JPS6211761B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a metal oxide varistor,
More particularly, the present invention relates to a varistor having a protective coating that improves the performance of such a varistor, particularly after assembly, including soldering operations. It has been found that the operation of soldering conductive leads to electrodes often has a detrimental effect on the leakage current of the varistor. This fact has not been clearly acknowledged until now,
Or, at least, it was not considered to be a cause of increased leakage current in the type of varistor that is the object of the present invention. Considering that the soldering temperature (several hundred degrees Celsius) is relatively lower than the varistor sintering temperature (over 1000 degrees Celsius), it is at least unlikely that the electrical properties of metal oxides will deteriorate during the soldering operation. It's quite a surprise. Now, to briefly describe one aspect of the present invention,
A metal oxide varistor is provided that not only has a low leakage current, but also maintains that low leakage current even after attachment of the leads by soldering. Such a varistor consists of a varistor body having first and second electrodes spaced apart from each other and a silicone polyimide layer covering the varistor body between the electrodes.
Electrical connections to the electrodes are made by soldering conductive lead wires, but the use of flux will not cause deterioration of the varistor characteristics. According to a preferred embodiment of the invention, the protective layer covers substantially the entire electrode-free surface of the varistor body as well as the outer periphery of the electrodes. In the present invention, one or more protective layers can be provided. In the case of a varistor having electrodes on opposite main surfaces of the varistor body, the protective layer is provided around the periphery of the varistor body. Alternatively, in the case of a varistor having two or more spaced apart electrodes on either surface of the varistor body, the protective layer is provided on the body surface between the electrodes. The present invention is best understood from the following description when taken in conjunction with the accompanying drawings. Turning first to FIG. 1, there is shown a metal oxide varistor 10 that constitutes one embodiment of the present invention. Such a varistor 10 includes a body 12, which is preferably a sintered body consisting essentially of a metal oxide, such as zinc oxide, and a plurality of predetermined additives. The method of manufacturing the varistor body 12 is known to those skilled in the art and will not be described in detail here. Generally speaking, by way of example and not by way of limitation, after mixing, sintering, and grinding the main components and additives, green barista pellets are pressed. If such pellets are then sintered at high temperatures, a sintered body having desired varistor properties can be obtained. A first electrode 14 and a second electrode attached to opposing main surfaces of the varistor 10 or the varistor body 12
It has an electrode 16 of. Again by way of example and not limitation, electrodes 14 and 16 are conveniently silver paint electrodes. That is, electrical contact to the varistor body 12 can be achieved by applying silver paint to the surface of the varistor body 12 using a silk screen method or the like and then firing it at a relatively high temperature (for example, 800° C.). When producing a varistor according to the invention, the steps described so far do not differ significantly from those previously known. The present invention resides in the steps described below. According to the conventional technology, the varistor body to which the electrodes are attached is attached to the varistor body by soldering or the like.
A pair of metal lead wires are joined. Typically, fluxing agents are used in soldering operations to improve adhesion. Prior to the present invention, the use of fluxing agents in soldering operations had never been associated with deleterious effects on varistor properties. Now, it has been discovered that the leakage current of metal oxide varistors increases during soldering operations. This increase appears to be due to the effect of the soldering operation on the varistor surface between the electrodes. Such effects are not eliminated by surface cleaning operations, such as those typically performed after soldering. Thus, not only the initial leakage current of the varistor is increased by the soldering operation, but also the tendency for the varistor to deteriorate after long-term application of the operating voltage is also increased by the soldering operation. According to the invention, after forming electrodes 14 and 16 and connecting them to leads 20 and 22,
A solder-resistant protective layer 18 is provided around the periphery of the varistor body 12 before soldering. It will be obvious to those skilled in the art that no special method is required to provide the protective layer 18, and that many suitable methods can be used. Although a variety of materials may provide satisfactory results, it is preferred in accordance with the present invention to use a silicone polyimide copolymer as the protective layer 18. This material can be easily applied in its uncured liquid state. For example, a varistor body with electrodes attached is mounted on a rotating jig and then silicone polyimide is applied using a small brush or dropper. Thereafter, the silicone polyimide may be cured by heating at a temperature of 120° C. or higher, preferably about 150° C., for 1 hour or more. It is more preferable to apply and cure the second silicone polyimide layer in the same manner. After curing of protective layer 18, solder layers 24 and 26
Lead wires 20 and 22 are attached to electrodes 14 and 16, respectively, by . The solder layers 24 and 26 are made of, for example, a lead-tin eutectic alloy and may also contain a small amount of silver. Such a solder layer can be applied by heating at a temperature of about 225°C for about 10 minutes. Various fluxing agents can be used in this case, but according to the present invention, a weakly acidic fluxing agent, such as Alpha Metals (U.S.A.)
Product name: Alpha manufactured by Metals
Preferably, RMA302 is used. After the leads have been attached by soldering, the varistor is subjected to a cleaning operation, for example by washing it one or more times in chlorotene. It should be emphasized that although specific soldering operations are described herein that yield good results, the invention is not limited thereby. That is, the benefits of the present invention can be achieved in a variety of varistors manufactured by a variety of operations without modifying current operations. The completed varistor is then encapsulated, for example, by forming an epoxy outer coating (not shown) covering the entire varistor, including the protective layer 18.
The protective layer of the present invention not only prevents deterioration of leakage current characteristics during soldering, but also improves the long-term stability of varistors, which have traditionally been adversely affected by the epoxy commonly used to encapsulate such varistors. It has also been found to be useful for improving sexual performance. As silicone polyimide copolymers, those having various compositions, ie, those having various chemical structures, can be used. Silicone polyimides are generally made from tetracarboxylic dianhydrides (e.g. benzophenonetetracarboxylic dianhydride), organic diamines (e.g. methylene dianiline) and silicone-containing diamines (e.g. bis(γ-aminopropyl)tetramethyldisiloxane). A silicone polyamic acid or silicone polyimide precursor is prepared by reacting with a mixture of the following: This silicone polyimide precursor is cured by heating to become silicone polyimide. In the practice of this invention, it is preferred that the ratio of organic diamine to silicone-containing diamine be 70:30 and that the solids content of the precursor in the organic solvent be 50%.
Silicone polyimide has its precursor typically about 125
It is produced by curing at a temperature of from 0.degree. C. to 400.degree. C., preferably below about 200.degree. The following Figure 2 is a flowchart showing steps for carrying out the present invention. Although the present invention can be advantageously implemented with respect to various types of varistors, it is particularly useful with respect to high voltage and large current varistors. First, a varistor body 12 is prepared, which has already been sintered prior to the various steps constituting the present invention. If desired, it can be precoated with a passivating coating, such as that described in U.S. Pat. No. 3,857,174. Electrodes are then formed on the varistor body. To do this, for example,
A silver electrode is formed by applying a silver paste by a silk screen method in an area where it is desired to form an electrode, and then firing the coated varistor body. Such electrode formation is a common technique, and the steps therefor do not form part of the present invention.
After forming the electrodes, a protective layer 18 of the invention is applied. Such a protective layer 18 covers the surfaces of the varistor body 12 not covered by the electrodes, and preferably also covers parts of the electrodes. Since the deterioration of the leakage current characteristics of the varistor during the soldering operation seems to be a surface rather than an internal phenomenon, the protective layer 18 protects the upper electrode 1.
It is only necessary that a continuous barrier be formed between 4 and the lower electrode 16. It is not necessarily required that the protective layer 18 completely cover the periphery of the varistor body 18 and partially overlap the upper electrode 14 and the lower electrode 16. However, if the protective layer 18 completely covers the periphery of the varistor body 18 and partially overlaps the upper electrode 14 and the lower electrode 16, the entire outer surface of the electrodes of the varistor body 18 can be soldered. This is preferable because it protects it from the effects of the above and the effects of the flux used at that time. It is also possible to provide a plurality of protective layers, but in that case it is preferable to cure each protective layer before applying the next protective layer. After applying the last protective layer, the varistor is completed following normal assembly procedures. That is, leads 20 and 22 are attached by a soldering operation that includes the use of a flux, followed by a cleaning operation as is known to those skilled in the art. Such a soldering operation may involve immersing the entire varistor, including the portion covered by the protective layer 18, in a solder bath as long as the temperature does not exceed about 400°C. After the leads 20 and 22 are attached, the varistor may be encapsulated, for example by epoxy encapsulation. The table below illustrates the leakage current improvement measured after soldering for varistors with and without the protective layer of the present invention.

[Table] Baristas who do not

[Table] Barista
In addition, the leakage current is a value measured at V ₁ /2 volts, α ₁ , α ₂ and α ₃ are each 0.1 to
Values measured in the range 1 mA, 1-10 mA and 10-100 mA, and V ₁ is the voltage across the varistor at a current of 1 mA. It can thus be seen that not only the leakage current of the varistor is significantly improved by the addition of the protective layer according to the invention, but also the other properties of the varistor are improved as well. The following FIG. 3 shows a varistor forming another embodiment of the invention. In this and subsequent figures, like components are designated with like reference numerals. A first electrode 14 and a second electrode 16 are attached to the upper and lower main surfaces of the varistor body 12, respectively. Preparation of the varistor body 12 and formation of the electrodes may be performed as described above in connection with FIG. A protective layer 30 surrounds the periphery of the varistor body 12 but does not overlap the electrodes 14 and 16. Such a protective layer 30 covers the electrode 1
This serves to prevent the surface current path of the varistor body 12 between 4 and 16 from becoming contaminated during the soldering operation. After installing the protective layer 30,
Conductive leads 20 and 22 are attached to electrodes 14 and 16 by solder layers 24 and 26, respectively. The application and curing of the protective layer 30 and the soldering of the lead wires 20 and 22 are performed in the first step.
This may be done as described above in connection with the figures. That is, when applying the protective layer 30, a slightly smaller amount of silicone polyimide is applied to the varistor body than is necessary to obtain a protective layer that covers the periphery of the varistor body 12 and a portion of the electrodes 14 and 16. It is only necessary to apply it to the peripheral area of 12. The following FIG. 4 shows a varistor constituting yet another embodiment of the present invention. Such a varistor 34 includes a body 12 as described above in connection with FIGS. 1-3, but with a first electrode 36 and a second electrode 38 attached to one major surface 40. electrodes 36 and 3
8 are spaced apart from each other, and the protective layer 42
are provided on main surface 40 between adjacent ends of electrodes 36 and 38. Then, by the same operation as described above, the conductive lead wire 44 is
and 46 are soldered to electrodes 36 and 38, respectively. Note that the protective layer 42 may be applied and cured in the same manner as described above, but
It goes without saying that there is no need to rotate the varistor body 12 during the application of step 2. Instead, silicone polyimide may be applied in a line between electrodes 36 and 38 using a brush or the like. barista 3
4 may also be provided with a peripheral protective layer 30 as described above in connection with FIG. 3, if desired. The following FIG. 5 shows a varistor similar to that of FIG. 4, except that the protective layer covers the entire exposed surface of the varistor body as well as part of the electrodes. Such a varistor 50 includes a body 12 having a first electrode 36 and a second electrode 38 attached to one major surface, and has a protective layer 52 consisting of a peripheral region 52A and an inner region 52B.
Note that the internal region 52B located between the electrodes 36 and 38 is an extension of the peripheral region 52A. Conductive leads 44 and 46 are attached to electrodes 36 and 38 by solder layers 54 and 56, respectively. As mentioned above, silicone polyimide is used to form the protective layer, but the details of the method for providing the protective layer on the varistor body will vary depending on the type of varistor and the package used with it. be able to.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a metal oxide varistor having a coating for improving solderability according to the present invention, FIG. 2 is a flowchart of a method for manufacturing a solderable varistor according to the present invention, and FIG. FIG. 4 is a cross-sectional view of a varistor constituting another embodiment of the present invention, FIG. 5 is a cross-sectional view of a varistor constituting still another embodiment of the present invention. FIG. In the figure, 12 is the varistor body, 14 and 16 are electrodes, 18 is a protective layer, 20 and 22 are lead wires, 24 and 26 are solder layers, 30 is a protective layer,
36 and 38 represent electrodes, 42 a protective layer, 44 and 46 lead wires, 52 a protective layer, and 54 and 56 a solder layer.

Claims (1)

[Scope of Claims] 1. A body consisting essentially of a metal oxide and one or more additives, first and second electrodes spaced apart from each other and directly attached to said body, said first and second electrodes. A metal oxide varistor having a protective layer provided on the surface of the main body in between, and a solder layer provided on the exposed surfaces of each of the first and second electrodes, the protective layer being made of silicone polyimide and resistant to solder and solder-containing contaminants, and wherein the protective layer covers the outer periphery of the electrode and provides solder attachment within the surface of each of the electrodes. A metal oxide varistor with low leakage current, characterized in that it has respective openings that expose and define areas. 2. A body 12 of varistor material is provided with first and second electrodes 14, 16 or 3 spaced apart from each other.
6, 38 are formed on said body, and then first and second conductive leads 20, 22 or 44,
46 to the first and second electrodes, prior to the soldering step, the first
and a layer 18, 3 of a protective material made of silicone polyimide on the body between the second electrodes.
A method for manufacturing a metal oxide varistor, comprising the step of providing 0, 42 or 52. 3. Claim 2, wherein the first and second electrodes 36, 38 are disposed on one major surface of the main body having first and second major surfaces.
A method for manufacturing a metal oxide varistor as described in . 4. The method for manufacturing a metal oxide varistor according to claim 2, wherein the first and second electrodes 14 and 16 are respectively arranged on the first and second main surfaces of the main body. 5. The method for manufacturing a metal oxide varistor according to claim 2, which comprises curing the silicone polyimide. 6. The silicone polyimide layer is provided on substantially the entire surface of the main body not covered by the first and second electrodes and on the outer periphery of the first and second electrodes. 2
The method for manufacturing a metal oxide varistor according to any one of items 1 to 5. 7. The method of manufacturing a metal oxide varistor according to claim 5, wherein the curing step comprises heating the main body provided with the silicone polyimide layer to a temperature higher than 125°C. 8. The method for manufacturing a metal oxide varistor according to claim 7, wherein a second silicone polyimide layer is provided on the silicone polyimide layer in substantially the same area as the silicone polyimide layer. 9 Heat the second silicone polyimide layer to 125°C.
9. A method of manufacturing a metal oxide varistor according to claim 8, wherein the metal oxide varistor is hardened by heating to a higher temperature. 10. The method for manufacturing a metal oxide varistor according to any one of claims 2 to 9, further comprising the step of encapsulating the varistor. 11. Claims 2-10, wherein the body consists essentially of zinc oxide and one or more additives.
A method for manufacturing a metal oxide varistor according to any one of the above.