematic cross section.

FIG. 2B shows the component from FIG. 1 in a schematic longitudinal section.

FIG. 3 shows the area of the contact layer of the component from FIG. 2B.

FIG. 4 shows the electrical characteristic curve of a component according to the invention according to FIG. 1 in comparison with the characteristic curve of a known component in a schematic illustration.

FIG. 5A shows a further example of a component according to the invention in the form of a varistor array.

FIG. 5B shows a first longitudinal section of the component from FIG. 5A.

FIG. 5C shows a second longitudinal section of the component from FIG. 5A.

FIG. 1 shows a ceramic component according to the invention, which has a base body 1 on which a first contact Layer 2 and a second contact layer 3 is arranged. The two contact layers 2, 3 are galvanically applied tin layers. With the help of these tin layers, the component can be soldered onto a printed circuit board in SMD assembly.

A protective layer 4, which contains ZnMn0 ₄ , is applied to the top of the base body 1. A further protective layer 7, which contains a PLZT ceramic and which is applied by screen printing, is applied to one side surface of the base body 1. The protective layer 4 is applied by laminating a ceramic green sheet on the top of the base body 1.

FIG. 2A shows the component from FIG. 1 in a schematic cross section. The base body 1 consists of a stack 5 of electrode layers 6 lying one above the other, which are separated from one another by ceramic layers 9. Palladium or silver is advantageously used as the material for the conductive electrode layers 6. These two precious metals have the advantage that they can be sintered in air, which means that an electrode-specific sintering atmosphere can be dispensed with.

The ceramic layers 9 contain a varistor ceramic, which is a zinc oxide ceramic doped with aluminum, cobalt and manganese oxide. Such a ceramic material has the advantage that it can be used in a varistor. A protective layer 4 is arranged on the top and on the bottom of the stack 5. The layers 6, 9 of the stack 5 are produced with the two protective layers 4 in a common sintering process. The protective layers 4 typically have a thickness of 20 to 40 μm

The principle of the varistor ceramic used for the base body is known per se to the person skilled in the art. The main component is zinc oxide, which is mixed with bismuth oxide, antimony oxide, cobalt oxide, manganese oxide and other metal oxides or 1 φ J n

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Starting from a zinc oxide ceramic doped with aluminum, cobalt and manganese oxide, the structure of which is controlled in the subsequent sintering process by further additives, especially Bi2θ ₃ Sb2θ ₃ , Siθ2, NiO and Cr2θ ₃ as well as some boric acid, the raw material mixture is processed into a slip and processed into ceramic green foils. The ceramic green foils are processed in a screen printing process using a precious metal paste suitable for sintering with the ceramic, with which the green foils are printed. This precious metal paste represents the sintered electrode layers 6.

The printed green foils are stacked, laminated and separated by cutting, from which green parts of a certain miniaturized design of multilayer varistors, essentially given by the geometric dimensions, result. In this process protective ceramic foils (protective layers 4) made of a spinel ceramic are used as the base and cover foil, which have a high electrical resistance, are chemically stable with respect to the electroplating baths to be used later and are adapted in their sintering behavior to the varistor ceramic.

After cutting, the green parts are printed on the respective cut sides with a screen printing paste consisting of the same passivating material. The green parts constructed in this way are sintered after careful debinding (removal of carbon residues) and then provided with a metal paste consisting of silver or a silver-containing alloy on the end faces in the area of the emerging electrode layers 6 by immersion. The silver or the silver-containing alloy is then burned in in a separate process step, as a result of which contact is made with the electrode layers 6 of the component. This silver layer also forms the starting layer 10 of the electroplating which, as shown in FIG. 3, is part of the second contact layer 3. The baked metal paste is then galvanically coated with a metal layer 8, which can be a nickel or tin layer or also a nickel / tin double layer, to produce the contact layers 2, 3, as a result of which the component's SMD capability is established. The protective layers 4, 7 prevent the caustic partial dissolution of the varistor ceramic and, moreover, the deposition of nickel or tin on the ceramic surface between the end faces of the component. During sintering, the protective layer 4 compresses largely free of tension and pores, so that the surfaces printed with the protective layer 4 are largely sealed and protect against the acid attack of an electroplating bath.

So that the protective layer can fulfill its protective function, it must have a layer thickness between 20 and 50 μm.

FIG. 4 shows the electrical characteristic curve 11 of a component according to the invention in comparison with the electrical characteristic curve 12 of a known component with a protective layer made of PLZT ceramic. In FIG. 4, the electrical current in the unit mA is plotted against the electrical voltage in the unit V. It can be clearly seen that the component according to the invention has a lower leakage current than the known component at low voltages, which is due to the reduced chemical influence of the varistor ceramic by the protective layer made of the spinel ceramic. In contrast, the characteristic curve 12 of the known component with a protective layer made of PLZT ceramic shows that the electrical resistance of the varistor is low at low voltages, which is disadvantageous due to the commonly used parallel connection of the varistor to the electrical circuit to be protected.

FIG. 5A shows a component according to the invention in the form of a varistor array in a perspective view. The construction ment has a base body 1, which is produced from stacked electrode layers and ceramic layers using multilayer technology. A protective layer 4, which contains a spinel ceramic of the composition AB ₂ 0 ₄ , is provided as the bottom or top layer of the layer stack. Several individual elements according to FIGS. 1 to 3 are now combined in a single ceramic body. For each of the individual elements, a pair of contact layers 2, 3 are provided, which are arranged on opposite outer surfaces of the base body 1.

FIG. 5B shows in a longitudinal section from FIG. 5A the arrangement of the electrode layers 6 which form the internal electrodes of the component. The electrode layers 6 are arranged in the form of four adjacent stacks 5. It can be seen from the longitudinal section according to FIG. 5C that the electrode layers 6 of each stack 5 lying in one plane are electrically conductively connected to an outer electrode 2. Each stack 5 of electrode layers 6 is assigned a pair of outer electrodes 2, 3. The outer electrodes 2, 3 are each electrically insulated from one another. FIG. 5C also shows the electrode layers arranged under the electrode layers 6 lying in the plane of the drawing in the form of dashed lines. The electrode layers marked with a dashed line are electrically conductively connected to contact layers 3, which are arranged on the side of the component opposite the contact layers 2. Electrodes lying one above the other partially overlap one another, with adjacent electrode layers being contacted with different contact layers 2, 3.

The protective layer shown in FIGS. 5A and 5B serves as passivation, which either protects the component from caustic electroplating baths or, if no galvanic reinforcement of the contact layers 2, 3 is necessary, contributes to increasing the surface resistance of the component.

The surfaces of the base body 1 free from the protective layers 4 can either remain free or can also be passivated by means of suitable processes using other materials.

The varistor array shown in FIGS. 5A to 5C contains several varistors, which are constructed according to FIGS. 1, 2A and 2B, side by side in one component.

The specified ceramic materials can optionally contain other conventional constituents in small amounts that do not impair the desired properties.

The invention is not limited to the exemplary embodiment shown, but is defined in its most general form by patent claim 1.

Claims

claims

1. Electrical component

- With a base body (1) containing a ceramic material,

- With at least two contact layers (2, 3) provided on the surface of the base body (1),

- In which the part of the surface of the base body (1) free of contact layers (2, 3) is at least partially covered by a protective layer (4) which contains a spinel ceramic of the composition AB2O4.

2. The component according to claim 1, wherein the ceramic material of the base body (1) contains a varistor ceramic.

3. Component according to one of claims 1 to 2, wherein the composition of the spinel ceramic is selected so that it is sinterable together with the ceramic material of the base body (1).

4. Component according to one of claims 1 to 3, wherein the composition of the spinel ceramic is selected so that the protective layer (4) has a specific electrical resistance greater than 1 MΩcm.

5. The component according to one of claims 1 to 4, wherein the composition of the spinel ceramic is ZnMn ₂ Ü4.

6. Component according to one of claims 1 to 5, which is produced by sintering a stack (5) of superimposed ceramic green foils and electrically conductive electrode layers (6), the top and bottom green foils of the stack (5) being a spinel ceramic of the composition contains AB2O4.

7. The component according to one of claims 1 to 6, in which one side surface of the stack (5) is covered by a further protective layer (7) which contains a ceramic of the composition (Pb, La) (Zr, Ti) 0 ₃ .

8. The component according to claim 7, wherein the further protective layer (7) is applied in the form of a screen printing paste.

9. The component according to one of claims 1 to 8, in which the protective layer (4) is designed such that it inhibits the access of components of an electroplating bath to the parts of the surface of the base body (1) it covers.

10. The component according to one of claims 1 to 9, wherein the contact layers (2, 3) contain galvanically applied metal layers (8).

11. The component according to one of claims 1 to 10, which contains arranged in the base body (1), one above the other electrically conductive electrode layers (6), which are separated by ceramic layers (9) and in which the adjacent electrode layers (6) with different contact layers ( 2, 3) are contacted.

12. The component according to one of claims 1 to 10, which contains a plurality of stacks (5) arranged in the base body (1) of superimposed, electrically conductive electrode layers (6) separated from one another by ceramic layers (9), in which the stack (5) side by side are arranged in the base body (1), in which adjacent electrode layers (6) of each stack (5) are contacted with different contact layers (2, 3) and in which each pair (5) is assigned a pair of contact layers (2, 3) which is electrically insulated from each further pair of contact layers (2, 3).