Silver paste and silver powder

Abstract

Provided is silver paste for forming an internal electrode of a multilayer inductor, the silver paste containing at least a silver powder, binder resin, and an organic solvent, wherein (a) to (c) are simultaneously satisfied, and at least one of (1) to (3) is satisfied.

Description

Silver paste and silver powder

[0001] This invention relates to a silver paste and silver powder for forming internal electrodes in a multilayer inductor. More specifically, it relates to a silver paste and silver powder suitable for forming internal electrodes in a multilayer inductor that is fired at a higher temperature than conventional methods.

[0002] Conductive pastes containing silver powder have been widely used for forming internal and external electrodes of electronic components. This is partly because, compared to conductive inks using organometallic compounds, conductive pastes containing metal powder can be used with various printing methods such as screen printing, offset printing, gravure printing, inkjet printing, dip printing, dispensing, brush coating, and spin coating, and can form a thick film in a single application, which is advantageous for obtaining high conductivity.

[0003] Sintered conductive pastes are designed with a composition suitable for the target sintering temperature. If the sintering of the metal powder begins at a temperature far below the target temperature, the shrinkage behavior of the metal powder and the surrounding substrate will not match, leading to crack formation. In addition, organic components in the paste may remain in the conductive film after sintering. Furthermore, if the sintering temperature is far above the melting point of the metal powder, the sintered conductive film will further liquefy and aggregate (forming spheres), resulting in a decrease in the continuity of the electrode film and a higher resistance value.

[0004] For example, the silver paste for forming internal electrodes in conventional multilayer inductors developed by the present applicant (Patent Documents 1-3) is designed on the premise that it will be fired at a temperature of 700°C or lower, and the most suitable results are obtained at a firing temperature of around 650°C. Such a silver paste is also disclosed in Patent Document 4.

[0005] International Publication No. 2020 / 137329, International Publication No. 2020 / 137330, International Publication No. 2020 / 137331, Japanese Patent Publication No. 2018-55883

[0006] In recent years, the manufacturing of internal electrodes for high-performance multilayer inductors, which have become rapidly popular, has required increasing firing temperatures for silver paste, with firing at temperatures of 750°C or higher, and in some cases even 850°C to 950°C.

[0007] The inventors have now revised the composition of the silver paste used for forming the internal electrodes of conventional multilayer inductors, and have redesigned the composition to enable proper firing even in high-temperature ranges of 750°C or higher. However, despite redesigning the paste to be suitable for firing at 750°C or higher, when the internal electrodes of a multilayer inductor were actually formed using the silver paste and fired at high temperatures of 750°C or higher, a completely unexpected new problem was discovered. Specifically, cracks that had not occurred when firing was performed at temperatures below 700°C in the past sometimes appeared in the fired body.

[0008] Figure 1 is a simplified cross-sectional view of a multilayer inductor in which cracks have occurred. As shown here, cracks 3 were observed to have occurred in the layer direction between the internal electrode layers 1 stacked in the magnetic material layer 2, and in some cases, cracks extended from end to end in the layer direction, as shown in Figure 1.

[0009] The reason for the above problems is not clear, but it is presumed to be as follows. However, the present invention is not limited by this presumption. When conventional silver paste is fired in a high-temperature range, first, in a low-temperature range of 200°C to 400°C, organic components and additives contained in the silver paste decompose, producing gas (CO2). ２ These particles disperse and fly off the coating during firing. In the subsequent heating process, the silver powder sintersects smoothly up to around 700°C. However, research by the inventors revealed that when the firing temperature is raised above 700°C to around 750°C, some kind of gas is again generated from the coating during sintering.

[0010] In such a high-temperature region, since the sintering of the silver powder already contained in the silver paste has progressed to some extent and the fired film is becoming densified, there is no escape route for the gas, and as a result, the internal pressure of the fired film rises, which is presumed to be a factor causing cracks.

[0011] When the inventors analyzed the gas generated in this high-temperature region in detail, it was found that trace amounts of SO ２ were contained. In the binder resin, organic solvent, and additives used in the silver paste, those containing sulfur components are also widely used. However, the sulfur components contained in these intentionally added ones decompose in the low-temperature region of 200°C or higher and 400°C or lower and disperse and scatter as SO ２ gas from the coating film during firing. Therefore, the SO ２ generated in the high-temperature region around 750°C does not originate from the binder resin, organic solvent, additives, etc. For confirmation, the inventors prepared and fired silver paste by combining various binder resins, organic solvents, and additives that do not contain any sulfur components. As a result, it was found that there are cases where SO ２ is still generated in the high-temperature region.

[0012] As one of the sources of SO ２ generated in such a high-temperature region, silver sulfide (Ag ２ S) is considered. That is, it is speculated that sulfur is directly bonded to the surface of the silver powder and silver sulfide (Ag ２ S) exists on the surface of the silver powder and is contained in the silver paste. When the silver powder was thoroughly analyzed, it was confirmed that some of them had trace amounts of silver sulfide (Ag ２ S) on the surface.

[0013] At present, it has not been possible to identify where this sulfur component has mixed in and bonded to the silver powder. However, for example, regarding silver powder manufactured by the wet method, it is possible that the raw materials for manufacturing the silver powder contained trace amounts of sulfur, and regarding silver powder manufactured by the vapor-phase method, it is also possible that trace amounts of sulfur have mixed in from somewhere (for example, combustion gas) during the manufacturing process or in the post-treatment process after the manufacturing process.

[0014] Silver sulfide, which is generated by decomposition during firing at high temperatures, detaches from the silver powder even in trace amounts at high temperatures. It is presumed that this process ionizes the silver, making it particularly prone to inducing short circuits between electrodes.

[0015] Although Patent Document 4 discloses that electrodes were formed by firing silver paste at temperatures of 800°C and 900°C, the firing was performed only on the surface of an alumina plate, and no experiments were conducted to actually use magnetic materials to laminate and confirm their use as internal electrodes in a laminated inductor.

[0016] In one embodiment, the problem that the present invention aims to solve is to provide a silver paste and silver powder that can suppress the occurrence of cracks in the fired body and short circuits between electrodes, even when fired in a high-temperature range.

[0017] One embodiment of the present invention includes the following embodiments:

[0018] <1> A silver paste containing at least silver powder, a binder resin, and an organic solvent, which simultaneously satisfies the following (a) to (c): (a) The content of the silver powder in the silver paste is in the range of 93.0% by mass or more and 97.0% by mass or less; (b) When the 10th and 50th percent values ​​of the volume-based cumulative fraction of the laser diffraction particle size distribution measurement of the silver powder are defined as D10 and D50, respectively, D10 and D50 are in the range of 2.30 μm or more and 3.20 μm or less, and 3.60 μm or more and 4.60 μm or less, respectively; (c) The total content of chlorine, phosphorus, and alkali metal elements contained in the silver powder is 10 ppm by mass or less; A silver paste for forming internal electrodes of a multilayer inductor that satisfies at least one of the following requirements (1) to (3): (1) Silver sulfide (Ag) contained in the silver powder ２ (1) The content of S is 40 ppm by mass or less in terms of sulfur; (2) The silver powder contains a metal and / or metal oxide with a higher melting point than silver, and its content relative to the silver powder is in the range of 10 ppm by mass or more and 1000 ppm by mass or less; (3) The dry film density of the silver paste is 6.70 g / cm³ ３ 7.50g / cm or more ３ It is within the range of less than.

[0019] <2> The silver paste according to <1>, wherein the porosity of the silver powder after 200 tappings is in the range of 0.31 to 0.42.

[0020] <3> The tap density of the silver powder after 200 taps was 6.30 g / cm³. ３ The above is the silver paste described in <1> or <2>.

[0021] <4> The silver paste according to any one of <1> to <3>, wherein the metal and / or metal oxide having a higher melting point than silver is the metal having a higher melting point than silver that alloys with silver.

[0022] <5> The silver paste according to any one of <1> to <4>, wherein a metal and / or metal oxide having a higher melting point than silver is contained in a state that is unevenly distributed on the surface of the silver powder.

[0023] <6> The silver paste according to any one of <1> to <5>, wherein the silver paste is fired at a temperature of 750°C or higher when forming the internal electrodes of a multilayer inductor.

[0024] <7> The silver paste according to any one of <1> to <6>, wherein the D10 / D50 of the silver powder is in the range of 0.50 or more and 0.80 or less.

[0025] <8> A silver paste according to any one of <1> to <7>, wherein the content of silver powder with a particle size of less than 1 μm relative to the silver powder is less than 2.5 volume percent.

[0026] <9> Silver powder for forming internal electrodes of a multilayer inductor, wherein when the 10th and 50th percent values ​​of the volume-based cumulative fraction of the laser diffraction particle size distribution measurement of the silver powder are defined as D10 and D50, respectively, D10 and D50 are in the range of 2.30 μm to 3.20 μm and 3.60 μm to 4.60 μm, respectively, the total content of chlorine, phosphorus, and alkali metal elements contained in the silver powder is 10 ppm by mass or less, and at least one of the following requirements (1) to (4) is satisfied: (1) The porosity after 200 taps is in the range of 0.31 to 0.42; (2) The tap density after 200 taps is 6.30 g / cm ３The above. (3) The content of silver sulfide (Ag ２ 2S) is 40 mass ppm or less in terms of sulfur. (4) D10 / D50 is within the range of 0.50 or more and 0.80 or less.

[0027] <10> The silver powder according to <9>, which contains a metal and / or metal oxide having a melting point higher than that of silver, and the content thereof with respect to the silver powder is within the range of 10 mass ppm or more and 1000 mass ppm or less.

[0028] <11> The silver powder according to <10>, wherein the metal and / or metal oxide having a melting point higher than that of silver is a metal having a melting point higher than that of silver which alloyizes with silver.

[0029] <12> The silver powder according to <10> or <11>, wherein the metal and / or metal oxide having a melting point higher than that of silver is contained in a state of being unevenly distributed on the surface of the silver powder.

[0030] <13> The silver powder according to any one of <9> to <12>, wherein the content of silver powder having a particle size of less than 1 μm with respect to the silver powder is less than 2.5% by volume.

[0031] <14> The silver powder according to any one of <9> to <13>, which is used for the purpose of forming an internal electrode of a multilayer inductor.

[0032] According to one embodiment of the present invention, even when fired in a high-temperature region, it is possible to provide a silver paste and silver powder capable of suppressing the generation of cracks and the generation of shorts between electrodes in a fired body.

[0033] FIG. 1 is a schematic cross-sectional view of a multilayer inductor in which cracks have occurred. FIG. 2 is an external view of the multilayer inductor. FIG. 3 is an exploded view of the element portion of the above multilayer inductor. FIG. 4 is a schematic cross-sectional view along the X-X axis of FIG. 2. FIG. 5 is a schematic enlarged view of a part of FIG. 4. FIG. 6 is an SEM image of a silver powder containing copper used in Experimental Example 8. FIG. 7 is an SEM image of a silver powder containing manganese used in Experimental Example 17.

[0034] The details of the present invention will be described below. The following descriptions of the constituent elements may be based on representative embodiments of the present invention, but the present invention is not limited to such embodiments.

[0035] (Definitions) In numerical ranges described in stages within this specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Also, in numerical ranges described in the present invention, the upper or lower limit of that numerical range may be replaced with the values ​​shown in the examples.

[0036] Furthermore, in the following, "magnetic powder" refers to powder that exhibits soft magnetism as a characteristic, and may be made from materials such as pure iron, as well as alloys of iron with elements such as silicon, cobalt, aluminum, and nickel, or from widely known soft magnetic materials such as amorphous or ferrite.

[0037] In this specification, "D10" and "D50" refer to the 10th and 50th percentile values, respectively, of the cumulative fraction of the volume basis in laser diffraction particle size distribution measurement. The "content of silver powder with a particle size of less than 1 μm relative to the total silver powder" is calculated from (volume of silver powder with a particle size of less than 1 μm / total volume of silver powder) × 100, based on the "volume of silver powder with a particle size of less than 1 μm" and the "total volume of silver powder" obtained by the above laser diffraction particle size distribution measurement. In the experimental examples described later, this was determined using a laser diffraction / scattering particle size distribution analyzer (for example, LA-960, manufactured by HORIBA).

[0038] In this specification, "dry film density" refers to the density of the dry film obtained by drying a silver paste coating without applying pressure. Specifically, the silver paste to be measured is applied to a polyethylene terephthalate (PET) film to a thickness of approximately 150 μm, pre-dried at 80°C for 10 minutes, then the PET film is punched out in a circular shape with a diameter of 15 mm, and after further drying at 150°C for 1 hour, the PET film is peeled off, and the mass W and volume V of the resulting dry film are measured to calculate the W / V ratio.

[0039] In this specification, "tap density" refers to the value obtained by measuring the mass W1 and volume V1 of the silver powder after tapping it 200 times in a tapping container and confirming that the volume does not decrease further, and then calculating W1 / V1. In this specification, "porosity" is calculated from 1 - (tap density / specific gravity of silver powder).

[0040] In this specification, the "silver powder content relative to the silver paste" is calculated from (silver powder content / mass of silver paste) × 100. Furthermore, in this specification, the "total content of chlorine, phosphorus, and alkali metal elements contained in the silver powder" is calculated from (total content of chlorine, phosphorus, and alkali metal elements / mass of silver powder) × 100. Furthermore, in this specification, the "silver sulfide (Ag) contained in the silver powder" is calculated from the silver powder content. ２ S) The sulfur content is calculated as (Ag content of silver powder). ２ The amount of sulfur contained in S / mass of silver powder is calculated from (amount of sulfur in S / mass of silver powder) × 100. Furthermore, in this specification, "content of metals and / or metal oxides with a higher melting point than silver relative to silver powder" is calculated from (total content of metals and their oxides with a higher melting point than silver / mass of silver powder) × 100.

[0041] In the present invention, a combination of two or more preferred embodiments is a more preferred embodiment.

[0042] The following describes the silver paste and silver powder for internal electrodes of the multilayer inductor of the present invention.

[0043] [Silver Paste for Internal Electrodes of Multilayer Inductors] The silver paste for internal electrodes of a multilayer inductor of the present invention (hereinafter also simply referred to as silver paste) contains at least silver powder, a binder resin, and an organic solvent, and simultaneously satisfies the following (a) to (c), and satisfies at least one of the following requirements (1) to (3).

[0044] (a) The content of the silver powder in the silver paste is within the range of 93.0% by mass or more and 97.0% by mass or less. (b) When the 10th and 50th percent values ​​of the volume-based cumulative fraction of the laser diffraction particle size distribution measurement of the silver powder are defined as D10 and D50, respectively, D10 and D50 are within the range of 2.30 μm or more and 3.20 μm or less, and 3.60 μm or more and 4.60 μm or less, respectively. (c) The total content of chlorine, phosphorus, and alkali metal elements contained in the silver powder is 10 ppm by mass or less.

[0045] (1) Silver sulfide (Ag) contained in silver powder ２ (1) The content of S is 40 ppm by mass or less in sulfur equivalent. (2) The silver powder contains a metal and / or metal oxide with a higher melting point than silver, and its content relative to the silver powder is in the range of 10 ppm by mass or more and 1000 ppm by mass or less. (3) The dry film density of the silver paste is 6.70 g / cm³. ３ 7.50g / cm or more ３ It is within the range of less than.

[0046] (Silver powder) The silver powder may contain two or more types of silver powder with different particle sizes. In this case, the requirements (a) to (c) and requirements (1) to (3) above only need to be met by the silver powder as a whole.

[0047] <Requirement (a)> From the viewpoint of achieving both excellent conductivity as an internal electrode and excellent printability of the silver paste, the total content of the silver powder in the silver paste is in the range of 93.0% by mass or more and 97.0% by mass or less, and more preferably in the range of 94.0% by mass or more and 96.0% by mass or less.

[0048] <Requirement (b)> From the viewpoint of suppressing crack formation and short circuit formation, the D10 of the silver powder is preferably in the range of 2.30 μm to 3.20 μm, preferably in the range of 2.40 μm to 3.10 μm, and particularly preferably in the range of 2.50 μm to 3.00 μm.

[0049] From the viewpoint of suppressing crack formation and short circuit formation, the D50 of the silver powder is preferably in the range of 3.60 μm to 4.60 μm, preferably in the range of 3.80 μm to 4.55 μm, and particularly preferably in the range of 3.90 μm to 4.50 μm.

[0050] By ensuring that silver powder D10 and D50 meet the above requirements and controlling the content of small-particle silver powder, migration can be effectively suppressed, thereby preventing short circuits. Furthermore, by ensuring that silver powder D10 and D50 meet the above requirements, the thermal shrinkage behavior can be extended to higher temperatures. In other words, by allowing the sintering of the silver powder to proceed at higher temperatures, even during firing in the high-temperature range of 750°C or higher, gas escape routes exist in the fired body, suppressing the rise in internal pressure and preventing crack formation.

[0051] In the silver paste of the present invention, the D50 of the silver powder is in the range of 3.60 μm to 4.60 μm, and the D10 is in the range of 2.30 μm to 3.20 μm. The ratio of D10 to D50 is relatively large compared to the silver powder contained in conventional silver pastes, and the particle size distribution on the smaller particle side is narrow. As a result, while ensuring a certain degree of packing of the silver powder (and thus conductivity), an appropriate gap is created between the silver particles, which allows gas generated during firing in the high-temperature region to escape, and it is presumed that the occurrence of cracks is suppressed. Although not included in requirement (b), from the viewpoint of crack suppression, the D10 / D50 is preferably in the range of 0.50 to 0.80, more preferably in the range of 0.55 to 0.75, and even more preferably in the range of 0.60 to 0.70.

[0052] In one preferred embodiment, from the viewpoint of suppressing crack formation and short circuit formation, the content of silver powder with a particle size of less than 1 μm is less than 2.5 volume%, more preferably less than 2.0 volume%, even more preferably less than 1.5 volume%, and particularly preferably less than 1.0 volume% of the total silver powder. The less silver powder with a particle size of less than 1 μm, the better; however, repeatedly classifying to completely eliminate fine powder would increase costs, so the lower limit of the above content may be 0.01 volume%. By adopting the above configuration, the content of small-particle silver powder can be controlled, migration can be effectively suppressed, and short circuit formation can be suppressed. Furthermore, by adopting the above configuration, the thermal shrinkage behavior can be extended to the high-temperature side. That is, the sintering of the silver powder can proceed at a higher temperature, and even when firing in a high-temperature region of 750°C or higher, there is a gas escape route in the fired body, suppressing the rise in pressure inside the fired body and suppressing crack formation.

[0053] <Requirement (c)> In the present invention, from the viewpoint of suppressing the occurrence of cracks and short circuits, the total content of chlorine, phosphorus, and alkali metal elements contained in the silver powder is set to 10 ppm by mass or less. Preferably, the total content of chlorine, phosphorus, and alkali metal elements contained in the silver powder is 7 ppm by mass or less, more preferably 5 ppm by mass or less, even more preferably 3 ppm by mass or less, particularly preferably 1 ppm by mass or less, and most preferably none.

[0054] <Requirement (1)> Silver sulfide (Ag) contained in silver powder ２ The content of S) is preferably 40 ppm by mass or less in terms of sulfur, more preferably 35 ppm by mass or less, even more preferably 30 ppm by mass or less, particularly preferably 25 ppm by mass or less, and most preferably 20 ppm by mass or less. The sulfur components contained in the binder resin, organic solvent, additives, etc. are converted to SO by firing in a low temperature range of approximately 200°C to 400°C. ２It is generated as such, and since the shrinkage of the fired film accompanying the sintering of the silver powder has not yet begun, it can easily escape to the outside. On the other hand, silver sulfide does not decompose in firing at low temperatures, SO ２ This occurs during firing in high-temperature regions, causing cracks. In particular, it is suspected that the detachment of sulfur from silver sulfide at high temperatures results in the ionization of silver, which also causes short circuits. Silver sulfide (Ag) in relation to silver powder ２ These can be suppressed by keeping the content of S) below 40 ppm by mass.

[0055] The silver sulfide content can be controlled using conventionally known desulfurization methods. For example, the silver sulfide content can be controlled by immersing aluminum foil in silver powder dissolved in acetic acid, baking soda, etc.

[0056] <Requirement (2)> The silver powder contains a metal and / or metal oxide with a higher melting point than silver, and the content of the metal and / or metal oxide is preferably in the range of 10 ppm by mass or more and 1000 ppm by mass or less relative to the silver powder, and more preferably in the range of 20 ppm by mass or more and 850 ppm by mass or less. By adopting the above configuration, the thermal shrinkage behavior can be extended to the high temperature side. That is, the completion of sintering of the silver powder is shifted to a higher temperature side, and even when firing in the high temperature range of 750°C or higher, there is a gas escape route in the fired body, which suppresses the rise in pressure inside the fired body and suppresses the occurrence of cracks. Furthermore, by adopting the above configuration, excellent conductivity can be achieved.

[0057] When the silver paste contains two or more types of silver powder, it is possible that only one of the silver powders contains a metal and / or metal oxide with a higher melting point than silver, or that all of the silver powders contain a metal and / or metal oxide with a higher melting point than silver. From the viewpoint of conductivity, it is preferable to ensure that only the silver powders with relatively small particle sizes contain a metal and / or metal oxide with a higher melting point, as this effectively extends the shrinkage behavior of the silver powder to the high-temperature side and also suppresses migration.

[0058] The metal and / or metal oxide with a higher melting point than silver is preferably 50°C to 1400°C higher than the melting point of silver. It is preferably one or more selected from the group consisting of manganese, iron, zirconium, titanium, copper, yttrium, cobalt, nickel, silicon and their oxides, lanthanum oxide, and alumina. In addition, precious metals such as gold, platinum, and palladium can also be used. Among these, it is more preferable that one or more be selected from the group consisting of manganese, zirconium, copper, and silica, with copper being particularly preferred. From the viewpoint of resistance, it is preferable to use a metal with a higher melting point than silver rather than a metal oxide with a higher melting point than silver. Furthermore, since it is preferable to include it in the silver powder as an alloy, it is particularly preferable to use a metal with a higher melting point than silver that alloys with silver. When a metal with a higher melting point than silver is included in the silver alloy powder, the content is in the range of 10 ppm to 1000 ppm by mass relative to the silver component in the silver alloy powder. If the silver powder contains a metal with a higher melting point than silver (preferably copper), migration is suppressed, and high conductivity can be maintained while suppressing the occurrence of short circuits. Furthermore, the surface of the silver powder may be coated with glass, ceramic, or the like. Specifically, a glassy thin film or ceramic layer can be formed on the surface of the silver powder using methods described in the applicant's Patent No. 3206496 and Patent No. 3475749.

[0059] From the viewpoint of extending the shrinkage behavior of silver powder to the high-temperature side, it is preferable that metals and / or metal oxides with a higher melting point than silver are included in a state where they are unevenly distributed on the surface of the silver powder. By adopting the above configuration, it is possible to effectively control the shrinkage behavior while reducing the content of metals and / or metal oxides with a higher melting point than silver, and to maintain high conductivity. The above state can be observed by SEM (scanning electron microscope). Furthermore, there are no particular limitations on the method of unevenly distributing the metals and / or metal oxides on the surface of the silver powder, and widely known methods can be used. Examples include the spray pyrolysis method described in the applicant's patent No. 3277823, and other methods such as the widely known sol-gel method, in which a film containing precursors of high-melting-point metals and / or metal oxides is formed on the surface of the silver powder and then heat-treated. In the experimental example described later, silver powder in which metals and / or metal oxides with a higher melting point than silver are unevenly distributed on the surface was prepared based on the manufacturing method described in the applicant's international publication number WO2024 / 204211. Specifically, a nitrate containing the metal to be unevenly distributed on the surface was attached to the surface of the raw material metal powder, which was silver powder, and then dispersed in the gas phase and subjected to heat treatment.

[0060] <Requirement (3)> The dry film density of the silver paste is 6.70 g / cm³. ３ 7.50g / cm or more ３ Preferably, it should be within the range of less than 6.75 g / cm³. ３ 7.30g / cm or more ３ It is even more preferable that the values ​​are within the following range. While a high dry film density is usually required for silver paste from the viewpoint of improving conductivity, in the silver paste of the present invention, by setting the dry film density within the above numerical range, even during firing in a high-temperature region, a gas escape route exists in the fired body, suppressing the rise in pressure inside the fired body and suppressing the occurrence of cracks. As for the method of controlling the dry film density, it can be controlled by widely known general methods. For example, it is possible to control the dry film density by controlling the particle size distribution of the silver powder and its surface condition (smoothness, presence or absence of surface treatment, etc.), by changing the binder resin or organic solvent used, or by changing the type and amount of dispersant added to the paste.

[0061] <Other Requirements> The following describes the other requirements for the silver paste of the present invention. From the viewpoint of improving conductivity and suppressing cracks, the porosity of the silver powder after 200 tapping cycles is preferably in the range of 0.31 to 0.42, and more preferably in the range of 0.32 to 0.40.

[0062] Furthermore, from the perspective of improving conductivity, the tap density of silver powder after 200 taps was 6.30 g / cm³. ３ Preferably, it is 6.40 g / cm³ or more. ３ The above is more preferable. There are no particular limitations, but from the viewpoint of suppressing crack formation, the upper limit of the tap density is 7.20 g / cm³. ３ The following is preferable:

[0063] The silver powder contained in the silver paste of the present invention exhibits a narrow particle size distribution with relatively close D10 and D50 values ​​and few small particle sizes, yet tends to have a low porosity and a high tap density. This is thought to be because when silver powder is manufactured by a gas phase method that heats the metal powder at or above its melting point, as described in Japanese Patent Publication No. 63-31522, the smoothness of the particle surface contained in the silver powder increases, and the shape of the particles approaches that of a perfect sphere. As a result, it becomes easier to maintain D10 and D50 within the above numerical range while bringing the porosity and tap density closer to the above numerical range by performing appropriate classification treatment as needed. Furthermore, by surface treating the silver powder with a carboxylic acid-based surface treatment agent, it is possible to further improve the smoothness of the silver powder.

[0064] The shape of the silver powder may be granular, flake-like, or irregular, but it is preferably spherical, and especially close to a perfect sphere. In this invention, "spherical" means that, when observed by SEM, the average aspect ratio of any 50 particles within the field of view is within the range of 1.0 to 1.5. Preferably, the average aspect ratio is within the range of 1.0 to 1.3. In the examples described later, silver powder with an average aspect ratio within the range of 1.0 to 1.3 was used.

[0065] Silver powder may be used in combination with silver and / or metal components other than metal oxides and / or conductive metal oxides that have a higher melting point than silver. One example is a mixed powder of silver powder and metal powders such as palladium, platinum, gold, and nickel. Another example is an alloy powder in which silver is alloyed with the said metal component, and furthermore, a composite powder in which the surface of silver particles is coated with a metal component. These powders may also be used in mixture form. However, from the viewpoint of balancing conductivity and cost, the content of metal components other than silver and / or conductive metal oxides is preferably 0.1% by mass or more and 30% by mass or less relative to the total amount of silver component.

[0066] The method for producing silver powder is not particularly limited, but for example, it can be produced by conventionally known methods such as atomization, wet reduction, CVD, the spray pyrolysis method described in Japanese Patent Publication No. 63-31522 by the present applicant, the "method for thermally decomposing a pyrolytic metal-containing compound in the gas phase" described in Japanese Patent No. 3812359, the PVD method described in Japanese Patent No. 3541939, and the method described in International Publication No. WO2024 / 204211. Among these, the spray pyrolysis method, the "method for thermally decomposing a pyrolytic metal-containing compound in the gas phase," and the PVD method are preferred because they easily produce spherical, highly crystalline silver powder with uniform particle size. Furthermore, the method described in International Publication No. WO2024 / 204211 is particularly preferred because it produces silver powder with a sufficiently controlled particle size distribution.

[0067] (Binder Resin) Examples of binder resins include cellulose resin, (meth)acrylic resin, phenolic resin, epoxy resin, urethane resin, polyester resin, polyethylene resin, etc. From the viewpoint of printability, cellulose resin, particularly ethylcellulose, is preferred. On the other hand, from the viewpoint of excellent thermal decomposition properties, reducing the amount of residual carbon in the fired product, and suppressing the occurrence of cracks, (meth)acrylic resin is preferred. The silver paste of the present invention may contain two or more types of binder resins.

[0068] From the viewpoint of improving conductivity and printability, the binder resin content relative to the silver powder is preferably in the range of 0.430% by mass or more and 0.750% by mass or less, and more preferably in the range of 0.440% by mass or more and 0.600% by mass or less.

[0069] (Organic Solvents) Any organic solvent commonly used in silver paste can be used without particular limitation, as long as it does not hinder the effects of the present invention. Examples of organic solvents include alcohol-based, ether-based, ester-based, hydrocarbon-based organic solvents, water, and mixtures thereof. The silver paste of the present invention may contain two or more organic solvents.

[0070] (Other components) The silver paste of the present invention may contain, as necessary, glass frit, inorganic compounds such as metal oxides, plasticizers, viscosity modifiers, surfactants, dispersants, oxidizing agents, etc., which are commonly used as additives in ordinary silver pastes, as long as they do not combine with silver to form silver sulfide.

[0071] The silver paste of the present invention is manufactured by kneading silver powder, a binder resin, an organic solvent, and inorganic compounds, additives, etc., as appropriate, according to conventional methods, and dispersing them uniformly to prepare a rheological paste suitable for screen printing and other printing methods.

[0072] The silver paste of the present invention is preferably fired at a temperature of 750°C or higher when forming the internal electrodes of a multilayer inductor, while the upper limit of the firing temperature is preferably 950°C or lower. In particular, the firing temperature is preferably within the range of 750°C or higher and 900°C or lower.

[0073] To facilitate understanding of the "printability" in the present invention, a multilayer inductor will be explained below as an example. Typically, a multilayer inductor 10 is composed of a base body 11 and a first external electrode 12 and a second external electrode 13 that cover a pair of end faces of the base body 11, as shown in Figure 2 as an example.

[0074] As shown in Figure 3, the base body 11 is constructed by laminating magnetic material layers A1 to A20 and internal electrode layers B1 to B17. Magnetic material layers A1 to A20 are obtained by preparing a magnetic paste by kneading magnetic powder with an appropriate binder resin and organic solvent, and then molding and drying this paste into a sheet. On the surface of each of the magnetic material layers A3 to A19, internal electrode layers B1 to B17 with predetermined patterns are formed by screen printing. The silver paste of the present invention is used to form these internal electrode layers. One end of internal electrode layer B1 is exposed to the end face of magnetic material layer A3 and electrically connected to the external electrode 12, and similarly, one end of internal electrode layer B17 is electrically connected to the external electrode 13.

[0075] Furthermore, each of the internal electrode layers B1 to B17 is electrically connected via through-hole electrodes C1 to C16 formed to penetrate the thickness direction of the magnetic material layers A3 to A19, and as a whole, the internal electrode layers B1 to B17 are configured in a coil shape in the stacking direction. When forming the internal electrode layers B1 to B17 on the magnetic material layers A3 to A19, it is preferable to further stack a magnetic material layer (not shown) with a shape that can fill the step difference caused by the thickness of the internal electrode layers B1 to B17 on top of the magnetic material layers A3 to A19. The base body 11, on which the magnetic material layers A1 to A20 and the internal electrode layers B1 to B17 are stacked, is fired at 750°C or higher after a thermocompression bonding process, and then external electrodes 12 and 13 are formed on a pair of ends to form a stacked inductor. The external electrodes may be formed using the silver paste of the present invention, or they may be formed using a conductive paste mainly composed of nickel or copper.

[0076] The cross-section of the multilayer inductor manufactured in this manner, as shown in Figure 2, along the X-X axis is as shown in Figure 4, and a magnified portion of it is shown in Figure 5. Note that both Figures 4 and 5 are simplified diagrams and do not necessarily correspond to the content and structure shown in Figure 3, including the number of layers. As shown in Figure 4, the cross-sectional shape of the internal electrode layer is ideally desired to be rectangular. However, in reality, the viscosity and fluidity of the silver paste have an effect when it is applied and printed onto the substrate, so the actual cross-sectional shape of the internal electrode layer is usually a roughly trapezoidal shape as shown in Figure 5. In order to make this closer to rectangular, it would be desirable to obtain a paste that is highly fluid when printed and quickly exhibits high viscosity after printing. However, there are a great many parameters that must be controlled in the paste, and they interact with each other in a complex way, so it is no exaggeration to say that there are virtually no cases in which the expected results and characteristics are obtained.

[0077] As described above, in this specification, "printability" means not only that the material exhibits appropriate fluidity during printing by screen printing, gravure printing, etc., but also that it quickly exhibits high viscosity after printing, resulting in a coating film (conductor film) that is closer to a rectangular shape.

[0078] [Silver powder for internal electrodes of multilayer inductors] The silver powder of the present invention has the following characteristics: when the 10th and 50th percent values ​​of the cumulative fraction based on volume measured by laser diffraction particle size distribution are defined as D10 and D50, respectively, D10 and D50 are within the range of 2.30 μm to 3.20 μm and 3.60 μm to 4.60 μm, respectively; the total content of chlorine, phosphorus, and alkali metal elements contained in the silver powder is 10 ppm by mass or less; and at least one of the following requirements (1) to (4) is met: (1) The porosity after 200 taps is within the range of 0.31 to 0.42; (2) The tap density after 200 taps is 6.30 g / cm³ ３ That is all. (3) Silver sulfide (Ag) contained in silver powder ２ (4) The content of S is 40 ppm by mass or less in terms of sulfur. (5) D10 / D50 is within the range of 0.50 or more and 0.80 or less.

[0079] The preferred embodiments of D10, D50, the content of silver powder with a particle size of less than 1 μm, the sum of the content of chlorine, phosphorus, and alkali metal elements, the porosity, tap density, and D10 / D50 are the same as those of the preferred embodiments of the silver powder contained in the silver paste of the present invention described above, and therefore are omitted here.

[0080] From the viewpoint of suppressing the occurrence of cracks and short circuits, silver sulfide (Ag) contained in silver powder ２ The content of S) is preferably 40 ppm by mass or less in terms of sulfur, more preferably 35 ppm by mass or less, even more preferably 30 ppm by mass or less, particularly preferably 25 ppm by mass or less, and most preferably 20 ppm by mass or less.

[0081] From the viewpoint of suppressing the occurrence of cracks, the silver powder of the present invention contains a metal and / or metal oxide with a higher melting point than silver, and preferably its content is in the range of 10 ppm by mass or more and 1000 ppm by mass or less, and more preferably in the range of 20 ppm by mass or more and 850 ppm by mass or less.

[0082] The metal and / or metal oxide with a higher melting point than silver preferably has a melting point about 50°C to 1400°C higher than the melting point of silver, and is preferably one or more selected from the group consisting of manganese, iron, zirconium, titanium, copper, yttrium, cobalt, nickel, silicon or their oxides, lanthanum oxide, and alumina. In addition, precious metals such as gold, platinum, and palladium can also be used. Among these, it is more preferable that one or more be selected from the group consisting of manganese, zirconium, copper, and silica, with copper being particularly preferred. From the viewpoint of resistance, it is preferable to use a metal with a higher melting point than silver rather than a metal oxide with a higher melting point than silver. Furthermore, since it is preferable to include it in the silver powder as an alloy, it is particularly preferable to use a metal with a higher melting point than silver that can be alloyed with silver. When a metal with a higher melting point than silver is included in the silver alloy powder, the content is in the range of 10 ppm to 1000 ppm by mass relative to the silver component in the silver alloy powder.

[0083] From the viewpoint of extending the shrinkage behavior of silver powder towards high temperatures, it is preferable that metals and / or metal oxides with a higher melting point than silver are included in a state where they are unevenly distributed on the surface of the silver powder.

[0084] In one preferred embodiment, from the viewpoint of suppressing crack formation and short circuit formation, the content of silver powder with a particle size of less than 1 μm is less than 2.5 volume%, more preferably less than 2.0 volume%, even more preferably less than 1.5 volume%, and particularly preferably less than 1.0 volume% of the total silver powder. The less silver powder with a particle size of less than 1 μm, the better; however, repeatedly classifying to completely eliminate fine powder would increase costs, so the lower limit of the above content may be 0.01 volume%. By adopting the above configuration, the content of small-particle silver powder can be controlled, migration can be effectively suppressed, and short circuit formation can be suppressed. Furthermore, by adopting the above configuration, the thermal shrinkage behavior can be extended to the high-temperature side. That is, the sintering of the silver powder can proceed at a higher temperature, and even when firing in a high-temperature region of 750°C or higher, there is a gas escape route in the fired body, suppressing the rise in pressure inside the fired body and suppressing crack formation.

[0085] The silver powder of the present invention is preferably used for forming internal electrodes in multilayer inductors.

[0086] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples. In the following, "%" refers to mass unless otherwise specified.

[0087] <Manufacturing of Silver Powder> Based on the method for manufacturing metal powders described in International Publication No. WO2024 / 204211, silver powder, which was produced as raw material metal powder by reducing silver ions in a liquid phase, was further dispersed in a gas phase using a carrier gas, heated, and then collected. Subsequently, the collected silver powder was subjected to a classification treatment to adjust the values ​​of D10 and D50, as well as the content of silver powder with a particle size of less than 1 μm relative to the total silver powder, and silver powders 1 to 21 as described in Table 1 were prepared. In the above manufacturing process, for silver powders 2, 4, 5, 8 to 11 and 13 to 20, salts of metals or metal oxides with a higher melting point than silver were weighed so that the content of such metals or metal oxides was as shown in the table, and after being attached to the surface of the raw material metal powder, they were heated in the gas phase to prepare the powders. In these silver powders, a portion of the silver forms an alloy with the high-melting-point metal. Furthermore, for silver powder 21, a surface treatment was performed by attaching zirconium octanoate to the surface of the silver powder after heating and collection.

[0088] Figures 6 and 7 show SEM images obtained by scanning microscopy-energy dispersive X-ray analysis (SEM-EDX) for silver powders 8 and 17, respectively. Here, Figure 6(B) is an image of the SEM image of Figure 6(A) mapped with the copper element, and similarly, Figure 7(B) is an image of Figure 7(A) mapped with the manganese element.

[0089] Furthermore, each of the collected silver powders was subjected to desulfurization treatment and then heated in a gas phase containing sulfur to adjust the silver sulfide content relative to the silver powder. In addition, for silver powders 12 and 13, the silver powders were heated in a gas phase containing chlorine to adjust the chlorine content relative to the silver powder.

[0090] In Table 1, the content of components other than silver in the silver powder was determined by analysis using ICP. Other parameters were determined by the method described above.

[0091]

[0092] <Preparation of Silver Paste (Samples)> Silver paste samples 1 to 21 were prepared by kneading the total amount of silver paste with silver powders 1 to 21 in the proportions shown in Table 2, ethyl cellulose with a solid content of 0.530% by mass, and the remainder of terpineol.

[0093] The various parameters listed in Table 2 were determined as follows. Each sample 1 to 21 was coated onto a PET film in a 20 mm × 20 mm × 150 μm area, pre-dried at 80°C for 10 minutes, then punched out in a 15 mm diameter circular shape along with the PET film, and further dried at 150°C for 1 hour. After peeling off the PET film, the mass W and volume V of the resulting dried film were measured, and the dry film density was calculated from the W / V formula. Each sample 1 to 21 was coated and printed onto a ceramic substrate in a 60 mm × 0.6 mm × 40 μm rectangular parallelepiped shape, fired in air (oxidizing atmosphere) at 870°C to form a conductive film, and the sheet resistance was determined using the four-terminal method with a digital multimeter. The resistance values ​​are converted to a film thickness of 10 μm after firing.

[0094]

[0095] <<Performance Evaluation>> A magnetic layer with a thickness of 30 μm was prepared in advance using a magnetic paste containing magnetic powder. A rectangular parallelepiped-shaped conductive film was formed on this layer using sample 1 by screen printing, and then a magnetic layer was printed to fill the step caused by the thickness of the pattern. Three sets of these were stacked, and then magnetic layers for covering were stacked on the top and bottom. Next, these were heat-pressed together, degreased in an oxidizing atmosphere, and then fired at 870°C to produce the laminate of Experimental Example 1. Twenty laminates were prepared. Similarly, twenty laminates were prepared for each of Experimental Examples 2 to 21 using the samples.

[0096] Each laminate was cut, and its cross-section was observed. A rating was assigned for zero cracks, a B rating for one crack, a C rating for two cracks, and a D rating for three or more cracks. The results are summarized in Table 3.

[0097] Electrical resistance measurements were taken between the top and middle layers, and between the middle and bottom layers, in the three conductive films of each laminate. This was repeated for 20 samples prepared for each experimental example. The ratio of the number of times conduction occurred out of the total number of measurements was defined as the short-circuit rate. A short-circuit rate of 2% or less was rated A, a rate between 2% and 4% was rated B, a rate between 4% and 6% was rated C, and a rate above 6% was rated D. These results are summarized in Table 3.

[0098] Furthermore, a sheet resistance of 3.0 mΩ / □ or less was considered acceptable, but 2.5 mΩ / □ or less was considered particularly good.

[0099]

Claims

1. A silver paste containing at least silver powder, a binder resin, and an organic solvent, which simultaneously satisfies the following (a) to (c): (a) The content of the silver powder in the silver paste is in the range of 93.0% by mass or more and 97.0% by mass or less; (b) When the 10th and 50th percent values ​​of the volume-based cumulative fraction of the laser diffraction particle size distribution measurement of the silver powder are defined as D10 and D50, respectively, D10 and D50 are in the range of 2.30 μm or more and 3.20 μm or less, and 3.60 μm or more and 4.60 μm or less, respectively; (c) The total content of chlorine, phosphorus, and alkali metal elements contained in the silver powder is 10 ppm by mass or less; A silver paste for forming internal electrodes of a multilayer inductor that satisfies at least one of the following requirements (1) to (3): (1) Silver sulfide (Ag) contained in the silver powder ２ (1) The content of S is 40 ppm by mass or less in terms of sulfur; (2) The silver powder contains a metal and / or metal oxide with a higher melting point than silver, and its content relative to the silver powder is in the range of 10 ppm by mass or more and 1000 ppm by mass or less; (3) The dry film density of the silver paste is 6.70 g / cm³ ３ 7.50g / cm or more ３ It is within the range of less than.

2. The silver paste according to claim 1, wherein the porosity of the silver powder after 200 tappings is in the range of 0.31 to 0.

42.

3. The tap density of the silver powder after 200 taps is 6.30 g / cm³. ３ The silver paste according to claim 1 or 2.

4. The silver paste according to claim 1 or 2, wherein the metal and / or metal oxide having a higher melting point than silver is a metal having a higher melting point than silver that alloys with silver.

5. The silver paste according to claim 1 or 2, wherein a metal and / or metal oxide having a higher melting point than silver is contained in a state of uneven distribution on the surface of the silver powder.

6. The silver paste according to claim 1 or 2, wherein the silver paste is fired at a temperature of 750°C or higher when forming the internal electrodes of a multilayer inductor.

7. The silver paste according to claim 1 or 2, wherein the D10 / D50 of the silver powder is in the range of 0.50 or more and 0.80 or less.

8. The silver paste according to claim 1 or 2, wherein the content of silver powder with a particle size of less than 1 μm relative to the silver powder is less than 2.5 volume percent.

9. Silver powder wherein, when the 10th and 50th percent values ​​of the volume-based cumulative fraction of the laser diffraction particle size distribution measurement of the silver powder are defined as D10 and D50, respectively, D10 and D50 are within the range of 2.30 μm to 3.20 μm and 3.60 μm to 4.60 μm, respectively, the total content of chlorine, phosphorus, and alkali metal elements contained in the silver powder is 10 ppm by mass or less, and the silver powder satisfies at least one of the following requirements (1) to (4): (1) The porosity after 200 taps is within the range of 0.31 to 0.42; (2) The tap density after 200 taps is 6.30 g / cm³ ３ That is all. (3) Silver sulfide (Ag) contained in silver powder ２ (4) The content of S is 40 ppm by mass or less in terms of sulfur. (5) D10 / D50 is within the range of 0.50 or more and 0.80 or less.

10. The silver powder according to claim 9, comprising a metal and / or metal oxide having a higher melting point than silver, wherein its content relative to the silver powder is in the range of 10 ppm by mass or more and 1000 ppm by mass or less.

11. The silver powder according to claim 10, wherein the metal and / or metal oxide having a higher melting point than silver is a metal having a higher melting point than silver that alloys with silver.

12. The silver powder according to claim 10, wherein a metal and / or metal oxide having a higher melting point than silver is contained in a state of being unevenly distributed on the surface of the silver powder.

13. The silver powder according to claim 9 or 10, wherein the content of silver powder with a particle size of less than 1 μm relative to the silver powder is less than 2.5 volume percent.

14. Silver powder according to claim 9 or 10, used for forming internal electrodes in a multilayer inductor.

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