Patent Grant
12100530
The present invention relates to a baked silver paste that contains a silver powder, and is used for forming an internal electrode or an external electrode included in a ceramic electronic component such as a multilayer ceramic capacitor, an inductor, or an actuator.
An electrically conductive paste containing a metal powder to be used for forming an internal electrode or an external electrode of an electronic component has been conventionally widely used. One of the reasons is that an electrically conductive paste containing a metal powder is applicable to various printing methods such as a screen printing method, an offset printing method, a gravure printing method, an inkjet method, a dipping method, a dispensing method, a brush coating method, and a spin coating method, and can form a thick film by performing coating printing once, and hence is advantageous for obtaining a high electrical conductivity as compared with, for example, an electrically conductive ink using an organic metal compound, or the like.
Besides, in order to obtain a high electrical conductivity, a content of the metal powder contained in the electrically conductive paste is preferably higher, and a coating film obtained by printing the paste is required to be dense and have a high density. When the content of the metal powder is high, definite conductivity can be obtained even in pattern formation that is further refined in the future.
For example, Patent Literature 1 (Japanese Patent Laid-Open No. 2005-174824) discloses the following invention: A metal colloidal particle is used instead of a metal powder for obtaining a highly dense and a highly conductive film, and an organic metal compound is used together to fill a gap between the metal colloidal particles with the organic metal compound, and thus, denseness is increased.
The metal colloidal particle and the organic metal compound contain a large amount of organic components, however, and therefore, if these are used as principal conductive components, a content of a metal component is lower than in a paste using a metal powder, and hence a conductor film with a low specific resistance cannot be obtained. In addition, it is difficult to apply to the above-described various printing methods, and if a large amount of a binder resin, such as a viscosity modifier or the like, is added to the paste for solving this problem, a metal ratio in a resultant coating film is further reduced.
Meanwhile, when a metal powder is used as an electrically conductive component of a paste, a conductor film having a low specific resistance can be obtained if the content of the metal powder is high, but printability is deteriorated as the content of the metal powder is higher. Therefore, in a silver paste, for example, a coating film density described in Patent Literature 2 is 5.4 g/cm3 at the most, and a coating film density described in Patent Literature 3 is about 5.70 g/cm3 at the most. Besides, Patent Literature 4 discloses an example of a nickel paste having a dry film density of 6.2 g/cm3.
In an electrically conductive paste, when a dense coating film is obtained for attaining a high electrical conductivity, printability is sacrificed as in these techniques, and thus, there is a trade-off (antinomic) relationship therebetween. Therefore, there is a demand for an electrically conductive paste capable of simultaneously attaining dense coating film formation and good printability.
There are known cases where two types of silver powders having different particle sizes, that is large and small sizes, are used for obtaining a dense coating film (Patent Literature 5 and Patent Literature 6). However, in particular, in a silver paste using a silver powder, a phenomenon designated as migration is known, and as the amount of the silver powder having a small size (of, for example, less than 0.5 μm) contained in the silver paste is larger, migration more easily occurs.
In order to inhibit the migration, various attempts have been conventionally made by addition of a migration inhibitor containing fluorine to a silver paste (Patent Literature 7), and addition of a mixed powder, an alloy powder, a compound powder or the like containing three elements of copper, tin and manganese to a silver powder (Patent Literature 8).
It is, however, not preferable for obtaining high electrical conductivity to add a large amount of a component different from a silver powder to a paste as described above for purpose of preventing migration.
Accordingly, an object of the present invention is to solve these problems. Specifically, an object of the present invention is to provide a silver paste that contains a silver powder in a high concentration and is excellent in printability, and to accordingly provide a silver conductor film that has a high filling factor and a high film density, exhibits high electrical conductivity, and is excellent in migration resistance.
As a result of earnest studies made for solving the above-described problems, the present inventors found the following: Even when a silver powder is contained in a paste in a high concentration, a dense dry film can be obtained without sacrificing printability by adjusting, respectively to specific ranges, [1] a content of the silver powder in the silver paste, [2] D10 and D50 of the silver powder, [3] a specific surface area of the silver powder, [4] a copper content of the silver powder, [5] a content of a binder resin in the silver powder, and [6] a dry film density of the silver paste. In addition, migration can be effectively inhibited merely by causing a minimum necessary amount of a copper component to be contained in the silver powder. Thus, the present invention was accomplished.
Specifically, a present invention (1) provides a silver paste, containing at least a silver powder, a binder resin, and an organic solvent, in which all of the following conditions [1] to [6] are satisfied: [1] a content CAG of the silver powder based on the silver paste is 80.00 to 97.00% by mass; [2] D10 is 1.00 to 3.00 μm and the D50 is 3.00 to 7.00 μm, where D10 and D50 respectively represent a 10% value and a 50% value of a volume-based cumulative fraction obtained by laser diffraction particle size distribution measurement of the silver powder; [3] the silver powder has a specific surface area SBET of 0.10 to 0.30 m2/g; [4] the silver powder has a copper content of 10 to 5000 ppm by mass; [5] a content CBND of the binder resin based on the silver powder is 0.430 to 0.750% by mass; and [6] the silver paste has a dry film density of 7.50 g/cm3 or more.
A present invention (2) provides the silver paste according to (1), in which the content CAG of the silver powder based on the silver paste is 92.00 to 96.00% by mass.
A present invention (3) provides the silver paste according to (1) or (2), further satisfying the following condition [7]: [7] the silver powder is a spherical powder having an aspect ratio of 1.0 to 1.3.
A present invention (4) provides the silver paste according to any one of (1) to (3), in which the dry film density is 7.60 g/cm3 or more.
A present invention (5) provides the silver paste according to any one of (1) to (4), in which the copper content of the silver powder is 30 to 500 ppm by mass.
A present invention (6) provides the silver paste according to any one of (1) to (5), further satisfying the following condition [8]: [8] the content CBND of the binder resin based on the silver powder is 0.440 to 0.600% by mass.
A present invention (7) provides the silver paste according to any one of (1) to (6), in which the D10 is 1.20 to 2.00 μm.
A present invention (8) provides the silver paste according to any one of (1) to (7), in which the D50 is 3.90 to 5.00 μm.
A present invention (9) provides the silver paste according to any one of (1) to (8), to be used for forming a conductor film by a heat treatment at 700° C. or less.
The present invention can provide a silver paste that contains a silver powder in a high concentration and is excellent in printability, and accordingly provide a silver conductor film that has a high filling factor and a high film density, exhibits high electrical conductivity, and is excellent in migration resistance.
A silver paste of the present invention is a silver paste containing at least a silver powder, a binder resin, and an organic solvent, in which [1] a content CAG of the silver powder based on the silver paste is 80 to 97% by mass, [2] D10 is 1.00 to 3.00 μm and D50 is 3.00 to 7.00 μm, where D10 and D50 respectively represent a 10% value and a 50% value of a volume-based cumulative fraction obtained by laser diffraction particle size distribution measurement of the silver powder, [3] the silver powder has a specific surface area SBET of 0.10 to 0.30 m2/g, [4] the silver powder has a copper content of 10 to 5000 ppm by mass, [5] a content CBND of the binder resin based on the silver powder is 0.430 to 0.750% by mass, and [6] the silver paste has a dry film density of 7.50 g/cm3 or more.
The silver paste of the present invention contains at least a silver powder, a binder resin, and an organic solvent.
The silver powder used in the silver paste of the present invention, namely, the silver powder contained in the silver paste of the present invention, is not especially limited in the shape, the particle size, the production method, and the like as long as requirements described below can be met.
The copper content of the silver powder used in the silver paste of the present invention is 10 to 5000 ppm by mass, and preferably 30 to 500 ppm by mass. When the copper content of the silver powder falls in this range, migration is difficult to occur. On the other hand, when the copper content of the silver powder is less than the range, migration easily occurs. From the viewpoint of migration resistance, there is no upper limit in the copper content, but when the copper content is over the range, a specific resistance is increased. Therefore, the copper content of the silver powder in the present invention is within a range of 10 to 5000 ppm by mass. It is noted that the term “to” used herein to indicate a numerical range indicates a range including values preceding and following the term “to” unless otherwise stated. Specifically, for example, a notation “10 to 5000” has the same meaning as “10 or more and 5000 or less” unless otherwise stated.
The silver powder used in the silver paste of the present invention has a D10 of 1.00 to 3.00 μm, and a D50 of 3.00 to 7.00 μm. The D10 of the silver powder is preferably 1.20 to 2.00 μm, and the D50 of the silver powder is preferably 3.90 to 5.00 μm. When the D10 and D50 of the silver powder meet the above-described requirements, the dry film density is increased, and in addition, the content of a silver powder having a small diameter in which migration easily occurs is small, and therefore, a short circuit between conductor films is more effectively inhibited. It is noted that the D10 and D50 of the silver powder respectively refer to a 10% value and a 50% value of a volume-based cumulative fraction obtained by a laser diffraction particle size distribution measurement.
The specific surface area SBET of the silver powder used in the silver paste of the present invention is 0.10 to 0.30 m2/g, and preferably 0.12 to 0.20 m2/g. When the specific surface area SBET of the silver powder falls in this range, the content of a silver powder having a small diameter in which migration easily occurs is small, and therefore, a short circuit between conductor films is effectively inhibited. It is noted that the SBET (m2/g) refers to a specific surface area obtained by a BET (Brunauer-Emmett-Teller) method with helium gas adsorbed onto a surface of the silver powder.
The shape of the silver powder may be a granular shape, a flake shape, or an indeterminate shape, and is preferably a spherical shape. In the present invention, the spherical shape refers to a powder in which an average of aspect ratios of arbitrary fifty particles found in the visual field through observation with an SEM (scanning electron microscope) is in a range of 1.0 to 1.5. The average of the aspect ratios is preferably in a range of 1.0 to 1.3.
Besides, the silver powder used in the silver paste of the present invention is preferably a mixed powder obtained by mixing two or more types of silver powders having different average particle sizes (D50). When a mixed powder containing two or more types of silver powders having different average particle sizes (D50) is used as the silver powder, a dense and highly electrically conductive conductor film having a high dry film density can be easily obtained.
The silver powder used in the silver paste of the present invention is preferably a mixed powder containing: (a) a first silver powder having a D50 of 3.50 to 7.50 μm, preferably 3.70 to 7.50 μm, and particularly preferably 4.00 to 6.00 μm, and (b) a second silver powder having a D50 that is 0.80 to 2.70 μm, may be 0.80 to 2.00 μm, and is preferably in a range of 0.80 to 1.80 μm. In this case, the above-described various physical property values such as the specific surface area SBET, the D50 and D10, and the copper content may fall respectively in the above-described numerical ranges as the whole mixed powder is obtained by mixing the two or more silver powders. Since a value of the D10 of the whole mixed powder is easily lowered by the second silver powder, the second silver powder having a D10 adjusted to 0.70 μm or more is preferably used.
The binder resin used in the silver paste of the present invention is not especially limited, and can be a binder resin used in a usual silver paste. Examples of the binder resin include celluloses, acrylic resins, phenol resins, epoxy resins, urethane resins, polyester resins and polyethylene resins.
The organic solvent used in the silver paste of the present invention is not especially limited, and can be an organic solvent used in a usual silver paste. Examples of the organic solvent include alcohol-based, ether-based, ester-based, and hydrocarbon-based organic solvents, water, and mixed solvents of these.
When a silver content of the silver paste is higher, a film with higher electrical conductivity can be obtained, but when the silver content is too high, printability is deteriorated. A content CAG of the silver powder in the silver paste of the present invention is 80.00 to 97.00% by mass, and preferably 92.00 to 96.00% by mass. When the content of the silver powder falls in this range, both electrical conductivity and printability can be increased. The content CAG (% by mass) of the silver powder in the silver paste refers to a content percentage of the silver powder based on the silver paste obtained in accordance with an equation “(content of silver powder/mass of silver paste)×100”.
In the silver paste of the present invention, the content percentage CBDN of the binder based on the silver powder is 0.430 to 0.750% by mass, and preferably 0.440 to 0.600% by mass. When the content percentage of the binder based on the silver powder falls in this range, good printability is obtained, and in addition, a silver content of the silver paste can be increased. The CBND (% by mass) refers to a content percentage (% by mass) of the binder resin based on the silver powder obtained in accordance with an equation “(content of binder resin in silver paste/content of silver powder in silver paste)×100”.
The dry film density of the silver paste of the present invention is 7.50 g/cm3 or more. A lower limit of the dry film density of the silver paste of the present invention is preferably 7.60 g/cm3, and particularly preferably 7.80 g/cm3. An upper limit of the dry film density of the silver paste of the present invention is preferably as high as possible as long as printability is good, but does not exceed a density of silver (10.5 g/cm3), and in consideration of workability, productivity, and the like, the upper limit is about 8.50 g/cm3 in reality. As a method for controlling the dry film density, the dry film density can be controlled by a widely known general method, and can be controlled by, for example, adjusting a particle size distribution or a surface state (such as smoothness, and whether surface-treated or not) of the silver powder, by changing the binder resin, the organic solvent or the like to be used, or if a dispersant is added to the paste, by changing the type or the amount of the dispersant to be added. In the present invention, the dry film density refers to a density of a dry film obtained by drying a coating film of the silver paste as it is without compression. In an example described later, a silver paste to be measured is applied on a PET (polyethylene terephthalate) film in a thickness of about 150 μm, the resultant is temporarily dried at 80° C. for 10 minutes, and then the resultant paste together with the PET film is punched into a circular shape with a diameter of 15 mm, the resultant is finally dried at 150° C. for 1 hour, and then the PET film is peeled. A mass W and a volume V of the thus obtained dry film are measured to calculate a value W/V as the dry film density.
In the silver paste of the present invention, a value “CBND/SBET” is preferably 2.00 to 3.40, and further preferably 2.5 to 3.1, where SBET (m2/g) represents the specific surface area of the silver powder, and CBND (% by mass) represents the content percentage of the binder resin based on the silver powder. When the value CBND/SBET falls in this range, good printability is obtained, and in addition, the silver content of the silver paste can be increased, and as a result, a conductor film having a high dry film density, and high denseness and conductivity can be easily obtained.
When all of the above-described requirements [1], [2], [3], [4], [5] and [6] are met in the silver paste of the present invention, a dense and highly conductive film can be obtained, and a conductor film having a low specific resistance can be formed, whereas a conductor film excellent in printability and excellent in coating film shape can be obtained, and in addition, excellent migration resistance can be obtained and occurrence of a short circuit between conductor films can be reduced. In an experimental example described below, the silver paste of the present invention is screen printed on a substrate to be printed, and the coating film shape (printing pattern) is evaluated to be preferable and printability is evaluated to be good as a value of a contact angle (rectangularity) between the substrate and the coating film is closer to 90°.
In the present invention, a production method for the silver powder is not especially limited, and the silver powder can be produced by, for example, conventionally known atomization method, wet reduction method and chemical vapor deposition (CVD) method, a physical vapor deposition (PVD) method as described in Japanese Patent No. 3541939, a spray pyrolysis method as described in Japanese Patent Publication No. 63-31522, a “method for pyrolyzing a pyrolytic metal-containing compound in a gas phase” as described in Japanese Patent No. 3812359, or the like. Among these methods, the PVD method, the spray pyrolysis method, and a production method by the “method for pyrolyzing a pyrolytic metal-containing compound in a gas phase” are preferred because a silver powder that is spherical, has a high crystallinity, and contains particles of uniform sizes can be easily obtained.
The silver paste of the present invention can appropriately contain, if necessary, an inorganic compound such as a glass frit or a metal oxide, and a plasticizer, a viscosity modifier, a surfactant, a dispersant, an oxidant or the like used as additives in a usual silver paste.
The silver paste of the present invention can be produced in accordance with a usual method by kneading and homogeneously dispersing the silver powder, the binder resin, the organic solvent, and an organic oxide, an additive and the like appropriately added if necessary, and then preparing the resultant into a paste rheology applicable to screen printing and other printing methods.
The silver paste of the present invention is used for forming an internal electrode, an external electrode, or a thick film conductor circuit included in an electronic component such as a multilayer ceramic capacitor, an inductor, or an actuator. In particular, the silver paste of the present invention is suitably used for forming an internal electrode of a multilayer inductor using, as a magnetic material, a dust core material, particularly a soft magnetic metal particle.
The term “printability” used herein will now be described to be more easily understood by exemplifying a multilayer inductor.
Usually, a multilayer inductor 10 includes, as in an example illustrated in
The base body 11 includes, as illustrated in
The multilayer inductor of
In this manner, the term “printability” herein means not only that printing can be performed with appropriate flowability in printing process by a screen printing method, a gravure printing method or the like but also that a high viscosity is rapidly exhibited after the printing to obtain a coating film (conductor film) in a shape closer to a rectangular shape.
The silver paste of the present invention is suitably used as a baked silver paste used for forming an internal electrode or an external electrode in a ceramic electronic component such as a multilayer ceramic capacitor, an inductor, or an actuator. The silver paste of the present invention is used, for example, for forming a conductor film by a heat treatment at 700° C. or less.
Now, the present invention will be described on the basis of specific experimental examples, and it is noted that the present invention is not limited to these examples.
<Production of Silver Powder>
First, silver powders 1 to 21 shown in Table 1 were prepared in accordance with the spray pyrolysis method described in Japanese Patent Publication No. 63-31522. Specifically, for each of the silver powders 6, 8, 13 to 17, and 19 to 21, an aqueous solution, which was obtained by dissolving silver salt and copper salt weighed so that a copper content of a resultant silver powder could be a value shown in a column of Whole Silver Powder of Table 1, was spray pyrolyzed, and the thus collected silver powder was subjected to a classification treatment to adjust values of D10 and D50. On the other hand, for the silver powders 1 to 5, 7, 9 to 12 and 18, an aqueous solution, which was obtained by dissolving silver salt and copper salt weighed so that a copper contained in a resultant silver powder could be a value shown in a column of First Silver Powder or Second Silver Powder of Table 1, was spray pyrolyzed, and the thus collected silver powder was subjected to a classification treatment to adjust values of D10 and D50, and then, the resultant first silver powder and second silver powder were mixed to obtain a mixed powder as the silver powder.
In each of these silver powders, a 10% value (D10) and a 50% value (D50) of a volume-based cumulative fraction were obtained with a laser diffraction particle size distribution measuring apparatus. Besides, a specific surface area (SBET) was measured by the BET method, and aspect ratios of fifty silver particles arbitrarily selected through observation of an SEM (scanning electron microscope) image were measured to obtain an average thereof. Results are shown in Table 1.
Each of silver paste samples a to u was produced by using each of the silver powders shown in Table 1 in a content CAG shown in Table 2, ethyl cellulose in a content CBND shown in Table 2, and terpineol (TPO) as the balance, and kneading them.
Next, each of the silver paste samples a to u was applied on a PET film into a size of 20 mm×20 mm×151 μm, the resultant was dried at 80° C. for 10 minutes, was punched with a punch of 15 mmΦ, and was further subjected to a drying treatment at 150° C. for 1 hour. Next, the mass W and the volume V of the thus obtained dry film were respectively measured to obtain a dry film density in accordance with the equation of W/V. Results are shown in Table 2. It is noted that the acceptable criterion of the dry film density was set to 7.50 g/cm3 or more.
Next, each of the silver paste samples was printed on a ceramic substrate by coating into a rectangular parallelepiped shape of 60 mm×0.6 mm×40 μm, the resultant was baked at 650° C. under an oxidative atmosphere (in the air) to form a conductor film, and an electrical resistance value was obtained by a four-terminal method to calculate a specific resistance. Results are shown in Table 2. It is noted that the acceptable criterion of the specific resistance was set to 2.00 μΩ·cm or less, and preferably 1.90 μΩ·cm or less.
1)Content of silver powder in silver paste: (silver powder/silver paste) × 100
2)Content of ethyl cellulose in silver powder: (ethyl cellulose/silver powder) × 100
On a magnetic layer having a thickness of 30 μm precedently prepared, a rectangular parallelepiped pattern was formed by a screen printing method with each of the silver paste samples a to u obtained as described above, and a magnetic layer was further printed thereon to fill a level difference caused by the thickness of the pattern. Assuming that the resultant was one set, three sets were stacked on one another, and covering magnetic layers were further stacked respectively on uppermost and lowermost portions. Next, the resultant was subjected to thermocompression bonding, was degreased under an oxidative atmosphere, and was baked at 650° C. to obtain a laminate.
Next, the thus obtained laminate was used to evaluate printability. Specifically, the laminate was cut as illustrated in
Besides, twenty-five laminates were produced in the same manner as described above, and short circuit ratios of the laminates were measured. Specifically, electrical resistance between the upper most layer and the middle layer, and between the middle layer and the lower most layer in the three conductor films of each laminate was measured, and this measurement was performed repeatedly on the prepared twenty-five laminates. A ratio of the number of times of electrical conduction out of the total number of times of the measurement was defined as a short circuit ratio. Results are shown in Table 3. It is noted that the acceptable criterion of the short circuit ratio was set to 3% or less.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-242806 | Dec 2018 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/046481 | 11/28/2019 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2020/137330 | 7/2/2020 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20170259333 | Nakanoya | Sep 2017 | A1 |
| 20200238388 | Michiaki | Jul 2020 | A1 |
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| Number | Date | Country | |
|---|---|---|---|
| 20220089885 A1 | Mar 2022 | US |
