JP2017076591A - Metallic paste - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metallic paste from which a sintered body with high sinterability is formed and which easily forms a sintered body with reduced occurrence of voids.SOLUTION: A metallic paste comprises metal powder in which elements constituting a particle surface mainly include silver, an epoxy compound, and a curing agent of the epoxy compound. The curing agent has three or more carboxylic acids as acid in one molecule. The metallic paste suitably has the curing agent in such an amount that the number of carboxylic acids as acid as becomes 0.2 or more and 1.5 or less per epoxy group of the epoxy compound.SELECTED DRAWING: None

Description

  The present invention relates to a metal paste containing metal powder.

  Various metal pastes containing metal powder and resin are known. For example, Patent Document 1 describes a conductive paste composition that fills a via hole. This paste composition contains conductor particles and an epoxy compound. The epoxy compound has one or more epoxy groups in one molecule, and the total amount of hydroxyl group, amino group and carboxyl group is 5 mol% or less of the epoxy group, and the epoxy equivalent is 100 to 350 g / eq It is. According to this document, according to this paste composition, it is possible to easily realize a double-sided printed board having a small-diameter inner via hole without using a through-hole plating technique.

  Patent Document 2 describes a conductive silver paste that contains silver powder, an epoxy resin, and a curing agent, and the blending ratio of the curing agent is 0.3 to 1.5 parts by weight with respect to 1 part by weight of the epoxy resin. Has been. As the curing agent, an amine curing agent, a urea curing agent, an acid anhydride curing agent and an aromatic amine curing agent are used. According to this document, it is described that according to this paste, the resistance value is sufficiently lowered in a short heating step while having a certain viscosity required for high-definition printing by screen printing.

  Patent Document 3 describes a thermally conductive paste containing low-temperature sinterable silver fine particles and a thermosetting binder, wherein the thermosetting binder is 2 to 7 parts by mass with respect to 100 parts by mass of the silver fine particles. The thermosetting binder is composed of an epoxy resin having an ester bond such as diglycidyl phthalate and a curing agent. It is described in this document that a silver film having improved mechanical strength and reliability can be obtained by using this heat conductive paste without using a special technique such as an impregnation process.

International Publication No. 2008/132732 JP 2012-248370 A JP 2013-214733 A

  According to the technique described in each patent document described above, by curing the paste, it is possible to obtain a cured body in which the metal particles are sintered and the resin is cured. However, in the conventional paste, the degree of sintering is not sufficient due to the fact that the resin hinders the sintering of metal particles, and voids are generated in the cured body even if the sintering proceeds to some extent. It had the problem that.

  Accordingly, an object of the present invention is to improve a paste containing metal powder, and more specifically, a cured body in which the sinterability of metal particles in the cured body formed from the paste is high and the generation of voids is suppressed. An object of the present invention is to provide a metal paste that can be easily formed.

The present invention is a metal paste containing a metal powder whose element constituting the particle surface is mainly silver, an epoxy compound, and a curing agent for the epoxy compound,
The curing agent provides a metal paste having three or more carboxylic acids as an acid per molecule.

  The metal paste of the present invention has a high sinterability of metal particles in a cured body formed from the paste, and it is easy to form a cured body in which generation of voids is suppressed.

  Hereinafter, the present invention will be described based on preferred embodiments thereof. The metal paste of the present invention includes metal powder, an epoxy resin, and a curing agent for the epoxy resin. Therefore, the metal paste of the present invention is a resin composition containing metal powder. The metal paste of the present invention has a category of fluidity called a paste in the art.

  The metal paste of the present invention is applied to, for example, the surface of an object to be coated, and the coating film formed thereby is heated to cause curing of the epoxy resin and to sinter the metal particles in the paste. To produce a sintered body and to form a cured body. The cured body thus obtained is preferably used as, for example, an electric conductor and a heat conductor. In this sense, in the following description, the cured metal paste is also referred to as “conductor”.

  The metal powder contained in the metal paste of the present invention is one in which the element constituting the particle surface is mainly silver. Typical examples of the metal powder in which the element constituting the particle surface is mainly silver are those in which silver is present on the outermost surface of a core material made of metal other than (i) silver powder itself and (ii) pure silver. As mentioned. In the case of (ii), examples of the core material include copper, copper alloy, nickel, zinc, titanium, and the like. From the viewpoint of high electrical conductivity and thermal conductivity of the core material, copper is preferable. . “The metal element constituting the particle surface is mainly silver” means that the outermost surface of the metal powder contained in the metal paste of the present invention is measured by X-ray photoelectric spectroscopy, and the amount ratio of silver on the outermost surface (( (Atom% of silver) / (Atom% of other metal elements))), the amount ratio of silver on the outermost surface is 2 or more. Within this range, the metal powder can sufficiently promote the sintering of the metal particles. From this viewpoint, the amount ratio of the silver is more preferably 4 or more, and further preferably 10 or more.

  In particular, as the metal powder, silver powder made of an aggregate of pure silver particles can be used. By the way, while silver powder is excellent in sinterability when heated, the degree of shrinkage during the sintering process is large, which may cause voids in the conductor. From this viewpoint, it is also preferable to use silver-coated metal powder in which the outermost surfaces of the various core materials described above are coated with silver. However, although the silver-coated metal powder has a smaller degree of shrinkage when heated than pure silver powder, there are cases where the electrical resistance is increased because there is less necking growth due to sintering of the particles. Therefore, it is more preferable to use a mixture containing silver powder and silver-coated metal powder as the metal powder. By using a mixture of both, it is possible to advance sintering between particles while suppressing excessive shrinkage of the metal particles when the metal paste is heated. As a result, the electrical conductivity and heat of the conductor are increased. This is advantageous in that the conductivity can be increased. When using the mixture of both, it is preferable that the usage-amount of the silver coat metal powder with respect to 100 mass parts of silver powder shall be 10 mass parts or more and 900 mass parts or less, and it is still more preferable to set it as 20 mass parts or more and 500 mass parts or less, 30 It is still more preferable to set it as mass parts or more and 300 mass parts or less.

Even if the form of the metal powder is any of the cases (i) and (ii) described above, the metal particles constituting the metal powder have a cumulative particle size of 50 by the laser diffraction scattering particle size distribution measurement method. When expressed by volume cumulative particle diameter D ₅₀ in volume%, it is preferably 0.005 μm or more and 20 μm or less, more preferably 0.01 μm or more and 15 μm or less, and further preferably 0.02 μm or more and 10 μm or less. preferable. When the particle size of the metal particles is within this range, the density of the intended conductor increases, and the dimensional stability during the production of the sintered body becomes good.

  Moreover, even if the form of the metal powder is any of the cases (i) and (ii) described above, the shape of the metal particles is, for example, spherical, flat (also called scale-shaped), dendritic, It may be a fixed form or the like. Alternatively, a combination of two or more of these shapes may be used.

  The metal powder can be produced by appropriately selecting a known method according to its shape and size. For example, when the metal powder is pure silver powder or silver alloy powder, it can be produced by a method such as a wet reduction method, a gas atomization method, a water atomization method, an electrolysis method, or a plasma method. Furthermore, an additional process such as a pulverization process can be performed. In the case where the metal powder is a silver-coated metal powder, a silver coat layer may be formed after the metal powder as a core material is manufactured by the above-described method. For the formation of the silver coat layer, for example, a plating method using a reducing agent or a displacement plating method using a difference in ionization tendency can be employed.

  The metal powder is preferably blended in the metal paste of the present invention in the range of 50% by mass to 98% by mass, more preferably in the range of 55% by mass to 96% by mass, and still more preferably in the range of 60% by mass. % To 94% by mass.

  An epoxy resin is blended in the metal paste together with the metal powder. The epoxy resin is a thermosetting synthetic resin having a reactive epoxy group at the end of the molecule. Epoxy resins include bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, tetramethylbiphenyl type epoxy resin, triphenylmethane type epoxy resin, tetraphenylethane type epoxy resin, dicyclopentadiene-phenol addition reaction Type epoxy resin, phenol aralkyl type epoxy resin, phenol novolac type epoxy resin, cresol novolac type epoxy resin, biphenyl type epoxy resin, hydroxynaphthalene type epoxy resin, anthracene type epoxy resin, naphthol-cresol co-condensed novolak type epoxy resin, aromatic Hydrocarbon formaldehyde resin modified phenolic resin type epoxy resin, biphenyl novolac type epoxy resin, glycidylamine type epoxy resin, Cyclopentadiene type epoxy resin, glycidyl ester type epoxy resin, or the like can be used cycloaliphatic epoxy resin. The epoxy resin can be used alone or in combination of two or more, and the viscosity characteristics of the metal paste, the curing speed, the viscoelasticity, heat resistance, thermal expansion coefficient, etc. of the cured body are set according to the blending ratio. be able to.

  The epoxy resin preferably has an epoxy equivalent of 90 g / mol to 500 g / mol, more preferably 95 g / mol to 450 g / mol, and even more preferably 97 g / mol to 420 g / mol. preferable. By using an epoxy resin having an epoxy equivalent in this range, a conductor having satisfactory physical property values can be obtained from the metal paste of the present invention.

  In particular, it is preferable to use a glycidyl ester type epoxy resin represented by the following formula (2) as the epoxy resin. When this epoxy resin is used, the electrical conductivity and thermal conductivity of the conductor obtained from the metal paste can be increased.

  In formula (2), ring A is more preferably a cyclohexane ring. In this cyclohexane ring, one or more of hydrogen atoms bonded to carbon atoms constituting the ring may be substituted with a substituent.

The metal paste also contains an epoxy resin curing agent. In the present invention, it is preferable to use a curing agent having three or more carboxylic acids as an acid per molecule. In this specification, the “carboxylic acid” may refer to a —COOH group itself or an organic compound having a —COOH group or a derivative thereof depending on the context. “Derivatives” include organic carboxylic acid salts represented by —COOX (X represents an anion), and organic carboxylic acid anhydrides represented by — (C═O) —O— (C═O) —. Etc. An organic carboxylic acid anhydride is formed by dehydration condensation of two —COOH groups, and is therefore a compound having two carboxylic acids as an acid in one molecule. In the present specification, “as an acid” means “as a proton (H ⁺ )”.

  In the present invention, the use of an epoxy resin curing agent having three or more carboxylic acids as an acid per molecule has the technical significance described below. That is, it is known that a metal powder whose element constituting the particle surface is mainly silver, such as silver powder and silver-coated metal powder, has a property of being easily sintered at a relatively low temperature. Therefore, when heating a metal paste containing these metal powders, when the curing rate of the epoxy resin is excessively high, the sintering of the metal particles tends to be hindered by the resin, and the electrical resistance of the conductor of the paste tends to increase. is there. In order to prevent this inconvenience, it is conceivable to slow down the curing rate of the epoxy resin by selecting the type of curing agent. An acid anhydride is known as such a curing agent. However, since an acid anhydride is generally highly volatile, the acid anhydride volatilizes during heating of the paste, and as a result, voids are likely to be generated in the conductor. Thus, the suppression of the increase in resistance of the conductor and the suppression of the generation of voids in the conductor are in a trade-off relationship. On the other hand, in this invention, volatilization of a hardening | curing agent can be suppressed by delaying hardening of an epoxy resin by using the specific hardening | curing agent mentioned above. Furthermore, since the curing agent having three or more carboxylic acids in the molecule as an acid has an action of promoting the sintering of silver located on the surface of the metal powder, the curing agent is used before the thermosetting reaction of the resin. A network of particle sintered bodies is significantly formed. That is, the electrical resistance and thermal resistance of the conductor, which is a cured body, can be reduced. Moreover, since a network of particle sintered bodies is significantly formed prior to the thermosetting reaction in the heating step, the resin flows without filling the periphery of the sintered body of metal particles. A structure will be obtained. By filling the resin around the sintered body of metal particles without gaps, in a thermal stress environment such as thermal cycle, the thermal stress is not excessively concentrated on the particle necking part, and the resin part covering the necking part is covered. Since it is dispersed, it is possible to obtain a conductor excellent in long-term stability of electrical conductivity and thermal conductivity.

  Here, the void related to the reliability of the conductor is intended for a conductor having a diameter of 0.5 μm or more in the cross section of the conductor. In order to stabilize the electrical resistance change and thermal conductivity change of the conductor in a long-term reliability environment, the cross-sectional area occupancy of the void of the size is preferably 13% or less, more preferably 7% or less. It is.

  As described above, a preferable curing agent that can be used in the present invention may be any one having three or more carboxylic acids as an acid per molecule. The curing agent preferably has one or more carboxyl groups (—COOH groups) in one molecule. For example, the curing agent preferably has (a) only a carboxyl group (—COOH group) in one molecule. The curing agent preferably has (b) one or more carboxyl groups (—COOH groups) and one or more acid anhydride sites in one molecule.

  In the case of (a), examples of the curing agent include cycloaliphatic tricarboxylic acid, chain aliphatic tricarboxylic acid, and aromatic tricarboxylic acid. Of these tricarboxylic acids, it is preferable to use a cycloaliphatic tricarboxylic acid from the viewpoints of low volatility and control of the curing rate of the epoxy resin. Examples of the cycloaliphatic tricarboxylic acid include cyclohexane-1,2,4-tricarboxylic acid.

  In the case of (b) above, examples of the curing agent include cycloaliphatic compounds and aromatic compounds represented by the following formula (1).

  Preferred examples of the compound represented by the formula (1) include cycloaliphatic from the viewpoint of reaction control in which the curing of the epoxy resin is started after sintering of the metal particles and prevention of volatilization in the curing reaction process. Examples include cyclohexane-1,2,4-tricarboxylic acid-1,2-anhydride, which is a compound, and trimellitic anhydride, which is an aromatic compound.

  The compounding amount of the curing agent in the metal paste is such that the number of carboxylic acid as the acid in the curing agent is 0.2 or more and 1.5 or less per epoxy group of the epoxy compound (hereinafter referred to as “epoxy group”). It is preferable that the amount be such that it is referred to as “acid equivalent ratio”. By setting the blending ratio of the epoxy resin and the curing agent in this way, it is possible to effectively prevent the formation of unreacted substances during curing, and the conductivity of the conductor formed from the metal paste is sufficient. Can be increased. Moreover, it can suppress effectively that a void generate | occur | produces in a conductor. From the viewpoint of making this effect even more remarkable, the amount of the curing agent is such that the number of carboxylic acids in the curing agent as an acid per epoxy group of the epoxy compound is 0.2 or more and 1.5 or less. In particular, the amount is preferably such that the ratio is 0.3 or more and 1.2 or less.

  It is preferable that the epoxy resin and the curing agent, which are organic components in the metal paste, are contained in a total amount of 1% by mass to 20% by mass with respect to the amount of the metal powder. By setting the ratio of the organic component and the metal powder in the metal paste within this range, the conductivity of the conductor formed from the metal paste can be made sufficiently high. From the viewpoint of making this effect even more remarkable, it is more preferable that the epoxy resin and the curing agent are contained in a total amount of 2% by mass or more and 17% by mass or less based on the amount of the metal powder. More preferably, the content is 3% by mass or more and 15% by mass or less.

  In the metal paste, other components can be blended as necessary in addition to the components described above. Examples of such components include reactive diluents and curing catalysts that are substances involved in the curing of epoxy resins, solvents, coupling agents, leveling agents, and dispersants. Furthermore, substances for adjusting the sintering of the metal powder, such as polyols, alcohols, sugar alcohols, fatty acids and the like, can be added to the extent that the reactivity of the resin is not impaired. For mixing the components constituting the metal paste, a rotating / revolving stirrer, a screw mixer, a Henschel mixer, an internal kneader, a butterfly mixer, etc. can be used. A kneading method in which crushing and mixing are performed at the same time can be employed.

  Various printing methods can be used for coating the metal paste. For example, screen printing, vacuum printing, roll coating, ink jetting, dispensing coating, etc. can be mentioned, and these are appropriately selected along with the viscosity adjustment of the paste based on the joint structure of the coated molded body and the electronic element. .

  In order to obtain the conductor by heating the metal paste of the present invention, for example, heating is performed at 150 ° C. or higher and 300 ° C. or lower, more preferably 180 ° C. or higher and 250 ° C. or lower in an inert gas atmosphere such as a nitrogen atmosphere. Just do it. The heating time may be adjusted according to the degree of curing of the epoxy compound. Since the metal paste of the present invention contains a curing agent having a relatively low curing rate, in the initial stage of heating, sintering of the metal particles occurs while the epoxy resin remains in a fluid state. Therefore, the sintering between the metal particles is hardly hindered by the epoxy resin. As heating progresses, the epoxy resin begins to cure, and the cured epoxy resin fills the portion other than the sintered body. In addition, the volatilization of the curing agent by heating is suppressed. Due to these reasons, the presence of voids in the conductor caused by curing is suppressed.

  For example, the metal paste of the present invention is heated to form a conductor, and the conductor can be used for filling a via hole in a printed wiring board or connecting between circuit wirings. It can also be used for electrical connection between a printed wiring board circuit and a surface-mounted electronic device. In these cases, the conductor functions as an electrical conductor. Or the conductor obtained by heating the metal paste of this invention can also be used as a die-bonding material for joining the die | dye of a semiconductor device, and a support body (for example, wiring body). Alternatively, it can be used as an adhesive between the semiconductor and a heat radiating metal member such as a heat spreader installed on the semiconductor. In these cases, the conductor functions as a thermal conductor. Also, the conductor can be filled into a via so as to function as a heat conductor. In these conductors, the particles are closely bonded to each other, and the presence of voids in the conductor is suppressed, so that the conductor has high thermal conductivity. As described above, according to the present invention, a printed wiring board including the conductor and an electronic device bonding structure including the conductor are also provided.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, “%” means “mass%”.

[Example 1]
As the metal powder, a mixture of pure silver powder and silver-coated copper powder with a mass ratio of 1: 1 was used. The pure silver powder used _was a spherical _{D 50} is 0.2 [mu] m. The silver-coated copper powder used was a spherical D ₅₀ is 3 [mu] m. The silver coat ratio with respect to copper powder was 10 mass%. The amount ratio of silver on the outermost surface of the silver-coated copper powder measured by X-ray photoelectron spectroscopy was 12.
As the epoxy resin, hexahydrophthalic acid diglycidyl ester (AK-601 manufactured by Nippon Kayaku Co., Ltd.) was used. This epoxy resin had an epoxy equivalent of 153 g / mol.
Cyclohexane-1,2,4-tricarboxylic acid-1,2-anhydride was used as a curing agent.
The compounding ratio as an “epoxy group-acid equivalent ratio” between the epoxy resin and the curing agent is as shown in Table 1 below. Moreover, the total amount and compounding ratio of metal powder, an epoxy resin, and a hardening | curing agent are as showing in the following Table 1.
The above raw materials were prepared and mixed with a spatula, and then pulverized and mixed using a three-roll mill to obtain a metal paste. At this time, an appropriate amount of ethyl carbitol acetate was added for the purpose of adjusting the viscosity of the metal paste.

[Examples 2 to 4]
A metal paste was obtained in the same manner as in Example 1 except that the formulation of the metal paste was as shown in Table 1 below.

Example 5
Trimellitic anhydride was used as a curing agent. Except this, it carried out similarly to Example 1, and obtained the metal paste.

Example 6
Cyclohexane-1,2,4-tricarboxylic acid was used as a curing agent. Except this, it carried out similarly to Example 1, and obtained the metal paste.

Example 7
As a metal powder, a metal paste was obtained in the same manner as in Example 1 except that the mass ratio of pure silver powder and silver-coated copper powder was 1: 0.

Example 8
As a metal powder, a metal paste was obtained in the same manner as in Example 2 except that the mass ratio of pure silver powder and silver-coated copper powder was 0: 1.

Example 9
Except that the epoxy resin used in Example 1 and the cyclohexene oxide type alicyclic epoxy resin (Celoxide 2021P manufactured by Daicel Corporation) were blended at a mass ratio of 7: 3 as the epoxy resin, the same as in Example 1. A metal paste was obtained.

[Comparative Example 1]
As a curing agent, a mixture of 4-methylhexhydrophthalic acid and hexahydrophthalic acid was used. As for the ratio of both, hexamethyl phthalic acid was 30% with respect to 70% of 4-methyl hexhydrophthalic acid. Except this, it carried out similarly to Example 1, and obtained the metal paste.

[Comparative Example 2]
As a curing agent, 3-dodecynyl succinic anhydride was used. Except this, it carried out similarly to Example 1, and obtained the metal paste.

[Comparative Example 3]
Endomethylenetetrahydrophthalic anhydride was used as a curing agent. Except this, it carried out similarly to Example 1, and obtained the metal paste.

[Comparative Example 4]
A tetraphenylborate salt of a 1,8-diaza-bicyclo [5.4.0] undec-7-ene derivative was used as a curing agent, and 5% was added to the epoxy resin in the same manner as in Example 1. A metal paste was obtained.

[Comparative Example 5]
A metal paste was obtained in the same manner as in Example 1 except that 1-cyanoethyl-2-undecylimidazole was used as a curing agent and 10% was added to the epoxy resin.

[Evaluation]
After the metal pastes obtained in Examples and Comparative Examples were applied and formed into a film, heat treatment was performed at 220 ° C. for 1 hour in an atmosphere in nitrogen to obtain a conductor having a thickness of 20 μm. About this conductor, the electrical resistivity, the thermal conductivity, the degree of suppression of voids, and the electrical resistance change rate after the thermal cycle test (TCT) were measured and evaluated by the following methods. The results are shown in Table 1 below.

[Electric resistivity]
The electrical resistivity of the conductor was measured by a four-terminal method (based on JIS-K7194 (1994)) using a resistivity meter (Loresta GP manufactured by Mitsubishi Chemical Corporation) in an environment of 25 ° C. and 60% RH.
The measured electrical resistivity was classified according to the following criteria.
A: The electrical resistivity is less than 9 μΩ · cm (good).
B: The electrical specific resistance is 9 μΩ · cm to 50 μΩ · cm (possible).
C: The electrical resistivity is more than 50 μΩ · cm (defect).

〔Thermal conductivity〕
A sample having a conductor film thickness of 1.0 mm was used as a sample for measuring thermal conductivity, and the measurement was performed according to the laser flash method (JIS-R1611 (2010)). The obtained measured values were classified according to the following criteria.
A: Thermal conductivity is 60 W / m · K or more (good).
B: Thermal conductivity is 15 W / m · K or more and less than 60 W / m · K (possible).
C: Thermal conductivity is less than 15 W / m · K (defect).

[Degree of void suppression]
A cross section of the obtained conductor was cut out, and an enlarged image (SEM image: 3000 times) of the cross section was obtained. From this enlarged image, a secondary image was created in which areas filled with metal particles or resin were painted white and areas where voids of 0.5 μm or more were observed were painted black. This secondary image was subjected to binarized image processing using software, and the ratio of the area of the void portion to the area of the observation field was calculated. The ratio was divided according to the following criteria.
A: The ratio of the cross-sectional area of the void portion is 7% or less (good).
B: The cross-sectional area ratio of the void portion is more than 7% and 13% or less (possible).
C: The ratio of the cross-sectional area of the void portion is greater than 13% (defective).

[TCT electrical resistance change rate]
In TCT, 500 cycles of an environment of −55 ° C. · 7 minutes to + 125 ° C. · 7 minutes were performed. When the electrical resistivity of the conductor before TCT is R1, and the electrical resistivity of the conductor after TCT is R2, the rate of change in electrical resistance ΔR (%) is defined as ΔR = (R2−R1) / R1 × 100 Is done. The electric resistance change rate ΔR obtained in this way was classified according to the following criteria.
A: Electric resistance change rate is 3% or less (good).
B: Electric resistance change rate is more than 3% and 10% or less (possible).
C: Electric resistance change rate is 10% or more (defect).

  As is clear from the results shown in Table 1, the conductors formed from the pastes obtained in the respective examples have low electrical specific resistance and high thermal conductivity as much as the conductors formed from the pastes in the respective comparative examples. It can be seen that the generation of voids is suppressed and the TCT electric resistance change rate is small.

Claims (11)

A metal paste containing a metal powder whose element constituting the particle surface is mainly silver, an epoxy compound, and a curing agent for the epoxy compound,
A metal paste in which the curing agent has three or more carboxylic acids as an acid per molecule.   2. The metal paste according to claim 1, which contains the curing agent in an amount such that the number of carboxylic acids as an acid is 0.2 to 1.5 per epoxy group of the epoxy compound. .   The metal paste according to claim 1 or 2, wherein the curing agent has one or more carboxyl groups.   The metal paste according to any one of claims 1 to 3, wherein the curing agent has one or more acid anhydride sites and one or more carboxyl groups. The metal paste according to any one of claims 1 to 4, wherein the curing agent is represented by the following formula (1).

The metal paste according to any one of claims 1 to 5, wherein the epoxy compound is represented by the following formula (2).

  The metal paste according to any one of claims 1 to 6, wherein the metal powder comprises an aggregate of silver particles.   The metal paste according to any one of claims 1 to 6, wherein the metal powder is made of a mixture containing silver-coated metal particles and silver particles.   A conductor obtained by curing the metal paste according to any one of claims 1 to 8.   A printed wiring board comprising the conductor according to claim 9.   An electronic device bonding structure comprising the conductor according to claim 9.