JPH0430722B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for manufacturing a voltage non-linear resistance element, and more particularly to a method for manufacturing a voltage non-linear resistance element such as a varistor including a semiconductor ceramic body mainly made of strontium titanate. [Prior Art] It has been well known to construct a varistor using a semiconductor ceramic body having voltage non-linear resistance characteristics. As this type of semiconductor ceramic, strontium titanate (SrTiO ₄ )-based materials are often used because they have the advantage of less variation in properties and are excellent in mass production. FIG. 1 is a diagram showing an example of a chip-type varistor using a semiconductor ceramic mainly composed of strontium titanate, which is the background of the present invention. Ceramic body BK is, for example, made by heat-treating a semiconductor ceramic mainly composed of strontium titanate in an oxidizing atmosphere to increase the resistance of the grain boundaries, or by adding a metal oxide to a semiconductor ceramic mainly composed of strontium titanate. , which is then heat-treated and diffused to make the grain boundaries high in resistance. A pair of electrodes EL1 and EL2 are formed on opposing end surfaces of the ceramic body BK. These electrodes EL1 and EL2 are formed, for example, by printing and applying an electrode paste of silver, zinc, or copper, and then baking it. [Problem to be solved by the invention] When evaluating the V-I characteristics of such a varistor, it was found that the characteristics varied widely, and the electrode
After soldering the lead wires to EL1 and EL2,
When measured again, the threshold voltage V ₁ mA is approximately −10 to 30
% change. These causes are caused by the ceramic body
When printing electrodes EL1 and EL2 on BK, as shown in Figure 2, the ceramic body BK
This is because barriers vr1 and Vr2 were formed at the interfaces between the electrodes EL1 and EL2, respectively. These barriers vr1 and vr2 exhibit voltage nonlinearity like the varistor VR formed of the ceramic body BK, but their nonlinear coefficients are inferior to the characteristics possessed by the ceramic body. Therefore, the electrode EL1 is attached to such a semiconductor ceramic body.
and EL2, the nonlinear coefficient as a whole becomes small. Also, not only that,
Since these barriers VR1 and VR2 are easily broken by thermal or electrical shock, there is a large variation in the varistor characteristics as a whole, and the threshold voltage varies before and after the processing to attach the lead wires. It will change drastically. In addition, the same problem occurs when a non-ohmic electrode is formed by a thermal spraying method, a vacuum evaporation method, a sputtering method, an electroless plating method, or the like. Furthermore, the same applies to multilayer varistors in which molten metal constituting internal electrodes is injected into a semiconductor ceramic body and solidified. In order to avoid the effects of the barriers vr1 and vr2 as described above, it is conceivable to form a so-called ohmic voltage that does not create a barrier at the interface with the semiconductor ceramic body (see Japanese Patent Laid-Open No. 57-2073195). Such ohmic electrodes are made of, for example, aluminum (Al), copper (Cu), nickel (Ni), zinc (Zn), tin (Sn), or the like. By forming such ohmic electrodes using baking, metal spraying, vacuum evaporation, plating, etc., surge resistance can be improved; however, the above-mentioned electrode materials are difficult to solder; Moreover, it has the disadvantage of poor moisture resistance, such as variations in its properties due to moisture absorption. Therefore, attempts have been made to form silver electrodes with excellent solderability and moisture resistance on top of ohmic electrodes, but this method has problems such as not only being expensive but also poor workability. There is. That is, in the case of a two-layer structure in which a silver electrode is formed on an ohmic electrode, the ohmic electrode layer and the silver electrode layer must be formed accurately so as not to be misaligned. This is because if a positional shift occurs between the two layers, variations in characteristics will be further aggravated. Therefore, another problem with the ohmic electrode of the conventional structure is that the process for processing it with high precision becomes complicated. Therefore, an object of the present invention is to provide a method for manufacturing a voltage nonlinear resistance element that is simpler and cheaper, and that can reduce changes before and after processing, such as soldering of lead wires. [Means for Solving the Problems] Simply put, the present invention includes the following steps: (a) preparing a semiconductor ceramic body having voltage non-linear resistance characteristics based on strontium titanate; (b)
a step of applying electrodes to the semiconductor ceramic body;
and (c) applying a predetermined pulse voltage to the electrode to electrically destroy a barrier formed between the voltage and the semiconductor ceramic body. [Operation] For example, when a pulse voltage is applied to the electrode with a current density sufficient to destroy the barrier formed between the electrode and the semiconductor ceramic body, but not enough to destroy the voltage nonlinear resistance characteristics of the semiconductor ceramic body. , that barrier is destroyed. Therefore,
Even when lead wires are soldered to the electrodes thereafter, the V-I characteristics do not vary due to the barrier. [Effects of the Invention] According to the present invention, when forming an electrode, the barrier formed between the semiconductor ceramic body and the electrode can be forcibly destroyed electrically by applying a pulse voltage. Therefore, the obtained voltage nonlinear resistance element can maintain stable characteristics even if it is subsequently subjected to processing such as attaching lead wires by soldering. The above objects and other objects and features of the invention will become more apparent from the following detailed description with reference to the drawings. [Example] A semiconductor ceramic body BK similar to that shown in FIG. 1 is prepared. This ceramic body BK is a semiconductor ceramic body mainly composed of strontium titanate and having voltage nonlinear resistance characteristics. Such semiconductor ceramics can be obtained by the following method. For example, by diffusing manganese etc., for example 1100 ~
By oxidizing in air at 1200℃, oxides such as manganese oxide are attached to the grain boundaries of strontium titanate, or by simply oxidizing by heat treatment in an oxidizing atmosphere, the grain boundaries are made to have high resistance. There are some things that have become. And its ceramic body
Print electrode paste such as silver, copper or zinc on BK and bake it to create baked electrodes EL1 and EL2 (first
form). Then, one or more pulse voltages with a half-width from 0 to the peak value (Fig. 3) of 0.1 to 100 μsec and a current density of 1 to 100 A/cm ² are applied between the electrodes EL1 and EL2 in both the positive and negative directions. Apply twice. The current density at this time is
The size is chosen to be dense enough to destroy vr1 and vr2 (Figure 2), but not to destroy the bulk BK itself. According to experiments conducted by the inventors of the present invention, for certain materials and sizes, the barrier cannot be destroyed at a current density of 1 A/cm ² or less, for example.
Moreover, at 100 A/cm ² or more, the inherent voltage nonlinear resistance characteristics of the chip or bulk itself are lost. After that, lead wires are soldered to the electrodes EL1 and EL2. In this way, a voltage non-linear resistance element such as a varistor is completed. In addition to baking, the electrodes EL1 and EL2 may be formed by a thermal spraying method, a vacuum evaporation method, a sputtering method, or an electroless plating method. In short, there are barriers vr1 and vr between the ceramic body BK and the electrodes EL1 and EL2.
The method of the present invention is applied to a varistor in which a varistor 2 is formed. Example 1 Twenty semiconductor ceramic bodies mainly made of strontium titanate whose crystal grain boundaries had been made highly resistive by heat treatment in an oxidizing atmosphere, in this example air, were prepared, and silver was applied to the surface of the ceramic bodies. The main paste was applied and baked at 700-900°C to form an electrode. Half of them were left as they were, and a triangular wave pulse voltage with a half-width of 10 μsec and a current density of 50 A/cm ² was applied to the remaining half five times in both the positive and negative directions.
The VI characteristics of each were measured. Furthermore, lead wires were attached to these electrodes by soldering, and the V-I characteristics were then measured again. The results are shown in Tables 1 and 2, respectively. V ₁ mA and α in Tables 1 and 2 are determined from each VI characteristic. Table 1 shows the results without applying a pulse, and Table 2 shows the results with applying a pulse. In addition, in the table below,
"Before processing" refers to the state before the lead wire is soldered to the electrode, and "after processing" refers to the state after the lead wire is soldered. and means the average value, and σ represents the variation. Furthermore, V ₁ mA is the threshold voltage when the current is 1 mA,
α is a nonlinear coefficient.

【table】

[Table] As is clear from Tables 1 and 2, by applying a pulse to forcibly destroy barriers VR1 and VR2 (Fig. 2), the threshold voltage before and after machining can be changed. The rate of change in the linearity coefficient can be significantly reduced, and a voltage nonlinear resistance element (for example, a varistor) with stable characteristics with little variation can be obtained. Example 2 A rectangular wave with a half-width of 50 μsec and a current density of 10 A/cm 2 was applied in both positive and negative directions to 10 devices (electrodes baked onto semiconductor ceramic bodies) prepared in the same manner as in Example ¹ . 10 was applied ^three times and the V-I characteristics were measured. Furthermore, after attaching a lead wire to the baked electrode by soldering, its VI characteristic was measured again. And from each V-I characteristic, V ₁ mA, α
The results are shown in Table 3.

[Table] From Table 3, it can be seen that even if a square wave pulse voltage is applied ¹⁰ times, the barrier between the electrode and the bulk is destroyed and the variations in threshold voltage and nonlinear coefficient are reduced. . Example 3 A semiconductor ceramic body mainly made of strontium titanate, whose crystal grain boundaries were made to have high resistance by heat-treating and diffusing metal oxides applied to the surface, was prepared.
The electrodes are prepared by printing a silver-based paste on their surfaces and baking them at 700 to 900 degrees Celsius. For half of them, a triangular wave pulse voltage with a half width of 10 μsec and a current density of 50 A/cm ² is applied five times in both the positive and negative directions. Then, first, the VI characteristics of these elements are measured.
Then, lead wires were attached to these elements by soldering, and the VI characteristics were measured again. Then, find V ₁ mA, α from each VI characteristic,
The results are shown in Tables 4 and 5. Table 4 shows the case when no pulse is applied, and Table 5 shows the case when the pulse is applied.

【table】

[Table] Example 4 A rectangular wave with a half-value width of 50 μsec and a current density of 10 A/cm 2 was applied to 10 elements (semiconductor ceramic bodies with baked-on electrodes) prepared in the same manner as in Example ³ . The voltage is applied ¹⁰³ times in both directions and the V-I characteristics are measured. After attaching lead wires to the electrodes, the V-I characteristics are measured. Then, V ₁ mA,α was determined from each VI characteristic, and the results are shown in Table 6.

Table 6 shows that even if a rectangular wave pulse voltage is applied, the barrier between the electrode and the bulk is destroyed, and the variations in the threshold voltage and nonlinear coefficient are reduced. Example 5 A semiconductor ceramic body mainly made of strontium titanate, whose crystal grain boundaries were made to have high resistance by heat-treating and diffusing the metal oxide applied to the surface, was made into 40
Prepare 20 ceramic bodies and print copper paste on the surface of the ceramic body, and print zinc paste on the surface of the remaining 20 ceramic bodies.
The electrodes are formed by baking at 600-800℃. Half of them are left as they are, and triangular wave pulses with a half-width of 10 μsec and a current density of 50 A/cm ² are applied five times in both the positive and negative directions to the other half, and the V-I characteristics are measured for each. Next, lead wires were attached by soldering to both those to which pulses were applied and those to which no pulses were applied, and the VI characteristics were measured again. Then, V ₁ mA, α was determined from each VI characteristic, and the results are shown in Tables 7 and 8. Table 7 shows the case when no pulse is applied, and Table 8 shows the case when the pulse is applied.
Each is shown below.

【table】

[Table] In both cases, for example, a pulse voltage was applied after electrodes were formed on the semiconductor ceramic body. However, even if, for example, a pulse voltage is applied between the lead wires after forming the electrodes and attaching the lead wires, the rate of change in the threshold voltage before and after processing can be similarly reduced, and variations can be suppressed. Furthermore, in each of the above embodiments, the ceramic body BK
Although the varistor in which electrodes EL1 and EL2 are formed on the surface of the varistor is shown, if the method of the present invention is applied to a varistor having a structure as shown in FIG. 4 or 5, the same effect will be obtained. In other words, in the example shown in Fig. 4, the electrode EL1 is placed on both sides of the ceramic body BK.
EL2 is formed, and an intermediate electrode IEL is further formed in the ceramic body BK. The example shown in Figure 5 is called a multilayer varistor, and the internal electrode
IEL1 and IEL2 are formed and electrically connected to electrodes EL1 and EL2 on the side surface of ceramic body BK. Therefore, by applying a pulse voltage to the electrodes EL1 and EL2, the intermediate electrodes IEL and IEL, respectively,
It is possible to destroy the barrier between the internal electrodes IEL1, IEL2 and the ceramic body BK. Note that the internal electrodes IEL1 and IEL2 are made by melting a base metal such as Pb and Sn, and injecting the base metal into the void layer of the ceramic body BK where the electrodes IEL1 and IEL2 have a void layer (porous layer). It may be something you have done.

[Brief explanation of drawings]

FIG. 1 is a structural diagram showing an example of a chip-type varistor, which is the background of the present invention. FIG. 2 is an equivalent circuit showing that a barrier is formed at the interface between the electrode and the bulk. FIG. 3 shows an example of a pulse voltage applied after electrode formation. FIG. 4 is a schematic cross-sectional view of another chip-shaped varistor. Fifth
The figure is a schematic cross-sectional view of another chip-shaped varistor. FIG. 6 is a schematic perspective view of the ring varistor. In the figure, BK is a ceramic body, EL1 and
EL2 is an electrode, and IEL, IEL1 and IEL2 are internal electrodes.

Claims (1)

[Scope of Claims] 1 (a) Step of preparing a semiconductor ceramic body having voltage non-linear resistance characteristics mainly based on strontium titanate, (b) Step of forming electrodes on the semiconductor ceramic body, and (c) ) A method for manufacturing a voltage nonlinear resistance element, comprising the step of applying a predetermined pulse voltage to the electrode to electrically destroy a barrier formed between the electrode and the semiconductor ceramic body. 2 The step C includes applying a pulse voltage with a current density sufficient to destroy the barrier formed between the electrode and the semiconductor ceramic body, but not destroy the voltage non-linear resistance characteristic of the semiconductor ceramic body. A method for manufacturing a voltage nonlinear resistance element according to claim 2, comprising: