IES84552Y1 - A varistor and production method - Google Patents

Description

A Varistor and Production Method INTRODUCTION Field of the Invention The invention relates to metal oxide varistors (MOVS).

Prior Art Discussion Manufacture of a MOV typically involves sintering metal oxide ceramic powder to provide a disc (may alternatively be square or other shapes) body, tiring electrodes onto the disc body, attaching leads typically by soldering, and encapsulating. It is well known that the choice of encapsulant is critical to ensure good electrical stability over time. Many encapsulation materials will lead to faults involving increased leakage and/or a drop in the nominal voltage when subject to a biased elevated temperature test, typically called an Accelerated Life Test, e.g. 125°C at rated bias voltage for l00Ohours.

A conventional approach to addressing this problem is to develop or select a specific encapsulation material which does not exhibit this problem when used with the particular ceramic material. However, this is not always possible, and it is often not possible for the manufacturing process to guarantee consistent avoidance of faults.

Also, developing a custom encapsulant material is time-consuming and often results in a non-standard material with associated impact on unit cost.

Another approach is to apply a passivation to the exposed surface of the disc to prevent encapsulant/ceramic surface interaction. However, applying a passivation material requires an awkward additional step in the manufacturing process, and it is often difficult to achieve good uniformity of passivation coverage. One known method for applying a passivation coating to a MOV disc involves rolling the discs on rollers which are in contact with a reservoir of the passivation material. Another method "' 384552 involves stacking the discs and rotating the stack in the path of a spray gun which is spraying the passivation material. These methods have the drawback of being difficult to control consistently. Also, handling and unit sticking can lead to defects in the passivation coating and even in the disc itself.

The invention is therefore directed towards providing an improved method of preventing encapsulant/ceramic interaction from causing faults.

SUMMARY OF THE INVENTION According to the invention, there is provided a method of manufacturing a varistor, the method comprising the steps of: providing a ceramic body having opposed planar faces with face edges; applying an electrode to each face while leaving a gap between each electrode and the edge of the face; applying passivation material to at least one face in said gap, the passivation material not extending from one electrode to the other around the ceramic body; applying leads to the electrodes, and encapsulating the body, the electrodes, and the passivation.

In one embodiment, the passivation material is applied in a band around at least one of the electrodes.

In one embodiment, the passivation material is applied as a band around each electrode.

In one embodiment, the passivation material also overlies at least part of an electrode.

In one embodiment, the passivation material comprises glass paste.

In a further embodiment, the passivation material is applied by printing.

In one embodiment, the passivation material is applied by screen printing.

In one embodiment, the ceramic body is supported in a nest plate during printing.

In one embodiment, the electrodes are applied by printing electrode paste and firing, and the ceramic body is supported during printing of the passivation material in the same nest plate as is used during printing of electrode paste.

In one embodiment, the method comprises the further step of stacking the varistor with at least one other varistor.

In one embodiment, the passivation has a depth to ensure avoidance of contact between a lead and a ceramic body in proximity to the lead.

According to another aspect, the invention provides a varistor comprising: a body of ceramic material having opposed faces with face edges: an electrode on each face, in which there is a gap between at least one electrode and the edge of the face; passivation material on at least one face in said gap, the passivation material not extending from one electrode to the other electrode around the ceramic body; leads connected to the electrodes, and encapsulant surrounding the ceramic body, the electrodes, and the passivation material.

In one embodiment, the passivation is in a band around at least one of the electrodes.

In one embodiment, there is a band on both of the opposed faces.

DETAILED DESCRIPTION OF THE INVENTION Brief Description of the Drawings The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:- Fig. l is a diagram showing our understanding of the cause of some faults in prior art varistors; Fig. 2 is a perspective view of a passivated disc before application of leads and encapsulation, showing the manner in which passivation has been applied to the disc; Fig. 3 is a diagrammatic cross-sectional view of the completed varistor (omitting the leads, for clarity); Fig. 4 is a diagrammatic cross-sectional view ofa protection product comprising a stack of varistors; Fig. 5 is a flow diagram illustrating steps for applying passivation in a manufacturing process for varistors; Figs. 6 is a photograph showing a nest plate containing discs, some before and some after screen printing; and Fig. 7 is a plot shovtdng a passivation ("glass") firing profile.

Description of the Embodiments Referring to Fig. I a prior art varistor 1 has a ceramic body ("disc") 2, disc-shaped silver electrodes 3 top and bottom. top and bottom leads 4 and 5, and encapsulation 6.

Our understanding of the problem set out in the Introduction above is that there is effectively a conductive path along the disc surface which is less resistive (R) than a path through the disc 2. This surface conductive path arises from interaction of the encapsulant and the exposed ceramic body surface from the edge of the top electrode 3 radially out, then down the curved disc edge, and then radially in to the other electrode 3, when the varistor is subjected to an accelerated life test. While the prior art approach of passivating all of the disc surfaces addresses this problem, it introduced awkward additional processing steps which themselves tended to cause other problems or at least added considerable additional expense to the manufacturing process.

In the invention, and referring to Figs. 2 and 3, a partially manufactured varistor 10 of the invention comprises a disc l2 and top and bottom electrodes 13 as is conventional.

However, the ring-shaped planar surfaces of the body l2 which are not covered by the electrodes 13 are coated with passivation material 14, and this passivation material does not extend from one face to the other. The passivation material 14 is screen printed and cured. It covers the band of exposed planar disc surface around each electrode 13, and also overlaps the electrode 3, as shown most clearly in Fig. 3.'l‘his diagram also shows encapsulation 15.

The planar passivation 14 breaks a link at both planar surfaces between the silver electrode 13 and the edge of the disc to break any conductive path which might exist between the electrodes around the ceramic/encapsulant interface. This diagram shows diagrammatically a resistive link R on the disc edge, however this is isolated from both electrodes 13 by the passivation 14.

In this embodiment, there is a band of passivation on both sides of the disc, however, in some cases only one band is sufficient to ensure that the conductive path is adequately broken. The decision depends on the nature of the encapsulant being used and the expected operating and test conditions of the varistor.

Referring to Fig. 4, in another embodiment, in building a stack of varistors there is a potential problem whereby the terminal between the elements may rest on the MOV surface at the edge of the unit. Given the conventional soldering assembly process there may also be a layer of flux material at this point. It has been found that this can lead to an electrical failure whereby there is a conduction path formed from the edge of the MOV surface, where the terminal is in contact, along the outside edge of the MOV element to the opposing terminal. This failure has been found to occur under electrical stress testing. Fig. 4 illustrates a stack 16 of varistors 17 having electrodes 18 and passivation 19 is applied to each disc. This ensures that terminals 20 are not in direct contact with the discs and also reduces the possibility for the flux material to flow to the edges of the discs.

Referring to Figs. 5 and 6 the passivation material is applied in a screen printing process. In this process the units are loaded (21) into a nest plate (Fig. 4). a plate which has multiple locations machined to suit the disc dimensions so that each disc will be precisely located on the plate. The screen design is such that the openings in the screen are in an annular pattern of such dimensions to match the size of the disc diameter and to ensure that the passivation deposited extends over the edge of the silver electrode on the disc, as shown in Fig. 3. The screen and nest plate are aligned in step 22. The location of the annular pattern matches the locations of the discs in the nest plates when both are registered on the printing machine. The screen mesh and emulsion parameters along with the passivation material, solids loading, and viscosity mainly determine the thickness of the passivation which will be laid down (23) on the disc surface. A typical emulsion thickness is 10pm. A typical passivation material has a solids loading of 60 — 80% and a viscosity of 25 — 45 Pas. Given these parameters the laid down thickness will be of the order of 1.2 e-4 g/sqmm.

Once printed (23,) with the passivation material the nest plate is placed in an oven (25) at 170°C for 2 mins 5 see. This removes the solvents and hardens the passivation layer so that the units can be handled without removing the coating. Once removed from the oven the nest plate is covered with another nest plate and the combined plates are inverted (26). This inverts the units such that the reverse sides of the discs can be printed (27) with the passivation material in a similar manner.

On completion of the printing process the units are dried (28) and fired (29) with a peak temperature of 610°C for a duration of c. 20 mins. A profile is shown in Fig. 7.

This firing process densifies the passivation material.

Example A ceramic disc body was produced by sintering in the conventional manner. The ceramic material is mainly ZnO, with bismuth, antimony and other oxides required to achieve the desired electrical performance. The disc dimensions were 20.5mm in diameter with a ~2mm thickness. Electrodes were fired onto the surfaces as follows: a silver paste material which contains binders and solvents and glass frit suitable to the printing process and the subsequent firing cycle was printed onto each fiat side of the disc. This was then subjected to a firing process with peak temperatures of 600-800°C for a total time of 1.5 — 8 hours. This silver electrode was approximately 17mm in diameter and so leaves a ring of exposed ceramic on each of the flat surfaces with a dimension in the radial direction of l.75mm. The thickness of this silver material was in the range of 4-18pm. The exposed planar surfaces (i.e. the areas of the flat surface not covered by the silver material) were passivated as follows: the units were loaded into similar nest plates as those used for the silver printing operation. A passivation material. of glass paste with appropriate binders and solvents is printed in an annular pattern which is determined by the screen pattern. The annular pattern was aligned with the silver electrode print such that the passivation material covers mostly the exposed disc between the silver electrode and the edge of the disc.

Each disc was then assembled with two terminals and was over-moulded with a nylon (encapsulant) material to produce a finished device.

The finished varistor device was tested as follows: devices were subjected to an accelerated life test with 125°C ambient temperature and continuous rated DC voltage applied for 1000 hours. The nominal varistor voltage (measured at lmAdc) was monitored at various time intervals. For MOV devices a definition of a failure is a MOV whose nominal varistor voltage varies by more than +/- 10% during this test.

Table 1 below shows the summary of the results from this test, showing the impact of having no passivation present.

Mould Ave Shift Material % Fails 1mA Nylon Pass 1 0 2.5 Glass 1 Pass 2 O 2.4 Glass 2 Pass 3 O 0.1 Silicone None 50 10.9 None Table 1 — Summary of number of failures following Accelerated Life Test.

In the above example the discs are loaded into nest plates for screen printing of the passivation material. However, in another embodiment, with use of a nest plate which can withstand the firing temperatures involved, it is possible to print the electrode paste, fire the electrodes, print the passivation material, and fire the passivation material while the discs remain in Situ in the nest plate. The discs can then be inverted as described above with reference to the process step 26 for application of both the electrode and the passivation on the other side.

It will be appreciated that the invention allows excellent protection against faults as it eliminates any potential resistive link between the electrodes. Also, it achieves this without need to coat the side surfaces of the disc body with passivation. Also, the passivation which is applied, is applied in a very simple manner because it is on planar surfaces. Screen printing is particularly convenient as nest plates are used anyway in the process for screen printing ofthe electrode paste.

The invention is not limited to the embodiments described but may be varied in construction and detail. For example the passivation may alternatively comprise a silicone or clay material. Also, it is not essential that passivation be applied to both opposed disc faces One face alone may be regarded as enough to break any potential conductive path, depending on the varistor rating. Further, while in the invention the side surface of the disc is left free of passivation material, the process for applying the passivation material onto the faces may result in some being applied to the side surface close to the edges.

Claims (1)

1.Claims A method of manufacturing a varistor, the method comprising the steps of: providing a ceramic body having opposed planar faces with face edges; applying an electrode to each face while leaving a gap between each electrode and the edge of the face; applying passivation material to at least one face in said gap, the passivation material not extending from one electrode to the other around the ceramic body; applying leads to the electrodes, and encapsulating the body, the electrodes, and the passivation material. A method of producing a varistor as claimed in claim 1, wherein the passivation material is applied as a band around each electrode, and it also overlies at least part of an electrode; and wherein the passivation material comprises glass paste. A method of producing a varistor as claimed in either of claims 1 or 2, wherein the passivation material is applied by printing. A method of producing a varistor as claimed in claim

1. wherein the passivation material is applied by screen printing; the ceramic body being supported in a nest plate during printing. A varistor comprising: a body of ceramic material having opposed faces with face edges; an electrode on each face, in which there is a gap between at least one electrode and the edge of the face; passivation material on at least one face in said gap, the passivation material not extending from one electrode to the other electrode around the ceramic body; leads connected to the electrodes, and encapsulant surrounding the ceramic body, the electrodes, and the passivation material.