TECHNICAL FIELD

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.

BACKGROUND ART

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 electronically 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 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/cm3at the most, and a coating film density described in Patent Literature 3 is about 5.70 g/cm3at 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).

CITATION LIST

Patent Literature

[Patent Literature 1] Japanese Patent Laid-Open No. 2005-174824

[Patent Literature 2] Japanese Patent Laid-Open No. 2007-131950

[Patent Literature 3] Japanese Patent Laid-Open No. 2008-192565

[Patent Literature 4] Japanese Patent Laid-Open No. 2004-220807

[Patent Literature 5] Japanese Patent Laid-Open No. 2003-280179

[Patent Literature 6] Japanese Patent Laid-Open No. 2005-203304

[Patent Literature 7] Japanese Patent Laid-Open No. 2014-197483

[Patent Literature 8] International Publication No. WO2014/061765

SUMMARY OF INVENTION

Technical Problem

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, the present invention aims 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.

Solution to Problem

As a result of earnest studies made for solving the above-described problems, the present inventors found the following: Even in a case where a silver powder is contained in a high concentration in a paste, a dense dry film can be obtained without sacrificing printability when the following conditions are satisfied: [1] the silver powder contains both of a first silver powder (a) having a D50 of 3.50 to 7.50 μm and a second silver powder (b) having a D50 of 0.80 to 2.70 μm, where D50 represents a 50% value of a volume-based cumulative fraction obtained by laser diffraction particle size distribution measurement; [2] a copper content of the whole silver powder is 10 to 5000 ppm by mass; [3] a copper content of the second silver powder (b) is 80 ppm by mass or more; and [4] the first silver powder (a) contains substantially no copper. In addition, migration can be effectively inhibited 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 the silver powder satisfies all of the following conditions [1] to [4]: [1] the silver powder contains a first silver powder (a) having a D50 of 3.50 to 7.50 μm and a second silver powder (b) having a D50 of 0.80 to 2.70 μm, where D50 represents a 50% value of a volume-based cumulative fraction obtained by laser diffraction particle size distribution measurement; [2] a copper content of the whole silver powder is 10 to 5000 ppm by mass; [3] a copper content of the second silver powder (b) is 80 ppm by mass or more; and [4] the first silver powder (a) contains substantially no copper.

A present invention (2) provides the silver paste according to (1), in which the second silver powder (a) has a D10 of 0.70 μm or more, where D10 represents a 10% value of a volume-based cumulative fraction obtained by laser diffraction particle size distribution measurement.

A present invention (3) provides the silver paste according to (1) or (2), in which a copper content of the first silver powder (a) is less than 50 ppm by mass.

A present invention (4) provides the silver paste according to any one of (1) to (3), in which the first silver powder (a) has a D50 of 3.70 to 7.50 μm.

A present invention (5) provides the silver paste according to any one of (1) to (3), in which the first silver powder (a) has a D50 of 4.00 to 6.00 μm.

A present invention (6) provides the silver paste according to any one of (1) to (5), in which the second silver powder (b) has a D50 of 0.80 to 2.00 μm.

A present invention (7) provides the silver paste according to any one of (1) to (6), in which the whole silver powder has a D10 of 1.00 to 3.00 μm, and a D50 of 3.00 to 7.00 μm.

A present invention (8) provides the silver paste according to (7), in which the whole silver powder has a D10 of 1.20 to 2.00 μm.

A present invention (9) provides the silver paste according to (7) or (8), in which the whole silver powder has a D50 of 3.90 to 5.00 μm.

A present invention (10) provides the silver paste according to any one of (1) to (9), in which a copper content of the whole silver powder is 30 to 500 ppm by mass.

A present invention (11) provides the silver paste according to any one of (1) to (10), further satisfying the following condition [5]: [5] a content CAGof the whole silver powder based on the silver paste is 80.00 to 97.00% by mass.

A present invention (12) provides the silver paste according to (11), in which the content CAGof the whole silver powder based on the silver paste is 92.00 to 96.00% by mass.

A present invention (13) provides the silver paste according to any one of (1) to (12), in which the silver paste has a dry film density of 7.50 g/cm3or more.

A present invention (14) provides the silver paste according to any one of (1) to (13), in which a value CBND/SBETis 2.00 to 3.40, where SBET(m2/g) represents a specific surface area of the whole silver powder, and CBND(% by mass) represents a content percentage of the binder resin based on the whole silver powder.

A present invention (15) provides the silver paste according to any one of (1) to (14), in which a specific surface area SBETof the whole silver powder is 0.10 to 0.30 m2/g.

A present invention (16) provides the silver paste according to any one of (1) to (15), to be used for forming a conductor film by a heat treatment at 700° C. or less.

Advantageous Effects of Invention

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.

