JP6889641B2 - Silver paste - Google Patents

Description

The present invention relates to a silver paste.

In electronic devices for electrical and electronic components, there is a wide range of techniques for forming a conductor film by printing powder made of a conductive material equivalent to a conductor on an insulating substrate without using a conductor, and wiring with this conductor film. It has been adopted. The conductive material for forming the conductor film is generally provided on the base material in the form of a conductive paste containing a conductive powder, a binder and an organic solvent.

Various types of conductive pastes are used depending on the application. For example, in applications that require particularly high electrical conductivity (low resistivity characteristics), a silver paste containing silver powder is used as a conductive material. Further, the more dense the conductor film formed from the conductive paste and the larger the cross-sectional area, the lower the resistance. Therefore, overprinting is performed twice or more at the same place to increase the cross-sectional area of the formed conductor film.

Japanese Unexamined Patent Publication No. 06-251618 Japanese Unexamined Patent Publication No. 07-220523 Japanese Unexamined Patent Publication No. 2003-59336

By the way, when printing is performed twice or more in order to form one conductor film, there is a problem that the process of forming the conductor film is more than doubled and the productivity is deteriorated. Therefore, there is a demand for a silver paste capable of forming a thicker thick film by one printing and firing. Here, when printing a thick coating film at one time, there arises a problem that the coating film cannot support its own weight and drips easily, that is, the corners of the cross section are easily rounded. If the cross-sectional shape of the coating film is rounded, the effect of reducing the resistance due to the expansion of the cross-sectional area is impaired. To address this problem, a paste design with a high silver particle content (ie, high content) and a low amount of binder and solvent is effective. However, reducing the amount of binder and solvent can lead to trade-offs such as extremely low fluidity and printability of the silver paste. For example, preparation of silver paste and printing itself became difficult, and even if printing was possible, the surface morphology of the formed coating film was disturbed, and there was a limit to reducing the resistance of the conductor film.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a silver paste capable of forming a coating film having a large thickness and a cross-sectional shape closer to a rectangle by one printing.

In order to solve the above-mentioned problems of the prior art, the technique disclosed herein provides a silver paste containing silver particles, a binder, and a solvent. In this silver paste, the binder contains ethyl cellulose (hereinafter, may be referred to as "EC") and an acrylic resin. Then, regarding the viscosity of the silver paste when the shear rate was changed in the order of 20 / s, 10 / s, and 0.01 / s, the rotation immediately after applying the shear at the shear rate of 10 / s for 1 second. The viscosity (i) and the rotational viscosity (ii) immediately after applying shear at a shear rate of 0.01 / s for 1 second were measured, and the amount of change in these viscosities: {rotary viscosity (ii) −rotary viscosity ( When i)}; is defined as the rising viscosity after resting, the rising viscosity after standing still is 300 Pa · s or more and 1500 Pa · s or less.

By using EC and an acrylic embroidery oil compatible with EC as a binder in the silver paste, the present inventors exhibit rheological characteristics suitable for thick film printing even when the amount of the binder is reduced. We have found that a rheological silver paste can be realized. Then, they have found that this rheological property can be more appropriately expressed by the above-mentioned rising viscosity after stationary, rather than simply blending a binder, and have completed the present invention. That is, the silver paste that satisfies the above-mentioned rising viscosity after standing still exhibits a viscosity decrease suitable for flattening (leveling) the coating film when left to stand after printing, and has a viscosity recovery characteristic suitable for suppressing sagging of the coating film. Is provided as provided. As a result, a silver paste capable of forming a coating film having a cross-sectional shape closer to a rectangle is realized even when a thick film is printed.

Patent Documents 1 to 3 disclose a configuration in which ethyl cellulose and an acrylic resin are used as a binder for the conductive paste. However, Patent Document 1 discloses a technique in which the thickness of the coating film formed by printing is evaluated at the maximum value, and the cross-sectional shape is not rectangular. Further, Patent Document 2 discloses that a coating film having a thickness of 250 μm is remarkably uneven with a flatness of 30 to 150 μm. Since Patent Document 3 targets only intaglio printing as a printing method, it does not disclose any conductive paste capable of forming a thick coating film.

A preferred embodiment of the silver paste disclosed herein is characterized in that the proportion of the silver particles in the entire silver paste is 94% by mass or more. With such a configuration, it is possible to increase the thickness of the coating film that can be formed by one printing (for example, 30 μm or more) while increasing the denseness of the formed electrodes. Even a highly content silver paste having a high proportion of silver particles is preferable in that good printability can be ensured.

In a preferred embodiment of the silver paste disclosed herein, the ratio of the acrylic resin to the total of the ethyl cellulose and the acrylic resin is 30% by mass or more and 90% by mass or less. In the technique disclosed herein, in order to print a coating film having a highly rectangular cross section, the ratio of ethyl cellulose to the acrylic resin is not absolute, but the ratio of ethyl cellulose to the acrylic resin is defined as described above. By setting the range, it is possible to preferably prepare a silver paste capable of printing a coating film having a highly rectangular cross section.

In a preferred embodiment of the silver paste disclosed herein, the acrylic resin has a weight average molecular weight (hereinafter, may be simply referred to as “Mw”) of 15,000 or less. According to the study of the present inventor, it is preferable that the Mw of the acrylic resin is relatively small. It is considered that this enhances the compatibility of the acrylic resin with ethyl cellulose and enhances the viscosity recovery characteristics of the silver paste. As a result, even if the amount of the binder is small, the resistance to the sagging of the coating film acts suitably, and even in the case of thick film printing, the sagging of the coating film can be suitably prevented.

In a preferred embodiment of the silver paste disclosed herein, the angle between the lower bottom of the trapezoid and the legs is 50 in a substantially trapezoidal cross section of the printed body obtained by printing the silver paste linearly and drying it. It is characterized by being greater than or equal to °. According to the technique disclosed herein, the cross-sectional shape of the coating film can be made closer to a rectangle in this way. As a result, the cross-sectional area of the coating film that can be formed by one printing can be expanded, and a conductor film having lower resistance can be formed.

In a preferred embodiment of the silver paste disclosed herein, a dispersant can be further included. In addition, a rheology modifier can be further included. This is preferable because the silver paste that realizes the rising viscosity after standing still can be easily adjusted.

It is sectional drawing which schematically explains the structure of the laminated chip inductor formed by using the silver powder which concerns on one Embodiment. It is a graph which illustrates the relationship between the shear rate of a silver paste, and the viscosity time dependence. (A) and (b) are diagrams for explaining the measurement of the rising viscosity of the silver paste after standing still.

Hereinafter, preferred embodiments of the present invention will be described. It should be noted that matters other than those specifically mentioned in the present specification (for example, the composition of silver paste) and necessary for carrying out the present invention (for example, a method for supplying silver paste, a method for manufacturing an electronic element, etc.) ) Can be understood based on the technical content taught in this specification and the general technical common knowledge of those skilled in the art in the art. The present invention can be carried out based on the contents disclosed in the present specification and common general technical knowledge in the art. In this specification, the notation "A to B" indicating the range means A or more and B or less.

[Silver paste]
The silver paste disclosed herein contains, as an essential composition, silver particles, a binder, and a solvent. By supplying silver particles to a desired base material in a desired form in the form of such a paste and firing the silver particles, a conductor film formed by sintering the silver particles can be formed. Here, the silver paste disclosed herein is supplied to a base material or the like, and while the surface of the silver paste is flattened, changes in the shape of the coating film, collapse, etc. are suppressed at the edges thereof. Is desired.

[Rising viscosity after rest]
Therefore, the silver paste disclosed here is formed with silver particles so that the rising viscosity after rest calculated based on the following shear history imitating the printing of the silver paste is 300 Pa · s or more and 1500 Pa · s or less. , The blending balance of the binder and the solvent is prepared.
Here, the "resting rising viscosity" is used to evaluate the viscosity recovery phenomenon in a low shear region, which represents the behavior such as leveling (flattening) and sagging on the substrate after the silver paste is printed. It is an index. In the present specification, in the viscosity measurement of silver paste, when the shear rate is changed in the order of 20 / s, 10 / s, and 0.01 / s in three steps and applied, the shear rate is 10 / s for 1 second. The rotational viscosity (i) immediately after applying shear and the rotational viscosity (ii) immediately after applying shear for 1 second at a shear rate of 0.01 / s were measured, and the amount of change in these viscosities: {rotary viscosity. Let (ii) -rotational viscosity (i)}; be the rising viscosity after resting.

When the rising viscosity after resting is 300 Pa · s or more, the viscosity of the silver paste in the resting state after printing is lowered to a sufficient degree for leveling, and the viscosity is sufficiently restored to prevent sagging. It can be evaluated as having characteristics. The rising viscosity after rest is preferably 350 Pa · s or more, more preferably 400 Pa · s or more, particularly preferably 450 Pa · s or more, and can be, for example, 500 Pa · s or more. The upper limit of the rise viscosity after rest is not particularly limited, but if the rise viscosity after rest is excessively high, the leveling property of flattening the surface of the silver paste after printing tends to deteriorate. From this point of view, the upper limit of the rising viscosity after rest can be, for example, 1500 Pa · s or less.

From various studies, the inventor has found that it is not possible to unambiguously determine the composition of silver particles, binder and solvent for a silver paste having such rheological properties. The silver particles, the binder and the solvent can be configured as follows, for example, so as to satisfy the above-mentioned static rising viscosity. Hereinafter, the components of the silver paste will be described.

[Silver powder]
The silver powder disclosed herein is a material for forming a conductor film having high electrical conductivity (hereinafter, may be simply referred to as "conductivity") as a conducting wire in an electronic device or the like. Silver (Ag) is not as expensive as gold (Au), is less likely to be oxidized, and has excellent conductivity, and is therefore preferable as a conductive material. The composition of the silver powder is not particularly limited as long as it is a powder containing silver as a main component (aggregation of particles), and a silver powder having desired conductivity and other physical properties can be used. Here, the main component means that it is the largest component among the components constituting the silver powder. Examples of the silver powder include silver and silver alloys and those composed of a mixture or a composite thereof. As the silver alloy, for example, a silver-palladium (Ag-Pd) alloy, a silver-platinum (Ag-Pt) alloy, a silver-copper (Ag-Cu) alloy and the like can be mentioned as preferable examples. For example, 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, can also be used. Since the higher the purity (content) of the silver powder, the higher the conductivity tends to be, it is preferable to use a silver powder having a high 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 technique disclosed here, it is possible to form a conductor film having extremely low resistance even by using silver powder having a purity of about 99.5% or more (for example, about 99.8% or more). Will be done. From this point of view, in the technique disclosed herein, even if silver powder having a purity of 99.99% or less (for example, 99.9% or less) is used, a conductor film having sufficiently low resistance is formed. It is possible.

The shape of the silver particles is not particularly limited, and may be various shapes such as a spherical shape, a flat plate shape, a needle shape, and an indefinite shape. For example, it is preferable to use a spherical paste for the purpose of forming a linear conductor film by a screen printing method.

Further, the silver particles can be used in the form of silver powder, which is an aggregate of a plurality of silver particles. Here, the average particle size of the silver powder is not particularly limited. However, from the viewpoint that it can be suitably used for manufacturing an electronic device at the present time, it is also a preferable embodiment that the average particle size is in a predetermined range. If the average particle size of the silver powder is too small, sintering proceeds at a lower temperature, but it tends to aggregate and the filling property of the silver particles at the time of firing is lowered, which is not preferable. Therefore, it is preferable that the average particle size of the silver powder is, for example, 0.5 μm or more as a guide. The average particle size is preferably 0.7 μm or more, more preferably 1 μm or more, and particularly preferably 1.2 μm or more. Further, if the average particle size of the silver powder is too large, it is necessary to expose it to a high temperature for a long time for sintering, and it is not preferable in that it does not satisfy the demand for realizing sintering at a low temperature. Therefore, the average particle size of the silver powder can be, for example, 5 μm or less as a guide. 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 the "average particle size" in the present specification, the particle size (D 50} ) at a cumulative volume of 50% in the volume-based particle size distribution based on the laser diffraction / scattering method is adopted.

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

In another aspect, the silver powder can also be used as a mixture of two particle groups having different average particle diameters. In this case, for example, the average particle size (D ₅₀ ) of the first particle group is in the range of 2 μm to 5 μm (for example, 2 μm), and the average particle size (D ₅₀ ) of the second particle group is 0.5 μm to 2 μm (for example). For example, a range of 0.5 μm) is given as a preferable 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 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, a silver powder having a good filling property can be prepared.
A silver powder having such an average particle size and a particle size distribution characteristic can form a dense conductor film with good filling property. This is advantageous in forming a conductor film having a lower resistivity.

Although not necessarily limited to this, it is preferable that the proportion of silver particles in the entire silver paste is high from the viewpoint of forming a dense conductor film and enhancing the shape-maintaining characteristics after the paste is supplied. .. For example, the content of silver particles in the silver paste is preferably 94% by mass or more, more preferably 94.5% by mass or more, further preferably 95% by mass or more, and particularly preferably 95.5% by mass or more. For example, it can be 96% by mass or more. By forming the silver paste with high content in this way, the silver particles exert an anchoring effect after being supplied to the base material or the like, and it is possible to suppress changes in the shape of the coating film, collapse and the like. On the other hand, with respect to such a high-content silver paste, it is necessary to impart fluidity and supplyability capable of printing silver powder with components of 6% by mass or less of the balance. In this case, the binder and the solvent are preferably configured as follows.

[Binder]
The binder is a component that plays a role of binding the silver particles to each other and the silver particles to the base material at the stage of forming or firing the silver paste by printing, drying, or the like. Then, after the silver particles are integrated by firing, the binder can become an unnecessary resistance component. In addition, the binder preferably adjusts the viscosity and rheological properties of the silver paste, and desires the fluidity of the silver paste when it is supplied to the base material and the shape stability of the silver paste after it is supplied to the base material. It also has the function of adjusting the paste. The silver paste disclosed herein is printable, and although the surface of the printed matter (which may be a coating film) after printing is substantially flattened, the shape of the printed matter is formed at the end thereof. Collapse, in other words, it is required that sagging is suppressed. In other words, it is required that the cross-sectional shape of the printed matter can be maintained in a shape close to a rectangle.

Therefore, the binder is preferably a component that disappears at a temperature lower than the firing temperature of the silver paste and does not remain in the conductor film. As such a binder, an organic compound having a binder function is generally used without particular limitation, but in the technique disclosed herein, ethyl cellulose (EC) and an acrylic resin are used in combination. Further, the silver paste disclosed herein preferably does not contain an inorganic binder (for example, glass frit) that does not disappear at the sintering temperature of silver particles.

EC is a cellulose derivative obtained by ethyl etherifying the hydroxyl group of cellulose and has compatibility with an acrylic resin. EC is also preferable as a binder for silver paste in that the firing residue is small. The degree of etherification of EC is not particularly limited. For example, those containing about 25% to 55% of ethoxy groups can be preferably used.

As the acrylic resin, a polymer containing an acrylic acid ester or a methacrylic acid ester as a main monomer component can be used. Typically, the acrylic resin can be a polymer containing an alkyl (meth) acrylate as a main monomer. The acrylic resin may further contain another monomer component having copolymerizability with the alkyl (meth) acrylate as a submonomer. Here, the term "(meth) acrylic" is used herein to mean acrylic and / or methacryl. Further, the main monomer means a component having the largest mass ratio among the monomer components constituting the acrylic resin. The main monomer is typically a component that occupies 50% by mass or more of the monomer components constituting the acrylic resin, and is, for example, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95. It can occupy 100% by mass or more, substantially 100% by mass. The sub-monomer component is a monomer component other than the main monomer component among the monomer components constituting the acrylic resin. Examples of such an acrylic resin include acrylic polymers such as polymethylmethacrylate, polyethylmethacrylate, polybutylmethacrylate, and polymethylacrylate. The acrylic resin may be introduced with a functional group that expresses a desired function within a range that does not deviate from the object of the present invention. However, the introduction of such a functional group is not essential, and for example, a characteristic functional group such as a carboxy group, an epoxy group, or an amino group may not be introduced.

Acrylic resins having a relatively small weight average molecular weight (Mw) are preferable because they can preferably satisfy the above-mentioned rising viscosity after rest. By reducing the Mw of the acrylic resin, the compatibility with ethyl cellulose is further enhanced, and the binder structure in the silver paste destroyed by shearing during printing is easily reconstructed, which contributes to the recovery of viscosity. It is thought that it will be done. The Mw of the acrylic resin is, for example, preferably 15,000 or less, preferably 10000 or less (for example, less than 10000), and more preferably 8000 or less, 7000 or less, 6000 or less, 5000 or less. An example is given. The lower limit of Mw of the acrylic resin is not strictly limited, but in order to fully exert the binder function (for example, the binding function of silver particles) with a smaller amount of binder, for example, Mw may be 1000 or more. preferable.

As the solvent, the above-mentioned ethyl cellulose and acrylic resin, which are binders, can be dissolved or dispersed, 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. The boiling point of the solvent is more preferably about 230 ° C. or higher (typically about 230 ° C. to 260 ° C.). Examples of such an organic solvent 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 spirit, turpineol, mentanol, and texanol can be preferably used. Particularly preferable solvent components include butyl carbitol (BC), butyl carbitol acetate (BCA), 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate and the like.

The silver paste disclosed herein may contain various additives other than the above as long as it does not deviate from the object of the present invention. Preferable examples of such additives include additives such as surfactants, antifoaming agents, antioxidants, dispersants, and rheology modifiers.

The silver particles, binder and solvent in the silver paste can be blended so as to satisfy the above-mentioned stand-up viscosity after quiescentness. Here, for example, in the case of a high-content silver paste in which the proportion of silver particles is 94% by mass or more of the total silver paste, it may be important to adjust the blending of the binder and the solvent.

Here, the binder and the solvent construct a dispersion system in which the silver particles are dispersed. The ratio of the binder to the total of the binder and the solvent is preferably, for example, 15% by mass or more and 75% by mass or less. By setting the binder to 15% by mass or more, silver particles can be bound at the time of forming the coating film, and the shape maintaining characteristics can be suitably improved. The binder is preferably 18% by mass or more, more preferably 20% by mass or more, further preferably 30% by mass or more, and particularly preferably 40% by mass or more. However, an increase in the amount of the binder leads to a decrease in the amount of the solvent, and an excessive content of the binder causes a decrease in the fluidity and printability of the silver paste. Therefore, 75% by mass or less is appropriate. The binder is more preferably 70% by mass or less, further preferably 60% by mass or less, and particularly preferably 50% by mass or less. At least a part of the binder and the solvent can be mixed in advance and contained in the silver paste in the form of a vehicle.

Further, in the binder, EC and the acrylic resin may be contained together, and for example, in the silver paste disclosed here, the blending ratio is not a decisive factor for satisfying the above-mentioned rising viscosity after resting. It has been confirmed that. However, as a preferable example, when the ratio of silver particles is 94% by mass or more and a high content silver paste is used, for example, the ratio of acrylic resin to the total of ethyl cellulose and acrylic resin is 30% by mass or more and 90% by mass. It is preferable to adjust in the range of about% or less. If the proportion of the acrylic resin is less than 30% by mass, the rising viscosity tends to be too low after standing, which is not preferable. The acrylic resin is more preferably 35% by mass or more, for example. If the proportion of the acrylic resin exceeds 90% by mass, the viscosity of the silver paste itself becomes too high and the printability deteriorates, which is not preferable. For example, the acrylic resin is more preferably 85% by mass or less.

Such a silver paste can be prepared by weighing the above-mentioned materials so as to have a predetermined composition (mass ratio) and mixing them so as to be homogeneous. Stirring and mixing of 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 strictly limited because it varies depending on the thickness of the target electrode (by extension, the thickness of the paste print) and the like. For example, when forming a coating film having a shape suitable for forming an internal electrode of a laminated part (for example, a thickness of about 50 μm), the viscosity of the silver paste at 25 ° C. and 10 rpm is 300 Pa · s or more and 500 Pa · s or less. , More preferably 350 Pa · s or more and 450 Pa · s or less. As a result, the electrode pattern can be printed with improved position accuracy and shape accuracy.

When this silver paste is supplied on a base material in a thick linear shape, for example, it is suppressed that the corners are broken (drip) and rounded as compared with the conventional silver paste. As a result, for example, the cross-sectional shape can be closer to a rectangle. For example, when the cross-sectional shape of a coating film (dried product) obtained by printing this silver paste in a line with a thickness of 50 μm or more is regarded as a trapezoid, the angle between the lower bottom of the trapezoid and the legs is 50 ° or more. It can be a steep rise. The rise angle can be, for example, 52 ° or more, 54 ° or more, 56 ° or more, 58 ° or more, 60 ° or more, 62 ° or more, and the like. According to the silver paste disclosed here, even when the thick film is printed in this way, a coating film having a high rectangular cross section can be obtained.

The silver paste can be supplied onto a base material and then fired to obtain a conductor film. The firing temperature can be determined according to the sintering temperature of the silver particles. For example, firing in a temperature range of about 600 ° C. to 900 ° C. for about 10 minutes to 1 hour is exemplified. Prior to firing, the solvent may be removed by allowing it to stand at 50 to 150 ° C. for about 15 to 30 minutes. As a result, a conductor film formed by densely sintering silver particles can be formed on the base material in a highly rectangular shape, for example, even if the thickness is thick.

The conductor film obtained by the above silver paste maintains a high rectangularity in cross-sectional shape even when it is thick. As a result, the conductor film can be a conductor film having a lower resistivity even if the resistivity is the same. Therefore, the silver paste disclosed herein can be preferably used, for example, as an electrode-forming paste for electronic devices having various configurations and applications. Such electronic elements are not particularly limited, but are, for example, multilayer inductors, multilayer chip beads, multilayer ceramic capacitors (MLCCs), multilayer ceramic varistores, multilayer PTC thermistors, and multilayer NTC thermistors. And so on.

FIG. 1 is a cross-sectional view schematically showing the structure of the laminated chip inductor 1. The dimensional relationship (length, width, thickness, etc.) in FIG. 1 does not necessarily reflect the actual dimensional relationship. Further, the reference numerals X and Z in the drawing represent the left-right direction and the up-down direction, respectively. However, this is just for convenience of explanation.

The laminated chip inductor 1 includes a main body portion 10 and external electrodes 20 provided on both side surface portions of the main body portion 10 in the left-right direction X. The main body 10 has a structure in which a plurality of magnetic material layers 12 are laminated and integrated in the vertical direction Z. The magnetic layer 12 includes, for example, spinel ferrite such as manganese-zinc ferrite, nickel-zinc ferrite, and copper-zinc ferrite, hexagonal ferrite such as barium ferrite and strontium ferrite, furnet ferrite such as yttrium iron garnet, and Fe-Si alloys. It is composed of metal-based materials such as Fe-Ni-based alloys, Fe-Co-based alloys, and Fe-Cr-based alloys. An internal electrode layer 14 constituting a coil conductor is provided between the magnetic material layers 12. The coil conductor is formed between the magnetic material layers 12 by using the above-mentioned conductive paste. Two coil conductors adjacent to each other in the vertical direction Z with the magnetic material layer 12 interposed therebetween are conducted through via holes provided in the magnetic material layer 12. As a result, the internal electrode layer 14 is formed in a three-dimensional coil shape (spiral shape). Both ends of the coil conductor are connected to the external electrode 20.

Such a laminated chip inductor 1 can be manufactured, for example, by the following procedure. That is, first, a magnetic paste containing the powder made of the above-mentioned metal-based material, a binder resin, and an organic solvent is prepared, and this is supplied onto a carrier sheet to form a green sheet. Then, a via hole is formed at a predetermined position on the green sheet by laser irradiation or the like. Next, the silver paste disclosed here is printed at a predetermined position with a predetermined electrode pattern (coil pattern). If necessary, the via holes may be printed with a silver paste prepared for through holes. A plurality of such green sheets with an electrode pattern (for example, 100 or more) are produced, these are laminated, crimped, and then cut to a desired size to produce an unfired main body 10. Then, by drying and firing these, the green sheet is integrally fired to form a monosilic magnetic material. As a result, the main body portion 10 in the form in which the internal electrodes 22 are sandwiched between the plurality of magnetic material layers 12 is formed. Then, an appropriate external electrode forming paste is applied to both ends of the main body 10 and fired to form the external electrode 20. In this way, the multilayer chip inductor 1 can be manufactured.

Here, the silver paste disclosed here can print a thick electrode pattern in a single print while maintaining a high rectangularity in the cross section. Therefore, it is possible to form an electrode pattern with high shape accuracy with high quality with a smaller number of printing steps. The internal electrode 22 created from this electrode pattern can realize a low resistivity of, for example, 2 μΩ · cm or less. The dielectric layer 12 is made of a dielectric material having excellent DC superimposition characteristics. Since the internal electrode 22 can realize a low resistivity of 2 μΩ · cm or less, for example, a chip inductor 1 used in a power supply circuit capable of passing a large current with a small Joule heat loss due to the electrode is provided. .. Further, since the chip inductor 1 has a low DC resistance, it can be applied to a power inductor, a choke inductor, and the like. The shape of the chip can be, for example, 1608 shape (1.6 mm × 0.8 mm), 2012 shape (2.0 mm × 1.25 mm), 2520 shape (2.5 mm × 2.0 mm), or the like.

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

[Preparation of silver paste]
The silver pastes of Examples 1 to 12 were prepared by blending silver powder, binder, solvent and additives in the ratios shown in Table 1 below and uniformly mixing them with a three-roll mill.
As the silver powder, a spherical powder having an average particle size of 2.6 μm was used. As the resin, ethyl cellulose (EC) and an acrylic resin having a weight average molecular weight Mw of 5000 were appropriately selected and used. However, in Example 11, a resin having a weight average molecular weight Mw of 14,000 was used as the acrylic resin. Diethylene glycol monobutyl ether was used as the solvent. The amount of resin and the amount of solvent were adjusted so that the 10 rpm viscosity of the silver paste described later was 400 ± 100 Pa · s, which was suitable for printing.

[10 rpm viscosity]
For the silver pastes of Examples 1 to 12, when the rotation speed was constant at 10 rpm with the SC4-14 spindle at room temperature (25 ° C.) using a digital rotational viscometer (DV-III ULTRA manufactured by Brookfield). The viscosity was measured and the results are shown in the "10 rpm viscosity" column of Table 1 below.

[Rheological characteristics]
For the silver pastes of Examples 1 to 12, the time of the viscosity of the silver paste when the shear rate was changed by the following program at room temperature (25 ° C.) using a rheometer (HAAKE RheoStress 6000 manufactured by Thermo Scientific). I investigated the dependency. Then, from the results, the "rise" and "slope" of the viscosity after rest defined as follows were calculated and shown in the relevant column of Table 1. As the sensor, a cone plate having a diameter of 25 mm was used, and the measurement was carried out under the conditions of a distance between the plates of 0.052 mm and a measurement sample volume of 0.04 mL.

<Step share rate program>
At the time of measurement, the shear rate was changed in three steps, and the time dependence (rotational viscosity curve) of the viscosity at each shear rate was examined. The shear rate was changed in the order of 20 / s, 10 / s, and 0.01 / s. The share time at each shear rate was 1 second. The rheometer performed the measurement in the rotation speed control (CR) mode in which the rotation speed of the sensor was constantly controlled at each step and the torque was detected. Here, the shear rate "20 / s" simulates a state in which a share (shear) is added to the paste during the printing process, and "10 / s" means that the paste is printed (transferred) from the printing apparatus to the medium and the share is released. "0.01 / s" mimics the leveling process of the printed paste. This step share rate program can accurately simulate the viscosity recovery behavior of silver paste during printing. Depending on the specifications of the rheometer, an interval of about 0 to 2 seconds (for example, about 1.2 to 1.8 seconds in this example) may occur when switching the shear rate. Intervention of such intervals in step share rate programs is acceptable.

<Data analysis>
First, according to the step share rate program, the rotational viscosity (i) immediately after applying the share for 1 second at a shear rate of 10 / s and the rotational viscosity immediately after applying the share for 1 second at a shear rate of 0.01 / s. (Ii) was measured, and the amount of change in these viscosities was defined as "viscosity rise after rest": {rotational viscosity (ii) -rotational viscosity (i)} ;.
Further, by dividing this rise in viscosity after rest by the measurement time interval t ₁ , "viscosity after rest": {rotational viscosity (ii) -rotational viscosity (i)} / t ₁ ; These values were calculated and shown in the relevant column of Table 1. The measurement time interval t ₁ includes the actually measured value of the interval.

For reference, the time dependence of the viscosity of the silver pastes of Examples 1, 3 and 11 is shown in FIG. Further, for reference, FIG. 3 shows a diagram for explaining the time dependence of the viscosities of (a) Example 1 and (b) Example 11 and how to calculate the viscosity rise and the slope after resting.

[Printed cross-section rectangularness]
Using the silver paste of Examples 1 to 12, linear electrodes were printed on an alumina substrate by a screen printing method. For screen plate making, a # 325 stainless mesh (opening ratio 43%) is used, and a fine line with a line width of 400 μm is printed only once so as to have a film thickness of about 50 μm, and dried at 120 ° C. for 10 minutes. The printed matter of Examples 1 to 12 was formed.
The cross-sectional shape of the obtained fine line-shaped printed matter in the width direction was measured using a laser microscope (manufactured by KEYENCE CORPORATION, VK-9510). Based on the result, the angle between the lower side of the substantially trapezoidal cross section and the leg was measured as follows and used as the "rise angle".

That is, when measuring the cross-sectional shape of the printed matter (electrode), first, the surface height information of the electrode was acquired by scanning the laser microscope, and the cross-sectional shape profile of the electrode was obtained. At this time, the cross-sectional shape profile includes the surface of the substrate on which the electrodes are not formed on both sides in the width direction of the electrodes.
Then, in the cross-sectional shape profile, the surface positions of the substrates at both ends of the cross-sectional shape are connected by a straight line to form a baseline. Moreover, this baseline was set as the lower side of the trapezoid.
Next, in the cross-sectional shape profile, when the position of the baseline is the minimum point and the position with the highest surface height is the maximum point, the tangent line at the extremum that appears between the minimum point and the maximum point is defined as an isosceles trapezoidal leg. .. The angle between the leg and the lower side (the angle equal to the inclination of the leg) was defined as the "rise angle". The results are shown in the relevant column of Table 1.
The pole change point is a point (a, f (a, f (a, f)) in which the sign of f''(x) changes before and after f''(a) = 0 when the cross-sectional shape profile is represented by the curve y = f (x). a)). When specifying the pole change point, the cross-sectional shape profile may be smoothed within a range that does not contradict the purpose of obtaining the above-mentioned "rise angle".

Further, the rise angle of the print body of Example 1 was used as a reference "Ref", and the rise angle of the print body of Examples 2 to 12 was evaluated, and the result is shown in the "evaluation" column of Table 1. The symbols described in the evaluation column indicate that the amount of increase in the rise angle with respect to the rise angle (Ref) of the printed matter of Example 1 was as follows.
×: Less than + 5 ° Δ: + 5 ° or more and 10 ° or less ○: + 10 ° or more

[Evaluation]
In order for an electronic paste such as a silver paste to be sufficiently leveled after coating, it is required to maintain a state in which the viscosity is lowered for a certain period of time for leveling due to shearing during coating. On the other hand, in order to prevent the thick-film printed matter from sagging, it is required that the viscosity is instantaneously restored with a shearing that is sufficiently smaller than that at the time of coating.

As shown in Table 1, the samples of Examples 1 to 12 are listed in ascending order of viscosity rise after rest. However, in the samples of Examples 1-12, the values of 10 rpm viscosity are arranged separately. From this, it was found that even if the silver paste has approximately the same viscosity at 10 rpm, the rising angle of the cross-sectional shape of the printed matter, that is, the viscosity recovery behavior differs depending on the composition. Although it is not completely correlated, the rise angle of the cross section of the printed matter tends to gradually increase as the rise of the viscosity after standing still increases, and then gradually decrease. In other words, it was confirmed that whether or not the printed matter is prone to sagging greatly correlates with the rise in viscosity after resting, in other words, the recoverability of viscosity. Specifically, it was found that the larger the rising edge of the viscosity after resting, the higher the rectangularity of the cross section of the printed matter, but when the rising edge of the viscosity after resting becomes excessively large, the rectangularness tends to be slightly lower. Then, it was found that when the rise in viscosity is about 300 Pa · s or more, good printing with high rectangularity of the cross section of the printed matter can be performed.

In detail, Example 1 is an example in which only EC, which has been generally used in the past, is used as the binder. On the other hand, Example 12 is an example in which only acrylic resin is used as the binder. In Example 12, it was found that the viscosity of the silver paste was too high to be used for printing. That is, it was found that the silver paste using the acrylic resin alone is not suitable for printing.

Further, as can be seen from the comparison of Examples 1 and 3, Example 3 is a paste to which an adjusting agent is added to Example 1. This adjusting agent is generally added to the paste (commercially available product) in order to exhibit the functions of preventing the sedimentation of particles in the paste and preventing the paste from sagging. Therefore, from the results in Table 1, it can be seen that the adjusting agent does not have a great influence on the "10 rpm viscosity" of the silver paste, but has a role of increasing the "viscosity rise". However, although the silver paste of Example 3 contains a relatively large amount of an adjusting agent of 0.2%, the rise in viscosity is only 203 Pa · s, and the addition of the adjusting agent effectively improves the rectangularness of the cross section of the printed matter. It turns out that it has not been possible to increase it.

Then, for example, from the comparison of Examples 1, 2 and 4, by using EC and an acrylic resin together as a binder, "rising of viscosity" can be achieved while maintaining the printability of the silver paste without adding an adjusting agent. It turns out that it tends to rise. It is considered that this is because the network structure of the binder in the silver paste cut by shearing makes it easier to form the resin network again by adding the acrylic resin. However, it was also confirmed that when the rise in viscosity was (Example 2) 88 Pa · s and (Example 4) 257 Pa · s, the rectangularity of the cross section of the printed matter could not be effectively enhanced.

Further, for example, as shown in Examples 4, 5 and 6, by using EC and an acrylic resin together as a binder and adjusting the ratio thereof, the rise in viscosity can be effectively increased to 300 Pa · s or more. As a result, it was confirmed that the rectangularness of the cross section of the printed matter was also improved. However, as can be seen from these examples, it was also confirmed that the rise in viscosity is not simply determined by the ratio of EC and acrylic resin. In other words, the rectangularity of the printed matter is not simply determined by the rotational viscosity and composition of the silver paste, but is strongly influenced by the rheological properties determined by the content of the silver powder and the blending of the binder and solvent. all right.

Further, from the comparison of Examples 4 and 9, the ratio of EC and the acrylic resin in these examples is the same, but by appropriately adjusting the amount of silver powder and the ratio of the binder and the solvent, 96% in Example 9. It was found that it is possible to print a high-content silver paste print that achieves high rectangularity.

On the other hand, from the comparison between Examples 5 and 6 and Examples 7 and 8 and the comparison between Example 4 and Example 11, it is preferable to add an adjusting agent in addition to the combined use of EC and acrylic resin to increase the viscosity. It was confirmed that it was possible to increase the rectangularness of the cross section of the printed matter. Although the addition of the adjusting agent is not essential, it has been found that the addition of the adjusting agent is suitable from the viewpoint of easily enhancing the rectangularness of the cross section of the printed matter.

Further, from the comparison of Examples 10 and 11, in Example 10 in which a resin having an Mw of 14000 was used as the acrylic resin, the viscosity at 10 rpm was high, but the rise in viscosity was slightly lowered, and the rectangularity of the printed matter was slightly inferior. The result was that. From this, it is considered that the smaller the molecular weight of the acrylic resin, the faster the viscosity recovery behavior. It has been found that the Mw of the acrylic resin may be 14,000 or less, but the Mw of the acrylic resin is preferably smaller, such as 10000 or less, for example, 5000 or less.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of claims. For example, in the above example, the composition of the silver paste is fixed, but the binder and the dispersant in the silver paste are components that are burnt down by firing, and are disclosed here because they depend on the printing method and printing conditions. It is understood by those skilled in the art that it does not have an essential effect on the technology. The techniques described in the claims include various modifications and modifications of the specific examples illustrated above.

1 Laminated chip inductor 10 Main body 12 Magnetic material layer 14 Internal electrode layer 20 External electrode

Claims (6)

A silver paste containing silver particles, a binder, and a solvent.
The binder contains ethyl cellulose and an acrylic resin.
Regarding the viscosity of the silver paste when the shear rate was changed in the order of 20 / s, 10 / s, and 0.01 / s, the rotational viscosity immediately after applying shear at a shear rate of 10 / s for 1 second. (I) and the rotational viscosity (ii) immediately after applying shearing for 1 second at a shear rate of 0.01 / s were measured, and the amount of change in these viscosities: {rotary viscosity (ii) − rotational viscosity (i). )}; Is the rising viscosity after resting,
The silver paste having a rising viscosity after standing still is 300 Pa · s or more and 1500 Pa · s or less. The silver paste according to claim 1, wherein the ratio of the silver particles to the whole of the silver paste is 94% by mass or more. The silver paste according to claim 1 or 2, wherein the ratio of the acrylic resin to the total of the ethyl cellulose and the acrylic resin is 30% by mass or more and 90% by mass or less. The silver paste according to any one of claims 1 to 3, wherein the acrylic resin has a weight average molecular weight of 15,000 or less. The silver paste is printed linearly and dried in a substantially trapezoidal cross section of the printed matter.
The silver paste according to any one of claims 1 to 4, wherein the angle between the lower bottom of the trapezoid and the legs is 50 ° or more. The silver paste according to any one of claims 1 to 5, further comprising a rheology modifier.

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