DE10110680A1 - Electrical component - Google Patents

Abstract

The invention relates to an electrical component with a base body (1) which contains a ceramic material, with at least two contact layers (2, 3) provided on the surface of the base body (1), in which the part free of contact layers (2, 3) Surface of the base body (1) is at least partially covered by a protective layer (4) which contains a spinel ceramic of the composition AB¶2¶O¶4¶. The component according to the invention with a protective layer made of a spinel ceramic has the advantage that the electrical properties of the ceramic material, in particular a varistor ceramic material, are not adversely affected.

Description

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

There are components of the type mentioned that can be used as varistors that have a high thermal Have stability and their base body is a mixture of Contains metal oxides. Such components are becoming more common wise in surface mounting on boards and serve the voltage protection of various electronic components in electrical devices and electrical engineering devices.

The possibility of surface mounting (SMD capability) of the Components are achieved by contacting the base body pers by baking a silver paste on the body and then electroplating the silver paste in one Nickel and tin bath. The outermost tin contact layer is guaranteed the SMD capability. Applying the contact layers occur in usually acidic baths can attack the ceramic component chemically. By a protective layer is not an acid attack on the Contact layer to be coated parts of the surface of the ceramic component prevented. Protection also serves layer to reduce other external influences on the Component. In particular, the protective layer prevents this Plate the be not from the baked silver paste covered surfaces of the component.

Ceramic materials based on the perovskite structure type and containing lead are known as protective layers. Examples include Pb (Zr, Ti) O ₃ (PZT ceramic), (Pb, La) (Zr, Ti) O ₃ (PLZT ceramic) or (Pb, La) (Zr, Sn, Ti) O ₃ ( PLZST ceramic). 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 ei have a high lead content. The lead can easily get into that Penetrate inside the ceramic base body and the elec negatively influence the characteristic values of the component. Dar in addition, this enriches and is not environmentally friendly toxic lead in the manufacture of ceramic components necessary sintering furnace.

Electrical components that form a protective layer from a PLZT ceramics also have the disadvantage that the protective layer the electrical properties of the construction can negatively affect elements due to chemical Reactions between the protective layer and the main body of the Component take place.

There are also electri from the publication DE 196 34 498 C2 cal components known, their protective coatings barium tita contain nat, silicon oxide or aluminum oxide. This Protective coverings have the disadvantage that they are covered with ceramics, which are used for example for varistors, not ge are sinterable together and therefore the manufacture of such construction elements is complex.

The aim of the present invention is therefore a component specify that has a protective layer that the Bauele protects against external influences and the minor Has lead content.

According to the invention, this goal is achieved by an electrical construction element achieved according to claim 1. Advantageous configurations  the invention are the further claims refer to.

The invention relates to an electrical component with a base body, which contains a ceramic material. The Keramikma 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 O ₄ .

The component according to the invention has the advantage that its Protective layer consists of a material that forms the basic body of the component can effectively protect against electroplating. Desweite Ren has the advantage of the component that is ne protective layer contains no lead. Under a spinel Ceramic is to be understood as a ceramic in which the A- Places and B places with the elements magnesium, manganese, Se len, cobalt, nickel, zinc, aluminum or iron can.

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

In particular, a varistor comes as the ceramic material ceramic into consideration. The use of a varistor ceramic in the component according to the invention has the advantage that a Spinel ceramic with a suitable chemical composition the electrical properties of the component according to the invention not negatively affected.  

It is also advantageous if the composition of the Spinel ceramic is chosen so that the spinel ceramic with the ceramic material of the base body can be sintered together. This enables the construction of the component according to the invention elements are significantly simplified since the sintering and the Production of the protective layer in a single standard step can take place. With the common sinterability of spinel ceramic and ceramic material of the base body in particular on the sintering temperature, the sintering curve (the is called the time course of the temperature during sintering), to pay attention to 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 has electrical resistance greater than 1 MΩcm. Thereby can provide effective insulation between the contact layer ten are taken care of, so that short circuits in the protective layer can be avoided. It also has a high elec trical resistance of the spinel ceramic has the advantage that the electrodeposition of metals on other than for the surfaces of the base body provided for the contact layers can be avoided.

In connection with a varistor ceramic based on ZnO, it is particularly advantageous to use a ceramic of the composition ZnMn ₂ O ₄ as spinel ceramic. This spinel ceramic is terbar together with the varistor ceramics mentioned, does not influence the varistor ceramics chemically to a certain extent and offers sufficient mechanical stability, as is necessary, for example, for the electroplating of several electrical components in the bulk material due to the friction occurring there. In addition, a spinel ceramic of the composition mentioned can be made sufficiently dense, that is to say sufficiently non-porous, 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.

It is also a particularly advantageous component that is produced by sintering a stack of superimposed ceramic green foils and electrically conductive electrode layers, the top and bottom green foils of the stack containing a spinel ceramic of the composition AB ₂ O 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 of the composition ZnMn ₂ O ₄ is particularly suitable 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) O ₃ . The use of such PLZT ceramic on a side surface of the layer stack has the part before 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 is achieved, the production of the further protective layer using a screen printing process is particularly advantageous. Larger layer thicknesses as would be necessary if a spinel ceramic were used 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 so is that they allow access to components of a Electroplating bath to the parts of the surface covered by it inhibits the body. This will provide effective protection of the basic body of the component in front of components of the galva nik, especially the caustic acid baths guaranteed. The  Execution of the protective layer as protection against the electroplating bath can be done in particular by making them as porous as possible free, that means without cavities if possible and also as possible is produced without continuous channels.

Furthermore, components are advantageous in which the Kon Clock layers galvanically applied to the base body contain layers. Such components have the front part that the contact layers for a large number of construction elements are simultaneously galvanically produced in the bulk material can be. This allows a large number of building elements can be manufactured easily and cheaply. Electroplated brought metal layers can, for example, a nickel or be a layer of tin. It also gets beyond that a double layer in question, with the inner layer made up of Nickel and the outer layer is made of tin.

In addition, it is advantageous if the base body of the Component superimposed, electrically conductive elec contains layers of ceramics separated by ceramic layers which are separated, with adjacent electrode layers with different contact layers are electrically contacted. By executing the electrode layers in the form of in interlocking combs, the electrical resistance of the electrical component can be greatly reduced, which at is advantageous for a varistor, for example.

Varistors are used as components for voltage protection parallel to electronic circuits on boards in the Computer and communication technology used. When it occurs they conduct the resulting currents from overvoltages me, which is why a low ohmic resistance is advantageous is.

In all of these applications, the surface mountability of electrical components of particular importance since only  thereby the assembly of boards with large quantities and succeed little effort.

The surface mountability of the component according to the invention Due to the protective layer, it can be easily obtained.

In the following, the invention is illustrated by means of an embodiment game and the associated figures explained in more detail.

Fig. 1 shows an example of a component of the invention in schematic-perspective view.

Fig. 2A shows the component of FIG. 1 in a schemati's 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 of a device according to the invention according to FIG. 1 in comparison with the characteristic of a known component in a schematic representation.

Fig. 5A shows a further example of an inventive SLI component 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 an inventive ceramic component, comprising a basic 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 to a printed circuit board in SMD assembly.

A protective layer 4 , which contains ZnMn ₂ O ₄ , is applied to the upper side of the base body 1 . On a side surface of the base body 1 , a further protective layer 7 is applied, which contains a PLZT ceramic and which is brought on by screen printing. 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 of Fig. 1 in schematic cross section. The basic body 1 consists of a stack 5 of electrode layers 6 lying one above the other, which are separated by ceramic layers 9 from one another. 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 made 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 Varistorkera used for the base body Mik is known to the person skilled in the art. As the main ingredient Zinc oxide is used, which with bismuth oxide, antimony oxide, cobalt oxide, manganese oxide and other metal oxides or  other metal compounds are added and sintered. It other types of recipes can still be used, where bismuth is replaced by praseodymium or barium. The mentioned materials have a strongly non-linear electrical cal current-voltage characteristic and can be more advantageous be used as a varistor.

In order not to have to apply excessive layer thicknesses on the side faces of the layer stack 5 , these are covered with further protective layers 7 made from a PLZT ceramic, in which the protective effect against an electroplating bath can be achieved with smaller layer thicknesses than with a spinel ceramic. Due to the smaller layer thickness (typically 5 to 15 μm), the further protective layers 7 can advantageously be applied using the screen printing method. It is also possible, however, to cover all parts of the surface of the base body 1 not covered by contact layers 2 , 3 with a protective layer 4 made of spinel ceramic.

Fig. 2B shows the ceramic component of FIG. 1 with egg nem ceramic base body 1. A first contact layer 2 and a second contact layer 3 are applied to two opposite sides of the base body 1 . These contact layers 2 , 3 give the component its SMD capability. To protect the base body 1 from the acids in the electroplating bath acting on the base body 1 during the application of the contact layers 2 , 3 , the protective layers 4 also visible in FIG. 2A are arranged on the top and bottom of the base body 1 .

Inside the base body 1 there are electrodes 6 which consist of a mixture of silver and palladium and which reduce the ohmic resistance of the component.

The component shown in FIGS. 2A and 2B is a varistor that is manufactured according to the method described below:
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 Bi ₂ O ₃ Sb ₂ O ₃ , SiO ₂ , NiO and Cr ₂ O ₃ and some boric acid. the raw material mixture is processed into a slip and processed into ceramic green foils. The processing of the ceramic green foils is done in a screen printing process with 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 th, essentially given by the geometric dimensions ge, miniaturized design of multilayer varistors arise. In this process, protective ceramic films (protective layers 4 ) made of a spinel ceramic are used as the base and cover film, 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 a separate process step, whereby the contact to the electrode layers 6 of the component is Herge.

This silver layer simultaneously forms the starting layer 10 of the electroplating, which, as shown in FIG. 3, is part of the two 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 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, while the electric current is plotted in units of mA versus the electric voltage in the unit of 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 an inventive device in the form of a varistor array in a perspective view. The component has a base body 1 which is made of multilayered electrode layers and ceramic layers in multilayer technology. As the bottom or top layer of the layer stack, a protective layer 4 is provided, which contains a spinel ceramic composition AB ₂ O ₄ . Here are now several individual elements shown in FIGS. 1 to summarized in a single ceramic body to 3. 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 of 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 . From the longitudinal section according to FIG. 5C shows that the electrode layers lying in a plane 6 of each stack 5 are connected electrically conductively with 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. In Fig. 5C are moreover shown below the sign in the plane of the electrode layers 6 electrode layers arranged in the form of dashed lines. The electrode layers marked with gestri Chelter line are electrically conductively connected to contact layers 3 formed on the contact layers 2 are arranged opposite side of the Bauele management. Overlapping electrodes 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 provides protection of the component against 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 free of the protective layers 4 surfaces of the base body 1 can either remain free or can also be passivated using suitable methods using other materials.

The varistor array shown in FIGS. 5A to 5C contains a plurality of 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 common ingredients in small quantities, that do not affect the desired properties.

The invention is not limited to the illustrated leadership example, but is in its most general form defined by claim 1.

Claims (12)

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 from contact layers ( 2 , 3 ) is at least partially covered by a protective layer ( 4 ) which contains a spinel ceramic of the composition AB ₂ O ₄ . 2. Component according to claim 1, wherein the ceramic material of the base body ( 1 ) contains a vari storkeramik. 3. Component according to one of claims 1 to 2, in which the composition of the spinel ceramic is selected such 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's 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 O ₄ . 6. The 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 -Ceramics of composition AB ₂ O ₄ contains. 7. The component according to one of claims 1 to 6, wherein a 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) O ₃ . 8. The component according to claim 7, wherein the further protective layer ( 7 ) in the form of a screen printing paste is applied. 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 ), superimposed electrically conductive electrode layers ( 6 ), which are separated from each other 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 electrically superimposed electrode layers ( 6 ), one above the other, separated by ceramic layers ( 9 ), in which the stack ( 5 ) are arranged side by side 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 stack ( 5 ) has a pair of contact layers ( 2 , 3 ) is assigned, which is electrically isolated from each further pair of contact layers ( 2 , 3 ).