JP6158461B1 - Silver powder and silver paste and use thereof - Google Patents

Abstract

A silver powder capable of forming a wiring having a low electric resistivity is provided. According to the present invention, a silver powder used for forming an electrode of an electronic device is provided. This silver powder has the following (1) to (4): (1) The loss on ignition when heated to 600 ° C. is 0.05% or less; (2) The tap density is 5 g / cm 3 or more; (3) The maximum aspect ratio is 1.4 or less; (4) the specific surface area based on the BET method is 0.8 m2 / g or less;

Description

The present invention relates to a silver powder and a silver paste that can be suitably used for forming a wiring in an electronic device.
This application claims the priority based on the Japan patent application 2015-253416 for which it applied on December 25, 2015, The whole content of the application is integrated in this specification as a reference.

  2. Description of the Related Art In electronic devices for electric / electronic devices, a technique of printing and wiring a powder made of a conductive material corresponding to a conductive wire on an insulating substrate without using a conductive wire is widely adopted. The conductive material for forming the wiring is generally used for printing in the form of a conductive paste dispersed in a dispersion medium together with a binder.

  Various types of conductive paste are used depending on the application. For example, in applications that require particularly high electrical conductivity (low resistivity characteristics), silver paste containing silver powder is used as the conductive material. On the other hand, conductive paste containing copper powder, aluminum powder, nickel powder, or the like is used in applications where the cost is relatively low. In addition, for example, for electronic elements in which the use environment and the production environment are high, a firing-type paste that is baked onto a substrate by firing is used. For electronic elements that cannot be exposed to high temperatures, heat-curable pastes that are fixed to a substrate by heat-curing at low temperatures are used. As a prior art regarding a baking type silver paste, the cited references 1-2 are mentioned, for example.

Japanese Patent Application Publication No. 2009-062558 Japanese Patent Application Publication No. 2011-181538

  Patent Document 1 relates to a conductive paste for a low temperature co-fired ceramics (LTCC) wiring board, and has a polyhedron with irregularities on the surface in order to suppress generation of cracks and delamination. The use of silver-like silver particles is disclosed. Patent Document 2 discloses a technique relating to a conductive paste for forming an electrode of a solar cell element, and discloses that the conductive paste contains silver particles having a predetermined property and glass frit.

  It is known that such a firing type conductive paste can form a silver electrode (wiring) having a lower resistivity when fired at a relatively high firing temperature. For example, in the case of the conventional conductive paste, the electrical resistivity (specific resistance) of the electrode obtained when firing at a relatively low temperature is 600 ° C. to 700 ° C. is about 2.3 μΩ · cm or more. On the other hand, the electrical resistivity of the electrode obtained when firing at a high temperature of 800 ° C. or higher is about 2.1 μΩ · cm or higher. This value can be said to be a sufficiently low electrical resistivity depending on the application. However, for example, since the theoretical value of the electrical resistivity of bulk silver is 1.6 μΩ · cm, the silver electrode is required to have a lower resistance at a higher level (for example, 2 μΩ · cm or less). Furthermore, it is required to achieve such a low resistance value at a lower firing temperature. This invention is made | formed in view of said situation, The objective is to provide the silver powder as an electroconductive material which can form wiring with a low electrical resistivity. Another object of the present invention is to provide a conductive paste using such silver powder.

In order to solve the above-described problems of the prior art, the technology disclosed herein provides a silver powder that is used to form an electrode of an electronic device. This silver powder has the following (1) to (4): (1) Loss on ignition when heated to 600 ° C for a sample dried at a drying temperature of 110 ° C is 0.05% or less; (2) Tap The density is 5 g / cm ³ or more; (3) the maximum aspect ratio is 1.4 or less; (4) the specific surface area based on the BET method is 0.8 m ² / g or less; .

  The inventors of the present invention have intensively studied and adjusting the above-mentioned four properties of the silver powder in a predetermined range to adjust the silver powder for electrode formation capable of realizing an unprecedented low electrical resistivity. As a result, the present invention has been completed. According to the above configuration, for example, by supplying silver powder onto the base material and firing, formation of pores in the electrode is suppressed, and an electrode (wiring) having a resistance lower than that of the conventional electrode can be formed. For example, a low-resistance electrode having an electrical resistivity of 2 μΩ · cm or less can be realized. Furthermore, such a low-resistance electrode can be formed by firing at a firing temperature lower than conventional.

Note that the "loss on ignition (Ig-loss)" as used herein, is an indicator showing the percent mass loss upon heating up to 6 00 ° C. silver powder was dried at a drying temperature of 110 ° C. . This loss on ignition can be measured in accordance with the chemical product weight loss and residue test methods specified in JIS K0067: 1992.
Further, in this specification, “tap density” is an index indicating the apparent density of powder after tapping silver powder 1000 times in a predetermined container. The tap density can be measured according to the metal powder-tap density measuring method specified in JIS Z2512: 2012.

Furthermore, in this specification, the “maximum aspect ratio” means an arithmetic average value of aspect ratios measured for three silver particles that are considered to have the highest aspect ratio in each of observation images of three or more fields in electron microscope observation. Means.
In this specification, the “specific surface area” is a specific surface area obtained by analyzing gas molecule adsorption isothermal characteristics of silver powder measured by the gas adsorption method based on the BET method. This specific surface area can be measured according to the specific surface area measurement method of powder (solid) by gas adsorption prescribed in JIS Z8830: 2013 (ISO 9277: 2010).

  In a preferred embodiment of the silver powder disclosed herein, (5) an average particle diameter based on observation with an electron microscope is 1 μm or more and 3 μm or less. Such a configuration is preferable because the above four properties are easily realized and a low-resistance electrode is easily formed.

In a preferred embodiment of the silver powder disclosed herein, (6) the specific gravity is 10.4 g / cm ³ or more. The technique disclosed herein aims to reduce the resistivity of the electrode at a higher level. Therefore, it is preferable that the silver particles themselves constituting the silver powder do not contain pores and have a high specific gravity. With such a configuration, a silver electrode having a low resistivity can be more reliably formed.

  In another aspect, the invention disclosed herein provides a silver paste. Such a silver paste contains any of the silver powders described above, a binder resin, and a dispersion medium. With such a configuration, the above-described silver powder can be suitably supplied to a desired substrate, and a low-resistance electrode can be formed efficiently and simply. For example, a plurality of fine electrode patterns can be printed on a single substrate by using a printing technique, which is efficient.

  In a preferred embodiment of the silver paste disclosed herein, the electric resistivity of the fired silver product obtained when fired in a temperature range of 700 ° C. or higher and 800 ° C. or lower is configured to achieve 2 μΩ · cm or lower. Thereby, an electrode having an electrical resistivity of 2 μΩ · cm or less can be stably formed.

  In a preferred embodiment of the silver paste disclosed herein, the electrical resistivity of the silver fired product obtained when fired in a temperature range of 600 ° C. or higher and 900 ° C. or lower is configured to achieve 2.1 μΩ · cm or lower. Yes. Thus, an electrode having an electrical resistivity of 2.1 μΩ · cm or less can be stably formed at a wide firing temperature from a relatively low temperature to a high temperature.

  In a preferred embodiment of the silver paste disclosed herein, the electrical resistivity of the silver fired product obtained when fired in a temperature range of 600 ° C. or higher and 800 ° C. or lower is configured to achieve 2 μΩ · cm or lower. Thereby, an electrode having an electric resistivity of 2 μΩ · cm or less can be stably formed even at a relatively low firing temperature of 600 ° C.

  As described above, according to the technology disclosed herein, it is possible to produce a low-resistance silver electrode in which pore formation is suppressed by firing. Moreover, this silver electrode can be simply formed by baking the printing body printed on the base material by arbitrary shapes. Therefore, it is an electronic device that is used at a high temperature or manufactured at a high temperature, and can be particularly suitably applied to an application that requires an electrode having a particularly low resistivity. From this point of view, the technology disclosed herein also provides an electronic device including this silver electrode.

FIG. 1 is a cross-sectional view schematically illustrating a configuration of a multilayer chip inductor formed using silver powder according to an embodiment. FIG. 2 is a cross-sectional SEM image of a silver electrode obtained by firing a silver paste according to an embodiment. FIG. 3 is a cross-sectional SEM image of a silver electrode obtained by firing a conventional silver paste.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification (for example, properties of silver powder) and matters necessary for the implementation of the present invention (for example, a method for supplying a silver paste, a configuration of an electronic device, etc.) Can be understood on the basis of the technical contents taught in the present specification and the general technical common knowledge of those skilled in the art. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field. In the present specification, the notation “A to B” indicating a range means A or more and B or less.

[Silver powder]
The silver powder disclosed here is a material for forming a wiring having high electrical conductivity (hereinafter sometimes simply referred to as “conductivity”) as a conductive wire in an electronic device or the like. Silver (Ag) is preferable as an electrode material because it is not as expensive as gold (Au), is hardly oxidized, and has excellent conductivity. The composition of the silver powder is not particularly limited as long as it is a powder (aggregation of particles) containing silver as a main component, and a silver powder having desired conductivity and other physical properties can be used. Here, the main component means that it is the maximum component among the components constituting the silver powder. Examples of the silver powder include silver and silver alloys, and mixtures or composites thereof. Preferred examples of the silver alloy include a silver-palladium (Ag—Pd) alloy, a silver-platinum (Ag—Pt) alloy, a silver-copper (Ag—Cu) alloy, and the like. For example, core-shell particles in which the core is made of metal other than silver, such as copper or silver alloy, and the shell covering the core is made of silver can also be used. Since the silver powder tends to have higher conductivity as its purity (content) is higher, it is preferable to use a silver powder having higher purity. The silver powder preferably has a purity of 95% or more, more preferably 97% or more, and particularly preferably 99% or more. According to the technology disclosed herein, for example, it is possible to form an extremely low resistance electrode even by using silver powder having a purity of about 99.5% or more (for example, about 99.8% or more). The From this point of view, in the technique disclosed herein, it is possible to form a sufficiently low-resistance electrode even when, for example, silver powder having a purity of 99.99% or less (99.9% or less) is used. It is.

  The silver powder is integrated by firing to form an electrode. When the silver particles constituting the silver powder are sintered, a plurality of particles are integrated with the sintering, and the apparent volume is reduced. That is, the position of silver particles moves during sintering. In addition, the shape of the silver particles changes with sintering so that the particle gap is lost. In the technology disclosed herein, various properties of the silver powder are adjusted so that the silver particles are more densely packed during firing to form an electrode with fewer pores. That is, the silver powder is determined such that (1) ignition loss, (2) tap density, (3) maximum aspect ratio, and (4) specific surface area are within a predetermined range. Below, each physical-property value is demonstrated.

(1) Loss on ignition (Ig-loss)
The ignition loss is an index showing the proportion (%) of the weight loss when heated silver powder was dried at a drying temperature of 110 ° C. until the 6 00 ° C.. The component which decreases by such heating is a component (volatile component) that burns out during firing, and can inhibit the smooth movement and filling of silver particles when the silver powder is fired. This volatile component is considered to be mainly composed of an organic substance, and may be a component derived from, for example, a dispersant or a surfactant attached to the surface of the silver powder in order to enhance the dispersibility of the silver powder. In the technique disclosed here, the loss on ignition of the silver powder is limited to 0.05% or less in order to suppress a decrease in the packing property of the silver particles. The ignition loss is preferably 0.045% or less, more preferably 0.04% or less, and particularly preferably 0.035% or less. The ignition loss depends on the performance of the measuring device, but may be substantially 0%.

(2) Tap density The tap density is an index indicating a bulk density in a lightly consolidated state when voids due to aggregation of powder naturally filled in a container are eliminated by impact under a predetermined tapping condition. Here, the tap density when the tap height is 5 cm, the tap speed is 100 times / minute, and the tapping frequency is 1000 times is adopted. If the tap density of the silver powder is too low, the arrangement of the silver particles when supplied onto the base material tends to be large in voids, and further, the filling property is difficult to increase when the silver particles move during firing. It is not preferable. From this point of view, in the technique disclosed herein, the tap density of the silver powder is regulated to 5 g / cm ³ or more. Tap density is preferably 5.1 g / cm ³ or more, more preferably 5.2 g / cm ³ or more, 5.3 g / cm ³ or more is particularly preferable. The upper limit of the tap density is not particularly limited. The density of the powder has a relationship of tap density <apparent density ≦ specific gravity. Therefore, it is preferable that it is closer to the closest packing density calculated from the average particle diameter of the silver powder or the specific gravity of silver (10.50 g / cm ³ ).

(3) Maximum aspect ratio Since the silver powder disclosed here is subjected to firing, the closer the silver particles constituting the silver powder are to the true spherical shape, the better the filling property, and the denser the electrode that is the fired product. Can do. Therefore, the presence of non-spherical particles that hinder the filling properties during firing is not preferable. In the technique disclosed here, the maximum aspect ratio of the silver particles is limited to 1.4 or less, thereby ensuring the filling property of the silver particles constituting the silver powder. In addition, the packing aspect of the particles is most disturbed by the particles whose shape is far from the true sphere, and the maximum aspect ratio is evaluated instead of the average aspect ratio from the viewpoint that this leads to the formation of larger pores in the electrode. As an indicator. The maximum aspect ratio of the silver powder is preferably 1.35 or less, more preferably 1.3 or less, and particularly preferably 1.25 or less.
Note that the maximum aspect ratio is the arithmetic of the aspect ratio measured for these silver particles by selecting the three silver particles that are considered to have the highest aspect ratio in each of the observation images of three or more fields in the electron microscope observation. Mean value. The aspect ratio is an index calculated as “a / b” where a is the maximum long diameter (maximum length) of silver particles in the observed image and b is the maximum width of silver particles orthogonal to the maximum long diameter. is there.

(4) Specific surface area The specific surface area is a value indicating the surface area of the silver powder per unit weight, and can be an index reflecting the size and surface morphology of the silver particles constituting the silver powder. In general, for powders having the same average particle diameter, the larger the specific surface area, the more likely the shape of the particles to be farther from a true sphere. Therefore, in the technique disclosed here, it is prescribed | regulated that the specific surface area based on BET method should use the silver powder of 0.8 m < ² > / g or less. The specific surface area of 0.8 m ² / g corresponds to the specific surface area of true spherical silver particles having a diameter of about 0.7 μm. In the present invention, the suitability of the silver powder used for forming the electrode of the electronic element is evaluated based on such a value. The specific surface area of silver powder is preferably 0.75 m ² / g or less, more preferably ^{0.65m 2 / g, 0.6m 2 /} g or less is particularly preferred. In addition, that a specific surface area is small also means that the average particle diameter of silver powder is coarse. Therefore, although it cannot be generally described because it depends on the use of the electronic device, the specific surface area based on the BET method is preferably about 0.1 m ² / g or more, and preferably about 0.15 m ² / g or more. More preferred.

  As described above, the technique disclosed herein employs a combination of the above four indices and adjusts the value to an optimum value in order to improve the filling property during the firing of the silver powder. Therefore, in order to form a lower resistance electrode, it is essential that the silver powder satisfies the above four requirements simultaneously. Thereby, when this silver powder is baked at a temperature lower than the melting point of bulk silver (about 962 ° C.), a denser baked product can be obtained. As a result, an electrode having a low resistivity can be formed.

(5) Average particle diameter In addition, the average particle diameter of silver powder is not specifically limited as long as the said requirements are satisfy | filled. However, it is also a preferable aspect that the average particle diameter is within a predetermined range from the viewpoint that it can be suitably used for the production of electronic devices at the present time. If the average particle size of the silver powder is too small, sintering proceeds at a lower temperature, but the primary particles tend to aggregate and the packing property of the silver particles during firing is lowered, which is not preferable. In view of this, the silver powder preferably has an average particle diameter of, for example, 1 μm or more based on an electron microscope capable of distinguishing and observing primary particles and secondary particles (aggregated particles). . The average particle diameter is preferably 1.1 μm or more, more preferably 1.2 μm or more, and particularly preferably 1.3 μm or more. For example, it can be 1.5 μm or more. In addition, if the average particle size of the primary particles of the silver powder is too large, it is not preferable in that it needs to be exposed to a high temperature for a long time for sintering and does not satisfy the desire to realize sintering at a low temperature. Therefore, the average particle diameter based on the electron microscope observation of the silver powder can be set to 5 μm or less, for example. The average particle size is preferably 4.5 μm or less, more preferably 4 μm or less, and particularly preferably 3.5 μm or less.

In this specification, “average particle diameter based on electron microscope observation” refers to an electron microscope represented by, for example, a scanning electron microscope (SEM), a transmission electron microscope (TEM), and the like. _{Is the} cumulative 50% particle size (D _EM50 ) in the particle size distribution (number basis) based on the equivalent circle diameter of 100 silver powder particles observed by the above.

The average particle diameter of the powder is also evaluated by a laser diffraction / scattering method. According to the laser diffraction / scattering method, the average particle diameter is measured without completely distinguishing primary particles and secondary particles. As for the silver powder disclosed here, the average particle diameter of the silver powder based on the laser diffraction / scattering method is preferably, for example, 0.5 μm or more. The average particle diameter based on the laser diffraction / scattering method is preferably 0.7 μm or more, more preferably 1 μm or more, and particularly preferably 1.2 μm or more. On the other hand, with respect to the upper limit of the average particle diameter of the silver powder, there is no significant difference between the laser diffraction / scattering method and the electron microscope observation method. The average particle size is preferably 4.5 μm or less, more preferably 4 μm or less, and particularly preferably 3.5 μm or less.
As used herein, “average particle size based on laser diffraction / scattering method” refers to the particle size (D _L50 ) when the cumulative volume is 50% in the volume-based particle size distribution measured by the laser diffraction / scattering method. doing.

Further, as the silver powder, those having a sharp (narrow) particle size distribution are preferable. For example, a silver powder that does not substantially contain particles having an average particle diameter of 10 μm or more can be preferably used. Furthermore, as an indicator of the sharpness of the particle size distribution, the particle size distribution ( _DL10 ) from the small particle size side in the particle size distribution based on the laser diffraction / scattering method and the particle size at the cumulative 90% volume _{(D L90)} and the ratio of _(D L10 _/ D _L90) can be employed. When all the particle sizes constituting the powder are equal, the value of D _L10 / D _L90 is 1, and conversely, the value of D _L10 / D _L90 approaches 0 as the particle size distribution becomes wider. It is preferable to use a powder having a relatively narrow particle size distribution such that the value of D _L10 / D _L90 is 0.15 or more, for example 0.15 or more and 0.5 or less.

In another aspect, the silver powder can be used by mixing two particle groups having different average particle diameters. In this case, for example, the average particle diameter (D _L50 ) of the first particle group is in the range of 2 μm to 5 μm (for example, 2 μm), and the average particle diameter (D _L50 ) of the second particle group is 0.5 μm to 2 μm ( For example, a preferable range is 0.5 μm). At this time, the particle size distribution of each particle group is preferably sharp as described above. For example, the first particle group has a ratio of 65 to 90% by mass (for example, 70% by mass) and the second particle group has a ratio of 35 to 10% by mass (for example, 30% by mass). Mix. Thereby, silver powder with favorable filling property can be prepared.
A silver powder having such an average particle size and particle size distribution characteristic can form a dense electrode with good filling properties. This is advantageous in forming a lower resistivity electrode.

(6) Specific gravity Although the specific gravity of the silver powder is not strictly limited, for example, it is more preferable that the ratio of pores contained in the silver particles themselves constituting the silver powder is small. Therefore, for example, it is preferable that the specific gravity (also referred to as true density) of silver powder measured by a constant volume expansion method is high. In this specification, the specific gravity of the silver powder adopts a value measured by a constant volume expansion method using helium gas. The specific gravity of the silver powder can be approximately 10.3 g / cm ³ or more, for example, 10.35 g / cm ³ or more. Furthermore, the specific gravity of the silver powder is preferably 10.4 g / cm ³ or more, more preferably 10.45 g / cm ³ or more, 10.5 g / cm ³ or more is particularly preferable.

  The above silver powder is fired after being supplied to an arbitrary base material, and the silver particles constituting the silver powder are integrally sintered to obtain a silver electrode (which may be a wiring) as a sintered product. be able to. Although the firing temperature depends on the composition of the silver powder, the silver powder that can be regarded as pure silver (for example, purity 99.9% or more) can be set to a temperature lower than 962 ° C., which is the melting point. Therefore, the firing temperature can be in the temperature range of about 800 ° C. to 900 ° C., for example, as in the case of conventional silver powder. However, the silver powder disclosed here can obtain a silver electrode having a lower resistance than when the conventional silver powder is fired at the same temperature. Furthermore, even when firing at a temperature lower than that of conventional silver powder, an electrode having a resistivity equivalent to or lower than that of the conventional one can be realized. Therefore, this silver powder is preferably fired at a temperature of 900 ° C. or lower (less than 900 ° C.), for example. The firing temperature is more preferably 850 ° C. or less (less than 850 ° C.), further preferably 800 ° C. or less (less than 800 ° C.), and particularly preferably 750 ° C. or less (less than 750 ° C.). For example, the firing temperature can be 700 ° C. or less (less than 700 ° C.), particularly 650 ° C. or less (less than 650 ° C.), for example, about 600 ° C. (typically 580 ° C. to 620 ° C.). It does not restrict | limit especially about the minimum of baking temperature, For example, setting it as 550 degreeC or more is illustrated.

[Silver paste]
The method for supplying the silver powder to the binder resin is not particularly limited. In the technique disclosed herein, the silver powder can be provided in the form of a silver paste dispersed in an organic vehicle component in order to improve the supply and handling properties of the silver powder. Such a silver paste essentially comprises silver powder and an organic vehicle component.
As the organic vehicle component, various materials conventionally used in this type of silver paste can be used without particular limitation depending on the desired purpose. Typically, the organic vehicle component is configured as a mixture of binder resins and dispersion media of various compositions. In such an organic vehicle component, the binder resin may be completely dissolved in the dispersion medium, or a part thereof may be dissolved or dispersed (may be a so-called emulsion type organic vehicle).

  The binder resin is a component that plays a role of bonding the silver particles and the silver particles and the base material at a stage where the prepared silver paste is formed into a film by printing, drying or the like. Therefore, after the silver particles are integrated by baking, the binder resin can be an unnecessary resistance component. Therefore, this binder resin is preferably a component that disappears at a temperature lower than the firing temperature and does not remain in the electrode. As such a binder resin, an organic compound having a binder function can be used without particular limitation. Specifically, for example, cellulose polymers such as ethyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, acrylic resins such as polybutyl methacrylate, polymethyl methacrylate, polyethyl methacrylate, epoxy resins, phenol resins, alkyd resins, polyvinyl alcohol, Binder resins based on vinyl resins such as polyvinyl butyral and rosin resins such as rosin and maleinized rosin are preferably used. In particular, the use of a cellulosic polymer (for example, ethyl cellulose) is preferable because viscosity characteristics capable of performing good screen printing can be suitably realized.

  In addition, as above-mentioned, this silver powder is excellent in sinterability and the filling property at the time of sintering. Therefore, for the purpose of forming an electrode having a lower resistivity, the silver paste preferably does not contain components other than silver powder and organic vehicle components. For example, it is a preferable aspect that this silver paste does not contain glass frit which can be said to be an inorganic binder in addition to the organic vehicle component.

  A preferable dispersion medium constituting the organic vehicle is an organic solvent having a boiling point of about 200 ° C. or higher (typically about 200 ° C. to 260 ° C.). An organic solvent having a boiling point of about 230 ° C. or higher (typically about 230 ° C. to 260 ° C.) is more preferably used. Examples of such organic solvents include ester solvents such as butyl cellosolve acetate and butyl carbitol acetate (BCA: diethylene glycol monobutyl ether acetate), ether solvents such as butyl carbitol (BC: diethylene glycol monobutyl ether), ethylene glycol and diethylene glycol. An organic solvent such as a derivative, toluene, xylene, mineral spirit, terpineol, mentanol, or texanol can be preferably used. Particularly preferred solvent components include butyl carbitol (BC), butyl carbitol acetate (BCA), 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate and the like.

  The blending ratio of each constituent component contained in the silver paste can be adjusted as appropriate by the electrode forming method, typically the printing method, and the like. It can comprise based on the mixing | blending ratio which followed. As an example, for example, the ratio of each component can be determined using the following formulation as a guide.

  That is, the content ratio of the silver powder in the silver paste is suitably about 80% by mass or more (typically 80% by mass to 98% by mass) when the entire paste is 100% by mass, More preferably, it is about 83 mass%-about 96 mass%, for example, it is preferable to set it as about 85 mass%-about 95 mass%. Increasing the silver powder content ratio is preferable from the viewpoint of reducing the binder resin ratio and forming a dense electrode pattern with few pores with high shape accuracy. On the other hand, if the content is too high, the handleability of the paste and the suitability for various printability may be reduced.

Of the organic vehicle components, the binder resin is contained in a proportion of about 10% by mass or less, typically about 0.3% by mass to 8% by mass, when the mass of the silver powder is 100% by mass. Is preferred. Particularly preferably, it is contained in a proportion of 0.5% by mass to 6% by mass with respect to 100% by mass of silver powder. In addition, this binder resin may contain the binder resin component which is melt | dissolving in the organic solvent, and the binder resin component which is not melt | dissolved in the organic solvent, for example. When the binder resin component dissolved in the organic solvent and the binder resin component not dissolved are included, the ratio thereof is not particularly limited, but for example, the binder resin component dissolved in the organic solvent is (10% to 10%) can be occupied.
In addition, the content ratio as a whole of the organic vehicle is variable in accordance with the properties of the obtained paste. As an approximate guide, when the entire conductive composition is 100% by mass, for example, 2% by mass to 20% by mass. % Is appropriate, preferably 5% by mass to 15% by mass, and more preferably 5% by mass to 10% by mass.

  Further, the conductive composition disclosed herein can contain various inorganic and / or organic additives other than those described above without departing from the object of the present invention. Preferable examples of such additives include additives such as surfactants, antifoaming agents, antioxidants, dispersants, viscosity modifiers and the like.

Such a silver paste can be prepared by weighing the above-described materials so as to have a predetermined composition (mass ratio) and mixing them uniformly. The materials can be stirred and mixed using various known stirring and mixing devices such as a three-roll mill, a roll mill, a magnetic stirrer, a planetary mixer, and a disper.
The suitable viscosity of the paste is not particularly limited because it varies depending on the thickness of the target electrode (and thus the thickness of the paste print). For example, when forming a printed material having a shape suitable for the internal electrode of the multilayer ceramic chip (for example, having a thickness of about 50 μm), the viscosity of the silver paste is 350 to 450 Pa · s (10 rpm, 25 ° C.). It may be prepared. As a result, the electrode pattern can be printed with increased positional accuracy and shape accuracy.

  Such a paste is preferably baked after being supplied onto the substrate and then allowed to stand at 50 to 150 ° C. for 15 to 30 minutes to remove the dispersion medium. The firing temperature can be determined in the same manner as the firing temperature of the silver powder. Thereby, the silver electrode formed by sintering silver particles densely is formed on the substrate.

  The fired product of the above silver paste has a particularly low resistivity (for example, 2 μΩ · cm or less), and therefore, it is considered that the silver paste is suitably used as an electrode for a use requiring a particularly low electrical resistivity. For example, it can be used as an electrode of an electronic device having various configurations and applications. As a suitable example, for example, a ceramic wiring board based on LTCC whose firing temperature has been lowered to about 900 ° C. or less is cited as a suitable example. When manufacturing such LTCC, it is preferable in that the ceramic substrate and the electrode can be co-fired by forming a wiring pattern with a silver paste on a green sheet of the ceramic substrate. Therefore, a multilayer ceramic chip in which green sheets on which such wiring patterns are printed is laminated and fired, and electrodes are provided as internal electrodes (inner layer wiring) is particularly desirable. The multilayer ceramic chip is not particularly limited, and examples thereof include a multilayer ceramic capacitor (MLCC), a multilayer ceramic inductor, a multilayer ceramic varistor, a multilayer PTC thermistor, and a multilayer NTC thermistor. Among these, in order to suppress the loss of Joule heat due to the internal electrode, a multilayer ceramic inductor in which a low resistivity is required for the electrode can be cited as a desirable application example.

FIG. 1 is a cross-sectional view schematically showing a multilayer chip inductor 1. Configurations such as dimensional relationships (length, width, thickness, etc.) and the number of laminated dielectric layers in this figure do not necessarily reflect actual dimensional relationships and modes.
The multilayer chip inductor 1 is a monolithic type multilayer ceramic chip formed by, for example, laminating and integrating a plurality of dielectric layers (ceramic layers) 12 formed using ferrite powder. A coil conductor as the internal electrode 22 is provided between the dielectric layers 12. In the coil conductor, a part of the coil is formed between each dielectric layer 12, and the two coil conductors sandwiching the dielectric layer 12 are conducted through via holes provided in the dielectric layer 12 sheet. . Thus, the entire internal electrode 22 is configured to have a three-dimensional coil shape (spiral). The multilayer chip inductor 1 includes an external electrode 20 at a portion corresponding to the side surface of the dielectric layer 12 in the outer surface.

  This multilayer chip inductor 1 can typically be manufactured by the following procedure. That is, first, a dispersion mainly composed of ferrite powder is supplied onto a carrier sheet to form a green sheet made of a dielectric material. The green sheet is fired at a firing temperature of about 900 ° C. or lower. A via hole is formed at a predetermined position of the green sheet by laser irradiation or the like. Next, the silver paste disclosed herein is printed at a predetermined position with a predetermined electrode pattern (coil pattern). If necessary, a silver paste prepared for a through hole may be printed on the via hole. A plurality of such green sheets with an electrode pattern (for example, 100 sheets or more) are produced, and these are laminated and pressure-bonded to produce an unfired electronic element body 10. Next, the laminated chip is dried and fired under a predetermined heating condition (maximum baking temperature is 900 ° C. or less) for a predetermined time (the time for maintaining the maximum baking temperature is, for example, about 10 minutes to 5 hours). As a result, the green sheet is fired and the green sheet is fired integrally to form the monolithic dielectric layer 12. The electrode paste is baked to form the internal electrode 22. Thereby, the electronic element body 10 of the multilayer chip inductor 1 in the form in which the internal electrode 22 is sandwiched between the plurality of dielectric layers 12 is manufactured. Thereafter, the external electrode 20 is formed by applying a conductive paste for forming an external electrode to a desired portion of the electronic element body 10 and baking it. In this way, the multilayer chip inductor 1 can be manufactured. In other words, the electronic element body 10 in the form of the multilayer chip inductor 1 in which the internal electrode is disposed inside the dielectric as the ceramic substrate is realized. Note that the construction process of the multilayer chip inductor 1 described above does not particularly characterize the present invention, and thus detailed description thereof is omitted.

  Here, the silver paste used for forming the internal electrode 22 is capable of forming a dense silver electrode with few pores because the ignition loss of the silver powder used is kept low. For example, it is possible to perform firing at a low temperature of about 600 ° C to 700 ° C. Even when firing at such a low temperature, the internal electrode 22 can achieve a low resistivity of, for example, 2 μΩ · cm or less. The dielectric layer 12 is made of a dielectric material having excellent direct current superposition characteristics. Since the internal electrode 22 can realize a low resistivity of 2 Ω · cm or less, the chip inductor 1 used in a power supply circuit capable of flowing a large current with a small loss of Joule heat due to the electrode is provided. For example, the shape of the chip can be realized in a size such as a 1608 shape (1.6 mm × 0.8 mm), a 2520 shape (2.5 mm × 2.0 mm), or the like.

  Hereinafter, some examples relating to the present invention will be described, but the present invention is not intended to be limited to those shown in the examples.

(Silver powder)
First, the silver powder of Examples 1-8 was prepared as a silver powder used as the main body of a silver paste.
The silver powder of Example 1 and Example 6 is a silver powder manufactured by the atomizing method. The powder of Example 1 having a relatively large average particle size and the powder of Example 6 having a relatively small average particle size were prepared.
The silver powders of Examples 2, 7 and 8 are silver powders produced by a wet method. The powder of Example 8 having a relatively small average particle diameter and the powders of Examples 2 and 7 having a relatively large average particle diameter were prepared.
The silver powders of Examples 3 to 5 are silver powders produced by the PVD method. The powder of Example 3 having a relatively small average particle size, the powder of Example 5 having a relatively large average particle size, and the powder of Example 4 in the middle thereof were prepared.
The ignition loss, tap density, maximum aspect ratio, specific surface area, average particle diameter, bulk density and dry density of these powders were measured by the following procedure.

[Ignition loss (Ig-loss)]
About 25 mg of each silver powder was weighed, and the ignition loss was measured using a differential thermal balance (TG8120, manufactured by Rigaku Corporation). The measurement conditions were such that the drying temperature was 110 ° C., and the ratio (%) of the mass loss when heated from room temperature to 600 ° C. with respect to the mass of the dried sample was the ignition loss. The measurement atmosphere was dry air, and the temperature rising rate was 10 ° C./min. The obtained loss on ignition is shown in the “Ig-loss” column of Table 1.

[Tap density]
Each silver powder was weighed by 20 g (20.00 ± 0.02 g), put into a 20 mL measuring cylinder, and then tapped with a tapping device. The tapping conditions were tap height: 5 cm, tap speed: 100 times / min, and tap number: 1000 times. Then, the powder volume after tapping was measured, and the tap density was calculated by dividing the mass of the silver powder by the powder volume (apparent volume) after tapping. The tap density was measured according to the metal powder-tap density measuring method defined in JIS Z2512: 2012. The obtained tap density is shown in the “Tap density” column of Table 1.

[Maximum aspect ratio]
Each silver powder was observed with a scanning electron microscope (manufactured by Keyence Corporation, VE-9800), and observation images with a magnification of 10000 times were obtained for three visual fields. And about each of these observation images, three silver particles judged to be the largest were selected, and the aspect ratio was measured. The average value of the aspect ratios obtained for a total of nine particles was taken as the maximum aspect ratio. The aspect ratio is an index calculated as “a / b” where a is the maximum major axis (maximum length) of silver particles in the observed image and b is the width orthogonal to the maximum major axis. The obtained maximum aspect ratio is shown in the “maximum aspect ratio” column of Table 1.

[Specific surface area]
The specific surface area of each silver powder was measured using an automatic specific surface area / pore distribution measuring device (Macsorb HM model-1210, manufactured by Mountec Co., Ltd.). Nitrogen gas was used as the adsorption gas. The specific surface area was calculated by the BET 1-point method. The specific surface area obtained is shown in the column “BET specific surface area” in Table 1.

[Average particle size]
The average particle diameter of each silver powder was measured in two ways: an electron microscope observation method and a laser diffraction / scattering method.
In the electron microscope observation method, first, a double-sided tape was applied to a sample stage for SEM observation, and silver powder to be measured was supplied thinly and sparsely thereon, and the powder was fixed to the stage. Next, gold deposition was performed on the stage on which the silver powder was fixed, and the stage was set on an electron microscope (manufactured by Keyence Co., Ltd., VE-9800) and fixed on the stage under the conditions of an acceleration voltage of 20 kV and a magnification of 10,000 times. The powder was photographed in the powder end region where the particles could be sparse. Then, using an image analysis type particle size distribution measurement software (manufactured by Mountec Co., Ltd., Mac-View Ver. 4), the particle size distribution was automatically measured based on arbitrary 100 particles from the obtained SEM particle image. A cumulative 50% particle size was determined from the particle size distribution, and was taken as the average particle size based on electron microscope observation. The results are shown in the column of “D _SEM 50” in Table 1.

In the laser diffraction / scattering method, the particle size distribution was automatically measured using a laser diffraction / scattering particle size distribution measuring apparatus (LA-920, manufactured by Horiba, Ltd.). The average particle size was set to a 50% cumulative particle size in the volume-based particle size distribution obtained by particle size distribution measurement. The results are shown in the column “D _L 50” in Table 1.

[specific gravity]
The specific gravity of each silver powder was measured by a constant volume expansion method using a dry automatic densimeter (manufactured by Shimadzu Corporation, Micromeritics Accupic II 1340). Helium (He) was used as a replacement gas.
The true specific gravity obtained is shown in the “specific gravity” column of Table 1.

(Silver paste)
By blending 1.5 parts by mass of ethyl cellulose as a binder and 8.5 parts by mass of butyl carbitol as a dispersion medium with 90 parts by mass of the prepared silver powder, and uniformly mixing with a three-roll mill The silver pastes of Examples 1-8 were prepared. In addition, in this embodiment, in order to arrange the printability of the silver paste of each example, it adjusted so that the viscosity of a paste might be 350-450 Pa.s (10 rpm, 25 degreeC).

[Dry density]
Each silver paste was supplied to a thickness of about 150 μm on the substrate using an applicator and dried at 130 ° C. for 1 hour to form a dry coating film. Then, five samples for measurement were prepared by cutting out the dried coating film into a disk shape having a diameter of 15 mm. And the density (dry density) of the dry coating film was computed based on the following Formula by measuring the weight of this sample for a measurement, a radius, and thickness.
(Dry density) = (Weight) / {π × (Radius) ² × (Thickness)};
The weight and radius were measured once for each measurement sample. The thickness was measured at three locations for each measurement sample using a digital electronic micrometer (K351C, manufactured by Anritsu Corporation), and the average value was adopted. As the dry density, an average value (n = 5) of the values obtained for the five measurement samples was adopted and shown in the “dry density” column of Table 1.

(electrode)
This silver paste was subjected to pattern printing on a substrate by a screen printing method, dried at 130 ° C. for 30 minutes, and then fired to produce a silver line electrode (fired product) on the substrate. An alumina plate was used as the substrate. The silver paste was printed so as to have a stripe shape with a line width after firing of 200 μm, a fired thickness of 20 to 40 μm, and a pitch between lines of 200 μm. There were four firing temperatures of 600 ° C., 700 ° C., 800 ° C., and 900 ° C.
For reference, a cross section of a silver electrode obtained by firing the silver paste of Example 4 (or Example 5) in FIG. 2 and the silver paste of Example 7 (or Examples 3 and 6) in FIG. 3 at 900 ° C. An SEM image was shown.

[Electric resistivity]
The electrical resistivity of the silver pattern wiring produced as described above was measured using a digital multimeter (manufactured by Iwatatsu Measurement Co., Ltd., SC-7401). The obtained electrical resistivity was shown in the “Resistivity” column of Table 1 for each firing temperature.

  As shown in Table 1, when the relationship between the firing temperature and the electrical resistivity of all the silver pastes is observed, it can be seen that an electrode having a low electrical resistivity tends to be formed by firing at a high temperature. However, when firing at 900 ° C., which is close to 962 ° C., which is the melting point of bulk silver, it can be seen that the resistivity of some silver pastes (Example 7) increases rapidly. This is considered to be due to organic matter and gas contained in the electrode, and a large amount of large voids are formed in the electrode by burning and expanding them. Moreover, it is thought that the organic substance and gas inside an electrode originate from the unburned residue of the used silver powder and the binder resin at the time of baking.

  Regarding the silver paste using the silver powders of Examples 2, 4 and 5 having the properties disclosed herein, it was found that an electrode having a low electrical resistivity can be formed by firing at a relatively low temperature. In addition, as shown in FIG. 2, it can be seen that the electrode formed of the silver paste disclosed here has a small gap left in the electrode. On the other hand, as shown in FIG. 3, it can be seen that the electrode formed from the conventional silver paste has many large voids formed in the electrode. Large voids are undesirable because they greatly hinder the formation of conductive paths in the electrodes and increase the resistivity of the electrodes.

More specifically, for example, in the case of the silver paste of Example 2, in the range where the firing temperature is lower than 900 ° C., the resistivity of the fired product obtained by firing is 2.0 μΩ · cm or less even at a low temperature of 600 ° C. It was found that a low resistivity can be realized. It was also found that when the firing temperature is in the range of 700 ° C. to 800 ° C., the resistivity of the fired product can be as low as 1.9 μΩ · cm or less.
Moreover, about the silver paste of Example 4 and Example 5, in the range whose baking temperature is higher than 600 degreeC, the resistivity of the baked product obtained by baking is called 2.0 microhm * cm or less (1.9 microohm * cm or less), It has been found that low resistivity can be achieved. In addition, it was confirmed that the silver pastes of Examples 4 and 5 also had a relatively low resistivity of 2.1 μΩ · cm when fired at 600 ° C.

  In contrast, for the silver paste of Example 1, a fired product having a relatively low resistance can be obtained by baking at a high temperature of 800 ° C. to 900 ° C., but compared with other examples at a low temperature of 600 ° C. to 700 ° C. As a result, the resistivity increased. In particular, it has been found that the resistivity of the fired product is the highest in all cases when firing at 600 ° C. This is because silver particles with a large aspect ratio are mixed in, so the movement of particles with a large aspect ratio is hindered during firing, and the filling of the silver powder is impaired, resulting in the formation of relatively large voids in the fired product. This is thought to be due to this.

  About the silver paste of Example 3, it turned out that it is not received to the influence of the resistivity baking temperature of a baked product so much, and a low resistivity can be implement | achieved stably at 2.1-2.3 microhm * cm. However, in order to realize a low resistance of 2.1 μΩ · cm, firing at 900 ° C. is necessary, and it has been found that it is impossible to form a low resistance electrode by low-temperature firing. The silver powder used in the paste of Example 3 has a high specific surface area as can be seen from the small average particle size, and the tap density is low because the silver powder tends to aggregate. This is considered to be due to the fact that voids are easily formed between the silver particles even in the electrode being fired due to such properties.

  As for the silver paste of Example 6, a fired product having a relatively low resistance can be obtained by baking at a high temperature of 800 ° C. to 900 ° C., but it is not as high as Example 1, but another example is obtained by baking at 600 ° C. It was found that the resistivity of the fired product was higher than that. Although the silver powder of Example 6 is not so small in average particle diameter, it can be said that there are particles having a high aspect ratio, a high specific surface area, and few particles close to a true sphere. For this reason, it is considered that the tap density is low, and even in the electrode being fired, it is easy to remain with voids formed between the silver particles.

  For the silver pastes of Examples 7 and 8, a fired product having a high resistivity as a whole is obtained regardless of the firing temperature. This is presumably because the silver powders of Examples 7 and 8 have a high Ig-loss and the amount of volatilization of the organic component during firing of the electrode is large, so that voids are easily formed in the electrode. Low Ig-loss is preferable because, for example, it can be an advantageous feature in the production of MLCCs. In addition, in the comparison between the silver pastes of Example 7 and Example 8, the paste of Example 8 uses finer silver powder. Therefore, it is considered that the silver paste of Example 8 has higher sinterability and a slightly lower resistivity was achieved.

  The silver powder is characterized to some extent by its manufacturing method. For example, when a large amount of a protective material or the like is included in the raw material liquid for forming silver powder by a wet method, an atomizing method, or the like, the Ig-loss of the manufactured silver powder can be increased. Further, according to the PVD method, a silver powder that is relatively spherical and has a small maximum aspect ratio can be easily produced. However, as shown in the above example, it is understood that the silver powder for electrode formation is not limited only to the production method, and suitability can be determined by the properties of the silver particles themselves.

The silver powders of Examples 2, 4 and 5 that satisfy the properties disclosed herein have a specific gravity of 10.4 g / cm ³ or more. Moreover, it turns out that the silver paste using the silver powder of Example 2, Example 4, Example 5 implement | achieves the high value whose dry density is 6.9 g / cm < ³ > or more when a dry film is formed. Thus, it can be seen that a silver powder satisfying the properties disclosed herein is particularly useful for applications where it is supplied on a substrate in the form of a paste to form an electrode.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. For example, in the above example, the composition of the silver paste is constant, but the binder and the dispersant in the silver paste are components that are burned out by firing, and are also disclosed here because they depend on the printing method and printing conditions. It will be understood by those skilled in the art that it has no essential effect on the technology. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

DESCRIPTION OF SYMBOLS 1 Multilayer chip inductor 10 Electronic element main body 12 Dielectric layer 20 External electrode 22 Internal electrode

Claims (10)

A silver powder used to form an electrode of an electronic device,
The following (1) to (4):
(1) The loss on ignition when the sample dried at a drying temperature of 110 ° C. is heated to 600 ° C. is 0.05% or less;
(2) The tap density is 5 g / cm ³ or more;
(3) the maximum aspect ratio is 1.4 or less;
(4) The specific surface area based on the BET method is 0.8 m ² / g or less;
Silver powder that satisfies all of these conditions.   (5) The silver powder according to claim 1, wherein an average particle diameter based on observation with an electron microscope is 1 μm or more and 3 μm or less. (6) The silver powder according to claim 1 or 2, wherein the specific gravity is 10.4 g / cm ³ or more.   The silver paste containing the silver powder of any one of Claims 1-3, binder resin, and a dispersion medium.   The silver paste of Claim 4 comprised so that the electrical resistivity of a silver baked product obtained when it bakes in the temperature range of 700 degreeC or more and 800 degrees C or less will achieve 2 microhm * cm or less.   The silver paste of Claim 4 or 5 comprised so that the electrical resistivity of 2.1 micrometers ohm * cm or less may be achieved when it bakes in the temperature range of 600 degreeC or more and 900 degrees C or less.   It is comprised so that the electrical resistivity of the silver baked product obtained when it bakes in the temperature range of 600 degreeC or more and 800 degrees C or less will achieve 2 microohm * cm or less. Silver paste.   An electronic device comprising the fired product of the silver paste according to any one of claims 4 to 7 as an electrode.   The electronic device according to claim 8, wherein the electrical resistivity of the electrode is 2 μΩ · cm or less. A ceramic base material, and an internal electrode disposed inside the ceramic base material,
The electronic device according to claim 8, wherein the electrode is provided as the internal electrode.