JP4159997B2 - Method for producing aluminum oxide-coated fine silver powder - Google Patents

Description

The present invention relates to a method for producing a fine silver powder is used as conductive powder, and more particularly to a method for producing a predominantly aluminum oxide-coated fine silver particles for silver paste for use in the electronic circuit board wiring.

  In recent years, small electronic devices such as mobile phones have become widespread. In order to manufacture such a small electronic device, a substrate for an electronic circuit using a low-temperature fired ceramic (LTCC) as a substrate material is often used. In order to arrange small electronic components such as a chip capacitor and a chip inductor on a ceramic multilayer substrate of a package using the LTCC (LTCC package), a wiring portion for an electronic circuit having a fine pitch is formed.

  In the LTCC package, it is necessary to improve the conductivity of the wiring part because of the above-mentioned demand for fine pitch, and fine silver powder is used as a raw material in the wiring part as follows.

  That is, first, a conductive paste (silver paste) obtained by kneading fine silver powder and an organic agent or the like is prepared. Next, using this paste, a wiring pattern is printed on a green sheet made of a ceramic material by a screen printing method or the like. Then, the conductive paste and the green sheet are fired at the same time in a firing furnace to manufacture an LTCC package in which a wiring portion is formed of a silver material.

  At this time, the following phenomenon occurs in the firing process of the LTCC package. Although the melting point of silver is 960 ° C., sintering of fine silver powder particles (powder particles) starts around 500 ° C. Along with this, thermal shrinkage of silver in the conductive paste begins. On the other hand, the LTCC ceramic material also thermally shrinks in a behavior different from that of silver. Therefore, because there is a difference in thermal shrinkage behavior between the silver of the fine silver powder and the ceramic material during the firing process, defects such as cracks and delamination are likely to occur in the electronic circuit board, and both heat shrinkage to prevent such defects There is a request to perform the firing process while adjusting the behavior of the.

  According to Patent Document 1 or 2, in order to meet the demand, a dry or wet process in which fine particles of silver powder are coated with an inorganic substance and the sintering (firing) temperature of the particles of fine silver powder is shifted to a higher temperature. Inorganic coating methods have been proposed.

  On the other hand, according to Patent Document 3, there is provided a method for producing a metal oxide film-coated powder, in which the powder is dispersed in a metal alkoxide solution, and the metal alkoxide is hydrolyzed to obtain powder particles (core material) surfaces. A method for forming a metal oxide film has been proposed.

JP 2001-240901 A JP 2001-335805 A Japanese Patent Laid-Open No. 10-67503

  However, Patent Documents 1 and 2 do not disclose a production method in which aluminum alkoxide is hydrolyzed and aluminum oxide is coated on the particles of the granular silver powder, and the granular silver powder obtained by the production method is not disclosed. Furthermore, in Patent Document 3, the method of forming a metal oxide film by hydrolyzing a metal alkoxide is merely to improve the characteristics of the magnetic toner, and sintering (firing) between metal (granular silver powder) particles. There is no disclosure or even suggestion of shifting the heat shrinkage temperature to the high temperature side.

Accordingly, an object of the present invention is to hydrolyze an aluminum alkoxide, which is a metal alkoxide, to coat fine silver powder particles with aluminum oxide, so that the conductive paste containing the fine aluminum powder coated with aluminum oxide is formed on the substrate. An object of the present invention is to provide a method for producing fine silver powder which does not cause the above-mentioned defects such as cracks and delamination when fired simultaneously with a ceramic material.

In such a situation, the present inventors hydrolyze an aluminum alkoxide compound in an organic solvent in a production method in which aluminum oxide (Al ₂ O ₃ ), which is an inorganic substance, is deposited on the surface of fine silver powder particles, and oxidize on the surface of fine silver powder particles. The inventors have earnestly studied suitable production conditions for the alumina-coated alkoxide method that can coat aluminum. As a result, the inventors have obtained the knowledge that the heat shrinkage start temperature of the aluminum oxide-coated fine silver powder produced by this method is greatly shifted to the high temperature side as compared with the heat shrinkage start temperature of the original powder, and the present invention has been completed. .

  The main reason why aluminum oxide is selected as the coating material in the present invention is because the silver paste containing aluminum oxide is primarily intended for ceramics such as LTCC that is fired at a high temperature. By using the material, the heat resistance is higher (the thermal decomposition temperature is higher). For example, the thermal decomposition temperatures of titanium oxide, zinc oxide, and tin oxide are 1640 ° C., 1725 ° C., and 1127 ° C., respectively, but the thermal decomposition temperature of aluminum oxide is 2015 ° C. It can be seen that the heat resistance is particularly excellent. Second, considering the compatibility of material properties with LTCC ceramic materials, it is superior in terms of affinity.

Hereinafter, the manufacturing method of the aluminum oxide covering fine silver powder concerning the present invention is explained.

<Method for producing aluminum oxide-coated fine silver powder>
The method for producing aluminum oxide-coated fine silver powder of the present invention includes the following steps a to e.
Step a. An alkoxide solution preparation step in which aluminum diisopropylate monosecondary butyrate is dissolved in isopropyl alcohol to prepare an aluminum alkoxide solution;
Step b. A slurrying step of adding fine silver powder to the aluminum alkoxide solution to make the solution into a slurry,
Step c. A coating step of performing hydrolysis reaction on the solution by adding water to the slurry solution and coating aluminum oxide on the fine silver powder particles,
Step d. A washing step of washing the fine silver powder coated with the aluminum oxide with acetone;
as well as,
Step e. A drying step of drying the aluminum oxide-coated fine silver powder washed with acetone.

According to the present invention, since the coating process is performed by a wet method (in a solvent), aluminum oxide can be uniformly coated (deposited) on the particle surface of the fine silver powder as compared with the dry method. In the dry method, the film thickness is not uniform, that is, for example, the coating film tends to be in the form of islands or scales, and aluminum oxide tends to cover the surface of the fine silver powder particles. Therefore, the aluminum oxide coating was easily peeled off when the particles were in contact with each other during kneading during the production of the conductive paste.

Thus, according to the dry method, there may be a portion where the aluminum oxide coating is peeled off, and the portion where the aluminum oxide is peeled off and exposed is silver itself as a core material. Therefore, under the influence of the peeling portion, as will be described later (see the thermal shrinkage curve of Comparative Example 2 (dry type) in FIG. 2), unlike the thermal shrinkage behavior of the wet process such as the present invention, In this case, the heat shrinkage starts from 400 ° C. and results in a heat shrinkage rate of 15% over 800 ° C., and defects such as cracks and delamination are likely to be caused when firing the ceramic substrate. Therefore, the aluminum oxide-coated fine silver powder obtained by the dry method is not practical as a fine silver powder for a conductive paste applied to a ceramic substrate used in an electronic circuit or the like.

However, according to the present invention, since the aluminum oxide is uniformly coated on the silver powder particles as a whole, the peeling phenomenon during the kneading can be prevented, and the heat shrinkage due to the uncoated portion as described above. There is no problem of increasing the starting temperature. Furthermore, according to the present invention, the specific resistance of the wiring portion formed when the conductive paste is baked due to an unnecessarily large amount of aluminum oxide and uneven coating on the surface of the silver dust particles. There is no significant decline.

  By the above-mentioned alumina coat alkoxide method, aluminum oxide-coated fine silver powder uniformly coated with aluminum oxide is produced.

  Further, in the method for producing the aluminum oxide-coated fine silver powder described above, the aluminum oxide-coated fine silver powder may be further sieved, and the particle size may be adjusted so that the particle sizes of the aluminum oxide-coated fine silver powder are appropriately matched.

Here, the silver powder used as the core material will be described. With respect to this silver powder, there is no particular limitation on the powder shape, the wet method of the powder and the manufacturing method of the dry method, etc., as long as the aluminum oxide fine silver powder having the powder characteristics described in this application can be obtained. Silver powder can be used.

Furthermore, it will be as follows when the suitable conditions of the manufacturing method of the said aluminum oxide covering fine grain silver powder are described.
Step a. An alkoxide solution preparation step in which 0.3 to 5.0 g of aluminum diisopropylate monosecondary butyrate is dissolved in 1000 mL of isopropyl alcohol to prepare an aluminum alkoxide solution. In this step a, aluminum diisopropylate monosecondary butyrate Since the specific resistance at the time of baking becomes low when it is 0.3-3.0 g, this range is suitable.
Step b. A slurrying step of adding 100 g of fine silver powder to the aluminum alkoxide solution to make the solution into a slurry,
Step c. A coating step of adding 5 g of water to the slurry solution and subjecting the solution to hydrolysis reaction at 30 ° C. for 60 minutes to coat aluminum oxide on fine silver powder particles (note that water is limited to 5 g) If the amount of water is less than 1 g, the hydrolysis reaction is unlikely to occur, whereas if the amount of water is more than 10 g, moisture is likely to remain in the fine silver powder and agglomeration is likely to occur. The reason why the reaction temperature during the hydrolysis is 30 ° C. is that if it is less than 20 ° C., the reaction rate becomes slow and the cooling cost increases, while heating is required to increase the reaction rate above 40 ° C. The reaction time is set to 60 minutes because an unreacted content may remain if the reaction time is shorter than 30 minutes. Reacting with a long as is because sufficiently complete to be wasteful heating regardless of which is performed productivity is deteriorated.)
Step d. A washing step of washing the fine silver powder coated with the aluminum oxide with acetone;
as well as,
Step e. A drying step of drying the aluminum oxide-coated fine silver powder washed with acetone.

  According to this manufacturing method, the aluminum oxide-coated fine silver powder uniformly coated with aluminum oxide is produced under more favorable conditions.

Hereinafter, the aluminum oxide-coated fine silver powder obtained by the method for producing an aluminum oxide-coated fine silver powder according to the present invention will be described.

<Aluminum oxide coated fine silver powder>
The aluminum oxide-coated fine silver powder obtained by the present invention is a fine silver powder in which aluminum oxide is coated on the surface of the fine silver powder by hydrolyzing an aluminum alkoxide compound (the “fine silver powder” particles in this case have a primary particle size of 0). .Mu.m order silver particles in the order of 50 to 3.0 .mu.m) having the following powder characteristics.
a. Specific surface area (m ² / G) is 0.5 to 4.0
b.D ₅₀ (Μm) is 0.5 to 10.0
c.D _max (Μm) is 1.0 to 30.0

When aluminum oxide-coated fine silver powder of alumina-coated alkoxide method with the above powder characteristics is measured using a thermomechanical analyzer (TMA), thermal shrinkage starts at a temperature higher than the thermal shrinkage start temperature of uncoated silver powder. The heat shrinkage rate is 0 to 5% from the heat shrinkage start temperature to 800 ° C. That is, since the heat shrinkage rate is only 5% at most up to 800 ° C., when the wiring portion is formed on the electronic circuit board with the conductive paste containing the aluminum oxide coated fine silver powder of the present invention, the aluminum oxide coating is applied. Compared with fine silver powder that has not been made, defects such as cracks and delamination are less likely to occur during substrate firing.

Here, the powder characteristics will be described in any order. First, the specific surface area of the aluminum oxide coated fine silver powder is 0.5 m. ² /G-4.0m ² The target is in the range of / g. D ₅₀ However, it is intended for those in the range of 0.5 μm to 10.0 μm. Furthermore, D _max However, it is intended for those in the range of 1.0 μm to 30.0 μm.

That is, the specific surface area is 4.0 m. ² Greater than / g, D ₅₀ Is smaller than 0.5 μm and D _max In the case of fine silver powder having a particle size smaller than 1.0 μm, the amount of oil absorbed from the paste material tends to be large, making it difficult to form a paste.

On the other hand, D ₅₀ Is greater than 10.0 μm and D _max In the case of a fine silver powder having a particle size larger than 30.0 μm, aggregation is strong at the time of producing a conductive paste, making it difficult to form a paste and making it difficult to form fine pitches in electronic circuit wiring. The specific surface area is 0.5m ² Silver powder smaller than / g tends to start from a relatively high temperature because the primary particle size is large, and there is not much need for coating with aluminum oxide or the like.

The aluminum oxide-coated fine silver powder produced according to the present invention has a delamination or crack in the electronic circuit board when a conductive paste containing the fine silver powder is applied to the wiring part on the electronic circuit board and fired simultaneously with the ceramic of the circuit board. It is possible to prevent problems such as these. In addition, since the coating method by a wet method is employ | adopted, an inorganic material can be uniformly coat | covered on the whole particle | grain surface irrespective of the particle | grain shape of fine silver powder. Therefore, particularly when the conductive paste containing fine silver powder is produced when the particle shape is substantially spherical, there is also a secondary effect that the aluminum oxide coating is difficult to peel off even if kneaded with a strong force by a kneader or the like.

  Hereinafter, the best embodiment of the present invention will be described in detail through Examples 1, 2, 3, and 4 while comparing with Comparative Examples 1 and 2. In addition, this invention is not limited to the following Example.

  Hereinafter, with reference to FIG. 1, the manufacturing method of the 1st aluminum oxide coating | cover fine grain silver powder by which the coating of aluminum oxide is given by the wet method is demonstrated.

  As shown in FIG. 1, the manufacturing method according to Example 1 includes a process a (dissolution process), a process b (slurry process), a process c (hydrolysis process), a process d (cleaning process), and a process e. (Drying process) is included. In order to adjust the particle size of the fine silver powder after drying, a sieving step is added. By these steps, an aluminum oxide-coated fine silver powder whose surface is coated with aluminum oxide is obtained.

In more detail,
Step a: 3.5 g of aluminum diisopropylate monosecondary butyrate (in the case of Example 1) was dissolved in 1000 ml of isopropyl alcohol (that is, the concentration was 3.5 g / L. Examples 2, 3, and 4) In this case, the concentration of the solution was 1.8 g / L, 0.35 g / L, and 0.47 g / L, respectively.)
Step b: 100 g of fine silver powder was added to this solution (100 g / L), and the fine silver powder and the solution were mixed to form a slurry while stirring so that the fine silver powder was dispersed throughout the solution.
Step c: While maintaining the slurry state, 5 g of water was added to the slurry solution, stirred, and aged at 30 ° C. for 60 minutes for hydrolysis reaction. “Water” should be “pure water”.
Step d: Finally, the fine silver powder coated with aluminum oxide was washed with several tens mL of acetone.
Step e. Acetone was evaporated to dry the fine silver powder.
By the above production method, the aluminum oxide-coated fine silver powder of the present invention was obtained.

  In addition, in this manufacturing method, since an organic solvent is basically used for the solution, the sodium aluminate is coated by neutralization as disclosed in Patent Document 1 described above, and in the process like the water-based wet coating method. Since there is no need for water washing or dehydration, the cost of water is not required, and the total time from the start to the end of the process can be shortened.

  The production method according to Example 2 is basically the same as the production method according to Example 1, except that the input weight of aluminum diisopropylate monosecondary butyrate in step a is 1.8 g / L. It is.

  The production method according to Example 3 is basically the same as the production method according to Example 1, except that the input weight of aluminum diisopropylate monosecondary butyrate in step a is 0.35 g / L. It is.

  The production method according to Example 4 is basically the same as the production method according to Example 1, except that the input weight of aluminum diisopropylate monosecondary butyrate in step a is 0.47 g / L. It is.

  Next, with reference to Table 1, various powder characteristics of the aluminum oxide-coated fine silver powder obtained by the above production method will be described.

  Table 1 shows the results of various powder characteristic values of the aluminum oxide-coated fine silver powders obtained in Examples 1 to 4 (wet method) and various powder characteristic values of Comparative Examples 1 and 2. The column of Comparative Example 1 shows the powder characteristics of the charged granular silver powder, and the column of Comparative Example 2 shows the aluminum oxide obtained by mechanochemical dry method. The powder properties of the coated granular silver powder are shown. The production method of the dry method is to put uncoated silver powder and aluminum oxide into a hybridizer (manufactured by Nara Machinery Co., Ltd.) in order to obtain aluminum oxide-coated fine silver powder by fixing aluminum oxide to uncoated silver powder, and at 8000 rpm. Circulation was performed for 5 minutes.

Specifically, in order from the left, specific surface area (SSA), particle size (D ₅₀ ) at 50% cumulative volume obtained using laser diffraction scattering particle size measurement method, maximum particle size (D _max ), aluminum oxide coating Rate (%), and thermal shrinkage rate (%) at 800 ° C. by thermomechanical analysis (TMA) (see FIG. 2).

SSA (m ² / g) is a surface area per unit weight of silver powder, and was measured by a known method (BET method).

D ₅₀ and D _max (μm) are particle diameters when the volume accumulation obtained by using the laser diffraction scattering particle size measurement method reaches 50% and the maximum, and this D ₅₀ and D _max (μm). The value of) is not a value obtained by directly observing the diameter of each particle, but the particle diameter is calculated by regarding the aggregated particles as one particle (aggregated particle).

  The amount of aluminum oxide coating (%) is obtained by dissolving the obtained aluminum oxide-coated powdered silver powder in a nitric acid solution by heating, adding hydrogen fluoride and boric acid, further heating, cooling to room temperature, and diluting the solution. By analyzing with an ICP analyzer such as ICP8000E, the content of aluminum oxide in the fine silver powder was determined.

  The above powder characteristic values are summarized in Table 1 for each of the characteristic values of Examples 1 to 4 and Comparative Examples 1 and 2.

  FIG. 2 shows the thermal contraction rate of the aluminum oxide-coated fine silver powder according to Examples 1 to 4 and the uncoated silver powder as the base powder according to Comparative Example 1 and the aluminum oxide-coated fine silver powder obtained by the dry method according to Comparative Example 2 ( %). The result of this experiment was that a pressure of 98 MPa was applied to 0.5 g of each silver powder to form a pellet having a diameter of 5 mm and a height of about 5 mm, and this pellet was heated in a nitrogen gas atmosphere using a thermomechanical analysis (TMA) apparatus. It was obtained by heating to 900 ° C. at 20 ° C./min and measuring the heat shrinkage rate (%).

<Comparison of thermal shrinkage of Examples 1 to 4 and Comparative Examples 1 and 2>
(1) According to the graph of thermal contraction rate by TMA shown in FIG. 2, the thermal contraction according to Examples 1 to 4 exhibits a contraction rate of 0 to 5% from the thermal contraction start temperature to 800 ° C. . On the other hand, the thermal contraction according to Comparative Example 1 (original powder (uncoated silver powder)) starts abruptly from the thermal contraction start temperature (400 ° C.) and exhibits a thermal contraction rate of about 16% up to 800 ° C. From this result, it can be clearly seen that the coating of aluminum oxide formed on the surface of the fine silver powder particles by the alumina coat alkoxide method significantly contributes to the suppression of heat shrinkage of silver.

(2) Further, according to the same graph shown in FIG. 2, when the heat shrinkage curve of Comparative Example 2 by the dry method and the heat shrinkage curve of Comparative Example 1 are compared, the aluminum oxide-coated fine silver powder prepared by the dry method is Although the heat shrinkage rate is improved to about 4% from the heat starting temperature as a whole compared to the original powder, the aluminum oxide-coated fine silver powder by the alumina coat alkoxide method according to Examples 1 to 4 is significantly more heat shrinkable. It can be seen that the rate has not improved.

(3) Further, according to the heat shrinkage curves according to Examples 1 to 4 of the same graph shown in FIG. 2, the heat shrinkage start temperature shifts to a higher temperature side as the aluminum oxide coverage increases, and the heat shrinkage is higher. You can see that it starts with. However, if the aluminum oxide coverage on the particle surface of the fine silver powder increases, the resistivity decreases when the wiring of the electronic circuit is formed using a conductive paste containing the aluminum oxide-coated fine silver powder. Therefore, when determining a suitable aluminum oxide coverage, it is needless to say that the specific resistivity should not be made smaller than a desired value.

Method for producing a silver powder according to the present invention can be used in the conductive powder and a manufacturing method thereof are mixed into the conductive paste for wiring patterns of ceramic electronic circuit board.

The flowchart which concerns on the alumina coat | court alkoxide method for producing the aluminum oxide coating | coated silver powder which concerns on Examples 1-4 is shown. The heat contraction curve which concerns on Examples 1-4 and the aluminum oxide coating | coated silver powder which concerns on the comparative example 1 and the comparative example 2 is shown.

Claims (3)

The manufacturing method of the aluminum oxide covering fine silver powder including the process a-the process e shown below.
Step a. An alkoxide solution preparation step in which aluminum diisopropylate monosecondary butyrate is dissolved in isopropyl alcohol to prepare an aluminum alkoxide solution;
Step b. A slurrying step of adding fine silver powder to the aluminum alkoxide solution to make the solution into a slurry,
Step c. A coating step of performing hydrolysis reaction on the solution by adding water to the slurry solution and coating aluminum oxide on the fine silver powder particles,
Step d. A washing step of washing the fine silver powder coated with the aluminum oxide with acetone;
as well as,
Step e. A drying step of drying the aluminum oxide-coated fine silver powder washed with acetone. The manufacturing method of the aluminum oxide covering fine silver powder including the process a-the process e shown below.
Step a. An alkoxide solution preparation step of dissolving 0.3 to 5.0 g of aluminum diisopropylate monosecondary butyrate in 1000 mL of isopropyl alcohol to prepare an aluminum alkoxide solution;
Step b. A slurrying step of adding 100 g of fine silver powder to the aluminum alkoxide solution to make the solution into a slurry,
Step c. A coating step in which 1 to 10 g of water is added to the slurry solution to perform a hydrolysis reaction for 30 to 90 minutes at a temperature of 20 to 40 ° C., and to coat aluminum oxide on fine silver powder particles;
Step d. A washing step of washing the fine silver powder coated with the aluminum oxide with acetone;
as well as,
Step e. A drying step of drying the aluminum oxide-coated fine silver powder washed with acetone. A method for producing the aluminum oxide-coated fine silver powder according to claim 2,
The production method according to claim 2 , wherein the amount of aluminum diisopropylate monosecondary butyrate dissolved in step a is 0.3 to 3.0 g.

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