Flake copper powder and conductive paste containing the same

Abstract

The flake copper powder has a low apparent density and is strongly oriented horizontally in the coating film. Therefore, a conductive paste using the flake copper powder as a conductive filler has many contact points between the conductive fillers. Therefore, even if the ratio of the conductive filler is low, the flake copper powder can be fired at a low temperature of about 300°C to form a conductive network and become a conductive paste that exhibits high conductivity. [Solution] 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 / cm 3 or more and 1.8g / cm 3 A flaky copper powder having the following structure, which is strongly oriented horizontally in a coating film:

Description

[Technical Field]

[0001] The present invention relates to a flaky copper powder suitable for use as a conductive filler in a conductive paste. Specifically, the flaky copper powder has a low apparent density and is strongly oriented horizontally in a coating film. Therefore, a conductive paste containing the flaky copper powder as a conductive filler has many contact points between the conductive filler particles in the coating film, and therefore can be fired at a low temperature of about 300°C to form a conductive network, even if the proportion of conductive filler is low, resulting in a conductive paste with high conductivity. [Background technology]

[0002] 2. Description of the Related Art With the increasing demand for electronic components, conductive pastes for forming conductive circuits have been attracting attention.

[0003] As the conductive filler in the conductive paste, copper powder is often used, which has excellent conductivity and is inexpensive.

[0004] The shape of the copper powder is important for the conductive paste to exhibit high conductivity, and flake-shaped copper powder is more advantageous than spherical powder in that it has more contact points between the copper powder particles and forms a conductive network.

[0005] Flake-shaped copper powder is likely to exhibit high conductivity even at a low content ratio, and is therefore suitable as a conductive filler for conductive pastes used in conductive circuits of expandable substrates.

[0006] Furthermore, conductive pastes that use flake copper powder as a conductive filler can be fired at relatively low temperatures to form a conductive network, which contributes to reducing CO2 emissions.

[0007] Furthermore, as electronic components become more and more sophisticated, uniform application and excellent hiding power for the underlying surface are required. A conductive paste containing flake copper powder as the conductive filler contributes to achieving uniform application and excellent hiding power for the underlying surface.

[0008] Therefore, in order to produce flake copper powder suitable for use as a conductive filler in conductive paste, development is being carried out with a focus on particle size distribution, average particle diameter, etc.

[0009] However, the flake copper powders currently in widespread use have thick particles or are nearly spherical in shape, which results in poor horizontal orientation of the flake copper powder in the coating film and few contact points between the flake copper powder particles.

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

[0011] Therefore, there is a need to develop flake copper powder that can be fired at a low temperature of around 300°C to form a conductive network and produce a conductive paste that exhibits high conductivity, even if the conductive filler content is low. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Patent Publication No. 2005-200734 [Patent Document 2] Patent Publication No. 2005-314755 [Patent Document 3] Patent Publication No. 2017-025393 Summary of the Invention [Problem to be solved by the invention]

[0013] Patent Document 1 describes a flake copper powder for use as a conductive filler in a conductive paste, focusing on the thickness, aspect ratio, particle size distribution, and the like 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 high conductivity can be achieved when fired at a low temperature of around 300°C.

[0015] Patent Document 2 describes a flake copper powder for use as a conductive filler in a conductive paste, focusing on the aspect ratio, crystallite size, P amount, and the like of the powder.

[0016] However, there is no description regarding the firing temperature.

[0017] Patent Document 3 describes a flake copper powder for conductive paste that focuses on the distribution of the aspect ratio of the powder and the amount of C.

[0018] However, the material is fired at a high temperature of about 845°C, and there is no mention of whether high conductivity can be achieved by firing at a low temperature of about 300°C.

[0019] The inventors set out to develop a technical problem of producing a conductive filler that can be fired at a low temperature of around 300°C to produce a conductive paste that exhibits high conductivity, even when the conductive filler ratio is low. As a result of numerous prototypes and experiments, they were able to produce a conductive paste with 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, as measured by a laser diffraction particle size distribution analyzer. 3 or more and 1.8g / cm 3A flaky copper powder having a content of 10% by weight or more and 15% by weight or less of a resin soluble in an organic solvent is dispersed in 80% by weight of a transparent organic solvent-based lacquer, and the flaky copper powder is dispersed in 20% by weight of the flaky copper powder in accordance with JIS (K 5101-4:2004), if the flake copper powder has a hiding power of 90% or more, calculated using the formula (1) "(1-ΔL / ΔL0) x 100 (ΔL0 = difference in lightness L value of the black part of the hiding power test paper and the white part, ΔL = difference in lightness L value of the black part of the hiding power test paper and the white part)", the apparent density is low and the powder is strongly oriented horizontally in the coating film. Therefore, a conductive paste using this flake copper powder as the conductive filler will have many contact points between the conductive fillers in the coating film, and therefore even a conductive paste with a low ratio of conductive filler can be fired at a low temperature of around 300°C to form a conductive network and become a conductive paste with high conductivity. This finding has enabled the achievement of the above technical objective. [Means for solving the problem]

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

[0021] The present invention relates to a particle size distribution analyzer having 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 1.8g / cm 3 The flake copper powder is as follows: The flake copper powder has a content of organic solvent-soluble resin of 10% by weight or more, and has a hiding power of 90% or more when the hiding power is calculated by the hiding power measurement method specified in JIS (K 5101-4:2004) using the following formula (1): (Equation 1) (1-ΔL / ΔL0) × 100 ΔL = Difference in brightness L value between the black part and the white part of the opacity test paper coated with dispersion paste ΔL0 = Difference in brightness L value between the black part and the white part of the opacity test paper

[0022] The present invention also provides a conductive paste containing the above-mentioned flaky copper powder.

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

[0024] The flaky copper powder of 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 1.8g / cm 3 The flake copper powder described below has a shape that allows the flake copper powder particles to easily form contact points with each other.

[0025] Furthermore, since the hiding power is 90% or more and the coating film is strongly oriented horizontally, a conductive paste in which the flaky copper powder of the present invention is used as a conductive filler has many contact points between the conductive fillers, and therefore, even if the ratio of the conductive filler is low, a conductive network is formed by firing at a low temperature of around 300°C, thereby exhibiting high conductivity.

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

[0027] [Figure 1] FIG. 1 is a scanning electron microscope (SEM) image (19,000x magnification) of the flaky copper powder of Example 1, highlighting the shape of the powder. [Figure 2] FIG. 2 is a scanning electron microscope (SEM) image (19,000x magnification) of the flaky copper powder of Comparative Example 1, with the shape of the powder emphasized. DETAILED DESCRIPTION OF THE INVENTION

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

[0029] The median diameter (hereinafter referred to as "D50") of the flaky copper powder of the present invention measured with a laser diffraction particle size distribution analyzer is preferably 0.3 μm to 35 μm, more preferably 0.5 μm to 30 μm, and even more preferably 0.7 μm to 30 μm.

[0030] If D50 is greater than 35 μm, a certain number of coarse particles will be present, which may make it impossible to form circuits or the like by printing.

[0031] Furthermore, if D50 is greater than 35 μm, it is difficult to densify the fired film, and there is a risk that high conductivity may not be obtained.

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

[0033] This is because there is a risk that the fine particles generated by pulverization may aggregate together.

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

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

[0036] The particle shape of flaky copper powder with low AD becomes irregular. This is because the lower the AD, the more voids there are between the copper powder particles.

[0037] As shown in Figure 1, the flaky copper powder of the present invention has a shape in which the edges of the powder particles are not rounded but have multiple depressions toward the inside of the powder (a jagged saw-like shape), which suggests that the number of contact points between the flaky copper powder particles in the coating film will increase.

[0038] Therefore, the lower the AD, the better. 3 Smaller flake copper powders are difficult to produce.

[0039] AD is 1.8g / cm 3 Above this temperature, the powder particle shape becomes rounded as shown in Figure 2, which reduces the number of contact points between the flake copper powder particles. Therefore, if the conductive paste has a low ratio of conductive filler, it is difficult to form a conductive network at a low temperature of around 300°C, and high conductivity may not be achieved.

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

[0041] The flaky copper powder of the present invention can be prepared by pulverizing raw copper powder by mechanical pulverization.

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

[0043] The pulverization method may be either dry pulverization or wet pulverization, but considering the post-treatment after pulverization, dry pulverization is preferred.

[0044] The pulverizer is not particularly limited, and examples thereof include a ball mill, a vibration mill, and a stamp mill.

[0045] During the pulverization, a lubricant may be used to prevent the copper powder from agglomerating and to promote the pulverization.

[0046] The lubricant is not particularly limited, and examples thereof 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 flaky copper powder of the present invention may contain unavoidable impurities during the manufacturing process.

[0048] (Concealment rate) The flaky copper powder of the present invention is strongly oriented horizontally in the coating film.

[0049] If the flake copper powder is oriented in various directions, there is a risk that there will not be enough contact points in the coating.

[0050] The horizontal orientation of the flaky copper powder in the coating film can be measured by the hiding power defined in JIS (K5101-4:2004).

[0051] If the flake copper powder in the coating film has a strong horizontal orientation and is aligned without gaps, the hiding power will be high.

[0052] In the present invention, the hiding power is measured using a dispersion paste prepared by dispersing 20% ​​by weight of the above-mentioned flake 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] An example of a transparent organic solvent-based lacquer is nitrocellulose lacquer (Selva (registered trademark) 26 / resin content ratio 10% by weight to 15% by weight / manufactured by Kansai Paint Co., Ltd.).

[0055] The hiding power is measured by printing the above dispersion paste onto a hiding power test paper using a bar coater (#18) according to the procedure specified in JIS (K5101-4:2004).

[0056] As printing methods, we will show two examples: a manual printing method in which paint is dripped in front of the bar, then both ends are pressed so that the force is applied evenly over the entire bar, and the bar is pulled toward you at a constant speed without rotating, and an automatic printing method using a desktop test coater (e.g., K Control Coater / RK Print-Coat Instruments).

[0057] The hiding rate is calculated using the following formula (1): ΔL0 is the difference in the lightness L value of the black part of a hiding rate test paper having a white part and a black part and the lightness L value of the white part, and ΔL is the difference in the lightness L value of the black part of a hiding rate test paper to which a dispersion paste has been applied and the lightness L value of the white part. When ΔL is equal to ΔL0, it is 0%, and when there is no difference (ΔL = 0), it is 100%.

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

[0059] The hiding rate of the dispersion paste is preferably 90% or more, more preferably 93% or more, and even more preferably 98% or more.

[0060] If the hiding power of the dispersion paste is less than 90%, there will be many gaps on the coating surface and few contact points between the flake copper powder particles, so if used as a conductive filler in a conductive paste, high conductivity may not be achieved.

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

[0062] The varnish is not particularly limited, but examples thereof include varnishes prepared by dissolving acrylic resin, polyester resin, or polyvinyl butyral resin in a solvent.

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

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

[0065] The proportion of flaky copper powder in the conductive paste may be calculated by dividing the weight of the flaky copper powder by the total weight of the flaky copper powder and the weight of the resin contained in the varnish.

[0066] This is because the solvent component in the varnish is decomposed and volatilized by baking, but the resin component and metal component remain.

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

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

[0069] The drying method is not particularly limited, and an example is a method in which the film is dried by standing in the air at 60° C. for 10 minutes.

[0070] The firing method is not particularly limited, and examples thereof include a heat reduction method using a reducing atmosphere and a photo-sintering method using a xenon lamp.

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

[0072] The volume resistivity of the fired film can be calculated by calculating the resistance value with a resistance measuring instrument, and then measuring the film thickness and line width with a surface roughness measuring instrument. [Example]

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

[0074] (Flake copper powder) 10 kg to 20 kg of steel balls with a diameter of 1.6 mm to 6.4 mm were added to 1 kg to 2 kg of spherical copper powder (D50: 2 μm to 150 μm) produced by the atomization method or reduction method, and pulverized using stearic acid as a lubricant at a rotation speed of 40 rpm to 70 rpm for 5 hours to 400 hours to obtain each of the flake copper powders of the examples and comparative examples.

[0075] (D50 and AD measurements) 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), and the apparent density (AD: g / cm) was measured 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 each flake copper powder of the Examples and Comparative Examples with 80% by weight of nitrocellulose lacquer (Selva (registered trademark) 26 / resin content ratio 10% by weight to 15% by weight / manufactured by Kansai Paint Co., Ltd.) in a paint conditioner.

[0077] The prepared dispersion paste was printed on coated paper (ΔL0 = 79.23) for hiding power test using bar coater #18. After dropping the paint in front of the bar, both ends were pressed to apply a uniform force to the entire surface, and the bar was pulled toward the user at a constant speed without rotating.

[0078] Using a color difference meter (SE6000 / manufactured by Nippon Denshoku Industries Co., Ltd.), the lightness L value of the black part and the lightness L value of the white part of each of the coated paper for the hiding rate test and the coated paper for the hiding rate test printed with the dispersion paste were measured, and the hiding rate was calculated using the above (Equation 1).

[0079] (Conductive paste) 100 parts by weight of each flake copper powder of the Examples and Comparative Examples and 44 parts by weight or 100 parts by weight of a varnish consisting of polyvinyl butyral resin (S-LEC (registered trademark) manufactured by Sekisui Chemical Co., Ltd.) and γ-BL (manufactured by Mitsubishi Chemical Corporation) with a resin content of 25% by weight were kneaded with a painting knife to prepare conductive pastes with a flake copper powder content of 90% by weight and 80% by weight, respectively, after firing.

[0080] The prepared conductive paste was used to form a coating film with a line width of 4 mm and a film thickness of 50 μm on a glass substrate using a metal spatula.

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

[0082] The dried coating film was baked at 300°C for 60 minutes in a nitrogen atmosphere to prepare a baked film.

[0083] The resistance of the fired film was measured using a resistance meter (RM3545, manufactured by Hioki E.E. Corporation) at an electrode distance of 40 mm, and the film thickness and line width were measured using a surface roughness measuring instrument (SE500A, manufactured by Kosaka Laboratory Co., Ltd.), and the volume resistivity was calculated.

[0084] The results are shown in Table 1.

[0085] [Table 1]

[0086] Table 1 proves that a conductive paste containing the flaky copper powder of the present invention can achieve low volume resistivity and high conductivity even when the proportion of flaky copper powder after firing is 80% by weight.

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

[0088] It can be seen that the edges of the particles in the flake copper powder of the examples have a shape with multiple depressions toward the inside of the powder, while the edges of the particles in the flake copper powder of the comparative examples are rounded. [Industrial Applicability]

[0089] The flaky copper powder in the present invention has a low apparent density and is strongly oriented horizontally in the coating film. Therefore, a conductive paste in which the flaky copper powder is the conductive filler has many contact points between the conductive fillers in the coating film. Therefore, even if the ratio of the conductive filler is low, the conductive paste can be fired at a low temperature of around 300°C to form a conductive network and exhibit high conductivity. Therefore, the present 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 more and 35 μm or less, and the apparent density (AD) is 0.1 g / cm ３ or more and 1.8 g / cm ３ 1. A flaky copper powder comprising: The flaky copper powder has a hiding ratio of 90% or more, as calculated by the following formula (1) using the hiding ratio measurement method specified in JIS (K 5101-4:2004), of a dispersion paste prepared by dispersing 20% ​​by weight of the flaky 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. (Formula 1) (1 - ΔL / ΔL ０ ) × 100 ΔL = Difference in brightness L value between the black part and the white part of the hiding power test paper coated with the dispersion paste ΔL ０ = Difference in brightness L value between the black part and the white part of the opacity test paper

2. A conductive paste containing the flaky copper powder according to claim 1.

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

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