Conductive paste for external electrode, and multilayer ceramic electronic component comprising external electrode formed using same

Abstract

Provided is a conductive paste having high heat resistance making it possible to withstand heat from solder with a high melting temperature, while also having flexibility making it possible to relax stress even against rapid expansion of a moisture absorbing material.　This conductive paste for an external electrode contains (A) conductive particles, (B) epoxy resin, (C) a curing agent, and (D) a bismaleimide compound.

Description

Conductive paste for external electrodes and multilayer ceramic electronic component having external electrodes formed using the same

[0001] The present invention relates to a conductive paste for external electrodes and a multilayer ceramic electronic component having external electrodes formed using the same.

[0002] Chip-type multilayer ceramic electronic components, each consisting of a large number of laminated dielectric layers and electrodes, are widely used in electronic circuits in small devices such as mobile phones and tablet computers, where cost reduction and miniaturization are strongly desired. A conventional multilayer ceramic electronic component will be described using the multilayer ceramic capacitor shown in FIG. 1 as an example. Multilayer ceramic capacitors are miniaturized by providing external electrode layers instead of leads on a structure consisting of a large number of laminated dielectric layers and internal electrodes, significantly contributing to the miniaturization and weight reduction of electronic devices. Multilayer ceramic capacitor 1 has a structure in which ceramic dielectric layers 2 and internal electrode layers 3 are alternately laminated, and an external electrode layer 4 is provided on the internal electrode contact surface of the ceramic composite. Typically, the external electrode has a structure in which a plating layer 5 is applied to the external electrode layer 4. The plating layer 5 typically comprises a nickel plating layer and a tin plating layer. When mounting a multilayer ceramic capacitor on a circuit board 7, the external electrodes of the multilayer ceramic capacitor are connected to the wiring electrodes of the circuit board via solder layers 6.

[0003] Causes of failure in electronic circuits using multilayer ceramic capacitors include physical factors such as external force applied to a circuit board on which the multilayer ceramic capacitor is mounted or deflection of the circuit board. Such external force is transmitted as stress to the laminate composite via the solder layer and external electrode, which can cause separation between the external electrode and the ceramic composite or cracks in the ceramic composite. Providing an electrode layer as a buffer material on the external electrode layer 4 has been proposed as a means of improving the reliability of electronic circuits against external physical force (Patent Document 1). In this approach, a multi-layer external electrode is formed, with a copper substrate as the first conductor layer 11 serving as an external electrode connected to the internal electrode of the multilayer capacitor, and a resin-containing conductive paste as a buffer material laminated on top of the first conductor layer 12.

[0004] AEC Q-200, the standard for automotive passive components, requires solder heat resistance. Furthermore, in recent years, the melting temperature of solder itself has increased with the trend toward lead-free soldering. For example, while the melting temperature (solidus temperature) of conventional lead solder is approximately 183°C, the melting temperature (solidus temperature) of lead-free solder is often 200°C or higher. Therefore, when soldering with lead-free solder, higher heat resistance is required for external electrodes. Furthermore, for automotive passive components, it is necessary for the coating film to not crack even when subjected to a high-temperature, high-humidity test (humidity resistance test) followed by a solder heat resistance test. In addition to heat resistance, stress relaxation properties against the sudden expansion of moisture-absorbed materials are also required. One method for imparting heat resistance has been reported, for example, by increasing the glass transition temperature (Tg) using naphthylene ether epoxy resins (Patent Document 2).

[0005] JP 2011-233452 A JP 2018-145410 A

[0006] Conductive pastes for forming external electrodes of electronic components are required to be safe and highly reliable, and therefore must have flexibility capable of withstanding bending of the substrate and high heat resistance. However, as in Patent Document 2, simply increasing the glass transition point of the conductive paste to improve heat resistance results in a hard and brittle cured product, which can crack when subjected to rapid expansion of the moisture-absorbing material. Therefore, the present invention aims to provide a conductive paste that has high heat resistance capable of withstanding the heat of high-melting-temperature solder and flexibility capable of stress relaxation even when subjected to rapid expansion of the moisture-absorbing material.

[0007] The present inventors have conducted extensive research to solve the above problems and have arrived at the present invention. That is, the present invention includes the following inventions, but is not limited to the following.

[0008] 1. A conductive paste for external electrodes, comprising: (A) conductive particles; (B) an epoxy resin; (C) a curing agent; and (D) a bismaleimide compound.

[0009] 2. The conductive paste according to the preceding paragraph 1, wherein the bismaleimide compound (D) is a liquid at room temperature.

[0010] 3. The conductive paste according to item 1 or 2 above, wherein the bismaleimide compound (D) has a viscosity of 20 Pa·s or less when measured at 25° C. with a Brookfield viscometer.

[0011] 4. The conductive paste according to any one of items 1 to 3 above, wherein the weight-average molecular weight of the bismaleimide compound (D) is in the range of 1,000 to 10,000.

[0012] 5. The conductive paste according to any one of items 1 to 4 above, wherein the (D) bismaleimide compound is a dimer acid-modified bismaleimide compound.

[0013] 6. The (D) bismaleimide compound is represented by the following formula (1): (wherein X represents a divalent aliphatic hydrocarbon group having 1 to 100 carbon atoms which may contain a heteroatom between the carbon atoms).

[0014] 7. The conductive paste according to any one of items 1 to 3 and 6 above, wherein the (D) bismaleimide compound is an aliphatic bismaleimide compound having 2 to 50 carbon atoms.

[0015] 8. The (D) bismaleimide compound is represented by the following formula (2): (-C ３６ H ７０ 8. The conductive paste according to any one of items 1 to 3, 6, and 7 above, wherein the bismaleimide is represented by the formula:

[0016] 9. The conductive paste according to any one of items 1 to 8 above, wherein the (D) bismaleimide compound includes a bismaleimide compound having an acid value of 2 mg-KOH / g or less.

[0017] 10. The conductive paste according to any one of items 1 to 9 above, wherein the content of the (D) bismaleimide compound is in the range of 1 part by mass to 10 parts by mass per 100 parts by mass of the (A) component.

[0018] 11. The conductive paste according to any one of items 1 to 10 above, wherein the (C) curing agent contains an allyl phenol resin.

[0019] 12. The conductive paste according to any one of items 1 to 11 above, further comprising dicyandiamide.

[0020] 13. The conductive paste according to any one of items 1 to 12, wherein the conductive paste has a flexural modulus of 3 GPa to 10 GPa when dried at 120°C for 60 minutes and then cured in an air dryer at 180°C for 30 minutes.

[0021] 14. The conductive paste according to any one of items 1 to 13, having a thixotropic index value, which is the ratio of viscosity at a rotation speed of 10 rpm to viscosity at a rotation speed of 100 rpm, measured at 25°C with an HB type viscometer, of 7 or less.

[0022] 15. The conductive paste according to any one of items 1 to 14 above, wherein the external electrodes are used for connection with lead-free solder.

[0023] 16. A multilayer ceramic electronic component having external electrodes formed using the conductive paste according to any one of items 1 to 15 above.

[0024] 17. A multilayer ceramic electronic component comprising external electrodes formed by curing a conductive paste, the conductive paste containing (A) conductive particles, (B) an epoxy resin, (C) a curing agent, and (D) a bismaleimide compound.

[0025] 18. The multilayer ceramic electronic component according to item 17 above, wherein the conductive paste further contains dicyandiamide.

[0026] 19. The multilayer ceramic electronic component according to item 17 or 18, further comprising a circuit board on which wiring electrodes are formed, the external electrodes being connected to the wiring electrodes by soldering layers, and the soldering layers being formed of lead-free solder.

[0027] The conductive paste of the present invention can be used as the second conductor layer of an external electrode, thereby providing a conductive paste that exhibits high adhesion, has high heat resistance capable of withstanding the heat of high-melting-temperature solder, and is flexible enough to relieve stress even when subjected to rapid expansion of moisture-absorbing materials.

[0028] FIG. 1 is a schematic diagram showing the structure of a ceramic capacitor having an external electrode made up of multiple conductor layers, in which a second conductor layer containing resin is stacked on top of a first conductor layer connected to an internal electrode.

[0029] Hereinafter, embodiments of the present invention will be described in detail. In the present invention, the term "first conductor layer" refers to a conductor layer that constitutes an external electrode and is directly connected to an internal electrode. The term "second conductor layer" refers to a conductor layer that constitutes an external electrode and is stacked on the first conductor layer.

[0030] The conductive paste for external electrodes of the present invention contains (A) conductive particles, (B) an epoxy resin, (C) a curing agent, and (D) a bismaleimide compound. Components (A) to (D) are described in detail below.

[0031] (A) Conductive Particles The conductive particles are components for imparting conductivity to the external electrodes, and are metal particles. Examples of the metal particles include Ag, Cu, Ni, Pd, Au, and Pt metal particles. Ag metal particles are preferred because they can relatively easily provide excellent conductivity.

[0032] The conductive particles are preferably metal particles having a melting point of 700°C or higher. The melting point of the conductive particles is more preferably 800°C or higher. The upper limit of the melting point is not particularly limited, but is usually 1800°C or lower, preferably 1600°C or lower. The conductive particles can be used alone or in combination of two or more types.

[0033] Examples of conductive particles include alloys of Ag, Cu, Ni, Pd, Au, and Pt, and among these, metal particles having a melting point of 700° C. or higher are preferred. Ag alloy particles are preferred because excellent conductivity can be obtained relatively easily.

[0034] Examples of alloy particles include metal particles of an alloy composed of two or more elements selected from the group consisting of Ag, Cu, Ni, Pd, Au, and Pt. Examples of binary Ag alloys include AgCu alloys, AgAu alloys, AgPd alloys, and AgNi alloys. Examples of ternary Ag alloys include AgPdCu alloys and AgCuNi alloys.

[0035] Furthermore, examples of alloy particles include metal particles of an alloy composed of one or more elements selected from Ag, Cu, Ni, Pd, Au, and Pt and one or more other elements, and among these, metal particles having a melting point of 700° C. or higher as an alloy are preferred. Examples of other elements include Zn, Al, and Sn, and in the case of a binary alloy of Sn and Ag, an AgSn alloy can be used in which the mass ratio of Sn to Ag is higher than 25.5:74.5.

[0036] Furthermore, multilayered particles, in which the surfaces of metal particles are coated with other metal atoms or metal compounds, can be used as conductive particles. By disposing expensive metals such as Ag, Pd, Au, and Pt on the surfaces of relatively inexpensive metal particles such as Cu, the overall cost of the conductive particles can be reduced while utilizing the physical properties of these metals, such as high conductivity, migration resistance, and corrosion resistance. Multilayered particles can be manufactured using known manufacturing methods tailored to the particle shape. For example, core-shell particles with an outer layer having a certain thickness ratio relative to the entire particle can be produced by, for example, precipitating a metal on the surface of an inner layer metal particle through a metal precipitation reaction caused by a reduction reaction of a compound containing the metal on the outer layer. Alternatively, particles can be produced in which a thin coating film is formed on the surface of the inner layer metal particle by immersing the inner layer metal particle in a solution of a metal compound and then drying the resulting particles.

[0037] The thickness and mass ratio of the outer layer and inner layer portions of multilayer particles are not particularly limited. From the viewpoint of placing greater importance on the properties of the metal on the outer layer side, a larger thickness and mass ratio of the outer layer are preferred, while from the viewpoint of cost, a smaller thickness and mass ratio of the outer layer are preferred. The thickness of the outer layer portion is preferably 1 to 100 nm or less and can be controlled by the manufacturing method. Furthermore, it is not necessary for the outer layer portion to cover the entire surface of the metal on the inner layer side; particularly in coated particles, a structure in which the metal of the outer layer coats a portion of the metal particle on the inner layer side may be used. In the case of coated particles, the mass ratio of the outer layer portion is preferably 0.01 to 1.0 parts by mass per 100 parts by mass of the conductive particles. In the case of core-shell particles, it is preferably 5 to 40 parts by mass per 100 parts by mass of the conductive particles.

[0038] Furthermore, as the conductive particles, it is possible to use low-melting-point metal particles such as Sn, In, and Bi, whose melting point is 200° C. or more and less than 700° C. The low-melting-point metal particles are preferably non-Pb.

[0039] Furthermore, as the low-melting-point metal particles, metal particles of an alloy of Sn, In, and Bi, having a melting point of 200°C or more and less than 700°C, can also be used. Sn alloys are preferred because they can relatively easily provide excellent conductivity. Examples of alloy particles include metal particles of an alloy composed of two or more elements selected from the group consisting of Sn, In, and Bi, and examples of binary alloys include SnIn alloys.

[0040] The conductive particles may be of any shape, such as spherical, flake-like, scale-like, or needle-like. The average particle diameter of these particles is preferably 0.015 to 30 μm, as this ensures a good surface condition after printing or coating and can impart excellent conductivity to the formed electrode layer. When the conductive particles are spherical, the average particle diameter is more preferably in the range of 0.2 to 5 μm. When the metal particles are flaky, the average particle diameter is more preferably in the range of 5 to 30 μm. In this specification, the average particle diameter refers to the particle diameter in the case of spherical particles, the diameter of the longest part in the case of flaky particles, the major axis of the particle flakes in the case of flaky particles, and the average length in the case of needle-like particles. Here, the average particle diameter of metal particles is a value determined by image analysis after observation with a scanning electron microscope (SEM).

[0041] The conductive particles preferably comprise a combination of (A1) flaky silver particles and (A2) spherical silver particles. The mass ratio of the silver particles (A1) to (A2) is preferably 5:95 to 95:5. By setting the ratio of flaky silver powder to spherical silver powder within the above range, the specific resistance can be reduced, and the thixotropic index (TI) value can be reduced, resulting in improved coating shape and plating properties. A ratio of 15:85 to 80:20 is more preferred, and a ratio of 20:80 to 60:40 is even more preferred. The metal particles used in the present invention may be commercially available or may be prepared by methods known to those skilled in the art.

[0042] (B) Epoxy Resin: Epoxy resin functions as a binder for conductive pastes. Epoxy resin is a general term for thermosetting resins that can be cured by forming a crosslinked network with epoxy groups present in the compound, and includes prepolymer compounds before crosslinking. Epoxy resins have excellent dimensional stability, insulating properties, and chemical resistance. The type of epoxy resin is not particularly limited as long as it has two or more epoxy groups. From the standpoint of curing speed and reliability, compounds with 2 to 6 epoxy groups are more preferred, and bifunctional epoxy resins with two epoxy groups are even more preferred. Using a thermosetting resin that is liquid at room temperature is preferable because it allows for a reduction in the amount of organic solvent used as a diluent. Furthermore, in relation to the curing agent described below, epoxy resins that are cured with phenolic resins are particularly preferred.

[0043] Examples of epoxy resins include bisphenol A type epoxy resins having an average molecular weight of about 400 or less; branched polyfunctional bisphenol A type epoxy resins such as p-glycidyloxyphenyldimethyltrisbisphenol A diglycidyl ether; bisphenol F type epoxy resins; phenol novolac type epoxy resins having an average molecular weight of about 570 or less; alicyclic epoxy resins such as vinyl(3,4-cyclohexene) dioxide, 3,4-epoxycyclohexylcarboxylate (3,4-epoxycyclohexyl)methyl, bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate, and 2-(3,4-epoxycyclohexyl)5,1-spiro(3,4-epoxycyclohexyl)-m-dioxane; glycidyl ester type epoxy resins such as diglycidyl hexahydrophthalate, diglycidyl 3-methylhexahydrophthalate, and diglycidyl hexahydroterephthalate; glycidyl amine type epoxy resins such as diglycidyl aniline, diglycidyl toluidine, triglycidyl-p-aminophenol, tetraglycidyl-m-xylylenediamine, and tetraglycidyl bis(aminomethyl)cyclohexane; hydantoin type epoxy resins such as 1,3-diglycidyl-5-methyl-5-ethylhydantoin; and naphthalene ring-containing epoxy resins, particularly naphthylene ether type epoxy resins. Other examples include epoxy resins having a silicone skeleton such as 1,3-bis(3-glycidoxypropyl)-1,1,3,3-tetramethyldisiloxane; diepoxide compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, butanediol diglycidyl ether, neopentyl glycol diglycidyl ether, and cyclohexanedimethanol diglycidyl ether; and triepoxide compounds such as trimethylolpropane triglycidyl ether and glycerin triglycidyl ether.Furthermore, solid epoxy resins such as bisphenol A epoxy resins, bisphenol F epoxy resins, bisphenol S epoxy resins, naphthalene epoxy resins, phenol novolac epoxy resins, phenol aralkyl epoxy resins, cresol novolac epoxy resins, biphenyl epoxy resins, biphenyl novolac epoxy resins, biphenyl aralkyl epoxy resins, triphenylmethane epoxy resins, and dicyclopentadiene epoxy resins may be used in combination in an amount such that the entirety of (B) is liquid. Bisphenol A epoxy resins, bisphenol F epoxy resins, and naphthylene ether epoxy resins are more preferred epoxy resins, with naphthylene ether epoxy resins being particularly preferred.

[0044] Naphthylene ether type epoxy resins are epoxy resins having a structure in which naphthalene structures are linked by ether groups, and are considered to have better chemical heat resistance than conventional high heat resistant resins. Naphthylene ether type epoxy resins are, for example, those represented by the following general formula: [In the formula, R １ and R ２ are each independently a hydrogen atom, a benzyl group, an alkyl group, or a group represented by the following formula: (wherein each Ar is independently a phenylene group or a naphthylene group, m is an integer of 1 or 2), and n is an integer of 1 to 20).

[0045] Specific examples of other naphthylene ether type epoxy resins include naphthylene ether type epoxy resins represented by formula (6) or formula (7) disclosed in "Network Polymer," Vol. 30, No. 4, p. 192 (2009). Commercially available naphthylene ether type epoxy resins include naphthylene ether type epoxy resins manufactured by DIC (product name: Epiclon HP-6000 (epoxy equivalent weight: 240 to 270)), (product names: EXA-7310 (epoxy equivalent weight: 247), EXA-7311 (epoxy equivalent weight: 277), EXA-7311L (epoxy equivalent weight: 262), EXA7311-G3 (epoxy equivalent weight: 250)), and the like.

[0046] (B) The epoxy resin is preferably used so that its epoxy equivalent is 2000 g / eq or less. Here, "epoxy equivalent" refers to the value obtained by dividing the molecular weight of the resin by the number of epoxy groups in the molecule. By setting the epoxy equivalent within this range, it is possible to keep the specific resistance low and improve the adhesive strength. The epoxy equivalent range is preferably 50 to 2000 g / eq, more preferably 70 to 1000 g / eq, and even more preferably 80 to 700 g / eq.

[0047] The content of the (B) epoxy resin in the conductive paste of the present invention is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and particularly preferably 5 parts by mass or more, per 100 parts by mass of the (A) conductive particles. Furthermore, the content is preferably 25 parts by mass or less, more preferably 20 parts by mass or less, and particularly preferably 15 parts by mass or less, per 100 parts by mass of the (A) conductive particles. By setting the content of the (B) epoxy resin within this range, sufficient adhesion to the copper undercoat and solder layer can be achieved without impairing the conductivity originally required of the conductive paste.

[0048] (C) Curing Agent Any curing agent can be used without particular limitations as long as it has the function of curing the epoxy resin (B). As the curing mechanism of the epoxy resin, a self-curing resin may be used, or a curing agent or curing catalyst such as an amine, imidazole, acid anhydride, or onium salt may be used. An amino resin or a phenol resin may also function as a curing agent for the epoxy resin. In particular, it is preferable to use a phenol resin.

[0049] Phenolic resin is a general term for thermosetting resins that can be crosslinked into a network via phenolic hydroxyl groups present in the compound, including prepolymer compounds prior to crosslinking. Phenolic resins are epoxy resin curing agents that react with the epoxy groups of epoxy resins and contribute to imparting solder wettability. Examples of phenolic resins include phenol novolac resins and their alkylated or allylated derivatives, cresol novolac resins, phenol aralkyl (including phenylene and biphenylene skeleton) resins, naphthol aralkyl resins, triphenolmethane resins, and dicyclopentadiene-type phenolic resins. Among these, cresol novolac resins and phenol aralkyl resins are preferred due to their ability to enhance solder wettability.

[0050] The phenolic resin may be a phenolic resin initial condensate that is commonly used as a curing agent for epoxy resins, and may be a resol type or a novolac type. However, in order to obtain excellent heat cycle resistance, it is preferable that 50 mass % or more of the phenolic resin be an alkyl resol type, alkyl novolac type, or aralkyl novolac type phenolic resin, a xylene resin, or an allyl phenol resin represented by the following general formula: Also preferred are aralkyl novolac phenolic resins, which are phenol-p-xylylene glycol dimethyl ether polycondensates represented by the formula: (wherein n is 0 to 300). Furthermore, in the case of alkyl resole phenolic resins, it is preferable that the average molecular weight is 2,000 or more in order to obtain excellent printability. In these alkyl resole or alkyl novolac phenolic resins, the alkyl group may have 1 to 18 carbon atoms, and preferred are those having 2 to 10 carbon atoms, such as ethyl, propyl, butyl, pentyl, hexyl, octyl, nonyl, and decyl.

[0051] Among these, a combination of an epoxy resin with an aralkyl novolac phenolic resin, a resol phenolic resin, a xylene resin, or an allyl phenolic resin is preferred because it provides excellent adhesion and heat resistance. Polyimide resins are also effective in terms of heat resistance.

[0052] Furthermore, from the viewpoint of further improving heat resistance and toughness, the conductive paste of the present invention preferably contains an allylphenol resin and a (D) bismaleimide compound. That is, by using an allylphenol resin in combination with a (D) bismaleimide compound, heat resistance and toughness can be further improved. This is presumably because the allyl group of the allylphenol resin and the maleimide group of the bismaleimide compound undergo a radical reaction to improve curability, thereby further improving heat resistance and toughness. However, the present invention is not bound by this reasoning.

[0053] The content of the (C) curing agent in the conductive paste of the present invention may be an amount that can sufficiently cure the (B) epoxy resin. When the (C) curing agent is a phenolic resin, an appropriate weight ratio can be calculated based on the epoxy equivalent of the epoxy resin and the active hydrogen equivalent of the curing agent. When a combination of an epoxy resin with an aralkyl novolac phenolic resin, a resol phenolic resin, a xylene resin, or an allyl phenolic resin is used, the mass ratio of the epoxy resin to the phenolic resin is preferably in the range of 5:1 to 1:5, and more preferably 4:1 to 1:2.

[0054] (D) Bismaleimide Compound Bismaleimides are generally represented by the following formula (1): (wherein X is a divalent organic group). Bismaleimides have a rigid chemical structure and versatile, high reactivity toward radical reactions, addition reactions, and other reactions. While bismaleimides themselves can be used as monomers, they are also used as additives to various polymers to impart strength and heat resistance. Furthermore, bismaleimides are compounds whose properties can be altered by the divalent organic group (X in the above formula), which is the structure that connects the two maleimide groups.

[0055] The bismaleimide compound may be solid at room temperature (25°C), but solid bismaleimides have low solubility in common organic solvents and may require dilution with a large amount of organic solvent. Therefore, from the viewpoint of workability, it is preferable that the bismaleimide compound is liquid at room temperature. The viscosity of the liquid bismaleimide compound at 25°C is preferably 20 Pa s or less, more preferably 10 Pa s or less, and even more preferably 5 Pa s or less. The viscosity here is the value measured with a Brookfield viscometer at 25°C.

[0056] Methods for producing bismaleimides are well known, and they can generally be obtained by adding two molar equivalents of maleic acid to a diamine, followed by cyclodehydration in the presence of a catalyst. Therefore, by controlling the structure of the diamine, bismaleimides with various properties can be obtained. The structure of the diamine used as the raw material, with the two amino groups removed, corresponds to X in the above formula (1).

[0057] X in the above formula (1) is not particularly limited as long as it is a divalent organic group. It can be a linear, branched, or cyclic hydrocarbon group, an aromatic group, a heterocycle, or a structure in which a plurality of these structures are combined, and a part of the hydrocarbon group, or the part where the hydrocarbon groups, aromatic groups, or heterocycles are connected to each other, may be interrupted by a heteroatom such as oxygen, sulfur, or nitrogen, or a carboxyl group, and the main skeleton part may have a substituent such as a short-chain alkyl group, a hydroxyl group, a halogen, or a cyano group. Examples of structures that can constitute the group X include, but are not limited to, aliphatic hydrocarbon groups such as methylene, ethylene, propylene, and butylene; cyclic hydrocarbon groups such as cyclopentanediyl, cyclohexanediyl, cycloheptanediyl, and cyclooctanediyl; divalent groups based on hydrocarbon compounds having unsaturated bonds such as 2-butene and acetylene; divalent groups based on aromatic hydrocarbon groups such as benzene, naphthalene, phenanthrene, anthracene, and pyrene; and divalent groups based on heterocyclic compounds such as furan, pyrrole, thiophene, pyrazole, oxazole, thiazole, triazole, pyridine, pyran, piperazine, triazine, azepane, indole, quinoline, isoquinoline, benzothiazole, 1,3-dioxoisoindole, xanthene, carbazole, cyclopentane-1,3-dione, lactones, and lactams. Although there are usually multiple possible positions at which these divalent groups can be bonded to the nitrogen atom of the maleimide or to other moieties constituting X, the positions can be any positions, and the combination of bonding positions can also be any. The moiety represented by X may have an asymmetric carbon atom or a symmetric plane or axis, and when stereoisomers are considered as bismaleimide, any of the isomers or a mixture of the isomers can be used.

[0058] In the above formula (1), X preferably has a chain-like skeleton, but its length and size are not particularly limited, and X may be a polymer, with molecular weight fluctuations for each molecule. When the divalent organic group moiety represented by X is polymeric, the weight-average molecular weight of the X moiety can be in the range of 800 to 10,000, preferably in the range of 900 to 5,000, and more preferably in the range of 1,000 to 2,900. Therefore, when the bismaleimide compound has a polymer skeleton, the weight-average molecular weight of the bismaleimide compound is preferably in the range of 1,000 to 10,000, more preferably in the range of 1,100 to 5,000, and even more preferably in the range of 1,200 to 3,000. Furthermore, in relation to the acid value described below, the weight-average molecular weight of the bismaleimide compound is particularly preferably in the range of 1,400 to 1,800, and most preferably in the range of 1,450 to 1,700.

[0059] Examples of bismaleimide compounds having a high molecular weight are shown below, but the bismaleimide compounds that can be used in the present invention are not limited to these. (In the above formula, each p is a value of 1 to 10 and does not have to be an integer. When p is not an integer, it means that the respective compound is a polymer.)

[0060] In another preferred embodiment, the bismaleimide compound is a dimer acid-modified bismaleimide compound. Dimer acid-modified bismaleimide is a modified bismaleimide in which the amino group of a dimer diamine having 24 to 48 carbon atoms is maleimidized. Use of a dimer acid-modified bismaleimide can reduce the elastic modulus of the conductive paste and inhibit the occurrence of cracks in the cured product. Of the above examples, an example of a dimer acid-modified bismaleimide is one having an aliphatic hydrocarbon skeleton having 36 carbon atoms.

[0061] The divalent organic group portion in the bismaleimide compound may be based on a low-molecular-weight organic compound. One preferred embodiment of the bismaleimide compound is a bismaleimide compound represented by the above formula (1), in which X represents a divalent aliphatic hydrocarbon group having 1 to 100 carbon atoms, preferably 2 to 50 carbon atoms, which may contain a heteroatom between the carbon atoms, and more preferably an aliphatic bismaleimide compound in which X is an aliphatic hydrocarbon group. By using an aliphatic bismaleimide compound, it is possible to impart high flexibility to the conductive paste. Examples of the aliphatic hydrocarbon group include -(CH ２ ) ｎ In addition to the linear hydrocarbon group represented by -, branched hydrocarbon groups and / or hydrocarbon groups having one or more unsaturated bonds can also be mentioned.

[0062] Examples of bismaleimide compounds based on low-molecular-weight organic compounds other than those in which X is an aliphatic hydrocarbon group are shown below, but the bismaleimide compounds that can be used in the present invention are not limited to these. (wherein n is an integer from 1 to 100)

[0063] More preferred aliphatic bismaleimide compounds are those in which X in the above formula (1) contains an aliphatic hydrocarbon group having 36 carbon atoms derived from dimer diamine. Here, X may have any branching or double bond position in the dimer acid, and the number of hydrogen atoms may vary accordingly. Particularly preferred bismaleimides are those represented by the following formula (2): The use of the bismaleimide compound of the formula (2) not only enables stress relaxation even in the event of sudden expansion of a moisture-absorbing material, but also provides toughness and further improves electrical conductivity and adhesive strength.

[0064] The bismaleimide compound may be used alone or as a mixture of two or more compounds. The bismaleimide compound preferably contains a bismaleimide compound having an acid value of 2 mg-KOH / g or less. The acid value of a bismaleimide compound is a parameter that quantitatively represents the amount of acid component remaining in the bismaleimide. In this specification, the acid value of a bismaleimide is a value measured by neutralization titration in accordance with JIS K0070 (1992). It is more preferable to contain a bismaleimide compound having an acid value of 1.5 mg-KOH / g or less, even more preferable to contain a bismaleimide compound having an acid value of 1 mg-KOH / g or less, and particularly preferable to contain a bismaleimide compound having an acid value of 0.5 mg-KOH / g or less. By using a bismaleimide compound having an acid value of 2 mg-KOH / g or less, moisture absorption can be suppressed.

[0065] The amount of the bismaleimide compound contained in the conductive paste is preferably 1 part by mass or more, more preferably 1.5 parts by mass or more, and particularly preferably 2 parts by mass or more, per 100 parts by mass of the component (A). The upper limit of the amount of the bismaleimide compound is preferably 10 parts by mass or less, more preferably 8 parts by mass or less, and particularly preferably 7 parts by mass or less. By adjusting the amount within this range, it is possible to impart toughness and improve electrical conductivity and adhesion.

[0066] In addition to the components (A) to (D), the conductive paste of the present invention may optionally contain additional components such as other resins and rubber particles, solvents, and known additives, provided that the effects of the present invention are not impaired. Examples of such additives include imidazoles such as 2-phenyl-4-methyl-5-hydroxymethylimidazole, curing catalysts such as dicyandiamide, and dispersing aids such as aluminum chelate compounds such as diisopropoxy(ethylacetoacetato)aluminum, titanate esters such as isopropyl triisostearoyl titanate, aliphatic polycarboxylic acid esters, unsaturated fatty acid amine salts, surfactants such as sorbitan monooleate, and polymeric compounds such as polyesteramine salts and polyamides. Inorganic and organic pigments, silane coupling agents, leveling agents, thixotropic agents, antifoaming agents, thixotropic agents, dispersants, and the like may also be added.

[0067] The conductive paste of the present invention preferably contains a curing catalyst, and particularly preferably contains dicyandiamide as the curing catalyst. Using the curing catalyst in combination with the component (D) further improves the heat resistance of the conductive paste. The amount of curing catalyst contained in the conductive paste is preferably 0.01 parts by mass or more, more preferably 0.015 parts by mass or more, and particularly preferably 0.02 parts by mass or more, per 100 parts by mass of the component (A). The upper limit of the amount of curing catalyst is preferably 1 part by mass or less, more preferably 0.9 parts by mass or less, and particularly preferably 0.8 parts by mass or less, per 100 parts by mass of the component (A).

[0068] The conductive paste may contain a thermosetting resin other than epoxy resin. The thermosetting resin that can be added is not particularly limited as long as it undergoes a polymerization reaction and hardens when heated, and examples include amino resins such as urea resin, melamine resin, and guanamine resin; oxetane resin; phenoxy resin; silicone-modified organic resins such as silicone epoxy and silicone polyester; and polyimide resin. For example, BT resin can also be used. As the phenoxy resin, bisphenol A phenoxy resin can be used, and is available as a commercially available product (e.g., bisphenol A phenoxy resin 4250 (manufactured by Mitsubishi Chemical), bisphenol A phenoxy resin Fx316 (manufactured by Nippon Steel & Sumitomo Metal), bisphenol A phenoxy resin YP50 (manufactured by Nippon Steel & Sumitomo Metal), etc.). Furthermore, a thermoplastic resin may be used in addition to the epoxy resin. Examples of the thermoplastic resin include polysulfone, polyethersulfone, polyvinyl butyral, and maleimide resin. These resins may be used alone or in combination.

[0069] The conductive paste of the present invention may contain rubber particles to relieve stress in the external electrodes. The rubber particles preferably have an average particle size of 1 μm or less, since this improves adhesion to the copper substrate. Here, the average particle size of the rubber particles is the average primary particle size, and is a value determined by image analysis after observation with a scanning electron microscope (SEM).

[0070] Examples of usable rubber particles include rubbers containing a molecular chain formed by the linkage of carbon atoms, preferably those in which the proportion of carbon atoms among the atoms constituting the rubber particles is 50% or more, and which may contain a functional group containing a heteroatom in the molecular skeleton, such as urethane rubber or acrylic rubber. More specifically, examples include hydrocarbon-derived rubbers such as natural rubber, and rubbers containing a functional group containing a heteroatom in the skeleton, such as acrylic rubber and urethane rubber. It is preferable to use at least one rubber selected from the group consisting of diene-based rubber, (meth)acrylate-based rubber, urethane-based rubber, isoprene-based rubber, fluorine-based rubber, and chloroprene-based rubber. In addition to these rubbers, hydrocarbon-derived rubbers mainly composed of polyolefins, such as ethylene-propylene rubber and styrene rubber, can also be used. These rubber particles may also be crosslinked. These can be used alone or in combination of two or more types.

[0071] Rubber particles having a core-shell particle structure can be used. For example, by placing a highly dispersible rubber on the surface (shell) of core particles with good physical properties such as toughness, modified rubber particles can be obtained that possess both properties, making it possible to design rubber particles that are more suitable for conductive pastes. The core-shell particle structure may be a simple two-layer structure, or may have three or more layers stacked together. There are no particular restrictions on the type of rubber particles selected for each layer, the thickness ratio of each layer, the mass ratio, etc., and any type and range can be designed.

[0072] From the viewpoint of ease of handling and application, the conductive paste of the present invention preferably has a viscosity measured with an HB-type viscometer at 25°C and 10 rpm of 60 Pa s or less, more preferably 40 Pa s or less, and even more preferably 20 Pa s or less. As long as a paste having a uniform composition can be obtained and has fluidity, there is no lower limit to the viscosity, but a viscosity of 5 Pa s or more is preferred.

[0073] Furthermore, the conductive paste of the present invention can be blended with an organic solvent to adjust the viscosity. Examples of organic solvents include aromatic hydrocarbons such as toluene, xylene, mesitylene, and tetralin; ethers such as tetrahydrofuran; ketones such as methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, and isophorone; lactones such as 2-pyrrolidone and 1-methyl-2-pyrrolidone; ether alcohols such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, and diethylene glycol monobutyl ether, as well as corresponding propylene glycol derivatives, phenoxyethanol, and ethylene glycol monophenyl ether; esters such as corresponding acetates; diesters such as methyl esters and ethyl esters of dicarboxylic acids such as malonic acid and succinic acid; dialkyl carbonates; and carbonates such as ethylene carbonate and propylene carbonate. The amount of organic solvent used is selected arbitrarily depending on the method for printing or applying the paste, etc., but in the case of screen printing, for example, it is preferably used in an amount such that the apparent viscosity of the paste at room temperature is 10 to 500 Pa s, more preferably 15 to 300 Pa s.

[0074] The fluidity of the conductive paste can be evaluated by the thixotropic index (TI). From this viewpoint, the thixotropic index, which is the ratio of the viscosity at a rotation speed of 10 rpm to the viscosity at a rotation speed of 100 rpm measured at 25°C with an HB type viscometer, is preferably 7 or less, more preferably 5 or less, and even more preferably 3 or less. The lower limit of TI is not particularly limited, but is 1 or more. By keeping TI within the above range, the coating shape and plating property can be improved. The TI of the conductive paste can be calculated by the following formula: TI=η １ / η １０ (In the formula, η １ is the viscosity measured using a rotational viscometer at a temperature of 25°C and a rotation speed of 10 rpm, and η１０ is η except that the rotation speed is 100 rpm. １ (The viscosity is measured under the same conditions as in Example 1.) When TI is within the above range, the conductive paste can be smoothly discharged from the application means. Furthermore, in the manufacturing process of a multilayer ceramic capacitor, it takes a certain amount of time from applying the conductive paste to the multilayer ceramic chip until it is hardened by a process such as baking, and since the external electrodes are not applied in a flat shape, if the conductive paste has high fluidity, it may naturally spread to some extent between application and the baking process. However, when TI is within the above range, manufacturing can be performed while maintaining the original dimensions.

[0075] According to the above definition of TI, when the TI of a conductive paste is 1 or less, the viscosity of the conductive paste under a relatively high shear stress is the same as or higher than the viscosity of the conductive paste under a relatively low shear stress. However, since the viscosity of a conductive paste varies depending on the measurement conditions, the behavior of the conductive paste under actual compression molding conditions may not match the behavior predicted from the TI defined above.

[0076] The conductive paste of the present invention can be prepared by uniformly mixing the ingredients using a mixing means such as a mixing machine, a propeller mixer, a kneader, a roll, a pot mill, etc. The preparation temperature is not particularly limited, but the paste can be prepared at, for example, 10 to 40°C.

[0077] The conductive paste of the present invention can be used to form a second conductor layer of an external electrode of a multilayer ceramic electronic component, which has a first conductor layer connected to an internal electrode layer and a second conductor layer laminated on the first conductor layer. By using the conductive paste of the present invention as the second conductor layer, an external electrode composed of multiple layers can be formed. The formation method is not particularly limited, and known methods can be used. For example, the paste of the present invention can be printed or applied onto the first conductor layer of the external electrode of a multilayer ceramic electronic component, optionally dried, and then heated and cured to form the second conductor layer.

[0078] The first conductor layer connected to the internal electrode layer may be formed, for example, by applying an electrode paste containing, for example, silver as a main component and glass frit to the internal electrode contact surface of the multilayer ceramic composite, optionally drying the paste, and then firing the paste. Methods for printing, applying, drying, and firing the electrode paste may be any method known to those skilled in the art.

[0079] The coating thickness in the printing / coating step is usually 10 to 200 μm, preferably 20 to 100 μm. The drying step is mainly carried out when an organic solvent is used, and can be carried out at room temperature or by heating (for example, heating at 80 to 160°C). The curing step can usually be carried out at 150 to 250°C. The curing temperature is preferably 150°C or higher, more preferably 180°C or higher. In addition, to eliminate adverse effects of heat on added components, the curing temperature is preferably 250°C or lower, more preferably 220°C or lower.

[0080] The curing time can be changed by changing the curing temperature, etc., but from the viewpoint of workability, 1 to 60 minutes is preferable. For example, when the resin in the paste is an epoxy resin using a phenolic resin as a curing agent, curing is carried out at 150 to 250°C for 10 to 60 minutes, and the second conductor layer constituting the external electrode can be formed.

[0081] When a cured product of a conventional conductive paste is placed in a high-temperature, high-humidity environment and absorbs moisture, it may expand due to moisture absorption or crack due to subsequent heating. However, the conductive paste of the present invention has high heat resistance that can withstand the heat of high-melting-temperature solder, and also has flexibility that allows stress relaxation even when the moisture-absorbing material expands rapidly. These properties of the conductive paste of the present invention can be evaluated as high crack resistance. The crack resistance was evaluated according to the method described in the examples.

[0082] Furthermore, the conductive paste of the present invention has high heat resistance, allowing it to withstand processing in high-heat environments. Conductive pastes are sometimes used in components that require high reliability, and such components are required to minimize peeling and cracking of the external electrodes when exposed to high heat during the manufacturing process. One indicator of heat resistance is reflow resistance, which is the resistance to heat when reflowing is performed under conditions that reproduce the mounting process of electronic components. The conductive paste of the present invention has extremely high reflow resistance. The reflow resistance was evaluated using the method described in the Examples.

[0083] The conductive paste of the present invention has flexibility when cured and has excellent stress relaxation capability against physical external forces applied to multilayer ceramic electronic components. The flexibility is evaluated based on the flexural modulus, and the conductive paste of the present invention preferably has a flexural modulus of 3 GPa to 15 GPa, more preferably 4 GPa to 12 GPa, and even more preferably 5 GPa to 10 GPa when dried at 120°C for 60 minutes and then cured in an air dryer at 180°C for 30 minutes.

[0084] The conductive paste of the present invention also exhibits high adhesion to the first conductor layer that constitutes the external electrode and is in contact with the first conductor layer. The metal typically used for the first conductor layer is copper, and the conductive paste of the present invention has an adhesive strength to a copper substrate of 1.0 kN / cm when dried at 120°C for 60 minutes and then cured in an air drier at 180°C for 30 minutes. ２ It is preferable that the strength is 1.5 kN / cm or more. ２ More preferably, it is 2.0 kN / cm or more. ２ More preferably, it is 2.5 kN / cm or more. ２ The adhesive strength is measured based on the test method in the examples described below, and when a force that releases adhesion is applied, it is preferable that the adhesive layer breaks down inside (cohesive failure) rather than at the bonding interface with the adhesive layer (interfacial failure).

[0085] If necessary, the external electrodes thus formed may be plated with nickel, tin, or the like to further increase the adhesive strength when soldered onto a circuit board or the like.

[0086] The use of the conductive paste of this embodiment is not particularly limited, but it is preferably used to form external electrodes of multilayer ceramic electronic components. Furthermore, the conductive paste of this embodiment can suppress the occurrence of cracks after a solder heat resistance test after moisture absorption, even when a lead-free solder with a high melting temperature is used. Therefore, the conductive paste for external electrodes of this embodiment is preferably used to connect with lead-free solder.

[0087] In this specification, lead-free solder refers to solder that contains substantially no lead components. The melting temperature (solidus temperature) of lead-free solder is preferably 200°C or higher, more preferably 210°C or higher. When soldering with lead-free solder, higher heat resistance than conventional solder is required for external electrodes. However, the conductive paste of this embodiment can suppress the occurrence of cracks after a solder heat resistance test after moisture absorption, even when a lead-free solder with a high melting temperature is used. For this reason, the external electrodes formed by hardening the conductive paste are preferably connected to wiring electrodes of a circuit board of an electronic component by a soldering layer, and the soldering layer is preferably formed of lead-free solder.

[0088] The multilayer ceramic electronic component of this embodiment includes electrodes formed by curing the conductive paste of this embodiment. In particular, multilayer ceramic electronic components including electrodes formed by curing the conductive paste of this embodiment as external electrodes are preferred. Examples of multilayer ceramic electronic components equipped with external electrodes formed using the conductive paste of the present invention include capacitors, capacitor arrays, thermistors, varistors, inductors, and LC, CR, LR, and LCR composite components. Because the conductive paste of the present invention has extremely high heat resistance, it can be preferably used for components requiring high reliability.

[0089] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0090] [Preparation of Conductive Paste] The components in Tables 1 and 2 were blended to prepare conductive pastes of Examples and Comparative Examples (numbers in the tables relating to blend amounts are in parts by mass unless otherwise specified).

[0091] Conductive particles (component (A)) The following were used as component (A) in the examples and comparative examples: (A1) Flake silver powder (FA 202, manufactured by DOWA Electronics Co., Ltd.), average particle size 15 μm (A1-2) Flake silver powder (T956, manufactured by Toyo Chemical Industry Co., Ltd.), average particle size 7.0 μm (A1-3) Flake silver powder (AGC B4, manufactured by Fukuda Metal Foil & Powder Co., Ltd.), average particle size 4.0 μm (A2) Spherical silver powder (AG-SNA-149, manufactured by DOWA Electronics Co., Ltd.), average particle size 2.5 μm

[0092] Epoxy Resin (Component (B)) In the Examples and Comparative Examples, the compounds used as component (B) are as follows: (B1) Naphthalene-type epoxy resin (EPICLON HP-6000 manufactured by DIC; epoxy equivalent: 250 g / eq) (B2) Bisphenol A-type epoxy resin (jER 1003F manufactured by Mitsubishi Chemical Corporation; epoxy equivalent: 700 to 800 g / eq)

[0093] Curing Agent (Component (C)) In the Examples and Comparative Examples, the compounds used as component (C) are as follows: (C1) Phenolic resin (MEH8000H manufactured by Meiwa Kasei Co., Ltd.) allylated phenol novolac resin (C2) Phenolic resin (MEHC7800H manufactured by Meiwa Kasei Co., Ltd.) aralkyl novolac phenolic resin (C3) Phenolic resin (PSM 4324 manufactured by Gunei Chemical Industry Co., Ltd.) novolac phenolic resin (C4) Phenolic resin (BRM 595 manufactured by Aica Kogyo Co., Ltd.) high molecular weight novolac phenolic resin

[0094] Bismaleimide Compound (Component (D)) The compounds used as component (D) in the examples and comparative examples are as follows: (D1) Dimer acid-modified bismaleimide compound represented by the following chemical formula: (D2) Dimer acid-modified bismaleimide compound represented by the following chemical formula (BMI-1500 manufactured by Designer Molecules Inc., weight average molecular weight: 1,500, liquid at room temperature)

[0095] In addition to the above four components, the following optional components were used: Rubber component: Butadiene rubber particles (core-shell particles with an average primary particle size of 100 to 200 nm, the core containing butadiene rubber and the shell being a polymer of glycidyl methacrylate (10 parts by mass) and methyl methacrylate (90 parts by mass)) as the rubber component. Note that these core-shell type rubber particles were dispersed in bisphenol A type epoxy resin. Curing catalyst: EH-3842 (dicyandiamide) manufactured by ADEKA Corporation; 2P4MHZ-PW manufactured by Shikoku Chemical Industry Co., Ltd. Solvent: Ethylene glycol monophenyl ether (Hisorb EPH manufactured by Toho Chemical Industry Co., Ltd.); Diethylene glycol monoethyl ether (EC manufactured by Taishin Chemical Co., Ltd.) Coupling agent: Silane coupling agent (KBM-403 manufactured by Shin-Etsu Chemical Co., Ltd.); Polysulfide-based silane coupling agent (CABRUS (registered trademark) 4 manufactured by Osaka Soda Co., Ltd.) Resin (butyral): Polyvinyl butyral resin (BX-3Z manufactured by Sekisui Chemical Co., Ltd.)

[0096] In the examples and comparative examples, the properties of the conductive paste were measured as follows.

[0097] (Viscosity) The viscosity of the evaluation sample immediately after preparation was measured using an HB type viscometer (manufactured by Brookfield) (SC4-14 spindle) at a temperature of 25° C. and 10 rpm.

[0098] (Thixotropic Index Value TI) The ratio of viscosity at a rotation speed of 10 rpm to viscosity at a rotation speed of 100 rpm measured at 25° C. using an HB type viscometer was calculated.

[0099] (Specific Resistivity Value) For the conductive pastes of the Examples and Comparative Examples, a zigzag pattern of 71 mm in length, 1 mm in width, and 20 μm in thickness was printed on an alumina substrate of 20 mm in width, 20 mm in length, and 1 mm in thickness using a 250-mesh stainless steel screen. The pattern was dried at 120°C for 60 minutes and then cured in air at 180°C for 30 minutes to form an external electrode. The thickness of the zigzag pattern was determined by averaging the values ​​at six points intersecting the pattern using a Tokyo Seimitsu surface roughness and shape measuring instrument (product name: Surfcom 1400). After curing, the specific resistance was measured using an LCR meter using a four-terminal method.

[0100] (Adhesion strength) 1: Preparation of substrate: Copper was printed as an undercoat on an alumina substrate measuring 20 mm in width, 20 mm in length, and 1 mm in thickness using a 250-mesh stainless steel screen to a width of 20 mm, length of 20 mm, and thickness of approximately 20 μm after firing. After drying at 150° C. for 10 minutes, it was fired at 900° C. for 60 minutes in a nitrogen atmosphere to form a fired copper undercoat on the alumina substrate.

[0101] 2: Measurement of Adhesion Strength: For the conductive pastes of the Examples and Comparative Examples, a pattern was printed on the alumina substrate on which the fired copper underlayer was formed using a 250-mesh stainless steel screen, with a 5x5 pattern of 1.5 mm width, 1.5 mm length, and a thickness of approximately 25 μm after curing. Ten 3216-size alumina chips were placed arbitrarily on the pattern. After drying at 120°C for 60 minutes, the chips were cured in air at 180°C for 30 minutes to form test pieces. After curing, the bond strength (shear strength) between the alumina chips and the fired copper underlayer was measured using an Aiko Engineering benchtop strength tester (model number: 1605VC) at a displacement rate of 12 mm / min. The adhesive strength was 3.0 kN / cm. ２ or more, 3.0 kN / cm ２ Less than 1.0 kN / cm ２ or more, 1.0 kN / cm ２ The results were evaluated on a three-point scale, with the following three levels: 1. When the alumina chip peeled off, the state of the adhesive surface was judged visually to determine whether cohesive failure had occurred or whether partial interfacial failure had occurred, and the failure mode was judged by visual inspection. When the failure mode was confirmed to be cohesive failure, it was rated as ◯, and when interfacial peeling was observed in part of the adhesive surface, it was rated as △. The results are shown in Table 1.

[0102] (Flexural modulus) The conductive pastes of the examples and comparative examples were applied to a substrate, dried at 120°C for 60 minutes, and then cured in the atmosphere at 180°C for 30 minutes using a blower dryer. Three test pieces measuring 40 x 10 x 0.13 mm were then peeled off from the substrate to prepare three test pieces. A two-point support, one-point load bending test was performed using a Shimadzu autograph at a displacement rate of 1 mm / min, and the flexural modulus was calculated from the stress-strain curve.

[0103] (Toughness) Whether toughness was imparted was judged by whether three of the test pieces broke after one minute had passed at a displacement rate of 1 mm / min after exceeding the strain point (yield point) at which the maximum stress was calculated from the stress-strain curve (S-S curve) calculated by measuring the flexural modulus. If none of the three test pieces broke after one minute at a displacement rate of 1 mm / min after exceeding the yield point, the test piece was rated as "no break." If one or two of the three test pieces did not break after one minute at a displacement rate of 1 mm / min after exceeding the yield point, the test piece was rated as "partial break." If all three test pieces broke before exceeding the yield point, the test piece was rated as "fracture." The test piece was rated on a three-point scale: break, partial break, or no break.

[0104] (Evaluation of Cracks After Solder Heat Resistance Test After Moisture Absorption) A conductive paste having the composition shown in Table 1 was dip-coated onto the first conductor layer connected to the internal electrode of the ceramic composite of a multilayer ceramic capacitor to a cured thickness of approximately 30 to 40 μm. The paste was then dried at 120°C for 60 minutes and then cured in air at 180°C for 30 minutes using a fan dryer to form an external electrode. The resulting multilayer ceramic capacitor was left in an environment of 85°C and 85% RH for 24 hours and then immersed for 10 seconds in a solder bath containing M705 solder (lead-free solder manufactured by Senju Metal Industry Co., Ltd.) heated to 310°C. Scanning electron microscope (SEM) photographs of the cross section near the external electrode formed using the conductive paste were examined and evaluated as follows. A rating of ◯ (Good) was given for cases where no cracks were observed, and a rating of × (Bad) was given for cases where cracks were present.

[0105]

[0106] Table 1 shows that the conductive paste of the present invention, which contains a bismaleimide compound, has high heat resistance that can withstand the heat of high-melting solder and flexibility that can relieve stress even when moisture is rapidly expanded. Furthermore, comparing Example 1 and Example 2, it can be seen that the conductive paste of Example 1, which uses a conductive paste containing the bismaleimide compound (D1), also has excellent toughness and conductivity. On the other hand, Comparative Example 1, which does not contain a bismaleimide compound, showed cracks in the solder heat resistance test, and was hard and brittle due to its high flexural modulus, so it did not satisfy the high reliability requirements for electronic components at a higher level.

[0107] (Reflow Resistance) The components listed in Table 2 were blended to prepare conductive pastes for Examples and Comparative Examples. The resulting conductive paste was dip-coated onto the first conductor layer connected to the internal electrode of the ceramic composite of a multilayer ceramic capacitor to a cured thickness of approximately 30 to 40 μm. The paste was then dried at 120°C for 60 minutes and then cured in an air dryer at 200°C for 30 minutes to form external electrodes. The resulting multilayer ceramic capacitors were then mounted on substrates using M705 solder, passing them through a reflow oven (TNR15-225LN-M, manufactured by Tamura Corporation) once at a 10-minute in-out time and a peak temperature of 260°C. More than 30 identical test pieces were prepared, and the mounted substrates were passed through a reflow oven three times at a 10-minute in-out time and a peak temperature of 260°C. Scanning electron microscope (SEM) photographs of the cross section near the external electrodes were examined, and the number of samples showing peeling or cracks was counted and evaluated as follows.・Less than 10% of the samples had peeling or cracks: Excellent ・30-10% of the samples had peeling or cracks: Good ・70-30% of the samples had peeling or cracks: Fair ・More than 70% of the samples had peeling or cracks: Bad

[0108]

[0109] Table 2 shows that the conductive paste of the present invention, which contains a bismaleimide compound, exhibits high heat resistance and little peeling or cracking even in a reflow test. Furthermore, Example 14 shows that the conductive paste using dicyandiamide as a curing catalyst has extremely high reflow resistance. On the other hand, in Comparative Example 2, which does not contain a bismaleimide compound, peeling or cracking was observed in more than 80% of the samples, and the paste did not satisfy the high reliability required for electronic components at a higher level.

[0110] The conductive paste of the present invention is a material that exhibits high adhesion, has high heat resistance that can withstand the heat of high-melting-temperature solder, and is flexible enough to relieve stress even when moisture is absorbed, and can be suitably used for multilayer ceramic electronic components such as multilayer ceramic capacitors and external electrodes of multilayer inductors.

[0111] REFERENCE SIGNS LIST 1 Multilayer ceramic capacitor 2 Ceramic dielectric 3 Internal electrode layer 4 External electrode layer 5 Plated layer 6 Soldered layer 7 Substrate 11 First conductor layer connected to internal electrode 12 Second conductor layer containing resin

Claims

1. A conductive paste for external electrodes, comprising: (A) conductive particles; (B) an epoxy resin; (C) a curing agent; and (D) a bismaleimide compound.

2. The conductive paste according to claim 1, wherein the bismaleimide compound (D) is liquid at room temperature.

3. The conductive paste according to claim 1 or 2, wherein the viscosity of the bismaleimide compound (D) measured at 25°C with a Brookfield viscometer is 20 Pa·s or less.

4. The conductive paste according to any one of claims 1 to 3, wherein the weight average molecular weight of the bismaleimide compound (D) is in the range of 1,000 to 10,000.

5. The conductive paste according to any one of claims 1 to 4, wherein the bismaleimide compound (D) is a dimer acid-modified bismaleimide compound.

6. The bismaleimide compound (D) is represented by the following formula (1): (wherein X represents a divalent aliphatic hydrocarbon group having 1 to 100 carbon atoms which may contain a heteroatom between the carbon atoms).

7. The conductive paste according to any one of claims 1 to 3 and 6, wherein the bismaleimide compound (D) is an aliphatic bismaleimide compound having 2 to 50 carbon atoms.

8. The bismaleimide compound (D) is represented by the following formula (2): (-C ３６ H ７０ The conductive paste according to any one of claims 1 to 3, 6 and 7, wherein the bismaleimide is represented by the following formula: wherein - represents a hydrocarbon group derived from a dimer acid skeleton.

9. The conductive paste according to any one of claims 1 to 8, wherein the bismaleimide compound (D) includes a bismaleimide compound having an acid value of 2 mg-KOH / g or less.

10. A conductive paste according to any one of claims 1 to 9, wherein the content of the (D) bismaleimide compound is in the range of 1 to 10 parts by mass per 100 parts by mass of the (A) component.

11. The conductive paste according to any one of claims 1 to 10, wherein the (C) curing agent contains an allyl phenol resin.

12. The conductive paste according to any one of claims 1 to 11, further comprising dicyandiamide.

13. The conductive paste according to any one of claims 1 to 12, which has a flexural modulus of 3 GPa to 10 GPa when dried at 120°C for 60 minutes and then cured in an air dryer at 180°C for 30 minutes.

14. The conductive paste according to any one of claims 1 to 13, which has a thixotropic index value, which is the ratio of viscosity at a rotation speed of 10 rpm to viscosity at a rotation speed of 100 rpm, measured at 25°C using an HB type viscometer, of 7 or less.

15. The conductive paste according to any one of claims 1 to 14, wherein the external electrodes are used for connection with lead-free solder.

16. A multilayer ceramic electronic component having external electrodes formed using the conductive paste according to any one of claims 1 to 15.

17. A multilayer ceramic electronic component comprising external electrodes formed by hardening a conductive paste, the conductive paste containing (A) conductive particles, (B) an epoxy resin, (C) a hardener, and (D) a bismaleimide compound.

18. The multilayer ceramic electronic component according to claim 17, wherein the conductive paste further contains dicyandiamide.

19. The multilayer ceramic electronic component according to claim 17 or 18, comprising a circuit board on which wiring electrodes are formed, the external electrodes are connected to the wiring electrodes by soldering layers, and the soldering layers are formed of lead-free solder.

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