Conductive paste

Abstract

This conductive paste is used for forming a cured product with which holes of a circuit board are filled and comprises an epoxy resin, a curing agent, and conductive particles. At least the surface of the conductive particles is formed of a metal. The conductive particles are spherical and have a D50 of 1-7 μm in a particle size distribution.

Description

conductive paste Cross-reference of related applications

[0001] This application claims priority under Japanese Patent Application No. 2025-047019, which is incorporated into the description of this application by reference.

[0002] This invention relates to a conductive paste.

[0003] Conventionally, conductive pastes have been used as fillers for via formation in circuit boards, adhesives for bonding components together while electrically connecting them, and so on. Generally, this type of conductive paste contains epoxy resin, a curing agent, and conductive particles, and functions to electrically connect the conductive particles by utilizing the curing shrinkage of the epoxy resin, thereby forming a conductive path. For example, Patent Document 1 describes a conductive adhesive composition for connecting busbars in a solar panel. In Patent Document 1, flake-shaped conductive particles are used to increase the contact area between particles in order to impart good conductivity to the cured product.

[0004] Japan Special Table No. 2024-534163

[0005] On the other hand, in the formation of vias to achieve electrical conductivity (interlayer connection) between layers of multilayer substrates, holes with micro-order diameters are formed in the substrate, and conductive paste is filled into these holes and cured to form a cured material in the holes. With the miniaturization of circuit boards in recent years, the holes have also become smaller, so there is a need for a conductive paste that exhibits good fluidity even when filling small-diameter holes and can form a cured material with excellent conductivity. The object of the present invention is to provide a conductive paste that can satisfy such needs.

[0006] A conductive paste used for forming a cured material to fill holes in a circuit board, comprising epoxy resin, a curing agent, and conductive particles, wherein at least the surface of the conductive particles is made of metal, the conductive particles are spherical, and the D50 in the particle size distribution is 1 μm or more and 7 μm or less.

[0007] According to the present invention, a conductive paste is provided that, by using specific spherical conductive particles, can form a cured product that exhibits good fluidity and has excellent conductivity.

[0008] This is a schematic cross-sectional view of a glass substrate according to one embodiment, showing a state in which a hardened conductive paste has formed in the hole.

[0009] An embodiment of the present invention of a conductive paste will be described with an example of forming an electrical conduction path in a hole in a glass substrate that serves as a circuit board. For conductive pastes that can fill small-diameter holes, using spherical conductive particles is advantageous because it suppresses the increase in viscosity of the conductive paste compared to using conductive particles with a large surface area, such as flake-shaped or dendritic particles. On the other hand, spherical conductive particles have a small surface area, which is not advantageous for achieving excellent conductivity in the cured product of the conductive paste. Therefore, in the conductive paste of this embodiment, specific spherical conductive particles are used.

[0010] As shown in Figure 1, the glass substrate B according to this embodiment has a hole H into which the conductive paste is filled. The hole H may or may not have plating formed on its peripheral wall. The hole diameter (diameter) of the hole H is, for example, 20 μm to 150 μm. The hole diameter of the hole H may be 20 μm to 100 μm, or 20 μm to 50 μm.

[0011] In the formation of conductive paths using conductive paste in this embodiment, the conductive paste is filled into the hole H by a printing method such as screen printing, and then cured to form a cured material C. The conductive path is then formed in the hole by the filler material, which is the cured material C.

[0012] The conductive paste of this embodiment comprises a thermosetting resin composition that hardens with heat and conductive particles. The thermosetting resin composition comprises an epoxy resin and a curing agent that promotes the curing reaction of the epoxy resin.

[0013] The epoxy resin preferably contains at least one of the following: a polyfunctional glycidyl ether type epoxy resin, a polyfunctional glycidylamine type epoxy resin, and a polyfunctional alicyclic epoxy resin. It is more preferable to contain two or more of these, and even more preferable to contain all of them. These epoxy resins are preferred because they can form cured products with excellent heat resistance. In particular, glass substrates have excellent heat resistance among substrates. Therefore, in the manufacturing process of circuit boards using glass substrates, heat treatments that take advantage of the heat resistance of the glass substrate may be considered. It is preferable to include these epoxy resins so that the filler material in the holes does not impair the advantage of the glass substrate based on its heat resistance.

[0014] Examples of the polyfunctional glycidyl ether type epoxy resins include bisphenol type epoxy resins such as bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, bisphenol AF type epoxy resin, bisphenol Z type epoxy resin, and bisphenol fluorene type epoxy resin, as well as diphenyl sulfide type epoxy resin, diphenyl ether type epoxy resin, naphthalene type epoxy resin, hydroquinone type epoxy resin, resorcinol type epoxy resin, phenol novolac type epoxy resin, cresol novolac type epoxy resin, alkyl novolac type epoxy resin, styrene-phenol novolac type epoxy resin, bisphenol novolac type epoxy resin, naphthol novolac type epoxy resin, phenol aralkyl type epoxy resin, α-naphthol aralkyl type epoxy resin, β-naphthol aralkyl type epoxy resin, naphthalenediol aralkyl type epoxy resin, biphenyl aralkylphenol type epoxy resin, biphenyl type epoxy resin, and triphenylmethane type epoxy resin.

[0015] Examples of the polyfunctional glycidylamine-type epoxy resins include aniline-type epoxy resins such as 4,4'-methylenebis(N,N-diglycidylaniline) and N,N-diglycidyl-4-glycidyloxyaniline, as well as diaminodiphenylmethane-type epoxy resins and metaxylenediamine-type epoxy resins.

[0016] The polyfunctional alicyclic epoxy resin may have epoxycycloalkyl groups such as epoxycyclopentyl groups and epoxycyclohexyl groups. Alternatively, the polyfunctional alicyclic epoxy resin may have two or more glycidyl ethers bonded to a fused ring skeleton such as a bicycloalkane or tricycloalkane.

[0017] It is preferable that each of the polyfunctional glycidyl ether-type epoxy resin, the polyfunctional glycidylamine-type epoxy resin, and the polyfunctional alicyclic epoxy resin is liquid at room temperature (25°C). This makes it possible to lower the viscosity of the conductive filler.

[0018] The epoxy equivalents of the polyfunctional glycidyl ether type epoxy resin, the polyfunctional glycidylamine type epoxy resin, and the polyfunctional alicyclic epoxy resin are preferably 90 g / eq or more and 200 g / eq or less. This increases the crosslinking density in the filler and improves the heat resistance of the filler.

[0019] The epoxy resin preferably comprises at least one of the polyfunctional glycidyl ether type epoxy resin, the polyfunctional glycidylamine type epoxy resin, and the polyfunctional alicyclic epoxy resin, and at least one of the dimer acid modified epoxy resin and rubber modified epoxy resin; more preferably comprises at least one of the polyfunctional glycidylamine type epoxy resin and the polyfunctional alicyclic epoxy resin, and at least one of the dimer acid modified epoxy resin and rubber modified epoxy resin. The epoxy resin preferably comprises both the dimer acid modified epoxy resin and the rubber modified epoxy resin.

[0020] The aforementioned dimer acid-modified epoxy resin is a reaction product of dimer acid, which is a dimer of an unsaturated fatty acid, and an epoxy compound.

[0021] Examples of the rubber-modified epoxy resins include NBR-modified epoxy resins and SBR-modified epoxy resins.

[0022] It is preferable that both the dimer acid-modified epoxy resin and the rubber-modified epoxy resin are liquid at room temperature (25°C). This allows for a lower viscosity of the conductive filler.

[0023] The epoxy equivalents of the dimer acid-modified epoxy resin and the rubber-modified epoxy resin are preferably 300 g / eq or more and 500 g / eq or less.

[0024] The epoxy resin may contain an epoxy resin as a reactive diluent. Examples of such epoxy resins include monofunctional types having C4 to C18 alkyl groups, such as butyl glycidyl ether and 2-ethylhexyl glycidyl ether, and polyfunctional types having C4 to C18 alkylene groups, such as 1,6-hexanediol diglycidyl ether. Such reactive diluents are liquid at room temperature (25°C) and can reduce the viscosity of the conductive paste and improve its ability to fill holes.

[0025] The total content of the polyfunctional glycidyl ether type epoxy resin, the polyfunctional glycidylamine type epoxy resin, and the polyfunctional alicyclic epoxy resin, relative to the mass of the epoxy resin, is preferably 45% by mass or more and 75% by mass or less, and more preferably 55% by mass or more and 75% by mass or less. The total content of the dimer acid modified epoxy resin and the rubber modified epoxy resin is preferably 5% by mass or more and 20% by mass or less, and more preferably 5% by mass or more and 10% by mass or less. The content of the reactive diluent is preferably 15% by mass or more and 40% by mass or less, and more preferably 20% by mass or more and 35% by mass or less. This provides a good balance of heat resistance of the filler, suppression of cracks in the filler, and filling ability of the conductive paste into holes.

[0026] The proportion of epoxy resin that is liquid at room temperature (25°C) relative to the mass of the epoxy resin is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. The epoxy resin may consist substantially only of epoxy resin that is liquid at room temperature (25°C).

[0027] The thermosetting resin composition may contain a thermosetting monomer copolymerized with the epoxy resin. Examples of the thermosetting monomer include benzoxazine compounds. Examples of the benzoxazine compounds include polymers of phenol, diaminodiphenylmethane, and formaldehyde, referred to as P-d type benzoxazine compounds, and polymers of bisphenol F, aniline, and formaldehyde, referred to as F-a type benzoxazine compounds. Benzoxazine compounds having two benzoxazine groups are considered to have lower heat resistance than those having three or more benzoxazine groups, but are thought to be able to suppress the occurrence of cracks in the filler. Furthermore, benzoxazine compounds having two benzoxazine groups are considered to be able to suppress the increase in viscosity of the conductive paste compared to those having three or more benzoxazine groups. From the viewpoint of suppressing viscosity increase, benzoxazine compounds having two or fewer benzoxazine groups are preferred.

[0028] When the conductive paste contains the benzoxazine compound, the content of the benzoxazine compound is preferably 1 to 12 parts by mass, and more preferably 5 to 9 parts by mass, per 100 parts by mass of the epoxy resin. In this case, since the heat resistance of the benzoxazine compound is imparted to the filler, the total content of the polyfunctional glycidyl ether type epoxy resin, the polyfunctional glycidylamine type epoxy resin, and the polyfunctional alicyclic epoxy resin, which can contribute to improving the heat resistance of the filler, may be 40% to 60% by mass, or 45% to 55% by mass, relative to the mass of the epoxy resin. On the other hand, when the conductive paste does not contain the benzoxazine compound, it is preferable that the total content of the polyfunctional glycidyl ether type epoxy resin, the polyfunctional glycidylamine type epoxy resin, and the polyfunctional alicyclic epoxy resin is larger than when it contains the benzoxazine compound. For example, the total content of these is preferably more than 60% by mass and 75% by mass or less, and more preferably 65% ​​to 75% by mass, relative to the mass of the epoxy resin.

[0029] The curing agent preferably contains at least one of an imidazole-based curing agent and a phenol-based curing agent, and more preferably contains both.

[0030] Examples of the imidazole-based curing agents include alkyl-containing imidazoles such as 1-methylimidazole (1MZ), 2-methylimidazole (2MZ), 2-undecylimidazole (C11Z), 2-heptadecylimidazole (C17Z), 1,2-dimethylimidazole (1,2DMZ), 2-ethyl-4-methylimidazole (2E4MZ), 2-phenyl-4-methylimidazole (2P4MZ), and 1- Aryl group-containing imidazoles such as 2-methylimidazole (1B2MZ) and 2-phenylimidazole (2PZ), 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine (2MZ-A), 2,4-diamino-6-(2'-undecylimidazolyl)-ethyl-s-triazine (C11Z-A), 2,4-diamino-6-[2'-ethyl-4-methylimidazolyl-(1') Triazine ring-containing imidazoles such as 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine isocyanuric acid adduct (2MAOK-PW), 1-cyanoethyl-2-methylimidazole (2MZ-CN), 1-cyanoethyl-2-ethyl-4-methylimidazole (2E4MZ-CN), and 1-cyanoethyl-2-undecylimidazole Examples of cyano group-containing imidazoles include zole (C11Z-CN), 1-cyanoethyl-2-phenylimidazole (2PZ-CN), 1-cyanoethyl-2-ethyl-4-methylimidazolium trimellitate (2E4MZCNS), 1-cyanoethyl-2-undecylimidazolium trimellitate (C11ZCNS), and 1-cyanoethyl-2-phenylimidazole trimellitate (2PZCNS-PW). Furthermore, the imidazole-based curing agent may be 2-phenylimidazole isocyanuric acid adduct, 2-methylimidazole isocyanuric acid adduct, 2-phenyl-4,5-dihydroxymethylimidazole (2PHZ-PW), 2-phenyl-4-methyl-5-hydroxymethylimidazole (2P4MHZ), 1-cyanoethyl-2-phenyl-4,5-di(2-cyanoethoxy)methylimidazole, etc.

[0031] The imidazole-based curing agent is preferably an alkyl-containing imidazole, and among these, long-chain alkyl-containing imidazoles having C10 or more alkyl groups, such as 2-undecylimidazole (C11Z) and 2-heptadecylimidazole (C17Z), and dialkyl-containing imidazoles, such as 1,2-dimethylimidazole (1,2DMZ) and 2-ethyl-4-methylimidazole (2E4MZ), are preferred. These alkyl-containing imidazoles can gradually cure from a low temperature range of around 90°C, thus suppressing the occurrence of cracks when the conductive paste placed in the hole changes into a filler (cured product). Furthermore, these alkyl-containing imidazoles produce effective curing shrinkage in combination with the epoxy resin and conductive particles of this embodiment, thereby improving the conductivity of the conductive paste.

[0032] The phenolic curing agent is preferably a novolac-type phenolic resin such as a cresol novolac resin, a naphthol novolac resin, a naphthol-phenol cocondensed novolac resin, or a naphthol-cresol cocondensed novolac resin.

[0033] The content of the curing agent is preferably 1 to 10 parts by mass, more preferably 2 to 8 parts by mass, and even more preferably 3 to 7 parts by mass, per 100 parts by mass of the epoxy resin.

[0034] The conductive particles have a surface formed of metal. Examples of such conductive particles include silver particles, copper particles, nickel particles, silver-coated copper particles, silver-coated nickel particles, gold-coated copper particles, and gold-coated nickel particles. Among these, silver particles, silver-coated copper particles, and silver-coated nickel particles, which have a surface formed of silver, are preferred. The conductive paste may contain only one type of conductive particle, or it may contain two or more types of conductive particles.

[0035] The conductive particles are spherical. Specifically, when the insulating filler is observed at magnification with an SEM or the like, the maximum diameter and minimum diameter of 20 arbitrarily selected particles are measured, and the average value of sphericity calculated as minimum diameter / maximum diameter for the 20 particles is preferably 0.7 or more, more preferably 0.8 or more, and still more preferably 0.9 or more.

[0036] The D50 in the particle size distribution of the conductive particles is 1 µm or more and 7 µm or less. The D50 of the conductive particles is preferably 1 µm or more and 5 µm or less, more preferably 2 µm or more and 4 µm or less. In the present specification, the particle size distribution means a volume-based particle size distribution measured by a laser diffraction scattering particle size distribution measurement method. In addition, when the conductive paste contains two or more types of conductive particles, the D50 can be obtained by simply averaging the D50 of each type. In this case, the proportion of conductive particles having a D50 of 1 µm or more and 7 µm or less among all the conductive particles is preferably 20% by mass or more, more preferably 25% by mass or more. The proportion of conductive particles having a D50 of 1 µm or more and 7 µm or less among all the conductive particles may be 30% by mass or more, 40% by mass or more, or 50% by mass or more.

[0037] The BET specific surface area of the conductive particles is 0.1 m ２ / g or more and 0.5 m ２ / g or less, preferably 0.2 m ２ / g or more and 0.5 m ２ / g or less, more preferably. This makes it possible to suppress an increase in the viscosity of the conductive paste while improving the conductivity of the filler. The BET specific surface area can be determined from a nitrogen adsorption isotherm measured in accordance with the BET method.

[0038] The content of the conductive particles is preferably 85% by mass or more and 95% by mass or less, more preferably 87% by mass or more and 93% by mass or less, based on the mass of the conductive paste. On the other hand, the content of the epoxy resin is preferably 5% by mass or more and 10% by mass or less, more preferably 6% by mass or more and 10% by mass or less, based on the mass of the conductive paste. This makes it possible to impart conductivity provided by the conductive particles to the filler while improving the effect of curing shrinkage caused by the epoxy resin.

[0039] Preferably, the conductive paste contains a silane coupling agent as an optional additive. Preferred examples of the silane coupling agent include glycidyl group-containing silane coupling agents such as 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, and 3-glycidoxypropylmethyldimethoxysilane; more specifically, a silane coupling agent having a monofunctional glycidyl group is preferred. This enables forming a filler with excellent adhesion to a glass substrate and excellent heat resistance.

[0040] The content of the silane coupling agent is, for example, 1 part by mass or more and 10 parts by mass or less relative to 100 parts by mass of the epoxy resin, and is preferably 1 part by mass or more and 5 parts by mass or less.

[0041] The conductive paste may contain other additives. For example, the conductive paste may contain an antifoaming agent, a flame retardant, an antioxidant, a leveling agent, a rheology control agent, and the like. Examples of the antifoaming agent include those containing acrylic resins, vinyl ether resins, silicone resins, fluorine-based resins, and modified resins thereof.

[0042] Preferably, the conductive paste does not contain a solvent. That is, the conductive paste is preferably solvent-free. This can suppress the generation of voids caused by volatilization of the solvent during thermosetting.

[0043] While exemplary embodiments of the present invention have been shown, the conductive paste according to the present invention is not limited to the configuration of the above embodiments. Furthermore, the conductive paste according to the present invention is not limited by the above-described functions and effects. Various modifications can be made to the conductive paste according to the present invention without departing from the gist of the present invention.

[0044] For example, the conductive paste according to the present invention can also be suitably used for forming vias in circuit boards other than glass substrates, and can also be used for applications other than forming vias.

[0045] This disclosure includes the following: [1] A conductive paste used for forming a cured product to fill holes in a circuit board, comprising an epoxy resin, a curing agent, and conductive particles, wherein at least the surface of the conductive particles is made of metal, the conductive particles are spherical, and the D50 in the particle size distribution is 1 μm or more and 7 μm or less.

[0046] [2] The conductive paste according to [1] above, wherein the content of the conductive particles is 85% by mass or more and 95% by mass or less.

[0047] [3] The conductive paste according to [1] or [2] above, wherein the epoxy resin comprises at least one of a glycidylamine type epoxy resin and an alicyclic epoxy resin, and at least one of a dimer acid modified epoxy resin and a rubber modified epoxy resin.

[0048] [4] The conductive paste according to any one of [1] to [3] above, wherein the epoxy resin contains a reactive diluent, and the content of the reactive diluent is 15% by mass or more and 40% by mass or less based on the mass of the epoxy resin.

[0049] [5] The conductive paste according to any one of [1] to [4] above, further comprising a silane coupling agent, wherein the circuit board is a glass substrate.

[0050] The present invention will be further described below with reference to examples, but the present invention is not limited thereto.

[0051] [Raw Materials Used] Epoxy Resin A: Trifunctional glycidylamine type epoxy resin (manufactured by Mitsubishi Chemical Corporation, jER630, liquid, epoxy equivalent 90-106 g / eq) Epoxy Resin B: Bifunctional dimer acid modified epoxy resin (manufactured by Mitsubishi Chemical Corporation, jER871, liquid, epoxy equivalent 390-470 g / eq) Epoxy Resin C: NBR modified epoxy resin (manufactured by ADEKA Corporation, EPR4030, liquid, epoxy equivalent 365 g / eq) Epoxy Resin D: Alicyclic epoxy resin (epoxy resin with two glycidyl ether groups bonded to a tricyclodecane skeleton, manufactured by ADEKA Corporation, EP-4088S, liquid, epoxy equivalent 170 g / eq) Epoxy Resin E (Reactive Diluent): 1,6-Hexanediol diglycidyl ether (manufactured by Shikoku Synthetic Co., Ltd., EpoGose® HD) Epoxy resin F (reactive diluent): C12-13 alkoxyglycidyl ether (manufactured by ADEKA Corporation, ED502, epoxy equivalent 320 g / eq) Epoxy resin G (reactive diluent): 2-ethylhexylglycidyl ether (manufactured by Mitsubishi Chemical Corporation, YED188, liquid) Thermosetting monomer: Pd-type benzoxazine compound (manufactured by Shikoku Chemicals, Inc., benzoxazine P-d) Curing agent A (C11 alkyl group-containing imidazole curing agent): 2-undecylimidazole (manufactured by Shikoku Chemicals, Inc., C11Z) Curing agent B (C17 alkyl group-containing imidazole curing agent): 2-heptadecylimidazole (manufactured by Shikoku Chemicals, Inc., C11Z) Curing agent C: 4,5-bis(hydroxymethyl)-2-phenylimidazole (manufactured by Shikoku Chemicals Corporation, 2PHZ) Curing agent D: 2-phenyl-4-methyl-5-hydroxymethylimidazole (manufactured by Shikoku Chemicals Corporation, 2P4MHZ) Curing agent E (naphthol-phenol co-condensed novolac resin): naphthol-cresol-formaldehyde polymer (MEH7000) Silane coupling agent: 3-glycidoxypropyltrimethoxysilane (manufactured by Shin-Etsu Silicone Co., Ltd., KBM-403) Leveling agent: xylene solution of acrylic polymer (manufactured by Kyoeisha Chemical Co., Ltd., Polyflow, No. 85HF) Silver-coated copper particles A: spherical, 1.6 μm (D50), TAP density 5.5 g / mL Silver-coated copper particles B: spherical, 2.2 μm (D50), TAP density 5.6 g / mL, BET specific surface area 0.44 m, ２ / g Silver-coated copper particles C: spherical, 3.0 μm (D50), TAP density 6.2 g / mL, BET specific surface area 0.30 m ２ / g Silver-coated copper particles D: flake-shaped, 4.6 μm (D50), TAP density 5.8 g / mL, BET specific surface area 0.31 m ２ / g Silver-coated copper particles E: spherical, 8.3 μm (D50), TAP density 6.5 g / mL, BET specific surface area 0.10 m ２ / g Silver particles A: spherical, 8.8 μm (D50), TAP density 7.3 g / mL, BET specific surface area 0.11 m ２ / g Silver particles B: spherical, 2.7 μm (D50), TAP density 5.8 g / mL, BET specific surface area 0.22 m ２ / g Silver particles C: spherical, 1.9 μm (D50), TAP density 5.0 g / mL, BET specific surface area 0.43 m ２ / g Silver particles D: spherical, 0.8 μm (D50), TAP density 4.8 g / mL, BET specific surface area 1.00 m ２ / g

[0052] [Production Examples] Each component shown in Tables 1 to 3 was kneaded with a three-roll mill to prepare a conductive paste.

[0053] [Evaluation 1: Conductivity] Conductive paste was printed in a line (60 mm long, 1 mm wide, approximately 100 μm thick) onto a glass substrate. The printed conductive paste was heated at 90°C for 60 minutes for partial curing, and then heated at 200°C for 60 minutes for full curing to create an evaluation substrate with a conductive pattern. Next, the resistance value (R) between both ends of the conductive pattern was measured using a tester, and the cross-sectional area (S, cm²) was measured. ２ The resistivity was calculated from the length (L, cm) using the following formula (1). Ten conductive patterns were formed by printing five lines on each of two glass substrates, and the average resistivity of these patterns was calculated. The conductivity of the conductive paste was evaluated based on the following evaluation criteria using resistivity as an indicator. Resistivity = (S / L) × R ... (1) (Evaluation criteria) A: If the resistivity is 6.0E-05Ω·cm or less, the conductivity is excellent. B: If the resistivity is greater than 6.0E-05Ω·cm and 8.0E-05Ω·cm or less, the conductivity is moderately good. C: If the resistivity is greater than 8.0E-05Ω·cm, the conductivity is poor.

[0054] [Evaluation 2: Filling Performance] A 1.0 mm thick glass epoxy substrate with a copper pattern was prepared with a φ0.15 mm diameter through-hole. Conductive paste was filled into the through-hole using a printing method with a Microtech MT-320 printer. The conductive paste was heated at 90°C for 60 minutes, and then heated at 200°C for another 60 minutes to cure it and prepare a sample. The obtained sample was observed before and after curing using an X-ray transmission device Y.Cheetah μHD manufactured by Exlon International under the following measurement conditions to check for the presence of cracks, voids, and the filling performance of the through-hole. The filling performance of the conductive paste was evaluated based on the following evaluation criteria. <Measurement Conditions> Voltage: 50 kV Current: 80 μA Power: 4 W (Evaluation Criterion 1: Presence or Absence of Cracks) ○: No cracks were observed. ×: At least one crack was observed. (Evaluation Criterion 2: Presence or Absence of Voids) ○: No voids are found. ×: At least one void is found. (Evaluation Criterion 3: Filling Condition) ○: No unfilled areas are found. ×: Unfilled areas are found.

[0055] [Evaluation 3: Paste Formation] After mixing each component from Tables 1-3, those that formed a paste were evaluated as "○", and those that did not form a paste were evaluated as "×".

[0056]

[0057]

[0058]

[0059] B: Glass substrate, H: Hole, C: Cured material

Claims

1. A conductive paste used for forming a cured material to fill holes in a circuit board, comprising epoxy resin, a curing agent, and conductive particles, wherein at least the surface of the conductive particles is made of metal, the conductive particles are spherical, and the D50 in the particle size distribution is 1 μm or more and 7 μm or less.

2. The conductive paste according to claim 1, wherein the content of the conductive particles is 85% by mass or more and 95% by mass or less.

3. The conductive paste according to claim 1 or 2, wherein the epoxy resin comprises at least one of a glycidylamine-type epoxy resin and an alicyclic epoxy resin, and at least one of a dimer acid-modified epoxy resin and a rubber-modified epoxy resin.

4. The conductive paste according to claim 1 or 2, wherein the epoxy resin contains a reactive diluent, and the content of the reactive diluent is 15% by mass or more and 40% by mass or less, based on the mass of the epoxy resin.

5. The conductive paste according to claim 1 or 2, further comprising a silane coupling agent, wherein the circuit board is a glass substrate.

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