Copper paste

Abstract

To provide a copper paste that shows excellent oxidation resistance, has excellent electrical conductivity, thermal conductivity, and storage stability, and expresses high joint strength.SOLUTION: Provided is a copper paste containing a copper powder and an organic solvent, wherein the organic solvent is an alcohol-based solvent containing one or more first alcohols selected from the group consisting of monohydric and dihydric alcohols having a viscosity at 20°C of 3-70 mPa s, and one or more second alcohols selected from the group consisting of dihydric and trihydric alcohols having a viscosity at 20°C of 300-1000 mPa s.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a novel copper paste. [Background technology]

[0002] Conductive pastes are used to form wiring in electronic components such as chip resistors, chip capacitors, and solar cells, as well as in electronic packaging devices such as printed wiring boards and substrates with through-holes. They can also be used for electrodes and wiring connected to transistors that control pixel switching in displays, and for power modules that operate electric motors and other devices with high power efficiency. In power modules, semiconductor chips made of silicon, silicon carbide, gallium nitride, or other materials are bonded to heat-dissipating substrates. However, solder, which has low thermal conductivity, makes it difficult to dissipate heat generated by the power module, preventing efficient operation at high temperatures. Therefore, the use of conductive pastes is particularly important in power modules.

[0003] Most current conductive pastes use silver paste, which has excellent oxidation resistance. However, silver is expensive and prone to migration defects in fine-pitch wiring. Therefore, in recent years, attempts have been made to use copper paste, which is low-cost and has good thermal conductivity. However, while copper is cheaper than silver, it is prone to oxidation. Therefore, copper paste must be fired using reducing gases such as hydrogen or formic acid or inert gases such as nitrogen, otherwise the copper oxidizes and its dispersion stability and conductivity decrease. To ensure paste printability and wiring density, copper powder containing fine particles is required, but oxidation is particularly problematic with such copper powder. Simplifying the manufacturing process and reducing costs requires firing at relatively low temperatures and short times, as well as improving bonding strength. To address these issues, various technologies are being investigated to prevent copper powder oxidation and improve its dispersibility.

[0004] For example, Patent Document 1 discloses copper nanoparticles coated with collagen peptides, and reports that these copper nanoparticles have excellent oxidation resistance and dispersion stability. Patent Document 2 describes a technique for improving the dispersibility of metal microparticles by using a mixture of an amide organic solvent, an amine organic solvent, and an alcohol as the solvent component of the paste. Patent Document 3 discloses a copper paste containing copper powder with an average particle size of 0.1 to 1 μm and an alcohol solvent. Furthermore, Non-Patent Document 1 describes the formation of a low-resistance sintered body by coating the surfaces of fine copper particles with a gelatin layer to prepare a paste, and then performing oxidative firing in air and reductive firing in N2 and 3% H2 gas. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5450725 [Patent Document 2] Patent No. 6097477 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-53216 [Non-patent literature]

[0006] [Non-Patent Document 1] Yonezawa et al., RSC Advances, 2015, 5, 61290-61297(2015) Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the copper pastes described in Patent Document 1 and Non-Patent Document 1, the copper particles may not be sufficiently sintered due to the coating of collagen peptides or gelatin on the surfaces of the copper particles. As a result, the bonds between the copper particles after sintering are weak, resulting in a problem of not being able to maintain the mechanical strength of the wiring. As mentioned above, in recent years, conductive pastes have been required to be sintered at low temperatures and in short periods of time, and to have high bonding strength. For example, they are required to be fired at 300°C or less for 10 minutes or less, and to achieve bonding strength of approximately 40 MPa or more. However, the copper pastes described in Patent Document 1 and Non-Patent Document 1 are difficult to meet these requirements. In the copper paste described in Patent Document 2, the main solvent, an amide-based organic solvent, may remain in the wiring after firing, reducing conductivity. In addition, the odor caused by the amine-based organic solvent may worsen the working environment. The paste described in Patent Document 3 cannot sufficiently prevent oxidation of the copper powder during firing, and may also cause problems such as viscosity changes during storage due to the evaporation of low-boiling-point solvents and residual high-boiling-point solvents.

[0008] In view of the above problems, an object of the present invention is to provide a copper paste that exhibits good oxidation resistance, has high electrical conductivity and thermal conductivity, and is excellent in storage stability and workability, and in particular, a copper paste that can be fired at a low temperature in a short time and exhibits high bonding strength. [Means for solving the problem]

[0009] As a result of extensive research, the present inventors have discovered that by using two or more different types of alcohols as dispersion media in a copper paste, oxidation of copper powder during firing can be suppressed, viscosity does not change during storage, and a copper paste with excellent workability can be obtained, leading to the completion of the present invention. That is, the present invention relates to the following copper pastes (1) to (7).

[0010] (1) A copper paste containing copper powder and an organic solvent, The organic solvent is one or more first alcohols selected from the group consisting of monohydric and dihydric alcohols having a viscosity at 20°C of 3 mPa·s or more and 70 mPa·s or less; one or more second alcohols selected from the group consisting of dihydric and trihydric alcohols having a viscosity at 20°C of 300 mPa·s or more and 1000 mPa·s or less; Copper paste, which is an alcohol-based solvent containing

[0011] (2) In alcohol-based solvents, the boiling point of the first alcohol at atmospheric pressure is 150°C or higher and 240°C or lower; and the boiling point of the second alcohol at atmospheric pressure is 190°C or higher and 320°C or lower; and The boiling point of the first alcohol is lower than the boiling point of the second alcohol. The copper paste according to (1) above.

[0012] (3) The copper paste according to any one of (1) to (2) above, wherein the ratio (X / Y) of the mass of the first alcohol (X) to the mass of the second alcohol (Y) in the organic solvent is 0.2 or more and 8.0 or less.

[0013] (4) the first alcohol is one or more alcohols selected from the group consisting of 1-hexanol, 1-heptanol, 2-heptanol, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, benzyl alcohol, hexylene glycol, 1,2-propanediol, 1,3-propanediol, and ethylene glycol; and The copper paste according to any one of (1) to (3) above, wherein the second alcohol is one or more alcohols selected from the group consisting of 2-ethyl-1,3-hexanediol and glycerol.

[0014] (5) The copper paste according to any one of (1) to (4) above, which is substantially free of resin components.

[0015] (6) The copper paste according to any one of (1) to (5) above, wherein the copper powder has a coating layer made of at least one compound selected from polysaccharides and fatty acids on at least a part of the surface thereof.

[0016] (7) The copper paste according to (6), wherein the copper powder having a coating layer contains 0.05% by mass or more and 0.8% by mass or less of carbon and 0.05% by mass or more and 1.5% by mass or less of oxygen, relative to 100% by mass of the copper powder. [Effects of the Invention]

[0017] According to the present invention, there is provided a copper paste that exhibits good oxidation resistance, has high electrical and thermal conductivity, and is excellent in storage stability and workability. The copper paste of the present invention can also be fired at a low temperature in a short time, and is capable of exhibiting high bonding strength. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the description of the embodiments.

[0019] The copper paste of the present invention is a copper paste containing copper powder and an organic solvent, The organic solvent is one or more first alcohols selected from the group consisting of monohydric and dihydric alcohols having a viscosity at 20°C of 3 mPa·s or more and 70 mPa·s or less; one or more second alcohols selected from the group consisting of dihydric and trihydric alcohols having a viscosity at 20°C of 300 mPa·s or more and 1000 mPa·s or less; It is a copper paste that is an alcohol-based solvent containing

[0020] Here, an important requirement of the present invention is the combined use of two or more different types of alcohol as the organic solvent of the dispersion medium. This suppresses oxidation of copper powder during firing, and provides a copper paste with excellent workability, with viscosity that does not change during storage. In this specification, the term "viscosity" refers to the viscosity at any shear rate, since the organic solvent is a Newtonian viscous body and the viscosity is independent of the shear rate. On the other hand, copper paste is a non-Newtonian viscous body, so it is measured using a cone-plate type dynamic viscoelasticity device (e.g., RST Cone-Plate Rheometer, manufactured by Brookfield), and the viscosity is measured at a shear rate of 1 s -1 Hereinafter, each component of the copper paste of the present invention will be described in detail.

[0021] (alcohol-based solvent) The organic solvent in the copper paste of the present invention is an alcohol-based solvent that combines two or more monohydric to trihydric alcohols with different viscosities, as described above. When a tetrahydric or higher polyhydric alcohol is used as the solvent, it may remain in the sintered body, reducing electrical conductivity and bonding strength, especially when firing is performed at low temperatures below approximately 300°C in a reducing or nitrogen atmosphere. Using only a monohydric alcohol as the solvent can easily volatilize the copper paste during storage or printing, causing changes in the viscosity of the copper paste and worsening workability. The present invention avoids these problems and, by using an alcohol with the above-described viscosity, provides a copper paste with uniformly dispersed copper powder and excellent physical properties and workability. In particular, the inclusion of a second alcohol with a high viscosity suppresses deformation from the desired shape due to sagging of the paste after application, and, as described below, allows the viscosity of the copper paste to be adjusted to an appropriate value without the need for a binder component such as a resin. A resin-free copper paste eliminates the need to consider the generation of carbon residue derived from the resin component, and firing can be performed in a non-oxidizing atmosphere at a relatively low temperature.

[0022] In the present invention, the term "alcohol-based solvent" refers to a mixed solvent primarily composed of alcohol. It may also include mixed solvents containing small amounts of water or organic solvents other than alcohol, such as ethers, ketones, esters, etc., in an amount of about 1 to 20% by mass, particularly about 5 to 10% by mass. Other solvents may also be included, such as hydrocarbon solvents and halogenated hydrocarbon solvents. However, because nitrogen-containing solvents such as amines and amides tend to remain in the dried product, it is preferable to contain no such solvents or, if present, to limit their content to about 5% by mass or less. The total mass of the first and second alcohols is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more, of the total solvent in the copper paste. Because alcohols, particularly trihydric alcohols, have a reducing effect, increasing their content in the copper paste solvent can more effectively suppress the oxidation of copper powder.

[0023] The first and second alcohols in the present invention may be any alcohols having the above-mentioned hydricity and viscosity, but alcohols with a boiling point of 150°C or higher are preferred. In this specification, boiling point refers to the boiling point at atmospheric pressure unless otherwise specified. If the boiling point of the alcohol is below 150°C, bumping may occur during heating, creating voids in the paste and deteriorating sinterability. On the other hand, if an alcohol with a boiling point of 150°C or higher is used, the copper paste can be fired without these problems, and the electrical and thermal conductivity of the sintered body can be improved. Furthermore, if the boiling point of the alcohol is 150°C or higher, the solvent will not volatilize and the viscosity will not change within a short period of time, even if the copper paste is stored at room temperature. This eliminates the need for refrigeration or freezing, thereby reducing storage costs.

[0024] To further enhance the antioxidant effect of the copper powder of the present invention, it is preferable to select a second alcohol with a higher boiling point than the first alcohol. The inclusion of a low-viscosity first alcohol provides the copper paste of the present invention with an appropriate viscosity and good workability. However, viscosity adjustment is not necessary after paste application, and from the viewpoint of preventing sagging of the copper paste, it is preferable that the first alcohol is removed. On the other hand, among alcohols, dihydric and trihydric alcohols, especially trihydric alcohols, have a high reducing effect, so it is preferable that the second alcohol containing these alcohols be present at a high concentration during firing. Therefore, by using a second alcohol with a higher boiling point than the first alcohol and evaporating near the firing temperature of the copper paste, good workability can be maintained and oxidation of the copper powder can be more effectively suppressed. In a more preferred embodiment of the present invention, the boiling point of these first alcohols is preferably 150°C or higher and 250°C or lower, particularly 240°C or lower, and the boiling point of the second alcohol is preferably 190°C or higher and 320°C or lower.

[0025] In the copper paste of the present invention, the content of the first alcohol and the second alcohol is not particularly limited. However, it is preferable that the ratio (X / Y) of the mass of the first alcohol (X) to the mass of the second alcohol (Y) in the organic solvent be 0.2 or more and 8.0 or less, particularly 0.5 or more and 5.0 or less. To ensure sufficient bonding strength of the copper paste, such as the die shear strength between the chip and the substrate, a copper paste layer must be printed with a substantially uniform thickness at the interface between the chip and the substrate. If the ratio is 0.2 or more, appropriate viscosity is easily achieved, resulting in sufficient bonding strength. If the ratio is 8.0 or less, the reduction effect is fully expressed, resulting in particularly good sinterability, and high electrical conductivity and bonding strength are obtained. The content of the organic solvent (alcohol-based solvent) is also not particularly limited and can be set as desired depending on the viscosity of the copper paste. However, a content of 5% by mass or more and 40% by mass or less, particularly 8% by mass or more and 20% by mass or less, based on 100% by mass of the total copper paste, is preferred for achieving a typical copper paste viscosity. If the solvent concentration is about 5% by mass or more, the copper paste can be spread to the interface residue with a uniform layer thickness, resulting in good bonding strength.Also, if the solvent concentration is about 40% by mass or less, no solvent remains during firing, preventing a decrease in electrical conductivity or bonding strength.

[0026] (First Alcohol) The first alcohol in the present invention is one or more alcohols selected from the group consisting of monohydric and dihydric alcohols having a viscosity of 3 mPa·s or more and 70 mPa·s or less, as described above. If the viscosity of the first alcohol is within this range, the application of the copper paste becomes easy and good workability is ensured. Furthermore, the boiling point of the first alcohol is preferably 150°C or more. More preferably, an alcohol with a boiling point that is more than 50°C lower than the firing temperature of the copper paste is used. While the firing temperature of copper paste is not particularly limited, it is generally around 250 to 300°C for general joining applications. Therefore, the boiling point of the first alcohol in the present invention is preferably 150°C or more and 250°C or less, more preferably 150°C or more and 240°C or less, particularly preferably 150°C or more and 230°C or less, and particularly preferably 170°C or more and 200°C or less. Furthermore, when the vapor pressure at around room temperature, for example 20°C, is 0.1 Pa or more and 100 Pa or less, further 1 Pa or more and 50 Pa or less, particularly 3 Pa or more and 30 Pa or less, storage stability and workability are improved, which is preferable. Specific examples of such first alcohols include 1-hexanol (viscosity 4.58 mPa·s, boiling point 158°C, vapor pressure 80 Pa), 1-heptanol (viscosity 5.81 mPa·s, boiling point 176°C, vapor pressure 44 Pa), 2-heptanol (viscosity 3.96 mPa·s, boiling point 159°C, vapor pressure 78 Pa), 1-octanol (viscosity 7.29 mPa·s, boiling point 195°C, vapor pressure 24 Pa), 2-octanol (viscosity 6.49 mPa·s, boiling point 180°C, vapor pressure 42 Pa), 2-ethyl-1-hexanol (viscosity 6.27 mPa·s, boiling point 185°C, vapor pressure 35 Pa), benzyl alcohol (viscosity Examples of suitable alcohols include, but are not limited to, monohydric alcohols such as ethylene glycol (viscosity 16.1 mPa·s, boiling point 197°C, vapor pressure 20 Pa), 1,2-propanediol (viscosity 40.4 mPa·s, boiling point 188°C, vapor pressure 28 Pa), 1,3-propanediol (viscosity 47 mPa·s, boiling point 214°C, vapor pressure 5 Pa), 2,3-butanediol (viscosity 45 mPa·s, boiling point 182°C, vapor pressure <100 Pa), and hexylene glycol (viscosity 34.4 mPa·s, boiling point 197°C, vapor pressure 19 Pa).A mixture of two or more of these alcohols may also be used. The above viscosity and vapor pressure values ​​are all measured at 20°C or 25°C. In the present invention, it is particularly preferable to use 1-octanol, 2-octanol, 2-ethyl-1-hexanol, ethylene glycol, 1,2-propanediol, or hexylene glycol as the first alcohol. As described above, these first alcohols have low viscosity, so the viscosity of the copper paste can be adjusted to an appropriate value by adding a smaller amount. This reduces the total amount of organic solvent in the copper paste, making it possible to suppress the residual organic solvent components during firing.

[0027] (Second Alcohol) The second alcohol in the present invention is one or more alcohols selected from the group consisting of dihydric and trihydric alcohols having a viscosity of 300 mPa·s or more and 1000 mPa·s or less, as described above. If the viscosity of the second alcohol is within this range, it is possible to prevent the copper paste from sagging before sintering, making it impossible to form the desired shape, and it also does not impair the workability of the copper paste. Furthermore, the boiling point of the second alcohol is preferably 190°C or higher, and more preferably, a temperature equal to or higher than the firing temperature of the copper paste minus 50°C. Considering the firing temperature of typical copper pastes, a temperature of 200°C or higher, particularly 240°C or higher, is preferred. There is no particular upper limit to the boiling point, but considering workability and the heat resistance temperature of the substrate or other object to be coated, a temperature of 320°C or lower, or even 300°C or lower, is preferred. Alcohols with a boiling point within this range do not remain in the gaps between copper particles in the sintered body even after low-temperature firing, thereby preventing a decrease in electrical conductivity. Furthermore, a vapor pressure of 1 mPa or more and 5 Pa or less, preferably 1.5 Pa or less, and particularly 1 Pa or less at room temperature (e.g., 20°C) is preferable because it improves storage stability and further enhances the oxidation suppression effect during baking. This effect is particularly pronounced when the vapor pressure of the second alcohol is lower than that of the first alcohol. Specific examples of such second alcohols include, but are not limited to, dihydric alcohols such as 2-ethyl-1,3-hexanediol (viscosity 323 mPa·s, boiling point 244°C, vapor pressure <1.4 Pa) and trihydric alcohols such as glycerol (viscosity 934 mPa·s, boiling point 290°C, vapor pressure 0.01 Pa). Mixtures of these alcohols are also acceptable.

[0028] (copper powder) The copper powder contained in the copper paste of the present invention is not particularly limited, and any of various commercially available products may be used. However, in the present invention, it is preferable that the total content of elements other than copper in the copper powder is 1 mass% or less relative to 100 mass% of the copper powder. Components other than copper, particularly metal elements, may segregate on the surface of the copper powder or form oxides, thereby impairing sinterability, and may also dissolve in the copper powder, reducing the electrical conductivity of the sintered body. If the content of elements other than copper, particularly elements such as As, Co, Cr, Fe, Ir, P, S, Sb, Se, Te, Ti, V, and Zr, is 1 mass% or less, the electrical resistivity of the copper paste sintered body can be approximately 5 μΩcm or less, and a thermal conductivity of approximately 130 W / m·K or more can be achieved. Such a thermal conductivity allows, for example, heat generated by a power module to be efficiently dissipated to the outside. It is even more preferable that the content of impurities, particularly the above elements, is 0.5 mass% or less, resulting in an electrical resistivity of approximately 4 μΩcm or less and a thermal conductivity of approximately 167 W / m·K or more.

[0029] Such copper powder can be produced using methods such as the high-pressure water atomization method described in International Publication No. 99 / 11407 and the wet reduction precipitation method described in International Publication No. 2014 / 80662. The high-pressure water atomization method is a method for producing metal powder (e.g., copper) from molten metal. The gas flows through the center of a nozzle through which the molten metal flows, fragmenting the molten metal near the nozzle outlet. The resulting fragments are then further fragmented by a liquid ejected in an inverted cone shape. This method allows for the continuous gas- and liquid-based fragmentation of the molten metal, resulting in the industrial, large-scale, and low-cost production of metal powder (e.g., copper) with fine particle size, spherical or granular shape, and low oxygen content. The wet reduction precipitation method involves the wet reduction of copper ions using a reducing agent such as hydrazine, using an organic solvent that is compatible with water and can reduce the surface tension of water as a solvent. Specifically, this method uses water and the organic solvent as a liquid medium, mixes a reaction solution containing monovalent or divalent copper ions with a reducing agent, and reduces the copper ions to produce copper particles. Generally, high-pressure water atomization can produce particles of 0.7 μm or larger. To produce finer particles smaller than this, wet reduction precipitation is suitable.

[0030] The copper powder (copper particles) preferably has an average particle size of 0.05 μm or more and 2.0 μm or less. By setting the average particle size of the copper particles to 2.0 μm or less, the surface area of ​​the copper particles increases relatively, and sintering at low temperatures tends to be easier. On the other hand, if the average particle size of the copper particles is less than 0.05 μm, the price of the copper particles used as raw material rises, and the copper wiring formed by the method of the present invention tends to become less of a low-cost substitute for silver wiring. Furthermore, if the average particle size is less than 0.05 μm, many particles tend to aggregate, resulting in a sintering property that is essentially equivalent to that of a paste made of coarse particles. Here, the average particle size refers to the 50% particle size (d 50 ) and is the median value in the particle diameter distribution measured using a laser particle size distribution analyzer or the like. More preferably, copper powder having an average particle diameter of 0.3 μm or more and 0.7 μm or less is used.

[0031] Copper powder (copper particles) may have a surface coating material to the extent that it does not affect sinterability. However, because the gelatin layer described in Non-Patent Document 1 tends to cause insufficient sintering, the copper powder in the present invention is preferably one that does not have a gelatin coating. In the present invention, the copper powder preferably has a coating layer made of a polysaccharide or fatty acid compound on at least a portion of its surface. When polysaccharide molecules coat the copper powder, the outer side (the side that comes into contact with the solvent) becomes hydrophilic, interacting with the hydroxyl groups of the organic solvent in the copper paste to provide appropriate viscosity. On the other hand, fatty acids have carboxyl groups that bond to the copper particle surface, and the opposite end of the fatty acid becomes hydrophobic, thereby improving the dispersibility of the copper particles and suppressing particle aggregation. As a result of these effects of polysaccharides and fatty acids, the copper paste can be distributed throughout the entire interface with a uniform layer thickness, resulting in good bonding strength. Examples of polysaccharides include, but are not limited to, gum arabic, carboxymethylcellulose, hydroxyethylcellulose, cellulose nanofiber, starch, glycogen, agarose (agar), pectin, alginic acid, and salts thereof, and may also include sulfur-containing polysaccharides such as carrageenan. Among these, gum arabic and sodium alginate are particularly preferred. Examples of fatty acids include medium-chain fatty acids such as pentanoic acid, octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, and tetradecanoic acid, with octanoic acid, nonanoic acid, and decanoic acid being particularly preferred.

[0032] While there are no particular limitations on the surface coverage ratio of these coating layers, it is preferable that the copper powder be coated at a ratio such that the carbon content is 0.05% to 0.8% by mass and the oxygen content is 0.05% to 1.5% by mass relative to 100% by mass of the copper powder containing the coating layer. If the carbon content or oxygen content is less than 0.05% by mass, the hydrophilicity of the polysaccharide molecules on the copper powder surface is not fully expressed, reducing the viscosity of the copper paste and making it difficult to form a uniform paste layer, which may result in reduced bonding strength. If the carbon content exceeds 0.8% by mass or the oxygen content exceeds 1.5% by mass, carbon- and oxygen-containing components may remain inside the sintered body, for example, during firing in a nitrogen atmosphere, potentially reducing electrical conductivity and bonding strength. More preferably, the carbon content is 0.1 to 0.5% by mass and the oxygen content is 0.1 to 1.0% by mass. The surface coverage ratio, carbon content, and oxygen content of the fatty acid are similar to those of polysaccharides.

[0033] (Preparation of copper paste) The copper paste of the present invention can be prepared by mixing the copper powder and solvent as described above and kneading them using a device such as a planetary mixer, if desired. It is also preferable to use a three-roll mill to enhance the dispersibility of the copper powder, if necessary. There is no particular restriction on the viscosity of the copper paste, and it can be set as desired depending on the intended application. For example, the viscosity of the copper paste can be adjusted to a value greater than 1000 kJ / s at a shear rate of 1 s. -1 By setting the viscosity of the copper paste at around room temperature, for example, 25°C, to 30 Pa·s or more and 2000 Pa·s or less, or 100 Pa·s or more and 1000 Pa·s or more, particularly 150 Pa·s or more and 800 Pa·s or less, the copper paste can be easily applied uniformly to a substrate or the like, and the thermal conductivity and bonding strength of the resulting copper sintered body can be further improved.

[0034] (Other ingredients) In addition to the above-mentioned components, the copper paste of the present invention may contain a dispersant or surfactant made of amines, an antioxidant, a reducing agent such as hydrazine, glass frit, a binder including a resin component, etc. As the resin component, for example, a cellulose-based resin such as methyl cellulose, ethyl cellulose, or carboxymethyl cellulose, an acrylic resin, a butyral resin, an alkyd resin, an epoxy resin, or a phenolic resin may be contained in an amount of about 0.05 to 5 mass % relative to the mass of the copper particles.

[0035] However, the copper paste of the present invention preferably contains substantially no resin component. If the copper paste contains a resin component, sinterability, particularly at temperatures below 350°C, may be impaired. For example, thermosetting resins such as epoxy resins tend to remain in the copper paste sintered body even after sintering. Even when cellulose resins are used, thermal decomposition begins at around 300°C, but complete thermal decomposition requires temperatures above 400°C. Furthermore, firing in an oxygen atmosphere is required to remove the carbon residue produced by thermal decomposition, which can lead to the oxidation of the copper powder. Resin-free copper pastes can be fired at relatively low temperatures in a non-oxidizing atmosphere, resulting in the formation of high-density copper sintered bodies without the risk of copper powder oxidation causing a decrease in conductivity. The copper paste of the present invention contains a second alcohol with high viscosity, making it possible to adjust the viscosity to an appropriate value even without a resin component.

[0036] (Copper paste applications) As described above, the copper paste of the present invention has high electrical and thermal conductivity and is also excellent in terms of storage stability and workability. The copper paste of the present invention can also be fired at low temperatures and in short periods of time, enabling it to exhibit high bonding strength. Therefore, it is suitable for use in forming wiring in electronic components such as power modules, chip resistors, chip capacitors, and solar cells, as well as in electronic packaging devices such as printed wiring boards and substrates with through-holes. For example, the copper paste of the present invention can be applied and fired to power modules, solar cell substrates, substrates for mounting electronic packaging devices, printed wiring boards, and substrates with through-holes. Examples of substrate materials that can be used include silicon substrates, oxide substrates such as silicate glass, alumina, and quartz, nitride substrates such as silicon nitride and aluminum nitride, carbide substrates such as silicon carbide and titanium carbide, resin substrates such as polyimide, polyethylene terephthalate, and polyethylene naphthalate, and even substrates having a transparent conductive film (TCO) or a metal film on their surfaces.

[0037] (Firing of copper paste) When firing the copper paste of the present invention, there are no particular limitations on the method or conditions, and any method can be used depending on the desired product and the material to which the paste is applied. However, it is preferable to dry and remove the first alcohol prior to firing the copper paste of the present invention. This increases the proportion of the second alcohol around the copper powder during firing, thereby more effectively preventing oxidation of the copper powder during firing. There are no particular limitations on the drying conditions, which can be set arbitrarily depending on the boiling point of the first alcohol used and the desired product, but heating in an air atmosphere at a temperature of 50 to 200°C, particularly 60 to 150°C, for 1 to 60 minutes is preferred. Drying can also be performed under reduced pressure to further reduce the heating temperature. Heat drying can also be performed in a reducing atmosphere.

[0038] The copper paste of the present invention can be fired at low temperatures and in short periods of time, allowing for a variety of firing conditions. For example, firing can be performed in an inert gas atmosphere such as nitrogen or argon gas, or in a reducing atmosphere containing approximately 0.1 to 30% by volume of hydrogen, ammonia, carbon monoxide, alcohol, or the like, at a temperature of 150 to 400°C, or 200 to 350°C, particularly 250 to 300°C, for 10 seconds to 60 minutes, particularly 2 minutes to 30 minutes, to produce a high-strength sintered body with excellent electrical and thermal conductivity. [Example]

[0039] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0040] Example 1 A copper paste was prepared by mixing copper powder coated with the polysaccharide gum arabic, with a 50% particle size of approximately 0.4 μm, a carbon content of 0.3% by mass, an oxygen content of 0.7% by mass, and metal elements other than copper of 0.2% by mass, ethylene glycol as the first alcohol, and glycerol as the second alcohol in a mass ratio of 87.0:6.5:6.5. Here, the mass ratio refers to the mass of each component when the weight of the copper paste is taken as 100. The viscosity (η0) of the prepared paste was measured within 2 hours after preparation using a dynamic viscoelastic meter (Brookfield Rheometer). The viscosity (η7) was then measured after 7 days of storage at 10°C in the atmosphere, and the rate of change in viscosity over time was calculated. The rate of change in viscosity (%) was defined as (η7 - η0) / η0 x 100.

[0041] This paste was applied to a glass substrate using a metal mask so as to form a 20 mm square, and after drying in air at 60°C for 5 minutes, it was pressurized and fired in a nitrogen atmosphere at 280°C for 2 minutes under a load of 20 MPa using a high-temperature press to produce a copper paste sintered body with a thickness of approximately 20 μm.The electrical resistivity of this sintered body was measured using a DC four-probe electrical resistance measurement device with the probe spacing set to 1 mm. In addition, a 1 mm thick copper plate was used instead of the glass substrate, and copper paste was applied to a thickness of 100 μm. A silicon carbide (SiC) semiconductor chip measuring 2 mm × 2 mm × 0.4 mm was then placed on top of the copper paste. A 500 nm Ti layer and a 500 nm Cu layer were deposited on the surface of the SiC chip that contacted the copper paste using a sputtering method. The resulting laminate was subjected to a 20 MPa load using a high-temperature press in a nitrogen atmosphere, and pressure firing was performed at 280°C for 2 minutes. After cooling the sample to room temperature, the die shear strength between the SiC chip and the copper substrate was measured using a die shear tester (Nordson DAGE4000). Regarding the above measurement results, a viscosity change rate of 10% or less, an electrical resistivity of 5 μΩcm or less, and a die shear strength of 40 MPa or more were all evaluated as passing, with grade A being given if all items passed, grade B being given if two items passed, and grade C being given if one item passed or all items failed. Furthermore, a grade A result and a die shear strength of 70 MPa or more were evaluated as passing, with grade AA being given. The measurement results and the determination results are shown in Table 1 below, together with the type of the first alcohol.

[0042] (Examples 2 to 7, Comparative Examples 1 to 3) Except for changing the type of the first alcohol, copper pastes were prepared and experiments were carried out in the same manner as in Example 1. Table 1 shows the results.

[0043] [Table 1]

[0044] In Examples 1 to 7, in which a first alcohol having a viscosity in the range of 3 mPa·s or more and 70 mPa·s or less was used in combination with a second alcohol according to the present invention, favorable results were obtained, with the results being rated AA, A, or B according to the above criteria. Furthermore, in Examples 1, 3 to 5, and 7, in which a first alcohol having a boiling point of 150°C or more and 240°C or less and a vapor pressure at 20°C of 3 Pa or more and 30 Pa or less was used, the results were even more favorable, with the results being rated AA or A.

[0045] Example 8 In the experiment shown in Table 1, the same experiment was performed except that the surface coating layer of the copper powder was changed from the polysaccharide gum arabic to decanoic acid, a fatty acid.Compared to the case of gum arabic-coated copper powder, the viscosity of the copper paste within 2 hours after preparation was approximately 8 to 25% lower and the applied thickness of the copper paste was approximately 60 to 85 μm.Other than this, the same results as those judged in Table 1 were obtained. This indicates that the surface coating layer of the copper powder is not limited to polysaccharides, but may also be a medium-chain fatty acid.

[0046] (Example 9, Comparative Examples 4 and 5) A copper paste was prepared under the same conditions as in Example 1, except that the first alcohol was changed to ethylene glycol and the type of the second alcohol was changed. The results are shown in Table 2.

[0047] [Table 2]

[0048] In accordance with the present invention, Examples 1 and 9, in which a dihydric or trihydric alcohol with a viscosity of 300 mPa·s or more and 1000 mPa or less was used as the second alcohol, obtained favorable results, rated as AA or A. On the other hand, Comparative Examples 4 and 5, in which an alcohol with a viscosity outside the above range was used, obtained a rating of C. In Examples 1 and 9, the boiling point of the second alcohol was 240°C or more, which is thought to have contributed to the more favorable results. Furthermore, Example 1, in which glycerol with a vapor pressure of 1 Pa or less at 20°C was used, obtained a particularly favorable result, rated as AA.

[0049] (Examples 10 to 13, Comparative Examples 6 and 7) Copper pastes were prepared and experiments were conducted under the same conditions as in Example 1 above, except that the first alcohol was ethylene glycol, the second alcohol was glycerol, and the mass ratio (X / Y) of each was changed. The results are shown in Table 3. The pass criteria in Table 3 are the same as in Tables 1 and 2. However, since there were two evaluation items this time, if both of them passed, it was judged as Grade A, and if one or both of them failed, it was judged as Grade C. The criteria for Grade AA are the same as in Tables 1 and 2.

[0050] [Table 3]

[0051] It was found that when X / Y is 0.2 (1 / 5) or more and 8 (8 / 1) or less, the electrical resistivity and die shear strength are AA or A, and favorable results are obtained. In particular, when X / Y is 0.5 (1 / 2) or more and 5 (5 / 1) or less, the die shear strength is very high at 70 MPa or more, and the grade is AA, which is a more favorable result.

[0052] (Examples 14 to 17, Comparative Examples 8 to 11) The same procedure as in Example 1 was carried out, except that the carbon and oxygen contents in the gum arabic-coated copper powder were varied as shown in Table 4 and the amount of metal impurities in the copper powder was set to 0.2 mass%. The composition and test results of each copper paste sample are shown in Table 4.

[0053] [Table 4]

[0054] All copper pastes exhibited die shear strengths exceeding 30 MPa, but copper powders with low carbon and oxygen contents tended to have softer paste viscosities. On the other hand, paste viscosity tended to harden as the carbon and oxygen contents increased. Examples 14 to 17, which used coated copper powders with carbon contents of 0.05 to 0.8% by mass and oxygen contents of 0.05 to 1.5% by mass, demonstrated low electrical resistivity of less than 4 μΩcm and high die shear strengths exceeding 40 MPa.

[0055] As described above, the copper paste of the present invention has high electrical conductivity, excellent storage stability, and exhibits high bonding strength. The copper paste of the present invention also exhibits good oxidation resistance, high thermal conductivity, and can be fired at low temperatures and in short periods of time.

Claims

1. A copper paste containing copper powder and an organic solvent, The organic solvent is one or more first alcohols selected from the group consisting of monohydric and dihydric alcohols having a viscosity at 20°C of 3 mPa·s or more and 70 mPa·s or less; one or more second alcohols selected from the group consisting of dihydric and trihydric alcohols having a viscosity at 20°C of 300 mPa·s or more and 1000 mPa·s or less; an alcohol-based solvent containing a ratio (X / Y) of the mass (X) of the first alcohol to the mass (Y) of the second alcohol in the organic solvent is 1.0 or more and 8.0 or less; A copper paste that is substantially resin-free.

2. In the alcohol-based solvent, the boiling point of the first alcohol at atmospheric pressure is 150°C or higher and 240°C or lower; and the boiling point of the second alcohol at atmospheric pressure is 190°C or higher and 320°C or lower; and The boiling point of the first alcohol is lower than the boiling point of the second alcohol. The copper paste according to claim 1 .

3. 2. The copper paste of claim 1, wherein the first alcohol is one or more alcohols selected from the group consisting of 1-hexanol, 1-heptanol, 2-heptanol, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, benzyl alcohol, hexylene glycol, 1,2-propanediol, 1,3-propanediol, and ethylene glycol.

4. 2. The copper paste of claim 1, wherein the second alcohol is one or more alcohols selected from the group consisting of 2-ethyl-1,3-hexanediol and glycerol.

5. The copper paste according to any one of claims 1 to 4, wherein the copper powder has a coating layer on at least a portion of its surface, the coating layer comprising at least one compound selected from polysaccharides and fatty acids.

6. 6. The copper paste according to claim 5, wherein the copper powder having the coating layer contains 0.05% by mass or more and 0.8% by mass or less of carbon and 0.05% by mass or more and 1.5% by mass or less of oxygen, relative to 100% by mass of the copper powder.

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