JP2022134171A - Silver paste, and use of the same - Google Patents

Abstract

To provide a silver paste for reducing exfoliation of an inner part of a silver film which can be generated when forming a silver film.SOLUTION: The silver paste herein disclosed is used for a silver film formed on a gold film, and includes at least a silver particle and a glass, where the glass has a softening point of 550°C or higher and 900°C or lower, and when calcining a disk-like test piece composed of the glass having a diameter of 15 mm and a height of 6 mm at 830°C under an atmospheric pressure, the diameter of the test piece after calcination is 0.9 times or more and 2.6 times or less.SELECTED DRAWING: Figure 3D

Description

The present invention relates to a silver paste, an electrode comprising a fired body of such silver paste, and a method for producing the electrode.

2. Description of the Related Art In electronic elements for electric/electronic parts, a technique of forming a conductive film on an insulating substrate using a conductive material without using a conductive wire and wiring with this conductive film is widely adopted.

For example, gold (Au) is used as a conductor film (electrode) for heating a resistor in a thermal print head. However, from the viewpoint of cost reduction, it has been proposed to change part of the electrodes from gold to silver (Ag). In Patent Document 1, as an example of such a configuration, an electrode layer (first layer) containing gold is arranged on the electrode at a position in contact with the resistor, and the electrode layer (first layer) is spaced from the resistor and contains silver on the first layer. A configuration in which an electrode layer (second layer) is formed is disclosed. Since gold diffuses to a resistor (for example, a resistor made of ruthenium oxide) to a lesser extent than silver does, the electrode layer containing gold is used for the part in contact with the resistor, while the other part is an electrode layer containing silver. As a result, the cost can be reduced and the deterioration of the resistor and the electrode layer can be suppressed.

Japanese Unexamined Patent Application Publication No. 2018-103608

By the way, as an example of a method of forming an electrode layer (silver film) containing silver on an electrode layer (gold film) containing gold, there is a method of applying (including printing) a silver paste on the gold film and firing the paste. be done. However, as a result of investigation by the present inventors, it was found that there is a problem that peeling occurs inside the silver film due to such baking. Peeling off inside the silver film not only results in a poor appearance, but also causes problems such as an increase in electrical resistance and accelerated deterioration of the electrode, which is not preferable.

Accordingly, the present invention has been made in view of the circumstances described above, and an object thereof is to provide a silver paste that reduces peeling inside the silver film that may occur during the formation of the silver film. Another object of the present invention is to provide an electrode that is a baked product of such a silver paste and a method for producing such an electrode.

In order to achieve the above object, the inventors of the present invention conducted a study, and it was confirmed that peeling occurred inside the silver film by baking the silver paste applied on the gold film at a temperature of at least 550 ° C. or higher. . Therefore, the present inventors thought that it would be possible to form a silver film with reduced peeling by devising to bond (repair) the peeling that occurred during firing, instead of preventing such peeling from occurring. We made a thorough study. As a result, the inventors have found that a silver film with reduced peeling can be formed by adding a glass having a softening point within a predetermined range and a fluidity within a predetermined range to a silver paste.

That is, the silver paste disclosed herein is a silver paste that is used for a silver film formed on a gold film, and contains at least silver particles and glass, and in the glass, the softening point of the glass is When a disk-shaped test piece with a diameter of 15 mm and a height of 6 mm made of the above glass and having a temperature of 550 ° C. or more and 900 ° C. or less is fired at 830 ° C. under atmospheric pressure, the diameter of the test piece after firing is 0.9 times. 2.6 times or less.
According to this configuration, since the softening point of the glass contained in the silver paste is at least the temperature (550° C.) at which peeling occurs inside the silver film, the glass softens after peeling occurs inside the silver film. This creates a force that attracts the softened glasses to each other. As a result, the softened glass around the peeled portion adheres to the peeled portion, and peeling can be reduced.

In a preferred aspect of the silver paste disclosed herein, the glass is contained in an amount of 0.5 vol % or more when the entire silver paste is 100 vol %.
By including glass in such a range, the effect of adhering the peeled portion is enhanced, so peeling inside the silver film can be further reduced.

Further, in a preferred aspect of the silver paste disclosed herein, the silver particles are contained in an amount of 60 wt % or more when the entire silver paste is 100 wt %.
According to such a configuration, the silver particles are arranged together with the glass in the adhesive portion of the peeled portion, so a silver paste for forming a highly conductive silver film with a reduced peeled portion is provided.

Also, to achieve the above objects, an electrode and a method for manufacturing the electrode are provided. The electrodes disclosed herein comprise a fired body of the silver paste disclosed herein. Thereby, an electrode with reduced peeling is realized.
Further, the method for manufacturing an electrode disclosed herein includes the steps of applying the silver paste disclosed herein to a predetermined base material, and firing at a temperature higher than the softening point of the glass contained in the silver paste. including. Also, in a preferred aspect, the electrode is an electrode for a thermal print head. This makes it possible to manufacture an electrode in which peeling inside the silver film is reduced.

1 is a cross-sectional view schematically showing the configuration of an electrode provided with a fired body (silver film) of silver paste according to one embodiment; FIG. 10 is an FE-SEM cross-sectional secondary electron image at a magnification of 1000 times of the silver film after firing at 830° C. of the electrode of Example 5. FIG. 10 is an FE-SEM cross-sectional secondary electron image at a magnification of 1000 times of the silver film after firing at 830° C. of the electrode of Example 7. FIG. 10 is an FE-SEM cross-sectional secondary electron image at a magnification of 5000 times of the silver film after firing at 550° C. of the electrode of Example 5. FIG. 10 is an FE-SEM cross-sectional secondary electron image at a magnification of 5000 times of the silver film of the electrode of Example 5 after baking at 650° C. FIG. 10 is an FE-SEM cross-sectional secondary electron image at a magnification of 5000 times of the silver film of the electrode of Example 5 after sintering at 750° C. FIG. FIG. 10 is an FE-SEM cross-sectional secondary electron image at a magnification of 5000 times of the silver film of the electrode of Example 5 after sintering at 830° C. FIG.

Preferred embodiments of the technology disclosed herein are described below. Matters other than those specifically mentioned in this specification (for example, the composition of the silver paste) that are necessary for implementation (for example, the method of supplying the silver paste, the method of manufacturing the gold film, etc.) It can be understood based on the technical content taught by this specification and the general technical knowledge of those skilled in the art. The content of the technology disclosed here can be implemented based on the content disclosed in this specification and common general knowledge in the field. In this specification, the notation "A to B" indicating the range means from A to B. Therefore, the case above A and below B is included.

Since the silver paste disclosed herein is preferably used to form a silver film on a gold film, an electrode comprising a gold film and a silver film will be described below with reference to the drawings. In the drawings below, members and portions having the same function are denoted by the same reference numerals, and redundant description may be omitted or simplified. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not necessarily reflect the actual dimensional relationships.

[electrode]
FIG. 1 is a cross-sectional view schematically showing the configuration of an electrode provided with a sintered silver paste (silver film) disclosed herein. As shown in FIG. 1, electrodes 20 are formed on the surface of the substrate 10 . The electrode 20 comprises a gold film 22 and a silver film 24 , with the gold film 22 formed on the substrate 10 . Further, the electrode 20 has a portion having a layered structure (a two-layer structure in FIG. 1) in which a silver film 24 is formed on a gold film 22 . By forming the silver film 24 on the gold film 22, cost reduction and electric resistance reduction can be realized. In addition, the silver film 24 does not necessarily have to be formed only on the gold film 22 , and the silver film 24 may have a portion formed on the substrate 10 .

The substrate 10 is not particularly limited, but is typically made of an insulating material such as a ceramic material (eg, alumina, etc.). Further, the surface of the substrate 10 may be coated with a glass material (for example, amorphous glass) (so-called glaze substrate). The gold film 22 is a conductive film with high electrical conductivity (simply referred to as “conductivity”), and is formed, for example, by applying a resinate containing gold (Au) to the surface of the substrate 10 and baking the resinate. The silver film 24 is a highly conductive conductor film and can generally be manufactured at a lower cost than the gold film 22 . The silver film 24 is preferably formed using the silver paste disclosed herein. The silver paste disclosed herein will be described below.

[Silver paste]
The silver paste disclosed here contains silver particles and glass. In addition, typically, an organic binder, solvent, etc. are included in order to improve the feedability and handling of these. A silver film (silver electrode) can be formed by sintering silver particles together by supplying silver particles in the form of a paste to a desired substrate in a desired form and firing the paste.

[Silver particles]
The silver particles contained in the silver paste disclosed herein are materials for forming conductive films (electrodes) with high electrical conductivity, which are conducting wires in electronic devices and the like. Silver (Ag) is preferable as a conductive material because it is less expensive than gold (Au), is resistant to oxidation, and has excellent conductivity. Silver particles are typically used in the form of silver powder, which is an aggregate of a plurality of silver particles. 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 silver powder having desired conductivity and other physical properties can be used. Here, the main component means the largest component among the components that constitute the silver powder. Examples of silver powders include those composed of silver, silver alloys, mixtures or composites thereof, and the like. Preferred examples of silver alloys include silver-palladium (Ag-Pd) alloys, silver-platinum (Ag-Pt) alloys, silver-copper (Ag-Cu) alloys, and the like. For example, it is possible to use core-shell particles in which the core is made of a metal other than silver, such as copper or a silver alloy, and the shell covering the core is made of silver. Core-shell particles in which the core is made of silver and the shell covering the core is made of a material other than silver, such as copper or a silver alloy, may also be used. The higher the purity (content) of the silver powder, the higher the electrical conductivity. The purity of the silver powder is preferably 95% or higher, more preferably 97% or higher, even more preferably 98% or higher, and particularly preferably 99% or higher. According to the technology disclosed herein, it is possible to form a conductor film with extremely low resistance, for example, by using silver powder with a purity of about 99.5% or more (for example, about 99.8% or more). be done. From this point of view, the technology disclosed herein forms a sufficiently low-resistance conductor film even when silver powder having a purity of 99.99% or less (for example, 99.9% or less) is used. It is possible.

The shape of the silver particles is not particularly limited, and may be spherical, tabular, acicular, amorphous, and the like. Typically, spherical particles are used. In addition, the average particle size of the silver powder is not particularly limited, but if the average particle size of the silver powder is too small, although sintering proceeds at a lower temperature, it tends to aggregate and the silver particles are easily packed during firing. It is not preferable because it reduces the performance. Therefore, it is suitable that the average particle size of the silver powder is, for example, 0.2 μm or more. The average particle size may be 0.3 μm or more, preferably 0.5 μm or more, more preferably 0.7 μm or more, still more preferably 1 μm or more, and particularly preferably 1.2 μm or more. Moreover, when the average particle size of silver powder is too large, it is necessary to expose to high temperature for a long time for sintering. Therefore, the average particle size of the silver powder can be, for example, 5 μm or less. 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.
The "average particle size" used herein is the particle size (D50) when the cumulative volume is 50% in the volume-based particle size distribution based on the laser diffraction/scattering method.

Moreover, as silver powder, the thing with sharp (narrow) particle size distribution is preferable. For example, silver powder that does not substantially contain particles having an average particle size of 10 μm or more can be preferably used. Furthermore, as an index of the sharpness of the particle size distribution, the particle size (D10) at the cumulative 10% volume 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 ( D90) and the ratio (D10/D90) can be employed. When the particle diameters constituting the powder are all equal, the value of D10/D90 is 1, and conversely, the value of D10/D90 approaches 0 as the particle size distribution becomes wider. It is preferred to use powders with a relatively narrow particle size distribution such that the value of D10/D90 is greater than or equal to 0.15, such as greater than or equal to 0.15 and less than or equal to 0.5.

In another aspect, the silver powder can also be used by mixing two particle groups with different average particle sizes. In this case, for example, the average particle diameter (D50) of the first particle group is in the range of 2 μm to 5 μm (eg, 2 μm), and the average particle diameter (D50) of the second particle group is 0.5 μm to 2 μm (eg, 0 0.5 μm) is a preferred example. At this time, the particle size distribution of each particle group is preferably sharp as described above. Then, for example, the first particle group is mixed at a ratio of 65 to 90 wt% (eg, 70 wt%) and the second particle group is mixed at a ratio of 35 to 10 wt% (eg, 30 wt%). This makes it possible to prepare a silver powder with good filling properties.
A silver powder having such average particle size and particle size distribution characteristics can form a dense conductor film with good filling properties. This is advantageous in forming conductor films with lower resistivity.

The proportion of silver powder in the entire silver paste (100 wt%) is not particularly limited, but is typically 60 wt% or more, for example, may be 70 wt% or more, or 80 wt% or more. is preferred. As a result, when the peeled portion inside the silver film is adhered, not only the glass but also the silver particles are easily arranged at such positions. Therefore, high conductivity can be maintained even in the portion where the peeled portion is adhered. In addition, the higher the proportion of silver powder, the more dense the electrode is formed, and the higher the conductivity. is more preferred.

[Glass]
The glass contained in the silver paste disclosed herein is a material for reducing peeling that may occur inside the silver film, which is the fired body of the silver paste. Glass has the property of starting to deform under its own weight when it reaches its softening point. Therefore, the glass that reaches the softening point in the silver paste begins to wet the surface of the silver particles, enters between the silver particles and between the silver particles and the base material to which the silver paste is applied, binding them together. have the effect of At this time, a force (surface tension) that attracts the softened glasses due to an intermolecular force is generated. As a result, when peeling occurs inside the silver film, the softened glass existing around the peeled portion attracts each other, so that the peeled portion can be adhered (repaired).

In order to properly exhibit such effects, the softening point of the glass is preferably higher than the temperature at which peeling occurs inside the silver film. Although details will be described later, according to studies by the present inventors, peeling occurs when the silver paste applied to the gold film is baked at least at 550°C. Therefore, the softening point of glass is preferably 550° C. or higher, and may be, for example, 570° C. or higher or 590° C. or higher. On the other hand, if the softening point of the glass is higher than the firing temperature for forming the silver film, the softening of the glass is insufficient (or the glass is not softened), and the above effects cannot be obtained. Therefore, the softening point of glass is set lower than the firing temperature. The sintering temperature of the silver paste can be appropriately determined based on the sintering temperature of the silver powder. Therefore, although the firing temperature is not particularly limited, it can be, for example, about 700° C. to 900° C., and is typically set to about 830° C., similar to the firing temperature of conventional silver powder. be done. Therefore, the softening point of suitable glass is, for example, 900° C. or lower, preferably 830° C. or lower, more preferably 800° C. or lower, and 720° C. or lower (for example, 716° C. or lower). More preferred. Further, the larger the difference between the softening point of the glass and the firing temperature T° C., the more softened the glass, which is preferable. As the glass softens more, the surface tension works more easily, so that the peeled portion is adhered more favorably. Therefore, the softening point of the glass is preferably set lower than the firing temperature by 50°C or more, preferably 80°C or more, and more preferably 100°C or more.

In addition, if the fluidity of the softened glass is too low, the glass tends to stay on the surface of the silver particles, so the softened glass around the peeled portion cannot be sufficiently attracted to each other, and the reduction of the peeled portion may be insufficient. . On the other hand, if the fluidity is too high, the force of attraction between the softened glasses becomes insufficient, so that the peeled portion cannot be sufficiently adhered. Therefore, it is desirable that the fluidity when the glass is softened is within an appropriate range. The fluidity of the glass contained in the silver pastes disclosed herein is measured by the button flow test. Specifically, a disk-shaped test piece (glass button) with a diameter of 15 mm and a height of 6 mm was produced by press molding (typically, a press pressure of 20 kg/cm ² or more), and was heated at 830 ° C. under atmospheric pressure. Measure the diameter of the specimen when fired for a given time (typically 0.25 to 1 hour). Then, the diameter of the test piece before firing and the diameter of the test piece after firing are compared to calculate the rate of change. Glass having preferable fluidity has a diameter of the test piece after firing of 0.9 times or more and 2.6 times or less, more preferably 1 time or more and 2.6 times the diameter of the test piece before firing. or less, more preferably 1 to 1.5 times. If the fluidity is in this range, peeling inside the silver film can be suitably reduced. In addition, since high surface tension acts on glass with low fluidity, when the glass is softened, the test piece may shrink rather than expand in the outer diameter direction of the circle. In such cases, the diameter of the glass button may be smaller after firing than before firing. In the button flow test, the same glass is tested with a plurality of test pieces (for example, 4 or more), and the average value thereof is used. In addition, since the glass after firing may have an elliptical shape or the like, in such a case, the longest diameter in plan view is the major diameter, and the longest diameter of the lines perpendicular to the major diameter is the minor diameter. Then, the area of the ellipse consisting of the major axis and the minor axis is used to convert the equivalent circle diameter to obtain the diameter.

The shape of the glass is not particularly limited, but powdered ones are typically used. In addition, since the glass powder is uniformly mixed with the silver paste, even if peeling occurs at any position inside the silver film, such peeling can be adhered. Therefore, although not particularly limited, the size of the glass powder is preferably about the same as or smaller than the size of the silver particles. Therefore, the average particle size of the glass powder based on the laser scattering diffraction method is suitably 4 μm or less, preferably 3 μm or less, may be 2 μm or less, and may be, for example, 1.5 μm or less. Although not particularly limited, the average particle size of the glass powder is suitably 0.2 μm or more, preferably 0.5 μm or more, more preferably 0.8 μm or more, and may be, for example, 1 μm or more. .

The content of glass contained in the silver paste is preferably 0.5 vol % or more when the entire silver paste is 100 vol %. With such a ratio, the effect of adhering the peeled portion inside the silver film formed from the silver paste is enhanced, so peeling inside the silver film can be further reduced. Also, the higher the glass content, the higher the adhesive effect of peeling. Therefore, the glass content may be, for example, 1 vol % or more or 1.5 vol % or more. On the other hand, the higher the glass content, the higher the electrical resistance of the silver film. Therefore, the glass content is, for example, suitably 10 vol % or less, preferably 5 vol % or less, and more preferably 3 vol % or less.

As described above, the glass contained in the silver paste exhibits the effect of reducing peeling inside the silver film by having a softening point and fluidity within an appropriate range, so the constituent elements of the glass are not particularly limited. Instead, glasses of various compositions can be used. For example, as an approximate glass composition, so-called lead-based glass, lead-lithium-based glass, zinc-based glass, bismuth-based glass, borate-based glass, borosilicate-based glass, which are names commonly expressed by those skilled in the art, Alkaline glass, lead-free glass, tellurium glass, and glass containing barium oxide, bismuth oxide, or the like may be used. Needless to say, these glasses include Si, Pb, Zn, Ba, Bi, B, Al, Li, Na, K, Rb, Te, Ag, Zr, and Sn in addition to the main glass constituent elements appearing in the above names. , Ti, W, Cs, Ge, Ga, In, Ni, Ca, Cu, Mg, Sr, Se, Mo, Y, As, La, Nd, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu , Ta, V, Fe, Hf, Cr, Cd, Sb, F, Mn, P, Ce and Nb. Such glass may be, for example, general amorphous glass or crystallized glass partially containing crystals. As for the glass, glass having one composition may be used alone, or glass having two or more compositions may be mixed and used.

Although the glass composition is not particularly limited, examples of the glass composition contained in the silver paste disclosed herein include at least _two selected from the group consisting of _{Bi2O3 , SiO2 ,} and B2O3. It is preferable to contain oxides as main constituents, and for example, it is preferable that each oxide is contained in an amount of 25 mol % or more with respect to the whole glass (100 mol %). That is, it is preferable that 50 mol % or more of the constituent components of the glass be composed of at least two selected from the group consisting of SiO ₂ , B ₂ O ₃ and Bi ₂ O ₃ . These glasses also contain, as other components, alkali metal components (typically Li ₂ O, Na ₂ O, K ₂ O), broadly defined alkaline earth metal components (typically MgO, CaO, BaO, SrO), aluminum component (typically _Al2O3 ) _, zinc component (typically ZnO), transition metal component ₍ e.g., _TiO2 , ZrO2, _MnO2 , etc.), etc. obtain.

A silicon component (typically SiO ₂ ) is a component that constitutes the framework of glass, has high thermal stability, and is a component that raises the softening point. Although not particularly limited, the silicon component may be contained, for example, at a molar ratio of oxide conversion of 25 mol% or more (preferably 30 mol% or more) to the entire glass (100 mol%), or 50 mol%. % or less (preferably 40 mol % or less). As a result, the softening point of the glass can be easily adjusted to the peeling temperature (550° C.) or higher of the silver film, so that the peeling inside the silver film can be preferably repaired.

A boron component (typically B ₂ O ₃ ) is a component that increases the fluidity of the glass and lowers the softening point. Although not particularly limited, the boron component may be contained, for example, at a molar ratio of oxide conversion of 25 mol% or more (preferably 30 mol% or more) with respect to the entire glass (100 mol%), or 50 mol%. % or less (preferably 40 mol % or less). This makes it possible to easily adjust the difference between the softening point of the glass and the firing temperature at the time of forming the silver film so as to increase the difference, so that peeling inside the silver film can be preferably repaired.

A bismuth component (typically Bi ₂ O ₃ ) is a component that increases the fluidity of the glass and lowers the softening point. Although not particularly limited, the bismuth component may be contained, for example, at a molar ratio of oxide conversion of 25 mol% or more (preferably 40 mol% or more) to the entire glass (100 mol%), or 60 mol%. % or less (preferably 55 mol % or less). As a result, the difference between the softening point of the glass and the firing temperature at the time of forming the silver film can be adjusted to be large, so peeling inside the silver film can be suitably repaired.

Alkali metal components (typically Li ₂ O, Na ₂ O, K ₂ O) are components that have the effect of imparting fluidity to the glass and lowering the softening point. Although it is not particularly limited, the alkali metal component may be contained, for example, at a molar ratio of oxide conversion of 0.1 mol% or more (preferably 2 mol% or more) with respect to the entire glass (100 mol%). It is good that it is 10 mol % or less (preferably 8 mol % or less). As a result, the difference between the softening point of the glass and the firing temperature at the time of forming the silver film can be adjusted to be large, so that peeling inside the silver film can be preferably repaired. The alkali metal component may be contained singly or in combination of two or more.

Broadly defined alkaline earth metals other than barium (typically MgO, CaO, SrO) function as network modifiers. Thereby, the physical stability and thermal stability of the glass can be improved, and the softening point can be raised. Although not particularly limited, the alkaline earth metal component is contained, for example, at a molar ratio of 0.1 mol% or more (preferably 5 mol% or more) in terms of oxide with respect to the entire glass (100 mol%). 15 mol % or less (preferably 10 mol % or less). Thereby, the softening point and fluidity of the glass can be adjusted within suitable ranges. The alkaline earth metal component may be contained singly or in combination of two or more.

A barium component (typically BaO) is a component that can improve the thermal stability of the glass and increase the softening point. Although not particularly limited, the barium component may be contained, for example, at a molar ratio of oxide conversion of 0.1 mol% or more (preferably 5 mol% or more) with respect to the entire glass (100 mol%). , 20 mol % or less (preferably 15 mol % or less). Thereby, the softening point and fluidity of the glass can be adjusted within suitable ranges.

An aluminum component (typically Al ₂ O ₃ ) is a component that controls fluidity and contributes to adhesion stability. Although not particularly limited, the aluminum component may be contained, for example, at a molar ratio of oxide conversion of 0.1 mol% or more (preferably 2 mol% or more) with respect to the entire glass (100 mol%). , 10 mol % or less (preferably 8 mol % or less). Thereby, the softening point and fluidity of the glass can be adjusted within suitable ranges.

A zinc component (typically ZnO) is a component highly effective in improving the thermal stability of glass, and is a component capable of raising the softening point. Although not particularly limited, the zinc component may be contained, for example, at a molar ratio of oxide conversion of 0.1 mol% or more (preferably 5 mol% or more) with respect to the entire glass (100 mol%). , 15 mol % or less (preferably 10 mol % or less). Thereby, the softening point and fluidity of the glass can be adjusted within suitable ranges.

Transition metal components ₍ eg, _TiO2 , ZrO2, _MnO2 , etc.) can be used to tune thermal stability. Although not particularly limited, the transition metal component may be contained, for example, at a molar ratio of oxide conversion of 0.1 mol% or more (preferably 2 mol% or more) with respect to the entire glass (100 mol%). It is good that it is 10 mol % or less (preferably 8 mol % or less). The transition metal component may be contained singly or in combination of two or more.

[Organic binder]
The organic binder is a component that plays a role in binding silver particles in the silver paste together and binding the silver particles and the base material (eg, gold film, etc.) to which the silver paste is applied. This makes it possible to improve the shape stability from when the silver paste is applied (or may be printed) on the base material to when it is fired. On the other hand, the organic binder can become an unnecessary resistance component after the silver particles are integrated by firing. Therefore, it is preferable that the component disappears at a temperature lower than the firing temperature of the silver paste and does not remain in the silver film. As such an organic binder, an organic compound having a binder function can be used without particular limitation. For example, 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 Binder resins based on vinyl resins such as alcohol and polyvinyl butyral, and rosin resins such as rosin and maleated rosin are preferably used. In addition, an organic binder may be used individually by 1 type, and may be used in combination of 2 or more type.

From the viewpoint of improving the shape stability of the silver paste, the content of the organic binder is preferably 0.5 wt% or more, for example, 1 wt% or more, when the entire silver paste is 100 wt%. good. On the other hand, if the organic binder is excessively contained, the amount of the organic binder that does not disappear by firing may increase. Good to have.

[solvent]
The solvent can dissolve or disperse the organic binder, and an organic solvent having a boiling point of about 200° C. or higher (typically about 200° C. to 260° C.) can be preferably used. Also, 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. Organic solvents such as derivatives, toluene, xylene, mineral spirits, terpineol, dihydroterpineol and texanol can be preferably used. In addition, a solvent may be used individually by 1 type, and may be used in combination of 2 or more type.

The solvent and organic binder build a dispersion system (vehicle) that disperses the silver particles and glass. The total ratio of the solvent and the organic binder may be appropriately changed according to the properties of the silver paste. It is preferably 15 wt %, more preferably 5 wt % to 10 wt %. The organic binder may contain an organic binder component dissolved in the solvent and an organic binder component not dissolved.

The silver paste disclosed herein can contain various additives other than those described above within a range not departing from the object of the present invention. Preferable examples of such additives include additives such as surfactants, antifoaming agents, antioxidants, dispersants, and rheology modifiers.

Such a silver paste can be prepared by weighing the materials described above so as to have a predetermined composition (mass ratio) and mixing them homogeneously. Stirring and mixing of the materials can be carried out 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 the like.

The silver paste disclosed here can be suitably used as a silver paste for forming a silver film on a gold film. For example, it can be suitably used for the electrodes of a thermal print head, and can be suitably used for both the individual electrode portion and the common electrode portion of the thermal print head.

An example of a suitable method for manufacturing an electrode provided with the silver paste sintered body disclosed herein will be described below. In addition, the manufacturing method of such an electrode is not limited to the following method.

The electrode manufacturing method disclosed herein includes a step of applying the silver paste disclosed herein to a predetermined base material (hereinafter also referred to as an “application step”), and the softening point of the glass contained in the silver paste. and a step of firing at a higher temperature (hereinafter also referred to as a “firing step”).

First, the coating process will be described. In the application step, the silver paste disclosed herein is prepared and applied (typically printed) onto a predetermined substrate. Such a substrate is not particularly limited as long as it has heat resistance that can withstand the firing temperature in the firing step described later. Since the silver paste disclosed herein is suitably used for forming a silver film on a gold film, typically a gold film or a substrate having a gold film on its surface is used as such a substrate. The coating method may follow a known method, and for example, screen printing, gravure printing, offset printing, inkjet printing, etc. can be applied.

Next, the firing process will be described. In the firing step, the firing temperature of the silver paste is set to a temperature equal to or higher than the softening point of the glass contained in the silver paste. Also, typically, it is appropriately determined based on the sintering temperature of the silver powder. Although the firing temperature depends on the composition of the silver powder, it can be lower than the melting point of 962° C. for silver powder that can be regarded as pure silver (for example, purity of 99.9% or higher). Therefore, although the firing temperature is not particularly limited, it can typically be about 700° C. to 900° C., preferably 850° C. or less (for example, 830° C. degree). Moreover, since such a temperature can be determined based on the properties of the silver powder, it may be fired at a temperature lower than 800°C (for example, about 770°C).

Also, the difference between the firing temperature and the softening point of the glass is preferably large. ° C. or more is more preferable. As a result, the glass contained in the silver paste can be sufficiently softened during firing, so that an electrode having a silver film with reduced peeling can be manufactured.

Examples relating to the technology disclosed herein will be described below, but these examples are not intended to limit the present invention.

(Test 1)
<Preparation of silver paste>
Silver powder (spherical, average particle size: 2.5 μm), glass powder (average particle size: 1.2 μm), ethyl cellulose (EC) as an organic binder, and diethylene glycol monobutyl ether (BC) as a solvent were prepared. First, after EC was mixed with BC, the mixture was heated to about 100° C. for several hours to prepare a vehicle. This vehicle, silver powder and glass powder were uniformly mixed in a three-roll mill to prepare a silver paste.
At this time, 3 vol% of the glass was added to the entire silver paste, and 90 wt% or more of the silver particles and 1.5 wt% of the EC were added to the weight of the entire silver paste (100 wt%). was blended with Since the weight of glass differs depending on the composition of the glass used in each example, the amount of the solvent was adjusted to 100 wt %. The following glass powders were used in each example. The following Bi ₂ O ₃ -based glass refers to glass in which Bi ₂ O ₃ is the most abundant constituent of the glass and the second most abundant constituent is less than 25 mol %. In addition, XZ-based glass (where X and Z represent different oxides) means that oxide X and oxide Z are the first and second most common constituents of the glass, and each constitutes the glass. It refers to those that occupy 25 mol % or more of the component.
Example 1: Bi ₂ O ₃ system glass A
Example 2: Bi ₂ O ₃ system glass B
Example 3: Bi ₂ O ₃ —SiO ₂ based glass Example 4: Bi ₂ O ₃ —B ₂ O ₃ based glass Example 5: B ₂ O ₃ —SiO ₂ based glass A
Example 6: B ₂ O ₃ —SiO ₂ based glass B
Example 7: ZnO—SiO ₂ based glass

<Button flow test>
A disk-shaped test piece having a diameter of 15 mm and a height of 6 mm was produced by press-molding the glass powder of each example. The pressure during press molding was set to 20 kg/cm ² or more. The test piece is placed on an alumina substrate, heated in an electric furnace from room temperature to 830 ° C. at a heating rate of 70 ° C./min in an atmospheric atmosphere (under atmospheric pressure), held for 15 minutes, and then cooled. did. Then, the diameter of the test piece was measured. Four test pieces were prepared for each example, and the average value of these diameters was obtained. Table 1 shows the result of the change ratio of the diameter (that is, the value of "average diameter after firing/15 mm") obtained from the average value.

<Measurement of softening point>
The softening point of the glass powder of each example was measured using a common precision DTA (differential thermal analysis). Typically, a plurality of endothermic peaks appear in the DTA curve, so the temperature corresponding to the first peak is the glass transition point, the temperature corresponding to the second peak is the yield point, and the temperature corresponding to the third peak is the softening point. As, the softening point was measured. Table 1 shows the results.

<Production of test electrodes>
A glaze substrate having a gold film (thickness of about 0.4 μm) on its surface was prepared, and the silver paste prepared above was applied onto the gold film by screen printing. After coating, the silver film was formed on the gold film by heating to 830° C. at a rate of temperature increase of 70° C./min, holding for 15 minutes and firing. In this manner, a test electrode for each example was produced.

<Peeling evaluation inside the silver film>
The manufactured test electrode was cut in the lamination direction of the gold film and the silver film. Then, the cut surface was observed at a magnification of 1000 using a field emission scanning microscope (FE-SEM) to evaluate the presence or absence of peeling. At this time, when peeling with a length of 25 μm or more was not observed, “◯” was given, and when peeling with a length of 25 μm or more was observed, “X” was given. The results are shown in Table 1. 2A shows an FE-SEM cross-sectional secondary electron image of the electrode of Example 5, and FIG. 2B shows an FE-SEM cross-sectional secondary electron image of the electrode of Example 7 as representative views.

As shown in Table 1, Examples 3 to 6 reduced the peeling inside the silver film, while Examples 1, 2 and 7 did not sufficiently reduce the peeling. As is clear from the comparison between FIG. 2A and FIG. 2B shown as representative diagrams, peeling occurs in the silver film in Example 7 in which the peeling evaluation is "x", whereas peeling evaluation is "○". In Example 5, it can be seen that there is no peeling of 25 μm or more in the silver film. In particular, the peeling observed in Examples 3, 5, and 6 was less than 10 μm, and the effect of reducing peeling was high. From these results, when the softening point of the glass is 529 ° C. or less, or when the fluidity of the glass is too low, that is, when the button flow test at 830 ° C. firing is performed, the diameter change ratio is less than 0.90 (more specifically, 0.85 or less), the peeling reduction effect is insufficient. Therefore, the softening point of the glass is not 529 ° C. or lower (e.g., 550 ° C. or higher or 570 ° C. or higher), and the diameter change ratio in the button flow test at 830 ° C. firing is 0.9 or higher (e.g., 1 or higher). It can be seen that by using a glass having a fluidity of 3 or less (specifically, 2.55 or less), the peeling can be more preferably reduced.

(Test 2)
<Observation of peeling inside the silver film>
The silver paste of Example 5 was screen-printed onto an alumina substrate with a gold film, and fired stepwise in an electric furnace at 550°C, 650°C, 750°C, and 830°C in this order. After firing at each temperature, a part of the electrode was cut out for peeling evaluation, and the cross section was observed with FE-SEM at a magnification of 5000 to obtain a cross-sectional secondary electron image. Such cross-sectional secondary electron images are shown in FIGS. 3A to 3D. FIG. 3A is an FE-SEM cross-sectional secondary electron image of the silver film after sintering at 550°C. FIG. 3B is an FE-SEM cross-sectional secondary electron image of the silver film after baking at 650°C. FIG. 3C is an FE-SEM cross-sectional secondary electron image of the silver film after sintering at 750°C. FIG. 3D is an FE-SEM cross-sectional secondary electron image of the silver film after sintering at 830°C.

As shown in FIG. 3A, it can be seen that sintering at 550° C. causes peeling inside the silver film. In addition, as shown in FIG. 3B, it can be seen that the peeling inside the silver film is maintained even after firing at 650.degree. On the other hand, in FIG. 3C, it can be seen that the peeled portion is reduced. This is probably because the softening point of the glass in the silver paste of Example 5 is 690° C., so that the firing at 750° C. softened the glass and adhered the peeled portion. Furthermore, in FIG. 3D, it can be seen that the delamination portion is further reduced. It is believed that this is because the glass was further softened by firing at 830° C. and adhered more favorably.

(Test 3)
<Evaluation of amount of glass added>
The above tests were performed such that the proportions of the glass powder of Example 5 to the total silver paste were 0.2 vol%, 0.5 vol%, 1 vol%, 1.5 vol%, 2.5 vol%, 3.5 vol% and 10 vol%, respectively. Seven silver pastes were prepared in the same manner as in 1. The amount of silver powder was appropriately adjusted according to the amount of glass powder. Then, an electrode was produced in the same manner as in Test 1 above, and peeling of the inside of the silver film was evaluated. Table 2 shows the results.

As shown in Table 2, when the amount of glass added is 0.5 vol % or more, peeling inside the silver film can be reduced. Above all, when the amount of glass added was 1.5 vol % or more, the peeling inside the silver film was less than 10 μm. Therefore, it was confirmed that the silver paste containing glass at a rate of 1.5 vol % or more exhibits a more favorable effect of reducing peeling.

(Test 4)
<Evaluation of firing temperature and peeling>
Peeling inside the silver film was evaluated by setting the firing temperature to 770° C., 800° C., and 830° C. when manufacturing the electrode of Example 6.

As shown in Table 3, it can be seen that peeling inside the silver film can be reduced at any temperature by firing at a temperature higher than the softening point of glass. Moreover, it was confirmed that when the difference between the firing temperature and the softening point of the glass is 50° C. or more, peeling can be suitably reduced.

Combining the results of Tests 1 to 4 above, the softening point of the glass contained in the silver paste is 550 ° C. or higher, which is the temperature at which peeling occurs inside the silver film, and the firing temperature for forming the silver film ( For example, as a result of a button flow test using a disc-shaped test piece with a diameter of 15 mm and a height of 6 mm, the diameter of the test piece after firing is 0.9 times lower than 900 ° C. (here, 830 ° C.). It was confirmed that by using a glass having a fluidity that is 3 times or less, it is possible to reduce peeling that may occur inside the silver film.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. For example, in the above example, the composition of the organic binder was constant, but the organic binder in the silver paste is a component that burns off when fired, and also depends on the printing method and printing conditions. It is understood by those skilled in the art that it does not have an essential impact on the technology to be used. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

10 substrate 20 electrode 22 gold film 24 silver film

Claims (6)

A silver paste used for a silver film formed on a gold film,
including at least silver particles and glass,
In the glass,
The softening point of the glass is 550° C. or higher and 900° C. or lower,
When a disk-shaped test piece made of glass and having a diameter of 15 mm and a height of 6 mm is fired at 830 ° C. under atmospheric pressure, the diameter of the test piece after firing is 0.9 times or more and 2.6 times or less.
silver paste. The silver paste according to claim 1, wherein the glass is contained in an amount of 0.5 vol% or more when the entire silver paste is 100 vol%. 3. The silver paste according to claim 1, wherein the silver particles are contained in an amount of 60 wt % or more when the entire silver paste is 100 wt %. An electrode comprising the fired body of the silver paste according to any one of claims 1 to 3. A method for manufacturing an electrode, comprising:
A step of applying the silver paste according to any one of claims 1 to 3 to a predetermined base material;
and firing at a temperature higher than the softening point of the glass contained in the silver paste. 6. The electrode manufacturing method according to claim 5, wherein the electrode is an electrode for a thermal print head.

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