Surge absorption element

Abstract

The present disclosure addresses the problem of realizing a favorable and stable ESD suppression effect and tolerance against abnormal voltage and DC voltage. A surge-absorbing element (1) comprises: an element body (11) having a pair of end surfaces facing each other and a plurality of lateral surfaces adjacent to the end surfaces; at least a pair of internal electrodes (13) provided inside the element body; and at least a pair of external electrodes that are provided on the end surfaces and that are electrically connected to the internal electrodes (13). The element body (11) has: a function part (12) having a polycrystal structure composed of a plurality of crystal particles exhibiting voltage non-linearity and having voids; and an outer shell part for covering the function part. The internal electrodes (13) are provided so as to face each other with the function part (12) interposed therebetween. The external electrodes include: at least a pair of primary external electrodes (14) provided on the end surfaces; and at least a pair of secondary external electrodes (15) provided on the primary external electrodes (14) and electrically connected thereto. The elastic modulus of the secondary external electrodes (15) is lower than that of the primary external electrodes (14).

Description

[Technical Field]

[0001] This disclosure relates to a surge absorption element, and more specifically, to a surge absorption element having a functional part that exhibits voltage nonlinearity characteristics. [Background technology]

[0002] Semiconductor devices such as ICs and LSIs are damaged or their characteristics degrade due to electrostatic discharge (ESD). In recent years, with the increasing speed of operation, IC wiring patterns have become finer, making them even more vulnerable to ESD. Furthermore, with the certainty of increasing communication speeds, the demand for ESD and other abnormal voltage countermeasures in high-speed transmission lines is growing. A common ESD countermeasure is connecting a surge absorption element between the input / output terminal lines of the semiconductor device and ground. This bypasses high-voltage surges (hereinafter referred to as ESD voltage) caused by electrostatic discharge, protecting the semiconductor device. Multilayer varistors are widely used as surge absorption elements. Typically, a multilayer varistor has a ceramic layer, a pair of internal electrodes, a ceramic insulator, and external electrodes. The ceramic insulator may have the same composition as the ceramic layer. The main component of the ceramic layer with varistor properties is ZnO. The internal electrodes face each other across the ceramic layer, forming the varistor function. The external electrodes are drawn out from both ends of the ceramic insulator and electrically connected to the external electrodes. Such a surge absorption element is disclosed in Patent Document 1.

[0003] Conventional multilayer varistors, due to capacitance components resulting from the ceramic layer and ceramic insulator, cause waveform distortion of high-speed signals when used in high-speed signal lines. Therefore, the capacitance of multilayer varistors used for ESD countermeasures in high-speed transmission circuits must be extremely low. The design method for reducing capacitance was to reduce the overlap between internal electrodes and decrease the electrode area. However, as the electrode area decreases, the current density increases when high-voltage surges are applied, and the load increases, leading to deterioration or destruction of the varistor characteristics and a decrease in resistance to ESD. At the same time, the suppression voltage, which is the voltage after the surge voltage has been suppressed by the multilayer varistor, must be increased, reducing the protective effect. In this context, there is a need for a countermeasure device that combines low capacitance with high protection performance and strong resistance, which cannot be achieved with conventional surge absorption elements. To meet these demands, a voltage-specific linear functional section with an air gap between opposing internal electrodes is formed, and surface discharge is caused on the exposed surfaces of multiple crystal grains in the air gap, thereby achieving low capacitance and a high suppression effect. Furthermore, unlike conventional planar gap electrode systems, electrode wear does not occur when ESD is applied, and therefore there is no increase in suppression voltage. Such a surge absorption element is disclosed in Patent Document 2.

[0004] Conventional technologies related to baristas can be found in Patent Documents 1 and 2.

[0005] However, as shown in Patent Document 2, device designs that incorporate air gaps within the device tend to result in unstable suppression effects and resistance to ESD and DC voltage. During ESD suppression, currents exceeding 30A flow instantaneously. Due to the expansion of internal electrodes caused by heat generation and thermal shock to external electrodes and ceramics, there were issues unique to porous internal structures, such as fracture between internal electrodes and delamination of external electrodes. Furthermore, the device was also susceptible to external forces when ESD was applied, leading to fluctuations in the electrical characteristics of the device. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 11-3809 [Patent Document 2] International Publication No. 2010 / 122732 [Overview of the Initiative]

[0007] The objective of this disclosure is to provide a surge absorption element that can achieve good and stable ESD suppression effects and withstand abnormal voltages and DC voltages.

[0008] A surge absorbing element according to one aspect of the present disclosure comprises a body having a pair of end faces facing each other and a plurality of side surfaces adjacent to each of the pair of end faces, at least a pair of internal electrodes provided inside the body, and at least a pair of external electrodes provided on each of the pair of end faces and electrically connected to each of the internal electrodes. The body has a functional portion having a polycrystalline structure composed of a plurality of crystalline grains having voids and exhibiting voltage nonlinearity characteristics, and an outer shell portion covering the functional portion. The internal electrodes are provided facing each other via the functional portion. Each of the external electrodes includes at least a pair of primary external electrodes provided on the end face, and at least a pair of secondary external electrodes provided on the primary external electrodes and electrically connected to the primary external electrodes. The elastic modulus of the secondary external electrodes is lower than that of the primary external electrodes. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a cross-sectional view of a surge absorbing element in one embodiment of the present disclosure. [Figure 2] Figure 2 is a schematic diagram illustrating the method for measuring the suppression voltage. [Figure 3] Figure 3(A) is a pulse waveform diagram in an electrostatic discharge immunity test without a surge absorption element. Figure 3(B) is a pulse waveform diagram in an electrostatic discharge immunity test using a surge absorption element according to an embodiment of the present disclosure. [Figure 4] Figure 4 is a cross-sectional view of another surge absorbing element in one embodiment of the present disclosure. [Figure 5] FIG. 5 is a cross-sectional view of yet another surge absorption element in one embodiment of the present disclosure. [Figure 6] FIG. 6 is a cross-sectional view of yet another surge absorption element in one embodiment of the present disclosure.

MODE FOR CARRYING OUT THE INVENTION

[0010] 1. Overview Hereinafter, a surge absorption element in one embodiment of the present disclosure will be described with reference to the drawings. Note that each of the drawings described in the following embodiments is a schematic diagram, and the ratio of the size and thickness of each component in each drawing does not necessarily reflect the actual dimensional ratio.

[0011] In order to solve the above-described problems, the inventors have intensively studied each component of the surge absorption element. As a result, in a surge absorption element having a primary external electrode and a secondary external electrode as external electrodes, it has been found that there is a relationship between the relaxation of internal stress, the ESD suppression effect, and the reduction in tolerance to abnormal voltages and the like, and the present disclosure has been completed.

[0012] As shown in FIG. 1, the surge absorption element 1 according to the present embodiment includes a body 11, at least one pair of internal electrodes 13 provided inside the body 11, and at least one pair of external electrodes provided on each of the pair of end faces of the body 11 and electrically connected to each of the internal electrodes 13. The body 11 has a functional portion 12 and an outer shell portion that covers the functional portion 12. Each of the at least one pair of external electrodes includes at least one pair of primary external electrodes 14 provided on the end face of the body 11, and at least one pair of secondary external electrodes 15 provided on the primary external electrodes 14 and electrically connected to the primary external electrodes 14. The at least one pair of internal electrodes 13 are provided to face each other via the functional portion 12. The surge absorption element 1 is characterized in that the elastic modulus of the secondary external electrode 15 is lower than that of the primary external electrode 14.

[0013] The surge absorption element 1 can achieve a good and stable ESD suppression effect and withstand voltage against abnormal voltage and DC voltage with the above configuration. Regarding the reason why the surge absorption element 1 of the present embodiment exhibits the above effect, it is not necessarily clear, but it can be speculated as follows. In a conventional surge absorption element, the decrease and instability of the ESD suppression effect, and the decrease in withstand voltage against abnormal voltage and DC voltage are considered to occur when the influence of the internal stress of the surge absorption element is manifested by the thermal shock due to heat generation during the application of a large current during ESD suppression. In the surge absorption element 1 of the present embodiment, by making the elastic modulus of the secondary external electrode 15 lower than the elastic modulus of the primary external electrode 14, it becomes easier to relieve this internal stress, thereby maintaining a good and stable ESD suppression effect and withstand voltage against abnormal voltage and DC voltage, and thus it is considered that it can be realized.

[0014] 2. Details <Surge Absorption Element> The surge absorption element 1 of the present embodiment includes a body 11, an internal electrode 13, a primary external electrode 14, and a secondary external electrode 15.

[0015] At least one pair of the internal electrode 13, the primary external electrode 14, and the secondary external electrode 15 may be provided respectively. In the surge absorption element 1 of FIGS. 1 and 4 to 6, the numbers of the internal electrode 13, the primary external electrode 14, and the secondary external electrode 15 are each 2 (a pair). That is, the internal electrode 13 includes a first internal electrode 13a and a second internal electrode 13b. The primary external electrode 14 includes a first primary external electrode 14a and a second primary external electrode 14b.The secondary external electrode 15 includes a first secondary external electrode 15a and a second secondary external electrode 15b.

[0016] ]] The surge absorption element 1 is mounted on a substrate by joining the first secondary external electrode 15a and the second secondary external electrode 15b to the substrate using a bonding material such as solder. When a surge voltage is applied between the first secondary external electrode 15a and the second secondary external electrode 15b while mounted on the substrate, a surge current flows through the functional unit 12 between the first internal electrode 13a, which is electrically connected to the first secondary external electrode 15a, and the second internal electrode 13b, which is electrically connected to the second secondary external electrode 15b. This protects semiconductor devices and the like that have a substrate.

[0017] The base body 11 has a pair of opposite end faces and a number of side faces adjacent to each of the pair of end faces. The base body 11 is usually shaped like a rectangular parallelepiped with six faces, and the "end faces" refer to two opposite faces with small area (the right face and the left face in Figure 1). The other four faces adjacent to each of these two end faces are called "side faces".

[0018] Figure 1 is a cross-sectional view of a surge absorbing element 1 in an embodiment of the present disclosure. The surge absorbing element 1 in Figure 1 has a ceramic body 11 inside which a first internal electrode 13a and a second internal electrode 13b are provided opposite to the first internal electrode 13a, and the region sandwiched between the first internal electrode 13a and the second internal electrode 13b is the functional part 12. A first primary external electrode 14a and a second primary external electrode 14b (collectively referred to as primary external electrode 14) are provided on both end faces of the body 11, and a first secondary external electrode 15a is provided on the first primary external electrode 14a, and a second secondary external electrode 15b (collectively referred to as secondary external electrode 15) is provided on the second primary external electrode 14b. The first internal electrode 13a and the second internal electrode 13b are thin film sheets having a certain thickness and are made of an Ag-Pd alloy. In addition, metallic materials such as Pd, Au, Ag, or Pt are preferably used. The first internal electrode 13a and the second internal electrode 13b have main surfaces, and at least a portion of the main surfaces of the first internal electrode 13a and the second internal electrode 13b face each other with a gap between them, forming overlapping opposing regions. The first internal electrode 13a and the second internal electrode 13b are drawn out from the functional part 12 toward two opposing end faces of the base body 11, and are electrically connected to the first primary external electrode 14a and the second primary external electrode 14b at the end faces of the base body 11, respectively.

[0019] The functional part 12 is made of a varistor material, which is a voltage-dependent nonlinear resistance composition. Specifically, the functional part 12 is formed of a sintered body having a polycrystalline structure composed of multiple crystalline grains that exhibit voltage nonlinearity characteristics. Such crystalline grains are mainly composed of, for example, ZnO. Such a varistor material contains elements such as Sr, Ca, Co, Cr, Mn, and Al as minor components in addition to ZnO, and these minor components have higher melting points than ZnO. In this embodiment, the composition of the varistor material is 97.5 mol% ZnO and 2.5 mol% of the other minor components. The thickness of the functional part 12 is approximately 6 μm.

[0020] The outer shell may be made of the same material as the functional part 12, or it may be made of a different material. In other words, the main component of the functional part 12 may be different from the main component of the outer shell. When the main component of the outer shell is different from that of the functional part 12, examples of the main component of the outer shell include sintered bodies, epoxy resins, thermosetting resins such as phenolic resins, and other resins.

[0021] If the outer shell is a sintered body, glass ceramics may be used as the sintered body. As for the glass ceramics, a material with MgO, SiO2, and Gd2O3 added to alumina particles and borosilicate glass (with a relative permittivity of approximately 10) may be used. When the main component of the outer shell is glass ceramics, the outer shell contains elements with a smaller work function than the functional part 12, enabling discharge at a lower voltage and achieving a high surge absorption effect. When the main component of the outer shell is resin, internal stress in the base body 11 can be effectively relieved, preventing fracture of the functional part 12, etc. As for the resin used for the outer shell, a resin with high heat resistance is more preferable because a large current flows when ESD is applied, generating a large amount of heat.

[0022] Thus, by making the main component of the functional part 12 and the main component of the outer shell different, the surge absorbing element 1 can exhibit new effects such as fracture prevention and high surge absorption. In the surge absorbing element 1, the main component of the functional part may contain ZnO, and the main component of the outer shell may contain resin. Alternatively, in the surge absorbing element 1, the main component of the functional part 12 may contain ZnO, and the main component of the outer shell may contain glass ceramics.

[0023] The functional part 12 has a porous structure with internal voids, and the polycrystalline structure composed of multiple crystalline grains is connected in the region sandwiched between the first internal electrode 13a and the second internal electrode 13b. In this embodiment, the porosity of the functional part 12 is set to approximately 85%.

[0024] Here, the porosity is calculated by polishing the functional part 12 with Ar ions using the cross-section polishing (CP) method, observing the polished cross-section, and determining the area ratio occupied by voids. This is done for five cross-sections in the base body 11, and the average is used as the porosity.

[0025] With this configuration, when an ESD voltage is applied, a surface discharge occurs on the surface of the varistor material in contact with the void, propagating through the grain boundary barrier, and current is passed between the first internal electrode 13a and the second internal electrode 13b.

[0026] The primary external electrode 14 contains a conductive metal. In this embodiment, Ag is used as the metal powder of the conductive paste. The conductive metal may include at least one selected from Cu, Ni, Pd, Ag-Pd alloy, Au, etc. The glass component may also include at least one selected from B, Si, Zn, Ba, Mg, Al, and Li, etc. The primary external electrode 14 may consist of multiple layers. The thickness of the thickest part of the primary external electrode 14 is approximately 120 μm. This primary external electrode 14 is formed by coating it onto the end face of the base body 11 and then heat-treating it at around 800°C. The elastic modulus of the primary external electrode 14 used in this embodiment is approximately 83 GPa.

[0027] In this embodiment, the secondary external electrode 15 is formed to cover the primary external electrode 14 from the outside, is electrically connected to it, and is made of a low modulus material such as a resin in which metal particles are dispersed.

[0028] Thus, the primary external electrode 14 may not contain resin, while the secondary external electrode 15 may contain resin. This allows for more favorable control of the elastic moduli of the primary external electrode 14 and the secondary external electrode 15.

[0029] The secondary external electrode 15 is formed by applying a thermosetting conductive paste containing metal onto the primary external electrode 14 and curing it by heat treatment. In this embodiment, the metal powder contained in the thermosetting conductive paste is 1-10 μm Ag powder, with a content of 70 wt%. The resistivity of the cured secondary external electrode 15 is 4 × 10⁻⁶. -6 The elastic modulus is approximately 8 GPa, and the thickness of the thickest part is approximately 150 μm. Thus, it is preferable that the thickness of the secondary external electrode 15 is greater than the thickness of the primary external electrode 14. By making the secondary external electrode 15, which has a lower elastic modulus, thicker than the primary external electrode 14, which has a higher elastic modulus, the stress relaxation effect is further improved, and reliability can be enhanced.

[0030] The elastic modulus is evaluated by preparing test specimens from the primary external electrode 14 and secondary external electrode 15 of the surge absorption element 1, or by preparing test specimens from the thermosetting conductive paste used for the primary external electrode 14 and secondary external electrode 15 according to the heat treatment conditions during electrode formation, and then performing the evaluation in accordance with JIS Z2280. Since there is a correlation between elastic deformation and plastic deformation, after forming the primary external electrode 14 and secondary external electrode 15, the relative relationship of softness may be compared by applying an indenter in the same way as Vickers hardness and comparing the size of the resulting marks. By configuring the surge absorption element 1 so that the mark left by the secondary external electrode 15 is larger than that left by the primary external electrode 14 when an indenter is pressed with the same force at each part of the cross-section, the effect of stress relaxation can be obtained.

[0031] In this embodiment, the surge absorption element 1, for example, by setting the porosity of the functional part 12 to approximately 85%, allows surface discharge to occur on the surface where multiple crystal particles in the functional part 12 are exposed in the voids when a surge voltage is applied between the internal electrodes 13, thereby conducting electricity between the internal electrodes 13. This further improves the ESD suppression effect and resistance to abnormal voltages. For this purpose, it is desirable to set the porosity of the functional part 12 to 25% or more and 92% or less. If the porosity is less than 25%, the resistance to ESD decreases. If the porosity is greater than 92%, it becomes difficult to connect the polycrystalline structure of the functional part 12 between the internal electrodes 13, making it difficult to form an electrical current path by surface discharge inside the functional part 12. Furthermore, the porosity of the functional part 12 is more preferably 55% or more and 92% or less, and even more preferably 64% or more and 87% or less. By using this porosity, the suppression voltage can be significantly reduced, and resistance to static electricity can be further enhanced.

[0032] However, if the porosity of the functional part 12 becomes large in this way, the expansion of the internal electrodes 13 due to the heat generated when ESD is applied, as well as the thermal shock to the external electrodes 14, 15 and ceramics, may cause fracture between the internal electrodes 13 or peeling of the external electrodes 14, 15, potentially leading to deterioration or destruction of electrical properties.

[0033] In contrast, the surge absorbing element 1 of this disclosure has primary external electrodes 14 on both end faces of the base body 11, and secondary external electrodes 15 on top of them, with the elastic modulus of the primary external electrode 14 being approximately 83 GPa and the elastic modulus of the secondary external electrode 15 being approximately 8 GPa. By making the elastic modulus of the secondary external electrode 15 much smaller than that of the primary external electrode 14 in this way, a surge absorbing element 1 with excellent electrical performance and reliability can be obtained.

[0034] The elastic modulus of the primary external electrode 14 is E A The elastic modulus of the secondary external electrode 15 is E B In this case, the secondary external electrode material is E after curing. A / E BIt is desirable to use a thermosetting conductive paste having an elastic modulus of ≧3. This is because when heat shock occurs due to heat generation during ESD application, characteristic fluctuations and element damage occur, and stress relaxation becomes more important than preventing cracks during solder mounting compared to conventional multilayer ceramic elements. Further, in the surge absorption element 1 as in the present embodiment, a large current is applied during ESD suppression, and the influence of internal stress becomes apparent due to heat shock caused by heat generation. Therefore, it is important to form the secondary external electrode 15 having a lower elastic modulus. Further, when the change in outside air temperature is large, or when the ESD voltage is large (such as 20 kV or more) and the temperature becomes high during application, the influence of internal stress becomes more apparent. Therefore, A / E B it is more desirable to set it to ≧10. Since the linear expansion coefficient depends on temperature, it also changes depending on the outside air temperature. For example, for a 10°C change in ambient temperature, for 1 m on one side of the material, Ag changes by 0.189 mm. On the other hand, ceramics generally have a small linear thermal expansion coefficient, and a value in the vicinity of 0.05 mm is common (0.044 mm for silicon carbide). Thus, since internal stress is more likely to occur during operation at high temperatures, it is preferable to select a material having a lower elastic modulus for the secondary external electrode 15. However, if the secondary external electrode 15 becomes extremely soft, the vibration resistance and mechanical strength decrease. Therefore, A / E B it is desirable to set it to ≦2000. Thus, it is preferable that 3≦E A / E B ≦2000, and it is more preferable that 10≦E A / E B ≦2000. Further, the primary external electrode 14 and the secondary external electrode 15 may be composed of a plurality of materials as long as the overall elastic modulus of the external electrode portion satisfies the relationship of the present disclosure. To realize this configuration, the thermosetting conductive paste contains 30 wt% to 90 wt% of metal powder and 5 wt% to 70 wt% of thermosetting resin. In order to form the secondary external electrode 15 having a low elastic modulus in order to obtain the absorption effect of internal stress during electrostatic application or external stress during solder mounting, the resin content is preferably 25 wt% to 60 wt%.

[0035] Furthermore, by using Ag for the primary external electrode 14, electrode firing in air becomes possible with a relatively inexpensive metal. In this case, an Ag-Pd alloy was used for the internal electrode 13. It is preferable that both the internal electrode 13 and the primary external electrode 14 contain Ag. This configuration prevents oxidation during ESD suppression and heat treatment, resulting in a low-resistance electrode, thus suppressing the decrease in suppression effect even with repeated ESD application. The primary external electrode 14 may be fired simultaneously with the base body 11, or it may be fired after firing the base body 11. By sintering the primary external electrode 14 and the internal electrode 13 simultaneously, the bonding strength is increased, and a burn prevention effect during high-current inrush is obtained. The primary external electrode 14 may also be formed by plating. Furthermore, since a current of several tens of amperes flows instantaneously through the surge absorption element 1 when ESD is applied, it is desirable that the primary external electrode 14 be void-free and high-density. This configuration allows for sufficient relaxation of internal stress in the element when ESD is applied.

[0036] Furthermore, it is desirable that the secondary external electrode 15 also contains Ag. This allows for relatively inexpensive curing in air and suppresses the increase in resistance due to oxidation during ESD suppression. In addition, the resistivity of the cured secondary external electrode 15 is approximately 4 × 10⁻⁶. -6 The resistance is Ωcm. This configuration makes it possible to suppress heat generation caused by large currents of 10A or more flowing through the surge absorption element during ESD suppression. The resistivity of the secondary external electrode 15 is 5 × 10 -6 It is preferable that the resistance be Ωcm or less. By reducing the resistance of the secondary external electrode 15, more current can be passed during ESD suppression, thus achieving a high ESD suppression effect.

[0037] Furthermore, it is preferable that the relationship between the melting point T1 of the material of the internal electrode 13 and the melting point T2 of the material of the primary external electrode 14 is T1 > T2. That is, it is preferable that the melting point of the internal electrode 13 is higher than the melting point of the primary external electrode 14. The elastic modulus is a temperature-dependent constant, and as heat is generated by the inrush current of a large current, the elastic modulus decreases. The rate of decrease is in good agreement with the melting point of each material. With this configuration, the internal stress during inrush current of a large current is absorbed from the internal electrode 13 to the primary external electrode 14 with a lower elastic modulus, and then to the secondary external electrode 15, thereby preventing damage to the element.

[0038] In this embodiment, by using an Ag-Pd alloy with a higher melting point than Ag, the effect of preventing melting due to heat generation during ESD suppression is also obtained. The internal electrode 13, which is in direct contact with the functional part 12, is often made of a material with a high melting point, especially to prevent thermal damage. For example, if Ag-Pd is selected for the internal electrode 13, a combination in which the primary external electrode 14 is made of Ag or Ag-Pd with a lower Pd content than the internal electrode 13 is preferred. Also, if Pt is selected for the internal electrode 13, a combination in which the primary external electrode 14 is made of Cu or Ag-Pd is preferred.

[0039] The shape of the metal particles contained in the primary external electrode 14 and the secondary external electrode 15 may be any shape, such as spherical, flake-shaped, or needle-shaped. Furthermore, the particle size is not particularly limited. For example, smaller particle sizes allow sintering to proceed at lower temperatures, resulting in improved conductivity. Therefore, the particle size and shape are appropriately selected considering the influence of thermal history on process design and electrical properties. Depending on the shape of the metal particles, the resistance of the secondary external electrode 15 may be further reduced by performing particle orientation treatment, such as applying a magnetic field.

[0040] Furthermore, the resin used in the thermosetting conductive paste can be any resin that functions as a binding binder, and is selected appropriately depending on the manufacturing process adopted, considering factors such as printability and coatability. The resin used in the thermosetting conductive paste includes, for example, thermosetting resins. Examples of thermosetting resins include (i) amino resins such as urea resin, melamine resin, and guanamine resin; (ii) epoxy resins such as bisphenol A type, bisphenol F type, phenol novolac type, and alicyclic type; (iii) oxetane resin; (iv) phenol resins such as resol type and novolac type; and (v) silicone-modified organic resins such as silicone epoxy and silicone polyester. Only one of these materials may be used as the resin, or two or more of these materials may be used in combination.

[0041] When the surge absorption element 1 has a small and thin design with nominal external dimensions of 2.0 mm in length, 1.25 mm in width, and 1.25 mm in height or less, good surge absorption characteristics can be achieved by setting the spacing between the internal electrodes 13 in the region where the internal electrodes 13 face each other (thickness of the functional part) to 2 μm to 50 μm. In this embodiment, the thickness of the internal electrodes 13 is approximately 6 μm. It is preferable that the thickness of the internal electrodes 13 be 5 μm or more. By making it this thickness, burnout of the internal electrodes due to discharge can be prevented and resistance to static electricity can be improved. However, conventionally, increasing the electrode thickness increased the internal stress due to thermal expansion of the internal electrodes 13 when static electricity is applied, causing fracture and destruction of the element, and the thickness of the internal electrodes 13 was limited to 5 μm. By forming a secondary external electrode 15 as in this embodiment and relieving the internal stress, it becomes possible to make the thickness of the internal electrodes 13 thicker than 5 μm, and resistance to static electricity can also be improved.

[0042] Here, the evaluation method for the surge absorption element 1 in this embodiment will be described. The electrostatic discharge (ESD) test is performed using the measuring apparatus shown in Figure 2, based on the electrostatic discharge immunity test in accordance with IEC 61000-4-2. In the measuring apparatus shown in Figure 2, the surge absorption element 1 of the evaluation sample mounted on the evaluation board is connected between the line and GND. Then, an electrostatic pulse with a predetermined ESD voltage is output from a discharge gun connected to the electrostatic simulator to the line on the upstream side of the surge absorption element. When the surge absorption element operates, the electrostatic pulse is bypassed to GND and absorbed, and as a result, the pulse component of the line on the downstream side of the surge absorption element 1 is suppressed. When evaluating the suppression voltage, the pulse waveform on this downstream side is observed with an oscilloscope, and the peak voltage value of the pulse waveform is taken as the suppression voltage. The electrostatic simulator has a charging capacity of 150 pF and a discharge resistance of 330 Ω, and the oscilloscope is observed in a 50 Ω system. In the repeated ESD test, the application of the ESD voltage in accordance with the electrostatic discharge immunity test described above is repeated.

[0043] Figures 3(A) and 3(B) are pulse waveform diagrams observed with an oscilloscope during an electrostatic discharge immunity test with an ESD voltage of 8kV. The horizontal axis represents time (nsec), and the vertical axis represents voltage (V). Figure 3(A) shows the pulse waveform diagram when the surge absorption element is not attached, and Figure 3(B) shows the pulse waveform diagram of this embodiment. By attaching the surge absorption element, static electricity of 1kV or more is suppressed to 200V or less. Also, in Figure 3(B), only peak C is observed at the time corresponding to peak A, and no peak appears at the time corresponding to peak B. This indicates that surface discharge occurred in the functional part 12 when ESD was applied, and a short circuit was confirmed.

[0044] The ESD voltage was set to 15kV, and the lead wires were brought into contact with the surge absorption element 1. The application cycle was 100 times. In this embodiment, no cracks or fractures occurred in the element.

[0045] Figure 4 is a cross-sectional view of a surge absorbing element 1 in another embodiment of the present disclosure. The outer periphery of both end faces of the element 11 is not covered by the primary external electrode 14, but is covered by the secondary external electrode 15. The bonding strength between the element 11 and the secondary external electrode 15 is smaller than the bonding strength between the element 11 and the primary external electrode 14. Thus, it is preferable that the side surfaces of the element 11 are not covered by the primary external electrode 14, but are covered by the secondary external electrode 15, and that the bonding strength between the secondary external electrode 15 and the element 11 is smaller than the bonding strength between the primary external electrode 14 and the element 11. This prevents open fracture by causing delamination at the interface between the element 11 and the secondary external electrode 15, where the bonding strength is small, when internal and external stresses act on the surge absorbing element 1. The bonding strength is evaluated by either creating a device with lead wires soldered to the primary external electrode 14 and secondary external electrode 15 of the surge absorption element 1, or by applying a conductive paste containing glass and metal to the base body 11, baking it, applying the conductive paste for the primary external electrode 14 or the conductive paste for the secondary external electrode 15 to the end face of the base body 11, baking or hardening it under the same conditions as when the electrodes were formed, soldering the lead wires, and then performing a tensile test. By increasing the area in which the end face of the base body 11 and the secondary external electrode 15 are directly joined, internal stress can be absorbed more efficiently. Furthermore, if the thickness of the functional part 12 is reduced and the suppression effect is improved, the current when ESD is applied will increase, and thermal expansion of the internal electrode will be more likely to occur. However, this configuration further enhances the suppression effect of thermal deformation of the internal electrode 13, thus also showing an effect in maintaining insulation.

[0046] Figure 5 shows a cross-sectional view of yet another surge absorption element 1 in this embodiment. The primary external electrode 14 is electrically connected to the internal electrode 13 and the secondary external electrode 15, and has a region at its end that is not partially covered by the secondary external electrode 15. Thus, a portion of the primary external electrode 14 may have a region that is not partially covered by the secondary external electrode 15. Furthermore, it is preferable to solder to this region of the primary external electrode 14 that is not covered by the secondary external electrode 15. With this configuration, when soldered to the substrate, a portion of the solder is directly connected to the primary external electrode 14 without going through the secondary external electrode 15 which contains resin components. Therefore, even when a large current such as 30A flows instantaneously, surge absorption is possible without the intervention of the secondary external electrode 15 which contains resin components, thus preventing burnout of the external electrode portion. Furthermore, an electrode paste that will become the primary external electrode 14 may be formed and fired at the same time as the base body 11. The simultaneous sintering of the primary external electrode 14 and the internal electrode 13 increases the bonding strength and provides a burn prevention effect during high-current inrush. Alternatively, plated electrodes may be formed on the primary external electrode 14 and the secondary external electrode 15. In this case as well, current can be passed through without the interposition of a secondary external electrode containing resin components, and the same effect can be obtained.

[0047] Furthermore, as shown in Figure 6, it is even more desirable that the primary external electrode 14 has a region that is not partially covered by the secondary external electrode 15 in the area facing the end face of the base body 11. By doing so, the path that does not pass through the secondary external electrode 15 containing the resin component can be shortened, and an even greater burn prevention effect can be obtained.

[0048] Next, a method for manufacturing the surge absorption element of this disclosure will be described.

[0049] First, ceramic powder exhibiting voltage nonlinearity characteristics, an organic binder, and a solvent are uniformly mixed, more preferably with resin particles. A ceramic slurry or ceramic paste is prepared in this manner. The ceramic powder used in this embodiment consists of 97.5 mol% ZnO as the main component, with 2.5 mol% of other minor components such as Sr, Ca, Co, Cr, Mn, and Al, achieving a configuration with high discharge efficiency. Plasticizers may also be included in the ceramic slurry or ceramic paste. The resin particles are made of a polymer material that completes thermal decomposition at approximately 600°C or below. Thermoplastic resins are preferably used. The resin particles can be spherical or ellipsoidal in shape, or perfectly spherical. Examples of spherical or ellipsoidal shapes include those in which the ratio of the longest diameter to the shortest diameter is 1.25 or less for 95% or more of the particles. In this embodiment, spherical acrylic resin particles were used to improve dispersibility during paste mixing.

[0050] Next, the outer layer green sheet and the conductive paste that forms the conductive substrate are prepared. After firing, as described later, the outer layer green sheet becomes the base body 11. The conductive substrate becomes the internal electrode 13. The ceramic green body becomes the functional part 12. The outer layer green sheet is a low-temperature co-fired ceramic (LTCC) sheet containing alumina particles and borosilicate glass, and its dielectric constant after firing is approximately 10. This configuration makes it possible to reduce the stray capacitance of the surge absorption element 1. The alumina particles and borosilicate glass contain La2O3, CeO2, and Pr6O 11 A mixture containing at least one of Nd2O3, Sm2O3, MgO, SiO2, and Gd2O3 may also be used. In this embodiment, an LTCC was used in which MgO, SiO2, and Gd2O3 were added to alumina particles and borosilicate glass. In this way, by making the element 11 a configuration that includes elements with a small work function, discharge is promoted and the protective effect is improved.

[0051] Next, a conductive paste is applied to the outer layer green sheet by screen printing or the like and dried to form a conductive substrate in a predetermined shape. After firing, as described later, the conductive substrate becomes the internal electrode 13. In this embodiment, an Ag-Pd alloy (Ag / Pd ratio: 70 / 30) is used as the internal electrode 13, enabling heat treatment in air. Next, a ceramic green body and another outer layer green sheet are formed on top of the outer layer green sheet and the conductive substrate. After that, a conductive substrate is formed on top of the ceramic green body using conductive paste. Subsequently, the outer layer green sheet is laminated.

[0052] The ceramic green body contains multiple resin particles. The ceramic green body is formed on a conductive substrate by molding a ceramic slurry using a doctor blade method, reverse roll coater method, etc., or by molding a ceramic paste using screen printing, gravure printing, etc. Alternatively, the ceramic green body may be formed on the outer layer green sheet and the conductive substrate without using an outer layer green sheet, and then the conductive substrate may be formed on the ceramic green body and the outer layer green sheet. In this way, the ceramic green body and the conductive substrate are brought into contact and formed as a single unit. After firing, the ceramic green body and the conductive substrate form a varistor.

[0053] Next, the laminate is heat-treated by raising the temperature to a level where the organic binder and resin particles can be burned away, thereby decomposing and removing the organic binder and resin particles contained in the ceramic green body to form a functional part 12 with voids. In this embodiment, firing was performed at 900°C to 1000°C. By including multiple resin particles in the ceramic slurry or ceramic paste in this way, it becomes easier to form a structure in which the crystalline particles of the functional part 12 are connected and in contact with the voids. As a result, the suppression voltage can be lowered. Furthermore, by using resin particles, they are dispersed on the main surface of the internal electrode in the gap region, forming an opening surface for the voids. Therefore, the concentration of current density in the internal electrode due to ESD can be reduced, preventing burnt wear of the internal electrode due to discharge and lowering the suppression voltage. The volume ratio of the resin particles contained in the ceramic slurry or ceramic paste to the total volume of the ceramic powder and resin particles was set to 70%. The volume ratio of the resin particles is preferably 10% or more and 80% or less, which can significantly lower the suppression voltage. Furthermore, the average particle size of the resin particles was set to 1.8 μm, and the ceramic powder to 1.1 μm. It is preferable that the average particle size of the resin particles be larger than that of the ceramic powder. This makes it easier to form a structure in which the crystal particles of the functional part 12 are connected and in contact with the voids, and the suppression voltage can be reduced. It is also preferable that the average particle size of the resin particles be less than or equal to the thickness of the functional part 12. In this embodiment, the thickness of the functional part was set to approximately 6 μm. Here, the average particle size is the value of the cumulative distribution at 50% (D50) measured by a particle size distribution measuring device. After forming the ceramic green body and the conductive substrate that will become the internal electrode together by bringing them into contact, the organic binder and resin particles are removed to form voids with a porosity of approximately 85%. In this way, when ESD is applied between the internal electrodes 13, surface discharge can be generated on the surface of the crystal particles in contact with the voids provided in the functional part 12. Therefore, the suppression voltage can be significantly reduced.

[0054] Next, a paste containing conductive particles such as Ag and Cu is applied to both end faces of the base body 11 and then baked to form the primary external electrode 14. In this embodiment, Ag was used as the primary external electrode 14. By using Ag-Pd for the internal electrode 13 and Ag for the primary external electrode 14, internal stress can be relaxed. The elastic modulus of the primary external electrode 14 obtained in this way is approximately 83 GPa.

[0055] Furthermore, a thermosetting conductive paste is applied on top to form the secondary external electrode 15. In this example, epoxy resin was used as the thermosetting resin, and a thermosetting conductive paste containing 60 wt% Ag powder was used. The Ag powder is needle-shaped with a long axis of 2 μm to 20 μm and a short axis of 0.2 μm to 2 μm, and by using a long axis length / short axis length of 5 to 75, conductivity is facilitated without increasing the amount of Ag, and conductivity is improved while maintaining a low modulus of elasticity. The temperature is raised to the maximum temperature of 200°C with a temperature gradient of 7°C to 60°C per minute, the maximum temperature is held for 10 to 60 minutes, and then the temperature is lowered to room temperature with a temperature gradient of 7°C to 60°C per minute to form the secondary external electrode 15. The resistivity at this time is 4 × 10⁻⁶ -6 The elastic modulus was approximately 8 GPa, with a density of Ωcm. Furthermore, to prevent oxidation, nitrogen gas was introduced, creating a low oxygen concentration environment (<8.0 × 10⁻⁶). -1 The resistance may be further reduced by baking at ppm. Next, a nickel layer and a tin layer may be sequentially formed on the surface of this electrode by electroplating. In this way, the surge absorption element 1 is completed.

[0056] (summary) As is clear from the embodiments described above, the surge absorbing element (1) of the first embodiment comprises a body (11) having a pair of end faces facing each other and a plurality of side surfaces adjacent to each of the pair of end faces, at least a pair of internal electrodes (13) provided inside the body (11), and at least a pair of external electrodes provided on each of the pair of end faces and electrically connected to each of the internal electrodes (13). The body (11) has a functional part (12) having a polycrystalline structure composed of a plurality of crystalline grains having voids and exhibiting voltage nonlinearity characteristics, and an outer shell part covering the functional part (12). The internal electrodes (13) are provided facing each other via the functional part (12). Each of the external electrodes includes at least a pair of primary external electrodes (14) provided on the end face, and at least a pair of secondary external electrodes (15) provided on the primary external electrodes (14) and electrically connected to the primary external electrodes (14). The elastic modulus of the secondary external electrodes (15) is lower than that of the primary external electrodes (14).

[0057] According to the first embodiment, a good and stable ESD suppression effect and tolerance to abnormal voltages and DC voltages can be achieved.

[0058] In the surge absorption element (1) of the second embodiment, when a surge voltage is applied between the internal electrodes (13) as in the first embodiment, a surface discharge occurs on the surface where multiple crystal particles in the functional part (12) are exposed in the void, thereby conducting current between the internal electrodes (13).

[0059] According to the second embodiment, the ESD suppression effect and tolerance to abnormal voltages, etc., can be further improved.

[0060] In the surge absorption element (1) of the third embodiment, in the first or second embodiment, the internal electrode (13) and the primary external electrode (14) contain Ag.

[0061] According to the third embodiment, oxidation during ESD suppression and heat treatment can be prevented, resulting in a low-resistance electrode. Therefore, even with repeated application of ESD, the decrease in the suppression effect can be suppressed.

[0062] In the surge absorbing element (1) of the fourth embodiment, in any one of the first to third embodiments, the elastic modulus of the primary external electrode (14) is set to E A The elastic modulus of the secondary external electrode (15) is E B When this is the case, 3≦E A / E B The value is ≤2000.

[0063] According to the fourth embodiment, the effect of stress relaxation can be further improved.

[0064] In the surge absorption element (1) of the fifth embodiment, in the fourth embodiment, 10 ≤ E A / E B The value is ≤2000.

[0065] According to the fifth embodiment, the effect of stress relaxation can be further improved.

[0066] In the surge absorption element (1) of the sixth embodiment, in any one of the first to fifth embodiments, the porosity of the functional part (12) is 25% or more and 92% or less.

[0067] According to the sixth embodiment, the suppression voltage can be made significantly lower, and resistance to static electricity can be further improved.

[0068] In the surge absorption element (1) of the seventh embodiment, in any one of the first to sixth embodiments, the melting point of the internal electrode (13) is higher than the melting point of the primary external electrode (14).

[0069] According to the seventh embodiment, the internal stress during inrush current is absorbed from the internal electrode (13) to the primary external electrode (14) with a lower elastic modulus, and then to the secondary external electrode (15), thereby preventing damage to the element.

[0070] In the surge absorption element (1) of the eighth embodiment, in any one of the first to seventh embodiments, the thickness of the secondary external electrode (15) is greater than the thickness of the primary external electrode (14).

[0071] According to the eighth aspect, by making the secondary external electrode (15), which has a low elastic modulus, thicker than the primary external electrode (14), which has a high elastic modulus, the stress relaxation effect can be further improved, and reliability can be further enhanced.

[0072] In the surge absorption element (1) of the ninth embodiment, in any one of the first to eighth embodiments, a portion of the primary external electrode (14) has a region that is not partially covered by the secondary external electrode (15).

[0073] According to the ninth embodiment, when soldered to the substrate, a portion of the solder is directly connected to the external electrode (14) without going through the secondary external electrode (15) containing the resin component. Therefore, even when a large current such as 30A flows instantaneously, surge absorption is possible without the intervention of the secondary external electrode (15) containing the resin component, thus preventing burnout of the external electrode portion.

[0074] In the surge absorbing element (1) of the tenth embodiment, as in the ninth embodiment, the primary external electrode (14) has a region that is not partially covered by the secondary external electrode (15) in the region facing the end face of the base body (11).

[0075] According to the tenth embodiment, the path that does not pass through the secondary external electrode (15) containing the resin component can be made shorter, and a further burn prevention effect can be obtained.

[0076] In the surge absorbing element (1) of the eleventh embodiment, in any one of the first to tenth embodiments, the side surface of the element (11) is not covered by the primary external electrode (14) but is covered by the secondary external electrode (15), and the bonding strength between the secondary external electrode (15) and the element (11) is smaller than the bonding strength between the primary external electrode (14) and the element (11).

[0077] According to the eleventh embodiment, when internal and external stresses act on the surge absorption element (1), open fracture can be prevented by delamination at the interface between the element (11), which has low bonding strength, and the secondary external electrode (15).

[0078] In the surge absorption element (1) of the twelfth embodiment, in any one of the first to eleventh embodiments, the primary external electrode (14) does not contain resin, and the secondary external electrode (15) contains resin.

[0079] According to the twelfth embodiment, the elastic moduli of the primary external electrode (14) and the secondary external electrode (15) can be more favorably controlled.

[0080] In the surge absorbing element (1) of the 13th embodiment, in any one of the first to 12 embodiments, the main component of the functional part (12) is different from the main component of the outer shell.

[0081] According to the 13th embodiment, new effects such as fracture prevention and high surge protection can be achieved.

[0082] In the surge absorption element (1) of the 14th embodiment, in any one of the first to 13 embodiments, the main component of the functional part (12) includes ZnO, and the main component of the outer shell includes glass ceramics.

[0083] According to the 14th embodiment, a high surge absorption effect can be obtained. [Industrial applicability]

[0084] The surge absorption element 1 of this disclosure can achieve a good and stable ESD suppression effect and withstand abnormal voltages and DC voltages, and is industrially useful. [Explanation of symbols]

[0085] 11 Base Body 12 Functional Sections 13 Internal electrode 13a First internal electrode 13b Second internal electrode 14 Primary external electrode 14a First primary external electrode 14b Second primary external electrode 15 Secondary external electrode 15a First secondary external electrode 15b Second secondary external electrode

Claims

1. A base body having a pair of end faces facing each other, and a plurality of side faces adjacent to each of the pair of end faces, The above-mentioned body includes at least one pair of internal electrodes, The system comprises at least one pair of external electrodes provided on each of the pair of end faces and electrically connected to each of the internal electrodes, The aforementioned body has a functional part having voids and a polycrystalline structure composed of a plurality of crystalline particles exhibiting voltage nonlinearity, and an outer shell part covering the functional part. The internal electrodes are arranged facing each other via the functional section, Each of the external electrodes includes at least one pair of primary external electrodes provided on the end face, and at least one pair of secondary external electrodes provided on the primary external electrodes and electrically connected to the primary external electrodes. The elastic modulus of the secondary external electrode is lower than that of the primary external electrode. A surge absorption element in which a portion of the primary external electrode has a region that is not partially covered by the secondary external electrode.

2. A body having a pair of end faces facing each other and a plurality of side faces adjacent to each of the pair of end faces, The above-mentioned body includes at least one pair of internal electrodes, The system comprises at least one pair of external electrodes provided on each of the pair of end faces and electrically connected to each of the internal electrodes, The aforementioned body has a functional part having voids and a polycrystalline structure composed of a plurality of crystalline particles exhibiting voltage nonlinearity, and an outer shell part covering the functional part. The internal electrodes are arranged facing each other via the functional section, Each of the external electrodes includes at least one pair of primary external electrodes provided on the end face, and at least one pair of secondary external electrodes provided on the primary external electrodes and electrically connected to the primary external electrodes. The elastic modulus of the secondary external electrode is lower than that of the primary external electrode. The side surface of the aforementioned body is not covered by the primary external electrode, but is covered by the secondary external electrode. A surge absorbing element in which the bonding strength between the secondary external electrode and the base body is smaller than the bonding strength between the primary external electrode and the base body.

3. A surge absorbing element according to claim 1 or 2, wherein when a surge voltage is applied between the internal electrodes, the plurality of crystal particles in the functional part exhibit surface discharge on the surface exposed in the void, thereby conducting electricity between the internal electrodes.

4. The surge absorbing element according to claim 1 or 2, wherein the internal electrode and the primary external electrode contain Ag.

5. The elastic modulus of the primary external electrode is E Ａ The elastic modulus of the secondary external electrode is E Ｂ In this case, 3 ≤ E Ａ / E Ｂ A surge absorbing element according to claim 1 or 2, wherein the value is ≤ 2000.

6. 10 ≤ E Ａ / E Ｂ A surge absorbing element according to claim 5, wherein the value is ≤ 2000.

7. The surge absorbing element according to claim 1 or 2, wherein the porosity of the functional part is 25% or more and 92% or less.

8. The surge absorbing element according to claim 1 or 2, wherein the melting point of the internal electrode is higher than the melting point of the primary external electrode.

9. The surge absorbing element according to claim 1 or 2, wherein the thickness of the secondary external electrode is greater than the thickness of the primary external electrode.

10. The surge absorbing element according to claim 1 or 2, wherein the primary external electrode has a region that is not partially covered by the secondary external electrode in a region facing the end face of the base body.

11. The primary external electrode does not contain resin. The surge absorbing element according to claim 1 or 2, wherein the secondary external electrode comprises a resin.

12. The surge absorbing element according to claim 1 or 2, wherein the main component of the functional part is different from the main component of the outer shell part.

13. The main component of the functional part includes ZnO, The surge absorbing element according to claim 1 or 2, wherein the main component of the outer shell is glass ceramics.

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