Patent Grant
8044761
1. Field of the Invention
The present invention relates to a varistor integrally comprising a resistor and a varistor element.
2. Related Background Art
Varistors are used in, for example, controlling devices, communication devices and their parts, to absorb or eliminate external surges (abnormal voltage) such as static electricity or noise, for protection against such external surges or noise.
The use of zinc oxide as a major component and rare earth elements, calcium oxide and silicon oxide as accessory components has been proposed for the constituent components of varistor elements in varistors, in order to enhance the varistor characteristics such as voltage non-linearity, as well as the discharge withstand current rating (see Japanese Patent Publication No. 3493384, for example).
Serial connection of resistors in a varistor requires separate mounting of two elements on the printed board, and thus increases the mounting space and conflicts against high-density mounting. Varistors that integrate varistor elements and resistors have been proposed as means for realizing high-density mounting (see Japanese Patent Publication No. 3097332, for example).
Varistors integrally comprising varistor elements and resistors for high-density mounting are preferably varistors with low electrostatic capacity so that they can exhibit satisfactory varistor characteristics and minimal effects on signals even with high-speed digital signals and transmission speeds.
It has been found that the varistor characteristics are impaired when a varistor integrally comprising a varistor element and resistor is fabricated. Upon examining the cause of this, it was discovered that the reduction in varistor characteristics is due to reaction products produced by reaction between the components of the varistor element, the conductors formed on the varistor element, and the resistor.
The present invention has been accomplished in light of these circumstances, and its object is to provide a varistor capable of high-density mounting and exhibiting excellent varistor characteristics while having sufficiently reduced resistance fluctuation.
In order to achieve this object, the invention provides a varistor comprising a varistor element, a pair of external electrodes on one main side of the varistor element and a resistor on the same main side, the resistor being formed in connection with the pair of external electrodes, wherein the varistor element has a main component and accessory components, containing zinc oxide as the main component and calcium oxides, silicon oxides and rare earth metal oxides as the accessory components, the proportion X of the calcium oxides in terms of calcium atoms is 2-80 atomic percent with respect to 100 mol of the main component and the proportion Y of the silicon oxides in terms of silicon atoms is 1-40 atomic percent with respect to 100 mol of the main component, the ratio of X to Y (X/Y) satisfies the following formula (1), and the external electrode and resistor contain oxides other than bismuth oxide and copper oxide.
1≦X/Y<3 (1)
The varistor of the invention exhibits excellent varistor characteristics while adequately reducing fluctuations in resistance. The reason for this effect is conjectured by the present inventors to be as follows. Specifically, since the varistor element provided in the varistor of the invention comprises a resistor and an external electrode containing oxides different from bismuth oxide and copper oxide, it is possible to adequately prevent reaction between the external electrodes, varistor element and resistor during production and use of the varistor. This can minimize reaction products in the external electrode, varistor element and resistor. For this reason, it is conjectured, it is possible to maintain the original excellent varistor characteristics of the varistor element without any impairment, and adequately reduce fluctuation in the resistance value.
According to the invention, a base glass layer is preferably provided between the main side of the varistor element and either or both the pair of external electrodes and the resistor.
A varistor provided with a base glass layer between the varistor element and either or both the pair of external electrodes and the resistor has sufficiently reduced reaction between the external electrodes and varistor element, as well as sufficiently minimized reaction with either or both the resistor and the varistor element.
The resistor in the varistor of the invention is preferably provided so as to cover at least part of the sides of the external electrodes opposite their varistor element sides.
Using a resistor of this type can further improve the bonding strength between the resistor and the conductors. It is thus possible to further prevent fluctuations in the resistance value of the varistor while also improving the durability and reliability.
The varistor of the invention is preferably provided with a glass layer covering the resistor and the pair of external electrodes. Such a glass layer will also provide protection to the varistor. It can also further improve the durability and reliability of the varistor.
According to the invention it is possible to provide a varistor capable of high-density mounting and exhibiting excellent varistor characteristics while having sufficiently reduced resistance fluctuation.
Preferred embodiments of the invention will now be explained with reference to the accompanying drawings where necessary.
As shown in
The varistor element 10 contains zinc oxide (ZnO) as the main component, as well as a rare earth metal oxide, calcium oxide and silicon oxide as accessory components. The ZnO content with respect to the entire varistor element 10 is preferably 70-99 atomic percent from the viewpoint of obtaining excellent varistor characteristics. This will allow high level properties to be achieved, including high varistor characteristics and large surge resistance.
The rare earth metal oxide included as an accessory component in the varistor element 10 is preferably at least one type of oxide selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. The rare earth metal oxide content is preferably 0.01-10 atomic percent in terms of rare earth metal with respect to zinc oxide as the main component. Voltage non-linearity will not be easily exhibited if the rare earth metal element oxide content is too low, while the varistor voltage will tend to increase drastically if the content is too high. The rare earth oxide is more preferably an oxide of Pr.
The calcium oxide content of the varistor element 10 is 2-80 atomic percent in terms of calcium atoms with respect to zinc oxide. The silicon oxide content of the varistor element 10 is 1-40 atomic percent in terms of silicon atoms with respect to zinc oxide. The proportion of calcium oxide with respect to silicon oxide is the atomic ratio of calcium atoms to silicon atoms (Ca/Si), and it satisfies general formula (1) above.
The electrostatic capacity of the varistor is generally represented by:
C=∈0∈r(S/d). (2)
In this formula, C represents the electrostatic capacity, ∈0 represents the permittivity in a vacuum, ∈r represents the relative permittivity, S represents the area of the opposite electrodes, by which electrostatic capacity is exhibited, and d represents the thickness between the opposite electrodes. Care is necessary when dealing with the thickness d of a varistor containing zinc oxide as the main component, i.e. a zinc oxide-based varistor. A zinc oxide-based varistor exhibits its characteristics by its grain boundaries. That is, under steady state conditions a significant difference exists between the resistance at the grain boundaries and the resistance inside the grains, with the resistance at the grain boundaries being much greater than inside the grains. Thus, under steady state conditions not exceeding the breakdown voltage (build-up voltage), almost the entire applied electric field impacts the grain boundaries. The thickness d must therefore take this factor into account.
The thickness d is represented by:
d=n·2W. (3)
Here, n is the number of grain boundaries parallel to the opposite electrodes, and 2 W is the depletion layer width of a single grain boundary.
The relationship between the varistor voltage V1mA and the number of grain boundaries n is as follows:
n=V1mA/φ. (4)
Here, φ is the barrier height at the grain boundary, which is the value representing the varistor voltage per grain boundary.
Substituting formula (3) and formula (4) into formula (2) gives the following formula:
C·V1mA=∈0∈r·(φ·S/2W). (5)
Assuming optimal voltage non-linearity, φ and 2 W are constant values (for example, φ=0.8 eV, 2 W=30 nm), and therefore formula (5) is constant so long as the area S of the opposite electrodes is constant. Stated differently, the electrostatic capacity can be lowered while maintaining optimal voltage non-linearity by reducing the area S of the opposite electrodes.
The method for reducing the area S of the opposite electrodes may be direct reduction in the area of the opposite electrodes. However, if the area of the opposite electrodes is simply reduced, the maximum tolerated energy and maximum tolerated surge will be lower as a result, thus tending to lower the voltage non-linearity or the reliability of the element. It is therefore desirable to control the microstructure of the ceramic in order to minimize reduction in the maximum tolerated energy and maximum tolerated surge and reduce the electrostatic capacity. In other words, by providing a first phase composed mainly of zinc oxide and a second phase composed of an oxide other than zinc oxide, and controlling the volume fraction of the second phase, the area of the grain boundaries of zinc oxide that exhibit electrostatic capacity between the opposite electrodes is reduced. This can lower the electrostatic capacity without reducing the area of the opposite electrodes.
A varistor element in a varistor according to the invention contains calcium oxides and silicon oxides, as mentioned above. Consequently, the crystal structure of the varistor element has a second phase composed of complex oxides synthesized by reaction of Ca and Si (for example, CaSiO3 or Ca2SiO4) introduced into a first phase composed mainly of zinc oxide, and the volume fraction of the second phase is controlled to the desired value. The area of the zinc oxide grain boundaries is therefore low. Complex oxides of Ca and Si have lower permittivity than zinc oxide and do not inhibit the voltage non-linearity. The electrostatic capacity of the varistor element can be reduced as a result.
As calcium oxides to be included in the varistor element there may be mentioned CaO, and complex oxides containing calcium, silicon and oxygen, such as CaSiO3 or Ca2SiO4. As silicon oxides to be included in the varistor element there may be mentioned SiO2, and complex oxides containing calcium, silicon and oxygen such as CaSiO3 or Ca2SiO4, or Zn2SiO4.
The varistor element of this embodiment preferably contains at least one oxide selected from among Co oxides and Group IIIB elements, in addition to the accessory components mentioned above. As Group IIIB elements there are preferred B, Al, Ga or In.
The Co oxide content is preferably 0.05-10 atomic percent in terms of Co with respect to zinc oxide as the main component. If the content is less than 0.05 atomic percent it will tend to be difficult to obtain the desired varistor voltage, while if the content exceeds 10 atomic percent, the varistor voltage will tend to increase and the voltage non-linearity will tend to be reduced.
The content of one or more oxides selected from among Group IIIB elements is preferably 0.0005-0.5 atomic percent in terms of the selected Group IIIB element with respect to zinc oxide as the main component. If the content is less than 0.0005 atomic percent the varistor voltage will tend to increase, while if the content exceeds 0.5 atomic percent, the resistance will tend to be lower and it may not be possible to obtain the intended varistor voltage.
The varistor element of this embodiment also preferably contains at least one oxide selected from among Group IA elements, as another accessory component. As Group IA elements there are preferred Na, K, Rb or Cs.
The content of one or more oxides selected from among Group IA elements is preferably less than 5 atomic percent in terms of the selected Group IA element with respect to zinc oxide as the main component. If the content is 5 atomic percent or greater, the melting point of the ceramic will be lowered and it will tend to melt during firing.
The varistor element of this embodiment also preferably contains at least one oxide selected from among Group IIA elements other than Ca, as another accessory component. As Group IIA elements there are preferred Mg, Sr or Ba.
The content of one or more oxides selected from among Group IIA elements other than Ca is preferably less than 1 atomic percent in terms of the selected Group IIA element with respect to zinc oxide as the main component. If the content is 1 atomic percent or greater, the varistor voltage will tend to be increased.
The varistor element of this embodiment also preferably contains at least one oxide selected from among Cr and Mo, as another accessory component. The content of this oxide is preferably less than 10 atomic percent in terms of Cr atoms and Mo atoms each with respect to zinc oxide as the main component. If the content is greater than 10 atomic percent, the varistor voltage will tend to be increased.
The external electrodes 30a, 30b are conductors, and they contain oxides different from bismuth oxide and copper oxide (CuO). Examples of such oxides include SiO2, NiO, MnO and Al2O3. The external electrodes 30a, 30b preferably contain simple metals in addition to the aforementioned oxides. Such simple metals are preferably Ag, Pd, Pt and the like.
The total content of oxides in each of the external electrodes 30a, 30b is preferably 0.01-20 wt % with respect to the total external electrode. If the total content of oxides is less than 0.01 wt % the bonding strength for the base material will tend to be low, and if it exceeds 20 wt % the electrical conductivity will tend to be impaired. The thickness of each external electrode 30a, 30b may be 1-30 μm, for example.
The resistor 60 may contain a conductive oxide such as RuO2, SnO2 or LaB6, an oxide such as Al2O3, B2O3 or SiO2, and a simple metal such as Pd, Ag or Pt.
The resistor 60 contains an oxide different from bismuth oxide and copper oxide (CuO). The oxide content of the resistor 60 is preferably 50-99 wt %. This will help to further prevent fluctuations in the resistance value. The thickness of the resistor 60 may be 1-30 μm, for example.
The base glass layer 12 contains an oxide commonly included in glass, such as HfO2, CaO, Al2O3, SiO2, ZnO, BaO or B2O3. The content of bismuth oxide and copper oxide (CuO) in the base glass layer 12 is preferably no greater than 1 wt % and more preferably no greater than 0.5 wt % each, with respect to the total base glass layer 12. The base glass layer 12 preferably also contains an oxide different from bismuth oxide and copper oxide. By reducing the content of bismuth oxide and copper oxide in the base glass layer 12, it is possible to further prevent reaction between the varistor element 10 and the external electrodes 30a, 30b and resistor 60. This can help further minimize reaction products in the varistor element 10, external electrodes 30a, 30b and resistor 60. The thickness of the base glass layer 12 may be 1-30 μm, for example.
The protective layer 60 is formed in order to protect the varistor element 10, external electrodes 30a, 30b and resistor 60. The protective layer 60 contains a glass or ceramic as the main component. The thickness of the protective layer may be 1-30 μm, for example.
As mentioned above, the external electrodes 30a, 30b of the varistor 1 according to this embodiment contain oxides other than bismuth oxide and copper oxide. It is thus possible to prevent reaction between the varistor 1 and external electrodes 30a, 30b, and between the external electrodes 30a, 30b and resistor 60. When the external electrodes contain bismuth oxide or copper oxide, the bismuth oxide or copper oxide reacts with the varistor element 10 components to form reaction products. Because bismuth can form a trivalent cation, a semiconductor may be formed as a reaction product. This can potentially lower the varistor characteristics.
According to
In the varistor of
(See
A varistor according to a second embodiment of the invention will now be explained with reference to
As shown in
The varistor element 23 is constructed as a laminated body, obtained by laminating a plurality of varistor layers and a plurality of first to third internal electrode layers 31 (
As shown in
Each first internal electrode 33 is electrically connected to the external electrode 25 via a lead conductor 37a, and electrically connected to the external electrode 30a via a lead conductor 37b. The lead conductors 37a, 37b are formed integrally with the first internal electrode 33. Each second internal electrode 35 is electrically connected to the external electrode 29 via a lead conductor 39a, and electrically connected to the external electrode 30b via a lead conductor 39b. The lead conductors 39a, 39b are formed integrally with the second internal electrode 35.
As shown in
As also shown in
Each fourth internal electrode 53 is electrically connected to the external electrode 26 via a lead conductor 57a, and electrically connected to the external electrode 30c via a lead conductor 57b. The lead conductors 57a, 57b are formed integrally with the fourth internal electrode 53. Each fifth internal electrode 55 is electrically connected to the external electrode 28 via a lead conductor 59a, and electrically connected to the external electrode 30d via a lead conductor 59b. The lead conductors 59a, 59b are formed integrally with the fifth internal electrode 55.
The first to fifth internal electrodes 33, 35, 43, 53, 55 contain the same components as the external electrode of the first embodiment described above. The internal electrodes contain the same components as the external electrodes of the first embodiment described above. The lead conductors 37a, 37b, 39a, 39b, 47, 57a, 57b, 59a, 59b also contain the same components as the external electrodes of the first embodiment. The internal electrodes and lead conductors preferably also contain an oxide different from bismuth oxide and copper oxide.
The external electrode 30a and external electrode 30b are situated on the main side 23b of the varistor element, perpendicular to the lamination direction of the varistor layer and at a prescribed spacing in the direction parallel to the main side 23b (
The external electrodes 30a-30d contain the same components as the external electrodes of the first embodiment described above. That is, they contain an oxide different from bismuth oxide and copper oxide. The external electrodes 30a-30d may be formed, for example, by burning conductive paste containing the metal and metal oxide in the external electrode of the first embodiment. The conductive paste used may be a mixture of an ordinary commercially available glass frit, an organic binder and an organic solvent with the metal or oxide powder. There are no particular restrictions on the organic binder, and it may be selected from among various binders such as ethylcellulose or polyvinyl butyral. The organic solvent may be appropriately selected from among various organic solvents such as terpineol, butylcarbitol, acetone or toluene. The mixing ratios in the conductive paste are not particularly restricted, and for example, the organic binder may be used at 1-20 parts by weight and the organic solvent at 1-40 parts by weight with respect to 100 parts by weight as the total weight of the metal and oxide powder. The mixing ratios may be modified as appropriate for adjustment of the flow property of the conductive paste.
A resistor 61 is formed on the main side 23b of the varistor element, spanning the external electrode 30a and external electrode 30b, while a resistor 63 is formed spanning the external electrode 30c and external electrode 30d. The resistors 61, 63 contain the same components as the resistor 60 of the first embodiment described above. That is, the resistors 61, 63 contain an oxide different from bismuth oxide and copper oxide. The resistors 61, 63 may be formed by firing a resistive paste comprising a mixture of glass such as Al2O3—B2O3—SiO2 with the metal and metal oxide in the resistor of the first embodiment.
One end of the resistor 61 is electrically connected to the first internal electrode 33 via the external electrode 30a and lead conductor 37b. The other end of the resistor 61 is electrically connected to the second internal electrode 35 via the external electrode 30b and lead conductor 39b. One end of the resistor 63 is electrically connected to the fourth internal electrode 53 via the external electrode 30c and lead conductor 57b. The other end of the resistor 63 is electrically connected to the fifth internal electrode 55 via the external electrode 30d and lead conductor 59b.
The external electrodes 25-29 (
The external electrodes 25-29 are formed on the outermost surface of the varistor element 23, and have the same composition as the external electrodes of the first embodiment. The external electrodes 25-29 may be formed by firing a conductive paste in the same manner as for the external electrodes 30a-30d described above.
As mentioned above, the third internal electrode 43 is formed overlapping with the first and second internal electrodes 33,35 as viewed from the lamination direction. Consequently, the region overlapping the first internal electrode 33 and third internal electrode 43 in the varistor layer functions as a region exhibiting varistor characteristics, and the region overlapping the second internal electrode 35 and third internal electrode 43 in the varistor layer also functions as a region exhibiting varistor characteristics.
Also, the third internal electrode 43 is formed overlapping with the fourth and fifth internal electrodes 53, 55 as viewed from the lamination direction. Consequently, the region overlapping the fourth internal electrode 53 and third internal electrode 43 in the varistor layer functions as a region exhibiting varistor characteristics, and the region overlapping the fifth internal electrode 55 and third internal electrode 43 in the varistor layer also functions as a region exhibiting varistor characteristics.
In a stacked chip varistor 21 having such a construction, the resistance R, varistor B1 and varistor B2 are connected in a “π”-shaped fashion, as shown in
A process for fabrication of a stacked chip varistor 21 according to this second embodiment of the invention will now be explained with reference to
First, amounts of the zinc oxide as the main component of the varistor layer and of the rare earth metal oxide, calcium oxide, silicon oxide and other components are weighed out, and are then combined to prepare a varistor starting material. The paste for formation of the varistor layer may be an organic paste obtained by kneading the varistor starting material with an organic vehicle, or it may be a water-soluble paste. The organic vehicle is obtained by dissolving a binder in an organic solvent. There are no particular restrictions on the binder used for an organic vehicle, and it may be selected from among various ordinary binders such as ethylcellulose or polyvinyl butyral. There are also no particular restrictions on the organic solvent used, and it may be appropriately selected from among terpineol, butylcarbitol, acetone and toluene, depending on the type of method used to form the varistor layer, such as a printing method or sheeting method. The contents of the organic vehicle and varistor starting material in the paste are not particularly restricted. For example, the organic vehicle may be added so that the binder content is about 1-5 wt % and the organic solvent content is about 10-50 wt % with respect to the total paste. If necessary, additives such as dispersing agents, plasticizers, dielectric materials, insulators and the like may also be included in the paste. As water-soluble paste there may be mentioned solutions of water-soluble binders and dispersing agents in water. There are no particular restrictions on the water-soluble binder, and it may be appropriately selected from among polyvinyl alcohol, cellulose, water-soluble acrylic resins and emulsions.
The paste (slurry) used to form the varistor layer may be obtained by mixing and crushing the varistor starting material with materials such as the binder, solvent (organic solvent or water) and various additives, for about 20 hours using a ball mill or the like. The mixing ratio of the starting material for preparation of the slurry may be appropriately adjusted to modify the flow property of the slurry.
The slurry is coated onto a film composed of polyethylene terephthalate, for example, by a known method such as a doctor blade method, and then dried to form a coating with a thickness of about 30 μm. The obtained coating is released from the film to obtain a green sheet.
Several electrode sections (in a number corresponding to the number of partitioned chips) corresponding to the first and second internal electrodes 33, 35 are then formed on the green sheet. Similarly, several electrode sections (in a number corresponding to the number of partitioned chips) corresponding to the third internal electrode 43 are formed on a different green sheet. In addition, several electrode sections (in a number corresponding to the number of partitioned chips) corresponding to the fourth and fifth internal electrodes 53, 55 are then formed on a different green sheet.
The electrode sections corresponding to the first to fifth internal electrodes 33, 35, 43, 53, 55 may be formed, for example, by printing a conductive paste obtained by mixing an oxide different from bismuth oxide and copper oxide, metal powder such as Ag particles or Pd particles, glass frit, an organic binder and an organic solvent, using a printing method such as screen printing, and drying the paste. There are no particular restrictions on the organic binder, and it may be selected from among various binders such as ethylcellulose or polyvinyl butyral. The organic solvent may be appropriately selected from among various organic solvents such as terpineol, butylcarbitol, acetone or toluene. The mixing ratios in the conductive paste are not particularly restricted, and for example, the organic binder may be used at 1-20 parts by weight and the organic solvent at 1-40 parts by weight with respect to 100 parts by weight as the total weight of the metal and oxide powder. The mixing ratios may be modified as appropriate for adjustment of the flow property of the conductive paste.
A laminated sheet is then formed by stacking the green sheets with electrode sections formed thereon and green sheets without electrode sections formed thereon, in a prescribed order. The laminated sheet obtained in this manner is cut into chip units, for example, to obtain multiple partitioned green bodies LS2 (see
The green body LS2 is then subjected to heat treatment at 180-400° C. for about 0.5-24 hours to remove the binder, and then to firing at 850-1400° C. for about 0.5-8 hours to obtain a varistor element 23. The firing produces varistor layers from the green sheets GS11-GS14 in the plurality of green bodies LS2. The electrode sections EL2 serve as the first and second internal electrodes 33, 35 and the lead conductors 37a, 37b, 39a, 39b. The electrode sections EL3 serve as the third internal electrode 43 and lead conductor 47. The electrode sections EL4 serve as the fourth and fifth internal electrodes 53, 55 and the lead conductors 57a, 57b, 59a, 59b.
External electrodes 25-29 and external electrodes 30a-30d are then formed on the outer surface of the varistor element 23. Here, a conductive paste is printed onto one main side 23a of the varistor element 23 by a screen printing technique so that it is in contact with the corresponding electrode sections EL2-EL4, and it is dried to form electrode sections corresponding to the external electrodes 25-29. Also, a conductive paste is printed onto the other main side 23b of the varistor element 23 by a screen printing technique so that it is in contact with the corresponding electrode sections EL2, EL4, and it is dried to form electrode sections corresponding to the external electrodes 30a-30d. The electrode sections are then fired at 500-850° C. to obtain a varistor element 23 having external electrodes 25-29 and external electrodes 30a-30d formed thereon. The conductive paste for the external electrodes 25-29 and external electrodes 30a-30d may be one obtained by mixing an oxide different from copper oxide and bismuth oxide, metal powder such as Ag particles or Pd particles, glass frit, an organic binder and an organic solvent.
The resistors 61, 63 are then formed in the following manner. First, resistance regions corresponding to the resistors 61, 63 are formed on the main side 23b of the varistor element 23, spanning each external electrode 30a and external electrode 30b pair, and spanning each external electrode 30c and external electrode 30d pair. Each resistance region corresponding to the resistors 61, 63 is formed by printing resistive paste by a screen printing technique and drying it. The resistive paste is fired at, for example, 800-900° C., to obtain the resistors 61, 63. This procedure yields a stacked chip varistor 21. The external electrodes 25-29 and external electrodes 30a-30d may also be formed simultaneously with the resistors 61, 63.
The resistive paste is a paste containing an oxide different from bismuth oxide and copper oxide. Specifically, it may be a mixture of an ordinary commercially available organic binder and an organic solvent with glass powder. The glass powder used may be a mixture of glass such as Al2O3—B2O3—SiO2 with RuO2. A Sn-based resistive paste may be used, as a mixture of glass such as Al2O3—B2O3—SiO2 with SnO2. An La-based resistive paste may also be used, as a mixture of glass such as Al2O3—B2O3—SiO2 with LaB6. There are no particular restrictions on the organic binder used to produce the resistive paste, and it may be selected from among various binders such as ethylcellulose or polyvinyl butyral. The organic solvent may be appropriately selected from among various organic solvents such as terpineol, butylcarbitol, acetone or toluene. The mixing ratios in the conductive paste are not particularly restricted, and for example, the organic binder may be used at 1-20 parts by weight and the organic solvent at 1-40 parts by weight with respect to 100 parts by weight as the total weight of the metal and oxide powder. The mixing ratios may be modified as appropriate for adjustment of the flow property of the resistive paste.
After firing, the alkali metals (for example, Li, Na and the like) may be diffused from the surface of the varistor element 23. A protective layer (overglaze layer) may also be formed on the outer surface of the stacked chip varistor 21, except for the regions on which the external electrodes 25-29 have been formed. The protective layer may be formed by printing glaze glass (for example, glass composed of SiO2, ZnO, B, Al2O3 or the like) and firing it at 500-600° C.
Thus, the varistor of the second embodiment comprises a pair of external electrodes (30a and 30b or 30c and 30d) containing an oxide different from bismuth oxide and copper oxide on the main side 23b of a varistor element 23 that contains zinc oxide, a rare earth metal oxide, a calcium oxide and a silicon oxide, with a resistor 61 or 63 connecting the pair of external electrodes. The resistors 61, 63 also contain an oxide different from bismuth oxide and copper oxide. This sufficiently inhibits mutual reaction between the external electrodes, resistor and varistor element. This type of stacked chip varistor therefore has low electrostatic capacity while exhibiting excellent varistor characteristics and adequately reducing fluctuations in the resistance value.
Similar to the first embodiment, incidentally, a base glass layer may be formed between the main side 23b of the varistor element and the external electrodes 30a-30d, and/or between the main side 23b of the varistor element and the resistors 61, 63. The base glass layer may be formed by printing a paste containing an oxide ordinarily included in glass other than bismuth oxide and copper oxide, such as SiO2—ZnO—BaO—ZrO2—Al2O3, onto the main side 23b of the varistor element 23 by a screen printing technique, and then drying it and firing at 800-900° C., for example. Then, as mentioned above, the external electrodes 30a-30d and the resistors 61, 63 may be formed on the base glass layer.
The base glass layer-forming paste used to form the base glass layer is prepared by mixing an ordinary commercially available organic binder or organic solvent with the oxide. There are no particular restrictions on the organic binder, and it may be selected from among various binders such as ethylcellulose or polyvinyl butyral. The organic solvent may be appropriately selected from among various organic solvents such as terpineol, butylcarbitol, acetone or toluene. The mixing ratios in the conductive paste are not particularly restricted, and for example, the organic binder may be used at 1-20 parts by weight and the organic solvent at 1-40 parts by weight with respect to 100 parts by weight as the total weight of the metal and oxide powder. The mixing ratios may be modified as appropriate for adjustment of the flow property of the base glass layer-forming paste.
Incidentally, both the external electrodes 25, 26, 28, 29 that function as input/output terminal electrodes and the external electrode 27 that functions as a ground terminal electrode, in the stacked chip varistor 21 of the second embodiment, are situated on one main side 23a of the varistor element 23. That is, the stacked chip varistor 21 is a stacked chip varistor in the form of a BGA (Ball Grid Array) package. This stacked chip varistor 21 is mounted on an external board by electrically and mechanically connecting each of the external electrodes 25-29 and the external board lands corresponding to each of the external electrodes 25-29 using solder balls. When the stacked chip varistor 21 has been mounted on the external board, each of the internal electrodes 33, 35, 43, 53, 55 extend in the direction perpendicular to the external board.
The embodiments described above are preferred embodiments of the invention, but the invention is not necessarily limited thereto.
The present invention will now be explained in greater detail based on examples and comparative examples, with the understanding that these examples are in no way limitative on the invention.
First, a starting powder was prepared containing zinc oxide as the main component and the components listed in Table 1 as accessory components. The contents in Table 1 are proportions with respect to zinc oxide. The starting powder, organic binder, organic solvent and additives were mixed and crushed for 20 hours using a ball mill to obtain a slurry for the varistor element.
<Preparation of Conductive Paste for Formation of External Electrodes>
A conductive paste was prepared containing the components for “Electrode A” listed in Table 2. Specifically, each of the components of the “Electrode A” shown in Table 2 were mixed in the proportions shown in Table 2 to prepare a starting mixture.
The starting mixture was mixed with the organic binder and organic solvent for 20 hours using a ball mill to obtain a conductive paste for formation of external electrodes.
<Preparation of Resistive Paste>
A resistive paste was prepared containing the components for “Resistor a” listed in Table 2. Specifically, each of the components of the “Resistor a” shown in Table 2 were mixed in the proportions shown in Table 2 to prepare a starting mixture.
The prepared starting mixture was mixed with the organic binder and organic solvent for 20 hours using a ball mill to obtain a resistive paste for formation of the resistor.
<Fabrication of Stacked Chip Varistor>
The slurry and each paste prepared in the manner described above were used to fabricate a stacked chip varistor corresponding to the second embodiment described above. The procedure for production of the stacked chip varistor will now be explained with reference to
First, the varistor element slurry prepared in the manner described above was coated onto a film composed of polyethylene terephthalate by a doctor blade method, and then dried to form a coating with a thickness of 30 μm. The obtained coating was released from the film to obtain a green sheet.
Electrode sections were then formed on the green sheet, corresponding to the first and second internal electrodes 33, 35 (
The electrode sections corresponding to the first to fifth internal electrodes 33, 35, 43, 53, 55 were formed by printing ordinary conductive paste by a screen printing technique, and then drying it.
A laminated sheet was then formed by stacking the green sheets with electrode sections formed thereon and green sheets without electrode sections formed thereon, in a prescribed order. The laminated sheet obtained in this manner was cut into chip units to obtain multiple partitioned green bodies LS2 (see
The plurality of green bodies LS2 were then subjected to heat treatment to remove the binder, and then to firing to obtain a varistor element 23.
After then printing a commercially available Ag—Pt paste onto one main side 23a of the varistor element 23 by a screen printing technique and drying it, it was fired at 900-1100° C. to form electrode sections (Ag—Pt conductors) corresponding to external electrodes 25-29.
Then, after printing conductive paste prepared in the same manner onto the main side 23b of the varistor element 23 by a screen printing technique, it was dried to form electrode sections corresponding to external electrodes 30a-30d. These electrode sections were then fired at 850° C. to obtain a varistor element having external electrodes 30a-30d formed on the main side 23b.
A resistive paste prepared in the same manner as described above was then printed by a screen printing technique in such a fashion as to span each pair of external electrodes 30a and external electrode 30b, and each pair of external electrodes 30c and external electrode 30d. The resistive paste was dried and fired at 850° C. to form resistors 61, 63. This connected the external electrode 30a and external electrode 30b by the resistor 61, and connected the external electrode 30c and external electrode 30d by the resistor 63. The process described above produced a stacked chip varistor 21 as shown in
<Evaluation of Reactivity>
A cross-section of the fabricated stacked chip varistor was subjected to Electron probe microanalyzer analysis (EPMA) to determine whether reaction products were present in the varistor element. Based on the EPMA analysis, reactivity level A was judged when no reaction products were detected that could adversely affect the varistor characteristics (no elements absent in the starting material were detected), and reactivity level B was judged when reaction products were detected (elements absent in the starting material were detected). The results are shown in Table 2.
<Evaluation of Resistance Value>
The resistance value of the fabricated stacked chip varistor was measured. Specifically, the resistance value was measured between the external terminal electrode 25 (26) and the external terminal electrode 29 (28) in the equivalent circuit shown in
Stacked chip varistors were fabricated and evaluated in the same manner as Example 1, except that for the materials in the resistor-forming resistive paste, the “Resistor a” components shown in Table 2 were changed to the “Resistor b-g” components. The evaluation results are shown in Table 2. The composition of the conductive paste used to form the electrodes was the same as in Example 1.
A stacked chip varistor was fabricated and evaluated in the same manner as Example 1, except that for the materials in the external electrode-forming conductive paste, the “Electrode A” components shown in Table 2 were changed to the “Electrode B” components shown in Table 3. The evaluation results are shown in Table 3.
Stacked chip varistors were fabricated and evaluated in the same manner as Comparative Example 3, except that for the materials in the resistor-forming resistive paste, the “Resistor a” components shown in Table 3 were changed to the “Resistor b-i” components. The evaluation results are shown in Table 3. The composition of the conductive paste used to form the electrodes was the same as in Comparative Example 3.
A stacked chip varistor was fabricated and evaluated in the same manner as Example 2, except that for the materials in the external electrode-forming conductive paste, the “Electrode A” components shown in Table 2 were changed to the “Electrode C” components shown in Table 4. The evaluation results are shown in Table 4.
Stacked chip varistors were fabricated and evaluated in the same manner as Comparative Example 12, except that for the materials in the resistor-forming resistive paste, the “Resistor b” components shown in Table 4 were changed to the “Resistor c-e” and “Resistor g-i” components. The evaluation results are shown in Table 4. The composition of the conductive paste used to form the external electrodes was the same as in Comparative Example 12.
A stacked chip varistor was fabricated and evaluated in the same manner as Example 2, except that for the materials in the external electrode-forming conductive paste, the “Electrode A” components shown in Table 2 were changed to the “Electrode D” components shown in Table 5. The evaluation results are shown in Table 5.
Stacked chip varistors were fabricated and evaluated in the same manner as Comparative Example 19, except that for the materials in the resistor-forming resistive paste, the “Resistor b” components shown in Table 5 were changed to the “Resistor c-e” and “Resistor g-r” components. The evaluation results are shown in Table 5. The composition of the conductive paste used to form the external electrodes was the same as in Comparative Example 19.
A conductive paste was prepared containing the components for “Electrode A” listed in Table 6, in the same manner as Example 1. A resistive paste was also prepared containing the components for resistor h listed in Table 6.
<Preparation of Paste for Base Layer Glass>
A paste was prepared containing the components for “Base layer glass 1” listed in Table 6. Specifically, each of the components of the “Base layer glass 1” shown in Table 6 were mixed in the proportions shown in Table 6 to prepare a starting mixture. The prepared starting mixture was mixed with the organic binder and organic solvent for 20 hours using a ball mill to obtain a paste for formation of the base layer glass.
<Fabrication of Stacked Chip Varistor>
Each paste prepared in the manner described above was used to fabricate a varistor element 23 in the same manner as Example 1. After printing the base layer glass-forming paste prepared as described above onto the main side 23b of the varistor element 23 by a screen printing technique, it was dried and fired at 850° C. to form a base glass layer.
Then, after printing external electrode-forming conductive paste prepared as described above onto the formed base glass layer by a screen printing technique, it was dried to form electrode sections corresponding to external electrodes 30a-30d. These electrode sections were then fired at 850° C. to obtain a varistor element having external electrodes 30a-30d formed on the base glass layer (not shown).
A resistive paste prepared in the same manner described above was then printed by a screen printing technique in such a fashion as to span each pair of external electrodes 30a and external electrode 30b, and each pair of external electrodes 30c and external electrode 30d. The resistive paste was dried and fired at 850° C. to form resistors 61, 63. The process described above produced a stacked chip varistor 21 as shown in
The reactivity was evaluated in the same manner as Example 1. The results are shown in Table 6.
Stacked chip varistors were fabricated and evaluated in the same manner as Comparative Example 26, except that for the materials in the base layer glass-forming paste, the “Base layer glass 1” components shown in Table 6 were changed to the “Base layer glass 2-6” components. The evaluation results are shown in Table 6. The compositions of the pastes used to form the electrodes and resistor were the same as in Comparative Example 26.
The stacked chip varistors comprising the resistors 61, 63 and the external electrodes 30a-30d containing no bismuth oxide and copper oxide had no reaction products in the varistor element, and the resistance fluctuation was also minimal.
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