JP2019157271A - Silver-coated copper powder - Google Patents

Abstract

To provide a silver-coated copper powder that yields, for cases of use in a conductive paste for low-temperature firing, a low resistivity close to the case where a silver powder is used.SOLUTION: Provided is a silver-coated copper powder which is a spherical silver-coated copper powder in which silver is coated on a copper particle, and the silver-coating thickness calculated from the silver-coating amount is 60 nm or more. Furthermore, the average particle diameter obtained from an image observed with a scanning-type electron microscope preferably is 0.5 μm or more.SELECTED DRAWING: None

Description

  The present invention relates to a silver-coated copper powder suitable for forming a wiring of an electronic material.

  Conventionally, metal powder has been used as a wiring forming material for electronic components such as conductive paste for printed wiring, semiconductor internal wiring, connection between a printed wiring board and electronic components, and the like. In recent years, there has been an increasing demand for low-temperature firing and thinning of wiring, particularly in the field of solar cell electrodes and the like. Therefore, there is a demand for metal powders for conductive pastes that have low electrical resistance even after low-temperature firing (hereinafter, resistance refers to electrical resistance), and that can cope with thinning.

  Conventionally, silver powder has been used as a metal powder for such a conductive paste. However, the price of silver bullion is expensive. Therefore, an alternative to silver-coated copper powder that can be manufactured at a lower cost is being actively studied.

  As a characteristic required for copper powder for conductive paste for fine wires used for low-temperature firing at 300 ° C. or less, although depending on the application and use conditions, the resistance value when used as conductive paste is compared with silver powder that has been used conventionally It is required to be close.

  However, the current silver-coated copper powder has a resistance value when used as a conductive paste due to deterioration of the silver layer due to temperature history during low-temperature firing and the formation of copper oxide on the surface of the silver layer due to copper diffusion. It is inferior compared.

  For example, Patent Document 1 discloses a method for increasing the conductivity by bringing the specific surface area of silver-coated copper powder close to the specific surface area calculated from the particle size obtained from image analysis. It is shown only as a resistance value. As shown in Non-Patent Document 1, the electrical conductivity of the metal fine particle aggregate is represented by the product of the electrical conductivity of the constituent metal particles and the electrical conductivity of the contact portion. When the resistance value of the metal particles is the same and the resistance of the contact layer is negligible, assuming that the contact area between the particles is a circle, the conductivity of the metal fine particle aggregate is proportional to the radius. In the dust resistance measurement, a certain contact area is given by applying physical external force, and the electrical conductivity of the metal fine particle aggregate is the electrical conductivity of the conductive paste mainly composed of resin compressive stress as external force. Since it is different, even if the dust resistance value is low, it cannot be said that the resistivity of the conductive paste is low.

Japanese Patent No. 5785532

Hideo Ono, Complete Particle Engineering Volume 1, p85 (2001)

  In the case of silver powder conventionally used, it is known that necking due to specific surface area occurs at a low temperature around 150 ° C. at the contact portion between silver. However, when silver-coated copper powder is used as the conductive paste for low-temperature firing, the formation of necking is hindered by the deterioration of the silver layer due to the temperature history during low-temperature firing and the formation of copper oxide on the silver layer surface due to copper diffusion. It is thought that. In addition, silver mass transfer is essential for the formation of necking, but it is thought that necking formation failure occurs due to the absence of a sufficient amount of silver for mass transfer.

That is, if a silver amount sufficient for copper diffusion and mass transfer is coated, a resistance value close to that of silver powder can be obtained even when using silver-coated copper powder as a conductive paste for low-temperature firing. It is considered possible.
Then, in this invention, when using for the electrically conductive paste for low-temperature baking, it aims at providing the silver coat copper powder which obtains the low resistivity near the case where silver powder is used.

The inventors of the present invention have clarified the amount of silver coating that achieves this purpose by using a commercially available copper powder and performing silver coating treatment by a wet method.
That is, a spherical silver-coated copper powder in which silver is coated on copper particles, and the silver-coated copper powder is characterized in that the silver coating thickness obtained from the silver coating amount is 60 nm or more, more preferably the silver The silver-coated copper powder is characterized in that an average particle diameter obtained from an image of the coated copper powder observed with a scanning electron microscope is 0.5 μm or more.
The spherical shape is not only a true sphere, but also an ellipsoid having an elliptical shape in which a ratio of a minor axis to a major axis (minor axis / major axis) is in a range of 0.8 to 1.0 in a predetermined cross section. The substantially spherical shape is also included.

  When the silver-coated copper powder according to the present invention is used for a conductive paste for low-temperature firing, a resistance value substantially equivalent to that of silver powder can be obtained. Further, since the silver-coated copper powder according to the present invention has a spherical shape, there is no anisotropy, and when used as a wiring forming material for electronic parts, the wiring can be thinned.

  Hereinafter, although one embodiment of the present invention is described in detail, the present invention is not limited to the following embodiment, and can be appropriately changed without departing from the gist of the present invention.

  The silver-coated copper powder according to the present embodiment is spherical and has a silver coating thickness obtained from the silver coating amount of 60 nm or more, and more preferably, the silver-coated copper powder is observed with a scanning electron microscope. The average particle diameter obtained from the obtained image is 0.5 μm or more.

  As described above, when the contact portion between the particles is assumed to be a circle, the conductivity of the metal particle aggregate is proportional to the radius. When silver powder is used as the conductive particles of the conductive paste for low-temperature firing, depending on the particle size, necking between the particles occurs at low temperatures, forming a low-resistance contact site, and compressive stress only due to thermal contraction of the resin Even if it is below, a stable low resistance value can be obtained as a result. On the other hand, when silver-coated copper powder is used, although silver exists in the contact portion between the particles, the thermal history during low-temperature firing causes copper diffusion through grain boundaries and pinholes in the silver-coated layer. Copper oxide that inhibits silver necking is likely to be produced. Further, when the thickness of the silver coat layer is thin, the amount of silver necessary for causing necking is insufficient, and necking is difficult to occur. When the above events overlap, it is more difficult to obtain a low resistance value than when silver powder is used. Therefore, by increasing the silver coating amount, that is, by thickly covering the silver, it is possible to suppress copper diffusion and to secure the silver amount necessary for necking.

  On the other hand, increasing the amount of silver goes against cost reduction, which is important in using silver-coated copper powder. Therefore, by clarifying the minimum amount of silver coating that can obtain a low resistance value close to the case of using silver powder, it is possible to obtain a low resistance value that is close to silver powder at a lower cost than using silver powder. A silver-coated copper powder is provided.

  In the present invention, the silver coating thickness (the thickness of the silver coating layer) obtained from the silver coating amount is used as an index as the amount of the silver coating. Here, the silver coating amount can be obtained by, for example, analyzing the composition of silver-coated copper powder, and the silver coating thickness can be obtained using the specific surface area.

  Next, the thickness of the necessary silver coat layer is 60 nm or more, but when it is thinner than this, the resistance value is at least 1.5 times higher than when silver powder is used, and the resistance value is close to silver powder. I can not say. Here, the resistance value close to (substantially equivalent to) the silver powder is set to 1.0 to 1.5 times the resistance value of the silver powder. Moreover, although there is no upper limit in particular, it is thought that it is determined naturally in consideration of cost.

  Moreover, although the average particle size range is 0.5 μm or more, when the average particle size is less than 0.5 μm, in order to obtain 60 nm as the thickness of the silver coat layer, 60% by mass or more of silver is attached to the copper powder. This is because the use of silver powder and the cost advantage are reduced.

The silver-coated copper powder according to the present embodiment is preferably spherical. As the silver-coated copper powder, flaky and dendritic ones are also known, and when these are used as conductive particles of a conductive paste, it is easy to secure a contact portion between the particles, and to obtain a low resistance conductive film Is advantageous. However, because it is an electrically conductive particle with anisotropy, when used as a wiring material, it is difficult to stabilize the wiring width, and if the distance between adjacent wirings is not sufficiently widened, a wiring short circuit occurs. There is a fear. Therefore, it has been difficult to meet the demand for finer wiring of electronic components, that is, higher density of electronic components.
On the other hand, when the spherical conductive particles are used for the conductive paste, the wiring width is more stable than that of the flaky or dendritic conductive particles, so that the wiring can be easily thinned.
The silver coat copper powder which concerns on this Embodiment performs the process which forms a silver coat layer on the surface of a spherical copper particle. As a method for forming the silver coat layer, an electroless plating method that is a wet method is preferably used, and a displacement plating method or a reduction plating method that is an electroless plating method can be used.
The displacement plating method utilizes the difference in ionization tendency between copper and silver, and when the copper dissolves in the solution, the electrons released by the copper reduce the silver ions in the solution and deposit them on the copper surface. Is. Specifically, a publicly known method may be used. For example, spherical copper particles are made into a water slurry, and a silver salt and a complexing agent as a silver ion source are added to the water slurry for treatment. If necessary, a conductive salt, a pH adjuster, a surfactant, a brightener, a crystal adjuster, a stabilizer, a precipitation inhibitor, and the like can be added. The silver salt is preferably a water-soluble silver salt, preferably silver nitrate, and more preferably silver nitrate in the presence of ammonia.
In the reduction plating method, silver is precipitated on the copper surface by a reducing agent, and a known method can be used. Specifically, spherical copper particles are used as a water slurry, and a silver salt and a reducing agent as a silver ion source are added to the water slurry for treatment. Moreover, the chemical | medical agent listed by the said displacement plating method, such as a complexing agent, can also be added as needed.
In order to obtain a uniform thickness of the silver coat layer, the copper particles can be washed before forming the silver coat layer. The cleaning method is not particularly limited, but it is preferable to perform a cleaning process after making copper particles into a water slurry, and to form a silver coat layer on the water slurry after the cleaning process using an electroless plating method.
Furthermore, in order to suppress aggregation of the obtained silver coat copper powder, you may surface-treat after forming a silver coat layer. As the surface treatment agent, carboxylic acids such as stearic acid and oleic acid, and water-soluble polymer compounds used as dispersants such as polyvinyl alcohol can be used. These may be used in combination.
After the silver coat layer is formed or surface-treated, the treated slurry is washed, filtered and dried to obtain silver-coated copper powder. Known methods can be used for washing, filtration, and drying.

Hereinafter, the present embodiment will be described more specifically using examples, but the present invention is not limited to the following examples.
(Silver coating method)
Silver-coated copper powder was prepared by performing silver plating on atomized copper powder (manufactured by Kojundo Chemical Laboratory Co., Ltd., SEM diameter 4.5 μm). First, copper powder was dispersed in ion-exchanged water, and displacement plating was performed according to the following procedure.

  That is, 100 g of copper powder was stirred in a 3% aqueous tartaric acid solution for about 1 hour, filtered, washed with water, and dispersed in 2 liters of ion-exchanged water. To this, 5 g of tartaric acid, 5 g of glucose and 50 mL of ethanol were added, and 50 mL of 28% ammonia water was further added and stirred. Were gradually added at a rate at which 1 g of silver nitrate was mixed per minute until the desired amount of silver was reached. After the reaction was completed, the powder was filtered, washed with water, and dried through ethanol to obtain silver-coated copper powder.

  In the above-described method using the atomized copper powder, when the silver amount of the silver-coated copper powder to be obtained is within the range of 10% by mass or less, unreacted silver remains in the filtrate after the reaction is completed. It is confirmed that when the silver amount of the silver-coated copper powder exceeds 10.0% by mass, unreacted silver remains in the filtrate after the reaction is completed. Therefore, in the mass range where the silver amount of the silver-coated copper powder to be obtained is 10.0% by mass or less, a sample was obtained by adjusting the amount of the silver solution to be added according to the desired silver amount. For samples whose silver amount exceeds 10.0% by mass, a silver-coated copper powder having a silver amount of 10.0% by mass is first prepared by the above-described method, and further reduced silver plating is performed to obtain a desired silver amount. Powder was produced.

That is, 100 g of the silver-coated copper powder obtained by the above method was stirred for about 1 hour in a 3% aqueous tartaric acid solution, filtered, washed with water, and dispersed in 2 liters of ion-exchanged water. After adding 19 g of ascorbic acid and 6 g of citric acid here, an aqueous solution in which 30 g of silver nitrate is added to 200 mL of ion-exchanged water is prepared, and 1 g of silver nitrate is mixed per minute for the amount of silver required for the target amount of silver. Added at a rate. After the reaction was completed, the powder was filtered, washed with water, and dried through ethanol to obtain silver-coated copper powder.
(Evaluation methods)
<Average particle size>
For the average particle size, the particle size of 300 or more primary particles that can be observed uniformly is measured from an image observed using a scanning electron microscope (JSM-7100F, manufactured by JEOL Ltd.). By doing this, the average value was obtained and used as the average particle diameter (SEM diameter).

<Silver content and coated silver thickness>
About the silver content of the obtained silver coat copper powder, it calculated | required by the ICP emission-spectral-analysis method (Agilent Technology Co., Ltd. make, ICP-OES5100SVDV). From the obtained silver content and the average particle diameter of the copper powder, the coated silver thickness was calculated assuming that the particles were spheres.
<Paste resistivity>
The obtained silver-coated copper powder is dispersed again in ion-exchanged water, and a stearic acid emulsion (Cerosol 920, manufactured by Chukyo Yushi Co., Ltd.) is added so that the stearic acid is 0.4% by mass with respect to the silver-coated copper powder. Then, after mixing for 20 minutes, surface treatment was performed by filtering, washing with water and drying. Further, 90.19% by mass of silver-coated copper powder, 0.45% by mass of bisphenol A type epoxy resin as an epoxy resin, 0.27% by mass of phenol formaldehyde type novolak resin as a curing agent, and 9.0% by mass of dipropylene glycol as a solvent. Each agent was mixed so that it might become 0.09 mass% of 2-phenyl-4-methylimidazole of a hardening accelerator, and it paste-formed using the autorotation / revolution mixer (Shinky Co., Ltd. product Aritori Nertaro). Dipropylene glycol was added to the obtained paste, the viscosity was adjusted, printed on an alumina substrate, and thermally cured in air at 200 ° C. for 30 minutes to obtain a paste film for evaluation. The surface resistance value of the paste film was measured by a four-probe method, and the film thickness was multiplied to obtain the paste film resistivity (Ω · cm).
Example 1
By the above method, a silver-coated copper powder having a silver content of 9.2% by mass was obtained. The silver coating thickness was calculated to be 60 nm. The paste film resistivity of this sample was 86 μΩ · cm.
(Example 2)
By carrying out reduction plating after substitution plating by the above method, the amount of the silver solution was also adjusted to obtain a silver-coated copper powder having a silver content of 20.0% by mass. The silver coating thickness was calculated to be 145 nm. The paste film resistivity of this sample was 87 μΩ · cm.
(Comparative Example 1)
In the same manner as in Example 1, the amount of the silver solution was adjusted to obtain a silver-coated copper powder having a silver content of 3.0% by mass. The silver coating thickness was calculated to be 19 nm. The paste film resistivity of this sample was 404 μΩ · cm.
(Comparative Example 2)
The amount of the silver solution was adjusted in the same manner as in Example 1 to obtain a silver-coated copper powder having a silver content of 7.4% by mass. The silver coating thickness was calculated to be 46 nm. The paste film resistivity of this sample was 144 μΩ · cm.
Next, the copper powder to be used was changed to a smaller particle size powder (made by Nippon Atomizing Co., Ltd., atomized copper powder ATP-Cu, SEM diameter 2.0 μm), and silver coating was performed in the same manner as described above. However, in the case of the small particle size copper powder, it has been confirmed that silver coating can be performed by displacement plating up to 20.0 mass% as the silver coating amount, and the silver coating amount exceeds 20.0 mass% Reduced plating was added to silver-coated copper powder.
(Example 3)
By the above method, a silver-coated copper powder having a silver content of 19.0% by mass was obtained. The silver coating thickness was calculated to be 62 nm. The paste film resistivity of this sample was 88 μΩ · cm.
Example 4
Silver plating copper powder having a silver content of 26.0% by mass was obtained by reducing plating after substitution plating by the above method and adjusting the amount of silver solution. The silver coating thickness was calculated to be 90 nm. The paste film resistivity of this sample was 81 μΩ · cm.
(Comparative Example 3)
The amount of the silver solution was adjusted in the same manner as in Example 3 to obtain a silver-coated copper powder having a silver content of 12.0% by mass. The silver coating thickness was calculated to be 37 nm. The paste film resistivity of this sample was 184 μΩ · cm.
(Comparative Example 4)
The amount of the silver solution was adjusted in the same manner as in Example 3 to obtain a silver-coated copper powder having a silver content of 15.0% by mass. The silver coating thickness was calculated to be 47 nm. The paste film resistivity of this sample was 116 μΩ · cm.
(Reference example)
The silver powder (made by Fukuda Metal Foil Powder Industry Co., Ltd., atomized silver powder Ag-HWQ 5 μm) having a particle size close to that of the above comparative example and examples is used as the particles, and the steps after the paste preparation of Example 1 are evaluated at the time of silver coated copper powder evaluation. The paste film resistivity was measured in the same manner. The silver powder used had an SEM diameter of 4.4 μm and a paste film resistivity of 86 μΩ · cm.

  The above results are summarized in Table 1.

As mentioned above, when the silver coat copper powder of Examples 1-4 which concerns on this invention was used for the conductive paste for low temperature baking, the paste film resistivity close | similar to the silver powder of a reference example was able to be obtained.
Although the embodiments and examples of the present invention have been described in detail as described above, those skilled in the art can easily make many modifications that do not substantially depart from the novel matters and effects of the present invention. You can understand. Therefore, all such modifications are included in the scope of the present invention.

For example, a term described with a different term having a broader meaning or the same meaning at least once in the specification or the drawings can be replaced with the different term in any part of the specification or the drawings. Also, the configuration and operation of the manufacturing method are not limited to those described in the embodiments and examples of the present invention, and various modifications can be made.

Claims (2)

A silver-coated copper powder, which is a spherical silver-coated copper powder in which copper particles are coated with silver, and the silver coating thickness obtained from the silver coating amount is 60 nm or more. The silver-coated copper powder, wherein an average particle diameter obtained from an image of the silver-coated copper powder observed with a scanning electron microscope is 0.5 μm or more.