Varistor and manufacturing method thereof

Abstract

To provide a varistor that achieves high reliability by reducing occurrence of unexpected resin cracks, and a manufacturing method thereof.SOLUTION: A manufacturing method of a varistor 1000 includes the steps of: forming a flux layer on electrodes 1110, 1120 formed on a surface of an element 1400 and / or lead wires 1310, 1320 disposed to the respective electrodes; heating the electrodes and / or the lead wires where the flux layer is formed with electromagnetic waves; after the step of heating, connecting the electrodes and the lead wires by soldering; and after soldering, coating a part of the element 1400 with a resin layer 1200.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a varistor and a method for producing the same. [Background technology]

[0002] In many cases, electronic components including varistors are sealed with resin to prevent exposure to the outside world and to improve weather resistance, heat resistance, reliability, dust resistance, etc. In such cases, differences in the properties and characteristics of the coating resin and the electronic component itself can cause various undesirable interactions between the resin and metals, ceramics, etc., which can in some cases have a negative impact on the reliability of the electronic component.

[0003] For example, the following Patent Document 1 discloses a ring varistor in which a tab with a through hole is joined to an electrode of a varistor element by a bonding material, and a manufacturing method thereof, and also mentions a manufacturing method of a ring varistor including a step of preheating the surface of the electrode. This document describes that the effect of preheating is to prevent the varistor element from being damaged by thermal shock, but does not disclose any specific preheating method.

[0004] More specifically, Patent Document 2 below discloses that by including fluororesin porous flakes throughout the entire molding resin, stress can be absorbed and the occurrence of resin cracks due to water vapor generated by sudden heating during solder mounting can be prevented. However, the problem-solving method disclosed in this document does not mention, disclose or suggest at all a method for eliminating the fundamental cause of the problem, which is to reduce the amount of voids by evaporating water.

[0005] Furthermore, Patent Document 3 listed below discloses a manufacturing method for a varistor in which, after soldering, the material is reheated at a temperature above the melting point of the solder. However, this is intended to increase reliability by reducing the variation in breakdown voltage caused by variations in the solder temperature / silver electrode thickness, etc., and makes no mention of preheating or preventing the occurrence of resin cracks, and does not provide any measures or solutions to the fundamental cause of reducing the amount of voids by evaporating water, etc. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2007-088287 A [Patent Document 2] Japanese Patent Application Publication No. 07-022544 [Patent Document 3] Japanese Patent Publication No. 61-080802 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the past, there was a problem that resin cracks may occur when a surge is applied to a varistor at an environmental temperature, for example, a guaranteed temperature of 85°C. There was also a concern that such resin cracks may have a negative effect on the reliability, durability, and product life of the electronic component. The occurrence of such resin cracks does not necessarily occur only in accelerated life tests or accelerated durability tests, but it is also assumed that they may occur unexpectedly over time while the product is in continuous use. The object of the present invention is to solve the above problems and to provide a highly reliable varistor and a manufacturing method thereof by reducing the occurrence of unexpected resin cracks. [Means for solving the problem]

[0008] The method for manufacturing a varistor of the present invention is characterized by comprising the steps of: forming a flux layer on an electrode formed on the surface of an element and / or a lead wire placed on the electrode; heating the electrode and / or the lead wire on which the flux layer is formed by electromagnetic waves; connecting the electrode and the lead wire by soldering after the heating step; and covering a portion of the element with a resin layer after soldering.

[0009] The method for producing a varistor of the present invention is preferably characterized in that the electromagnetic waves are infrared rays, millimeter waves or microwaves.

[0010] The method for producing a varistor of the present invention is further preferably characterized in that the wavelength of the electromagnetic waves is 0.8 μm to 120 mm, and the heating time is 10 seconds to 180 seconds.

[0011] The method for producing a varistor of the present invention is further preferably characterized in that the heating is performed using an infrared heater having a wavelength of 0.8 μm to 2.5 μm for a heating time of 40 seconds to 180 seconds.

[0012] The varistor of the present invention is characterized in that the total sum of the void projected areas in the solder fillets relative to the element projected area in a plan view is 0.5% or less.

[0013] The varistor of the present invention is characterized in that the density of the number of voids in the solder fillet relative to the element projected area in a plan view is 0.2 voids / mm2 or less. Effect of the Invention

[0014] It is possible to provide a highly reliable varistor and a method for manufacturing the same by reducing the occurrence of unexpected resin cracks. [Brief description of the drawings]

[0015] [Figure 1](a) is a diagram explaining the internal structure of a portion of a varistor after peeling off the protective resin layer, (b) is a diagram showing a cross section of the varistor, and (c) is a conceptual cross-sectional diagram explaining the inside of a solder fillet arranged around a lead wire. [Diagram 2] 2 is a flowchart showing a manufacturing process of the varistor of the present invention. [Diagram 3] FIG. 1 is a schematic diagram conceptually illustrating the reason why the present inventors believe that a crack occurs inside a varistor. [Figure 4] 1 is a schematic diagram for explaining the image configuration of a projection of an element shown in an X-ray transmission projection image. FIG. [Diagram 5] FIG. 13 is a diagram illustrating a surge current waveform applied when generating resin cracks for a confirmation experiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] An embodiment of the present invention will be described with reference to Fig. 1 and Fig. 2. Fig. 1(a) is a diagram illustrating the external appearance of a varistor 1000 and the internal structure with a portion of a protective resin layer 1200 peeled off, Fig. 1(b) is a diagram illustrating a cross section of the varistor 1000, and Fig. 1(c) is a conceptual cross-sectional diagram illustrating the internal state of a solder fillet 1500 arranged around a lead wire 1310 (1320). The shape and structure of each varistor shown in Fig. 1 are merely examples, and the present invention is not limited to the embodiment shown in Fig. 1.

[0017] The varistor 1000 shown in Fig. 1 is an electronic component formed by covering an element 1400 with a protective resin layer 1200 (exterior film), and is typically a voltage nonlinear resistor or the like. The element 1400 is, for example, a voltage nonlinear resistance element (also called a "varistor element"), in which a silver electrode 1110 is disposed on the front side of a varistor element body 1410, and a silver electrode 1120 is disposed on the back side thereof. The varistor element body 1410 is, for example, disc-shaped, and the pair of silver electrodes 1110, 1120 are disposed so as to sandwich the varistor element body 1410, whose front and back sides are formed in parallel, from the front and back sides, respectively.

[0018] Lead wires 1310, 1320 for external connection are mounted on element 1400, and in this embodiment, lead wire 1310 is electrically connected to the silver electrode 1110 side, and lead wire 1320 is electrically connected to the silver electrode 1120 side. Therefore, between lead wires 1310, 1320, electrical characteristics such as resistance characteristics of varistor element body 1410 located between silver electrodes 1110, 1120 (typically voltage-current characteristics for surge avoidance) are expressed.

[0019] The element 1400 is covered with a protective resin layer 1200, which maintains insulation from the outside, prevents exposure, and protects the element 1400 and peripheral components from moisture, dust, fire, leakage, etc. The protective resin layer 1200 may be formed by applying an exterior film liquid material obtained by mixing an exterior film main material with an organic solvent, and the application here means that the liquid material is attached by immersion or coating and then fixed. The protective resin layer 1200 may be made of any resin, such as epoxy resin, silicone resin, or phenol resin, but epoxy resin may be used in particular as a commonly used resin.

[0020] The main exterior film material may contain a silicone resin or silicone elastomer and one or more of aluminum hydroxide, magnesium hydroxide, and calcium hydroxide in a weight ratio of 45 / 55 to 5 / 95, and the main exterior film material may be liquid before curing. The organic solvent may be, for example, isopropyl alcohol. The ratio of the organic solvent mixed into the main exterior film material may be in the range of 20 to 40 parts by weight when the main exterior film material is 100 parts by weight.

[0021] In addition, when the silicone resin or silicone elastomer and one or more of aluminum hydroxide, magnesium hydroxide, and calcium hydroxide can be mixed almost evenly, it is not prohibited to not contain an organic solvent, and in this case, the amount may be in the range of 0 to 40 parts by weight.

[0022] The exterior film liquid material may be prepared by mixing silicone resin or silicone elastomer, one or more of aluminum hydroxide, magnesium hydroxide, and calcium hydroxide, and an organic solvent in a predetermined ratio, or may be a mixture of these materials in advance. The protective resin layer 1200 may be an exterior film having excellent explosion-proof and flame-retardant properties, and in this case, may contain silicone resin or silicone elastomer and one or more of aluminum hydroxide, magnesium hydroxide, and calcium hydroxide in a weight ratio range of 100 / 0 to 50 / 50.

[0023] In addition, one or more of aluminum hydroxide, magnesium hydroxide, and calcium hydroxide may be dispersed almost uniformly in the silicone resin or silicone elastomer by mixing an organic solvent when mixing the main material of the exterior film. Explosion-proofing may be ensured by increasing the silicone ratio in the protective resin layer 1200. Here, explosion-proofing may be ensured by adding aluminum hydroxide or the like, so that the ceramic contents are not released or exposed to the outside beyond the protective resin layer 1200, and further flame retardancy may be ensured by the silicone itself and aluminum hydroxide or the like. Other known or well-known resins may be appropriately used for the protective resin layer 1200.

[0024] In addition, as illustrated in FIG. 1(c), in the present invention, the preheating process before solder-dipping the element 1400 is performed by non-contact heating using an infrared heater or microwaves instead of a thermostatic bath, and the preheating process is performed from the inside of the element 1400 (typically directly on the solvent, voids, moisture, etc. in the excess flux). If preheating (preheating) is performed in a thermostatic bath as in the past, heat is applied from the parts exposed to the outside and the outer surface, and the heat is transferred sequentially to the inside, and the temperature increases sequentially from the outside, so that a coating is formed first on the outermost layer of the solder flux, and the voids 1510 present in the solder flux and the solvent or water 1520 in the flux are trapped inside, and there is a concern that their removal / elimination or reduction does not proceed smoothly. As illustrated in FIG. 1(c), it is considered that the solvent or water 1520 in the flux may be taken into the cavity of the void 1510. Voids 1510 trapped in the solder flux and solvents or water 1520 in the flux may migrate into the solder during a solder dip process, etc., and there is a concern that heat generated during subsequent actual use may cause internal cracks in the protective resin layer 1200, etc.

[0025] On the other hand, if non-contact heating is performed using electromagnetic waves such as infrared heaters or microwaves as in the present invention, heating is performed from the inside, and it is possible to avoid a coating being formed on the outer surface of the solder flux first, so that the removal / elimination or reduction of voids 1510 present inside and solvent or water 1520 in the flux can be smoothly performed at the time of the pre-heating. As described later, if voids 1510 and solvent or water 1520 in the flux remain in the solder fillet when the product is used, they can be a cause of crack generation, etc., and can also affect the reliability and durability of the varistor 1000, so it is desirable to reduce them. As described above, in the varistor 1000 of the present invention, the voids 1510 and solvent or water 1520 in the flux can be significantly reduced compared to the conventional case.

[0026] Here, from the viewpoint of avoiding a case where a film is formed on the outer surface of the solder flux before the water in the flux is removed by heating from inside the element, it is possible to directly contact the lead wire by contacting a heating element with the lead wire and heating by thermal conduction from the lead wire. Also, even if it is not direct contact, it is possible to heat by thermal conduction through the lead wire by, for example, blowing hot air on the exposed part of the lead wire.

[0027] Next, an overview of the manufacturing process of the varistor 1000 according to the present invention will be explained with reference to Fig. 2. Fig. 2 is a flow chart showing the manufacturing process of the varistor 1000 according to the present invention. The flow chart in Fig. 2 is a typical example, and the present invention is not limited to the manufacturing process described here. Note that the symbols shown in Fig. 2 correspond to the symbols shown in Fig. 1.

[0028] In the varistor 1000, the varistor element 1410 may be, for example, a ceramic element made of a sintered body formed into a disk shape with zinc oxide as the main component and with added elements such as magnesium oxide, bismuth oxide, and cobalt oxide.

[0029] First, as shown in FIG. 2, the above-mentioned varistor element body 1410 is prepared (Step S210), a silver electrode 1110 is printed on the front side of the varistor element body 1410, and a silver electrode 1120 is printed on the back side of the varistor element body 1410 (Step S220), which is then fired to produce an element 1400 in which the silver electrodes 1110, 1120 are arranged and mounted on the front and back sides of the varistor element body 1410 (Step S230).

[0030] Next, lead wire 1310 is placed in contact with silver electrode 1110, and lead wire 1320 is placed in contact with silver electrode 1120 so that lead wires 1310, 1320 clamp element 1400 across both silver electrodes (step S240). Solder flux is then applied to both surfaces of silver electrodes 1110, 1120 and lead wires 1310, 1320 that are placed in contact therewith (step S250).

[0031] Then, as shown in step S260, preheating is performed in a non-contact manner using an infrared heater on element 1400 having lead wires 1310, 1320 to which solder flux is applied. Preheating can be performed, for example, at 900 W for 40 to 180 seconds for one element 1400 having a volume of 444 mm2.

[0032] In general, when a thermostatic bath is used, it is considered preferable to preheat the solder fillet at a temperature of 120° C. or higher and 200° C. or lower.

[0033] Next, as shown in step S270, the preheated element 1400 is solder-dipped before the preheating cools, and then the element 1400 is cooled (step S280). In the case of solder-dipping, it has been confirmed that the element 1400 can be heated at a solder temperature of 240°C to 260°C.

[0034] Next, in preparation for forming the protective resin layer 1200, a main exterior film material is prepared by mixing silicone resin or silicone elastomer with one or more of aluminum hydroxide, magnesium hydroxide, and calcium hydroxide in a weight ratio of 45 / 55 to 5 / 95 (this main exterior film material may be liquid), and then an organic solvent is mixed with the main exterior film material to prepare a liquid exterior film material. Isopropyl alcohol or the like is used as the organic solvent. When the main exterior film material is 100 parts by weight, the organic solvent is contained in an amount of 20 to 40 parts by weight. The liquid exterior film material (material for forming the protective resin layer) is applied to the element 1400, dried, and cured (step S290), thereby completing the formation of the protective resin layer 1200.

[0035] Figure 3 is a schematic diagram conceptually explaining the reason why the inventors believe that cracks occur inside the varistor 1000. The various symbols in Figure 3 correspond to those in Figure 1. When the varistor 1000 is cut at AB near the centre as shown in Figure 3(a) and the cut surface is observed, it can be seen that a lead wire 1310 is arranged on top of the silver electrode 1110 on the top surface of the varistor element body 1410, and a lead wire 1320 is arranged below the silver electrode 1120 on its underside, with the entire varistor element body 1410 being covered with a protective resin layer 1200, as shown in Figure 3(b).

[0036] Fig. 3(c) is a schematic enlarged view of the rectangular area shown slightly below and to the left in Fig. 3(b). As shown in Fig. 3(c), due to voids 1510 that were initially present in the solder fillet 1500 and solvent or water 1520 in the flux, spaces (which can also be called "floats" or "gaps") form between the element and the resin over time of actual use, or cracks occur in thin parts of the resin (parts with weak strength). This is an event that may have a negative effect on the reliability, durability, and product life of the varistor 1000 as an electronic component, so it is preferable to prevent the occurrence of such cracks and spaces as much as possible.

[0037] For this reason, the inventors performed X-ray transmission observation of varistor 1000 in the observation direction shown in Fig. 3(a) and confirmed the number, density and projected area of ​​voids 1510 from the projection and transmission X-ray diagram shown in Fig. 3(d), and through various trial and error and research, came up with a reduction measure for them. Note that what is observed as a slightly black x mark in the center in Fig. 3(d) are lead wires 1310, 1320 and solder fillet 1500 arranged around them.

[0038] The cracks shown in Figures 3(b) and 3(c) are those that occurred when a surge was applied to varistor 1000 in a temperature environment of 85°C (guaranteed temperature).The principle is that the solvent and water in the flux that was not able to escape to the outside during the solder dipping process remains in the solder fillet, and when a surge is applied, the temperature becomes high enough to melt the solder, so the remaining flux and water change phase from liquid to gas and expand in volume, creating spaces and gaps between the element and the resin, and it is thought that the stress directed outward from the resin causes cracks in the resin.

[0039] If the preheating is done in a thermostatic oven after applying solder flux as in the past, the heat is gradually transferred from the outermost part of the element to the inside by sequential thermal conduction, so that a coating is formed first on the outer surface of the flux, which is exposed to high temperature the earliest. This is thought to cause the solvent, water, voids (small air bubbles), etc. in the flux present inside to be trapped in the flux. For this reason, the inventors tried preheating with an infrared heater instead of a thermostatic oven and found that the voids and the solvent and water in the flux were reduced. Table 1 below is a summary of the experimental and evaluation results of the void area ratio, void number density, and the occurrence of resin cracks due to surge application when the preheating conditions were a thermostatic oven for 5 minutes and an infrared heater for 40 seconds (s).

[0040] <Comparison table between thermostatic chambers and infrared heaters> [Table 1]

[0041] As shown in Table 1 above, by evaporating the water, flux, its solvents, and voids present in the solder flux by preheating with an infrared heater or the like before solder dipping, the amount of voids remaining in the solder fillet is reduced, and it is believed that even if a surge is applied to an electronic component completed as a varistor or the like, the occurrence of resin cracks can be reduced. Specifically, as can be seen from Table 1, the area ratio of voids to the element projected area is reduced with the infrared heater, and the void number density is also reduced. When observed with an X-ray transmission image as described below, the water, flux, its solvents, and voids present in the solder flux are all observed as particulate voids without being separated from one another.

[0042] Furthermore, as can be seen from Table 1, when preheating was performed in a thermostatic bath, the total void area in the X-ray transmission projection 2D photograph was 1.046 mm2, the element projection area was 154.932 mm2, the total void area ratio to the element projection area was 0.675%, and the number of voids and the density of the number of voids to the element projection area were 36 and 0.232 / mm2, respectively, whereas when an infrared heater was used, the figures were 0.554 mm2, 139.045 mm2, 0.398%, 18, and 0.129 / mm2, respectively. Furthermore, the inventors have confirmed that, with regard to the total sum of the void projected area in the solder fillet relative to the element projected area in a planar view, a total void area ratio of 0.5% or less has a clear effect on reducing the occurrence of cracks, and that a ratio of 0.4% or less is the most effective.

[0043] By reducing remaining voids in advance by heating non-contact with electromagnetic waves, typically infrared heaters, it is believed that even if the temperature of electronic components such as varistors rises, the amount of expanding gas is reduced, suppressing the formation of voids and the occurrence of cracks in the resin. In other words, when heating in a thermostatic chamber, the outer surface is heated first, so the solvent on the flux surface, for example IPA (isopropyl alcohol), evaporates first and a film is formed on the flux surface, making it difficult for the moisture and IPA inside to evaporate, but when heating with infrared rays, it is heated from the inside, so the moisture and IPA inside evaporate first and make it difficult for a film to form, so the amount of remaining flux and moisture is reduced.

[0044] By preheating with infrared rays, it is possible to activate the flux and at the same time to properly evaporate the moisture inside it. This eliminates the need for additional steps or operations to evaporate the moisture, etc. Therefore, the total time and number of steps required for preheating can be reduced.

[0045] The preferred conditions for preheating after applying solder flux are as follows: Use an infrared heater with a wavelength of 0.8 to 2.5 μm and set the heating time to 40 to 180 seconds. - Use microwaves with a wavelength of 12 cm (typically a microwave oven) and heat for 10 to 20 seconds. In other words, if the flux is preheated for 10 to 180 seconds with electromagnetic waves with wavelengths in the range of 0.8 μm to 12 cm after application, it is believed that the moisture in the flux will be easily evaporated.

[0046] Fig. 4 is a schematic diagram for explaining the image configuration of the projection 410 of the element shown in the X-ray transmission projection image. In Fig. 4, the projection image of the void 1510 is observed as a grain in the projection image 1550 of the solder. As shown in Fig. 4, the projection images of the lead wires 1310 and 1320 are included near the central axis of the projection image 1550 of the solder.

[0047] Figure 5 is a diagram explaining the surge current waveform applied when generating resin cracks for a confirmation experiment. The resin cracks in this experiment were artificially generated by applying a surge according to the following procedure. The first step in this procedure is to measure the varistor voltage. The varistor voltage is the voltage when 1 mA of current is applied to the varistor. Next, the varistor is heated in a thermostatic chamber until it reaches 85°C. Next, a surge current waveform as shown in Figure 5 is applied, and then the varistor is allowed to cool naturally at room temperature for 5 minutes. Next, the varistor voltage is measured again. Then, the occurrence of resin cracks is confirmed by visual inspection from the outside, and the rate of change between the initial varistor voltage measurement and the second varistor voltage measurement is calculated. The varistor is judged to have been destroyed (typically due to the occurrence of cracks) when it is confirmed by visual inspection, etc. that a crack has occurred, or when the rate of change in the varistor voltage deviates from the range of ±10%.

[0048] As a result, it was possible to understand and confirm that resin cracks had occurred in areas where the resin was thin and relatively fragile, and it was also observed that spaces and gaps had formed at the boundary between the element and the resin, where the resin appeared to be floating.

[0049] A Softex X-ray TV inspection device (WORK-LEADER 100) was used to take the X-ray perspective projection diagrams described above. X-rays were taken with an intensity and cumulative number of times sufficient to penetrate the lead wires outside the element. When calculating the element projected area, the contrast and brightness were adjusted so that the white spots observed in the solder fillet (i.e., voids) were easily visible. The cumulative value of the projected areas of multiple such spots was then used to calculate the total void projected area. In this case, the image processing software ImageJ was used to calculate the element projected area, the (cumulative) projected area of ​​the voids, and the number of voids.

[0050] The varistors and electronic components described above and their manufacturing methods and soldering methods, etc. are not limited to the specific explanations that exemplify the numerical values, structures, configurations, etc. in the embodiments, but are broadly applicable to processes and products that involve soldering and its preheating, and the materials, process contents, and their order and procedures can be appropriately changed and arranged as would be obvious to a person skilled in the art and within the scope of the present invention. [Industrial Applicability]

[0051] The present invention relates to electronic components, typically varistor elements, and a mounting method thereof, and is particularly suitable for various electronic components that undergo various soldering-related processes and are subsequently covered with a protective resin. [Explanation of symbols]

[0052] 1000··varistor, 1110··silver electrode, 1120··silver electrode, 1200··protective resin layer, 1310··lead wire, 1320··lead wire, 1400··element, 1410··varistor body, 1500··solder fillet, 1510··void, 1520··solvent or water in flux.

Claims

1. forming a flux layer on electrodes formed on the surface of the element and / or on lead wires disposed on the electrodes; a step of heating the electrode and / or the lead wire on which the flux layer is formed by electromagnetic waves; a step of connecting the electrodes and the lead wires by soldering after the heating step; and covering a portion of the element with a resin layer after soldering. A method for manufacturing a varistor comprising the steps of:

2. 2. The method for producing a varistor according to claim 1, The electromagnetic wave is an infrared wave, a millimeter wave, or a microwave. A method for manufacturing a varistor comprising the steps of:

3. 3. The method for producing a varistor according to claim 2, The wavelength of the electromagnetic wave is 0.8 μm to 120 mm, and the heating time is 10 seconds to 180 seconds. A method for manufacturing a varistor comprising the steps of:

4. 4. The method for producing a varistor according to claim 3, The heating is performed using an infrared heater having a wavelength of 0.8 μm to 2.5 μm for a heating time of 40 seconds to 180 seconds. A method for manufacturing a varistor comprising the steps of:

5. The total sum of the void projected areas in the solder fillet relative to the element projected area in a plan view is 0.5% or less. A varistor characterized by:

6. The density of the number of voids in the solder fillet relative to the element projected area in a plan view is 0.2 voids / mm2 or less. A varistor characterized by:

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