Copper flake powder and conductive paste containing said copper flake powder

Abstract

[Problem] To provide a copper flake powder that has a low apparent density and is strongly oriented in the horizontal direction in a coating film, and therefore, in a conductive paste in which the copper flake powder is contained as a conductive filler, the number of contact points between the conductive fillers increases, and therefore, even when the ratio of the conductive filler is low, the copper flake powder can be fired at a low temperature of about 300°C to build a conductive network, and forms a conductive paste that exhibits high conductivity. [Solution] A copper flake powder which has a median diameter (D50) of 0.3 µm to 35 µm inclusive as measured by a laser diffraction particle size distribution measuring device and an apparent density (AD) of 0.1 g / cm3 to 1.8 g / cm3 inclusive, and which is strongly oriented in the horizontal direction in a coating film.

Description

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

[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.

[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 allows for more contact points between the powder 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 in which flake copper powder is a conductive filler can be fired at relatively low temperatures to form a conductive network, thus CO ２ It contributes to reducing emissions.

[0007] Furthermore, as electronic components become more high-performance, uniform coating properties and good opacity of the substrate 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 opacity of the substrate.

[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, a conductive network is difficult to form 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 a conductive paste that exhibits high conductivity by forming a conductive network when fired at a low temperature of around 300°C, even if the conductive paste has a low ratio of conductive fillers.

[0012] Japanese Patent Publication No. 2005-200734, Japanese Patent Publication No. 2005-314755, Japanese Patent Publication No. 2017-025393

[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 considered it a technical challenge to produce 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 in the conductive paste 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 is 0.3 μm or more 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 describes a dispersion paste made 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, according to the opacity measurement method specified in JIS (K 5101-4:2004) (Formula 1) "(1 - ΔL / ΔL ０ ) × 100 (ΔL ０ If the opacity of the flake-shaped copper powder is 90% or more, calculated using the formula (ΔL = difference in brightness L value between the black and white areas of the opacity test paper, ΔL = difference in brightness L value between the black and white areas of the opacity test paper coated with dispersion paste), then the apparent density is low and the flakes are strongly oriented horizontally in the coating film. Therefore, a conductive paste using this flake-shaped copper powder as a conductive filler will have many contact points between the conductive fillers in the coating film. As a result, even a conductive paste with a low ratio of conductive fillers can be fired at a low temperature of around 300°C to form a conductive network and exhibit high conductivity. This finding has thus achieved the aforementioned technical objective.

[0020] The aforementioned technical problems can be solved by the present invention as follows.

[0021] The present invention relates to a material in which the median diameter (D50) measured by a laser diffraction particle size distribution analyzer is 0.3 μm or more and 35 μm or less, and the apparent density (AD) is 0.1 g / cm³. ３ In addition, 1.8 g / cm³ ３Flaky copper powder as described below, in which the content ratio of a resin soluble in an organic solvent is 10% by weight or more and 15% by weight or less, and 20% by weight of the flaky copper powder is dispersed in 80% by weight of a transparent organic solvent-based lacquer. The flaky copper powder has a hiding power of 90% or more calculated by the following formula (1) according to the hiding power measurement method specified in JIS (K 5101-4:2004). (Formula 1) (1 - ΔL / ΔL ０ ) × 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 ΔL ０ = the difference in the lightness L value between the black part and the white part of the hiding power test paper

[0022] Further, the present invention is a conductive paste containing the above flaky copper powder.

[0023] Further, the present invention is the above conductive paste, which is a conductive paste obtained by dispersing 44 parts by weight or more and 100 parts by weight or less of a varnish in respect to 100 parts by weight of the flaky copper powder.

[0024] The flaky copper powder in the present 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 / cm ３ or more and 1.8 g / cm ３ or less. Therefore, the flaky copper powders are in a shape that easily forms contact points with each other.

[0025] Further, since the hiding power is 90% or more and the flakes are strongly oriented horizontally in the coating film, in the conductive paste in which the flaky copper powder according to the present invention is a conductive filler, the contact points between the conductive fillers increase. Therefore, even when the ratio of the conductive filler is low, firing at a low temperature of about 300 °C forms a conductive network and exhibits high conductivity.

[0026] Therefore, the conductive paste in which the flaky copper powder in the present invention is a conductive filler can be suitably used for forming circuits as a conductive material and mounting electronic components.

[0027] It is a figure emphasizing the shape of the flaky copper powder of Example 1, which is a scanning electron microscope (SEM) image (19,000 times magnification). It is a figure emphasizing the shape of the flaky copper powder of Comparative Example 1, which is a scanning electron microscope (SEM) image (19,000 times magnification).

[0028] (Flaky copper powder) The flaky copper powder in the present invention is a flaky copper powder having a low apparent density and strongly oriented horizontally in a coating film.

[0029] The median diameter (hereinafter referred to as "D50") measured by a laser diffraction particle size distribution apparatus of the flaky 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] When D50 is larger than 35 μm, there are a certain number of coarse particles, so there is a risk that circuits or the like cannot be formed by printing.

[0031] Further, when D50 is larger than 35 μm, it is difficult for the fired film to be densified, so there is a risk that high conductivity cannot be obtained.

[0032] On the other hand, it is difficult to produce flaky 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 flaky 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 is.

[0036] The particle shape of the flaky copper powder with a low AD becomes irregular. This is because as AD decreases, the voids between the copper powder particles increase.

[0037] In the flaky copper powder of the present invention, as shown in FIG. 1, since the edge portions of the powder particles have a shape having a plurality of depressions (a shape like a serrated saw blade) without rounding toward the inside of the powder, it is suggested that the contact points between the flaky copper powders increase in the coating film.

[0038] Therefore, it is better for AD to be lower, but it is difficult to produce flaky copper powder smaller than 0.1 g / cm ３ When AD exceeds 1.8 g / cm

[0039] as shown in FIG. 2, the powder particle shape becomes rounded, so the contact points between the flaky copper powders decrease. In the case of a conductive paste with a low ratio of conductive filler, it is difficult to form a conductive network at a low temperature of about 300 °C, and there is a possibility that high conductivity cannot be obtained. ３ AD may be measured in accordance with JIS Z2504:2020.

[0040] The flaky copper powder in the present invention can be produced by pulverizing the raw copper powder by a mechanical pulverization method.

[0041] The raw copper powder is not particularly limited, and copper powders produced by various methods such as an atomization method, a reduction method, and an electrolysis method can be used.

[0042] The pulverization method may be either dry pulverization or wet pulverization, but dry pulverization is preferable in consideration of the post-treatment after pulverization.

[0043] The pulverizer is not particularly limited, and examples include a ball mill, a vibration mill, and a stamp mill.

[0044] At the time of pulverization, a lubricant may be used to prevent aggregation of the copper powder and promote pulverization.

[0045] The lubricant is not particularly limited, and 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.

[0046]

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

[0048] (Opacity) The flake-shaped copper powder in this invention 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 / manufactured by RK Print-Coat Instruments).

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

[0058] (Formula 1) (1-ΔL / ΔL ０ ) × 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, if used as a conductive filler in a conductive paste, high conductivity may not be achieved.

[0061] (Conductive paste) The conductive paste in which the flake-shaped copper powder in the present invention is a conductive filler can be made by dispersing the flake-shaped 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 resin include gamma-butyrolactone (γ-BL), butylcarbitol 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, and then fired to form a fired film.

[0069] The drying method is not particularly limited, but one example is drying 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.

[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) 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 to obtain the respective flake-shaped copper powders for the examples and comparative examples.

[0075] (Measurement of D50 and AD) The median diameter (D50: μm) of each flake 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 measured using a funnel with an orifice diameter of 5.0 mm in accordance with JIS Z2504. ３ ) was measured.

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

[0077] The prepared dispersion paste was then applied to the coating paper (ΔL) 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. ０ It was printed on 79.23.

[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) 100 parts by weight of each flake copper powder from the Examples and Comparative Examples were mixed with 44 parts by weight or 100 parts by weight of a varnish with a resin content of 25% by weight, consisting 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]

[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 exhibits 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.

[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, the number of contact points between the conductive fillers in the coating film increases. As a result, even with a low ratio of conductive fillers, firing at a low temperature of around 300°C forms a conductive network, resulting in a conductive paste that exhibits high conductivity. Accordingly, this invention has high industrial applicability.

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 flake copper powder, wherein 20% by weight of the flake copper powder is dispersed 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, and the opacity of the dispersion paste calculated by the following formula (Formula 1) according to the opacity measurement method specified in JIS (K 5101-4:2004) is 90% or more. (Formula 1) (1 - ΔL / ΔL ０ ) × 100 ΔL = Difference in brightness L value of 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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