Flake-shaped copper powder and conductive paste containing the flake-shaped copper powder

Abstract

The present invention provides a flake-shaped copper powder that has a low apparent density and is strongly oriented horizontally in the coating film. As a result, a conductive paste in which this flake-shaped copper powder is used as a conductive filler has many contact points with each other. Therefore, even with a low ratio of conductive filler, the paste can be fired at a low temperature of about 300°C to form a conductive network and exhibit high conductivity. [Solution] The median diameter (D50) measured by a laser diffraction particle size distribution analyzer is 0.3 μm or greater and 35 μm or less, and the apparent density (AD) is 0.1 g / cm³. 3 In addition, 1.8 g / cm³ 3 The following is a flake-shaped copper powder, which is strongly oriented horizontally in the coating film.

Description

[Technical Field]

[0001] The present invention relates to flake-shaped copper powder that can be suitably used as a conductive filler in conductive pastes. More specifically, the flake-shaped copper powder has a low apparent density and is strongly oriented horizontally in the coating film. Therefore, a conductive paste in which the flake-shaped copper powder is a conductive filler has many contact points between the conductive fillers in the coating film. As a result, even with a low ratio of conductive fillers, the paste can be fired at a low temperature of about 300°C to form a conductive network, thus exhibiting high conductivity. [Background technology]

[0002] With the increasing demand for electronic components, conductive pastes for forming conductive circuits are attracting attention.

[0003] Copper powder is frequently used as a conductive filler in conductive pastes because it has excellent conductivity and is inexpensive.

[0004] For conductive paste to exhibit high conductivity, the shape of the copper powder is important. Flake-shaped copper powder is advantageous over spherical powder because it has more contact points with other copper particles, thus forming a conductive network.

[0005] Flake-shaped copper powder is suitable as a conductive filler for conductive pastes used in conductive circuits of stretchable substrates because it easily exhibits high conductivity even at low concentrations.

[0006] Furthermore, conductive pastes, which use flake-shaped copper powder as a conductive filler, can be fired at relatively low temperatures to form a conductive network, thus contributing to the reduction of CO2 emissions.

[0007] Furthermore, as electronic components become more high-performance, uniform coating properties and good substrate opacity are required. If the conductive filler is a conductive paste made of flake-shaped copper powder, it can contribute to achieving uniform coating properties and high substrate opacity.

[0008] Therefore, in order to produce flake-shaped copper powder suitable as a conductive filler for conductive paste, development is underway focusing on particle size distribution and average particle diameter.

[0009] However, currently available flake copper powders have problems such as thick powder particles or nearly spherical flakes, resulting in weak horizontal orientation of the flake copper powder in the coating film and fewer contact points between the flake copper powder particles.

[0010] Furthermore, if the copper powder is thick or in a nearly spherical flake shape, it is difficult to form a conductive network in a conductive paste with a low ratio of conductive fillers, and firing at low temperatures of around 300°C is difficult, making it challenging to achieve high conductivity.

[0011] Therefore, there is a need for the development of flake-shaped copper powder that can produce conductive pastes with a low ratio of conductive fillers, which can be fired at a low temperature of around 300°C to form a conductive network and exhibit high conductivity. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2005-200734 [Patent Document 2] Japanese Patent Publication No. 2005-314755 [Patent Document 3] Japanese Patent Publication No. 2017-025393 [Overview of the Initiative] [Problems that the invention aims to solve]

[0013] Patent Document 1 describes flake-shaped copper powder for conductive fillers in conductive pastes, focusing on the thickness, aspect ratio, and particle size distribution of the powder.

[0014] However, the conductive paste described in Patent Document 1 is fired at a high temperature of 840°C, and there is no mention of whether it exhibits high conductivity when fired at a low temperature of around 300°C.

[0015] Patent Document 2 describes flake copper powder for conductive fillers in conductive pastes, focusing on the aspect ratio, crystallite size, and P content of the powder.

[0016] However, there is no mention of the firing temperature.

[0017] Patent Document 3 describes flake-shaped copper powder for conductive paste, focusing on the distribution of the aspect ratio of the powder and the amount of carbon.

[0018] However, the firing process is performed at a high temperature of approximately 845°C, and there is no mention of whether firing at a lower temperature of around 300°C would result in high conductivity.

[0019] The inventors of this invention aimed to create a conductive filler that exhibits high conductivity even when fired at a low temperature of around 300°C, even if the proportion of conductive filler is low. As a result of numerous prototypes and experiments, they found that the median diameter (D50) measured by a laser diffraction particle size distribution analyzer was 0.3 μm or more and 35 μm or less, and the apparent density (AD) was 0.1 g / cm³. 3 In addition, 1.8 g / cm³ 3The following flaky copper powder, wherein a transparent organic solvent-based lacquer having a resin content ratio soluble in an organic solvent of 10% by weight or more and 15% by weight or less, and 80% by weight of the transparent organic solvent-based lacquer, and 20% by weight of the flaky copper powder are dispersed therein, and the hiding power calculated by the hiding power measurement method defined in JIS (K 5101-4:2004) (Formula 1) “(1 - ΔL / ΔL0) × 100 (ΔL0 = the difference in the lightness L value between the black part and the white part of the hiding power test paper, ΔL = the difference in the lightness L value between the black part and the white part of the hiding power test paper coated with the dispersion paste)” is 90% or more. Such flaky copper powder has a low apparent density and strongly orientates horizontally in the coating film. Therefore, in a conductive paste using such flaky copper powder as a conductive filler, since the contact points between the conductive fillers in the coating film increase, even a conductive paste with a low ratio of the conductive filler can be fired at a low temperature of about 300°C to form a conductive network and exhibit high conductivity, thus achieving the above technical problem.

Means for Solving the Problem

[0020] The above technical problem can be solved by the present invention as follows.

[0021] The present invention relates to a flaky copper powder having a median diameter (D50) measured by a laser diffraction particle size distribution measuring device of 0.3 μm or more and 35 μm or less, and an apparent density (AD) of 0.1 g / cm 3 or more and 1.8 g / cm 3 or less, wherein a transparent organic solvent-based lacquer having a resin content ratio soluble in an organic solvent of 10% by weight or more and 15% by weight or less, and 80% by weight of the transparent organic solvent-based lacquer, and 20% by weight of the flaky copper powder are dispersed therein, and the hiding power calculated by the following (Formula 1) according to the hiding power measurement method defined in JIS (K 5101-4:2004) is 90% or more. (Formula 1)(1 - ΔL / ΔL0) × 100 ΔL = the difference in the lightness L value between the black part and the white part of the hiding power test paper coated with the dispersion paste ΔL0 = the difference in the lightness L value between the black part and the white part of the hiding power test paper

[0022] Furthermore, the present invention relates to a conductive paste containing the aforementioned flake-shaped copper powder.

[0023] Furthermore, the present invention relates to the conductive paste wherein the conductive paste is obtained by dispersing 100 parts by weight of flake copper powder in 44 parts by weight or more and 100 parts by weight or less of varnish. [Effects of the Invention]

[0024] The flake copper powder in this invention has a median diameter (D50) of 0.3 μm or more and 35 μm or less, and an apparent density (AD) of 0.1 g / cm³. 3 In addition, 1.8 g / cm³ 3 Because it is a flake-shaped copper powder, the flake-shaped copper powder has a shape that makes it easy for contact points to form between them.

[0025] Furthermore, because the opacity is 90% or more and the flake-shaped copper powder is strongly oriented horizontally in the coating film, the conductive paste in which the present invention's flake-shaped copper powder is a conductive filler has many contact points between the conductive fillers. Therefore, even with a low ratio of conductive fillers, it can be fired at a low temperature of about 300°C to form a conductive network and exhibit high conductivity.

[0026] Therefore, the conductive paste in which flake copper powder is a conductive filler according to the present invention can be suitably used as a conductive material for circuit formation and mounting of electronic components. [Brief explanation of the drawing]

[0027] [Figure 1] This image shows a scanning electron microscope (SEM) image (19,000x magnification) of the flake-shaped copper powder of Example 1, and a diagram highlighting the shape of the same powder. [Figure 2] This image shows a scanning electron microscope (SEM) image (19,000x magnification) of the flake-shaped copper powder of Comparative Example 1, and a diagram emphasizing the shape of the same powder. [Modes for carrying out the invention]

[0028] (flake-shaped copper powder) The flake-shaped copper powder in the present invention has a low apparent density and is a flake-shaped copper powder that strongly orientates horizontally in the coating film.

[0029] The median diameter (hereinafter referred to as "D50") measured by a laser diffraction particle size distribution apparatus for the flake-shaped copper powder in the present invention is preferably 0.3 μm to 35 μm, more preferably 0.5 μm to 30 μm, and still more preferably 0.7 μm to 30 μm.

[0030] If D50 is larger than 35 μm, a certain number of coarse particles exist, so there is a risk that circuits or the like cannot be formed by printing.

[0031] [[ID=J16]]Also, if D50 is larger than 35 μm, it is difficult for the fired film to densify, so there is a risk that high conductivity cannot be obtained.

[0032] On the other hand, it is difficult to produce flake-shaped copper powder with D50 less than 0.3 μm by mechanical pulverization method.

[0033] This is because there is a risk that the fine particles generated by pulverization will aggregate with each other.

[0034] The laser diffraction particle size distribution measurement method is to obtain the equivalent spherical diameter and the volume-based particle size distribution by analyzing the intensity of the scattered light generated by irradiating laser light on the particles dispersed in a solvent.

[0035] The apparent density (hereinafter referred to as "AD") of the flake-shaped copper powder in the present invention is 0.1 g / cm 3 ~1.8 g / cm 3 is preferably, more preferably 0.1 g / cm 3 ~1.5 g / cm 3 and is.

[0036] <X The particle shape of the flake-shaped copper powder with low AD becomes irregular. This is because as AD decreases, it indicates that there are more voids between the copper powder particles.

[0037] As shown in Figure 1, the flake copper powder in the present invention has a shape in which the edges of the powder particles are not rounded but have multiple indentations toward the inside of the powder (a jagged, saw-tooth-like shape). Therefore, it is suggested that the number of contact points between the flake copper powder particles increases in the coating film.

[0038] Therefore, a lower AD is better, but 0.1 g / cm³ is preferable. 3 Smaller flake-shaped copper powder is difficult to produce.

[0039] AD is 1.8 g / cm³ 3 Beyond a certain temperature, as shown in Figure 2, the powder particle shape becomes rounded, reducing the number of contact points between flake-shaped copper powders. In conductive pastes with a low ratio of conductive filler, a conductive network may not form easily at low temperatures of around 300°C, potentially resulting in a failure to achieve high conductivity.

[0040] AD should be measured in accordance with JIS Z2504:2020.

[0041] The flake-shaped copper powder in this invention can be produced by mechanically grinding the raw copper powder.

[0042] The raw material copper powder is not particularly limited; copper powder produced by various methods such as atomization, reduction, and electrolysis can be used.

[0043] The grinding method can be either dry grinding or wet grinding, but dry grinding is preferable considering the post-grinding processing.

[0044] The type of grinder is not particularly limited, but examples include ball mills, vibratory mills, and stamp mills.

[0045] During grinding, a lubricant may be used to prevent the aggregation of copper powder and to promote grinding.

[0046] The lubricant is not particularly limited, but examples include saturated fatty acids such as stearic acid, palmitic acid, myristic acid, and lauric acid, unsaturated fatty acids such as oleic acid, alcohols, and graphite.

[0047] The flake-shaped copper powder in this invention may contain unavoidable impurities during the manufacturing process.

[0048] (Concealment rate) In this invention, the flake-shaped copper powder is strongly oriented horizontally in the coating film.

[0049] If the flake-shaped copper powder is oriented in various directions, there is a risk that sufficient contact points may not be obtained in the coating film.

[0050] The horizontal orientation of flake copper powder in a coating can be measured by the opacity ratio specified in JIS (K5101-4:2004).

[0051] When flake-shaped copper powder in a coating film is strongly oriented horizontally and aligned without gaps, the opacity rate increases.

[0052] For measuring the opacity in this invention, a dispersion paste is used, which is obtained by dispersing 20% ​​by weight of the flake-shaped copper powder in 80% by weight of a transparent organic solvent-based lacquer.

[0053] The resin content dissolved in the transparent organic solvent-based lacquer is preferably 10% by weight or more and 15% by weight or less.

[0054] As an example of a transparent organic solvent-based lacquer, we will use nitrated cotton lacquer (Selva® 26 / resin content ratio 10% to 15% by weight / manufactured by Kansai Paint Co., Ltd.).

[0055] The opacity rate is measured by using the aforementioned dispersion paste and printing it onto opacity test paper with a bar coater (#18) according to the procedure specified in JIS (K5101-4:2004).

[0056] Two printing methods are illustrated: one involves manually printing by dripping paint onto the bar, then holding both ends to apply even pressure, and pulling the bar forward at a constant speed without rotating it; and the other involves automatically printing using a desktop test coater (e.g., a K Control Coater from RK Print-Coat Instruments).

[0057] The opacity is calculated using the following formula (Equation 1), where ΔL0 is the difference in lightness L value between the black and white parts of an opacity test paper having both white and white parts, and ΔL is the difference in lightness L value between the black and white parts of an opacity test paper coated with dispersion paste. When ΔL is equal to ΔL0, the opacity is considered 0%, and when there is no difference (ΔL=0), it is considered 100%.

[0058] (Equation 1) (1 - ΔL / ΔL0) × 100

[0059] The opacity of the dispersed paste is preferably 90% or higher, more preferably 93% or higher, and even more preferably 98% or higher.

[0060] If the dispersion paste contains flake-shaped copper powder with an opacity of less than 90%, there will be many gaps on the surface of the coating and few contact points between the flake-shaped copper powders. Therefore, using it as a conductive filler in a conductive paste may result in insufficient conductivity.

[0061] (Conductive paste) The conductive paste in which flake copper powder is a conductive filler in the present invention can be prepared by dispersing flake copper powder in varnish using a mixer or a metal spatula.

[0062] While varnishes are not particularly limited, examples include varnishes made by dissolving acrylic resin, polyester resin, or polyvinyl butyral resin in a solvent.

[0063] Examples of solvents for dissolving the aforementioned resin include gamma-butyrolactone (γ-BL), butyl carbitol acetate, butyl cellosolve, and terpineol.

[0064] The ratio of flake copper powder in the conductive paste is not particularly limited.

[0065] The ratio of flake copper powder in a conductive paste can be calculated by dividing the weight of the flake copper powder by the sum of the weight of the flake copper powder and the weight of the resin contained in the varnish.

[0066] This is because the solvent components in the varnish decompose and volatilize during firing, but the resin and metal components remain.

[0067] In a varnish with a resin content of 25% by weight, it is preferable to disperse 100 parts by weight of flake copper powder in 44 to 100 parts by weight of varnish.

[0068] In the present invention, the conductive paste is applied to a substrate to form a dried coating film, and then fired to form a fired film.

[0069] The drying method is not particularly limited, but we will illustrate it by letting it stand in the air at 60°C for 10 minutes.

[0070] The firing method is not particularly limited, but examples include heating reduction using a reducing atmosphere and photosintering using a xenon lamp.

[0071] The firing temperature is preferably 250°C to 300°C, and more preferably 275°C to 300°C.

[0072] The volume resistivity of a fired film can be calculated by first determining the resistance using a resistance meter, and then measuring the film thickness and line width using a surface roughness meter. [Examples]

[0073] The present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited thereto.

[0074] (Flake-shaped copper powder) To obtain flake-shaped copper powders for the examples and comparative examples, 1 kg to 2 kg of spherical copper powder (D50: 2 μm to 150 μm) prepared by atomization or reduction was mixed with 10 kg to 20 kg of steel balls with a diameter of 1.6 mm to 6.4 mm. Stearic acid was used as a lubricant, and the mixture was ground at a rotation speed of 40 rpm to 70 rpm for 5 to 400 hours.

[0075] (Measurement of D50 and AD) The median diameter (D50: μm) of each flake-shaped copper powder in the examples and comparative examples was measured using a laser diffraction particle size distribution analyzer (SALD-2300 / Shimadzu Corporation). The apparent density (AD: g / cm³) was then calculated using a funnel with an orifice diameter of 5.0 mm in accordance with JIS Z2504. 3 ) was measured.

[0076] (Concealment rate) Each dispersion paste was prepared by mixing 20% ​​by weight of the flake copper powder of the examples and comparative examples with 80% by weight of nitrated cotton lacquer (Selva® 26 / resin content ratio 10% to 15% by weight / manufactured by Kansai Paint Co., Ltd.) using a paint conditioner.

[0077] The prepared dispersion paste was printed onto coated paper (ΔL0=79.23) for opacity testing using a bar coater #18. After dropping the paint onto the front of the bar, both ends were held down to ensure even pressure was applied, and the bar was pulled towards the user at a constant speed without rotating it.

[0078] Using a colorimeter (SE6000 / manufactured by Nippon Denshoku Industries Ltd.), the lightness L values ​​of the black areas and white areas of the coated paper for opacity testing and the coated paper printed with dispersion paste were measured, and the opacity was calculated using the above formula (Equation 1).

[0079] (Conductive paste) Each of the flake copper powders from the examples and comparative examples was mixed with 44 parts by weight or 100 parts by weight of a varnish containing 25% by weight of polyvinyl butyral resin (Eslec® / manufactured by Sekisui Chemical Co., Ltd.) and γ-BL (manufactured by Mitsubishi Chemical Corporation) using a painting knife to produce conductive pastes with flake copper powder content of 90% by weight and 80% by weight, respectively, after firing.

[0080] Using the prepared conductive paste, a coating film with a line width of 4 mm and a thickness of 50 μm was formed on a glass substrate using a metal spatula.

[0081] The formed coating film was dried in air at 60°C for 10 minutes to obtain a dried coating film.

[0082] The dried coating was fired at 300°C for 60 minutes under a nitrogen atmosphere to produce a fired film.

[0083] The resistivity of the fired film was measured using a resistivity meter (RM3545 / HIOKI E. CORPORATION) at an inter-electrode distance of 40 mm, and the film thickness and line width were measured using a surface roughness meter (SE500A / Kosaka Laboratory Co., Ltd.), and the volume resistivity was calculated.

[0084] The results are shown in Table 1.

[0085] [Table 1]

[0086] Table 1 demonstrates that, with the conductive paste containing the flake-shaped copper powder of the present invention, even a conductive paste with a flake-shaped copper powder content of 80% by weight after firing can achieve low volume resistivity and high conductivity.

[0087] Scanning electron microscope (SEM) images (19,000x magnification) were taken of the flake-shaped copper powders of Example 1 and Comparative Example 1, and their shapes were compared (Figures 1 and 2).

[0088] In the flake copper powder of the example, the edges of the particles have a shape with multiple indentations directed inward, whereas in the flake copper powder of the comparative example, the edges of the particles are rounded. [Industrial applicability]

[0089] The flake-shaped copper powder in this invention has a low apparent density and is strongly oriented horizontally in the coating film. Therefore, in a conductive paste in which this flake-shaped copper powder is a conductive filler, there are many contact points between the conductive fillers in the coating film. As a result, even with a low ratio of conductive fillers, firing at a low temperature of about 300°C forms a conductive network and results in a conductive paste that exhibits high conductivity. Therefore, the present invention is highly applicable to industrial use.

Claims

1. The median diameter (D50) measured by a laser diffraction particle size distribution analyzer is 0.3 μm or greater and 35 μm or less, and the apparent density (AD) is 0.1 g / cm³. ３ In addition, 1.8 g / cm³ ３ The following is a flake-shaped copper powder: A dispersion paste obtained by dispersing 20% ​​by weight of the flake-shaped copper powder in 80% by weight of a transparent organic solvent-based lacquer having an organic solvent-soluble resin content of 10% by weight or more and 15% by weight or less, wherein the opacity of the dispersion paste is 90% or more, as calculated by the following formula (Formula 1) according to the opacity measurement method specified in JIS (K 5101-4:2004). (Formula 1) (1 - ΔL / ΔL ０ ) × 100 ΔL = Difference in lightness L value between the black area and the white area of ​​the opacity test paper coated with dispersion paste. ΔL ０ = Difference in brightness L value between the black area and the white area of ​​the opacity test paper.

2. A conductive paste containing the flake-shaped copper powder described in claim 1.

3. The conductive paste according to claim 2, wherein the conductive paste is obtained by dispersing 100 parts by weight of flake copper powder in 44 parts by weight or more and 100 parts by weight or less of varnish.

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