WO2013010492A1 - Conductive adhesive for capacitor and the corresponding capacitors - Google Patents

Conductive adhesive for capacitor and the corresponding capacitors Download PDF

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Publication number
WO2013010492A1
WO2013010492A1 PCT/CN2012/078872 CN2012078872W WO2013010492A1 WO 2013010492 A1 WO2013010492 A1 WO 2013010492A1 CN 2012078872 W CN2012078872 W CN 2012078872W WO 2013010492 A1 WO2013010492 A1 WO 2013010492A1
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WIPO (PCT)
Prior art keywords
silver
conductive adhesive
coated
adhesive according
particles
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Application number
PCT/CN2012/078872
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French (fr)
Inventor
Changjing CHEN
Original Assignee
Henkel (China) Company Limited
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Publication of WO2013010492A1 publication Critical patent/WO2013010492A1/en

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    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09J—ADHESIVES; NON-MECHANICAL ASPECTS OF ADHESIVE PROCESSES IN GENERAL; ADHESIVE PROCESSES NOT PROVIDED FOR ELSEWHERE; USE OF MATERIALS AS ADHESIVES
    • C09J9/00—Adhesives characterised by their physical nature or the effects produced, e.g. glue sticks
    • C09J9/02—Electrically-conducting adhesives
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
    • C08G59/00—Polycondensates containing more than one epoxy group per molecule; Macromolecules obtained by polymerising compounds containing more than one epoxy group per molecule using curing agents or catalysts which react with the epoxy groups
    • C08G59/18—Macromolecules obtained by polymerising compounds containing more than one epoxy group per molecule using curing agents or catalysts which react with the epoxy groups ; e.g. general methods of curing
    • C08G59/40—Macromolecules obtained by polymerising compounds containing more than one epoxy group per molecule using curing agents or catalysts which react with the epoxy groups ; e.g. general methods of curing characterised by the curing agents used
    • C08G59/42—Polycarboxylic acids; Anhydrides, halides or low molecular weight esters thereof
    • C08G59/4215—Polycarboxylic acids; Anhydrides, halides or low molecular weight esters thereof cycloaliphatic
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09J—ADHESIVES; NON-MECHANICAL ASPECTS OF ADHESIVE PROCESSES IN GENERAL; ADHESIVE PROCESSES NOT PROVIDED FOR ELSEWHERE; USE OF MATERIALS AS ADHESIVES
    • C09J163/00—Adhesives based on epoxy resins; Adhesives based on derivatives of epoxy resins
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09J—ADHESIVES; NON-MECHANICAL ASPECTS OF ADHESIVE PROCESSES IN GENERAL; ADHESIVE PROCESSES NOT PROVIDED FOR ELSEWHERE; USE OF MATERIALS AS ADHESIVES
    • C09J4/00—Adhesives based on organic non-macromolecular compounds having at least one polymerisable carbon-to-carbon unsaturated bond ; adhesives, based on monomers of macromolecular compounds of groups C09J183/00 - C09J183/16
    • C09J4/06—Organic non-macromolecular compounds having at least one polymerisable carbon-to-carbon unsaturated bond in combination with a macromolecular compound other than an unsaturated polymer of groups C09J159/00 - C09J187/00
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-SENSITIVE DEVICES, OF THE ELECTROLYTIC TYPE
    • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
    • H01G9/004—Details
    • H01G9/008—Terminals
    • H01G9/012—Terminals specially adapted for solid capacitors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-SENSITIVE DEVICES, OF THE ELECTROLYTIC TYPE
    • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
    • H01G9/004—Details
    • H01G9/04—Electrodes or formation of dielectric layers thereon
    • H01G9/042—Electrodes or formation of dielectric layers thereon characterised by the material
    • H01G9/0425—Electrodes or formation of dielectric layers thereon characterised by the material specially adapted for cathode
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-SENSITIVE DEVICES, OF THE ELECTROLYTIC TYPE
    • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
    • H01G9/15—Solid electrolytic capacitors
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F220/00—Copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical or a salt, anhydride ester, amide, imide or nitrile thereof
    • C08F220/02—Monocarboxylic acids having less than ten carbon atoms; Derivatives thereof
    • C08F220/10—Esters
    • C08F220/12—Esters of monohydric alcohols or phenols
    • C08F220/16—Esters of monohydric alcohols or phenols of phenols or of alcohols containing two or more carbon atoms
    • C08F220/18—Esters of monohydric alcohols or phenols of phenols or of alcohols containing two or more carbon atoms with acrylic or methacrylic acids
    • C08F220/1811—C10or C11-(Meth)acrylate, e.g. isodecyl (meth)acrylate, isobornyl (meth)acrylate or 2-naphthyl (meth)acrylate
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F222/00—Copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a carboxyl radical and containing at least one other carboxyl radical in the molecule; Salts, anhydrides, esters, amides, imides, or nitriles thereof
    • C08F222/10—Esters
    • C08F222/1006—Esters of polyhydric alcohols or polyhydric phenols
    • C08F222/102—Esters of polyhydric alcohols or polyhydric phenols of dialcohols, e.g. ethylene glycol di(meth)acrylate or 1,4-butanediol dimethacrylate

Definitions

  • the present invention relates to a conductive adhesive for capacitors, and to capacitors comprising said conductive adhesive.
  • Conductive adhesives are widely used to bond components at a relatively low temperature or at ambient temperature. Compared with traditional tin-soldering technologies and the use of lead-containing solder, conductive adhesives have the advantages of being easily applicable and environmentally friendly.
  • a conductive adhesive is essentially composed of a conductive filler, a resin, a curing agent and an additive.
  • the adhesion property, conductivity and reliability are key performances for a conductive adhesive used for capacitors. Furthermore, it is an ongoing effort in the industry to reduce the costs of a conductive adhesive and hence to reduce the production costs of capacitors while ensuring these key performances.
  • Conductive adhesives commercially available at the present time which are used in the fabrication of capacitor available are mainly conductive adhesives comprising silver powder.
  • Conductive adhesives comprising silver powder (also known as “silver pastes” or “silver glues”) can achieve stable bonding and effectively reduce the Equivalent Series Resistance (ESR) of electrolytic capacitors; while the performances thereof are stable.
  • ESR Equivalent Series Resistance
  • the price of silver is high and the costs of pure silver as the conductive filler material is approximately 1000 dollars per kg, based on a silver price of 29 dollars/Toz. Therefore, the high price of the silver glue makes this adhesive uncompetitive for applications in the capacitor market.
  • Such conductive adhesive should have excellent adhesion properties, a good conductivity and reliability including a good moisture-heat stability; while the production costs of said adhesive are low.
  • the present invention provides a new conductive adhesive suitable for use in capacitors.
  • a conductive adhesive for capacitors which comprises: 10 to 60 wt.% of an epoxy resin, 0.5 to 6 wt.% of a curing agent for the epoxy resin, 0 to 60 wt.% of silver-powder particles, and 15 to 50 wt.% of silver-coated non-metal particles.
  • Another aspect of the present invention is a conductive adhesive for capacitors, comprising: 10 to 40 wt.% of a vinyl resin, 0 to 2 wt.% of a peroxide initiator, 0 to 60 wt.% of silver-powder particles, and 15 to 50 wt.% of silver-coated non-metal particles.
  • the non-metal material used in the silver-coated non-metal particles is preferably one or more material(s) selected from glass, boron nitride, calcium carbonate, carbon black, carbon fiber, aluminum oxide and polymers.
  • the conductive adhesive of the present invention not only has an excellent electro-conductivity, but also has a high stability in a wet-hot environment.
  • the present invention also provides a capacitor comprising the above mentioned conductive adhesive for bonding at least one part of the capacitor.
  • the present invention relates to the use of silver-coated non-metal particles in the production of conductive adhesives for capacitors.
  • the present invention further relates to the use of the conductive adhesive of the present invention in the fabrication of capacitors and to the use of said adhesive for attaching an electrode to the leadframe of a capacitor.
  • the capacitor comprising the conductive adhesive of the present invention may be selected from aluminum electrolytic capacitors, tantalum electrolytic capacitors and niobium electrolytic capacitors.
  • capacitor means a device used to store charge and energy, which is composed of two electrodes and a dielectric sandwiched between the electrodes.
  • the capacitor is a main element in an electric circuit, and is widely used for blocking direct current, decoupling, bypassing, filtering waves, tuning the circuit, conversing energy, controlling the circuit and the like.
  • the conductive adhesive also known as conductive glue
  • the adhesion property, conductivity and reliability are three key performances for the conductive glue used for this purpose.
  • the tantalum body accounts for the largest part of the cost of the capacitor; and the conductive adhesive and the conductive coating account for the second largest part of the costs. It is hard to reduce the costs of the tantalum body, since the tantalum body can not be replaced. Therefore, more efforts for reducing the capacitor costs focus on the conductive adhesive and the conductive coating.
  • low cost conductive fillers to replace the traditional silver conductive agents.
  • These low cost conductive fillers comprise graphite, silver-coated particles (such as, silver-coated copper, silver-coated aluminum, silver-coated mica, silver-coated glass, silver-coated boron nitride and the like).
  • the prices of these conductive fillers are 10% to 80% of the silver price.
  • Another way is to lower the density of the adhesive, as the adhesive is consumed in volume.
  • the conductive adhesive used for capacitors has to meet the intrinsic requirement for reliability including the moisture-heat stability Aiming at the defects of the existing adhesives and considering the above factors, the inventors of the present invention developed a conductive adhesive comprising silver-coated non-metal particles.
  • a significant advantage of using silver-coated non-metal particles instead of silver is the dramatic reduction of costs compared with conductive filler which are based on pure silver.
  • the silver-coated boron nitride particle and silver-coated glass particle are much cheaper than silver. Based on the price of silver of 29 dollars/Toz, the costs for conductive silver fillers are approximately 1000 dollars/kg; while the price of silver-coated boron nitride particles is about 750 to 800 dollars/kg, which is 20 to 25% cheaper than conductive fillers based on pure silver; the price of silver-coated glass particles is about 500 dollars/kg, which is 50% cheaper than conductive fillers based on pure silver.
  • the density of silver-coated boron nitride or of silver-coated glass is relatively low (usually about 3 to 5 g/cm 3 ; typically for silver-coated boron nitride 30-103, the density of which is 3.92 g/cm 3 ); while the density of silver-powder/silver-flakes is about 10 to 11 g/cm 3 .
  • the density of typical adhesives comprising pure silver fillers is from 3.2 g/cm 3 to 3.6 g/cm 3
  • the density of an adhesive comprising silver-coated boron nitride fillers or silver-coated glass fillers is usually less than 2.8 g/cm 3 .
  • the inventors of the present invention found that the electro-conductivities of the obtained adhesives are surprisingly in the same level.
  • the typical volume resistivity of a silver-containing adhesive used for tantalum capacitors is about 0.001 ohm-cm; while the typical volume resistivity of an adhesive for tantalum capacitors comprising silver-coated boron nitride fillers or silver-coated glass fillers is about 0.001 to 0.01 ohm-cm, which definitely meets the requirement for capacitor adhesives ( ⁇ 0.1 ohm-cm).
  • the boron nitride and glass have an extremely high chemical and physical stability as well as a high temperature resistance, which imparts a high stability to the fillers.
  • the inventors surprisingly found that the adhesives using the above-mentioned conductive fillers have a comparable stability to those adhesives using pure silver.
  • the present invention provides adhesives with two resin systems: epoxy resin-based conductive adhesives and bismaleimide-acrylate-based conductive adhesives.
  • an epoxy resin-based conductive adhesive which comprises: 10 to 60 wt.% of an epoxy resin, 0.5 to 6 wt.% of a curing agent for the epoxy resin, 0 to 60 wt.% of silver-powder particles, and 15 to 50 wt.% of silver-coated non-metal particles.
  • the advantages of the epoxy resin-based conductive adhesive are good bonding properties and the low costs of the resin.
  • bisphenol A epoxy resin may be used, and the examples of which are Epiclon 850S from DIC Dainippon Ink & Chemicals, jER 828US from Japan Epoxy Resins Co., Ltd, as well as RAS-1 from Henkel Corporation.
  • the curing agent for the epoxy resin is also known as hardener, which is a substance or mixture controlling or facilitating the curing reaction of the epoxy resin.
  • the curing agent for the epoxy resin reacts with the epoxy resin to form a netlike stereoscopic polymer.
  • Curing agents suitable for use in the present invention include imidazole curing agents or acid anhydride curing agents, such as, dodecenyl succinic anhydride, methylhexahydrophthalic anhydride, 1 -cyanoethyl-2-ethyl-4-methylimidazole and the like.
  • silver-coated non-metal particle means a structure having a silver coating on the surface of a particle, wherein the particle is formed from a non-metal material.
  • non-metal material used in the silver-coated non-metal particle, as long as the material can stably exist in the conductive adhesive in the working environment of capacitors.
  • one or more materials selected from glass, boron nitride, calcium carbonate, carbon black, carbon fiber, aluminum oxide and polymers can be used as the non-metal material.
  • Silver can be coated onto the surface of the non-metal particle by means of traditional technologies, such as electroplating, spray coating and the like.
  • the density of the silver-coated non-metal particle is preferably close to the overall density of the conductive adhesive so as to avoid the deterioration of the adhesive due to the float or precipitation of particles.
  • the density of the silver-coated non-metal particle is preferably from 3 to 5 g/cm 3 .
  • the average particle size of the silver-coated non-metal particle used herein is preferably 5 to 100 micrometers, more preferably 10 to 40 micrometers, even more preferably 10 to 20 micrometers.
  • the amount of silver coated on the silver-coated non-metal particles is preferably 20 to 60wt.%; for silver-coated glass particles, the amount of silver is more preferably 35 to 40wt.%; for the silver-coated boron nitride particles, the amount of silver is more preferably 45 to 55wt.%.
  • the silver amount or amount of silver here means the proportion of the mass of silver based on the total mass of the silver-coated non-metal particle.
  • silver-coated glass particles or silver-coated boron nitride particles as conductive fillers in the conductive adhesive of the present invention.
  • the silver-coated boron nitride particle may be, for example, silver-coated boron nitride 30-103, commercially from Technic Inc.
  • the silver-coated glass particle has a lower cost than the silver-coated boron nitride particle.
  • the silver-coated glass particle may bring undesired metal ions into the conductive adhesive.
  • an ion exchanger may be, for example, IXE 100 or IXE770F from Toagosei Co., Ltd.
  • the amount of the epoxy resin is 10 to 60wt.%.
  • the amount of the curing agent varies depending on different epoxy resins. Generally, the amount of the curing agent for the epoxy resin is 0.5 to 6wt.%.
  • the amount of silver-powder particles is 0 to 60wt.%.
  • the amount of silver-coated non-metal particles is 15 to 50wt.%.
  • the conductive adhesive of the present invention may contain an additive if desired, such as, an adhesion promoter, a dispersing agent, a thixotropic agent and the like.
  • Said adhesion promoter may be an adhesion promoter of siloxanes terminated with a reactive functional group, such as, Silane A-187, Z-6040 and the like.
  • Said dispersing agent may be a dispersing agent of silicones, such as, BYK W940 and BYK-333.
  • Said thixotropic agent may be fumed silica, such as TS720 and R202.
  • the epoxy resin-based conductive adhesive of the present invention may be formulated according to methods well known to those skilled in the art.
  • the conductive adhesive of the invention may be formulated according to the following steps: firstly, a solid conductive promoter is dissolved in the epoxy resin, preferably at a temperature of 60 ° C ; secondly, the curing agent is added under stirring; thirdly, silver-coated boron nitride particles (or the silver-coated glass particles) and optionally silver-powder particles are added, and the mixture is stirred mechanically, preferably at a speed of 3000 rpm for 30 minutes.
  • the epoxy resin-based conductive adhesive of the present invention may be applied onto the surface to be adhered by a common method, such as printing, dispensing and the like, followed by drying and curing at a suitable temperature and moisture.
  • the epoxy resin-based conductive adhesive can be dried and cured in an oven, at a temperature of 150 ° C to 200 ° C with a thermostatic curing time of 30 to 60 minutes (the curing temperature and the curing time may be suitably selected according to the curing agent used).
  • the curing may also be achieved by a rapid curing, which is conducted at a relatively high temperature (250 ° C to 300 ° C ) with a curing time of 10 to 30 seconds.
  • a vinyl resin-based conductive adhesive which comprises: 10 to 40 wt.% of a vinyl resin, 0 to 2 wt.% of a peroxide initiator, 0 to 60 wt.% of silver-powder particles, and 15 to 50 wt.% of silver-coated non-metal particles.
  • the vinyl resin-based conductive adhesive has an excellent water resistance, a strong adhesion towards certain metal substrates, and may achieve a rapid curing at a relatively low temperature.
  • the vinyl resin may be an acrylic resin, a (poly)butadiene resin, an aliphatic resin containing a cyclic double bond structure, or a polyimide resin containing a heterocyclic double bond structure and the like; for example, bismaleimide resin 24-405A from Henkel Corp.; polybutadiene modified Ricon 131 MA10 resin, acrylate resin SR248, SR423 from Sartomer, U.S.A; and the like.
  • the silver-coated non-metal particles described above for the epoxy resin-based conductive adhesive are also suitable for the vinyl resin-based conductive adhesive.
  • the amount of vinyl rein is 10 to 40wt.%.
  • the amount of the silver-coated non-metal particles is 15 to 50wt.%.
  • the conductive adhesive of the present invention may contain an additive if desired, such as, an adhesion promoter, a dispersing agent, a thixotropic agent and the like.
  • Said adhesion promoter may be an adhesion promoter of siloxanes terminated with a reactive functional group, such as, Silane A-187, Z-6040 and the like.
  • Said dispersing agent may be a dispersing agent of silicones, such as, BYK W940 and BYK-333.
  • Said thixotropic agent may be fumed silica, such as TS720 and R202.
  • the vinyl resin-based conductive adhesive of the present invention may be formulated according to methods well known to those skilled in the art.
  • the vinyl resin-based conductive adhesive of the present invention may be formulated according to the following steps: firstly, different vinyl resins are homogeneously mixed under stirring; secondly, the curing agent, additives and so on are added, and the mixture is stirred for 1 minute, then the solid substances are dispersed into the system by a three-roller mill; thirdly, silver-coated boron nitride particles (or silver-coated glass particles) and optionally silver-powder is added, and the resulting mixture is stirred, preferably mechanically at a speed of 3000 rpm for 30 minutes.
  • the vinyl resin-based conductive adhesive of the present invention may be applied onto the surface to be adhered by a common method, such as printing, dispensing and the like, followed by driying and curing at a suitable temperature and moisture.
  • the vinyl resin-based conductive adhesive can be dried and cured in an oven, at a temperature of 100 ° C to 175 ° C with a thermostatic time of 1 to 60 minutes (the curing temperature and the curing time may be suitably selected according to the curing agent used).
  • the curing may also be achieved by rapid curing, which is conducted at 100 ° C to 280 ° C with a thermostatic time of 10 seconds to 1 minute.
  • capacitors such as ceramic capacitors, aluminum electrolytic capacitors, mica capacitors, paper capacitors, tantalum electrolytic capacitors and film capacitors and the like.
  • Different capacitors have different structures; however, they have in common that they all have two electrodes separated by a dielectric (media).
  • the conductive adhesive of the present invention may be used for any capacitor requiring the attachment of the anode or cathode to the substrate, especially for capacitors having high stability requirements in a wet-hot environment.
  • the conductive adhesive of the present invention is especially suitable for aluminum electrolytic capacitors, tantalum electrolytic capacitors and niobium electrolytic capacitors.
  • the solid tantalum electrolytic capacitor comprises a sintered body obtained by briquetting the tantalum powder and sintering, a tantalum oxide film formed on the surface of the sintered body, a manganese dioxide layer and a conductive layer on the manganese dioxide layer.
  • the conductive adhesive of the present invention may be used in capacitors for bonding the components of the capacitor.
  • the bonding formed by the conductive adhesive of the present invention is not only firm, of an excellent electro-conductivity, but also is highly reliable, especially of an excellent moisture-heat stability.
  • - SG15F35 silver-coated glass flakes with a silver content of 35wt.%, and an average particle size of 15 ⁇ , from Potter Industries Inc..
  • - 24-405A aliphatic alkane modified bismaleimide, from Henkel Corporation.
  • SR248 neopentyl glycol dimethacrylate, from Sartomer.
  • - Ricon 131 MA10 an oligomer of 2,5-furandione with 1 ,3-butadiene, from Sartomer.
  • IXE770F aluminum manganese oxide (an ion exchanger), from Toagosei.
  • RAS-1 2,6-biepoxypropyl-glycidyl phenol, from Henkel Corporation.
  • PN-H addition product of an epoxy rein and imidazole, from Ajinomoto co. Ltd.
  • a series of measurements including a conductivity measurement, a bonding strength measurement, a density measurement and a moisture-heat resistance measurement were conducted to prove the excellent performances of the conductive adhesive of the present invention.
  • the density of the cured conductive adhesive was measured in accordance with ATM-001 :
  • the primary apparatus used in this measurement was a densimeter.
  • the moisture-heat resistance of the cured conductive adhesive was measured in accordance with the following procedure:
  • the sample to be tested was put into a thermostatic chamber having a relative moisture of 85% and a temperature of 85 ° C . To measure variations in the volume resistivity and to determine the moisture-heat resistance several samples were removed from the chamber after a given period of time.
  • the volume resistivity of the dried and cured conductive adhesive was measured in accordance with ATM-0020:
  • the conductive adhesive was coated on a glass carrier to form a cuboid-like adhesive layer having a length of 7.5 cm and a width of 1 .25cm to produce a test sample.
  • the height of the sample varied and was usually between 0.001 cm to 0.01 cm.
  • the adhesive layer was cured and then placed on an electric bridge to measure the resistance.
  • the bonding strength of the dried and cured conductive adhesive layer was measured in accordance with ATM-0052:
  • Measurement apparatus Shear strength tester Dage 4000
  • An amount of the conductive adhesive was placed as a glue dot onto a silver substrate, and then the anode of a capacitor was attached to the glue dot. By pressing slightly the glue layer fully covered the lower surface of the capacitor. The amount of the conductive glue was controlled so as to exactly cover the lower surface of the capacitor. The glue layer was cured, and then the lateral shear strength of the glue layer, which can be used to express the bonding strength of the glue layer, was measured using a Dage 4000 instrument.
  • the conductive adhesives of examples 2-9 were formulated in the same manner as that in example 1 except that the formulations of the conductive adhesives were changed.
  • Table 1 examples based on bismaleimide (BMI) resin and acrylate resin system
  • Comparative example 26 the pure silver glue
  • Comparative example 27 the conductive adhesive using the silver-coated copper particle as the conductive filler
  • Total RH means the weight percentage of the semi-product in which the conductive filler (silver powder or silver-coated boro nitride particle) was not added.
  • the conductive adhesive of the present invention not only has excellent adhesion properties and an excellent electro-conductivity, but also exhibits an excellent moisture-heat stability.
  • the electro-conductivity of the conductive adhesive of the present invention is comparable to that of the silver glue (comparative example 26).
  • the moisture-heat stability of the conductive adhesive of the present invention is much better than that of the conductive adhesive using silver-coated copper as the conductive filler (comparative example 27).

Abstract

The present invention relates to a conductive adhesive for capacitors and to capacitors comprising the conductive adhesive. The conductive adhesive of the present invention comprises an epoxy resin, a curing agent for the epoxy resin, a silver powder and silver-coated non-metal particles; or comprises a vinyl resin, a peroxide initiator, a silver powder and silver-coated non-metal particles.

Description

Conductive adhesive for capacitor and the corresponding capacitors
The present invention relates to a conductive adhesive for capacitors, and to capacitors comprising said conductive adhesive.
Background Art
Conductive adhesives are widely used to bond components at a relatively low temperature or at ambient temperature. Compared with traditional tin-soldering technologies and the use of lead-containing solder, conductive adhesives have the advantages of being easily applicable and environmentally friendly.
A conductive adhesive is essentially composed of a conductive filler, a resin, a curing agent and an additive.
The adhesion property, conductivity and reliability (including the moisture-heat stability) are key performances for a conductive adhesive used for capacitors. Furthermore, it is an ongoing effort in the industry to reduce the costs of a conductive adhesive and hence to reduce the production costs of capacitors while ensuring these key performances.
Conductive adhesives commercially available at the present time which are used in the fabrication of capacitor available are mainly conductive adhesives comprising silver powder.
Conductive adhesives comprising silver powder (also known as "silver pastes" or "silver glues") can achieve stable bonding and effectively reduce the Equivalent Series Resistance (ESR) of electrolytic capacitors; while the performances thereof are stable. However, the price of silver is high and the costs of pure silver as the conductive filler material is approximately 1000 dollars per kg, based on a silver price of 29 dollars/Toz. Therefore, the high price of the silver glue makes this adhesive uncompetitive for applications in the capacitor market. Even though a silver-coated copper filler reduces the costs of a conductive adhesive, restrictions in the electroplating technology leads to a dramatic decrease in the thermal conductivity and electro-conductivity of these conductive adhesive, because the surface of copper powder is hardly to be fully coated, and the uncoated copper is reactive and is easily oxidized to copper oxide. Therefore, its practical use is seriously affected.
Up to now, there is no report regarding the use of silver-coated non-metal fillers in the production of conductive adhesives for capacitors. As for the conductive adhesives used in the electronic industry, the composition and the producing process thereof are largely different depending on their specific applications.
Therefore, there is a need to develop a conductive adhesive for capacitors. Such conductive adhesive should have excellent adhesion properties, a good conductivity and reliability including a good moisture-heat stability; while the production costs of said adhesive are low.
Description of the Invention
To solve the problem existing in the prior art, the present invention provides a new conductive adhesive suitable for use in capacitors.
According to one aspect of the invention, a conductive adhesive for capacitors is provided, which comprises: 10 to 60 wt.% of an epoxy resin, 0.5 to 6 wt.% of a curing agent for the epoxy resin, 0 to 60 wt.% of silver-powder particles, and 15 to 50 wt.% of silver-coated non-metal particles.
Another aspect of the present invention is a conductive adhesive for capacitors, comprising: 10 to 40 wt.% of a vinyl resin, 0 to 2 wt.% of a peroxide initiator, 0 to 60 wt.% of silver-powder particles, and 15 to 50 wt.% of silver-coated non-metal particles. The non-metal material used in the silver-coated non-metal particles is preferably one or more material(s) selected from glass, boron nitride, calcium carbonate, carbon black, carbon fiber, aluminum oxide and polymers.
The conductive adhesive of the present invention not only has an excellent electro-conductivity, but also has a high stability in a wet-hot environment.
The present invention also provides a capacitor comprising the above mentioned conductive adhesive for bonding at least one part of the capacitor.
In a further aspect the present invention relates to the use of silver-coated non-metal particles in the production of conductive adhesives for capacitors.
The present invention further relates to the use of the conductive adhesive of the present invention in the fabrication of capacitors and to the use of said adhesive for attaching an electrode to the leadframe of a capacitor.
The capacitor comprising the conductive adhesive of the present invention may be selected from aluminum electrolytic capacitors, tantalum electrolytic capacitors and niobium electrolytic capacitors.
All technical terms and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art, unless indicated otherwise. In case that the meanings understood by those skilled in the art conflict with the meanings defined herein, the meanings defined herein shall be used as the criterion.
All percentages, parts, proportions and the like mentioned herein are based on weight unless indicated otherwise. The term "capacitor", as used in the present invention, means a device used to store charge and energy, which is composed of two electrodes and a dielectric sandwiched between the electrodes. The capacitor is a main element in an electric circuit, and is widely used for blocking direct current, decoupling, bypassing, filtering waves, tuning the circuit, conversing energy, controlling the circuit and the like.
The conductive adhesive (also known as conductive glue) is used to bond various components in the capacitor industry, for example, attach the coated anode to the leadframe. The adhesion property, conductivity and reliability are three key performances for the conductive glue used for this purpose.
Another important factor are the costs besides the three key performances. In solid capacitors, like tantalum capacitors for example, the tantalum body accounts for the largest part of the cost of the capacitor; and the conductive adhesive and the conductive coating account for the second largest part of the costs. It is hard to reduce the costs of the tantalum body, since the tantalum body can not be replaced. Therefore, more efforts for reducing the capacitor costs focus on the conductive adhesive and the conductive coating.
There are several ways to reduce the costs of the adhesive. One is to use low cost conductive fillers to replace the traditional silver conductive agents. These low cost conductive fillers comprise graphite, silver-coated particles (such as, silver-coated copper, silver-coated aluminum, silver-coated mica, silver-coated glass, silver-coated boron nitride and the like). The prices of these conductive fillers are 10% to 80% of the silver price. Another way is to lower the density of the adhesive, as the adhesive is consumed in volume.
In addition, the conductive adhesive used for capacitors has to meet the intrinsic requirement for reliability including the moisture-heat stability Aiming at the defects of the existing adhesives and considering the above factors, the inventors of the present invention developed a conductive adhesive comprising silver-coated non-metal particles.
A significant advantage of using silver-coated non-metal particles instead of silver is the dramatic reduction of costs compared with conductive filler which are based on pure silver.
Taking the silver-coated boron nitride particle and silver-coated glass particle for example, they are much cheaper than silver. Based on the price of silver of 29 dollars/Toz, the costs for conductive silver fillers are approximately 1000 dollars/kg; while the price of silver-coated boron nitride particles is about 750 to 800 dollars/kg, which is 20 to 25% cheaper than conductive fillers based on pure silver; the price of silver-coated glass particles is about 500 dollars/kg, which is 50% cheaper than conductive fillers based on pure silver.
The density of silver-coated boron nitride or of silver-coated glass is relatively low (usually about 3 to 5 g/cm3; typically for silver-coated boron nitride 30-103, the density of which is 3.92 g/cm3); while the density of silver-powder/silver-flakes is about 10 to 11 g/cm3. As a consequence the density of typical adhesives comprising pure silver fillers is from 3.2 g/cm3 to 3.6 g/cm3, while the density of an adhesive comprising silver-coated boron nitride fillers or silver-coated glass fillers is usually less than 2.8 g/cm3.
Even though the performance, such as the electro-conductivity, of silver-coated boron nitride and silver-coated glass is not as good as the performance of pure silver, the inventors of the present invention found that the electro-conductivities of the obtained adhesives are surprisingly in the same level. For example, the typical volume resistivity of a silver-containing adhesive used for tantalum capacitors is about 0.001 ohm-cm; while the typical volume resistivity of an adhesive for tantalum capacitors comprising silver-coated boron nitride fillers or silver-coated glass fillers is about 0.001 to 0.01 ohm-cm, which definitely meets the requirement for capacitor adhesives (< 0.1 ohm-cm).
The boron nitride and glass have an extremely high chemical and physical stability as well as a high temperature resistance, which imparts a high stability to the fillers. During the reliability tests, the inventors surprisingly found that the adhesives using the above-mentioned conductive fillers have a comparable stability to those adhesives using pure silver.
Aiming at different applications, the present invention provides adhesives with two resin systems: epoxy resin-based conductive adhesives and bismaleimide-acrylate-based conductive adhesives.
Epoxy resin-based conductive adhesives
According to one embodiment of the present invention, an epoxy resin-based conductive adhesive is provided, which comprises: 10 to 60 wt.% of an epoxy resin, 0.5 to 6 wt.% of a curing agent for the epoxy resin, 0 to 60 wt.% of silver-powder particles, and 15 to 50 wt.% of silver-coated non-metal particles.
The advantages of the epoxy resin-based conductive adhesive are good bonding properties and the low costs of the resin.
The term "epoxy resin", as used herein, refers to polymers containing an epoxy group in the molecular structure. The cured epoxy resin has excellent chemical and physical properties, an excellent bonding strength towards the surfaces of metal and non-metal materials, a high hardness, a better flexibility, a good resistance to most solvents and alkali. Epoxy resins suitable for use in the present invention may comprise aromatic glycidol epoxy resins or aliphatic epoxy resins, such as bisphenol-type epoxy resins or phenolic-type epoxy resins. Examples of suitable epoxy resin are, for example, bisphenol A epoxy resins, bisphenol S epoxy resins, bisphenol F epoxy resins, phenol-novolak epoxy resins, and cresol-novolak epoxy resins.
In the epoxy resin-based conductive adhesive of the present invention, bisphenol A epoxy resin may be used, and the examples of which are Epiclon 850S from DIC Dainippon Ink & Chemicals, jER 828US from Japan Epoxy Resins Co., Ltd, as well as RAS-1 from Henkel Corporation.
The curing agent for the epoxy resin is also known as hardener, which is a substance or mixture controlling or facilitating the curing reaction of the epoxy resin. The curing agent for the epoxy resin reacts with the epoxy resin to form a netlike stereoscopic polymer. Curing agents suitable for use in the present invention include imidazole curing agents or acid anhydride curing agents, such as, dodecenyl succinic anhydride, methylhexahydrophthalic anhydride, 1 -cyanoethyl-2-ethyl-4-methylimidazole and the like.
The term "silver-coated non-metal particle", as used herein, means a structure having a silver coating on the surface of a particle, wherein the particle is formed from a non-metal material.
In principle, there is no special limitation on the non-metal material used in the silver-coated non-metal particle, as long as the material can stably exist in the conductive adhesive in the working environment of capacitors. For example, one or more materials selected from glass, boron nitride, calcium carbonate, carbon black, carbon fiber, aluminum oxide and polymers can be used as the non-metal material.
Silver can be coated onto the surface of the non-metal particle by means of traditional technologies, such as electroplating, spray coating and the like.
The density of the silver-coated non-metal particle is preferably close to the overall density of the conductive adhesive so as to avoid the deterioration of the adhesive due to the float or precipitation of particles. Specifically, the density of the silver-coated non-metal particle is preferably from 3 to 5 g/cm3.
The average particle size of the silver-coated non-metal particle used herein is preferably 5 to 100 micrometers, more preferably 10 to 40 micrometers, even more preferably 10 to 20 micrometers.
The amount of silver coated on the silver-coated non-metal particles is preferably 20 to 60wt.%; for silver-coated glass particles, the amount of silver is more preferably 35 to 40wt.%; for the silver-coated boron nitride particles, the amount of silver is more preferably 45 to 55wt.%. The silver amount or amount of silver here means the proportion of the mass of silver based on the total mass of the silver-coated non-metal particle.
In order to obtain a good compatibility with other ingredients as well as a suitable density it is preferred to use silver-coated glass particles or silver-coated boron nitride particles as conductive fillers in the conductive adhesive of the present invention.
The silver-coated boron nitride particle may be, for example, silver-coated boron nitride 30-103, commercially from Technic Inc.
The silver-coated glass particle has a lower cost than the silver-coated boron nitride particle. However, the silver-coated glass particle may bring undesired metal ions into the conductive adhesive. Thus, it is preferable to add an ion exchanger to the conductive adhesive when the conductive adhesive is used in an application sensitive to metal ion impurities. The ion exchanger may be, for example, IXE 100 or IXE770F from Toagosei Co., Ltd.
In the conductive adhesive according to the present invention, the amount of the epoxy resin is 10 to 60wt.%.
The amount of the curing agent varies depending on different epoxy resins. Generally, the amount of the curing agent for the epoxy resin is 0.5 to 6wt.%.
The amount of silver-powder particles is 0 to 60wt.%.
The amount of silver-coated non-metal particles is 15 to 50wt.%.
Besides the above main ingredients, the conductive adhesive of the present invention may contain an additive if desired, such as, an adhesion promoter, a dispersing agent, a thixotropic agent and the like. Said adhesion promoter may be an adhesion promoter of siloxanes terminated with a reactive functional group, such as, Silane A-187, Z-6040 and the like. Said dispersing agent may be a dispersing agent of silicones, such as, BYK W940 and BYK-333. Said thixotropic agent may be fumed silica, such as TS720 and R202.
The epoxy resin-based conductive adhesive of the present invention may be formulated according to methods well known to those skilled in the art. As an example, the conductive adhesive of the invention may be formulated according to the following steps: firstly, a solid conductive promoter is dissolved in the epoxy resin, preferably at a temperature of 60°C ; secondly, the curing agent is added under stirring; thirdly, silver-coated boron nitride particles (or the silver-coated glass particles) and optionally silver-powder particles are added, and the mixture is stirred mechanically, preferably at a speed of 3000 rpm for 30 minutes.
The epoxy resin-based conductive adhesive of the present invention may be applied onto the surface to be adhered by a common method, such as printing, dispensing and the like, followed by drying and curing at a suitable temperature and moisture. The epoxy resin-based conductive adhesive, can be dried and cured in an oven, at a temperature of 150°C to 200°C with a thermostatic curing time of 30 to 60 minutes (the curing temperature and the curing time may be suitably selected according to the curing agent used). The curing may also be achieved by a rapid curing, which is conducted at a relatively high temperature (250 °C to 300 °C ) with a curing time of 10 to 30 seconds.
Vinyl resin-based conductive adhesives
According to another embodiment of the present invention, a vinyl resin-based conductive adhesive is provided, which comprises: 10 to 40 wt.% of a vinyl resin, 0 to 2 wt.% of a peroxide initiator, 0 to 60 wt.% of silver-powder particles, and 15 to 50 wt.% of silver-coated non-metal particles.
The vinyl resin-based conductive adhesive has an excellent water resistance, a strong adhesion towards certain metal substrates, and may achieve a rapid curing at a relatively low temperature.
The term "vinyl resin", as used herein, means a resin containing a vinyl group (C=C) in the molecular structure which is capable of undergoing a free radical polymerization initiated by a peroxide or an azo compound to give an oligomer or a polymer (which may contain a residual vinyl group, as an intermediate for further grafting). The vinyl resin may be an acrylic resin, a (poly)butadiene resin, an aliphatic resin containing a cyclic double bond structure, or a polyimide resin containing a heterocyclic double bond structure and the like; for example, bismaleimide resin 24-405A from Henkel Corp.; polybutadiene modified Ricon 131 MA10 resin, acrylate resin SR248, SR423 from Sartomer, U.S.A; and the like.
The silver-coated non-metal particles described above for the epoxy resin-based conductive adhesive are also suitable for the vinyl resin-based conductive adhesive. In the vinyl resin-based conductive adhesive according to the present invention, the amount of vinyl rein is 10 to 40wt.%.
In the vinyl resin-based conductive adhesive according to the present invention, the amount of the silver-coated non-metal particles is 15 to 50wt.%.
Besides the above-mentioned main ingredients, the conductive adhesive of the present invention may contain an additive if desired, such as, an adhesion promoter, a dispersing agent, a thixotropic agent and the like. Said adhesion promoter may be an adhesion promoter of siloxanes terminated with a reactive functional group, such as, Silane A-187, Z-6040 and the like. Said dispersing agent may be a dispersing agent of silicones, such as, BYK W940 and BYK-333. Said thixotropic agent may be fumed silica, such as TS720 and R202.
The vinyl resin-based conductive adhesive of the present invention may be formulated according to methods well known to those skilled in the art. As an example, the vinyl resin-based conductive adhesive of the present invention may be formulated according to the following steps: firstly, different vinyl resins are homogeneously mixed under stirring; secondly, the curing agent, additives and so on are added, and the mixture is stirred for 1 minute, then the solid substances are dispersed into the system by a three-roller mill; thirdly, silver-coated boron nitride particles (or silver-coated glass particles) and optionally silver-powder is added, and the resulting mixture is stirred, preferably mechanically at a speed of 3000 rpm for 30 minutes.
The vinyl resin-based conductive adhesive of the present invention may be applied onto the surface to be adhered by a common method, such as printing, dispensing and the like, followed by driying and curing at a suitable temperature and moisture.
The vinyl resin-based conductive adhesive can be dried and cured in an oven, at a temperature of 100°C to 175°C with a thermostatic time of 1 to 60 minutes (the curing temperature and the curing time may be suitably selected according to the curing agent used). The curing may also be achieved by rapid curing, which is conducted at 100°C to 280 °C with a thermostatic time of 10 seconds to 1 minute.
Capacitor
There are many kind of capacitors, such as ceramic capacitors, aluminum electrolytic capacitors, mica capacitors, paper capacitors, tantalum electrolytic capacitors and film capacitors and the like. Different capacitors have different structures; however, they have in common that they all have two electrodes separated by a dielectric (media).
The conductive adhesive of the present invention may be used for any capacitor requiring the attachment of the anode or cathode to the substrate, especially for capacitors having high stability requirements in a wet-hot environment. The conductive adhesive of the present invention is especially suitable for aluminum electrolytic capacitors, tantalum electrolytic capacitors and niobium electrolytic capacitors.
Taking the solid tantalum electrolytic capacitor as an example, it comprises a sintered body obtained by briquetting the tantalum powder and sintering, a tantalum oxide film formed on the surface of the sintered body, a manganese dioxide layer and a conductive layer on the manganese dioxide layer.
The conductive adhesive of the present invention may be used in capacitors for bonding the components of the capacitor. The bonding formed by the conductive adhesive of the present invention is not only firm, of an excellent electro-conductivity, but also is highly reliable, especially of an excellent moisture-heat stability.
Examples
Materials used in the examples - jER 828US: bisphenol A epoxy resin, from Japan Epoxy Resins Co., Ltd.
- SG15F35: silver-coated glass flakes with a silver content of 35wt.%, and an average particle size of 15μηη, from Potter Industries Inc..
- SG05TF40: silver-coated glass flakes with a silver content of 40wt.%, and an average particle size of 5μηη, from Potter Industries Inc.
- 30-103: silver-coated boron nitride flakes with a silver content of 53wt.%, and an average particle size of 12μηη, from Technic Inc.
- 24-405A: aliphatic alkane modified bismaleimide, from Henkel Corporation.
- SR423A: isobornyl methylacrylate, from Sartomer.
- SR248: neopentyl glycol dimethacrylate, from Sartomer.
- Ricon 131 MA10: an oligomer of 2,5-furandione with 1 ,3-butadiene, from Sartomer.
- Perkadox 16: bis(4-tert-butyl-cyclohexyl)peroxydicarbonate, from Akzo Nobel.
- Perkadox CH-50: benzoyl peroxide, from Akzo Nobel.
- IXE770F: aluminum manganese oxide (an ion exchanger), from Toagosei.
- EA 101 : silver powder, from Metalor Technologies.
- GA 23825: silver powder, from Metalor Technologies.
- Epiclon 850S: bisphenoal A epoxy resin, from DIC Dainippon Ink & Chemicals.
- RAS-1 : 2,6-biepoxypropyl-glycidyl phenol, from Henkel Corporation.
- SP3006: 1 ,4- butanediol biglycidyl ether, from HenkelCorporation.
- Ajicure PN-H: addition product of an epoxy rein and imidazole, from Ajinomoto co. Ltd.
- EMI 24 Cn: 1 -cyanoethyl-2-ethyl-4-methylimidazole, from PCI Synthesis.
- A-187: 3-glycidoxypropyltrimethoxysilane, from Momentive Performance Materials.
- SPAA9829: flake-like silver powder particles, from Metalor.
- DDSA: 2- dodecenyl-succinic anhydride, from Krahn co, Ltd.
- MHHPA: methylhexahydrophthalic anhydride, from Dixie Chemical.
Methods of measurements:
A series of measurements including a conductivity measurement, a bonding strength measurement, a density measurement and a moisture-heat resistance measurement were conducted to prove the excellent performances of the conductive adhesive of the present invention.
Density measurement:
The density of the cured conductive adhesive was measured in accordance with ATM-001 :
The primary apparatus used in this measurement was a densimeter.
An empty densimeter was weighted, then filled with pure water, and weighted again to obtain the mass of the pure water, which was recorded as ml . Subsequently, the densimeter was emptied, and then the conductive adhesive sample to be tested was filled in, and weighted to obtain the mass of the sample, which was recorded as m2. The density of the conductive ahhesive sample was obtained by the following equation: D = m2/m1 *1 .0(g/cm3).
Moisture-heat resistance measurement:
The moisture-heat resistance of the cured conductive adhesive was measured in accordance with the following procedure:
The sample to be tested was put into a thermostatic chamber having a relative moisture of 85% and a temperature of 85°C . To measure variations in the volume resistivity and to determine the moisture-heat resistance several samples were removed from the chamber after a given period of time.
Volume resistivity measurement:
The volume resistivity of the dried and cured conductive adhesive was measured in accordance with ATM-0020:
Measurement apparatus: precise digital electric bridge Gen Rad 1689 RLC
The conductive adhesive was coated on a glass carrier to form a cuboid-like adhesive layer having a length of 7.5 cm and a width of 1 .25cm to produce a test sample. The height of the sample varied and was usually between 0.001 cm to 0.01 cm. The adhesive layer was cured and then placed on an electric bridge to measure the resistance. The volume resistivity was calculated in accordance with the following equation: p=0.254R/L wherein p is the volume resistivity, R is the resistance measured, and L is the height of the sample.
Bonding strength measurement:
The bonding strength of the dried and cured conductive adhesive layer was measured in accordance with ATM-0052:
Measurement apparatus: Shear strength tester Dage 4000
An amount of the conductive adhesive was placed as a glue dot onto a silver substrate, and then the anode of a capacitor was attached to the glue dot. By pressing slightly the glue layer fully covered the lower surface of the capacitor. The amount of the conductive glue was controlled so as to exactly cover the lower surface of the capacitor. The glue layer was cured, and then the lateral shear strength of the glue layer, which can be used to express the bonding strength of the glue layer, was measured using a Dage 4000 instrument.
Example 1
Firstly, 9.68g of bismaleimide resin 24-405A, 4.84g of acrylate diluent SR423A, 3.23g of acrylate diluent SR248, and 9.68g of acrylate oligomer Ricon 131 MA10 were added to a 100 ml vessel and homogeneously mixed with stirring by hand for about 1 minute. Secondly, to the mixed solution, 0.03g of inhibitor hydroquinone, 0.65g of ion exchanger IXE 770F, 0.77g of initiator bis(4-tert-butyl-cyclohexyl)peroxydicarbonate, and 0.08g of another initiator benzoyl peroxide were added, stirring by hand for about 1 minute; followed by dispersing the solid substances into the system by a three-roller mill, to give a homogeneous resin-curing agent system. Thirdly, to the resin-curing agent system, 29.08g of silver-coated boron nitride 30-103 and 41 .98g of silver powder EA101 were added, and then the mixture was transferred into a rotary mixer, and was stirred at a speed of 3000 rpm for 30 minutes to give the final product.
The conductive adhesives of examples 2-9 were formulated in the same manner as that in example 1 except that the formulations of the conductive adhesives were changed.
The specific formulations of the conductive adhesives of examples 1 -9 are listed in table 1 .
Table 1 : examples based on bismaleimide (BMI) resin and acrylate resin system
Figure imgf000019_0001
Examples of the pure silver glue and the conductive adhesive using the silver-coated copper particle as the conductive filler are given below for comparison.
Comparative example 26: the pure silver glue
Figure imgf000020_0001
Comparative example 27: the conductive adhesive using the silver-coated copper particle as the conductive filler
Figure imgf000021_0001
Example 10
Firstly, 1 .55g of solid 8-quinolinoi was added to 19.62g of liquid epoxy resin Epiclon850S, and then the mixture was heated to 60°C , and mechanically stirred at a speed of 3000 rpm for 10 minutes, to completely dissolve the solid. Secondly, to the above epoxy rein, 5.16g of diluent SP3006, 0.21 g of adhesion promoter A187 and 0.52g of curing agent EMI24CN were added, and stirred by hand for 1 minute to give a homogeneous solution. Thirdly, to the solution, 2.58g of another curing agent Ajicure PN-H, 25.82g of silver-coated boron nitride 30-103, and 44.54g of silver powder EA101 were added, and then the mixture was transferred into a rotary mixer, and was stirred at a speed of 3000 rpm for 30 minute to give the final product. The conductive adhesives of examples 11 -25 were formulated in a same manner as that in example 10 except that the formulations of the conductive adhesives were changed.
The specific formulations of the conductive adhesives of examples 10-25 are listed in tables 2 and 3.
Table 2: Examples based on epoxy resins
Figure imgf000023_0001
"Total RH" means the weight percentage of the semi-product in which the conductive filler (silver powder or silver-coated boro nitride particle) was not added.
Table 3: Formulations based epoxy resins
Figure imgf000024_0001
Results of the shear strength measurement and the moisture-heat resistance measurement of examples 1 -25 and comparative examples 28-27 are shown in tables 4 and 5 below.
Table 4: Results of the shear strength measurement
Figure imgf000026_0001
Table 5: Results of the moisture-heat resistance measurement
Figure imgf000027_0001
Figure imgf000028_0001
From the above test data, it can be seen that the conductive adhesive of the present invention not only has excellent adhesion properties and an excellent electro-conductivity, but also exhibits an excellent moisture-heat stability. The electro-conductivity of the conductive adhesive of the present invention is comparable to that of the silver glue (comparative example 26). The moisture-heat stability of the conductive adhesive of the present invention is much better than that of the conductive adhesive using silver-coated copper as the conductive filler (comparative example 27).
The present invention is illustrated in details in the embodiments; however, it is apparent for those skilled in the art to modify and change the embodiments without deviating from the spirit of the invention. All the modifications and changes should fall in the scope of the appended claims of the present application

Claims

1 . A conductive adhesive for capacitors, comprising:
10 to 60 wt.% of an epoxy resin ,
0.5 to 6 wt.% of a curing agent for the epoxy resin,
0 to 60 wt.% of silver-powder particles, and
15 to 50 wt.% of silver-coated non-metal particles.
2. The conductive adhesive according to claim 1 , wherein the epoxy resin is a bisphenol-type epoxy resin or a phenolic-type epoxy resin.
3. The conductive adhesive according to claim 2, wherein the epoxy resin is a bisphenol A epoxy resin.
4. The conductive adhesive according to any one of claims 1 to 3, wherein the curing agent is selected from amine curing agents, imidazole curing agents, and acid anhydride curing agents.
5. The conductive adhesive according to claim 4, wherein the curing agent is selected from imidazole curing agents, and acid anhydride curing agents.
6. The conductive adhesive according to any one of claims 1 to 5, wherein the silver-coated non-metal particles satisfy at least one of the following:
• a density of 3 to 5 g/cm3,
• an average particle size of 5 to 100 micrometers, and
• an amount of coated silver of 20 to 60 wt.%, said amount of coated silver means the percentage of the mass of silver based on the total mass of the silver-coated non-metal particle.
7. The conductive adhesive according to any one of claims 1 to 6, wherein the non-metal material used in the silver-coated non-metal particles is one or more material(s) selected from glass, boron nitride, calcium carbonate, carbon black, carbon fiber, aluminum oxide and polymers.
8. The conductive adhesive according to claim 7, wherein the silver-coated non-metal particles are silver-coated glass particles or silver-coated boron nitride particles.
9. The conductive adhesive according to claim 8, wherein the silver-coated non-metal particles are silver-coated boron nitride particles.
10. The conductive adhesive according to any one of claims 1 to 9, further comprising one or more additives selected from an adhesion promoter, a dispersing agent, a defoaming agent, and a thixotropic agent.
1 1 . A conductive adhesive for capacitors, comprising:
10 to 40 wt.% of a vinyl resin ,
0 to 2 wt.% of a peroxide initiator ,
0 to 60 wt.% of silver-powder particles, and
15 to 50 wt.% of silver-coated non-metal particles.
12. The conductive adhesive according to claim 11 , wherein the vinyl resin is selected from bismaleimide resins, acrylate resins, acrylic acid resins and butadiene resins.
13. The conductive adhesive according to claim 11 or 12, wherein the silver-coated non-metal particles satisfy at least one of the following:
• a density of 3 to 5 g/cm3,
• an average particle size of 5 to 100 micrometers, and
• an amount of coated silver of 20 to 60 wt.%, said amount of coated silver means the percentage of the mass of silver based on the total mass of the silver-coated non-metal particle.
14. The conductive adhesive according to any one of claims 11 to 13, wherein the non-metal material used in the silver-coated non-metal particles is one or more material(s) selected from glass, boron nitride, calcium carbonate, carbon black, carbon fiber, aluminum oxide and polymers.
15. The conductive adhesive according to claim 14, wherein the silver-coated non-metal particles are silver-coated glass particles or silver-coated boron nitride particles.
1 6. The conductive adhesive according to claim 15, wherein the silver-coated non-metal particle are silver-coated boron nitride particles.
17. The conductive adhesive according to any one of claims 11 to 1 6, further comprising one or more additives selected from an adhesion promoter, a dispersing agent, a defoaming agent, and a thixotropic agent.
18. A capacitor, comprising the conductive adhesive according to any one of claims 1 to 17.
19. The capacitor according to claim 18, which is an aluminum electrolytic capacitor, a tantalum electrolytic capacitor or a niobium electrolytic capacitor.
20. Use of silver-coated non-metal particles in the production of conductive adhesives for capacitors.
21 . The use of claim 20, wherein the silver-coated non-metal particles are silver-coated glass particles or silver-coated boron nitride particles.
22. Use of a conductive of any one of claims 1 to 17 in the fabrication of capacitors.
23. Use of a conductive of any one of claims 1 to 17 for attaching an electrode to the leadframe of a capacitor.
24. The use of claim 22 or 23, wherein the capacitor is an aluminum electrolytic capacitor, a tantalum electrolytic capacitor or a niobium electrolytic capacitor.
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