Electrically conductive paste, laminated body, and method for bonding cu substrate or cu electrode to electrical conductor

Abstract

An object of the present invention is to provide an electrically conductive paste having excellent bonding strength when bonded to an electronic substrate and the like, a laminated body, and a method for bonding a Cu substrate or Cu electrode to an electrical conductor. An electrically conductive paste comprising: a flake-like silver powder A having a particle size in the range of 1 μm or more and 15 μm or less and having a median diameter D50 of 2 μm or more and 5 μm or less; a silver powder B having a particle size in the range of 25 μm or more and 100 μm or less and having a median diameter D50 of 30 μm or more and 40 μm or less; a silver powder C having a particle size in the range of 10 nm or more and 190 nm or less and having a median diameter D50 of 50 nm or more and 150 nm or less; and a solvent, wherein the content of the silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass based on 100 parts by mass in total of the flake-like silver powder A, the silver powder B, and the silver powder C.

Description

Conductive paste, laminate, and method for joining Cu substrate or Cu electrode to conductor

[0001] The present invention relates to a conductive paste, a laminate, and a method for joining a Cu substrate or a Cu electrode to a conductor.

[0002] Conventionally, conductive pastes using μm-sized (hereinafter sometimes referred to as “micron-sized”) or nm-sized (hereinafter sometimes referred to as “nano-sized”) metal powders have been widely used as conductive materials. Conductive pastes using such micron-sized or nano-sized metal powders are used, for example, to form electronic circuits such as electrodes and wiring on printed wiring boards, and to bond between electronic boards and electronic components.

[0003] Ag (silver)-plated Cu (copper) substrates are sometimes used as substrates to which the conductive paste is applied. Currently, Ag-plated Cu electrodes are the mainstream of electrodes provided on electronic substrates. However, in recent years, to reduce costs and shorten manufacturing processes, Cu substrates or Cu electrodes without a plating layer, which are manufactured without a plating process, have begun to be used. When joining such Cu substrates or Cu electrodes without a plating layer to other components such as electronic components using a conductive paste, if the conductive paste is applied directly to the surface of the Cu substrate or Cu electrode and then sintered in an air atmosphere, the Cu is oxidized on the surface of the Cu substrate or Cu electrode, forming a copper oxide layer. The formation of this copper oxide layer on the surface of the Cu substrate or Cu electrode results in insufficient bonding at the interface between the Cu substrate or Cu electrode and the sintered body of the conductive paste. In such cases, to prevent the formation of a copper oxide layer on the surface of the Cu substrate or Cu electrode, the conductive paste is sintered in a nitrogen atmosphere rather than an air atmosphere.

[0004] As conductive pastes using metal powders, Patent Document 1, filed by the same applicant as the present application, proposes a conductive paste containing flaky silver powder having a median diameter D50 of 15 μm or less, silver powder having a median diameter D50 of 25 μm or more, and a solvent, as well as a conductive paste containing flaky silver powder having a median diameter D50 of 15 μm or less, silver powder having a median diameter D50 of 25 μm or more, silver powder having a bimodal particle size distribution with peaks at 0.1 μm to 0.5 μm and 1 μm to 15 μm, and a solvent. However, as will be described in Comparative Example 13 of the present application, it has been found that the conductive paste described in Patent Document 1 has a problem in that, when bonding a copper substrate without a plating layer to an electronic component (a Si chip) with the conductive paste, if the conductive paste is applied directly to the surface of the copper substrate and sintered in a nitrogen atmosphere, sufficient bonding strength between the copper substrate and the electronic component cannot be obtained.

[0005] International Publication No. 2020 / 008689

[0006] As described above, conventional conductive pastes have a problem in that when a Cu substrate or Cu electrode having no plating layer is bonded to another component such as an electronic component using the conductive paste, if the conductive paste is applied directly to the surface of the Cu substrate or Cu electrode and sintered in a nitrogen atmosphere, the bonding strength between the Cu substrate or Cu electrode and the component is insufficient.

[0007] Therefore, there is a need for a conductive paste that can solve the above problems and has excellent bonding strength when bonding a Cu substrate or Cu electrode to other components such as electronic components.

[0008] An object of the present invention is to provide a conductive paste and laminate that provide excellent bonding strength when bonding a Cu substrate or Cu electrode to another component such as an electronic component, and a method for bonding a Cu substrate or Cu electrode to a conductor.

[0009] As a result of extensive research aimed at solving the above problems, the inventors discovered that the above problems could be solved by optimizing the combination of multiple silver powders having different particle size ranges and median diameters D50, and thus completed the present invention. Specific aspects of the present invention are as follows. In this application, when a numerical range is expressed using "X to Y," the range includes the values ​​at both ends, X and Y.

[0010] [1] A conductive paste comprising: flake silver powder A having a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 2 μm or more and 5 μm or less; silver powder B having a particle size in the range of 25 μm or more and 100 μm or less and a median diameter D50 of 30 μm or more and 40 μm or less; silver powder C having a particle size in the range of 10 nm or more and 190 nm or less and a median diameter D50 of 50 nm or more and 150 nm or less; and a solvent, wherein the content of silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass per 100 parts by mass of the total of flake silver powder A, silver powder B, and silver powder C. [2] A conductive paste comprising: flake silver powder A having a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 2 μm or more and 5 μm or less; spherical silver powder D having a bimodal particle size distribution with a peak having a particle size in the range of 200 nm or more and 800 nm or less and a median diameter D50 of 300 nm or more and 600 nm or less, and a peak having a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 1.5 μm or more and 4 μm or less; silver powder C having a particle size in the range of 10 nm or more and 190 nm or less and a median diameter D50 of 50 nm or more and 150 nm or less; and a solvent, wherein the content of silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass per 100 parts by mass of the total of flake silver powder A, spherical silver powder D, and silver powder C.[3] Flake silver powder A having a particle size in the range of 1 μm to 15 μm and a median diameter D50 of 2 μm to 5 μm; silver powder B having a particle size in the range of 25 μm to 100 μm and a median diameter D50 of 30 μm to 40 μm; spherical silver powder D having a bimodal particle size distribution with a peak having a particle size in the range of 200 nm to 800 nm and a median diameter D50 of 300 nm to 600 nm, and a peak having a particle size in the range of 1 μm to 15 μm and a median diameter D50 of 1.5 μm to 4 μm; silver powder C having a particle size in the range of 10 nm to 190 nm and a median diameter D50 of 50 nm to 150 nm; and a solvent, A conductive paste characterized in that the content of silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass per 100 parts by mass of the total of the flaky silver powder A, the silver powder B, the spherical silver powder D, and the silver powder C. [4] The conductive paste according to [1], characterized in that the content of silver powder C is 20.0 parts by mass or more and 65.0 parts by mass or less per 100 parts by mass of the total of the flaky silver powder A, the silver powder B, and the silver powder C. [5] The conductive paste according to [2], characterized in that the content of silver powder C is 25.0 parts by mass or more and 65.0 parts by mass or less per 100 parts by mass of the total of the flaky silver powder A, the spherical silver powder D, and the silver powder C. [6] The conductive paste according to [3], characterized in that the content of the silver powder C is 20.0 parts by mass or more and 65.0 parts by mass or less relative to a total of 100 parts by mass of the flaky silver powder A, the silver powder B, the spherical silver powder D, and the silver powder C. [7] A laminate comprising a Cu substrate or a Cu electrode, the conductive paste according to any one of [1] to [6], and a conductor.[8] A method for joining a Cu substrate or a Cu electrode with a conductor, comprising: a step of applying the conductive paste according to any one of [1] to [6] to a Cu substrate or a Cu electrode; a step of overlaying a conductor on the conductive paste on the Cu substrate or the Cu electrode to form a laminate including the Cu substrate or the Cu electrode, the conductive paste, and the conductor; and a step of sintering the conductive paste by heating the laminate at a temperature of 150 to 350°C for a time of 1 minute or more and less than 100 minutes.

[0011] Conventional conductive silver pastes may not sinter sufficiently in a nitrogen atmosphere, resulting in insufficient bonding at the interface between the Cu substrate or Cu electrode and the sintered conductive paste. This is thought to be because, when sintered in a nitrogen atmosphere, the metal powder particles in the conductive paste do not fuse together as well as in an air atmosphere, leaving many gaps between the metal powder particles. Without being bound by theory, it is presumed that the silver powder C contained in the conductive paste of the present invention, having a particle size in the range of 10 nm to 190 nm and a median diameter D50 of 50 nm to 150 nm, has a small particle size and is therefore capable of filling gaps between multiple other silver powder particles and gaps between other silver powder particles and the substrate or electrode to be joined. Therefore, it is presumed that the conductive paste of the present invention can fill these gaps and achieve sufficient bonding with the Cu substrate or Cu electrode, even when sintering is performed in a nitrogen atmosphere or when a Cu substrate or Cu electrode without a plating layer is used.

[0012] The conductive paste of the present invention has excellent bonding strength when bonding a Cu substrate or Cu electrode to another component such as an electronic component.

[0013] FIG. 1 is a schematic diagram of a sample used in evaluating shear strength in the examples. FIG. 2 is a schematic diagram of wiring used in evaluating electrical resistance in the examples. FIG. 3 is an image of the surface of silver powder (A) in the examples, taken with a scanning electron microscope (magnification: 5000x). FIG. 4 is an image of the surface of silver powder (A') in the comparative example, taken with a scanning electron microscope (magnification: 5000x). FIG. 5 is an image of the surface of silver powder (B) in the examples, taken with a scanning electron microscope (magnification: 1000x). FIG. 6 is an image of the surface of silver powder (D) in the examples, taken with a scanning electron microscope (magnification: 5000x). FIG. 7 is an image of the surface of silver powder (C) in the examples, taken with a scanning electron microscope (magnification: 5000x). FIG. 8 is an image of the surface of silver powder (C) in the examples, taken with a scanning electron microscope (magnification: 20000x). FIG. 9 is the measurement results of the particle size distribution of silver powders (C) and (D) in the examples. 1 is a graph showing the shear strength of conductive pastes (1) to (3) in the examples (Examples 1 to 21 and Comparative Examples 18 to 23). FIG. 2 is a graph showing the shear strength for each sintering temperature of conductive pastes (1) to (3) in the examples (Examples 3, 10, 17, and 25 to 33). FIG. 3 is an image of a cross section of a shear strength measurement sample after sintering in Example 11 taken with a scanning electron microscope (magnification: 3000x). FIG. 4 is an image of a cross section of a shear strength measurement sample after sintering in Example 11 taken with a scanning electron microscope (magnification: 10000x). FIG. 5 is an image showing the results of mapping measurements of each element in a cross section of a shear strength measurement sample after sintering in Example 3.

[0014] The conductive paste, laminate, and method for joining a Cu substrate or Cu electrode to a conductor of the present invention will be described below.

[0015] [First Embodiment (Conductive Paste)] The conductive paste of the first embodiment comprises: flaky silver powder A having a particle size of 1 μm or more and 15 μm or less and a median diameter D50 of 2 μm or more and 5 μm or less; silver powder B having a particle size of 25 μm or more and 100 μm or less and a median diameter D50 of 30 μm or more and 40 μm or less; silver powder C having a particle size of 10 nm or more and 190 nm or less and a median diameter D50 of 50 nm or more and 150 nm or less; and a solvent, wherein the content of silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass per 100 parts by mass of the total of flaky silver powder A, silver powder B, and silver powder C. The conductive paste of the first embodiment comprises flaky silver powder A, silver powder B, and silver powder C, and the content of silver powder C is within a specific range, thereby achieving excellent bonding strength when bonding a Cu substrate or Cu electrode to another component such as an electronic component. The conductive paste of the first embodiment may be composed of flaky silver powder A, silver powder B, and silver powder C as the metal components, or may contain other metal components.

[0016] The particle size range of the flaky silver powder A is from 1 μm to 15 μm, preferably from 1 μm to 10 μm, and more preferably from 2 μm to 8 μm. The median diameter D50 of the flaky silver powder A is from 2 μm to 5 μm, preferably from 2.5 μm to 4.5 μm, and more preferably from 2.5 μm to 3.5 μm. By ensuring that the particle size range and median diameter D50 of the flaky silver powder A are within the above numerical ranges, the viscosity of the conductive paste can be adjusted to an appropriate level, and the packing density of the silver powder in the conductive paste can be increased.

[0017] In the present invention, the flaky silver powder A may be any powder that is recognized as flake-shaped (thin or flake-shaped) when observed with an SEM. The magnification of the SEM can be appropriately set to 5,000x, 10,000x, or the like so that the shape of the flaky silver powder A, which has a particle size in the range of 1 μm to 15 μm and a median diameter D50 of 2 μm to 5 μm, can be seen. Here, flaky (thin or flake-shaped) particles refer to particles having an aspect ratio (ratio of longest major axis to shortest minor axis) of 5 to 300. By including not only spherical silver powder but also flaky silver powder A, the conductive paste of the present invention is able to fill the gaps between adjacent spherical particles with flaky silver particles, forming continuous contact points between the silver powder particles, thereby improving conductivity and enabling uniform dispersion of each silver powder in the conductive paste.

[0018] The particle size range of silver powder B is from 25 μm to 100 μm, preferably from 25 μm to 80 μm, and more preferably from 25 μm to 70 μm. The median diameter D50 of silver powder B is from 30 μm to 40 μm, preferably from 30 μm to 38 μm, and more preferably from 33 μm to 37 μm. By ensuring that the particle size range and median diameter D50 of silver powder B are within the above numerical ranges, the packing density of the silver powder in the conductive paste can be increased, the thickness when the conductive paste is applied to a Cu substrate or Cu electrode can be maintained within a sufficient range, and the thermal properties and electrical properties of the joint between the Cu substrate or Cu electrode and the conductor can be improved.

[0019] The shape of the silver powder B can be confirmed by observation with an SEM. The magnification of the SEM can be appropriately set to 5,000x, 10,000x, etc. so that the shape of the silver powder B, which has a particle size in the range of 25 μm to 100 μm and a median diameter D50 of 30 μm to 40 μm, can be seen. The shape of the silver powder B is not particularly limited, but may be either spherical or non-spherical. Examples of non-spherical shapes include flake, needle, angular, dendritic, granular, irregular, teardrop, plate, ultra-thin plate, hexagonal plate, columnar, rod, porous, fibrous, lumpy, spongy, hexagonal, rounded, or a combination thereof. Among these shapes, spherical, columnar, or a combination thereof is preferred.

[0020] The particle size range of the silver powder C is 10 nm or more and 190 nm or less, preferably 10 nm or more and 150 nm or less, and more preferably 30 nm or more and 120 nm or less. The median diameter D50 of the silver powder C is 50 nm or more and 150 nm or less, preferably 60 nm or more and 130 nm or less, and more preferably 70 nm or more and 120 nm or less.

[0021] The shape of the silver powder C can be confirmed by observation with an SEM. The magnification of the SEM can be appropriately set to 5,000x, 10,000x, etc. so that the shape of the silver powder C having a particle size in the range of 10 nm to 190 nm and a median diameter D50 of 50 nm to 150 nm can be seen. The shape of the silver powder C is not particularly limited, but examples include spherical, non-spherical (e.g., flake-like, acicular, angular, plate-like, etc.), and combinations thereof. Among these shapes, spherical, flake-like, acicular, plate-like, and combinations thereof are preferred.

[0022] The content of silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass, preferably 10.0 to 70.0 parts by mass, and more preferably 20.0 to 65.0 parts by mass, per 100 parts by mass of the total of flaky silver powder A, silver powder B, and silver powder C.

[0023] By ensuring that the particle size range, median diameter D50, and content of silver powder C are within the above-mentioned numerical ranges, the viscosity of the conductive paste and the packing density of the silver powder can be adjusted appropriately, the sintering of the conductive paste can be sufficiently promoted even at low temperatures, and the thermal and electrical properties of the joint between the Cu substrate or Cu electrode and the conductor can be improved.

[0024] Furthermore, by setting the content of silver powder C to 20.0 parts by mass or more and 65.0 parts by mass or less relative to 100 parts by mass of the total of flaky silver powder A, silver powder B, and silver powder C, it is possible to improve the bonding strength when bonding a substrate or electrode to another component such as an electronic component.

[0025] The content of the flaky silver powder A is not particularly limited, but is preferably 3 to 83 parts by mass, more preferably 10 to 70 parts by mass, and most preferably 20 to 60 parts by mass, per 100 parts by mass of the total of the flaky silver powder A, silver powder B, and silver powder C.

[0026] The content of silver powder B is not particularly limited, but is preferably 3 to 83 parts by mass, more preferably 10 to 75 parts by mass, and most preferably 15 to 60 parts by mass, per 100 parts by mass of the total of flaky silver powder A, silver powder B, and silver powder C.

[0027] The total content of flaky silver powder A, silver powder B, and silver powder C per 100 parts by mass of the conductive paste is preferably 40 to 98 parts by mass, more preferably 60 to 95 parts by mass, and most preferably 80 to 95 parts by mass.

[0028] Examples of the solvent contained in the conductive paste include water, alcohol-based solvents, glycol ether-based solvents, terpineols, and mixtures thereof. Examples of the alcohol-based solvent include isopropyl alcohol, 1,2-butanediol, isobornylcyclohexanol, 2,4-diethyl-1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 2,5-dimethyl-2,5-hexanediol, 2,5-dimethyl-3-hexyne-2,5-diol, 2,3-dimethyl-2,3-butanediol, 1,1,1-tris(hydroxymethyl)ethane, 2-ethyl-2-hydroxymethyl-1,3-propanediol, 2,2'-oxybis(methylene)bis(2-ethyl-1,3 2,2-bis(hydroxymethyl)-1,3-propanediol, 1,2,6-trihydroxyhexane, bis[2,2,2-tris(hydroxymethyl)ethyl]ether, 1-ethynyl-1-cyclohexanol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, erythritol, threitol, guaiacol glycerol ether, 3,6-dimethyl-4-octyne-3,6-diol, 2,4,7,9-tetramethyl-5-decyne-4,7-diol, or a mixture thereof. Among these, from the viewpoints of printability and wettability of the paste, ethylene glycol or ethanol is preferred, and ethylene glycol is more preferred. Examples of glycol ether solvents include diethylene glycol mono-2-ethylhexyl ether, ethylene glycol monophenyl ether, 2-methylpentane-2,4-diol, diethylene glycol monohexyl ether, diethylene glycol dibutyl ether, triethylene glycol monobutyl ether, or a mixture thereof. Terpineols include cajeput oil, pine oil, petitgrain oil, or mixtures thereof.

[0029] The content of the solvent per 100 parts by mass of the conductive paste is preferably 5 to 30 parts by mass, more preferably 5 to 15 parts by mass, and most preferably 5 to 10 parts by mass. By keeping the content of the solvent within the above range, it is possible to adjust the viscosity to an appropriate level that improves the coatability (printability) of the conductive paste.

[0030] [Second Embodiment (Conductive Paste)] The conductive paste of the second embodiment comprises: flaky silver powder A having a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 2 μm or more and 5 μm or less; spherical silver powder D having a bimodal particle size distribution with a peak (first peak) having a particle size in the range of 200 nm or more and 800 nm or less and a median diameter D50 of 300 nm or more and 600 nm or less and a peak (second peak) having a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 1.5 μm or more and 4 μm or less; silver powder C having a particle size in the range of 10 nm or more and 190 nm or less and a median diameter D50 of 50 nm or more and 150 nm or less; and a solvent, wherein the content of silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass relative to a total of 100 parts by mass of the flaky silver powder A, the spherical silver powder D, and the silver powder C. The conductive paste of the second embodiment contains flaky silver powder A, spherical silver powder D, and silver powder C, and because the content of silver powder C is within a specific range, the conductive paste has excellent bonding strength when bonding a Cu substrate or Cu electrode to other components such as electronic components. The conductive paste of the second embodiment may consist of flaky silver powder A, spherical silver powder D, and silver powder C as metal components, or may contain other metal components.

[0031] The particle size range, median diameter D50, and flake shape definition of flaky silver powder A, the particle size range, median diameter D50, and shape of silver powder C, and the type and content of solvent may be those shown in the first embodiment above.

[0032] The particle size range of the first peak of spherical silver powder D is 200 nm or more and 800 nm or less, preferably 200 nm or more and 700 nm or less, and more preferably 300 nm or more and 500 nm or less. The particle size range of the second peak of spherical silver powder D is 1 μm or more and 15 μm or less, preferably 1.5 μm or more and 10 μm or less, and more preferably 2 μm or more and 8 μm or less. The median diameter D50 of the first peak of spherical silver powder D is 300 nm or more and 600 nm or less, preferably 300 nm or more and 500 nm or less, and more preferably 350 nm or more and 450 nm or less. The median diameter D50 of the second peak of spherical silver powder D is 1.5 μm or more and 4 μm or less, preferably 1.5 μm or more and 3.5 μm or less, and more preferably 2 μm or more and 3 μm or less. By ensuring that the particle size ranges of the first peak and second peak and the median diameter D50 of the spherical silver powder D are within the above-mentioned numerical ranges, the viscosity of the conductive paste and the packing density of the silver powder can be adjusted to appropriate values, the sintering of the conductive paste can be sufficiently advanced, and the thermal and electrical properties of the joint between the Cu substrate or Cu electrode and the conductor can be improved.

[0033] The content of the first peak per 100 parts by mass of spherical silver powder D is preferably 5 to 50 parts by mass, more preferably 10 to 30 parts by mass, and most preferably 15 to 25 parts by mass. The content of the second peak per 100 parts by mass of spherical silver powder D is preferably 50 to 95 parts by mass, more preferably 70 to 90 parts by mass, and most preferably 75 to 85 parts by mass. When the contents of the first peak and the second peak are within the above numerical ranges, the packing density of the silver powder in the conductive paste can be increased, and a denser structure can be formed when the powder is sintered.

[0034] The shape of the spherical silver powder D can be confirmed by observation with an SEM. The magnification of the SEM can be appropriately set to 5000x, 10000x, etc. so that the shape of the spherical silver powder D can be seen. The shape of the spherical silver powder D is not limited to a perfect sphere, and it is sufficient if the aspect ratio (ratio of major axis / minor axis) is 1.0 to 1.3.

[0035] The content of silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass, preferably more than 10.0 parts by mass and less than 80.0 parts by mass, more preferably 20.0 to 70.0 parts by mass, and most preferably 25.0 to 60.0 parts by mass, relative to 100 parts by mass of the total of flaky silver powder A, spherical silver powder D, and silver powder C. By having the content of silver powder C within the above numerical range, the viscosity of the conductive paste and the packing density of the silver powder can be adjusted to appropriate values, sintering of the conductive paste can be sufficiently promoted even at low temperatures, and the thermal properties and electrical properties of the joint between the Cu substrate or Cu electrode and the conductor can be improved.

[0036] Furthermore, by setting the content of silver powder C to 25.0 parts by mass or more and 65.0 parts by mass or less relative to 100 parts by mass of the total of flaky silver powder A, spherical silver powder D, and silver powder C, it is possible to improve the bonding strength when bonding a substrate or electrode to another component such as an electronic component.

[0037] The content of the flaky silver powder A is not particularly limited, but is preferably 3 to 78 parts by mass, more preferably 10 to 65 parts by mass, and most preferably 20 to 55 parts by mass, per 100 parts by mass of the total of the flaky silver powder A, the spherical silver powder D, and the silver powder C.

[0038] The content of spherical silver powder D is not particularly limited, but is preferably 3 to 78 parts by mass, more preferably 10 to 65 parts by mass, and most preferably 20 to 55 parts by mass, per 100 parts by mass of the total of flaky silver powder A, spherical silver powder D, and silver powder C.

[0039] The total content of the flaky silver powder A, spherical silver powder D, and silver powder C per 100 parts by mass of the conductive paste is preferably 40 to 98 parts by mass, more preferably 60 to 95 parts by mass, and most preferably 80 to 95 parts by mass.

[0040] [Third embodiment (conductive paste)] The conductive paste of the third embodiment comprises flake silver powder A having a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 2 μm or more and 5 μm or less, silver powder B having a particle size in the range of 25 μm or more and 100 μm or less and a median diameter D50 of 30 μm or more and 40 μm or less, silver powder C having a particle size in the range of 200 nm or more and 800 nm or less and a peak (first peak) having a median diameter D50 of 300 nm or more and 600 nm or less, and silver powder D having a particle size in the range of 1 μm or more and 15 μm or less. a silver powder C having a particle size within the range of 10 nm or more and 190 nm or less and a median diameter D50 of 50 nm or more and 150 nm or less, and a solvent, wherein the content of the silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass per 100 parts by mass of the total of the flaky silver powder A, the silver powder B, the spherical silver powder D, and the silver powder C. The conductive paste of the third embodiment comprises flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C, and the content of silver powder C is within a specific range, thereby achieving excellent bonding strength when bonding a Cu substrate or a Cu electrode to another component such as an electronic component. The conductive paste of the third embodiment may be composed of flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C as metal components, or may contain other metal components.

[0041] The particle size range, median diameter D50, and flake shape definition of flaky silver powder A, the particle size range, median diameter D50, and shape of silver powder B, the particle size range and median diameter D50 of the first peak and second peak of spherical silver powder D, the definition of sphericity of spherical silver powder D, the contents of the first peak and second peak per 100 parts by mass of spherical silver powder D, the particle size range, median diameter D50, and shape of silver powder C, and the type and content of solvent can be those shown in the first embodiment and / or the second embodiment.

[0042] The content of silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass, preferably 10.0 to 80.0 parts by mass, and more preferably 20.0 to 65.0 parts by mass, relative to 100 parts by mass of the total of flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C. By having the content of silver powder C within the above numerical range, the viscosity of the conductive paste and the packing density of the silver powder can be adjusted to appropriate values, sintering of the conductive paste can be sufficiently promoted even at low temperatures, and the thermal properties and electrical properties of the joint between the Cu substrate or Cu electrode and the conductor can be improved.

[0043] Furthermore, by having the content of silver powder C be 20.0 parts by mass or more and 65.0 parts by mass or less relative to 100 parts by mass of the total of flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C, the bonding strength when bonding a substrate or electrode to another component such as an electronic component can be made even better.

[0044] The content of flaky silver powder A is not particularly limited, but is preferably 3 to 80 parts by mass, more preferably 10 to 70 parts by mass, and most preferably 15 to 65 parts by mass, per 100 parts by mass of the total of flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C.

[0045] The content of silver powder B is not particularly limited, but is preferably 3 to 80 parts by mass, more preferably 10 to 70 parts by mass, and most preferably 15 to 50 parts by mass, per 100 parts by mass of the total of flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C.

[0046] The content of spherical silver powder D is not particularly limited, but is preferably 3 to 80 parts by mass, more preferably 10 to 70 parts by mass, and most preferably 20 to 60 parts by mass, per 100 parts by mass of the total of flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C.

[0047] The total content of flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C per 100 parts by mass of the conductive paste is preferably 40 to 98 parts by mass, more preferably 60 to 95 parts by mass, and most preferably 80 to 95 parts by mass.

[0048] In the conductive pastes of the first to third embodiments, the particle size range and median diameter D50 of each silver powder are measured by laser diffraction using a SALD-2300 (manufactured by Shimadzu Corporation) under the following conditions. (Measurement conditions) 0.1 g of silver powder is dissolved in 10 ml of ethanol and measured while being dispersed ultrasonically. The obtained measurement results are analyzed using software WingSALD II (manufactured by Shimadzu Corporation) to obtain the particle size range and median diameter D50.

[0049] The conductive pastes of the first to third embodiments can be prepared by kneading the above-mentioned silver powders and solvents using, for example, an ARV-310 device (manufactured by Thinky Corporation).

[0050] The conductive pastes of the first to third embodiments can be used for both sintering in a nitrogen atmosphere and sintering in an air atmosphere. The conductive pastes of the first to third embodiments can be applied directly to a Cu substrate or a Cu electrode without a plating layer and sintered to bond a conductor to the Cu substrate or the Cu electrode. The atmospheric conditions for sintering may be a vacuum, a nitrogen atmosphere, or an air atmosphere. The conductive pastes of the first to third embodiments can be used to bond a Cu substrate or a Cu electrode to a conductor in a nitrogen atmosphere without a plating layer.

[0051] A conductive paste can be applied (printed) onto a Cu substrate or onto a Cu electrode provided on a Cu substrate, and then sintered. The sintered body obtained by sintering the conductive paste is conductive and therefore functions as wiring (electronic circuit) on the Cu substrate or Cu electrode, or as a conductive joint (adhesive part). The method for applying (printing) the conductive paste is not particularly limited, but examples include dispense application, screen printing, spray application, brush application, injection, and combinations thereof.

[0052] The Cu substrate is not particularly limited, and examples thereof include ceramic substrates such as DBC (Direct Bonded Copper), Cu lead frames, etc. Other components such as electronic components can be mounted on the Cu substrate. It is preferable to remove the copper oxide layer present on the surface of the Cu substrate or Cu electrode by cleaning, polishing, etc., before applying the conductive paste and sintering. By removing the copper oxide layer present on the surface of the Cu substrate or Cu electrode, sufficient bonding can be achieved between the Cu substrate or Cu electrode and the sintered body of the conductive paste. Although an Ag plating layer can be provided on the Cu substrate or Cu electrode, it is preferable not to provide a plating layer from the viewpoints of reducing costs and shortening the manufacturing process.

[0053] The conductive paste of the present invention does not require a plating layer, and even when applied directly to the surface of a Cu substrate or Cu electrode from which the copper oxide layer has been removed and sintered, sufficient bonding can be obtained at the interface between the Cu substrate or Cu electrode and the sintered body. Of course, the conductive paste of the present invention can also provide sufficient bonding to the Cu substrate or Cu electrode with equal or greater bonding strength when applied to the Cu substrate or Cu electrode via an Ag plating layer. When the conductive paste is applied to a Cu substrate or Cu electrode via an Ag plating layer and then sintered in air, the presence of the plating layer prevents the formation of a copper oxide layer on the surface of the Cu substrate or Cu electrode, and sintering proceeds more easily than in a nitrogen atmosphere, resulting in superior bonding strength at the interface between the Cu substrate or Cu electrode and the sintered body of the conductive paste.

[0054] The atmospheric conditions for sintering the conductive paste are not particularly limited, and can be a nitrogen atmosphere, air atmosphere, vacuum, or the like. Among these, sintering in a nitrogen atmosphere is preferred from the viewpoint of suppressing the formation of a copper oxide layer due to oxidation of Cu and improving the bonding strength with the Cu substrate or Cu electrode. The heating temperature for sintering the conductive paste is not particularly limited, but from the viewpoint of obtaining a sintered body with a dense structure, it is preferably 150 to 350°C, more preferably 180 to 300°C, and most preferably 200 to 300°C. The heating time for sintering the conductive paste is not particularly limited, but from the viewpoint of obtaining a sintered body with a dense structure, it is preferably 1 minute or more and less than 100 minutes, more preferably 5 minutes or more and less than 60 minutes, and most preferably 10 minutes or more and less than 45 minutes. The pressure conditions for sintering the conductive paste are not particularly limited, but can be either no pressure or pressure.

[0055] The shape of the wiring or joint obtained by sintering the conductive paste is not particularly limited, but examples include sheet-like, film-like, tape-like, linear, disk-like, block-like, spot-like, irregular-shaped, and combinations thereof.

[0056] The electrical resistance of the wiring or joint obtained by sintering the conductive paste is preferably 8.5 μΩ cm or less, more preferably 7.0 μΩ cm or less, and most preferably 5.0 μΩ cm or less. The electrical resistance of the sintered body can be measured by the method described later in "1. Evaluation Methods" - "(2) Evaluation of Electrical Resistance Value."

[0057] The conductive paste may be a laminate including a Cu substrate or a Cu electrode, the conductive paste of any one of the first to third embodiments, and a conductor. The Cu substrate or the Cu electrode and the conductor can be joined by a method including the steps of: applying the conductive paste of any one of the first to third embodiments to a Cu substrate or a Cu electrode; overlaying a conductor on the conductive paste on the Cu substrate or the Cu electrode to form a laminate including the Cu substrate or the Cu electrode, the conductive paste, and the conductor; and heating the laminate at a temperature of 150 to 350°C for 1 minute or more and less than 100 minutes to sinter the conductive paste.

[0058] The sintered body obtained by sintering the conductive paste is conductive and functions as a conductive joint (adhesive joint) because it bonds the Cu substrate or Cu electrode to the conductor. The bonded conductor may be a chip component such as a capacitor or resistor, a chip obtained by cutting out sections of each semiconductor element from a wafer on which semiconductor elements such as resistors, transistors, capacitors, and integrated circuits are formed, or a combination thereof.

[0059] In the above-mentioned method for bonding to a Cu substrate or Cu electrode, the conditions for sintering the conductive paste (atmospheric conditions, heating temperature, heating time, pressure conditions, etc.) are not particularly limited, and the above-mentioned conditions can be used.

[0060] The shear strength of the joint between the Cu substrate or Cu electrode and the conductor is preferably 12.0 MPa or more, more preferably 15.0 MPa or more, more preferably 20.0 MPa or more, and most preferably 30.0 MPa or more. In the present invention, the shear strength means a strength measured by a method conforming to JIS Z 3198, and the bond strength (shear strength) of the laminate after sintering can be measured by the method described later in "1. Evaluation Methods" - "(1) Evaluation of Shear Strength".

[0061] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the contents described in the examples.

[0062] 1. Evaluation method (1) Evaluation of shear strength

[0063] (1-1) Shear Strength of Examples 1 to 21 and Comparative Examples 1 to 23 A copper substrate (length x width: 50 mm x 50 mm) was immersed in an ethanol solution containing 5% by weight of hydrochloric acid and left in the immersion state for 5 minutes. The immersed copper substrate was removed from the ethanol solution and left at room temperature (25°C) for 1 minute to remove the ethanol solution and dry, yielding a washed copper substrate (hereinafter referred to as test piece (1)). The copper oxide on the surface of the test piece (1) thus obtained was removed. A conductive paste was screen-printed directly onto the surface of the test piece (1) using a metal mask (thickness: 100 μm). Meanwhile, a Si chip (length x width: 3 mm x 3 mm) (corresponding to an electronic component) (hereinafter referred to as test piece (2)) was prepared, on which, starting from the Si chip side, a Ti layer (thickness: 40 nm) and an Ag layer (thickness: 1000 nm) were laminated by sputtering. Then, using a chip mounter (product name SMT-64RH, manufactured by Okuhara Electric Co., Ltd.), the Ag layer side of test piece (2) was laminated on the conductive paste applied to test piece (1), thereby obtaining a sample including test piece (1), conductive paste, and test piece (2). The obtained sample was heated at 250°C for 60 minutes under a nitrogen atmosphere and atmospheric pressure, and sintered. A schematic diagram of the obtained sintered sample is shown in Figure 1. Then, for the sintered sample, the shear strength (MPa) between test piece (1) and test piece (2) was measured at room temperature (25°C) using a universal bond tester Nordson DAGE Series 4000 (manufactured by Nordson Corporation) according to a method in accordance with JIS Z 3198. Eight measurements were performed for each sample, and the average shear strength (MPa) was calculated. If the average shear strength of the samples is 12.0 MPa or more, it can be said that the bondability between the test pieces is good. Also, if the average shear strength of the samples is 5.0 MPa or less, the test pieces are not sufficiently bonded to each other, and the measured values ​​vary, so there is no significant difference in the calculated average values. Therefore, it is expressed as 5.0 MPa or less rather than a specific numerical value.However, in Comparative Example 7, in which ethanol was used as the solvent, when a chip mounter was used, the solvent evaporated before the test piece (2) was laminated on the conductive paste, making it impossible to bond the test piece (2), and when sintering was performed in a nitrogen atmosphere under atmospheric pressure, the solvent evaporated too quickly to bond the test piece (2). Therefore, in Comparative Example 7, the test piece (2) was quickly laminated on the conductive paste by hand, the pressure during sintering was set to 0.4 MPa, and the other conditions were the same as above to obtain a sintered sample, and the shear strength was measured.

[0064] (1-2) Shear Strength of Examples 22 to 24 In Example 22, a copper substrate (length x width dimensions: 50 mm x 50 mm) (hereinafter referred to as test piece (1')) on which a Ti layer (thickness 40 nm) and an Ag layer (thickness 1000 nm) (corresponding to a plating layer) were laminated by sputtering, in this order from the copper substrate side, was used instead of test piece (1). A sintered sample was obtained in the same manner as in Examples 1 to 21 and Comparative Examples 1 to 23. The shear strength (MPa) between test piece (1') and test piece (2) of the sample was measured. In Example 23, a sintered sample was obtained in the same manner as in Examples 1 to 21 and Comparative Examples 1 to 23, except that an air atmosphere was used instead of a nitrogen atmosphere as the atmospheric condition during sintering. The shear strength (MPa) between test piece (1) and test piece (2) of the sample was measured. In Example 24, the above-mentioned test piece (1') was used instead of the test piece (1), and the sintering was carried out in the same manner as in Examples 1 to 21 and Comparative Examples 1 to 23, except that an air atmosphere was used instead of a nitrogen atmosphere as the atmospheric condition during sintering. The shear strength (MPa) between the test piece (1') and the test piece (2) in the sample was measured.

[0065] (1-3) Shear Strength of Examples 25 to 33 In Examples 25 to 27, sintered samples were obtained in the same manner as in Example 3, except that the sintering temperature was 200°C, 280°C, and 300°C, respectively, instead of 250°C. The shear strength (MPa) between the test piece (1) and the test piece (2) of the samples was measured. In Examples 28 to 30, sintered samples were obtained in the same manner as in Example 10, except that the sintering temperature was 200°C, 280°C, and 300°C, respectively, instead of 250°C. The shear strength (MPa) between the test piece (1) and the test piece (2) of the samples was measured. In Examples 31 to 33, sintered samples were obtained in the same manner as in Example 17, except that the sintering temperature was 200°C, 280°C, and 300°C, respectively, instead of 250°C. The shear strength (MPa) between the test piece (1) and the test piece (2) of the samples was measured.

[0066] (2) Evaluation of Electrical Resistance Values ​​The conductive pastes of Examples 3, 10, and 17, and Comparative Example 13, were applied to glass substrates (2 mm thick) by screen printing using a metal mask (100 μm thick). The glass substrates coated with the conductive pastes were heated at 250°C for 30 minutes in an air atmosphere and at atmospheric pressure to form sintered wiring. The wiring formed from the sintered body of the conductive paste thus obtained had dimensions of 3 mm x 30 mm. A schematic diagram of the resulting wiring is shown in FIG. 2. The thickness of the wiring formed from the sintered body of Examples 3, 10, and 17, and Comparative Example 13 thus obtained was measured using a standard outside micrometer M110 (manufactured by Mitutoyo Corporation) at room temperature (25°C) and a relative humidity of 50%. The electrical resistance (μΩ cm) of each wiring was measured by a four-terminal method using a lozesta-GP MCP-T610 (manufactured by Mitsubishi Chemical Corporation) under conditions of room temperature (25°C) and relative humidity of 50%. An ESP probe (manufactured by Mitsubishi Chemical Corporation, MCP-TP08P, No. 014052B) was used as the four-terminal probe for measuring the electrical resistance. Measurements were performed three times for each wiring, and the average electrical resistance (μΩ cm) was calculated. If the average electrical resistance of the wiring is 8.5 μΩ cm or less, it can be said that the conductivity is good.

[0067] 2. Regarding the Silver Powder: The flake silver powder used was a commercially available product named AgC239 (manufactured by Fukuda Metal Foil & Powder Co., Ltd., particle size range: 1 μm to 15 μm, median diameter D50: 2.5 μm, flake shape) (hereinafter referred to as "silver powder (A)"). An image of silver powder (A) taken with a field emission scanning electron microscope (FE-SEM) (JEOL Ltd., JSM-7000F) is shown in FIG. 3A. FIG. 3A is an image of the surface of silver powder (A) taken at a magnification of 5000x.

[0068] In addition, non-flake silver powder having a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 2 μm or more and 5 μm or less (hereinafter referred to as "silver powder (A')"), silver powder having a particle size in the range of 25 μm or more and 100 μm or less and a median diameter D50 of 30 μm or more and 40 μm or less (hereinafter referred to as "silver powder (B)"), silver powder having a particle size in the range less than 25 μm and a median diameter D50 of less than 30 μm (hereinafter referred to as "silver powder (B')"), spherical silver powder having a bimodal particle size distribution (hereinafter referred to as "silver powder (D)"), and silver powder having a particle size in the range of 10 nm or more and 190 nm or less and a median diameter D50 of 50 nm or more and 150 nm or less (hereinafter referred to as "silver powder (C)") were prepared by the following procedures.

[0069] Production of Silver Powder (A') Silver powder (A') was produced by the following production method. 1 g of commercially available silver powder S211A-10 (manufactured by Daiken Chemical Co., Ltd., median diameter D50: 0.54 μm, spherical (non-spherical)) and 1 g of polyvinylpyrrolidone were dispersed in 150 g of ethylene glycol. The resulting dispersion was heated to 150°C (reaction temperature), and then 20 g of ethylene glycol solution (containing 5 g of silver nitrate) was added and mixed. The resulting mixture was reacted for 3 hours while stirring at a rotation speed of 500 rpm while maintaining the temperature at 150°C. The resulting reaction solution was cooled to room temperature, and the precipitate (including silver powder) contained in the cooled reaction solution was separated using a centrifuge. The separated precipitate was washed three times with ethanol to obtain silver powder (A').

[0070] - Production of Silver Powder (B) A spherical silver powder (silver powder (B)) with a particle size in the range of 25 μm or more and 100 μm or less and a median diameter D50 of 36.34 μm was produced by a method in which silver was heated to a molten state, and the molten silver was sprayed and cooled (melt spray method).

[0071] Production of Silver Powder (B') A spherical silver powder (silver powder (B')) with a particle size of less than 25 μm and a median diameter D50 of 13.67 μm was produced by a method in which silver was heated to a molten state, and the molten silver was sprayed and cooled (melt spray method).

[0072] Images of the resulting silver powders (A') and (B) taken with a field emission scanning electron microscope (FE-SEM) (JEOL Ltd., JSM-7000F) are shown in Figures 3B and 4, respectively. Figure 3B is an image of the surface of silver powder (A') taken at a magnification of 5000x, and Figure 4 is an image of the surface of silver powder (B) taken at a magnification of 1000x.

[0073] - Production of Silver Powder (D) A spherical silver powder (silver powder (D)) having a bimodal particle size distribution with a peak (first peak) having a particle size in the range of 200 nm or more and 800 nm or less and a median diameter D50 of 300 nm or more and 600 nm or less, and a peak (second peak) having a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 1.5 μm or more and 4 μm or less was produced by the following production method.

[0074] 1.0 g of silver nitrate (a silver compound) was dissolved in 50 g of ethylene glycol (a polyhydric alcohol), and 0.5 g of polyvinylpyrrolidone (a dispersant) was dispersed in the ethylene glycol. The resulting ethylene glycol solution was heated to 150°C (reaction temperature), and then reacted for 1 hour while maintaining the temperature at 150°C and stirring at a rotation speed of 500 rpm. The silver powder contained in the resulting reaction solution was separated from the ethylene glycol (a polyhydric alcohol) using a metal mesh, and the separated silver powder was washed with ethanol to obtain silver powder (I). 2.0 g of silver nitrate was dissolved in 50 g of ethylene glycol while stirring at a rotation speed of 500 rpm, and 0.05 g of silver powder (I) was further dispersed in the ethylene glycol. The resulting ethylene glycol solution was heated to 150°C (reaction temperature), and then reacted for 1 hour while maintaining the reaction temperature at 150°C and stirring at a rotation speed of 500 rpm. The silver powder contained in the resulting reaction solution was separated from the ethylene glycol using a metal mesh, and the separated silver powder was washed with ethanol to obtain silver powder (D).

[0075] An image of the obtained silver powder (D) taken with a field emission scanning electron microscope (FE-SEM) (JSM-7000F manufactured by JEOL Ltd.) is shown in Figure 5. Figure 5 shows an image of the surface of silver powder (D) taken at a magnification of 5000 times. The contents of the first peak and the second peak per 100 parts by mass of silver powder (D) were approximately 20 parts by mass and approximately 80 parts by mass, respectively.

[0076] - Production of Silver Powder (C) Silver powder (C) was produced by the following production method. 0.25 g of polyvinylpyrrolidone (PVP), 100 g of pure water, 1.1 g of silver nitrate, and 150 g of N,N-dimethylformamide (DMF) were charged into a reaction vessel and mixed. The resulting mixture was heated to 100°C and then reacted for 1 hour while maintaining the temperature at 100°C and stirring at a rotation speed of 400 rpm. The resulting reaction solution was cooled to room temperature, and the precipitate (including silver powder) contained in the cooled reaction solution was separated using a centrifuge. The separated precipitate was washed three times with ethanol to obtain silver powder (C).

[0077] Images of the obtained silver powder (C) taken with a field emission scanning electron microscope (FE-SEM) (JEOL Ltd., JSM-7000F) are shown in Figures 6A and 6B. Figures 6A and 6B are images of the surface of silver powder (C) taken at magnifications of 5,000x and 20,000x, respectively.

[0078] The particle size distributions of silver powders (C) and (D) were measured by laser diffraction using a SALD-2300 (Shimadzu Corporation) under the following conditions, and are shown in Figure 7. (Measurement conditions) 0.1 g of silver powder was dissolved in 10 ml of ethanol, and the solution was dispersed ultrasonically while the measurement was performed. The measurement results were analyzed using software WingSALD II (Shimadzu Corporation) to obtain the particle size distribution.

[0079] The particle size ranges and median diameters D50 of the silver powders (A), (A'), (B), (B'), (D), and (C) used in the present examples and comparative examples were measured by laser diffraction using a SALD-2300 (manufactured by Shimadzu Corporation) under the following conditions. (Measurement conditions) 0.1 g of silver powder was dissolved in 10 ml of ethanol and dispersed ultrasonically while measurements were performed. The obtained measurement results were analyzed using software WingSALD II (manufactured by Shimadzu Corporation) to obtain the particle size ranges and median diameters D50.

[0080] 3. Production of Conductive Paste Conductive pastes of Examples 1 to 33 and Comparative Examples 1 to 23 were prepared with the compositions shown in the following Tables 1 to 8. The numerical values ​​of each component in the following Tables 1 to 8 represent the parts by mass of each component when the total amount of silver powder is taken as 100 parts by mass.

[0081] (1) Examples 1 to 21 and Comparative Examples 1 to 23 The above-described shear strength evaluation was carried out for each of the conductive pastes of Examples 1 to 21 and Comparative Examples 1 to 23. The evaluation results are shown in Tables 1 to 6 below.

[0082]

[0083]

[0084]

[0085]

[0086]

[0087]

[0088] The conductive pastes of Examples 1 to 7 shown in Table 1 contain: flake silver powder A having a particle size of 1 μm to 15 μm and a median diameter D50 of 2 μm to 5 μm; silver powder B having a particle size of 25 μm to 100 μm and a median diameter D50 of 30 μm to 40 μm; silver powder C having a particle size of 10 nm to 190 nm and a median diameter D50 of 50 nm to 150 nm; and a solvent, in which the content of silver powder C is greater than 5.0 parts by mass and less than 90.0 parts by mass per 100 parts by mass of the total of flake silver powder A, silver powder B, and silver powder C. The results in Table 1 above demonstrate that the conductive pastes of Examples 1 to 7 produce sintered bodies with excellent shear strength and good bondability. Among these, the sintered bodies of Examples 2 to 6 exhibited shear strengths of 23.8 MPa or more, demonstrating even better bondability. Examples 2 to 6 are conductive pastes in which the content of silver powder C is 20.0 parts by mass or more and 65.0 parts by mass or less per 100 parts by mass of the total of flaky silver powder A, silver powder B, and silver powder C.

[0089] The conductive pastes of Examples 8 to 14 shown in Table 2 contain flaky silver powder A, spherical silver powder D, silver powder C, and a solvent, each of which has a bimodal particle size distribution with a peak having a particle size in the range of 200 nm to 800 nm and a median diameter D50 of 300 nm to 600 nm, and a peak having a particle size in the range of 1 μm to 15 μm and a median diameter D50 of 1.5 μm to 4 μm. The content of silver powder C is greater than 5.0 parts by mass and less than 90.0 parts by mass per 100 parts by mass of the total of flaky silver powder A, spherical silver powder D, and silver powder C. The results in Table 2 above demonstrate that the conductive pastes of Examples 8 to 14 produce sintered bodies with excellent shear strength and good bondability. Among these, the sintered bodies of Examples 10 to 13 exhibited shear strengths of 30.1 MPa or more, demonstrating even better bondability. Examples 10 to 13 are conductive pastes in which the content of silver powder C is 25.0 parts by mass or more and 65.0 parts by mass or less per 100 parts by mass of the total of flaky silver powder A, spherical silver powder D, and silver powder C.

[0090] The conductive pastes of Examples 15 to 21 shown in Table 3 contain flaky silver powder A, silver powder B, spherical silver powder D, silver powder C, and a solvent, and the content of silver powder C is greater than 5.0 parts by mass and less than 90.0 parts by mass per 100 parts by mass of the total of flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C. The results in Table 3 above demonstrate that the conductive pastes of Examples 15 to 21 produce sintered bodies with excellent shear strength and good bondability. Among these, the sintered bodies of Examples 16 to 20 exhibited a shear strength of 28.9 MPa or more, demonstrating even better bondability. Examples 16 to 20 are conductive pastes in which the content of silver powder C is greater than or equal to 20.0 parts by mass and less than or equal to 65.0 parts by mass per 100 parts by mass of the total of flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C.

[0091] On the other hand, Comparative Examples 1 to 4 shown in Table 4 are conductive pastes containing only one of flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C. Comparative Examples 5 to 7 and 12 to 15 shown in Table 5 are conductive pastes containing flaky silver powder A but not silver powder C. Comparative Example 8 shown in Table 5 is a conductive paste containing neither flaky silver powder A nor silver powder C. Comparative Examples 9 to 11 and 16 shown in Table 5 are conductive pastes containing silver powder C but not flaky silver powder A. Note that Comparative Examples 9 to 11 contain a silver powder (silver powder (B')) having a median diameter D50 of less than 30 μm, rather than silver powder B. Furthermore, Comparative Example 16 contains a non-flake silver powder (silver powder (A')), rather than flaky silver powder A. Comparative Example 17 shown in Table 5 is a conductive paste containing flaky silver powder A and silver powder C, but not silver powder B or spherical silver powder D. From the results in Tables 4 and 5 above, it can be seen that the conductive pastes of Comparative Examples 1 to 17 had low shear strength and poor bondability for the sintered bodies.

[0092] Comparative Examples 18 and 19 shown in Table 6 are conductive pastes containing flaky silver powder A, silver powder B, and silver powder C, but with a silver powder C content of 5.0 parts by mass or less or 90.0 parts by mass or more. Comparative Examples 20 and 21 shown in Table 6 are conductive pastes containing flaky silver powder A, spherical silver powder D, and silver powder C, but with a silver powder C content of 5.0 parts by mass or less or 90.0 parts by mass or more. Comparative Examples 22 and 23 shown in Table 6 are conductive pastes containing flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C, but with a silver powder C content of 5.0 parts by mass or less or 90.0 parts by mass or more. As can be seen from the results in Table 6 above, the conductive pastes of Comparative Examples 18 to 23 had lower shear strength of the sintered body and inferior bondability compared to the conductive pastes of Examples 1 to 21.

[0093] FIG. 8 shows a graph of the experimental data on shear strength for Examples 1 to 7 and Comparative Examples 18 and 19 relating to a conductive paste containing flaky silver powder A, silver powder B, silver powder C, and a solvent (hereinafter referred to as "conductive paste (1)"), Examples 8 to 14 and Comparative Examples 20 and 21 relating to a conductive paste containing flaky silver powder A, spherical silver powder D, silver powder C, and a solvent (hereinafter referred to as "conductive paste (2)"), and Examples 15 to 21 and Comparative Examples 22 and 23 relating to a conductive paste containing flaky silver powder A, silver powder B, spherical silver powder D, silver powder C, and a solvent (hereinafter referred to as "conductive paste (3)")

[0094] (2) Examples 22 to 24 The above-described shear strength was evaluated for each of the conductive pastes of Examples 22 to 24. The evaluation results are shown in Table 7 below.

[0095]

[0096] The conductive pastes of Examples 22 to 24 shown in Table 7 have the same composition as the conductive paste of Example 3. As described in "(1) Evaluation of Shear Strength" in "1. Evaluation Method" above, Example 22 uses Test Piece (1') instead of Test Piece (1). Example 23 uses an air atmosphere instead of a nitrogen atmosphere as the atmospheric condition during sintering. Example 24 uses Test Piece (1') instead of Test Piece (1), and an air atmosphere instead of a nitrogen atmosphere as the atmospheric condition during sintering. A comparison of Example 3 in Table 1 above with Examples 22 to 24 in Table 7 above reveals that when a copper substrate (Test Piece (1')) having an Ag layer instead of a copper substrate (Test Piece (1)) with no Ag plating layer and with copper oxide removed from the surface was used, and when an air atmosphere was used instead of a nitrogen atmosphere as the atmospheric condition during sintering, the sintered body exhibited even superior shear strength and better bondability. Among them, in particular, Example 24, which used the above-mentioned test piece (1') and sintered under air atmosphere conditions, showed a sintered body that exhibited remarkably excellent shear strength and extremely good bondability. From the above, it was found that the conductive paste of the present invention, which shows excellent shear strength when a copper substrate without an Ag plating layer is used and a nitrogen atmosphere is used as the atmospheric condition for sintering, also shows similar or even better shear strength when a copper substrate with an Ag layer is used or when air atmosphere is used as the atmospheric condition for sintering.

[0097] (3) Examples 25 to 33 The above-described shear strength evaluation was performed on each of the conductive pastes of Examples 25 to 33. The evaluation results are shown in Table 8 below.

[0098]

[0099] As described in "(1) Evaluation of Shear Strength" in "1. Evaluation Method" above, the conductive pastes of Examples 25 to 33 have the same composition as the conductive pastes of Examples 3, 10, and 17, respectively, but the sintering temperature was 200°C, 280°C, or 300°C instead of 250°C. The shear strength measurement results for Examples 3, 10, 17, and 25 to 33 are summarized in a graph shown in Figure 9 for the above-mentioned conductive pastes (1) to (3). From the results of Examples 3, 10, 17, and 25 to 33 shown in Tables 1 to 3 and 8, as well as Figure 9, it was found that good shear strength was exhibited at any sintering temperature between 200 and 300°C. In particular, it was confirmed that sintering proceeded even at a low sintering temperature of 200°C, and sufficient shear strength was exhibited.

[0100] (4) Electrical Resistance Value of Wiring Made of Sintered Body (Examples 3, 10, and 17, and Comparative Example 13) As described in "(2) Evaluation of Electrical Resistance Value" in "1. Evaluation Method" above, the electrical resistance value of wiring made of the sintered body was also evaluated for Examples 3, 10, and 17, and Comparative Example 13. The evaluation results are shown in Table 9 below.

[0101]

[0102] The results in Table 9 above show that the conductive pastes of Examples 3, 10, and 17 had low electrical resistance values ​​for the wiring formed by the sintered body and had good electrical conductivity. On the other hand, the conductive paste of Comparative Example 13 had high electrical resistance values ​​for the wiring formed by the sintered body and had insufficient electrical conductivity.

[0103] (5) State of each sample Images of the shear strength measurement sample of Example 11 taken with a field emission scanning electron microscope (FE-SEM) (JSM-7000F manufactured by JEOL Ltd.) are shown in Figures 10A to 10C. Figures 10A to 10C are images of the cross section of the sample of Example 11 taken at magnifications of 3000x, 10000x, and 10000x, respectively. Figures 10A to 10C confirm that the silver particles were in sufficient contact with and fused to each other, resulting in a good sintered body, and that the sintered body provided sufficient bonding between the Si chip and the copper substrate.

[0104] Furthermore, the shear strength measurement sample of Example 3 was measured using SEM energy dispersive X-ray analysis (EDS) at an accelerating voltage of 25 kV, and an image of elemental mapping is shown in Figure 11. As can be seen from Figure 11, oxygen atoms were not concentrated at the interface between the copper substrate located at the bottom of Figure 11 and the sintered body located at the center of Figure 11, and no copper atoms were observed at this interface. From the above, it was confirmed that copper oxide was not formed at this interface, and that strong bonding was achieved between the copper substrate and the sintered body.

[0105] From the above results, (1) a conductive paste containing flaky silver powder A, silver powder B, silver powder C, and a solvent, in which the content of silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass per 100 parts by mass of the total of flaky silver powder A, silver powder B, and silver powder C, and (2) a conductive paste containing flaky silver powder A, spherical silver powder D, silver powder C, and a solvent, in which the content of silver powder C is 5.0 parts by mass and less than 90.0 parts by mass per 100 parts by mass of the total of flaky silver powder A, spherical silver powder D, and silver powder C, It has been found that (1) a conductive paste containing more than 5.0 parts by mass and less than 90.0 parts by mass of flaky silver powder A, silver powder B, spherical silver powder D, silver powder C, and a solvent, and having a content of silver powder C of more than 5.0 parts by mass and less than 90.0 parts by mass per 100 parts by mass of the total of flaky silver powder A, silver powder B, spherical silver powder D, and silver powder C, exhibits good bonding properties when applied to a Cu substrate or Cu electrode and sintered.

Claims

1. Flaky silver powder A having a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 2 μm or more and 5 μm or less; silver powder B having a particle size in the range of 25 μm or more and 100 μm or less and a median diameter D50 of 30 μm or more and 40 μm or less; silver powder C having a particle size in the range of 10 nm or more and 190 nm or less and a median diameter D50 of 50 nm or more and 150 nm or less; and a solvent, wherein the content of the silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass with respect to a total of 100 parts by mass of the flaky silver powder A, the silver powder B, and the silver powder C. A conductive paste characterized by the above.

2. Flaky silver powder A having a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 2 μm or more and 5 μm or less; spherical silver powder D having a bimodal particle size distribution having a peak with a particle size in the range of 200 nm or more and 800 nm or less and a median diameter D50 of 300 nm or more and 600 nm or less and a peak with a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 1.5 μm or more and 4 μm or less; silver powder C having a particle size in the range of 10 nm or more and 190 nm or less and a median diameter D50 of 50 nm or more and 150 nm or less; and a solvent, wherein the content of the silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass with respect to a total of 100 parts by mass of the flaky silver powder A, the spherical silver powder D, and the silver powder C. A conductive paste characterized by the above.

3. Flaky silver powder A having a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 2 μm or more and 5 μm or less, silver powder B having a particle size in the range of 25 μm or more and 100 μm or less and a median diameter D50 of 30 μm or more and 40 μm or less, a spherical silver powder D having a bimodal particle size distribution having a peak with a particle size in the range of 200 nm or more and 800 nm or less and a median diameter D50 of 300 nm or more and 600 nm or less, and a peak with a particle size in the range of 1 μm or more and 15 μm or less and a median diameter D50 of 1.5 μm or more and 4 μm or less, silver powder C having a particle size in the range of 10 nm or more and 190 nm or less and a median diameter D50 of 50 nm or more and 150 nm or less, and a solvent, wherein the content of the silver powder C is more than 5.0 parts by mass and less than 90.0 parts by mass with respect to a total of 100 parts by mass of the flaky silver powder A, the silver powder B, the spherical silver powder D, and the silver powder C. A conductive paste characterized by that.

4. The conductive paste according to claim 1, wherein the content of the silver powder C is 20.0 parts by mass or more and 65.0 parts by mass or less with respect to a total of 100 parts by mass of the flaky silver powder A, the silver powder B, and the silver powder C.

5. The conductive paste according to claim 2, wherein the content of the silver powder C is 25.0 parts by mass or more and 65.0 parts by mass or less with respect to a total of 100 parts by mass of the flaky silver powder A, the spherical silver powder D, and the silver powder C.

6. The conductive paste according to claim 3, wherein the content of the silver powder C is 20.0 parts by mass or more and 65.0 parts by mass or less with respect to a total of 100 parts by mass of the flaky silver powder A, the silver powder B, the spherical silver powder D, and the silver powder C.

7. A laminate including a Cu substrate or a Cu electrode, the conductive paste according to any one of claims 1 to 6, and a conductor.

8. A method for joining a Cu substrate or a Cu electrode and a conductor, comprising: a step of applying the conductive paste according to any one of claims 1 to 6 to the Cu substrate or the Cu electrode; a step of overlapping the conductor on the conductive paste on the Cu substrate or the Cu electrode to form a laminate including the Cu substrate or the Cu electrode, the conductive paste, and the conductor; and a step of sintering the conductive paste by heating the laminate at a temperature of 150 to 350 °C for a time of 1 minute or more and less than 100 minutes. The method characterized by that.