Method for fabricating a varistor device and varistor device

Abstract

A method for fabricating a varistor device is presented. In an embodiment the method includes providing a base body for the varistor device, wherein the base body comprises a ceramic material, providing a basic material for a base metal electrode region on the base body, exposing the base body with the basic material to a temperature under a protective gas atmosphere such that the base metal electrode region is formed and firmly connected to the base body and completing the varistor device.

Claims

1. A method for fabricating a varistor device, the method comprising: providing a base body for the varistor device, wherein the base body comprises a ceramic material; providing the base body with a passivation, wherein the base body is provided with the passivation such that sites or surface regions of the base body remain free of the passivation; providing a basic material for a base metal electrode region on the base body, wherein the base body is provided with the passivation before the providing the basic material on the base body, and wherein the basic material is provided or applied in regions that are free of, or uncoated by, the passivation in order to provide one or more electrodes of the varistor device; exposing the base body with the basic material to a burn-in temperature, under a protective gas atmosphere, causing conversion of the base material into the base metal electrode region in the regions that are free of, or uncoated by, the passivation, such that the base metal electrode region is formed and firmly connected to the base body; and completing the varistor device by at least forming a coating or encapsulation.

2. The method according to claim 1, wherein, before providing the basic material on the base body, providing the base body with a passivation.

3. The method according to claim 2, wherein the base body is provided with the passivation such that sides or surface regions of the base body remain free and the basic material can, later on, be provided or applied in the free or uncoated regions in order to provide one or more electrodes of the varistor device.

4. The method according to claim 2, wherein providing the base body with a passivation comprises providing the base body with a raw material, and, after the base body is provided with the raw material, curing the raw material at temperatures from 300? C. to 600? C. in order to form the passivation.

5. The method according to claim 1, wherein providing the base body with the basic material comprises screen printing the base body with the basic material.

6. The method according to claim 1, wherein exposing the base body to the temperature comprises exposing the base body to the temperature in a furnace with zones of different temperatures.

7. The method according to claim 6, wherein the base body is exposed for a duration between 5 min and 30 min in a zone with temperatures between 450? C. and 800? C. such that the base metal electrode region is formed and firmly connected to the base body.

8. The method according to claim 1, wherein, after exposing the base body to the temperature, providing the base body with solder contacts and/or solder straps.

9. A varistor device comprising: a ceramic base body, wherein the ceramic base body comprises two base metal electrode regions each connected to a main surface of two or more main surfaces of the ceramic base body; an electrode comprising a base metal electrode region, wherein the base metal electrode region is directly connected to the ceramic base body; a passivation directly connected to the ceramic base body, wherein the passivation is disposed only at an edge surface of the ceramic base body, and wherein the edge surface connects the two or more main surfaces of the ceramic base body; and an outer coating provided on the ceramic base body.

10. The varistor device according to claim 9, wherein the base metal electrode region contains copper.

11. The varistor device according to claim 9, further comprising a passivation directly connected to the ceramic base body.

12. The varistor device according to claim 11, wherein the ceramic base body comprises two base metal electrode regions each connected to a main surface of the ceramic base body, wherein the passivation is only arranged at an edge surface of the ceramic base body, and wherein the edge surface connects the main surfaces of the ceramic base body.

13. The varistor device according to claim 11, wherein the passivation is a lead-free glass, a ceramic material and/or an inorganic material.

14. The varistor device according to claim 9, wherein the base metal electrode region is a layer with a thickness between 5 ?m and 30 ?m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic sectional view of a varistor device.

(2) Like elements, elements of the same kind and identically acting elements may be provided with the same reference numerals in the FIGURES. Additionally, the FIGURES may be not true to scale. Rather, certain features may be depicted in an exaggerated fashion for better illustration of important principles.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(3) FIG. 1 shows a schematic view of a varistor device 100 in a longitudinal section. The varistor device 100 may be a strap varistor and/or a disk varistor. The varistor device 100 comprises a base body 1. The base body 1 is, expediently made of a ceramic material. Furthermore, base body 1 comprises, preferably, a disc-like shape. A main extension direction of the disc may run horizontally in FIG. 1 and extend through main surfaces of the base body 1. The base body 1 comprises two main surfaces 7 (cf. e.g. left and right sides or faces in FIG. 1). The main surfaces 7 may relate to a front and back surface of the base body 1. The base body 1 further comprises one or more edge surfaces 6. Preferably, the edge surface 6 connects the main surfaces 7. According to the disk-like embodiment of the varistor device 100 or the base body, the edge surface 6 may further exhibit a circumferential surface of the base body 1.

(4) Additionally or alternatively, the base body 1 may comprise a plane shape. Preferably, the base body 1 comprises or consists of zinc oxide (ZnO). Actually, the varistor functionality such as the nonlinear resistive behaviour may be due to the ZnO.

(5) The varistor device 100 further comprises, preferably two, electrodes each of which applied to a main surface 7. Each of the electrodes may be constituted by a base metal electrode region 2. When it is referred to the electrode or base metal electrode region 2, it may automatically be referred to both of the electrodes 2 or base metal electrode region 2 shown in FIG. 1.

(6) The base metal electrode region 2 is, preferably, made of copper. Alternatively, the base metal electrode region 2 may be made of any other base metal. The base metal electrode region 2, preferably, comprises a thickness between 5 ?m and 30 ?m. The base metal electrode regions 2 are, preferably, not significantly oxidized and may comprise an oxygen content of less than 0.1 at % only.

(7) Although this is not explicitly indicated in FIG. 1, the electrode may also comprise further electrode materials or electrode layers, e.g. further metals which may act as a diffusion barrier for corrosive agents which may be present during the fabrication, e.g. during soldering of contacts to the varistor device 100. However, the base metal electrode region 2 is that region of the electrode which directly contacts the base body 1.

(8) The base body 1 of the varistor device 100 comprises an electrode surface with an area of 100 mm.sup.2 or more, preferably an area of 200 mm.sup.2 or more such as 400 mm.sup.2 or more. Said electrode surface (not explicitly indicated), preferably, pertains to the surface of the base body 1 which is connected to or covered by at least one of the base metal electrode regions 2. The electrode surface may coincide with the main surface 7 on each side of the base body 1.

(9) The varistor device 100 may further be designed for root mean square AC operating voltages of 25 V or more, preferably of 50 V or more such as 75 V or more.

(10) The varistor device 100 further comprises a passivation 3, preferably, a passivation layer, which is applied at the edge surface 6 of the base body 1, i.e. in FIG. 1 at the top and the bottom of the base body 1. The edge surface 6, preferably, comprise a smaller area as compared to the electrode surfaces or one the main surface 7 and may thus be more prone to degradation or corrosion during fabrication of the varistor device 100. The passivation 3, as shown in FIG. 1, is arranged at the edge surface 6 only.

(11) Alternatively, the passivation 3 mayalthough not being explicitly indicatedbe arranged at any site or outer side of the base body 1 except the sides or regions of the base body 1 in which the base metal electrode region is provided or applied to.

(12) The passivation may be or comprise a lead-free glass, a ceramic material and/or an inorganic material. The passivation is provisioned for a protection of the base body against chemical reactions and/or influences, e.g., of a protective gas or gas atmosphere such as chemical reduction during the fabrication of the varistor device 100.

(13) The varistor device 100 further comprises solder straps 4 which are soldered to the electrodes 2, e.g. at each side of the varistor device (cf. left and right lateral side in FIG. 1). The solder straps 4 are, preferably, made of tin (Sn). Although not explicitly indicated in FIG. 1, the electrodes 2 may comprise further electrode and/or solder materials. The varistor device 100 further comprises an outer coating 5.

(14) In the following, the fabrication method of the varistor device is described. Said fabrication comprises providing the base body 1 for the varistor device 100, providing the base body with a basic material for the base metal electrode region and exposing the base body 1 with the basic material to a temperature under a protective gas atmosphere such that the base metal electrode region 2 is formed and the base metal electrode region 2 is firmly connected to the base body 1 of the varistor device 100. To this effect, the basic material may be or comprise a metal paste. Preferably, the basic material further comprises a binder or binding agent.

(15) The basic material may be provided by screen printing or another printing method, for example.

(16) During fabrication of the varistor device, the base body 1 may subsequently be coated by a raw material for the passivation. Subsequently, the base body 1 may be cured or baked in order to form the passivation 3, then coated with the basic material for the base metal electrode region, dried, exposed to the temperature, soldered, e.g. to the solder straps 4, and coated with the outer coating 5.

(17) The solder straps 4 and/or said further solder contacts or layers can manually be soldered, soldered by dip soldering or reflow soldering, e.g. under evacuated and/or protective ambient or atmospheric conditions. Moreover, during soldering, flux materials and/or special lead-free solders, such as bars, pastes or wires may be used. In particular, the solder straps 4, may be bolts and/or bent or straight in shape. The method further comprises providing or coating of the so far fabricated or assembled components with the outer coating 5. The outer coating 5 may be an encapsulation and/or an organic or inorganic material, e.g. an epoxy resin.

(18) The exposing step can be or comprise a burn-in step for the basic material, by which said material is converted into the base metal electrode region, and at the same time mechanically connected to the base body 1. During the fabrication, further electrode materials may be deposited or applied to the base body 1.

(19) The exposing step is, preferably, carried out in a conveyor furnace or kiln, such as a belt-like kiln (not explicitly indicated in the FIGURE). Said furnace may expediently comprise a facility for applying a protective gas atmosphere, such as a high purity nitrogen with little air content. The conveyor furnace, preferably, comprises a heating zone, a high-temperature zone, a cooling zone and an outlet area. In the heating zone, the above-mentioned binder is preferably removed from the basic material. In the high-temperature zone, temperatures between 450? C. and 800? C. may expediently be applied, for the mentioned exposure or burning-in of the basic material. Preferably, the pre-fabricated base body is exposed to temperatures of the mentioned range for a duration between 5 min and 30 min. Duration and temperature may depend on the size of the respective device or base body. The thermal impact may need to be greater for larger devices as compared to smaller ones. In the cooling zone, the respective products may be cooled from the temperatures of the high-temperature zone, for example.

(20) Particularly, the passivation may be curedas mentioned aboveat temperatures between 300? C. and 600? C. for 10 min to 4 h, e.g. at 560? C. for 1 h.

(21) Particularly, the basic material may be dried in ambient air at temperatures between 100? C. and 300? C. for a duration of 2 min to 15 min, for example.

(22) In an embodiment, the varistor device may have a length of 33.7 mm, a diameter of more than 32 mm, a varistor voltage of 216 V to 264 V, a leakage current of 2 ?A, a flow capacity or voltage pulse shape of 8/20 ?s and/or an energy absorption tolerance of 2 ms.

(23) In an alternative embodiment, the varistor device may have a varistor voltage of 675 V to 825 V and/or a leakage current of more than 19 pA.

(24) The scope of protection is not limited to the examples given herein above. The invention is embodied in each novel characteristic and each combination of characteristics, which particularly includes every combination of any features which are stated in the claims, even if this feature or this combination of features is not explicitly stated in the claims or in the examples.