Mixed powder for conductive paste and conductive paste mixture containing the same

Abstract

This invention provides a mixed powder for conductive pastes that can achieve high conductivity even when copper powder is sintered at a low firing temperature without prior surface treatment. [Solution] A mixed powder for conductive paste, comprising 0.5% to 5% by mass of organic compound zinc salt powder having a melting point of 290°C or lower, and the remainder being copper powder with an average particle size D50 of 2 μm or less, wherein the zinc content in the copper powder is 0.05% by mass or less.

Description

Technical Field

[0001] The present invention relates to a mixed powder for a conductive paste and a mixture for a conductive paste containing the same.

Background Art

[0002] In addition to being able to be fired at 350°C or lower, which is lower than the general sintering temperature of copper, the copper-based conductive paste is required to have high conductivity. In recent years, with the development of power electronics, the opportunity to use copper-based conductive paste as a replacement for expensive silver has increased, and there is a demand for further lowering of the firing temperature and achieving high conductivity at a low firing temperature. In order to satisfy such requirements, Patent Document 1 discloses a method of applying an organic surface treatment agent to the surface of copper particles in advance as a method of lowering the firing temperature of a conductive paste. Further, Patent Document 2 discloses a thick film conductive paste composition containing a conductor, a polymer binder, and a metal salt of a higher fatty acid (particularly, zinc stearate).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, the copper powder disclosed in Patent Document 1 has a high powder production cost because the organic surface treatment agent is heated to a temperature above its melting point for surface treatment. Furthermore, Patent Document 2 deals with base metals, precious metals, and mixtures and alloys thereof as conductors, and does not specify the elements or proportions of unavoidable impurities. In particular, the examples in Patent Document 2 only disclose a thick-film conductive paste composition mainly composed of silver as the conductor, and the firing temperature is 850°C. In other words, there is no disclosure of a thick-film conductive paste composition containing copper powder and higher fatty acid metal salts. Therefore, the object of this disclosure is to provide a mixed powder for conductive paste that can be sintered at a low firing temperature and still achieve high conductivity. [Means for solving the problem]

[0005] The aforementioned technical problems can be solved by the present invention as follows. The conductive paste mixed powder of the present invention is a mixed powder consisting of 0.5 to 5% by mass of organic compound zinc salt powder having a melting point of 290°C or lower, and the remainder being copper powder with an average particle size D50 of 2 μm or less, characterized in that the zinc content in the copper powder is 0.05% by mass or less. The conductive paste mixed powder of the present invention contains a predetermined organic compound zinc salt, and therefore, even when sintered at a low firing temperature, the sintered body exhibits high conductivity. In particular, since the above properties can be obtained simply by mixing copper powder with a predetermined organic compound zinc salt, the manufacturing cost can be reduced and productivity can be increased.

[0006] In the conductive paste mixed powder of the present invention, it is preferable that 95% or more by mass of the organic compound zinc salt has a particle size of 45 μm or less. In this case, when printing a conductive paste containing this powder onto a substrate, printing defects such as streaking caused by the organic compound zinc salt particles are less likely to occur. In the conductive paste mixed powder of the present invention, it is preferable that the organic compound zinc salt is a fatty acid zinc. The conductive paste mixture of the present invention is characterized by containing the conductive paste mixed powder of the present invention, a solvent, and a resin binder. [Effects of the Invention]

[0007] The conductive paste mixed powder of the present invention exhibits high conductivity even when sintered at low firing temperatures. Furthermore, the conductive paste mixed powder of the present invention can be manufactured at low costs and has high productivity. Therefore, by mounting a conductive paste containing the conductive paste mixed powder of the present invention onto a component, the risk of damaging the component is reduced, and the quality of the mounted component is also high. In this invention, a low firing temperature (low-temperature firing) means 350°C or lower, and preferably 300°C or lower. [Modes for carrying out the invention]

[0008] The present invention will be described in detail below with reference to embodiments. The present invention is not limited to the specific embodiments listed below.

[0009] "Mixed powder for conductive paste" The conductive paste mixed powder of the present invention is a mixed powder consisting of 0.5 to 5% by mass of organic compound zinc salt powder having a melting point of 290°C or lower, and the remainder being copper powder with an average particle size D50 (μm) of 2 μm or less, wherein the zinc content in the copper powder is 0.05% by mass or less.

[0010] "Copper powder" The copper powder used in this invention is an aggregate of copper particles, or an aggregate of copper particles treated with an organic surface treatment agent. Copper particles can be manufactured by known methods such as chemical reduction, atomization, electrolysis, and pulverization. Copper particles inevitably contain impurities that are mixed in during the raw material production or manufacturing process and cannot be removed even if efforts are made to remove them. Examples of these unavoidable impurities include zinc (Zn), tin (Sn), nickel (Ni), chromium (Cr), phosphorus (P), silicon (Si), manganese (Mn), iron (Fe), cobalt (Co), silver (Ag), gold (Au), aluminum (Al), lead (Pb), bismuth (Bi), indium (In), tellurium (Te), titanium (Ti), and boron (B). Preferably, the total amount of unavoidable impurities is 0.5% by mass or less. Examples of organic surface treatment agents include benzotriazoles, benzothiazoles, and alkanolamines. For example, commercially available organic rust inhibitors can be used.

[0011] The zinc content in the copper powder is 0.05% by mass or less. This zinc in the copper powder includes zinc, which is an unavoidable impurity of copper particles, and preferably an unavoidable impurity of copper particles. Because the conductive paste mixed powder of the present invention contains an organic compound zinc salt, the diffusion of zinc in the copper powder caused by the zinc concentration gradient during firing acts as a driving force to form metallurgical paths between the copper powder particles. Therefore, if the amount of zinc (Zn) as an unavoidable impurity in the copper powder is greater than 0.05% by mass, the driving force due to the concentration gradient decreases, and low-temperature sinterability cannot be achieved.

[0012] The average particle size D50 of the copper powder is preferably 2 μm or less, and more preferably 1 μm or less. For low-temperature firing, it is preferable that the particle size of the copper powder be fine. If the average particle size D50 of the copper powder is larger than 2 μm, sintering at 350°C or below may be insufficient, and the paste coating after firing may not be electrically conductive. The lower limit of the average particle size D50 of the copper powder is 0.1 μm or more. Here, the average particle size D50 refers to the volume-based median diameter determined by laser diffraction particle size distribution measurement.

[0013] "Organic compound zinc salt" The organic zinc salt undergoes thermal decomposition during firing, and the diffusion of zinc into the copper powder acts as a driving force, contributing to the bonding of the copper powder particles and resulting in low-temperature sinterability of the copper powder. Therefore, the melting point of the organic zinc salt is 290°C or lower, preferably 200°C or lower, and particularly preferably 150°C or lower. If the melting point is higher than 290°C, it may be difficult to obtain low-temperature sinterability due to zinc diffusion. Furthermore, the melting point of the organic zinc salt is preferably lower than the firing temperature. In addition, the organic zinc salt needs to be solid when mixed with copper powder in order to facilitate overall particle size control of the conductive paste mixture, and its melting point is preferably higher than room temperature. Examples of such zinc salts of organic compounds include zinc fatty acids such as zinc laurate, zinc stearate, and zinc oleate.

[0014] The particle size of the zinc salt of the organic compound is preferably such that 95% by mass or more is 45 μm or less, more preferably 97% by mass or more is 45 μm or less. When the amount of the zinc salt of the organic compound having a particle size of 45 μm or less is less than 95% by mass, printing defects such as streaks are likely to occur when making a conductive paste.

[0015] The content of the zinc salt powder of the organic compound is 0.5% to 5% by mass based on the total of the mixed powder. The preferable lower limit of the content of the zinc salt powder of the organic compound is 1% by mass, particularly 1.5% by mass. On the other hand, the preferable upper limit of the zinc salt powder of the organic compound is 3% by mass or less, particularly 2% by mass or less. When it is more than 5% by mass, the conductivity of the copper powder is likely to be reduced by zinc, and sintering may be inhibited by the residue of the organic compound. On the other hand, when it is less than 0.5% by mass, low-temperature sinterability cannot be obtained.

Examples

[0016] Next, examples and comparative examples of the present invention will be shown, but the present invention is not limited thereto.

[0017] <Copper powder> As the copper powder, the following copper powders A to E were prepared. "Copper powder A": Copper powder having an average particle size D50 of 0.5 μm produced by a chemical reduction method (Zinc content is 0.001% by mass) (Manufactured by Fukuda Metal Foil & Powder Industry Co., Ltd. EFC-20) (Examples 3, 4, 6, 8, 9, Comparative Examples 1, 5) "Copper powder B": Copper powder having an average particle size D50 of 1 μm produced by a chemical reduction method (Zinc content is 0.002% by mass) (Manufactured by Fukuda Metal Foil & Powder Industry Co., Ltd. EFC-09) (Examples 1, 5, Comparative Examples 2, 7, 8) "Copper Powder C": Copper powder with an average particle size D50 of 1.5 μm, manufactured by the atomization method. (Zinc content is 0.04% by mass) (Manufactured by Fukuda Metal Foil & Powder Industry Co., Ltd. - Cu-HWQ 1.5μm) (Examples 2, 7, Comparative Example 3) "Copper Powder D": Copper powder with an average particle size D50 of 3.0 μm, manufactured by the atomization method. (Zinc content is 0.005% by mass) (Manufactured by Fukuda Metal Foil & Powder Industry Co., Ltd. - Cu-HWQ 3μm) (Comparative Example 4) "Copper Powder E": Copper powder with an average particle size D50 of 1.5 μm, manufactured by the atomization method, with zinc added as an impurity during the manufacturing process. (Zinc content is 0.065% by mass) (Comparative Example 6)

[0018] The 50% particle size (average particle size D50) of the manufactured copper powder was measured using a laser diffraction particle size distribution analyzer SALD-2300 (manufactured by Shimadzu Corporation), and the zinc content was quantified using an ICP emission spectrometer iCAP7600 (manufactured by Thermo Fisher Scientific K.K.).

[0019] <Organic compound zinc salt powder> The following compositions a to e were prepared as organic zinc salt powders. "a": Zinc laurate (melting point 130℃) (Powder Base L manufactured by NOF Corporation) (Examples 1 and 2) "b": Zinc stearate (melting point 124℃) (Zinc stearate manufactured by NOF Corporation) (Examples 3-7, 9, Comparative Examples 4-6) "c": a 50% by mass + b 50% by mass (melting point <130℃) (Example 8) "d": Zinc benzoate (melting point 297℃) (Zinc benzoate, Grade 1, manufactured by Kanto Chemical Co., Ltd.) (Comparative Example 7) "e": Calcium stearate (melting point 165℃) (Calcium stearate manufactured by NOF Corporation) (Comparative Example 8) For each of the above a-e, the percentage of particles with a diameter of 45 μm or less was measured using a sieve-type particle size distribution analyzer. In Example 8, Example 9, and Comparative Example 7, the percentage of particles with a diameter of 45 μm or less was reduced by granulation, and then measured.

[0020] <Mixed> Copper powder and organic zinc salt compounds were weighed out so that the mixing ratio matched the composition shown in Table 1, and these were mixed in a rocking mixer cooled to below 40°C to prepare a mixed powder for conductive paste.

[0021] <Preparation of conductive paste> A varnish was prepared by dissolving gamma-butyrolactone (GBL, manufactured by Mitsubishi Chemical Corporation) in polyvinyl acetal resin (S-Rec BL-S, manufactured by Sekisui Chemical Co., Ltd.) in a mass ratio of 3:1. 30% by mass of this varnish was then mixed with 70% by mass of a mixed powder for conductive paste.

[0022] <Rating> A glass plate was masked to 75 mm x 4 mm, the conductive paste was printed on it, and it was fired for 1 hour under a nitrogen atmosphere at the temperature shown in Table 1. For the coating film after firing, the electrical resistance at a terminal distance of 40 mm was measured using a resistance meter RM3545 (manufactured by HIOKI E.E. CORPORATION), and the thickness of the coating film was measured using a surface roughness measuring instrument SE500A (manufactured by Kosaka Research Institute Co., Ltd.), and the electrical resistivity was calculated. Compared to the comparative examples corresponding to the examples (the correspondence is shown in Table 1), we evaluated the results as follows: ○ if the electrical resistivity improved and no printing defects such as scribing occurred, △ if the electrical resistivity improved but printing defects occurred, and × if the electrical resistivity did not improve. The results for each example and comparative example are shown in Table 1.

[0023] [Table 1]

[0024] The conductive pastes using the conductive paste mixed powders of Examples 1 to 8 were found to have lower electrical resistivity compared to the conductive pastes of the comparative subjects (Comparative Examples 1 to 3) at the same low-temperature firing temperature of 350°C or below. In Example 9, however, streaking occurred during printing of the conductive paste, and these streaks increased the electrical resistance. Furthermore, the conductive pastes using the conductive paste mixed powders of Examples 1 to 8 all exhibited good print quality. In Comparative Example 4 (copper powder D), the 50% particle size of the copper powder was larger than 2 μm, so conductivity could not be confirmed even after adding the organic compound zinc salt powder, and therefore the electrical resistivity could not be measured. In Comparative Example 5, the amount of organic compound zinc salt powder added was 7% by mass, which is greater than 5% by mass. As a result, conductivity could not be confirmed, and the electrical resistivity could not be measured. In Comparative Example 6 (copper powder E), the zinc content in the copper powder was 0.065% by mass, which is higher than 0.05% by mass. Therefore, even after adding the organic compound zinc salt powder, conductivity could not be confirmed, and the electrical resistivity could not be measured. In Comparative Example 7, the melting point of zinc benzoate, which is the organic zinc compound salt (d), was 297°C, which is higher than 290°C. Therefore, even after adding the organic zinc compound salt powder, conductivity could not be confirmed, and the electrical resistivity could not be measured. In Comparative Example 8, calcium stearate(e) was used instead of the organic zinc compound salt, but the electrical resistivity did not improve even with the same low-temperature firing at 250°C as in Comparative Example 2.

Claims

1. 0.5% to 5% by mass of powder of an organic compound zinc salt having a melting point of 290°C or lower, The remainder is a mixed powder consisting of copper powder with an average particle size D50 of 2 μm or less. The zinc content in the copper powder is 0.05% by mass or less. Mixed powder for conductive paste.

2. The aforementioned organic compound zinc salt has a particle size of 45 μm or less, with 95% by mass or more being the same. The mixed powder for conductive paste according to claim 1.

3. The aforementioned organic compound zinc salt is fatty acid zinc. The mixed powder for conductive paste according to claim 1.

4. A mixture for conductive paste comprising the conductive paste mixed powder according to any one of claims 1 to 3, a solvent, and a resin binder.

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