JP7774352B2 - Copper paste - Google Patents

Description

The present invention relates to copper paste.

A power module is composed of a semiconductor element that controls power, an insulating heat dissipation substrate, and cooling fins, all of which are bonded together. Materials such as Si, SiC, GaN, _{and Ga2O3} are used for the semiconductor element. Ceramic materials with excellent thermal conductivity, such as _Al2O3 , AlN, _{and Si3N4 ,} are used for the insulating heat dissipation substrate. Furthermore, Al is used for the heat dissipation fins. To bond these components, a thin metal film of Cu, Ni, Ag, or the like is formed on the surface to which the semiconductor element is bonded, and a thin Cu plate called Direct Bonding of Copper (DBC) or Active Metal Brazing (AMB) is formed on the surface of the insulating heat dissipation substrate. An additional thin Ag film may also be formed on the surface of this thin Cu plate.

The process of joining a semiconductor device to an insulating heat dissipation substrate is generally called "die bonding." This die bond is formed by a process that involves the following steps: applying a conductive paste to the surface of a thin copper plate formed on an insulating heat dissipation substrate using a screen printing or dispensing method (application process); drying the paste (drying process); placing a semiconductor device on the applied conductive paste (die mounting process); and heating the conductive paste while applying a stress of approximately 20 MPa in the stacking direction of the resulting laminate (pressure sintering process). This results in a thin metal film formed as a sintered body of the conductive paste on the surface of the semiconductor device, which is then bonded to the substrate. Typical sintering conditions are 250-300°C for 3-10 minutes, and the atmosphere can be air, nitrogen, hydrogen, or other suitable materials, depending on the paste used. The die bond formed in this manner requires a die shear strength of at least 30 MPa.

Here, the sintered body formed from the conductive paste has excellent thermal conductivity, allowing the heat generated when semiconductor elements are operated under high voltage and current to be efficiently transferred to the insulating heat dissipation substrate and dissipated through the cooling fins. While high-concentration lead-containing solder paste has traditionally been used for die bonding, silver paste, which has better thermal conductivity, has replaced it in order to accommodate the rising operating temperatures of elements, and in recent years, copper paste, which can be used at lower cost, has also been attracting attention.

However, when copper paste is pressure sintered under the above-mentioned sintering conditions, the temperature is lower and the time is shorter than normal sintering conditions, so it is not sintered sufficiently, making it difficult to achieve the die shear strength required for the product. Therefore, to promote sintering, it has been proposed to create a paste using fine copper particles with a large surface area.

For example, Non-Patent Document 1 proposes the use of copper nanoparticles with an average particle size of 50 to 60 nm, obtained by reducing a mixed solution of copper hydroxide and disodium nitrilotriacetate with hydrazine, in a copper paste. It has been reported that such copper paste forms a sintered body when heated at a low temperature of 200°C for 30 minutes in a nitrogen atmosphere, and that the die shear strength of the die bond is a maximum of 39 MPa.

Y. Kamikoriyama, H. Imamura, A. Muramatsu, K. Kanie, Sci. Rep., 9, 899 (2019).

However, when the particles are miniaturized as in Non-Patent Document 1, the particles tend to aggregate and form holes, protrusions, or cracks after the coating and drying processes. These defects remain as defects in the bonded body even after the pressure sintering process, and can cause a decrease in die shear strength. Furthermore, when the particles are miniaturized, the specific surface area increases, which causes the particle surface to oxidize during storage of the copper particles or in the drying process after applying the copper paste, degrading sinterability and ultimately degrading die shear strength and the thermal conductivity required for heat dissipation.

The present invention was made in consideration of the above situation, and aims to provide a copper paste that has good oxidation resistance and produces a sintered body with excellent die shear strength.

The present inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that a copper paste containing copper powder and an alcohol solvent, wherein the alcohol solvent contains one or more first alcohols selected from the group consisting of monohydric alcohols and dihydric alcohols, each having a viscosity at 25°C of 3 mPa·s to 70 mPa·s, and one or more second alcohols selected from the group consisting of dihydric alcohols and trihydric alcohols, each having a viscosity at 25°C of 300 mPa·s to 1000 mPa·s, and wherein the viscosity _η10 at 25°C and a shear rate of 10 s ^-1 is 1 Pa·s to 50 Pa·s, and the square root _√σ0 of the Casson yield stress _σ0 is 10 ^Pa½ or less, has good oxidation resistance and produces a sintered body with excellent die shear strength, leading to the completion of the present invention. That is, the present invention provides the following.

(1) A copper paste comprising copper powder and an alcohol solvent, wherein the alcohol solvent comprises one or more first alcohols selected from the group consisting of monohydric alcohols and dihydric alcohols, each having a viscosity at 25°C of 3 mPa·s or more and 70 mPa·s or less, and one or more second alcohols selected from the group consisting of dihydric alcohols and trihydric alcohols, each having a viscosity at 25°C of 300 mPa·s or more and 1000 mPa·s or less, wherein the viscosity _η10 at 25°C and a shear rate of 10 s ^-1 is 1 Pa·s or more and 50 Pa·s or less, and the square root _√σ0 of the Casson yield stress _σ0 is 10 ^Pa½ or less.

(2) The copper paste according to (1), wherein the square root √η _∞ of Casson viscosity η _∞ is 1 (Pa·s) ^1/2 or less.

(3) A copper paste described in (1) or (2), wherein the copper powder includes first copper particles having an average particle diameter of 50 nm or more and 900 nm or less, second copper particles having an average particle diameter of 150 nm or more and 1 μm or less and having an average particle diameter 100 nm or more larger than that of the first copper particles, and third copper particles having a plate-like, scale-like, flat or flake-like shape and an average particle diameter of 1.5 μm or more and 20 μm or less.

(4) The copper powder contains 1% by mass or more and 30% by mass or less of the second copper particles and 5% by mass or more and 60% by mass or less of the third copper particles relative to 100% by mass of the copper powder.

(5) A copper paste described in (1) or (2), in which the total amount of the first alcohol and the second alcohol is 5% by mass or more and 50% by mass or less relative to 100% by mass of the total amount of the copper powder, the first alcohol, and the second alcohol.

(6) A copper paste described in (1) or (2) that does not contain resin or that contains more than 0% by mass and not more than 10% by mass of the resin relative to 100% by mass of the copper powder.

(7) A copper paste described in (1) or (2) that does not contain a silane coupling agent having an epoxy group and a silane coupling agent having an amino group, or that contains a total amount of the silane coupling agent having an epoxy group and the silane coupling agent having an amino group of more than 0% by mass and not more than 0.05% by mass relative to 100% by mass of the copper powder.

(8) A copper paste described in (1) or (2), wherein the first alcohol includes one or more selected from the group consisting of 1-hexanol, 1-heptanol, 2-heptanol, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, benzyl alcohol, ethylene glycol, 1,2-propanediol, 1,3-propanediol, and 2,3-butanediol.

(9) A copper paste as described in (3), in which at least one of the first copper particles and the second copper particles has at least a portion of its surface coated with a polysaccharide.

(10) A copper paste as described in (3), wherein at least one of the first copper particles and the second copper particles contains, on at least a portion of its surface, one or more selected from the group consisting of octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, and tetradecanoic acid.

The present invention provides a copper paste that has good oxidation resistance and produces a sintered body with excellent die shear strength.

1 is a Casson plot of the paste of Comparative Example 2 (η ₁₀ =185 Pa·s). 1 is a Casson plot of the paste of Comparative Example 1 (η ₁₀ =11 Pa·s). 1 is a Casson plot of the paste of Example 4 (η ₁₀ =8 Pa·s). FIG. 2 is an optical microscope photograph of the dried paste of Comparative Example 1. FIG. 1 is an optical microscope photograph of the dried paste of Example 4.

The following describes embodiments of the present invention, but the present invention is not limited in any way to the description of the embodiments and can be implemented with appropriate modifications.

<Copper paste>
The copper paste according to this embodiment includes copper powder and an alcohol solvent. The alcohol solvent includes one or more first alcohols selected from the group consisting of monohydric alcohols and dihydric alcohols, each having a viscosity at 25°C of 3 mPa·s to 70 mPa·s, and one or more second alcohols selected from the group consisting of dihydric alcohols and trihydric alcohols, each having a viscosity at 25°C of 300 mPa·s to 1000 mPa·s, wherein the viscosity _η10 at 25°C and a shear rate of 10 s ^-1 is 1 Pa·s to 50 Pa·s, and the square root _√σ0 of the Casson yield stress _σ0 is 10 ^Pa½ or less.

Regarding the viscosity of the first alcohol and the second alcohol, these two types of alcohol are both Newtonian viscous bodies and their viscosity does not depend on the shear rate, so this refers to the viscosity at any shear rate.

In the case of copper paste, which is a non-Newtonian viscous body, it is measured using a cone-plate type dynamic viscoelasticity measuring device (for example, RST cone-plate Rheometer manufactured by Brookfield), and means the viscosity of the copper paste at a shear rate of 10 s ⁻¹ .

In the copper paste of this embodiment, at least two types of alcohol are used in combination as the organic solvent of the dispersion medium. This prevents oxidation of the copper powder during storage and sintering. The sintered body obtained in this way has a reduced amount of oxide present within it, resulting in excellent thermal conductivity and bonding strength.

In the copper paste according to this embodiment, the viscosity _η10 at a shear rate of 10 s ^-1 is 1 Pa·s or more and 50 Pa·s or less. Having a viscosity _η10 at a shear rate of 10 s ^-1 equal to or greater than the required value prevents the paste from seeping under the printing plate and becoming smeared during application by printing. Furthermore, the paste is prevented from flowing after application by dispensing, allowing the desired shape to be maintained. Furthermore, having a viscosity _η10 equal to or less than the required value allows the paste to be uniformly distributed in the gap between the semiconductor element and the insulating heat dissipation substrate during the die mounting process, suppressing variations in sintering of each part of the pressure-sintered body and suppressing variations depending on the part of the die shear strength.

The viscosity _η10 at a shear rate of 10 s ⁻¹ is not particularly limited as long as it is 1 Pa·s or more and 50 Pa·s or less, but is preferably, for example, 1.5 Pa·s or more, 2 Pa·s or more, 2.5 Pa·s or more, 3 Pa·s or more, 3.5 Pa·s or more, 4 Pa·s or more, 4.5 Pa·s or more, 5 Pa·s or more, 5.5 Pa·s or more, 6 Pa·s or more, 6.5 Pa·s or more, 7 Pa·s or more, 7.5 Pa·s or more, 8 Pa·s or more, 8.5 Pa·s or more, 9 Pa·s or more, 9.5 Pa·s or more, or 10 Pa·s or more. On the other hand, the viscosity η ₁₀ at a shear rate of 10 s ⁻¹ is preferably 49 Pa·s or less, 47 Pa·s or less, 45 Pa·s or less, 42 Pa·s or less, 40 Pa·s or less, 37 Pa·s or less, 35 Pa·s or less, 32 Pa·s or less, or 30 Pa·s or less.

In the copper paste according to this embodiment, the square root √σ ₀ of the Casson yield stress σ ₀ is 10 Pa ^1/2 or less. The square root √σ ₀ of the Casson yield stress σ ₀ is an index of the size and cohesion strength of the aggregates, and by having √σ ₀ equal to or less than a required value, the aggregates are prevented from becoming large or cohesion strongly, and therefore it is possible to prevent holes or protrusions from being formed on the paste surface after drying, which would reduce the die shear strength after pressure sintering.

The square root √σ ₀ of the Casson yield stress σ ₀ is not particularly limited as long as it is 10 Pa ^1/2 or less, and may be, for example, 9.7 ^{Pa 1/2} or less, 9.5 Pa ^1/2 or less, 9.2 Pa ^1/2 or less, 9 Pa ^1/2 or less, 8.7 Pa ^1/2 or less, 8.5 ^{Pa 1/2} or less, 8.2 Pa ^1/2 or less, 8 Pa ^1/2 or less, 7.7 ^{Pa 1/2} or less, 7.5 ^{Pa 1/2} or less, 7.2 Pa 1/2 or less, 7 Pa ^1/2 or less, 6.7 Pa ^1/2 or less, 6.5 ^{Pa 1/2} or less, 6.2 ^{Pa 1/2} or less, 6 Pa ^1/2 or less, 5.7 ^{Pa 1/2} or less, 5.5 ^{Pa 1/2} or less, 5.2 Pa ^1/2 or less, It is preferably ^1/2 or less.

In the copper paste according to this embodiment, the Casson viscosity _η∞ is not particularly limited, but its square root _√η∞ is 1 (Pa·s) ^1/2 or less, 0.97 (Pa·s) ^1/2 or less, 0.95 (Pa·s) ^1/2 or less, 0.92 (Pa·s) ^1/2 or less, 0.9 (Pa·s) ^1/2 or less, 0.87 (Pa·s) ^1/2 or less, 0.85 (Pa·s) ^1/2 or less, 0.82 (Pa·s) ^1/2 or less, 0.8 (Pa·s) ^1/2 or less, 0.77 (Pa·s) ^1/2 or less, 0.75 (Pa·s) ^1/2 or less, 0.72 (Pa·s) ^1/2 or less, 0.7 (Pa·s) ^1/2 or less, 0.67 (Pa·s) Preferably, the viscosity is ^0.65 (Pa s) ^1/2 or less, 0.62 (Pa s) ^1/2 or less, 0.6 (Pa s) ^1/2 or less, 0.57 (Pa s) ^1/2 or less, 0.55 (Pa s) ^1/2 or less, 0.52 (Pa s) ^1/2 or less, or 0.5 (Pa s) ^1/2 or less. The square root √η _∞ of the Casson viscosity η _∞ can be used as an index of the ease of paste movement in the paste application step, and when √η _∞ is equal to or less than the required value, the paste can easily follow the movement of the squeegee during screen printing, preventing defects in the paste application area and uneven paste thickness.

(Measurement of dynamic viscoelastic behavior of copper paste)
Measurements were taken using a dynamic viscoelasticity measuring device equipped with a cone-plate spindle (e.g., Brookfield's RST Cone-Plate Rheometer), and the viscosity of the copper paste at a shear rate of ¹⁰ s was defined as η _10. A Casson plot was also obtained, with the square root (√σ) of shear stress (σ) on the vertical axis and the square root (√γ) of shear rate (γ) on the horizontal axis. When an approximate straight line is obtained from this plot, the intercept of the approximate straight line with the vertical axis in the shear rate region of 10 ^s or more is the square root of the Casson yield stress (√σ ₀ ), and the slope is the square root of the Casson viscosity (√η _∞ ).

(alcohol solvent)
The organic solvent used in the copper paste according to this embodiment is an alcohol solvent that combines a monohydric or dihydric alcohol with a dihydric or trihydric alcohol, each with different viscosities. When a polyhydric alcohol with a valence of 4 or more is used as the solvent, the alcohol may remain in the sintered body, reducing electrical conductivity and adhesion strength, especially when sintering is performed at low temperatures of approximately 300°C or less in a reducing or nitrogen atmosphere. On the other hand, when only a monohydric alcohol is used as the solvent, the copper paste is prone to volatilization during storage and printing, which can change the viscosity of the copper paste and worsen workability. In the copper paste according to this embodiment, the combination of a monohydric or dihydric alcohol with a dihydric or trihydric alcohol with different viscosities avoids these problems, providing a copper paste with excellent physical properties and workability, with copper powder uniformly dispersed. 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, the viscosity of the copper paste can be adjusted to an appropriate value without the need for binder components such as resins. If the copper paste does not contain a resin component, there is no need to consider the generation of carbon residue derived from the resin component, and sintering can be carried out in a non-oxidizing atmosphere at a relatively low temperature.

In this specification, "alcohol solvent" refers to a mixed solvent primarily composed of alcohol, and may include mixed solvents containing small amounts of water or organic solvents other than alcohol, such as 1 to 20% by mass, 2 to 17% by mass, 3 to 15% by mass, 4 to 12% by mass, or 5 to 10% by mass of one or more selected from ethers, ketones, esters, etc. 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 that they are not included, or, if they are included, that their content be 5% by mass or less, 4% by mass or less, 3% by mass or less, 2% by mass or less, 1% by mass or less, 0.7% by mass or less, 0.5% by mass or less, 0.2% by mass or less, or 0.1% by mass or less.

The total amount of the first alcohol and the second alcohol is not particularly limited, but is preferably 5% by mass or more, 5.5% by mass or more, 6% by mass or more, 6.5% by mass or more, 7% by mass or more, 7.5% by mass or more, or 8% by mass or more relative to the total amount of copper paste (100% by mass). On the other hand, the total amount of the first alcohol and the second alcohol is preferably 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, or 20% by mass or less relative to the total amount of copper paste (100% by mass). By ensuring that the total amount of the first alcohol and the second alcohol is greater than the required amount, the copper paste can be applied to the entire interface with a uniform layer thickness, resulting in excellent bonding strength. Furthermore, by ensuring that the total amount of the first alcohol and the second alcohol is less than the required amount, no solvent remains during firing, preventing a decrease in electrical conductivity and bonding strength.

The total amount of the first alcohol and the second alcohol is not particularly limited, but is preferably 5% by mass or more, 7% by mass or more, 10% by mass or more, 12% by mass or more, 15% by mass or more, 17% by mass or more, 20% by mass or more, 22% by mass or more, 25% by mass or more, 27% by mass or more, or 30% by mass or more relative to 100% by mass of the total amount of the copper powder, the first alcohol, and the second alcohol. On the other hand, the total amount of the first alcohol and the second alcohol is preferably 50% by mass or less, 47% by mass or less, 45% by mass or less, 42% by mass or less, 40% by mass or less, 37% by mass or less, 35% by mass or less, 32% by mass or less, 30% by mass or less, 27% by mass or less, or 25% by mass or less relative to 100% by mass of the total amount of the copper powder, the first alcohol, and the second alcohol.

The total amount of the first alcohol and the second alcohol is not particularly limited, but is preferably 70% by mass or more, 75% by mass or more, 80% by mass or more, 85% by mass or more, 90% by mass or more, 95% by mass or more, 97% by mass or more, 98% by mass or more, 99% by mass or more, 99.9% by mass or more, or 99.99% by mass or more, relative to 100% by mass of the total solvent in the copper paste. Because alcohols, particularly trihydric alcohols, have a reducing effect, increasing their content in the solvent of the copper paste can more effectively suppress the oxidation of the copper powder.

The ratio (X/Y) of the mass of the first alcohol (X) to the mass of the second alcohol (Y) in the copper paste is not particularly limited, but is preferably 0.2 or greater, 0.3 or greater, 0.4 or greater, or 0.5 or greater. On the other hand, the ratio (X/Y) of the mass of the first alcohol (X) to the mass of the second alcohol (Y) is preferably 8 or less, 7 or less, 6 or less, or 5 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 of approximately uniform thickness must be printed at the interface between the chip and the substrate. A ratio (X/Y) equal to or greater than the required value results in a viscosity suitable for application, resulting in excellent bonding strength. A ratio (X/Y) equal to or less than the required value results in sufficient reduction activity derived from the alcohol, resulting in particularly good sinterability of the resulting sintered body, resulting in high electrical conductivity and bonding strength.

(First Alcohol)
The first alcohol is one or more alcohols selected from the group consisting of monohydric alcohols and dihydric alcohols, each having a viscosity of 3 mPa·s or more and 70 mPa·s or less at 25° C. If the viscosity of the first alcohol is within this range, application of the copper paste becomes easy, and good workability is ensured.

The boiling point of the first alcohol is not particularly limited, but is preferably 150°C or higher, 155°C or higher, 160°C or higher, 165°C or higher, 170°C or higher, 175°C or higher, 180°C or higher, 185°C or higher, or 190°C or higher. On the other hand, the boiling point of the first alcohol is preferably 250°C or lower, 245°C or lower, 240°C or lower, 235°C or lower, 230°C or lower, 225°C or lower, 220°C or lower, 215°C or lower, 210°C or lower, 205°C or lower, or 200°C or lower. Having a boiling point of 150°C or higher prevents bumping during heating, which can create voids in the paste and reduce sinterability, thereby improving the thermal conductivity and bonding strength of the sintered body. Furthermore, if the boiling point of the first alcohol is 150°C or higher, the solvent will not evaporate and cause a change in viscosity over a short period of time, even when the copper paste is stored at room temperature. Therefore, there is no need to store the product in a refrigerator or freezer, which reduces storage costs.In this specification, the "boiling point" refers to the boiling point at atmospheric pressure unless otherwise specified.

The boiling point of the first alcohol is not particularly limited, but it is preferably more than 50°C lower than the firing temperature of the copper paste.

The vapor pressure of the first alcohol is not particularly limited, but is preferably 0.1 Pa or more, 0.2 Pa or more, 0.3 Pa or more, 0.4 Pa or more, 0.5 Pa or more, 0.6 Pa or more, 0.7 Pa or more, 0.8 Pa or more, 0.9 Pa or more, 1 Pa or more, 1.2 Pa or more, 1.5 Pa or more, 1.7 Pa or more, 2 Pa or more, 2.2 Pa or more, 2.5 Pa or more, 2.7 Pa or more, or 3 Pa or more at around room temperature, for example, 25°C. On the other hand, the vapor pressure of the first alcohol is not particularly limited, but is preferably 100 Pa or less, 90 Pa or less, 80 Pa or less, 70 Pa or less, 60 Pa or less, 50 Pa or less, 40 Pa or less, or 30 Pa or less. Having the vapor pressure of the first alcohol within the required range ensures excellent storage stability and printability.

Specific examples of the first alcohol 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), and 2-ethyl-1-hexanol (viscosity 6.27 mPa·s, boiling point 1 Monohydric alcohols such as benzyl alcohol (viscosity 5.47 mPa s, boiling point 205°C, vapor pressure 18 Pa), and 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), and 2,3-butanediol (viscosity 45 mPa s, boiling point 182°C, vapor pressure <100 Pa) can be used. Among these, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, ethylene glycol, and 1,2-propanediol are preferably used as the first alcohol. The first alcohol may be used alone or in combination of two or more types as long as it satisfies the above-described requirements for the first alcohol. The viscosity and vapor pressure are both measured at 25°C. As described above, the first alcohol has a 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.

(Second Alcohol)
The second alcohol is one or more alcohols selected from the group consisting of dihydric alcohols and trihydric alcohols, each having a viscosity of 300 mPa·s or more and 1000 mPa·s or less at 25° C. If the viscosity of the second alcohol is within this range, it is possible to prevent the copper paste before sintering from sagging, which would make it impossible to form the desired shape, and also to prevent the workability of the copper paste from being impaired.

The boiling point of the second alcohol is not particularly limited, but is preferably 150°C or higher, 160°C or higher, 170°C or higher, 180°C or higher, 190°C or higher, 195°C or higher, 200°C or higher, 205°C or higher, 210°C or higher, 215°C or higher, 220°C or higher, 225°C or higher, 230°C or higher, 235°C or higher, 240°C or higher, 245°C or higher, 250°C or higher, 255°C or higher, 260°C or higher, 265°C or higher, 270°C or higher, 275°C or higher, 280°C or higher, or 285°C or higher. On the other hand, the boiling point of the second alcohol is preferably 320°C or lower, 315°C or lower, 310°C or lower, 305°C or lower, 300°C or lower, or 295°C or lower. Because the boiling point of the second alcohol is within this range, it does not remain in the gaps between the copper particles in the sintered body even after low-temperature sintering, thereby preventing a decrease in thermal conductivity. Because the boiling point of the second alcohol is above the required value, bumping during heating, which would cause voids in the paste and reduce sinterability, can be prevented, thereby improving the thermal conductivity and bonding strength of the sintered body. Furthermore, if the boiling point of the second alcohol is above the required value, even if the copper paste is stored at room temperature, the solvent will not volatilize and the viscosity will not change in a short period of time. This eliminates the need for refrigeration or freezing, thereby reducing storage costs.

The boiling point of the second alcohol is not particularly limited, but it is preferably more than 50°C lower than the firing temperature of the copper paste.

It is preferable to use 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 this embodiment with an appropriate viscosity and excellent workability. However, there is no need to adjust the viscosity after paste application, and from the perspective of preventing the copper paste from sagging, it is preferable for the first alcohol to disappear. Meanwhile, among alcohols, dihydric alcohols and trihydric alcohols, especially trihydric alcohols, have a high reducing effect. Therefore, it is preferable for the second alcohol, which contains at least one of these, to be present at a high concentration during firing. Therefore, by using a second alcohol with a higher boiling point than the first alcohol and which evaporates near the firing temperature of the copper paste, good workability can be maintained and oxidation of the copper powder can be more effectively suppressed.

Furthermore, it is preferable that the vapor pressure at around room temperature, for example 25°C, be 1 mPa or more and 5 Pa or less, preferably 1.5 Pa or less, and especially 1 Pa or less, because this 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.

The vapor pressure of the second alcohol is not particularly limited, but is preferably 1 mPa or more, 2 mPa or more, 3 mPa or more, 4 mPa or more, 5 mPa or more, 6 mPa or more, 7 mPa or more, 8 mPa or more, 9 mPa or more, or 10 mPa or more at around room temperature, for example, 25°C. On the other hand, the vapor pressure of the second alcohol is not particularly limited, but is preferably 100 Pa or less, 90 Pa or less, 80 Pa or less, 70 Pa or less, 60 Pa or less, 50 Pa or less, 40 Pa or less, or 30 Pa or less. Having the vapor pressure of the first alcohol within the required range results in excellent storage stability and printability.

Specific examples of the second alcohol that can be used include 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). As long as the second alcohol satisfies the above-mentioned requirements for the second alcohol, one type may be used alone, or two or more types may be mixed together.

(copper powder)
The copper powder is contained in a copper paste, and the paste is sintered to form a sintered body.

The copper powder is not particularly limited and may be any of various commercially available products. 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 produces 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 fragmented molten metal is then further fragmented by a liquid ejected in an inverted cone shape. This method allows for the continuous gas- and liquid-induced 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 reduces the surface tension of water. Specifically, this method uses water and an 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. For producing finer particles smaller than this, wet reduction precipitation is suitable.

In one embodiment, the average particle size of the copper powder (copper particles) is not particularly limited, but is preferably, for example, 0.05 μm or more, 0.06 μm or more, 0.07 μm or more, 0.08 μm or more, 0.09 μm or more, 0.1 μm or more, 0.12 μm or more, 0.15 μm or more, 0.17 μm or more, 0.2 μm or more, 0.22 μm or more, 0.25 μm or more, 0.27 μm or more, or 0.3 μm or more. On the other hand, the average particle size of the copper powder is preferably 2.0 μm or less, 1.9 μm or less, 1.8 μm or less, 1.7 μm or less, 1.6 μm or less, 1.5 μm or less, 1.4 μm or less, 1.3 μm or less, 1.2 μm or less, 1.1 μm or less, 1 μm or less, 0.9 μm or less, 0.8 μm or less, or 0.7 μm or less. When the average particle size of the copper powder is equal to or less than the required value, the surface area of the copper powder increases relatively, making it easier to sinter at low temperatures. On the other hand, when the average particle size of the copper powder is equal to or greater than the required value, it is possible to suppress an increase in the price of the copper powder. Furthermore, when the average particle size is equal to or greater than the required value, it is possible to prevent a large number of particles from agglomerating and forming defects in the sintered body. In this specification, the term "average particle size" refers to the 50% particle size (D50), and more specifically, it is the median value in the particle diameter distribution measured using a laser particle size distribution analyzer or the like.

In one embodiment, the copper powder preferably includes first copper particles having an average particle diameter of 50 nm to 900 nm; second copper particles having an average particle diameter of 150 nm to 1 μm, which is at least 100 nm larger than the first copper particles; and third copper particles having a plate-like, scale-like, flat, or flake-like shape and an average particle diameter of 1.5 μm to 20 μm. By having a difference in average particle diameter of 100 nm or more between the first and second copper particles, the gaps between the larger second copper particles are filled by the smaller first copper particles, resulting in a dense sintered body. Furthermore, by including third copper particles in a flake-like shape, the occurrence of cracks after the paste is applied and dried can be suppressed.

The shape of the first copper particles is not particularly limited, but it is preferable that the particles be spherical, ellipsoidal, polyhedral, irregular, wire-like, dendritic, etc.

The average particle diameter of the first copper particles is preferably, for example, 60 nm or more, 70 nm or more, 80 nm or more, 90 nm or more, 100 nm or more, 120 nm or more, 150 nm or more, 170 nm or more, 200 nm or more, 220 nm or more, or 250 nm or more. On the other hand, the average particle diameter of the first copper particles is preferably 850 nm or less, 800 nm or less, 750 nm or less, 700 nm or less, 650 nm or less, 600 nm or less, 550 nm or less, 500 nm or less, 450 nm or less, 400 nm or less, 350 nm or less, or 300 nm or less.

The content of the first copper particles is not particularly limited, and is preferably, for example, 20% by mass or more, 25% by mass or more, 30% by mass or more, 35% by mass or more, 40% by mass or more, 45% by mass or more, 50% by mass or more, 55% by mass or more, 60% by mass or more, 65% by mass or more, or 70% by mass or more relative to 100% by mass of the copper powder. On the other hand, the content of the first copper particles in the copper powder is preferably 90% by mass or less, 85% by mass or less, or 80% by mass or less relative to 100% by mass of the copper powder.

The shape of the second copper particles is not particularly limited, but it is preferable that they be spherical, ellipsoidal, polyhedral, irregular, wire-like, dendritic, etc.

The average particle diameter of the second copper particles is preferably, for example, 160 nm or more, 170 nm or more, 180 nm or more, 190 nm or more, 200 nm or more, 220 nm or more, 250 nm or more, 270 nm or more, 300 nm or more, 320 nm or more, or 350 nm or more. On the other hand, the average particle diameter of the first copper particles is preferably 950 nm or less, 900 nm or less, 850 nm or less, 800 nm or less, 750 nm or less, 700 nm or less, 650 nm or less, 600 nm or less, 550 nm or less, 500 nm or less, 450 nm or less, or 400 nm or less.

The average particle size of the second copper particles is preferably at least 100 nm larger than the average particle size of the first copper particles. On the other hand, the average particle size of the second copper particles is preferably at most 900 nm, 800 nm, 700 nm, 600 nm, 500 nm, 400 nm, 300 nm, or 200 nm larger than the average particle size of the first copper particles.

The content of the second copper particles is not particularly limited, and is preferably, for example, 1% by mass or more, 2% by mass or more, 3% by mass or more, 4% by mass or more, 5% by mass or more, 6% by mass or more, 7% by mass or more, 8% by mass or more, 9% by mass or more, 10% by mass or more, 11% by mass or more, or 12% by mass or more relative to 100% by mass of the copper powder. On the other hand, the content of the second copper particles in the copper powder is preferably 30% by mass or less, 27% by mass or less, 25% by mass or less, 22% by mass or less, 20% by mass or less, or 18% by mass or less relative to 100% by mass of the copper powder.

The average particle diameter of the third copper particles is preferably, for example, 1.5 μm or more, 2 μm or more, 2.5 μm or more, 3 μm or more, 3.5 μm or more, 4 μm or more, 4.5 μm or more, 5 μm or more, 5.5 μm or more, or 6 μm or more. On the other hand, the average particle diameter of the third copper particles is preferably 20 μm or less, 19 μm or less, 18 μm or less, 17 μm or less, 16 μm or less, 15 μm or less, 14 μm or less, 13 μm or less, 12 μm or less, 11 μm or less, 10 μm or less, 9 μm or less, or 8 μm or less.

The content of the third copper particles in the copper powder is not particularly limited, and is preferably, for example, 5% by mass or more, 5.5% by mass or more, 6% by mass or more, 6.5% by mass or more, 7% by mass or more, 7.5% by mass or more, 8% by mass or more, 8.5% by mass or more, 9% by mass or more, or 9.5% by mass or more, relative to 100% by mass of copper powder. On the other hand, the content of the third copper particles in the copper powder is preferably 60% by mass or less, 55% by mass or less, 50% by mass or less, 45% by mass or less, 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, or 12% by mass or less, relative to 100% by mass of copper powder.

The copper powder (copper particles) may have a portion of its surface coated with an organic substance. The organic substance is preferably a polysaccharide or fatty acid compound. When polysaccharide molecules coat the copper powder, the outer side (the side in contact with the solvent) becomes hydrophilic, interacting with the hydroxyl groups of the organic solvent in the copper paste to provide appropriate viscosity. Meanwhile, the carboxyl groups of fatty acids bond to the copper particle surface, making the opposite end of the fatty acid hydrophobic, thereby improving the dispersibility of the copper particles and suppressing particle aggregation. These effects of polysaccharides and fatty acids allow the copper paste to spread uniformly across the entire interface, resulting in excellent bonding strength.

Specific examples of polysaccharides that can be used include, but are not limited to, one or more selected from gum arabic, carboxymethyl cellulose, hydroxyethyl cellulose, cellulose nanofiber, starch, glycogen, agarose (agar), pectin, alginic acid, and salts thereof. Sulfur-containing polysaccharides such as carrageenan can also be used. Among these, it is preferable to use one or more selected from gum arabic and sodium alginate.

Specific examples of fatty acids that can be used include medium-chain fatty acids such as pentanoic acid, octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, and tetradecanoic acid. Among these, it is preferable to use one or more selected from octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, and tetradecanoic acid.

While the surface coverage of the organic material is not particularly limited, it is preferable that the carbon content be 0.05% to 0.8% by mass, preferably 0.1% to 0.5% by mass, and the oxygen content be 0.05% to 1.5% by mass, preferably 0.1% to 1% by mass, relative to 100% by mass of copper powder with a coating layer. By ensuring that the carbon and oxygen contents are above the required levels, the hydrophilic properties of the polysaccharide molecules on the copper powder surface are fully expressed, reducing the viscosity of the copper paste and forming a uniform paste layer, resulting in excellent bond strength for the resulting sintered body. On the other hand, by ensuring that the carbon and oxygen contents are below the required levels, carbon- and oxygen-containing components are prevented from remaining inside the sintered body, for example, during firing in a nitrogen atmosphere, resulting in excellent thermal conductivity and bond strength.

When the copper powder is made of the three types of copper particles described above, namely, the first copper particles, the second copper particles, and the third copper particles, it is preferable that at least one of the first copper particles and the second copper particles has at least a portion of its surface coated with an organic substance. Polysaccharides and fatty acids can be used as the organic substance, with polysaccharides being preferred. The third copper particles may also have at least a portion of their surface coated with an organic substance (polysaccharides, fatty acids, etc.).

In the copper paste according to this embodiment, the total content of elements other than copper in the copper powder is not particularly limited, but is preferably, for example, 1% by mass or less, 0.5% by mass or less, or 0.1% by mass or less, relative to 100% by mass of copper powder. Among the components other than copper, metal elements, in particular, may segregate on the surface of the copper powder or form oxides, impairing sinterability and potentially reducing the electrical conductivity of the sintered body by dissolving in the copper powder. If the total content of metal elements such as As, Co, Cr, Fe, Ir, P, S, Sb, Se, Te, Ti, V, and Zr is below the required level, the electrical resistivity of the copper paste sintered body can be reduced and the sintered body will exhibit superior thermal conductivity. Such thermal conductivity allows, for example, heat generated by a power module to be efficiently dissipated to the outside.

(Other ingredients)
In addition to the components described above, the copper paste according to this embodiment may contain a dispersant made of amines, a surfactant, an antioxidant, a reducing agent such as hydrazine, glass frit, a binder including a resin component, and the like.

The resin component is not particularly limited, but can be one or more selected from cellulose-based resins such as methyl cellulose, ethyl cellulose, and carboxymethyl cellulose, acrylic resins, butyral resins, alkyd resins, epoxy resins, and phenolic resins.

The resin content is not particularly limited, but is preferably greater than 0%, 0.01%, 0.05%, or 0.1% by mass relative to 100% by mass of copper powder. The resin content may be 10% by mass or less, 9% by mass or less, 8% by mass or less, 7% by mass or less, 6% by mass or less, 5% by mass or less, 4% by mass or less, 3% by mass or less, 2% by mass or less, 1% by mass or less, 0.5% by mass or less, 0.1% by mass or less, 0.05% by mass or less, or 0.01% by mass or less relative to 100% by mass of copper particles. However, it is preferable for the resin content to be zero, relative to 100% by mass of copper particles. If the copper paste contains a resin component, sinterability (especially at temperatures below 350°C) may be impaired. For example, thermosetting resins such as epoxy resins remain in the sintered copper paste compact even after sintering. Even when cellulose resins are used, although thermal decomposition begins at around 300°C, complete thermal decomposition requires heating at temperatures above 400°C. Furthermore, sintering in an oxygen atmosphere is required to remove the carbon residue produced by thermal decomposition, which may oxidize the copper powder. Resin-free copper pastes can be sintered at relatively low temperatures in a non-oxidizing atmosphere, resulting in the formation of high-density sintered copper bodies without the risk of copper powder oxidation causing a decrease in conductivity. The copper paste according to this embodiment contains a second alcohol with high viscosity, making it possible to adjust the viscosity to an appropriate value without the need for a resin component.

Furthermore, it is preferable that the copper paste does not contain either a silane coupling agent having an epoxy group or a silane coupling agent having an amino group, or contains a silane coupling agent having an epoxy group and a silane coupling agent having an amino group in a total amount of more than 0% by mass and 0.05% by mass or less per 100% by mass of copper powder.

(Uses of copper paste)
As described above, the copper paste according to this embodiment has high electrical and thermal conductivity, and is excellent in storage stability and workability. Furthermore, the copper paste according to this embodiment can be fired at low temperatures and in short periods of time, exhibiting high die shear strength. Therefore, it can be used for wiring formation 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, a copper sintered body can be formed by applying the copper paste according to this embodiment to a power module, a solar cell substrate, a substrate for mounting an electronic packaging device, a printed wiring board, or a substrate with through-holes, and then sintering it. 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 substrates having a transparent conductive film (TCO) or a metal film on their surfaces.

(Manufacturing of copper paste)
The copper paste according to this embodiment can be produced by mixing the copper powder and the solvent described above and kneading the mixture using a device such as a planetary mixer, if necessary. Furthermore, the dispersibility of the copper powder can be improved using a three-roll mill, if necessary. Furthermore, the paste may be filtered or degassed.

(Firing of copper paste)
When firing the copper paste according to this embodiment, the method and conditions are not particularly limited, and any method can be used depending on the desired product and the mating material to which the paste is applied. However, it is preferable to dry and remove the first alcohol prior to firing the copper paste according to this embodiment. 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. The drying conditions are not particularly limited and can be set as desired depending on the boiling point of the first alcohol and the desired product. However, for example, heating in an air atmosphere at 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 an inert gas atmosphere or a reducing atmosphere.

The copper paste according to this embodiment can be fired at low temperatures and in short periods of time, so the firing conditions are not particularly limited. For example, sintering 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 vapor, or the like, at 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 obtain a sintered body with excellent electrical conductivity, thermal conductivity, and die shear strength.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

[Evaluation of sample properties and performance]
The pastes prepared under the respective compositions and conditions described below were evaluated for their physical properties and performance according to the evaluation methods described below.

(Measurement of Viscosity, and Measurement of Casson Yield Stress and Casson Viscosity)
The viscosity of the prepared paste was measured using a dynamic viscoelasticity measuring device (manufactured by Brookfield, RST Cone Plate Rheometer) equipped with a cone-plate type spindle, and the viscosity of the copper paste at a shear rate of 10 s ^-1 was defined as η _10. In addition, a Casson plot was obtained with the square root (√σ) of shear stress (σ) on the vertical axis and the square root (√γ) of shear rate (γ) on the horizontal axis. When an approximate straight line is obtained in this plot, a Casson plot is possible, and the intercept of the ordinate of the approximate straight line in the shear rate region of 10 s ^-1 or more is the square root of the Casson yield stress (√σ ₀ ), and the slope of the approximate straight line is the square root of the Casson viscosity (√η _∞ ). Here, obtaining an approximate straight line means that the R ² value (coefficient of determination) is 0.9 or more.

Below, we will explain this by showing specific examples. As representative results, Casson plots were created for three types of copper paste (Comparative Example 2, Comparative Example 1, and Example 4, details of which will be described later). Figure 1 is a Casson plot for the paste of Comparative Example 2 (η ₁₀ =185 Pa·s). The R ² value in the region where the shear rate is 10 s ⁻¹ or more is 0.0615, and as is clear from Figure 1, there is no linearity, so fitting using the Casson equation is not possible.

On the other hand, Figure 2 is a Casson plot of the paste of Comparative Example 1 (η ₁₀ =11 Pa·s). Figure 3 is a Casson plot of the paste of Example 4 (η ₁₀ =8 Pa·s). In the Casson plots shown in Figures 2 and 3, the R ² values in the shear rate region of 10 s ^-1 or more are 0.9994 and 0.9996, respectively, making it possible to linearly approximate the data. In both cases, there is a rapid increase in √σ in the slow shear rate region and a gradual increase in √σ in the fast shear rate region. The rapid increase corresponds to the phenomenon of breaking down agglomerates of the fine powder contained in the paste, so an intercept of √σ ₀ and a slope of √η _∞ were obtained from the approximation line in the fast shear rate region (10 s ^-1 or more) as reflecting the characteristics of a homogeneous paste.

(Observation of tissue defects)
The prepared paste was applied to a glass substrate by stencil printing using a metal mask so as to form a square with sides of 20 mm, and then dried in the atmosphere at 100° C. for 5 minutes. The surface structure of the dried paste was observed with a stereoscopic optical microscope at 10x magnification to check for the presence or absence of structural defects such as holes, protrusions, and cracks.

The following explains this with specific examples. The pastes of Comparative Example 1 and Example 4, which were able to be linearly approximated in the Casson plot, were printed on a glass substrate, dried, and the surface of the resulting dried product was observed. Figure 4 shows an optical microscope photograph of the dried product of the paste of Comparative Example 1. Figure 5 shows an optical microscope photograph of the dried product of the paste of Example 4. Numerous holes were observed in the dried product of the paste of Comparative Example 1 (positions indicated by arrows in Figure 4). On the other hand, no holes or other structural defects were observed in the dried product of the paste of Example 4. Similar tests were performed on pastes prepared under various process conditions to check for the presence or absence of surface defects and printing defects.

(Measurement of electrical resistivity)
The prepared paste was applied to a glass substrate using a metal mask and stencil printed to form a square with sides of 20 mm, and then dried in air at 100 °C for 5 minutes. Subsequently, pressure sintering was performed in a nitrogen atmosphere using a high-temperature press under a load of 20 MPa at a heating temperature of 280 °C for 2 minutes, obtaining 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 a probe spacing set to 1 mm. When the electrical resistivity is converted to thermal conductivity according to the Wiedemann-Franz law, it corresponds to 134 Wm ^-1 K ^-1 or more.

(Measurement of die shear strength)
A copper paste was applied to a substrate of 1 mm thick copper plate 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 in contact with the copper paste using a sputtering method. A load of 20 MPa was applied to the resulting laminate using a high-temperature press in a nitrogen atmosphere, and the firing temperature was set to 280 °C, followed by pressure sintering for 3 minutes. After cooling the sample to room temperature, the adhesion strength between the SiC chip and the copper substrate was measured as die shear strength using a die shear tester (Nordson DAGE 4000).

(Evaluation criteria)
Regarding these measurement results, cases that satisfied all four conditions, (1) there were no surface defects in the dried paste, (2) there were no printing defects in the dried paste, (3) the electrical resistivity of the sintered body was 5 μΩcm or less, and (4) the die shear strength of the sintered body was 30 MPa or more, were evaluated as AA. Cases that satisfied three conditions were evaluated as A, cases that satisfied two conditions were evaluated as B, and cases that satisfied one condition or none of the conditions were evaluated as C.

Test 1: Effect of Paste Viscosity and Casson Yield Stress
Example 1
The copper particles used were first copper particles having a D50 of 270 nm and a roughly spherical shape, second copper particles having a D50 of 380 nm and a roughly spherical shape, and third copper particles having a D50 of 7 μm and a flake shape. The surfaces of the two types of copper particles other than the flake-shaped particles were coated with gum arabic, a polysaccharide.

First, the first copper particles, second copper particles, and third copper particles were mixed in a mass ratio of 65:30:5. The impurities contained in each of the first to third copper particles were 0.3% carbon, 0.7% oxygen, and 0.2% metal elements other than copper. Ethylene glycol as the first alcohol and glycerol as the second alcohol were prepared and weighed out to a mass ratio of copper particles, ethylene glycol, and glycerol of 60:20:20, and then kneaded in a planetary mixer to produce a copper paste. The evaluation results are shown in Table 1.

(Examples 2 to 7 and Comparative Examples 1 to 4)
Copper pastes were prepared under the same conditions as in Example 1, except that the mass ratio of the copper powder to the solvent and the mass ratio of the first to third copper particles were changed. The evaluation results are shown in Table 1. In Table 1, "N/A" indicates that a good approximation line (fitting) was not obtained in the Casson plot.

It was found that the values of η 10 , √σ 0 , and √η ∞ can be controlled by using as a solvent a first alcohol (ethylene glycol) having a viscosity in the range of 3 mPa·s or more and 70 mPa·s or less and a second alcohol (glycerol) having a viscosity in the range of ₃₀₀ mPa· _s or more and 1000 mPa·s or _less , and adjusting the paste composition. It was also found that a paste with good evaluation could be obtained by adjusting η ₁₀ and √σ ₀ .

Test 2: Effect of the first alcohol species
(Examples 8 to 13 and Comparative Examples 5 to 7)
Pastes were prepared and evaluated in the same manner as in Example 3, except that the ethylene glycol used as the first alcohol was changed to the alcohol shown in Table 2. The evaluation results are shown in Table 2.

Table 2 shows that the first alcohol is not limited to ethylene glycol, and that a sintered body with the desired performance can be obtained by using a copper paste that uses a monohydric or dihydric alcohol with a viscosity within the required viscosity range.

Test 3: Effect of a Second Alcohol Species
(Example 14, Comparative Examples 8 to 9)
Pastes were prepared and evaluated in the same manner as in Example 3, except that the glycerol used as the second alcohol was changed to the alcohol shown in Table 3. The evaluation results are shown in Table 3.

From Table 3, it was found that the second alcohol is not limited to ethylene glycol, and that a sintered body having the desired performance can be obtained by using a copper paste that uses a dihydric or trihydric alcohol having a viscosity within the required viscosity range.

Claims (10)

Contains copper powder and alcohol solvent,
The alcohol solvent is
a first alcohol, which is one or more selected from the group consisting of monohydric alcohols and dihydric alcohols, having a viscosity at 25°C of 3 mPa·s or more and 70 mPa·s or less;
and a second alcohol, which is one or more selected from the group consisting of dihydric alcohols and trihydric alcohols, having a viscosity at 25°C of 300 mPa s or more and 1000 mPa s or less,
The viscosity _η10 at 25°C and a shear rate of 10 s ^-1 is 1 Pa s or more and 50 Pa s or less,
A copper paste having a square root √σ ₀ of Casson yield stress σ ₀ of 10 Pa ^1/2 or less. The copper paste according to claim 1, wherein the square root √η _∞ of Casson viscosity η _∞ is 1 (Pa·s) ^1/2 or less. The copper powder is
First copper particles having an average particle diameter of 50 nm or more and 900 nm or less;
second copper particles having an average particle diameter of 150 nm or more and 1 μm or less, and an average particle diameter 100 nm or more larger than that of the first copper particles;
The copper paste according to claim 1 or 2, further comprising third copper particles that are plate-like, scale-like, flat, or flake-like and have an average particle diameter of 1.5 μm or more and 20 μm or less. The copper paste according to claim 3 , wherein the copper powder contains 1% by mass or more and 30% by mass or less of the second copper particles and 5% by mass or more and 60% by mass or less of the third copper particles relative to 100% by mass of the copper powder. The copper paste according to claim 1 or 2, wherein a total amount of the first alcohol and the second alcohol is 5% by mass or more and 50% by mass or less, relative to 100% by mass of a total amount of the copper powder, the first alcohol, and the second alcohol. The copper paste according to claim 1 or 2, which does not contain a resin or contains the resin in an amount of more than 0 mass % and 10 mass % or less relative to 100 mass % of the copper powder. 3. The copper paste according to claim 1, wherein the paste does not contain a silane coupling agent having an epoxy group and a silane coupling agent having an amino group, or contains the silane coupling agent having an epoxy group and the silane coupling agent having an amino group in a total amount of more than 0% by mass and 0.05% by mass or less, relative to 100% by mass of the copper powder. The copper paste according to claim 1 or 2, wherein the first alcohol includes one or more selected from the group consisting of 1-hexanol, 1-heptanol, 2-heptanol, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, benzyl alcohol, ethylene glycol, 1,2-propanediol, 1,3-propanediol, and 2,3-butanediol. The copper paste according to claim 3 , wherein at least one of the first copper particles and the second copper particles has at least a portion of its surface coated with a polysaccharide. 4. The copper paste according to claim 3, wherein at least one of the first copper particles and the second copper particles contains, on at least a portion of a surface thereof, one or more selected from the group consisting of octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, and tetradecanoic acid.