JP4473620B2 - Silver paste - Google Patents

Description

  The invention according to the present application relates to a silver paste. In particular, the present invention provides a silver paste that is suitable for low-temperature sintering performance and fine pitch circuit formation and is suitable for reducing the resistance of a conductor formed by sintering.

  Conventionally, the silver paste and silver ink are used for firing at a relatively high temperature such as performing circuit formation by simultaneous firing with a ceramic substrate, as well as for printed wiring boards as disclosed in Patent Document 1. Applications exist such that they are mixed with various resin components such as wiring circuits, via-hole filling, and component mounting adhesives and cured. In applications such as the latter, it has been common to obtain electrical conductivity only by contact between the powder particles without sintering the powder particles of the silver powder as the conductive filler.

  However, in recent years, the circuit wiring width and wiring film thickness have become extremely fine, so it is required not only to reduce electrical resistance to conductors formed using silver powder, but also to obtain high connection reliability. It has come to be. Since the silver paste and silver ink of the conventional method have acquired electroconductivity by the contact of powder particles, when a fine wiring is formed at low temperature, high connection reliability cannot be obtained. Accordingly, there has been an increasing demand for silver pastes and silver inks in which silver powder particles are sintered at low temperatures to exhibit conductivity. In general, in order to meet such a requirement, it is natural to consider lowering the sintering temperature by atomizing silver powder as a conductive filler.

For the production of conventional silver powder, as described in Patent Document 2, a wet reduction process in which a silver ammine complex aqueous solution is produced with a silver nitrate solution and aqueous ammonia and an organic reducing agent is added thereto is employed, and this is dispersed. It has been used by processing into a silver paste or silver ink using an agent. And silver ink containing silver nanoparticles, as disclosed in Patent Document 3, has been proposed to secure a low temperature sintering property superior to the case of using this conventional silver powder, and the formed conductor It is disclosed that a conductor having a specific resistance of about 2.8 × 10 ⁻⁵ Ω · cm can be formed.

  In addition, in order to reduce the resistance of the conductor obtained by sintering the silver paste, as another means, as disclosed in Patent Document 4, flake silver powder (flaky silver powder) having a large contact area between the powder grains can be obtained. Use has also been considered. Flake silver powder is produced by physically plastically processing and crushing silver powder particles, and may be expressed as scaly silver powder. Certainly, the flake silver powder is effective in reducing the resistance of the sintered conductor because it can secure a wide contact area between the powder grains, as can be easily considered from its shape.

JP 2001-107101 A JP 2002-334618 A JP 2002-324966 A JP-A-10-183209

  However, the conventional silver paste technology has the following problems and cannot satisfy market demands.

Problems when using silver nanoparticles: It is said that metal powder such as silver powder is generally difficult to achieve both fine atomization and dispersibility in the sense that the powder is closer to monodispersion. For example, in the case of a silver ink containing silver nanoparticles as disclosed in Patent Document 1, a large amount of a dispersant is added as a protective colloid in order to stabilize the dispersibility of the nanoparticles. It is common. In such a case, the decomposition temperature of the dispersant is generally higher than the sintering temperature of the silver nanoparticles, and the dispersant remains between the silver nanoparticles. At this time, since the silver nanoparticles have a remarkably fine particle size, it is difficult to ensure contact between the powder particles, and there is a high tendency that the low-temperature sintering characteristics inherent in the silver nanoparticles cannot be fully utilized.

  In addition, in the case of silver ink containing silver nanoparticles, the filler content is significantly lower than before, so it is difficult to form a thick film even if it is easy to form a thin film. Even if possible, the specific resistance of the film becomes extremely high, etc., so that it can be used for the formation of a wiring circuit with a large circuit cross section that can be used for a power supply circuit that allows a relatively large current to flow, or for a low resistance circuit Is difficult to apply. Furthermore, in adhesive applications for mounting parts, there are strict requirements for conductivity and adhesive strength, and it is indispensable to add a certain amount or more of resin that exhibits strong adhesive strength by curing, so there are parts that can not be handled with silver nanoparticle ink There were many.

Problems in powder characteristics of conventional silver powder: In circuit formation using conventional silver paste containing silver powder, there are many uses of non-fired or low-temperature sintered molds where the heating temperature is 300 ° C. or less, and high temperature at low temperature In order to obtain sintering performance, low crystalline silver powder has been preferred. However, in order to obtain a low crystalline silver powder, a reaction system having a high reduction must be employed in terms of production conditions, and as a result, only a silver powder with remarkable agglomeration was obtained although the crystallinity was low. Accordingly, there has been a demand in the market for the supply of silver powder that has low-temperature sinterability and has a fine dispersibility that is fine and unprecedented, and that has good dispersibility with little aggregation of the powder.

  On the other hand, silver powder has been required to have a small amount of impurities. That is, the above-described wet reduction process is employed for the production of silver powder, and the reducing agent used in the process remains on the surface of the silver powder particles. Therefore, as long as the conventional manufacturing method is adopted, it is an inevitable problem. And if the impurity amount of silver powder increases, the electrical resistance of the conductor formed using the silver powder will increase.

Problems with the conventional flake silver powder: Further, when the conventional flake silver powder is used, only a silver paste that is completely incompatible with the recent fine pitch circuit formation can be obtained. This is because the silver powder obtained by the conventional manufacturing method contains coarse particles having a particle diameter exceeding 10 μm. And, when trying to produce flake silver powder by physically plastic deformation using the silver powder, it is possible to obtain only flake silver powder whose powder characteristics are further deteriorated by promoting the dispersion of the powder particles of the original silver powder. . In addition, when trying to physically plastically deform silver powder, it is normal to use a lubricant from the viewpoint of suppressing aggregation of powder particles and preventing adhesion to processing tools such as media. In general, the surface of the powder is contaminated with a lubricant. As long as such a flake silver powder is used, the quality of the flake silver powder itself is not stabilized, and as a result, the viscosity of the silver paste is not stabilized, resulting in lack of handleability and screen formation when a circuit is formed using the screen printing method. It is difficult to reduce the electrical resistance of the formed conductor. Therefore, industrial use could not be expected at all.

  As a result, a fine pitch circuit can be formed with respect to silver paste containing silver powder in the market, and it has been desired to have a low-temperature sinterability unprecedented and to further reduce the resistance of the formed circuit. In particular, it has been desired that the resistance of the formed circuit can be reduced by using fine particles and high-dispersion silver powder that can accurately draw a fine circuit by using a screen printing method. In addition, the silver paste has been required to have a low impurity content for realizing low resistance.

  Therefore, as a result of intensive studies, the present inventors have solved the above-mentioned problems by adopting the silver paste described below. Hereinafter, the description will be divided into “silver paste” and “silver paste production method”.

<Silver paste>
The silver paste according to the present invention is a silver paste comprising a silver powder as a filler, a resin component, and an organic solvent. The silver powder is an ultrathin plate formed directly by a wet method and a microspherical silver powder having a substantially spherical particle shape. There use mixed mixed silver powder with a Jo silver powder, the fine spherical silver powder, the average particle size of the scanning average particle diameter D _IA of the primary particles obtained by the image analysis of an electron microscope image is 0.6μm or less, and primary particles and D _IA, and the average particle size D ₅₀ and the aggregation degree which is represented by D _{50 /} D _IA using by a laser diffraction scattering particle size distribution measuring method of 1.5 or less, the electrode thin plate silver powder, primary particles The basic feature is that the average thickness of the film is 50 nm or less and the average particle diameter D ₅₀ by the laser diffraction / scattering particle size distribution measurement method is 4.0 μm or less . Hereinafter, the silver powder and the organic agent referred to here will be described in detail.

Fine spherical silver powder constituting silver paste: First, the fine spherical silver powder will be described. Small spherical silver powder used in the silver paste according to the present invention, the average particle diameter D _IA of the primary particles obtained by image analysis of a scanning electron microscope image is provided with a powder characteristic of 0.6μm or less. This is because if the silver powder itself is not fine, no matter how the organic agent composition can be devised, low temperature sinterability cannot be obtained. The average primary particle diameter _DIA of 0.6 μm is low This is a critical particle size that can dramatically improve the sinterability. FIG. 1 shows a scanning electron microscope image of the fine spherical silver powder. Here, “average particle diameter D _IA of primary particles obtained by image analysis of scanning electron microscope image” means an observation image of fine spherical silver powder observed with a scanning electron microscope (SEM) (in the present invention) In the case of such fine spherical silver powder, it is preferably observed at a magnification of 10,000 times, and in the case of conventional silver powder, it is preferably observed at a magnification of 3000 to 5000 times). In addition, the image analysis of the fine spherical silver powder observed using a scanning electron microscope (SEM) in the present specification uses an IP-1000PC manufactured by Asahi Engineering Co., Ltd., and has a circularity threshold of 10 and an overlap of 20 The circular particle analysis was performed to determine the average particle diameter _DIA . Since the average particle diameter _DIA obtained by image processing the observation image of the fine spherical silver powder is obtained directly from the SEM observation image, the average particle diameter of the primary particles is surely captured. D _IA of small spherical silver powder as referred to in the present invention has come most in the range of 0.01μm~0.6μm unless present inventors have observed, it can be confirmed that further a fine particle size in reality In some cases, the lower limit is not explicitly stated.

Furthermore, small spherical silver powder is fine spherical silver powder is a silver powder with low cohesive fine, with an average particle diameter D _IA of the primary particles, the average particle size D ₅₀ by laser diffraction scattering particle size distribution measurement method It has a powder characteristic that the degree of aggregation represented by D ₅₀ / D _IA is 1.5 or less. Particulate of microscopic spherical silver powder used in the present matter invention, in fine enough not to conventional silver powder, because it exhibits high dispersibility, it was using "aggregation degree" as an index indicating the dispersibility. The degree of aggregation referred to in the present specification is a value represented by D ₅₀ / D _IA using the average particle diameter D _IA of the primary particles and the average particle diameter D ₅₀ obtained by a laser diffraction / scattering particle size distribution measurement method. That is. Here, D ₅₀ is a particle size at a volume accumulation of 50% obtained by using a laser diffraction / scattering particle size distribution measurement method, and the value of the average particle size D ₅₀ is truly one of powder particles. It can be said that the average particle diameter is calculated by capturing the aggregated particles as one particle (aggregated particle) instead of directly observing one diameter. That is, it is considered that the actual silver powder is not a so-called monodispersed powder in which individual particles are completely separated, but a plurality of powders are usually aggregated. However, little aggregation state of granular, closer to monodisperse, the value of the average particle diameter D ₅₀ is becoming small things usually. The D _{50 of the} fine spherical silver powder used in the present invention is in the range of about 0.25 μm to 0.80 μm, and becomes a fine spherical silver powder having an average particle diameter D ₅₀ in a range that could not be obtained at all by the conventional manufacturing method. . In the present invention, the laser diffraction / scattering particle size distribution measurement method is performed by mixing 0.1 g of silver powder with a 0.1% aqueous solution of SN Dispersant 5468 (manufactured by San Nopco), and ultrasonic homogenizer (US-made by Nippon Seiki Seisakusho). 300T) for 5 minutes, 1.51 is adopted as the refractive index, and measurement is performed using LS-230 manufactured by Beckman Coulter.

On the other hand, “average particle diameter D _IA of primary particles obtained by image analysis of scanning electron microscope image” is an observation image of fine spherical silver powder observed using a scanning electron microscope (SEM) as described above. This is the average particle size obtained by image analysis, and the average particle size of the primary particles is reliably captured without considering the aggregation state.

Therefore, the present inventors agglomerate the value calculated by D ₅₀ / D _IA using the average particle diameter D ₅₀ of the laser diffraction scattering particle size distribution measurement method and the average particle diameter D _IA obtained by image analysis. It was decided to take it as a degree. That is, assuming that the values of D ₅₀ and D _IA can be measured with the same accuracy in the same lot of fine spherical silver powder, considering the above-described theory, the D ₅₀ The value is considered to be larger than the value of _DIA . At this time, the value of D _50, the more granular aggregation state of the fine spherical silver powder is eliminated, Yuki approaching the value of the infinitely D _IA, the value of a degree of aggregation D _{50 /} D _IA, be close to 1 become. When the degree of aggregation becomes 1, it can be said to be a monodisperse powder having no agglomerated state of particles.

  And the present inventors examined the correlation between the degree of aggregation and the viscosity of the fine silver paste produced using the fine spherical silver powder of each aggregation degree, the surface smoothness of the conductor obtained by sintering, and the like. . As a result, it was found that a very good correlation was obtained. As can be seen from this, it can be determined that if the degree of aggregation of the fine spherical silver powder is controlled, the viscosity of the silver paste produced using the fine spherical silver powder can be freely controlled. Moreover, it has been found that if the degree of aggregation is set to 1.5 or less, it is possible to keep fluctuations in the viscosity of the silver paste, the surface smoothness after the sintering, and the like in a very narrow region. In addition, the more the aggregated state is resolved, the higher the film density of the conductor obtained by sintering with the silver paste, and the lower the electrical resistance of the resulting sintered conductor. It becomes.

Note that when the degree of aggregation is actually calculated, a value of less than 1 may be indicated. This makes the D _IA used for calculating the degree of aggregation is considered because it is assumed that true sphere, but in theory is not become a value less than 1, in reality, since not a true sphere It appears that a cohesion value of less than 1 is obtained.

Further, the fine spherical silver powder is a fine silver powder having low cohesiveness. In addition to the powder characteristics described above, the following A. And B. Thus, the circuit shape drawn with the silver paste can be made into a fine pitch, and the surface roughness of the sintered conductor when sintered is made appropriate and smooth.

A. The characteristic is that the specific surface area is 4.0 m ² / g or less. It can be said that the smaller the specific surface area value is, the silver powder having a smoother and smoother surface. The smoother the specific surface area, the lower the paste viscosity when processed into a paste. In the present invention, since it is used by mixing with the deformed ultrathin plate-like silver powder described below, the paste viscosity is likely to increase, and it is desirable to reduce the specific surface area of the powder as much as possible. In the present invention, the lower limit of the specific surface area is not provided, but the process capability when adopting the manufacturing method described later is about 1.5 m ² / g, and the specific surface area of most fine spherical silver powder is 2. made to be within the range of 0m ² /g~3.0m ² / g. The specific surface area referred to in this specification is measured by a BET one-point method using a monosorb (manufactured by Kantachrome Co., Ltd.) after subjecting a sample of 3.00 g to deaeration treatment at 70 ° C. for 10 minutes.

B. The characteristic is that the crystallite diameter of the powder grains is 10 nm or less, and the crystallite diameter and the sintering start temperature have a very close relationship. In other words, if silver powders having the same average particle diameter are compared with each other, the smaller the crystallite diameter, the lower the sintering possible. Therefore, since the particles are fine like the fine spherical silver powder according to the present invention, the surface energy is large, and the small crystallite diameter of 10 nm or less is provided, whereby the sintering start temperature can be lowered. Here, although no lower limit is set for the crystallite diameter, it is because a certain measurement error occurs depending on the measurement apparatus, measurement conditions, and the like. In addition, it is difficult to obtain a high reliability for the measured value in the range where the crystallite diameter is less than 10 nm. If the lower limit value is deliberately determined, according to the results of research by the present inventors, it is about 2 nm. I think there is.

  The silver paste using the fine spherical silver powder having the powder characteristics described above has a low-temperature sinterability that enables sintering at a temperature of 250 ° C. or less, from the viewpoint of the sintering temperature. In addition, there is no specific lower limit for the sintering temperature, but considering the research conducted by the inventors and general technical common sense, it is almost impossible to obtain a sintering start temperature lower than 150 ° C. It is impossible and is considered to be a temperature corresponding to the lower limit.

Ultra-thin plate-like silver powder constituting silver paste: Next, the ultra-thin plate-like silver powder will be described. The irregularly shaped silver powder such as conventional flakes is likely to cause clogging of the screen because of the presence of coarse particles having a particle size exceeding 10 μm. Therefore, in the present invention, an ultrathin plate-like silver powder directly formed by a wet manufacturing method described later is used. If the wet manufacturing method is used in this way, there is no contamination of the powder particles by the lubricant used in the plastic working method, so that the amount of impurities contained in the formed conductor can be eliminated and the conductor resistance can be lowered. The greatest feature of the ultrathin plate-like silver powder is that the average thickness of the powder is 50 nm or less.

Furthermore, very thin plate-like silver powder used in the present invention, without containing coarse particles, the average particle size D ₅₀ by laser diffraction scattering particle size distribution measuring method are those comprising powder characteristics hereinafter referred 4.0 .mu.m, substantially In particular, there are no coarse grains exceeding 4.0 μm. A scanning electron microscope image of the ultrathin plate-like silver powder is shown in FIG. In FIG. 2, it can be observed that small white spots are observed as very fine silver grains produced as a by-product, and as long as the production method described later is employed, this occurs at a constant rate. Is ignored and is called ultrathin plate-like silver powder. As can be seen from FIG. 2, this ultrathin plate-like silver powder is observed as if the beam of a scanning electron microscope was transmitted therethrough and it can be inferred that the thickness of the powder grain is very thin. Naturally, the particle diameter is considered to be a value reflecting the major axis of the ultrathin plate-like silver powder. In contrast, conventional flake silver powder is shown in FIG. As can be seen from a comparison between FIG. 2 and FIG. 3, it can be seen that the ultrathin plate-like silver powder referred to in the present invention has the same size and uniform thickness. Therefore, the paste viscosity can be easily controlled by using the ultrathin plate-like silver powder according to the present invention.

  Here, when the particle sizes of the ultrathin plate-like silver powder and the fine spherical silver powder constituting the mixed silver powder are compared, it can be said that the particle diameter of the fine spherical silver powder (0.6 μm or less) is extremely small. This is a mixture of microspherical silver powder that does not exceed the particle size of ultrathin plate-like silver powder, making it easy to enter between the particles of ultrathin plate-like silver powder and processing it into paste, etc. This is because the film density of the conductor formed in this manner is properly maintained to contribute to the reduction of electric resistance. In the present invention, the lower limit of the average particle size of the ultrathin plate-like silver powder is not provided, but it is about 1.0 μm from the process capability when the production method described later is adopted, and the average particle size of most ultrathin plate-like silver powders Falls within the range of 3.0 μm to 3.5 μm.

In addition to the powder characteristics described above, the ultrathin plate-like silver powder used in the present invention has a powder characteristic of a specific surface area of 3.5 m ² / g or less. This idea regarding the specific surface area reflects the technical idea that the smaller the value of the specific surface area, the smoother the viscosity of the paste when processed into a paste. In the present invention, the lower limit of the specific surface area is not provided, but it is about 1.5 m ² / g from the process capability when the production method described later is adopted, and the specific surface area of most ultrathin plate-like silver powder is: made to be within the range of 2.0m ² /g~2.5m ² / g.

Mixed silver powder constituting silver paste: The mixed silver powder used in the present invention is obtained by mixing the fine spherical silver powder and the ultrathin plate-like silver powder described above. The mixing ratio of the fine spherical silver powder and the ultrathin plate-like silver powder is preferably such that the content of the ultrathin plate-like silver powder is 1 wt% to 40 wt% based on the weight of the mixed silver powder. Strictly speaking, the content of the fine spherical silver powder is usually determined in consideration of the combination of the particle size distributions of the fine spherical silver powder and the ultrathin plate-like silver powder. That is, the idea in the present invention adopts the idea that the fine spherical silver powder that does not exceed the particle diameter of the ultrathin plate-like silver powder is mixed and used. And when the particle size of the ultrathin plate-like silver powder becomes as fine as 0.6 μm or less, the fine spherical silver powder having a particle size of about 1/8 of the ultrathin plate-like silver powder enters between the particles of the ultrathin plate-like silver powder. Therefore, it is possible to appropriately maintain the film density of the conductor formed by processing into a paste or the like and contribute to the reduction of electric resistance. On the contrary, the larger the particle size of the fine spherical silver powder, the harder it is to enter between the particles of the ultrathin plate-like silver powder unless it contains a large amount of the fine spherical silver powder. The film density of the conductor to be formed becomes low and cannot contribute to the reduction of electric resistance. Therefore, considering the particle size of the fine spherical silver powder and the ultrathin plate-like silver powder, the content of the ultrathin plate-like silver powder is preferably 1 wt% to 40 wt%.

  Accordingly, the lower limit value of 1 wt% means the minimum amount that can contribute to reduction of electrical resistance when a silver paste is produced and a conductor is formed in the combination of the ultrathin plate-like silver powder and the fine spherical silver powder. . The upper limit of 40 wt% is formed by processing into a silver paste or the like even when the mixing ratio of the ultrathin plate-like silver powder is further increased when the combination of the fine spherical silver powder and the ultrathin plate-like silver powder is used. The effect of reducing the electrical resistance of the conducting conductor is saturated.

Resin component constituting the paste: As far as the resin component constituting the paste is concerned, the resin component is not particularly required to be originally limited. This is because, if the paste resin components are the same, the resistance of the conductor formed inevitably can be reduced by using the mixed silver powder described above. However, on the premise that fine silver powder as described above is used as a filler, a composition that can ensure good dispersibility of the silver powder must be employed. The silver paste according to the present invention is intended to ensure low-temperature sinterability and to form a fine pitch circuit, and to reduce the resistance of the formed conductor. Therefore, it is necessary to employ a resin composition that can remove the solvent below the employed low temperature sintering temperature and that can ensure paste performance suitable for fine pitch circuit formation.

  In view of these matters, it is desirable that the resin component employs a composition containing at least one selected from an epoxy resin, a polyester resin, a silicon resin, a urea resin, an acrylic resin, and a cellulose resin.

  If these resin components are specified more specifically, they are as follows. a) The epoxy resin includes bisphenol A type, bisphenol F type, bisphenol AD type, novolac type, cresol novolac type, glycidylamine type, glycidyl ether type, aliphatic type, and heterocyclic type epoxy resin.

  b) The polyester resin is a polymer of dicarboxylic acid and glycol such as polyethylene terephthalate, polypropylene glycol maleate phthalate, polypropylene glycol fumarate phthalate, polypropylene glycol maleate, polypropylene glycol fumarate, polypropylene glycol adipate maleate, and the like.

  And it is desirable for the synthesis | combination of the polyester said here to obtain by carrying out the condensation reaction of the dicarboxylic acids and glycols which are described below. Dicarboxylic acids include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid, and 2,6-naphthalenedicarboxylic acid, succinic acid, glutaric acid, adipic acid, sebacic acid, dodecanedicarboxylic acid, and azelaic acid. Dicarboxylic acid, dibasic acid having 12 to 28 carbon atoms, 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, 4-methylhexahydrophthalic anhydride, 3-methylhexa Fats such as hydrophthalic anhydride, 2-methylhexahydrophthalic anhydride, dicarboxy hydrogenated bisphenol A, dicarboxy hydrogenated bisphenol S, dimer acid, hydrogenated dimer acid, hydrogenated naphthalenedicarboxylic acid, tricyclodecane dicarboxylic acid Cyclic dicarboxylic acid, etc. A.

  The glycol includes ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 3-methyl-1,5- Pentanediol, 2-methyl-1,5-pentanediol, 2,2-diethyl-1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, 1,9-nonanediol, 1 , 10-decanediol, 1,4-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,2-cyclohexanedimethanol, dimer diol, and the like are used.

  c) Silicone resin means amino-modified silicone, epoxy-modified silicone, carboxyl-modified silicone, carbinol-modified silicone, methacryl-modified silicone, mercapto-modified silicone, phenol-modified silicone, polyether-modified silicone, polyester-modified silicone, alkyd-modified silicone, acrylic Modified silicone, methylstyryl-modified silicone, alkyl-modified silicone, fluorine-modified silicone, and the like.

  d) The urea resin is an amine-modified urea resin, a butanol-modified urea resin, or the like.

  e) Acrylic resin includes polyacrylic acid, polymethacrylic acid, polymethyl acrylate, polymethyl methacrylate, polyitaconic acid, polyacrylate, polymethacrylate, polyitaconate, and the like.

  f) The cellulose resin includes methyl cellulose, ethyl cellulose, propyl cellulose, butyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose, hydroxybutyl cellulose, acetyl cellulose and the like.

  These may be used alone or in combination of two or more. Also in these respects, the silver powder to be dispersed can be easily dispersed, and alteration of the surface of the fine spherical silver powder can be prevented.

Blending balance of silver powder and paste: Furthermore, unless the blending balance of the mixed silver powder of the silver paste according to the present invention and the organic agent is made appropriate, a good circuit shape or the like cannot be formed. Therefore, as a result of intensive studies, the inventors of the present invention have determined that it is possible to form a conductor such as a good circuit if the mixed silver powder content is 80 wt% or more, more preferably in the range of 80 wt% to 93 wt%. is there. When the content of the mixed silver powder is less than 80 wt%, no matter how highly dispersible and fine silver powder is used, the film density of a circuit formed by sintering is lowered and the specific resistance is increased. And when content of mixed silver powder exceeds 93 wt%, a silver paste viscosity will rise rapidly and will thicken to the level which is hard to use to form a fine pitch circuit shape etc.

  In the silver paste according to the present invention, the mixed silver powder included is fine and highly dispersed, fine spherical silver powder excellent in low-temperature sintering performance, and an ultrathin plate shape excellent in particle size distribution and thickness uniformity. Since it is composed of silver powder, it exhibits excellent dispersibility in organic agents, is excellent in low-temperature sinterability, and is optimal for the formation of fine pitch circuits. Moreover, since the mixed silver powder itself included in the paste in the present invention is a fine particle, the surface roughness of the conductor after sintering becomes smooth. Moreover, since the amount of impurities contained in the silver powder is small, the specific resistance of the sintered conductor can be reduced.

  Hereinafter, the best mode of the present invention will be described, and detailed description will be given while comparing the examples and comparative examples. First, embodiments of “a method for producing a fine spherical silver powder used for a silver paste” and “a method for producing an ultrathin plate-like silver powder used for a silver paste” will be described.

<Embodiment of fine spherical silver powder used for silver paste>
Silver paste according to the present invention, the silver powder, the average particle diameter D _IA of the primary particles obtained by image analysis of a scanning electron microscope image is assumed that the use of small spherical silver powder is 0.6μm or less. Moreover, as can be understood from the above description of the silver paste production method, it would be extremely advantageous if fine and highly dispersed silver powder can be used from the beginning. Therefore, it is preferable to use fine spherical silver powder obtained by the method described below.

Microspherical silver powder production method 1: The present inventors mixed a conventional silver nitrate aqueous solution and aqueous ammonia to react to obtain a silver ammine complex aqueous solution, and by adding a reducing agent to this, silver particles are reduced and precipitated. Based on the manufacturing method of filtration, washing, and drying, the inventors devised the manufacturing method to obtain a fine spherical silver powder that cannot be obtained by the conventional manufacturing method.

  This fine spherical silver powder is produced by mixing and reacting an aqueous silver nitrate solution and aqueous ammonia to obtain an aqueous silver ammine complex solution, which is contacted with an organic reducing agent to cause silver particles to be reduced and precipitated, filtered and washed. In the method for producing fine spherical silver powder by drying, the main feature is that the amount of reducing agent, the amount of silver nitrate, and the amount of aqueous ammonia are used so that the silver concentration in the reaction solution becomes dilute. Conventionally, the reducing agent solution and the silver ammine complex aqueous solution are generally mixed together in a tank, and the silver concentration is set to a concentration level of 10 g / l or more. And if the amount of ammonia water was not added, productivity for the scale of the equipment could not be secured.

  The most important feature in this production method is that the concentration of the organic reducing agent after contact reaction between the silver ammine complex aqueous solution and the organic reducing agent is low, and it remains adsorbed on the surface of the generated silver powder or the growth of the powder. In the process, the amount of the organic reducing agent taken into the particles can be reduced. Therefore, it is most preferable to maintain the organic reducing agent concentration at 1 g / l to 3 g / l while the silver concentration in the mixed solution is 1 g / l to 6 g / l.

  Here, there is a proportional relationship between the silver concentration and the amount of the reducing agent, and it is natural that a higher amount of silver powder can be obtained as the silver concentration is higher. However, if the silver concentration here exceeds 6 g / l, the precipitated silver particles tend to be coarse, and the particle size is not different from that of conventional silver powder, and has high dispersibility as referred to in the present invention. It becomes impossible to obtain fine silver powder. On the other hand, when the silver concentration here is less than 1 g / l, an extremely fine fine spherical silver powder can be obtained, but it becomes too fine to increase the oil absorption and increase the paste viscosity. It is necessary to increase the amount of the agent, and the film density of the finally formed sintered conductor is low, and the electric resistance tends to increase. In addition, the required industrial productivity is not satisfied.

  And while the silver concentration is 1 g / l to 6 g / l, maintaining the organic reducing agent concentration at 1 g / l to 3 g / l is most suitable for obtaining fine spherical silver powder with good yield. It becomes a condition. Here, the reason why the organic reducing agent concentration is set to 1 g / l to 3 g / l is that it is selected as the most suitable range for obtaining fine silver powder in relation to the silver concentration of the silver ammine complex aqueous solution. When the concentration of the organic reducing agent exceeds 3 g / l, the amount of the reducing agent added to the silver ammine complex aqueous solution decreases, but the progress of aggregation of silver powder particles that are reduced and precipitated begins to become remarkable and is contained in the powder particles. The amount of impurities (in the present invention, the amount of impurities is regarded as the carbon content) starts to increase rapidly. On the other hand, when the concentration of the organic reducing agent is less than 1 g / l, the total amount of reducing agent to be used increases, the amount of wastewater treatment increases, and industrial economic efficiency is not satisfied.

  The “organic reducing agent” mentioned here is hydroquinone, ascorbic acid, glucose and the like. Among these, it is desirable to selectively use hydroquinone as the organic reducing agent. In particular, hydroquinone is relatively excellent in reactivity as compared with other organic reducing agents, and can be said to have a reaction rate most suitable for obtaining a low crystalline microspherical silver powder having a small crystallite size.

  And it is also possible to use another additive in combination with the said organic reducing agent. Additives mentioned here are glues such as gelatin, amine-based polymer agents, celluloses, etc., which stabilize the reduction precipitation process of fine spherical silver powder and at the same time serve as a constant dispersant. It is desirable to use the organic reducing agent appropriately and selectively according to the type of the organic reducing agent and the process.

  And in the method of carrying out the contact reaction of the silver ammine complex aqueous solution obtained as mentioned above and a reducing agent, and carrying out the reduction | restoration precipitation of the fine spherical silver powder, in this invention, as shown in FIG. 4, the silver ammine complex aqueous solution S1 flows constant. And a second flow path b that joins in the middle of the first flow path a, and the organic reducing agent passes through the second flow path b. The additive S2 according to need is flowed into the first flow path a and contact-mixed at the junction m of the first flow path a and the second flow path b to reduce and precipitate silver particles (hereinafter, this method). Is preferably referred to as a “merging and mixing method”).

  By adopting such a merging and mixing method, the mixing time of the two liquids is completed in the shortest time, and the reaction proceeds in a uniform state in the system, so that uniform-shaped powder particles are formed. Moreover, when the amount of the organic reducing agent when viewed as the whole solution after mixing is low, the amount of the organic reducing agent that remains adsorbed on the surface of the fine spherical silver powder that is reduced and deposited decreases. As a result, it is possible to reduce the amount of impurities deposited on the fine spherical silver powder obtained by filtration and drying. By reducing the amount of impurities deposited on the fine spherical silver powder, the electric resistance of the sintered conductor formed through the silver paste can be reduced.

  Furthermore, when silver nitrate aqueous solution and ammonia water are contact-reacted to obtain a silver ammine complex aqueous solution, a silver nitrate aqueous solution having a silver nitrate concentration of 2.6 g / l to 48 g / l is used, and a silver concentration of 2 g / l to 12 g. It is desirable to obtain a silver ammine complex aqueous solution of / l. Here, prescribing the concentration of the silver nitrate aqueous solution is synonymous with prescribing the amount of the silver nitrate aqueous solution, and it is considered that the silver concentration of the silver ammine complex aqueous solution is 2 g / l to 12 g / l. In addition, the concentration and amount of ammonia water added thereto are inevitably determined. At the present stage, a clear technical reason is not known, but by using a silver nitrate aqueous solution having a silver nitrate concentration of 2.6 g / l to 48 g / l, the best production stability is shown and the quality is improved. Stable microspherical silver powder can be obtained.

Microspherical silver powder production method 2: In the microspherical silver powder production method 1 described above, the method for cleaning the obtained fine spherical silver powder is not particularly defined and employs a regular method. However, the amount of impurities remaining on the surface of the fine spherical silver powder is further reduced by devising the cleaning method of the fine spherical silver powder obtained by the fine spherical silver powder production method 1, and silver using the fine spherical silver powder is obtained. This can contribute to lowering the conductor resistance of the circuit formed of the paste.

  The cleaning at this time may be performed in combination with water cleaning and alcohol cleaning, or only alcohol cleaning may be used, but the cleaning at the time of cleaning with alcohol is strengthened. That is, when the reduced spherical silver powder is 40 g, it is usually washed with about 100 ml of pure water and then washed with about 50 ml of alcohol. On the other hand, here, when performing alcohol cleaning, 200 kg or more per 1 kg of fine spherical silver powder is washed with a large volume of alcohol of 5 L or more and dried.

  Impurities can be reduced by such cleaning strengthening. The contact reaction between the silver ammine complex aqueous solution and the reducing agent when obtaining fine spherical silver powder was adopted as a whole solution after mixing using a dilute concentration reaction system. This is because a technique for keeping the amount of the organic reducing agent low is employed.

Silver powder obtained by the minute spherical silver powder manufacturing method has been described above, the average particle diameter D _IA of the primary particles obtained by image analysis of a scanning electron microscope image is small spherical silver powder as 0.6μm or less, yet good Therefore, it is easy to adjust the silver powder content when the paste is processed, and it is easy to make a fine pitch in a circuit drawn using the silver paste. Moreover, the fine spherical silver powder obtained by the manufacturing method as described above has a great feature in that it has an impurity content capable of reducing the resistance of the conductor.

<Method for producing ultrathin plate-like silver powder used in silver paste>
The ultrathin plate-like silver powder is obtained by the production method described below. The silver salt ammine complex used in this production method can be prepared according to a known technique. For example, a silver salt aqueous solution, preferably a silver nitrate aqueous solution and aqueous ammonia are mixed to obtain a silver salt ammine complex slurry. This slurry is used as it is in this production method.

  In the metal powder production method by the wet neutralization reduction method, generally, a reducing agent such as hydrazine, hydrosulfide, sodium thiosulfate, sodium nitrite, formalin or the like is often used. However, when these reducing agents are used in the production of the ultrathin plate-like silver powder in the present invention, there is a tendency that the compatibility with the mother liquor tends to be poor, the reducing power may become incompatible, and the resulting silver powder is significantly aggregated, The result is that the reduction does not proceed.

  However, by using an aqueous solution of water-soluble sulfite such as potassium sulfite, potassium hydrogen sulfite, sodium sulfite, sodium hydrogen sulfite or ammonium sulfite, or glucose as a reducing agent in the production method of this ultrathin plate-like silver powder, The ammine complex is reduced, and ultrathin plate-like silver particles having an average thickness of 50 nm or less are produced, and an ultrathin plate-like silver powder is obtained. Further, aggregation does not occur and the particle size distribution does not become broad.

  For the production of ultrathin plate-like silver powder, proteins, high molecular amino compounds, rubbery polysaccharides and thiol compounds that can act as protective colloids when reducing silver salt ammine complexes, such as gelatin, glue, albumin, polyethyleneamine It is important to add a protective colloid selected from the group consisting of gum arabic and thiolcarbamic acid.

  In general, a hydrophilic colloid that maintains the stability of the hydrophobic colloid in the reaction solution is often used as a protective colloid. When such a protective colloid is not used, the resulting silver powder is remarkably aggregated or the particle size distribution varies. In addition, when a protective colloid other than a protective colloid selected from the group consisting of proteins, high molecular amino compounds, rubbery polysaccharides and thiol compounds that can act as protective colloids is used, an ultrathin plate having an average thickness of 50 nm or less It is impossible or extremely difficult to obtain silver powder containing silver particles.

  A protective colloid selected from the group consisting of proteins, polymeric amino compounds, rubbery polysaccharides and thiol compounds that can act as protective colloids is first mixed with a slurry of an ammine complex of silver salt, and then the mixture and water-soluble sulfite. An aqueous solution of a reducing agent selected from the group consisting of salt and glucose may be mixed at one time, or first mixed with an aqueous solution of a reducing agent selected from the group consisting of water-soluble sulfite and glucose, and the mixture A silver salt ammine complex slurry may be mixed at one time. Or you may mix said protective colloid, the slurry of the ammine complex of silver salt, and the aqueous solution of a reducing agent at once.

  In the method for producing ultrathin plate-like silver powder, the protective colloid is added to the protective colloid particles per liter of the total amount of the water in the protective colloid, the water in the slurry and the water in the reducing agent aqueous solution present in the reaction system. The amount is 2 to 5 g. When the amount of the protective colloid particles is less than 2 g or more than 5 g, the obtained silver powder is a mixture of granular silver particles and plate-like silver particles, and the thickness of the plate-like silver particles is 60 nm or more. Tend.

  In the method for producing ultrathin plate-like silver powder, the silver salt ammine complex is added to silver per liter of the total amount of water in the protective colloid, water in the slurry and water in the reducing agent aqueous solution present in the reaction system. Is used in an amount that is 50 g or less. When the amount of the silver salt ammine complex exceeds 50 g calculated as silver, the resulting silver powder tends to be aggregated silver particles. Further, the thickness of the plate-like silver particles tends to be 60 nm or more. Even if the amount of the silver salt ammine complex is more than 50 g calculated as silver, the amount of the protective colloid particles is increased so that the resulting silver powder becomes a mixture of granular silver particles and plate-like silver particles. The thickness of the silver particles tends to be 60 nm or more. Even if the amount of the silver salt ammine complex is decreased, ultrathin plate-like silver particles having an average thickness of 50 nm or less can be obtained. However, as the amount of the silver salt ammine complex decreases, the production efficiency deteriorates. The amount of the ammine complex of the salt is preferably 10 g or more, more preferably 20 g or more, calculated as silver.

  In the production method of ultrathin plate-like silver powder, it is important to mix a slurry of an ammine complex of a silver salt and an aqueous solution of a reducing agent at a time in the presence of a protective colloid. When it is added in a divided manner during the mixing, or when it is gradually added, a large amount of crystal nuclei are generated, the growth of silver particles is not only inhibited, but the ultrathin plate having a target average thickness of 50 nm or less There is a tendency that silver powder composed of silver-like particles cannot be obtained. It is preferable to carry out this mixing step at 35 to 55 ° C.

  In the production method of the present invention, the silver powder containing the ultrathin plate-like silver particles produced by the above-described steps is recovered according to a conventional method such as washing, filtration, and drying. Through the above steps, ultrathin plate-like silver powder containing ultrathin plate-like silver particles and having an average thickness of 50 nm or less can be obtained.

Production of fine spherical silver powder: First, fine spherical silver powder used for a silver paste was produced. The procedure will be described below. A silver ammine complex aqueous solution was obtained by dissolving 63.3 g of silver nitrate in 9.7 liters of pure water to prepare an aqueous silver nitrate solution, and adding 235 ml of 25 wt% ammonia water all at once and stirring. .

  Then, this silver ammine complex aqueous solution is introduced into the first flow path a having an inner diameter of 13 mm shown in FIG. 4 at a flow rate of 1500 ml / sec, and the reducing agent is flowed from the second flow path b at a flow rate of 1500 ml / sec. The fine spherical silver powder was reduced and precipitated by bringing the contact to a temperature of ° C. As the reducing agent used at this time, an aqueous hydroquinone solution in which 21 g of hydroquinone was dissolved in 10 liters of pure water was used. Accordingly, the hydroquinone concentration at the end of mixing is about 1.04 g / l, which is a very dilute concentration.

In order to fractionate the fine spherical silver powder obtained as described above, it is filtered using a Nutsche, washed with 100 ml of water and 50 ml of methanol, and further dried at 70 ° C. for 5 hours to obtain a substantially spherical shape. Of fine spherical silver powder was obtained. The average primary particle diameter _{D IA} of the minute spherical silver powder is 0.3 [mu] m, the _{D 10} by laser diffraction scattering particle size distribution measurement _{0.283μm, D 50} is _{0.371μm, D 90} is the 0.491μm and _{D max} The standard deviation was 0.791 μm. Furthermore, the degree of aggregation represented by D ₅₀ / D _IA was 1.24, the specific surface area was 2.54 m ² / g, and the crystallite diameter was 7 nm.

  The crystallite size was measured by the Wilson method (crystallite size measurement method by X-ray diffraction) using a RINT2000 X-ray diffractometer manufactured by Rigaku Corporation.

Production of ultrathin plate-like silver powder: The ultrathin plate-like silver powder used in the following examples was produced as follows.

50 g of silver nitrate was completely dissolved in 300 ml of pure water, and 46 ml of ammonia water (NH ₃ concentration: 25 wt%) was added to the solution, followed by stirring to form an ammine complex of silver nitrate, thereby obtaining a slurry containing this acyline complex. . Further, 3 g of gelatin dissolved in 300 ml of pure water was added to the slurry containing this ammine complex and stirred.

  On the other hand, 57 g of potassium sulfite was dissolved in 300 ml of pure water to obtain a reducing agent aqueous solution. The slurry containing the above-mentioned silver nitrate ammine complex and gelatin was put into this reducing agent aqueous solution at a time, and a reduction reaction was carried out for 150 minutes while maintaining the temperature at 40 ° C. The produced silver powder was recovered by conventional filtration, washing treatment and drying.

The average thickness of the primary particles of the obtained electrode thin plate silver powder 45 nm, _{D 10} is 0.6μm by a laser diffraction scattering particle size distribution measuring _{method, D 50} is 1.3 _{.mu.m, D 90} is 6.5μm and _{D max} is 13 The specific surface area was 2.2 m ² / g. The recovered ultrathin plate-like silver powder was observed using a scanning electron microscope. As shown in FIG. 2, most of the generated particles had an ultrathin plate shape.

  The thickness (nm) of the ultrathin plate-like silver powder was photographed using a field emission scanning electron microscope (FE-SEM) (JSM-6330F manufactured by JEOL Ltd.) and based on the photograph. The thickness was measured, and the average value of 50 measured values was calculated.

Production of silver paste: 15.3 g of bisphenol F type epoxy resin (Nippon Kayaku Co., Ltd .: RE-303SL), 2.7 g of acid anhydride curing agent (Kayahard MCD manufactured by Nippon Kayaku Co., Ltd.), and amine adduct type curing agent (Ajinomoto Fine Techno Co., Ltd .: Amicure MY-24) 0.9 g and α-terpineol (Yasuhara Chemical Co., Ltd.) 12.9 g as a viscosity modifier were kneaded for 5 minutes with a paddle type kneader, and then the fine spherical silver powder and electrode Thin plate-like silver powder was added and kneaded for an additional 10 minutes. And after knead | mixing the obtained kneaded material with 3 rolls, the bubble contained in a kneaded material was removed using the defoaming machine (made by Shinkey: AR-250), and the silver paste A was obtained. The fine spherical silver powder content in the silver paste A at this time was 141.9 g (60.51 wt%), and the ultrathin plate-like silver powder content was 60.8 g (25.93 wt%).

  The obtained silver paste A was printed on an alumina substrate with a wiring width of 50 μm and a wiring-to-wiring interval of 50 μm using a screen printing machine, and showed good printability without wiring disconnection or blurring. Moreover, as a result of observing the plate used for the screen printing machine with a microscope, it was confirmed that the plate was not clogged with silver powder at all.

Subsequently, the silver paste A was printed as a specific resistance measurement sample on an alumina substrate using a screen printing machine under the conditions of 4 cm long × 3 cm wide, and then dried for 2 hours at a temperature of 180 ° C. The surface resistance of the dried film thus obtained was measured with a 4-probe resistance measuring instrument (Mitsubishi Chemical Corporation: Loresta GP), and the film thickness of the dried film was measured with a digital film thickness meter. Was calculated. As a result, the specific resistance was 3.59 × 10 ⁻³ Ω · cm.

Manufacture of microspherical silver powder: The manufacture of microspherical silver powder here is common to Example 1 and will not be described here in order to avoid redundant description.

Production of ultra-thin plate-like silver powder: The production of ultra-thin plate-like silver powder here is the same as in Example 1, and the explanation here is omitted to avoid duplicate explanation.

Production of silver paste: 12.0 g of bisphenol F type epoxy resin (manufactured by Nippon Kayaku Co., Ltd .: RE-303SL), 2.1 g of acid anhydride type curing agent (Kayahard MCD manufactured by Nippon Kayaku Co., Ltd.), and amine adduct type curing agent (Ajinomoto Fine Techno Co., Ltd .: Amicure MY-24) 0.7 g and α-Terpineol (Yasuhara Chemical Co., Ltd.) 15.2 g as a viscosity modifier were kneaded in a paddle type kneader for 5 minutes, and then the fine spherical silver powder and electrode Thin plate-like silver powder was added and kneaded for an additional 10 minutes. And after kneading the obtained kneaded material with 3 rolls continuously, the bubble contained in a kneaded material was removed using the defoaming machine (made by Shinkey: AR-250), and the silver paste B was obtained. The fine spherical silver powder content in the silver paste B at this time was 133.6 g (60.49 wt%), and the ultrathin plate-like silver powder content was 57.3 g (25.93 wt%).

  When the obtained silver paste B was printed on an alumina substrate using a screen printer with a wiring width of 50 μm and a wiring gap of 50 μm, it showed good printability without disconnection or blurring of the wiring. Moreover, as a result of observing the plate used for the screen printing machine with a microscope, it was confirmed that the plate was not clogged with silver powder at all.

Furthermore, after printing silver paste B as a sample for measuring specific resistance on an alumina substrate using a screen printing machine under the conditions of 4 cm long × 3 cm wide, it was dried at a temperature of 180 ° C. for 2 hours. The surface resistance of the dried film obtained at this time was measured with a 4-probe resistance measuring instrument (Mitsubishi Chemical Corporation: Loresta GP), and the film thickness of the dried film was measured with a digital film thickness meter to calculate the specific resistance. did. As a result, the specific resistance was 1.74 × 10 ⁻³ Ω · cm.

Comparative example

In this comparative example, the mixed silver powder of Example 1 is used as the mixed silver powder of the fine spherical silver powder and the normal flake silver powder in the same resin composition as in Example 1 at the same blending ratio as in Example 1. A silver paste C containing 00 wt% was produced, and the same evaluation was performed. Flakes silver powder used herein is intended to include coarse particles shown in FIG. 3, the powder characteristics is _{D 10} by laser diffraction scattering particle size distribution measurement method 3.9 _{.mu.m, D 50} is 11.71Myuemu, D ₉₀ was 33.8 μm, D _max was 74.0 μm, and the specific surface area was 1.00 m ² / g.

  When the silver paste D thus obtained was printed on an alumina substrate with a wiring width of 50 μm and an inter-wiring gap of 50 μm using a screen printing machine, the wiring circuit was disconnected due to uneven printing. As a result of observing the screen used in the screen printing machine with a microscope, clogging occurs. As a result, it was attempted to measure the specific resistance of the circuit, but it was impossible to measure the specific resistance.

<Contrast between Example and Comparative Example>
As can be seen from the above, the silver paste using the mixed silver powder of the ultrathin plate-like silver powder and the fine spherical silver powder according to the example is more practical in the dry film after firing at a low temperature (180 ° C.). A specific resistance with no hindrance is obtained. On the other hand, as shown in the comparative example, in the silver paste using the fine spherical silver powder and the flake silver powder, good screen printing corresponding to the formation of the fine pitch circuit is impossible, and the circuit shape is processed into the silver paste. It can be seen that the specific resistance of the dried film after formation and low-temperature firing cannot be measured.

  The silver paste according to the present invention uses a mixed silver powder composed of an ultrathin plate-like silver powder and a fine spherical silver powder, thereby reducing the film density and increasing the impurity content of the conductor after the circuit shape is formed and dried. The low-temperature sintering characteristics of the encapsulated fine spherical silver powder can be effectively utilized. As a result, the sintering temperature of the sintered conductor obtained using the silver paste according to the present invention falls within the range of 150 ° C to 250 ° C. Since the mixed silver powder contained in the silver paste has very high dispersibility and low impurity content, high productivity of a circuit board or the like having a high-quality low-resistance conductor can be secured.

Scanning electron microscope image of fine spherical silver powder. Scanning electron microscope image of ultrathin plate-like silver powder. Scanning electron microscope image of conventional flake silver powder. The figure showing the mixing concept of silver ammine complex aqueous solution and a reducing agent.

S1 silver ammine complex aqueous solution S2 additive a first channel b second channel m confluence

Claims (6)

In silver paste consisting of silver powder as filler, resin component and organic solvent,
The silver powder, have use a microsphere silver powder substantially spherical granular shape, a mixed silver powder obtained by mixing a directly formed very thin plate silver powder by a wet method,
The fine spherical silver powder is scanning the average particle diameter D _IA of the primary particles obtained by the image analysis of an electron microscope image is 0.6μm or less and an average particle diameter D _IA of the primary particles, a laser diffraction scattering particle size distribution measurement method The degree of aggregation represented by D ₅₀ / D _IA using an average particle diameter D _{50 of}
The ultrathin plate-like silver powder is a silver paste characterized in that an average primary particle thickness is 50 nm or less and an average particle diameter D ₅₀ by a laser diffraction scattering type particle size distribution measurement method is 4.0 μm or less . The fine spherical silver powder has the following A. And B. The silver paste according to claim 1, which has the following powder characteristics.
A. The specific surface area is 4.0 m ² / g or less.
B. The crystallite diameter is 10 nm or less. The silver paste according to claim 1 or ² , wherein the ultrathin plate-like silver powder has a specific surface area of 3.5 m ² / g or less . The mixing ratio between the small spherical silver powder and the electrode thin plate silver, based on the weight of the mixed silver powder, any one of claims 1 to 3 content of the electrode thin plate silver powder is 1 wt% 40 wt% Silver paste described in 1. The silver paste according to any one of claims 1 to 4 , wherein the resin component includes one or more selected from an epoxy resin, a polyester resin, a silicon resin, a urea resin, an acrylic resin, and a cellulose resin. The silver paste according to any one of claims 1 to 5 , wherein the silver powder content is 80 wt% to 93 wt%.