JP2016054239A - Laminated variable varistor and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reliable laminated variable varistor superior in ESD resistance capable of lowering the voltage.SOLUTION: The laminated variable varistor includes: a voltage nonlinear resistive element 11 which is constituted of plural ZnO crystal particles and an oxide containing Sr and Co among the ZnO crystal particles; plural internal electrodes 12 in the voltage nonlinear resistive element 11; and a surface layer 14 containing Zn smaller amount than a region between the internal electrodes 12. The region between the internal electrodes 12 contains no Bi or B, but the surface layer 14 contains Bi or B.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a laminated varistor for protecting various electronic devices from abnormal voltage and a method for manufacturing the same.

  In recent years, ICs have become more vulnerable to static electricity (ESD) with higher integration, and the need for ESD countermeasures has increased. As this ESD countermeasure component, a multilayer varistor is widely used.

In order to further improve the protection performance of the varistor, it is essential that the operating voltage is reduced and that the ESD resistance is excellent so that the ESD is not destroyed. Such varistors are mainly composed of two types of systems in which Bi or Pr is added to ZnO for characteristic expression, but these systems have a high operating voltage and insufficient ESD resistance. Therefore, SrCoO ₃ has been studied as a new additive. Since SrCoO ₃ has a low barrier to ZnO and a high melting point, it has lower pressure and improved ESD resistance compared to conventional Bi and Pr systems.

  On the other hand, there is an increasing demand for reliability such as moisture resistance.

  As prior art document information related to the invention of this application, for example, Patent Document 1 is known.

JP 2011-216877 A

However, alkaline earth metals are generally characterized by high reactivity with water, and SrCoO ₃ is also decomposed by water to produce SrO. As a result, in addition to the decrease in the insulation resistance due to the decomposition of the grain boundary oxide, the volume expansion of the grain boundary portion occurs due to the generation of Sr (OH) ₂ hydrate due to the reaction of SrO and water, resulting in cracks. There is a possibility that water will penetrate into the interior and the characteristics will deteriorate.

  The present invention solves the above-described conventional problems, and an object thereof is to provide a multilayer varistor that can be reduced in pressure and is excellent in ESD resistance and reliability.

  In order to solve the above problems, the present invention provides a voltage non-linear resistor comprising a plurality of ZnO crystal particles and an oxide containing Sr and Co between the ZnO crystal particles, and the voltage non-linear resistor. A laminated varistor including a plurality of internal electrodes and a surface layer portion having a lower Zn content than a region sandwiched between the internal electrodes, wherein Bi and B are included in the region sandwiched between the internal electrodes However, this surface layer portion is configured to contain Bi or B.

  By reacting Sr at the outer periphery of the voltage non-linear resistor with Bi or B to produce reactive organisms that are insoluble in water, the multilayer varistor with improved moisture resistance is prevented by preventing decomposition of grain boundary additives by water. Can be obtained. By making the surface layer portion of the element porous with a small Zn content, the depth of reaction of Sr with Bi or B can be increased, diffusion is promoted, and moisture resistance is further improved.

Sectional drawing of the lamination | stacking varistor in one embodiment of this invention Sectional drawing which shows the element distribution measurement point of the lamination | stacking varistor in one embodiment of this invention

  Hereinafter, a laminated varistor according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a cross-sectional view of a multilayer varistor according to an embodiment of the present invention, in which a plurality of internal electrodes 12 are provided between voltage nonlinear resistors 11, and the internal electrodes 12 are electrically connected to both ends. A connected external electrode 13 is provided, and a plating layer 15 is formed on the external electrode 13. The voltage non-linear resistor 11 is a sintered body composed of a plurality of ZnO crystal particles and SrCoO ₃ precipitated between the ZnO crystal particles, and the varistor characteristics are expressed by configuring in this way. It is a thing. Since SrCoO ₃ has a low barrier to ZnO and a high melting point, it can be made to have a lower pressure than conventional Bi-based and Pr-based materials and can also improve ESD resistance. In addition, the element using the voltage non-linear resistor 11 has a surface layer portion 14 having a lower Zn content than the region where the outer peripheral portion is sandwiched between the internal electrodes. The Zn content in the surface layer portion 14 is 90% or less of the Zn content in the region sandwiched between the internal electrodes, and the surface layer portion 14 has pores corresponding to the reduced amount.

However, alkaline earth metals are generally characterized by high reactivity with water, and SrCoO ₃ is also decomposed by water to produce SrO. As a result, in addition to the decrease in the insulation resistance due to the decomposition of the grain boundary oxide, the volume expansion of the grain boundary portion occurs due to the generation of Sr (OH) ₂ hydrate by the reaction of SrO and water, and cracks occur. This is likely to occur, and water may penetrate into the interior, resulting in deterioration of properties.

On the other hand, in the present embodiment, the outer peripheral portion of the voltage nonlinear resistor is made a porous surface layer portion 14 having a small Zn content, and glass containing Bi ₂ O ₃ or B ₂ O ₃ is diffused. Since the surface layer portion 14 is in a porous state, the area where Sr reacts with Bi or B increases, and diffusion is promoted. In this surface layer portion 14, a reaction product of Sr and Bi or Sr and B is formed. This reaction product is hardly soluble in water, and the moisture resistance is improved. Therefore, the pore surface of the surface layer portion 14 is an insolubilized reaction product, and moisture resistance is guaranteed even in a porous state. On the other hand, Bi and B are not included in the region sandwiched between the internal electrodes 12. This is because when Bi or B is contained in addition to Sr, Bi or B combines with SrCoO ₃ and the barrier disappears, so that the varistor characteristics are impaired. Here, Bi and B are not included, which means that the atomic percentage is 0.01% or less in order to maintain the varistor characteristics.

  The surface layer portion 14 loses the varistor characteristics and has a low resistance. Therefore, in the worst case, conduction occurs between the internal electrode 12 and the external electrode 13 via the surface layer portion 14. In order not to impair the function as a varistor, the resistance value between the surface layer portion 14 and the internal electrode 12 needs to be greater than or equal to the resistance value between the internal electrodes 12. That is, the distance between the surface layer portion 14 and the internal electrode 12 is ensured so that a distance greater than the resistance value between the internal electrodes can be obtained. Further, it is preferable that the surface layer portion 14 has a thickness of 10 μm or more and is deeply distributed inside the element as long as the varistor characteristics are not impaired.

  In addition, the Sr content contained in the surface layer portion 14 is set lower than the Sr content in the region sandwiched between the internal electrodes. The region sandwiched between the internal electrodes 12 needs a predetermined Sr content in order to obtain a predetermined varistor characteristic, but the surface layer portion 14 maintains the varistor characteristic and prevents moisture from the outside from entering. Therefore, Sr is not necessary. Therefore, by reducing the content of Sr contained in the surface layer portion 14 and making the remaining Sr a reaction product with Bi or B, a laminated varistor that is strong against water can be obtained.

  Next, the manufacturing method of the laminated varistor in one embodiment of the present invention will be described.

First, as starting materials, ZnO powder as a main component, SrCO ₃ powder and Co ₂ O ₃ powder containing alkaline earth metal as subcomponents, and higher purity Al ₂ O ₃ are prepared and weighed to a desired composition To do. SrCO ₃ powder and Co ₂ O ₃ powder form a grain boundary layer.

  Next, these starting raw material powders weighed were put into a polyethylene ball mill, φ5 mm stabilized zirconia cobblestone and pure water were added, mixed for about 20 hours, and dehydrated and dried. The dried powder was pulverized, mixed with an organic binder and the like, and a stabilized zirconia cobblestone having a diameter of 10 mm was added and mixed to obtain a uniformly dispersed slurry. A ceramic sheet is produced using this slurry.

A Cu paste for internal electrodes was printed on the ceramic sheet by a screen printing method to form a predetermined number of conductor layers. These and a ceramic sheet on which the internal electrode conductive paste was not printed were laminated and pressed so as to have a predetermined shape, thereby producing a laminate block. Form a ceramic sintered body by the elements obtained by cutting and separating the desired dimensions which the oxygen partial pressure is sintered at 1000 to 1100 ° C. under a low oxygen atmosphere of 10- ¹ Pa from 10- ⁵ Pa did. By firing in an atmosphere having a low oxygen partial pressure in this manner, Zn near the surface of the element is easily evaporated, and the surface layer portion 14 having a lower Zn content than the region sandwiched between the internal electrodes is formed in the outer peripheral portion. .

Next, a mixed powder obtained by mixing glass powder containing at least one of Bi and B and Al ₂ O ₃ powder at a weight ratio of 1: 9 is used twice the weight of the ceramic sintered body element. Then, the device was muffled with the mixed powder, and glass diffusion heat treatment was performed at about 800 ° C. and an oxygen partial pressure of less than 4.24 × 10 ¹ Pa.

Here, Al ₂ O ₃ powder having an average particle diameter of about 50 μm was used, and glass powder having an average particle diameter of about 7 μm was used. Thus, by using a glass powder having a particle size smaller than that of Al ₂ O ₃ powder and using an average particle size of 10 μm or less, Bi or B in the glass powder is converted into a ceramic sintered body element. Can rapidly diffuse to the surface of the substrate. The diffused Bi or B reacts with Sr to form a reaction product insoluble in water. By doing so, Sr existing near the surface of the ceramic sintered body element reacts with Bi or B in the glass powder near the surface and is discharged out of the ceramic sintered body element. More desirably, the average particle size of the glass powder is preferably 1/3 or less of the average particle size of the Al ₂ O ₃ powder. By doing so, the movement of Bi or B and Sr can be further promoted.

Furthermore, it is desirable that the ratio of the Al ₂ O ₃ powder in the mixed powder is 80% or more and 95% or less by weight. When the proportion of Al ₂ O ₃ powder is less than 80%, the proportion of the glass powder in contact with the ceramic sintered body element becomes too large, and it becomes hard to adhere to the surface. On the other hand, when the proportion of Al ₂ O ₃ powder exceeds 95%, it becomes difficult to react with Sr near the surface of the ceramic sintered body element.

  By doing so, Sr contained in the surface layer part 14 of the element is discharged outside during the glass diffusion heat treatment, and the remaining Sr and Bi or B in the glass are reacted to form a reaction product. be able to.

Thereafter, heat treatment is performed at an oxygen partial pressure of 4.24 × 10 ¹ Pa or more and 500 to 750 ° C., thereby exhibiting varistor characteristics. At this time, the atmosphere is preferably close to the air atmosphere. After removing Cu oxide on the end face of the internal electrode 12 generated during the heat treatment to expose the Cu internal electrode, the external electrode paste is applied and then dried, and the external electrode is baked at 700 to 1100 ° C., and voltage nonlinearity A laminated varistor having a resistor as an element was obtained.

During the glass diffusion heat treatment, the mixed powder and the ceramic sintered body element were put in a sheath and heat-treated while being rotated at 1.5 rpm in a rotary furnace. By doing so, the glass diffusion treatment was made uniform. When this element was measured with an electron beam microanalyzer (EPMA) at a diameter of about 10 μm per part of the measurement site, glass diffusion of 10 μm or more was confirmed from the surface layer. As a result of conducting a voltage application test in the humidity (60 ° C., RH 95%, 3.5 V applied, 500 H) on the elements prepared as described above, the characteristic fluctuation range of all elements is ΔV _{1 mA} <± 10% at N = 25. Settled in. On the other hand, in the case of an element manufactured by the same method without providing the surface layer portion 14 having a low Zn content, the glass diffusion layer is shallower than the surface layer by 3 μm or less. In all of the devices, V _{1 mA} decreased by 40% or more. Since the moisture resistance performance of the element subjected to this treatment is improved, the diffusion of glass is promoted by making the surface layer porous, and the SrO is insolubilized by this glass diffusion treatment. I can say. Further, by performing glass diffusion heat treatment in a rotary furnace, moisture resistance can be improved without variation between elements.

  About the element which performed glass diffusion heat processing using the glass powder containing Bi on condition of 800 degreeC-15 minutes, in the component of glass, the diffusion state of Bi of a heavy element in EPMA is about 10 micrometers aperture diameter per part of measurement location. The measurement was performed. From the result of element mapping, it was found that Bi was distributed from the device surface layer to a depth of about 50 μm when heat treatment was performed in the atmosphere. In addition, it was found that the components contained in this glass are distributed at the same position as Sr, which is the cause of the deterioration in moisture, and react selectively. Further, when the Sr content was examined, it was found that the content of the glass diffused was lower than that of the portion sandwiched between the internal electrodes. For the number of elements N = 3, as shown in FIG. 2, positions 1, 2, and 3 corresponding to the order from the upper surface of the element toward the lower side in the surface layer portion 14 where the glass diffuses, and the region sandwiched between the internal electrodes 12 The contents of Zn, Sr, Co and Bi at positions 4, 5, and 6 inside a certain element were analyzed by EPMA. The positions of the points are 10 μm, 30 μm, 50 μm, 495 μm, 500 μm, and 505 μm from the top of the element in order from 1.

  (Table 1) describes the detected intensities of Zn, Sr, Co, and Bi at each measurement point. (Table 2) is an average of the three elements of the detection intensity of each element at each measurement point.

  It can be seen that at the measurement points 1 to 3, the detection intensity of 10 to 40% Zn is lower than the measurement points 4 to 6 inside the element. The decrease in Zn strength is due to evaporation of ZnO during the firing process, and a porous structure in which open pores corresponding to the decrease in strength are generated. For Co, there is no change at the measurement point, whereas for Sr, the detected intensity is reduced from 1 to 3 compared to 4 to 6, and the amount of reduction is 38% (N = 3 average). And 1-3 contain Bi which is a glass component, and Sr detection intensity | strength is reducing with the presence of Bi. In addition to the selective reaction of Sr and the glass component, it is clear that Sr was discharged as the glass component diffused. Similar results were obtained when glass powder containing B was used.

Moreover, the formation state of a glass layer changes with the atmosphere at the time of heat processing of glass diffusion. When the treatment is performed at an oxygen partial pressure of 2.02 Pa and 750 ° C. to 900 ° C., the diffusion layer thickness confirmed by EPMA element mapping is 10 μm. When the oxygen partial pressure was 4.24 × 10 ¹ Pa in the same heat treatment profile, the glass diffusion was 20 μm. In addition, since the internal electrode is oxidized, the varistor characteristics cannot be maintained. However, the glass diffusion layer of the element heat-treated in the atmosphere (oxygen partial pressure 1.01 × 10 ⁵ Pa) was 50 μm. It can be seen that the diffusion tends to proceed as the O ₂ concentration increases. The thickness of the surface layer portion 14 can also be controlled by the atmosphere during the diffusion treatment. Further, even when the atmosphere of the heat treatment is close to inert, the glass diffusion layer is formed, so that it is possible to treat even an element having a base metal electrode that is oxidized when heat-treated in the atmosphere. Moreover, about the element baked in air | atmosphere, even if it performed the glass diffusion process in air | atmosphere, it diffused only to the depth of 10 micrometers from the surface layer. It can be seen that by diffusing Zn on the device surface layer and making the surface layer porous, the surface area is increased, and thus the glass can be more easily diffused.

  The laminated varistor according to the present invention is industrially useful because it can reduce the pressure, obtain an ESD resistance and excellent reliability.

11 Voltage Nonlinear Resistor 12 Internal Electrode 13 External Electrode 14 Surface Layer 15 Plated Layer

Claims (8)

A voltage non-linear resistor comprising a plurality of ZnO crystal particles, an oxide containing Sr and Co between the ZnO crystal particles, a plurality of internal electrodes inside the voltage non-linear resistor, and the internal A multilayer varistor including a surface layer portion having a lower Zn content than a region sandwiched between electrodes, wherein Bi and B are not included in the region sandwiched between the internal electrodes, and Bi or B is included in the surface layer portion. A multilayer varistor comprising B. The multilayer varistor according to claim 1, wherein the Sr content in the surface layer portion is lower than the Sr content in a region sandwiched between the internal electrodes. The multilayer varistor according to claim 1, wherein Sr in the surface layer part forms a reaction product with Bi or B. A step of forming a ceramic sheet by mixing ZnO powder as a main component and SrCO ₃ powder and Co ₂ O ₃ powder as subcomponents, a step of forming an internal electrode on the ceramic sheet, and forming the internal electrode The surface layer portion having a lower Zn content than the region sandwiched between the internal electrodes by firing the laminated ceramic sheet and the ceramic sheet not forming the internal electrode, pressurizing and cutting the element in a low oxygen atmosphere And a mixed powder of a powder containing at least one of Bi and B and an Al ₂ O ₃ powder is muffled on the ceramic sintered body and heat-treated to obtain Bi or And a step of diffusing B into a surface layer portion of the ceramic sintered body. The method for producing a laminated varistor according to claim 4, wherein the Sr content in the surface layer portion is lower than the Sr content in a region sandwiched between the internal electrodes. 5. The method of manufacturing a laminated varistor according to claim 4, wherein Sr in the surface layer portion forms a reaction product with Bi or B. The method for producing a laminated varistor according to claim 4, wherein a ratio of the Al ₂ O ₃ powder in the mixed powder is 80% or more and 95% or less by weight. 5. The powder containing at least one of Bi and B uses glass powder, and the average particle diameter of the glass powder is smaller than the average particle diameter of the Al ₂ O ₃ powder. The manufacturing method of the lamination varistor of description.

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