JP2005149913A - Silver paste and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide silver paste for forming a sintering conductor excellent in low temperature sintering characteristics. <P>SOLUTION: On the silver paste consisting of silver powder as a filler, an organic agent and a solvent, the silver powder with an average particle diameter D<SB>IA</SB>of primary particles obtained by image analysis of a scanning electron microscope image of 0.6 μm or less, is adopted. In manufacturing the silver paste, the silver powder and the organic solvent are put under distributed processing with the use of a wet type dispersing machine to obtain the silver paste. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The invention according to the present application relates to a silver paste and a method for producing the 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: Metal powder such as silver powder is generally said to be 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 to the particles 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.

  In addition, when conventional silver powder and flake silver powder are 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.

  As a result, it has been desired that a fine pitch circuit can be formed on a silver paste containing silver powder in the market, and that low-temperature sinterability can be provided. In particular, in order to accurately draw a fine circuit using a screen printing method, it has been desired that the silver powder contained in the silver paste is fine and highly dispersed. 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 “primary particles obtained by image analysis of a scanning electron microscope image, in which the silver powder is composed of a silver powder as a filler, a resin component, and an organic solvent, and the powder particles of the silver powder are substantially spherical. a silver paste. ", wherein the average particle diameter D _IA of is 0.6μm or less.

Silver powder constituting paste: Silver powder is required to have a substantially spherical shape. For example, if the shape of the particle is an irregular shape such as flake, the plate is likely to be clogged no matter how good the powder characteristics are.

Then, then the average particle diameter D _IA of the primary particles obtained by image analysis of a scanning electron microscope image is being asked is powder characteristics hereinafter referred 0.6 .mu.m. 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. Here, “average particle diameter D _IA of primary particles obtained by image analysis of scanning electron microscope image” means an observation image of silver powder (fine particles according to the present invention) observed using a scanning electron microscope (SEM) In the case of 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, image analysis of fine silver powder observed using a scanning electron microscope (SEM) in the present specification uses an IP-1000PC manufactured by Asahi Engineering Co., Ltd. A circular particle analysis is performed to obtain an average particle diameter _DIA . Since the average particle diameter _DIA obtained by image processing the observation image of the fine silver powder is obtained directly from the SEM observation image, the average particle diameter of the primary particles is surely captured. D _IA of fine silver powder referred to in the present invention, mostly in the range of 0.01μm~0.6μm unless observing present inventors although incoming reality that can be confirmed even more of fine particle size There is also a lower limit that is not clearly stated.

  Furthermore, the silver powder is preferably a fine silver powder with 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. Since the silver powder used in the present invention is finer than conventional silver powder and exhibits high dispersibility, the “aggregation degree” is used 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. D ₅₀ of the fine silver powder used in the present invention is in the range of about 0.25 μm to 0.80 μm, and becomes a fine silver powder having an average particle diameter D ₅₀ in a range that could not be obtained at all by the conventional production method. In this specification, the laser diffraction / scattering particle size distribution measuring method is measured using LS-230 manufactured by Beckman Coulter, Inc. with a refractive index of 1.15.

On the other hand, “average particle diameter D _IA of primary particles obtained by image analysis of scanning electron microscope image” is an image analysis of an observation image of silver powder observed using a scanning electron microscope (SEM) as described above. This is the average particle size obtained by doing so, 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 fine silver powder of the same lot, the value of D ₅₀ that reflects the presence of an agglomerated state in the measured value is considered in the above theory. Is considered to be larger than the value of _DIA . At this time, the value of D ₅₀ approaches the value of D _IA as much as the aggregation state of the fine silver powder particles disappears, and the value of D ₅₀ / D _{IA as} the degree of aggregation approaches 1. Become. It can be said that it is a monodisperse powder in which the aggregation state of powder particles is completely absent when the aggregation degree becomes 1.

  The inventors of the present invention have investigated the correlation between the degree of aggregation and the viscosity of the fine silver paste produced using fine silver powder of each degree of aggregation, 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 silver powder is controlled, the viscosity of the silver paste produced using the fine 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.

  b. The crystallite diameter 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 it is fine like the fine silver powder according to the present invention, it has a large surface energy and a small crystallite diameter of 10 nm or less, so that 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 high reliability for the measurement value in the range where the crystallite diameter is less than 10 nm. If the lower limit value is deliberately determined, it is about 2 nm obtained as a result of the present inventors' research. I think there is.

  The silver paste using the fine silver powder having the above-mentioned powder characteristics has a low-temperature sinterability that allows sintering at a temperature of 250 ° C. or lower, 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.

Resin component constituting paste: Originally, the resin component is not particularly limited. However, on the premise that fine silver powder as described above is used as a filler, a composition capable of ensuring good dispersibility of fine silver powder must be employed. In addition, the silver paste according to the present invention is intended to ensure low-temperature sinterability and form a fine pitch circuit, and the solvent can be removed at a temperature lower than the employed low-temperature sintering temperature. It is necessary to adopt a resin composition that can ensure paste performance suitable for the above.

  In view of these matters, it is desirable to employ 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) A silicon 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 silver powder particles can be prevented.

Blending balance between silver powder and paste: Furthermore, unless the blending balance between the silver powder and the organic agent in the silver paste according to the present invention is 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 silver powder content is 80 wt% or more, more preferably in the range of 80 wt% to 93 wt%. . When the silver powder content 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 silver powder exceeds 93 wt%, a silver paste viscosity will rise rapidly and it will thicken to the level which is hard to use to form a fine pitch circuit shape etc.

<Method for producing 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 fine 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. Accordingly, it is preferable to use fine silver powder obtained by the method described below.

Silver powder production method 1: The present inventors mixed and reacted a conventional silver nitrate aqueous solution and aqueous ammonia to obtain a silver ammine complex aqueous solution. By adding a reducing agent to this, silver particles were reduced and precipitated, and filtered. Based on the manufacturing method of washing and drying, the inventors devised the manufacturing method to obtain fine silver powder at a level that cannot be obtained by the conventional manufacturing method.

  This 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, washed and dried. In the method for producing silver powder, the feature is that the amount of reducing agent, the amount of silver nitrate, and the amount of aqueous ammonia, which are diluted after addition, are used. Conventionally, the reducing agent solution and the silver ammine complex aqueous solution are generally mixed together in a tank. Therefore, in order to generally set the silver concentration to 10 g / l or more, a large amount of silver nitrate, If the amount of reducing agent and the amount of aqueous ammonia were not added, productivity for the scale of the equipment could not be ensured.

  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 silver powder can be obtained, but it becomes too fine and the oil absorption increases, leading to an increase in paste viscosity. It is necessary to increase the amount, 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 the most suitable condition for obtaining fine silver powder with good yield. It becomes. Here, the organic reducing agent concentration of 1 g / l to 3 g / l 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 the most suitable reaction rate for obtaining low crystalline silver powder having a small crystallite size.

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

Then, a method of precipitating the above manner was silver ammine complex solution thus obtained fine silver powder and a reducing agent are contacted reacting reduction, in the present invention, as shown in FIG. 1, a silver ammine complex solution S ₁ is flow 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. and flowing the additive S ₂ as needed in the first flow path a, in contact mixed at the merging point m between the first flow path a and the second channel b, a method of reducing precipitation of silver particles (hereinafter, this It is desirable to adopt a method called “joint 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 adsorbed and retained on the surface of the fine silver powder to be reduced and precipitated is reduced. As a result, it becomes possible to reduce the amount of impurities deposited on the fine silver powder obtained by filtration and drying. By reducing the amount of impurities deposited on the fine silver powder, the electrical 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 fine-grained silver powder can be obtained.

Silver powder production method 2: In the silver powder production method 1 described above, the regular method is adopted without any particular restriction regarding the washing method of the obtained fine silver powder. However, by adding a contrivance to the silver powder cleaning method obtained by the silver powder production method 1, the amount of impurities remaining on the surface of the silver powder particles is further reduced, and the circuit conductor formed with the silver paste using the fine silver powder This contributes to lowering the resistance.

  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, in the case of 40 g of finely precipitated fine silver powder, 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 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 when the reaction solution between the silver ammine complex aqueous solution and the reducing agent in obtaining fine silver powder is used 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.

The silver powder obtained by the silver powder production 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 fine silver powder of 0.6μm or less, yet good dispersibility Therefore, it is easy to adjust the silver powder content when the paste is processed, and it becomes easy to make a fine pitch in a circuit drawn using the silver paste. Moreover, the 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.

  In the silver paste according to the present invention, the encapsulated silver powder is an unprecedented fine and highly dispersed silver powder, so that it has excellent low-temperature sintering performance, and is fine and exhibits excellent dispersibility in an organic agent. It is optimal for forming fine pitch circuits. In addition, since the 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.

  Further, by using the method for producing a silver paste according to the present invention, a silver paste containing uniformly monodispersed silver particles in an organic agent can be obtained efficiently, and silver suitable for forming a sintered conductor. This makes it possible to supply the paste at a low cost to the market.

  Hereinafter, the best mode of the present invention will be described in detail through examples while comparing with comparative examples.

Production of silver powder: First, fine silver powder used for 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% aqueous ammonia 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. 1 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 silver powder was reduced and precipitated by contacting the mixture at a temperature of 0 ° 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 silver powder obtained as described above, it is filtered using 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. A fine silver powder was obtained. FIG. 2 shows a scanning electron micrograph of the obtained fine silver powder. The average particle diameter D _IA is 0.3μm of this silver powder, a particle size distribution was measured using employ 1.15 to refractive index laser diffraction scattering particle size distribution analyzer LS-230 (manufactured by Beckman Coulter, Inc.) _{where, D 10} is _0.283Myuemu, a _{D 50} of _{0.371μm, D 90} is 0.491μm and _{D max} is 0.791Myuemu, standard deviation was 0.06 .mu.m. Furthermore, the degree of aggregation represented by D ₅₀ / D _IA was 1.24, the crystallite diameter was 7 nm, and the carbon content was 0.21 wt%. FIG. 3 shows a scanning electron micrograph of the silver powder obtained by the conventional manufacturing method. By comparison with FIG. 2, it is understood that the silver powder used in the present invention is extremely fine. It can be done.

  Incidentally, the X-ray diffraction method was used for the measurement of the crystallite diameter. The carbon content is used as a measure of the amount of impurities adhering to the silver powder particles. Using EMIA-320V manufactured by Horiba, fine silver powder 0.5 g, tungsten powder 1.5 g, tin powder 0.3 g was mixed, put in a magnetic crucible, and measured by a combustion-infrared absorption method.

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 in a paddle kneader for 5 minutes, and then 170 g of the fine silver powder was added. The mixture was further kneaded for 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 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 1.1 × 10 ⁻³ Ω · cm.

Manufacture of silver powder : Since manufacture of silver powder here is common with Example 1, in order to avoid duplication description, description here is abbreviate | omitted.

Production of silver paste: 12.0 g of bisphenol F type epoxy resin (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 kneader for 5 minutes, and then 170 g of the fine silver powder was added. The mixture was further kneaded for 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 B was obtained.

  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 5.7 × 10 ⁻⁴ Ω · cm.

Manufacture of silver powder : Since manufacture of silver powder here is common with Example 1, in order to avoid duplication description, description here is abbreviate | omitted.

Production of silver paste: 1.5 g of ethyl cellulose ( manufactured by Wako Pure Chemical Industries), 28.5 g of α-terpineol (manufactured by Yasuhara Chemical) as a viscosity modifier and 170 g of the fine silver powder were kneaded for 10 minutes in a paddle kneader.

  After the kneaded material thus obtained was continuously kneaded with three rolls, bubbles contained in the kneaded material were removed using a defoaming machine (manufactured by Shinky: AR-250) to obtain a silver paste C. It was. When this silver paste C 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, 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 C 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 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 2.5 × 10 ⁻⁵ Ω · cm.

Manufacture of silver powder : Since manufacture of silver powder here is common with Example 1, in order to avoid duplication description, description here is abbreviate | omitted.

Production of silver paste: 0.6 g of acrylic acid ester polymer (manufactured by Nippon Pure Chemicals Co., Ltd .: Jurimer AT-510), 0.6 g of oxazoline group-containing polymer (manufactured by Nippon Shokubai Co., Ltd .: Epocros WS-500) and α-terpineol (Yasuhara Chemical Co., Ltd.) 28.7 g and 170 g of the above silver powder were kneaded for 10 minutes with a paddle type kneader to obtain silver paste D.

  The silver paste D obtained in this way was printed on an alumina substrate with a wiring width of 50 μm and a wiring-to-wiring spacing of 50 μm using a screen printing machine, and showed good printability without wiring disconnection or blurring. It was. 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.

Further, using a screen printer, a silver paste D was printed on a alumina substrate as a sample for measuring specific resistance 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 LP), 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 6.6 × 10 ⁻⁵ Ω · cm.

Comparative Example 1

Production of silver powder: In this comparative example, the silver powder was changed to “3050HDP”, which is a flaky silver powder manufactured by Mitsui Mining & Smelting Co., Ltd. The powder characteristics of this 3050HDP are as follows. The average particle diameter D _IA is 2.01μm of this silver powder, a particle size distribution was measured using employ 1.15 to refractive index laser diffraction scattering particle size distribution analyzer LS-230 (manufactured by Pekkuman Coulter, Inc.) However, D ₁₀ is 1.46 μm, D ₅₀ is 2.25 μm, D ₉₀ is 2.25 μm, D _max is 5.61 μm, the degree of aggregation represented by D ₅₀ / D _IA is 1.12, and the crystallite The diameter was 8 nm and the carbon content was 0.30 wt%.

Production of silver paste: Silver paste E was obtained in the same manner as in Example 1 using the above silver powder. When the silver paste E thus obtained was printed on an alumina substrate with a wiring width of 50 μm and a wiring gap of 50 μm using a screen printing machine, the wiring was disconnected. And as a result of observing the plate used in the screen printing machine with a microscope, the flaky silver powder clogged with the silver powder even though it had good powder characteristics, and this is considered to be the cause of disconnection became. Therefore, the specific resistance of the circuit was not measured.

  The silver paste according to the present invention can effectively utilize the low-temperature sintering characteristics of the encapsulated fine silver powder without causing a decrease in the film density of the conductor after sintering and an increase in the impurity content. 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 silver powder contained in the silver paste has a very high dispersibility and a low impurity content, high productivity of a circuit board or the like having a high-quality low-resistance conductor can be secured.

  In addition, by using the method for producing a silver paste according to the present invention, a silver paste excellent in dispersibility of silver particles in an organic agent can be efficiently obtained, and a silver paste for forming a high-quality sintered conductor is obtained. Can be produced stably and can be supplied to the market at a low cost.

The figure showing the mixing concept of silver ammine complex aqueous solution and a reducing agent. The scanning electron microscope image of the fine silver powder concerning this invention. The scanning electron microscope image of the fine silver powder concerning the conventional manufacturing method.

Explanation of symbols

S ₁ silver ammine complex aqueous solution S ₂ additive a 1st flow path b 2nd flow path m Confluence

Claims (4)

In silver paste consisting of silver powder as filler, resin component and organic solvent,
The particulate silver powder is substantially spherical, a silver paste having an average particle diameter D _IA of the primary particles obtained by image analysis of a scanning electron microscope image is characterized by using those 0.6μm or less. 2. The resin component includes one or more selected from an epoxy resin, a polyester resin, a polyamideimide resin, a urethane resin, a silicon resin, a phenol resin, a naphthalene resin, a urea resin, an alkyd resin, a furan resin, and a cellulose resin. Silver paste described in 1. The silver paste according to claim 1 or 2, wherein the silver powder content is 80 wt% to 93 wt%. The silver powder is a fine silver powder having low cohesiveness. And b. The silver paste for conductor formation in any one of Claims 1-3 using what was provided with the powder characteristic of this.
a. The degree of aggregation 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 is 1.5 or less.
b. The crystallite diameter is 10 nm or less.