Bonding composition, method for producing bonded structure, and method for temporarily securing body to be bonded

Abstract

Provided is a bonding composition with which it is possible to improve tackiness between a coating film and a body to be bonded and obtain, through a simple method, a bonded structure having high bonding strength. Also provided are a method for producing a bonded structure, and a method for temporarily securing a body to be bonded, in which the aforementioned bonding composition is used. The bonding composition contains (A) copper powder, (B) a solvent, and (C) carboxylic acid. The solvent contains a liquid organic substance that has a boiling point of 230-300°C and has a viscosity of at least 100 mPa·s at a temperature of 25°C and a shear rate of 10 s−1. The carboxylic acid contains a C3 or higher polycarboxylic acid.

Description

Bonding composition, method for manufacturing bonded structure, and method for temporarily fixing objects to be bonded

[0001] The present invention relates to a bonding composition, a method for producing a bonded structure, and a method for temporarily fixing objects to be bonded.

[0002] In recent years, semiconductor elements (semiconductor chips) known as power devices have been widely used as power conversion and control devices, such as inverters. During mounting, semiconductor elements are bonded to a substrate, such as a circuit board or ceramic substrate, or a wiring body, such as a lead frame. Meanwhile, solder has traditionally been widely used for mounting semiconductor elements. Furthermore, with the recent rise in environmental awareness, there has been a demand for the use of lead-free solder.

[0003] However, power devices generate a lot of heat during operation due to the nature of controlling high currents, and lead-free solder has low heat resistance, making it insufficient for mounting power devices.

[0004] Therefore, it has been proposed to use a paste-like bonding composition containing a metal powder such as copper powder instead of solder. When a bonding composition is used to bond a pair of objects to be bonded, consisting of a substrate and a semiconductor element, for example, the bonding composition is applied to one surface of one of the objects to be bonded (substrate) and dried to form a coating film, and the other object to be bonded (semiconductor element) is mounted on the resulting coating film to produce a laminate. Thereafter, when the laminate is heated, the metal powder in the bonding composition is sintered to form a bonding layer made of a sintered body, and the objects to be bonded are bonded together via this bonding layer.

[0005] It has also been proposed to heat the laminate under pressure when joining the members to be joined. Heat treatment under pressure increases the driving force for sintering of the metal powder, further accelerating sintering, which has the advantage of increasing the bonding strength of the joined structure.

[0006] Patent Documents that disclose pressure bonding using a bonding composition include Patent Document 1 and Patent Document 2. Patent Document 1 discloses bonding a first conductor and a second conductor using a bonding composition containing copper powder, a liquid medium, and a reducing agent, and that when firing is performed under pressure, a pressure of 0.001 MPa to 20 MPa is applied to assist sintering of the copper particles (claims 1 to 7 and

[0045] ). Patent Document 2 discloses bonding a substrate and a bonded object using a paste containing metal fine particles and an organic solvent on the substrate, and that pressure is applied using a pressure member during heating (paragraphs

[0042] to

[0048] ).

[0007] International Publication No. 2020 / 032161 International Publication No. 2021 / 193150

[0008] As described above, although pressure bonding techniques using bonding compositions containing metal powder such as copper powder have been known, there is still room for improvement in the conventional techniques.

[0009] That is, when a pair of objects to be joined is pressure-joined, as described above, a laminate consisting of one of the objects to be joined, a coating film formed from the joining composition, and the other object to be joined is produced, and then this laminate is heated under pressure to join the objects to be joined.

[0010] The process of producing the laminate and the bonding process of heating the laminate are usually performed in separate equipment. Therefore, the laminate must be transported between the two processes, which can subject the laminate to external forces such as vibrations. External forces are also applied to the laminate during the bonding process. However, in the laminate, one of the bonded objects (the semiconductor element) is simply mounted on the coating film and is not sufficiently fixed. Therefore, external forces applied during transportation or bonding can cause the mounted bonded object to shift position (chip rotation) or fall off (see Figure 1). If the bonded object shifts position or falls off, pressure bonding becomes difficult in the subsequent process, leading to reduced productivity.

[0011] To avoid such problems, a possible method is to lower the drying temperature during coating formation so that the solvent remains in the coating. If the solvent remains in the coating, it is expected that when a bonded object is placed on the coating, the tackiness (adhesion) between the coating and the bonded object will improve, which will lead to the prevention of misalignment and detachment.

[0012] However, if a large amount of low-viscosity solvent remains in the coating, the coating may become too soft and lose its shape retention. Specifically, the coating may not be able to maintain its shape during pressure bonding, resulting in the coating protruding from between the bonded objects (a phenomenon known as squeeze-out). Squeeze-out makes it impossible to form a high-strength bonding layer. Furthermore, even if a bonding layer is formed, the large amount of solvent contained in the coating may cause pores in the bonding layer, which may lead to bonding failure. Bonded structures used in mounting devices that generate a large amount of heat, such as power devices, are repeatedly subjected to thermal stress caused by device heat generation, so they require excellent bonding reliability. Therefore, bonding failure is undesirable.

[0013] Another possible measure is to apply a temporary fixing agent (tacking agent) to a coating film before mounting the objects to be bonded, thereby preventing displacement or detachment of the objects to be bonded. For example, Patent Document 2 proposes a method for manufacturing a bonded structure using a temporary fixing composition, which is said to effectively prevent displacement between bonded objects (paragraphs

[0045] and

[0046] ). However, although the method using a temporary fixing agent (temporary fixing composition) is effective in preventing displacement, it requires a separate step of applying the temporary fixing agent, which complicates the manufacturing process and increases manufacturing costs.

[0014] As described above, there is a trade-off between improving tackiness and improving bonding strength, and it has been difficult to simultaneously improve both (tackiness and bonding strength) while maintaining high productivity using a simple method.

[0015] The present inventors have conducted extensive research in light of these problems. As a result, they have developed a bonding composition that contains, in addition to copper powder, a liquid organic substance having a specific boiling point and viscosity, and a specific carboxylic acid. They have discovered that by using this bonding composition to bond objects, the tackiness between the coating film and the objects to be bonded is improved, and a bonded structure with high bonding strength can be obtained by a simple method.

[0016] The present invention was completed based on these findings, and an object of the present invention is to provide a bonding composition that improves the tackiness between a coating film and a bonded object, and that enables a bonded structure having high bonding strength to be obtained by a simple method. Another object of the present invention is to provide a method for manufacturing a bonded structure and a method for temporarily fixing bonded objects using the bonding composition.

[0017] The present invention includes the following aspects (1) to (6). In this specification, the expression "to" includes the numerical values ​​at both ends. In other words, "X to Y" is synonymous with "at least X and at most Y."

[0018] (1) A bonding composition comprising (A) copper powder, (B) a solvent, and (C) a carboxylic acid, wherein the solvent has a boiling point of 230°C or more and 300°C or less, and is soluble in water at 25°C and a shear rate of 10 s －１ a liquid organic substance having a viscosity of 100 mPa·s or more under the above conditions, wherein the carboxylic acid includes a polycarboxylic acid having 3 or more carbon atoms.

[0019] (2) The bonding composition according to (1), wherein the polycarboxylic acid is either one or both of a dicarboxylic acid and a tricarboxylic acid.

[0020] (3) The bonding composition according to (1) or (2), wherein the content of the polycarboxylic acid is 0.1 mass % or more based on the total amount of the copper powder.

[0021] (4) The bonding composition according to any one of (1) to (3), wherein the content of the liquid organic substance is 30% by mass or more and 100% by mass or less based on the total amount of the solvent.

[0022] (5) A method for manufacturing a bonded structure in which a first object to be bonded and a second object to be bonded are bonded via a bonding layer, the method comprising the steps of: applying the bonding composition according to any one of (1) to (4) above to one surface of the first object to be bonded to form a wet film; removing at least a part of the solvent from the wet film to obtain a coating film; temporarily fixing the second object to be bonded to the coating film by applying pressure to obtain a laminate in which the first object to be bonded, the coating film, and the second object to be bonded are laminated in this order; and heating the laminate under pressure to sinter the copper powder in the coating film, thereby forming a bonding layer to bond the first object to be bonded and the second object to be bonded via the resulting laminate.

[0023] (6) A method for temporarily fixing a first object to be joined and a second object to be joined, comprising the steps of: applying the bonding composition according to any one of (1) to (4) above to one surface of the first object to be joined to form a wet film; removing at least a part of the solvent from the wet film to obtain a coating film; and temporarily fixing the first object to be joined and the second object to be joined via the coating film by applying pressure.

[0024] According to the present invention, there is provided a bonding composition that improves the tackiness between a coating film and a bonded object and that enables a bonded structure having high bonding strength to be obtained by a simple method. Furthermore, according to the present invention, there are provided a method for manufacturing a bonded structure and a method for temporarily fixing objects to be bonded that use the bonding composition.

[0025] 1A and 1B are diagrams illustrating a manufacturing process of a conventional joint structure, and FIG. 1C are diagrams illustrating a manufacturing process of a joint structure according to an embodiment of the present invention.

[0026] Specific embodiments of the present invention (hereinafter referred to as "present embodiments") are described below. However, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the present invention. Furthermore, in this specification, any combination of preferred aspects can be adopted as long as technical consistency can be achieved. For example, one preferred numerical range and another preferred numerical range can be combined arbitrarily.

[0027] <<1. Bonding Composition>> The bonding composition of this embodiment (hereinafter may be simply referred to as the “composition”) contains (A) copper powder, (B) a solvent, and (C) a carboxylic acid. The solvent has a boiling point of 230°C or higher and 300°C or lower, and is soluble in water at 25°C and a shear rate of 10 s －１ The composition includes a liquid organic substance having a viscosity of 100 mPa·s or more under the conditions of (a) and (b). The carboxylic acid includes a polycarboxylic acid having 3 or more carbon atoms. Such a composition has excellent tackiness and enables a bonded structure having high bonding strength to be obtained through a simple bonding process.

[0028] <Copper Powder> (A) Copper powder is a powder containing copper as a main component, and serves as a constituent material for the bonding layer (bonding portion) obtained by firing the composition. That is, a sintered body of the copper powder contained in the composition constitutes the bonding layer. In this specification, the term "powder" refers to an aggregate of many particles. It can also be said that many particles constitute the powder.

[0029] The copper powder may have a composition containing elemental copper, with the remainder being unavoidable impurities. The unavoidable impurities may be, for example, oxides inevitably formed on the surfaces of copper particles constituting the copper powder. Typically, the content of elements other than elemental copper in the copper powder is 5% by mass or less. Alternatively, the copper powder may contain copper in a proportion of 50% by mass or more, with the remainder being other elements. An example of such copper powder is copper alloy powder. From the viewpoint of improving electrical conductivity, the copper content in the copper powder is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. The content of each element in the copper powder can be measured, for example, by ICP atomic emission spectroscopy or inert gas fusion / non-dispersive infrared absorption spectroscopy.

[0030] The shape of the copper particles constituting the copper powder is not particularly limited. For example, various shapes such as spherical, flake (scale-like), polyhedral, dendritic (branch-like), and columnar may be used. However, from the viewpoint of increasing the packing of particles and forming a bonding layer with high strength (bonding strength), it is preferable that the copper powder contains spherical copper particles. Whether or not the copper particles are spherical can be determined from the circularity coefficient of the particles. Specifically, the copper powder is observed using a scanning electron microscope (SEM), and the area S and perimeter L of randomly selected copper particles are measured, and the circularity coefficient 4πS / L is calculated from these. ２ The circularity coefficients of a plurality of copper particles are determined, and the average value is calculated. When the arithmetic mean value of the circularity coefficients is 0.85 or more, the copper particles are defined as spherical.

[0031] Preferably, the content of copper powder in the bonding composition is 60% by mass or more and 85% by mass or less. By appropriately increasing the amount of copper powder, the conductivity and strength of the bonding layer are further increased. Furthermore, by appropriately reducing the amount of copper powder, aggregation of copper particles in the composition is suppressed, improving the coatability of the composition. The content is the total amount of copper powder contained in the composition. That is, when the copper powder includes copper first powder and copper second powder, as described below, it is the total amount of copper first powder and copper second powder.

[0032] The average particle diameter (D50) of the copper powder is preferably 0.03 μm or more, more preferably 0.05 μm or more. By appropriately increasing the particle diameter of the copper powder, particle aggregation in the composition can be prevented and particle dispersibility can be improved. On the other hand, the average particle diameter (D50) of the copper powder is preferably 20 μm or less, more preferably 10 μm or less. By appropriately decreasing the particle diameter of the copper powder, the sinterability of the copper powder can be improved. From the viewpoint of achieving both dispersibility and sinterability, D50 is preferably 0.03 μm or more and 20 μm or less, more preferably 0.05 μm or more and 10 μm or less.

[0033] The average particle diameter (D50) of copper powder can be determined as follows. First, an organic solvent such as 2-propanol is added in an amount 10 times (by mass) equivalent to the bonding composition, thoroughly stirred, and then a washing procedure is repeated to remove only the supernatant liquid while leaving the solids (cake). The resulting cake is left to stand at room temperature and thoroughly dried, and the resulting dried product (copper powder) is observed using an SEM. For SEM observation, an SEM image is obtained at a magnification of 1,000 to 100,000 times. Then, 50 or more particles whose particle outlines are observable in the SEM image are randomly selected and their particle diameters (Heywood diameters) are measured. The particle volume of spherical particles is calculated from the Heywood diameter, and the volume-based particle size distribution is determined from the obtained data. Next, the particle diameter (50% cumulative volume diameter) at which the cumulative volume (cumulative volume) of the resulting particle size distribution, starting from the smallest particle size, is calculated and defined as the average particle diameter (D50).

[0034] The particle diameter D50 of the copper powder may be determined by measuring the particle diameter of the copper powder used in preparing the bonding composition. When the copper particles constituting the copper powder are spherical, D50 is determined by observation with a scanning electron microscope (SEM). Specifically, the copper powder is observed with a scanning electron microscope (SEM), 50 particles are randomly selected from the SEM image at a magnification of 1,000 to 100,000 times, and the particle diameter (Heywood diameter) of each particle is determined. Then, assuming that the copper particles are truly spherical, the volume is calculated from the obtained particle diameter. A volume-based particle size distribution is determined from the particle diameters of the 50 particles, and the diameter at 50% of the cumulative diameter (50% volume cumulative particle diameter) is defined as D50.

[0035] More preferably, the copper powder comprises a first copper powder having a cumulative volume 50% diameter (D50) of 0.05 μm or more but less than 1 μm in the region of less than 1 μm in the particle size distribution measured by SEM observation, and a second copper powder having a cumulative volume 50% diameter (D50) of 1 μm or more but less than 1 μm in the region of 1 μm or more in the particle size distribution. When the copper particles constituting the copper powder are not spherical, i.e., when they are flaky (scale-like), polyhedral, dendritic, or columnar, the particle diameter D50 is determined by laser diffraction / scattering particle size distribution measurement. Specifically, 0.1 g of the measurement sample is mixed with the dispersant dispersion and dispersed for 1 minute using an ultrasonic homogenizer. For example, a US-300T manufactured by Nippon Seiki Seisakusho Co., Ltd. or an equivalent is used as the ultrasonic homogenizer. Thereafter, a particle size distribution on a volume basis is determined using a laser diffraction scattering particle size distribution analyzer, and the diameter at which the cumulative 50% (50% volume cumulative particle size) is taken as D50. As the laser diffraction scattering particle size distribution analyzer, an MT-3300EXII manufactured by Microtrackbell Corporation or an equivalent product is used.

[0036] The D50 of the copper first powder is preferably 0.05 μm or more from the viewpoint of preventing aggregation in the composition and obtaining good particle dispersibility. The D50 of the copper first powder is preferably less than 1 μm, more preferably 0.8 μm or less, and even more preferably 0.6 μm or less from the viewpoint of sufficiently ensuring the sinterability of the copper powder. The D50 of the copper second powder is preferably 1 μm or more from the viewpoint of improving the strength of the bonding layer obtained by sintering the copper powder. The D50 of the copper second powder is preferably 10 μm or less, more preferably 8 μm or less, and even more preferably 6 μm or less from the viewpoint of improving the coatability of the composition.

[0037] The ratio of the copper (I) powder to the total mass of the copper (I) powder and the copper (II) powder in the bonding composition is preferably 10% by mass or more and 95% by mass or less, more preferably 15% by mass or more and 90% by mass or less, and even more preferably 20% by mass or more and 80% by mass or less. The ratio of the copper (II) powder to the total mass of the copper (I) powder and the copper (II) powder is preferably 5% by mass or more and 90% by mass or less, more preferably 10% by mass or more and 85% by mass or less, and even more preferably 20% by mass or more and 80% by mass or less. By setting the blending ratio within the above-mentioned range, the particle packing property can be improved, and the strength of the bonding layer can be sufficiently increased.

[0038] The copper powder may be unsurface-treated (surface-untreated copper powder), or may be surface-treated (surface-treated copper powder) to the extent that the effects of the present embodiment are not impaired. Examples of surface-treated copper powder include copper powders having a surface treatment layer formed on the surface of the copper powder, the surface of which is made of a fatty acid, a fatty acid copper salt, an aliphatic amine, a silane coupling agent, a titanate coupling agent, an aluminate coupling agent, or the like.

[0039] <Solvent> The (B) solvent has the function of imparting an appropriate viscosity and therefore good coatability to the composition, and also has the effect of ensuring good conductivity and strength of the bonding layer obtained by firing the composition by uniformly dispersing copper particles in the composition.

[0040] The solvent of this embodiment has a boiling point of 230° C. or higher and 300° C. or lower, and is soluble in water at 25° C. and a shear rate of 10 s －１The composition contains a liquid organic substance having a viscosity of 100 mPa·s or more under the above conditions (hereinafter, sometimes collectively referred to as a "high-boiling-point, high-viscosity liquid organic substance"). The inclusion of such a high-boiling-point, high-viscosity liquid organic substance in the composition makes it possible to improve the tackiness between the coating film and the bonded body when manufacturing a bonded structure. In other words, because the boiling point of this liquid organic substance is relatively high, at least a portion of the liquid organic substance remains in the coating film obtained by applying and drying the composition. The remaining high-viscosity liquid organic substance improves the tackiness (adhesion) between the coating film and the bonded body when the bonded body (such as a semiconductor element) is mounted on the coating film. As a result, problems such as misalignment and detachment of the bonded body are avoided. In this specification, the boiling point refers to the value at an atmospheric pressure of 1 atmosphere.

[0041] If the boiling point of the liquid organic substance at 1 atmosphere is less than 230°C, excessive volatilization of the liquid organic substance occurs during drying of the coating film. As a result, the liquid organic substance may not remain in the dried coating film, making it difficult to achieve the effect of improving tackiness. Furthermore, the liquid organic substance may be altered by high-temperature heating in the subsequent pressure bonding process, resulting in a loss of shape retention of the coating film. On the other hand, if the boiling point of the liquid organic substance at 1 atmosphere exceeds 300°C, an excessively large amount of liquid organic substance may remain in the coating film, resulting in a loss of shape retention of the coating film. Furthermore, a large amount of remaining liquid composition may inhibit sintering of the copper powder, resulting in the formation of pores (voids) in the bonding layer (sintered body) that reduce strength. From the viewpoint of achieving a balanced improvement in tackiness, shape retention, and bonding strength, the boiling point of the liquid organic substance is preferably 235°C or higher and 290°C or lower, more preferably 240°C or higher and 285°C or lower.

[0042] If the viscosity of the liquid organic material is less than 100 mPa·s, the viscosity is too low, making it difficult to achieve the effect of improving tackiness. Furthermore, the shape retention of the coating film is reduced, which may cause the coating film to squeeze out during the subsequent pressure bonding process. The upper limit of the viscosity of the liquid organic material is not limited. However, from the viewpoint of improving the coatability of the bonding composition, a viscosity of 10,000 mPa·s or less is preferred. From the viewpoint of improving tackiness, shape retention, and coatability in a balanced manner, the viscosity is preferably 105 mPa·s or more and 3,000 mPa·s or less, and more preferably 110 mPa·s or more and 2,000 mPa·s or less.

[0043] The high-boiling-point, high-viscosity liquid organic substance that satisfies the above-mentioned boiling point and viscosity requirements is preferably at least one selected from the group consisting of 2-ethyl-1,3-hexanediol (also known as octylene glycol (OG)), 3-methyl-1,5-pentanediol (MPD), 2,4-diethyl-1,5-pentanediol, 1,5-pentanediol, etc. The bonding composition may contain one type of high-boiling-point, high-viscosity liquid organic substance alone, or may contain two or more types of high-boiling-point, high-viscosity liquid organic substances in combination.

[0044] In order to improve the properties of the bonding composition, such as the coatability and dispersibility, the solvent may contain other solvent components in addition to the high-boiling-point, high-viscosity liquid organic substance. The other solvent components may be, for example, solvents having a boiling point of less than 230°C or solvents having a boiling point of less than 25°C and a shear rate of 10 s －１ In any case, from the viewpoint of improving shape retention and bonding strength, it is preferable that the boiling point of the other solvent components is 300° C. or less.

[0045] As the other solvent, a monohydric or polyhydric alcohol is preferred, and a polyhydric alcohol is more preferred. Examples of polyhydric alcohols include propylene glycol, ethylene glycol, hexylene glycol, diethylene glycol, 1,3-butanediol, 1,4-butanediol, dipropylene glycol, tripropylene glycol, and glycerin. The bonding composition may contain one other solvent component alone, or may contain two or more other solvent components in combination. The bonding composition may not contain any other solvent components.

[0046] The amount of solvent in the bonding composition is not limited. However, from the viewpoint of improving the coatability of the bonding composition and the tackiness and shape retention of the coating film in a balanced manner, the total content of the solvent in the bonding composition is preferably 30% by mass or more and 50% by mass or less, and more preferably 40% by mass or more and 47% by mass or less, based on the total amount of copper powder. Note that when the solvent is composed only of a high-boiling-point, high-viscosity liquid organic substance, the amount of the high-boiling-point, high-viscosity liquid organic substance is the solvent amount, and when the solvent contains other solvent components, the total amount of the high-boiling-point, high-viscosity liquid organic substance and the other solvent components is the solvent amount.

[0047] The proportion of the high-boiling-point, high-viscosity liquid organic substance in the solvent is not limited. However, by increasing the amount of the high-boiling-point, high-viscosity liquid organic substance to a certain extent, it is possible to more significantly exhibit the effect of improving tackiness. The content of the high-boiling-point, high-viscosity liquid organic substance is preferably 30% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, based on the total amount of the solvent.

[0048] <Carboxylic Acid> The bonding composition of this embodiment contains (C) a carboxylic acid. The carboxylic acid also contains a polycarboxylic acid having three or more carbon atoms (hereinafter, sometimes collectively referred to as a "high-carbon polycarboxylic acid"). By using such a high-carbon polycarboxylic acid, it is possible to increase the strength of the bonding layer of the bonded structure produced using the bonding composition. Although this should not be interpreted in a limiting manner, the following mechanism is thought to be the reason for this.

[0049] Polycarboxylic acids (polycarboxylic acids) are organic acids containing multiple carboxyl groups (—COOH) in one molecule. In the bonding composition, hydrogen (H) from each carboxyl group is released and adsorbed to the copper particles that make up the copper powder. High-carbon polycarboxylic acids also have relatively large molecular structures. Therefore, it is believed that high-carbon polycarboxylic acids form stable adsorption structures with copper particles. Copper particles that form stable adsorption structures with high-carbon polycarboxylic acids release fine copper nanoparticles when heated during the bonding process. The released fine copper nanoparticles have a large driving force for sintering, which is thought to promote copper powder sintering and contribute to improving the strength of the bonding layer, which is the copper powder sintered body. In contrast, monocarboxylic acids and polycarboxylic acids with two or fewer carbon atoms, such as oxalic acid, do not form stable adsorption structures with copper particles, and are therefore thought to be insufficient in improving bonding strength.

[0050] The number of carboxy groups contained in the polycarboxylic acid per molecule is not limited as long as it is two or more. However, polycarboxylic acids containing an excessively large number of carboxy groups are difficult to obtain. From the viewpoint of improving bonding strength and easy availability, the polycarboxylic acid is preferably either one or both of a dicarboxylic acid (dicarboxylic acid) and a tricarboxylic acid (tricarboxylic acid). Furthermore, the number of carbon atoms per molecule of the polycarboxylic acid is not limited as long as it is three or more. The number of carbon atoms may be 3, 4, 5, 6, 7, 8, 9, or 10 or more. However, polycarboxylic acids with an excessively large number of carbon atoms may cause deterioration of fillet strength and reduce reliability. Here, the fillet refers to a portion of the bonding composition coating film formed during the production of a bonded structure where the bonded body is not placed. To avoid this problem, the number of carbon atoms is preferably 20 or less.

[0051] More specifically, the high-carbon polycarboxylic acid is preferably at least one selected from the group consisting of malonic acid (3 carbon atoms), succinic acid (4 carbon atoms), maleic acid (4 carbon atoms), glutaric acid (5 carbon atoms), adipic acid (6 carbon atoms), pimelic acid (7 carbon atoms), suberic acid (8 carbon atoms), azelaic acid (9 carbon atoms), sebacic acid (10 carbon atoms), fumaric acid (4 carbon atoms), citric acid (6 carbon atoms), and aconitic acid (6 carbon atoms). The bonding composition may contain one high-carbon polycarboxylic acid alone, or may contain two or more high-carbon polycarboxylic acids in combination.

[0052] Examples of the structural formulae of some carboxylic acids, including high-carbon polycarboxylic acids, are shown in Table 1 below. Note that MMA-10R is a branched dibasic acid manufactured by Okamura Oil Mills, Ltd. Table 1 below shows the structural formula of octane-1,7-dicarboxylic acid, which is the main component of MMA-10R.

[0053]

[0054] From the viewpoint of increasing the bonding strength of the bonded structure, the content of the high-carbon polycarboxylic acid is preferably 0.1 mass% or more based on the total amount of copper powder. On the other hand, if the amount of high-carbon polycarboxylic acid is excessively large, there is a problem that the storage stability of the bonding composition is impaired. From the viewpoint of achieving both improved bonding strength and storage stability of the bonding composition, the content of the high-carbon polycarboxylic acid is preferably 0.15 mass% or more and 2 mass% or less, and more preferably 0.18 mass% or more and 1.5 mass% or less, based on the total amount of copper powder.

[0055] <Other Components> The bonding composition may contain components other than (A) copper powder, (B) solvent, and (C) carboxylic acid, as long as the effects of this embodiment are not impaired. Examples of other components include reducing agents, binder components, surface tension modifiers, antifoaming agents, and viscosity modifiers. More specifically, examples include terpene-based compositions such as isobornylcyclohexanol, fatty acids, fatty acid alcohols, and fatty acid esters, which are added to further enhance tackiness; polyethylene glycol, bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane (Bis-Tris), and other components that act as reducing agents for copper particles; gelling agents for enhancing thixotropy; and copper carboxylates for further enhancing bonding strength. The amount of other components is preferably 15% by mass or less relative to the copper powder. The amount of other components may even be 0% by mass. In particular, the bonding composition may be substantially free of organic polymer components (resins). The bonding composition of the present embodiment, even without containing any other components, sufficiently exhibits the effect of improving the tackiness between the coating film and the object to be bonded, and the effect of improving the bonding strength of the bonded structure.

[0056] <Viscosity> Preferably, the paste viscosity of the bonding composition is 10 Pa·s or more and 800 Pa·s or less. By increasing the viscosity to a certain extent, the effect of suppressing squeeze-out can be more reliably achieved. Furthermore, uniform dispersion of copper particles in the composition is possible, and their sedimentation can be suppressed. Furthermore, the filling of copper particles in the coating film obtained by applying the composition is improved, making it possible to form a bonding layer with superior conductivity and strength. On the other hand, by appropriately suppressing the viscosity, the coatability of the composition can be improved. Furthermore, when copper particles are densely packed in the composition, they tend to aggregate. By suppressing the viscosity, aggregation of copper particles is suppressed, making it possible to form a bonding layer with particularly excellent conductivity and strength. From these viewpoints, the paste viscosity of the bonding composition is preferably 12 Pa·s or more and 600 Pa·s or less, more preferably 15 Pa·s or more and 400 Pa·s or less.

[0057] The bonding composition of the present embodiment is characterized by containing a liquid organic substance having a specific boiling point and viscosity and a specific carboxylic acid. This bonding composition improves the tackiness between the coating film and the bonded object, and also makes it possible to obtain a bonded structure with high bonding strength by a simple method.

[0058] The bonding composition of this embodiment can be used for both bonding objects under pressure and bonding objects without pressure. However, it is particularly suitable for bonding objects under pressure. That is, the bonding composition is preferably a pressure bonding composition.

[0059] <<2. Manufacturing Method of Bonding Composition>> The manufacturing method of the bonding composition of this embodiment is not limited as long as the above-described requirements are satisfied. However, it is preferably manufactured by mixing (A) copper powder, (B) solvent, (C) carboxylic acid, and other additives as necessary. Copper powder manufactured by various methods such as wet reduction, atomization, and electrolytic reduction can be used. Spherical particles are easily obtained when using wet reduction or atomization. Dendritic or columnar particles are easily obtained when using electrolytic reduction. Flake-shaped particles are obtained by applying a mechanical external force to spherical particles to cause plastic deformation. Furthermore, when using copper powder containing fine copper first powder and coarse copper second powder, the copper first powder and the copper second powder may be prepared and mixed. At least a portion of the solvent may be a solvent having a boiling point of 230°C or higher and 300°C or lower, and capable of being heated at 25°C and a shear rate of 10 s －１ A liquid organic substance having a viscosity of 100 mPa·s or more under the above conditions is used. At least a part of the carboxylic acid is a polycarboxylic acid having 3 or more carbon atoms. Mixing may be carried out using a known mixing device such as a roll mill.

[0060] <<3. Manufacturing Method of Bonded Structure>> The manufacturing method of the bonded structure of this embodiment includes the following steps: a step of applying the bonding composition described above to one surface of a first bonded body to form a wet film (a coating step), a step of removing at least a portion of the solvent from the wet film to obtain a coating film (a solvent removal step), a step of temporarily fixing a second bonded body to the coating film by applying pressure to obtain a laminate in which the first bonded body, the coating film, and the second bonded body are stacked in this order (a lamination step), and a step of heating the laminate under pressure to sinter the copper powder in the coating film and thereby bonding the first bonded body and the second bonded body via the bonding layer formed (a bonding step). The bonded structure is formed by bonding the first bonded body and the second bonded body via the bonding layer. An example of the manufacturing process of this embodiment is schematically shown in FIG. 2.

[0061] <Bonded Structure> The bonded structure is composed of a first bonded body, a second bonded body, and a bonding layer disposed between the first and second bonded bodies to bond them. Here, the bonded bodies (first bonded body, second bonded body) are objects to be bonded, and examples thereof include spacers and heat sinks made of various conductive metals such as gold, silver, or copper, metal wires, substrates having metal wires on their surfaces, and conductors such as semiconductor elements (semiconductor chips). More specifically, examples include semiconductor elements such as power modules, transmitters, amplifiers, and LED modules, lead frames, metal-plate-attached ceramic substrates, semiconductor element mounting substrates, metal wiring, blocks, power supply members, heat sinks, water-cooled plates, and metal clips. Furthermore, there are no limitations on the combination of the first bonded body and the second bonded body. The first bonded body and the second bonded body may be made of the same material or different materials.

[0062] Specific examples of the bonding structure include a semiconductor device in which a substrate for mounting a semiconductor element is used as the first body to be bonded, a semiconductor element is used as the second body to be bonded, and the two are bonded together with a sintered body of the bonding composition. Other examples include a bonded body in which a substrate for mounting a semiconductor element is used as the first body to be bonded, a heat sink is used as the second body to be bonded, and the two are bonded together with a sintered body of the bonding composition, and a bonded body in which a semiconductor element is used as the first body to be bonded, and a metal clip is used as the second body to be bonded, and the two are bonded together with a sintered body of the bonding composition.

[0063] <Coating Step> In the coating step, the bonding composition is coated on one surface of the first object to be bonded to form a wet film. Coating may be performed by a known method such as screen printing, dispense printing, gravure printing, offset printing, reverse coating, or doctor blade. The wet film may be provided over the entire surface of the first object to be bonded, or may be provided discontinuously in a partial region. From the viewpoint of more reliably ensuring high bonding strength, the wet film may be provided not only on the first object to be bonded but also on one surface of the second object to be bonded.

[0064] From the viewpoint of increasing the strength of the bonding layer, it is preferable that the thickness of the wet film is relatively large. On the other hand, from the viewpoint of preventing cracks from occurring in the coating film after solvent removal, it is preferable that the thickness of the wet film is relatively small. From the viewpoint of improving the bonding strength and preventing cracks in the coating film, the average thickness of the wet film is preferably 20 μm or more and 500 μm or less, more preferably 30 μm or more and 450 μm or less, and even more preferably 35 μm or more and 400 μm or less. Note that the above-mentioned thickness refers to the thickness of the wet film when a wet film is formed only on the first bonded body, and refers to the total thickness of both wet films when a wet film is formed on both the first bonded body and the second bonded body.

[0065] <Solvent Removal Step> In the solvent removal step, at least a portion of the solvent is removed from the wet film to obtain a coating film. Removing a portion of the solvent to reduce the amount of solvent increases the shape retention of the coating film, and as a result, deformation of the coating film can be suppressed even when pressure is applied to the coating film in the subsequent bonding step. Furthermore, if the coating film used in the bonding step contains an excessive amount of solvent, pores may form during the heating step during bonding, which may reduce the strength of the bonding layer. Removing a portion of the solvent in advance can prevent the problem of reduced bonding strength. Solvent removal can be performed by means of natural drying, hot air drying, infrared irradiation, hot plate drying, or the like, which utilizes solvent evaporation. However, it is preferable to heat-dry the wet film to promote solvent evaporation and removal.

[0066] When the wet film is dried by heating, it is desirable to adjust the heating temperature and heating time to fall within an appropriate range. By controlling the heating temperature and heating time, excessive solvent can be prevented from remaining in the coating film, improving the shape retention of the coating film and preventing a decrease in bonding strength due to pore formation. In addition, because an appropriate amount of high-boiling-point, high-viscosity liquid organic substance remains in the coating film, it is possible to fully utilize the effect of improving tackiness (adhesion) due to the high-boiling-point, high-viscosity liquid organic substance.

[0067] From the viewpoint of improving tackiness, shape retention, and bonding strength in a balanced manner, the heating and drying temperature is preferably 100°C or higher and 200°C or lower, more preferably 105°C or higher and 190°C or lower, even more preferably 110°C or higher and 180°C or lower, and particularly preferably 110°C or higher and 160°C or lower. From the same viewpoint, the heating time is preferably 10 minutes or higher and 60 minutes or lower, more preferably 15 minutes or higher and 50 minutes or lower, and even more preferably 20 minutes or higher and 40 minutes or lower. Heating can be carried out in an atmosphere such as an inert gas atmosphere such as nitrogen gas or argon gas, air atmosphere, or reduced pressure atmosphere.

[0068] From the viewpoint of improving tackiness, shape retention, and bonding strength in a balanced manner, the content of the solvent in the coating film obtained through the solvent removal process is preferably 0.01 mass% or more and 10 mass% or less, and more preferably 0.1 mass% or more and 9 mass% or less, relative to the total amount of copper powder.

[0069] <Lamination Step> In the lamination step (tack step), the second object to be joined is temporarily fixed to the coating film by applying pressure to obtain a laminate in which the first object to be joined, the coating film, and the second object to be joined are laminated in this order. The bonding composition used in the manufacturing method of this embodiment has the effect of improving tackiness (adhesion). Therefore, by placing the second object to be joined so that it is in contact with the coating film and then applying a small amount of pressure, the second object to be joined can be temporarily fixed to the coating film without causing displacement or falling off.

[0070] When applying pressure, it is preferable to apply pressure so that the second bonded object is in close contact with the coating film. For example, pressure is applied in a direction perpendicular to the coating film surface. From the viewpoint of properly performing temporary fixation and increasing bonding strength, it is preferable that the pressure be relatively high. On the other hand, from the viewpoint of preventing component destruction, it is preferable that the pressure be relatively low. The pressure is preferably 0.1 MPa or more and 5.0 MPa or less, more preferably 0.2 MPa or more and 4.5 MPa or less, and even more preferably 0.21 MPa or more and 4.4 MPa or less. Furthermore, the pressure application time is preferably 0.01 seconds or more, more preferably 0.1 seconds or more. There is no upper limit to the pressure application time, but it can be, for example, 5 seconds or less.

[0071] The temporary fixation may be performed at room temperature or while heating. When the temporary fixation is performed under heating, it is preferable to avoid excessively high heating temperatures in order to maintain tackiness by leaving the high-boiling-point, high-viscosity liquid organic substance in the coating film. The heating temperature of the first bonded body and the coating film is preferably 15°C or higher and 270°C or lower, more preferably 18°C ​​or higher and 260°C or lower, and even more preferably 20°C or higher and 250°C or lower. The heating temperature of the second bonded body is preferably 15°C or higher and 300°C or lower, more preferably 18°C ​​or higher and 250°C or lower, and even more preferably 20°C or higher and 200°C or lower.

[0072] <Bonding Step> In the bonding step, the laminate is heated under pressure to sinter the copper powder in the coating, thereby bonding the first bonded body and the second bonded body via the bonding layer formed. The bonding layer is composed of a sintered body in which the copper particles constituting the copper powder are firmly bonded, and therefore has high conductivity and strength. Note that, in this specification, "pressure" refers to a mode in which force is applied from outside the laminate. In other words, this does not include force generated by the weight of the laminate or the members constituting the laminate (first bonded body, second bonded body, etc.).

[0073] From the viewpoint of increasing the strength of the bonding layer, the heating temperature during bonding is preferably 180°C or higher and 450°C or lower, more preferably 200°C or higher and 400°C or lower. From the same viewpoint, the heating time is preferably 1 minute or higher and 30 minutes or lower, more preferably 2 minutes or higher and 25 minutes or lower. The pressure applied during bonding is preferably 1 MPa or higher and 35 MPa or lower, more preferably 2 MPa or higher and 32 MPa or lower, and even more preferably 3 MPa or higher and 30 MPa or lower. Bonding may be performed, for example, in an air atmosphere, an inert gas atmosphere, or a reducing atmosphere, and is preferably performed in an inert gas atmosphere or a reducing atmosphere.

[0074] In this way, a bonded structure can be obtained. The bonded structure of this embodiment has a high strength of the bonding layer. A bonded structure having such characteristics is suitable for use in various electronic circuits, particularly electronic circuits used in high-temperature environments, such as automotive electronic circuits and electronic circuits equipped with power devices.

[0075] <<4. Method for Temporarily Fixing Objects to be Bonded>> This embodiment also relates to a method for temporarily fixing a first object to be bonded and a second object to be bonded. This method includes the following steps: a step of applying the above-described bonding composition to one surface of the first object to be bonded to form a wet film (application step), a step of removing at least a portion of the solvent from the wet film to obtain a coating film (solvent removal step), and a step of temporarily fixing the first object to be bonded and the second object to be bonded via the coating film by applying pressure (temporary fixing step). Details of the application step, the solvent removal step, and the temporary fixing step are as described in the method for manufacturing a bonded structure.

[0076] According to the temporary fixing method of the present embodiment, it is possible to improve the tackiness between the coating film and the object to be joined, and also to obtain a joined structure having high joining strength by a simple method.

[0077] The present invention will be described in more detail using the following examples, but the present invention is not limited to the following examples.

[0078] Example 1 (1) Preparation of Bonding Composition Copper powder was prepared by mixing spherical copper particles with an average particle diameter (D50) of 0.16 μm (spherical copper powder) and flat copper particles with an average particle diameter (D50) of 4.5 μm (flat copper powder) in a mass ratio of 70:30. Octylene glycol (OG) was prepared as the solvent, and glutaric acid was prepared as the carboxylic acid. Polyethylene glycol (PEG300) with a number-average molecular weight of 300 was also prepared as another component. As shown in Table 1 above, glutaric acid is a dicarboxylic acid with five carbon atoms.

[0079] Next, the raw materials were weighed so that the ratio of octylene glycol (OG), polyethylene glycol (PEG300), and glutaric acid was 26% by mass relative to the copper powder (spherical copper powder and flaky copper powder). The weighed raw materials (spherical copper powder, flaky copper powder, OG, PEG300, and glutaric acid) were mixed, and the resulting mixture was pre-kneaded with a spatula and then paste-formed using a centrifugal vacuum mixer (Thinky Corporation, ARE-500). Two cycles of stirring mode (1000 rpm x 1 minute) and degassing mode (2000 rpm x 30 seconds) were performed. The resulting paste was then dispersed and mixed using a three-roll mill to prepare a paste-like bonding composition.

[0080] (2) Preparation of Bonded Structure A substrate (20 mm x 20 mm, thickness 2 mm) made by laminating copper and ceramics in the thickness direction was prepared as a first bonded body. A bonding composition was printed on the copper side of this substrate using a metal mask (6 mm x 6 mm, thickness 100 μm) to form a rectangular wet coating film (coating step). The wet coating film was coated with oxygen (O ２ ) The coating was dried at 120° C. for 30 minutes in a nitrogen atmosphere with a concentration of 3% or less to remove part of the solvent, thereby obtaining a coating film (solvent removal step).

[0081] Assuming a model component of a semiconductor power device, an Ag-plated SiC chip (5 mm × 5 mm × 190 μm) was prepared as the second bonded object. The SiC chip was then adsorbed onto a chip mounter's adsorption collet heated to 170°C. Then, while maintaining the temperature (170°C), the SiC chip was placed at the center of the coating film so that the Ag-plated surface was in contact with the coating film, and at the same time, the SiC chip was pressed into the coating film. During the pressing, a force of 6 kgf (equivalent to a pressure of approximately 2.4 MPa) was applied for 0.5 seconds to the surface of the SiC chip opposite the Ag surface. In this way, the SiC chip was temporarily attached to the coating film, and a laminate was produced (lamination process).

[0082] Next, the obtained laminate was transported to a firing furnace, and the laminate was pressurized to 25 MPa under a nitrogen atmosphere, and then heated to 300°C and held at that temperature (300°C) for 5 minutes to fire the coating film into a bonding layer, thereby obtaining a bonded structure (bonding process).

[0083] Example 2 A bonding composition and a bonded structure were prepared in the same manner as in Example 1, except that the same amount of 3-methyl-1,5-pentanediol (MPD) was used instead of the solvent octylene glycol (OG).

[0084] [Example 3] Instead of glutaric acid, which is a carboxylic acid, the same amount of pimelic acid was used. Otherwise, a bonding composition and a bonded structure were produced using the same procedures as in Example 1. As shown in Table 1 above, pimelic acid is a dicarboxylic acid having 7 carbon atoms.

[0085] Example 4 The same amount of MMA-10R was used in place of glutaric acid, a carboxylic acid. Otherwise, a bonding composition and a bonded structure were produced using the same procedures as in Example 1. As shown in Table 1 above, the main component of MMA-10R (octane-1,7-dicarboxylic acid) is a dicarboxylic acid with 9 carbon atoms.

[0086] Example 5: A copper powder was prepared by mixing a copper powder (spherical copper powder) consisting of spherical copper particles with an average particle diameter (D50) of 0.16 μm and a copper powder (flat copper powder) consisting of flat copper particles with an average particle diameter (D50) of 4.5 μm in a mass ratio of 70:30. Octylene glycol (OG) was prepared as the solvent, and glutaric acid and citric acid were prepared as carboxylic acids. Furthermore, polyethylene glycol (PEG300) with a number-average molecular weight of 300 was prepared as another component. As shown in Table 1 above, glutaric acid is a dicarboxylic acid having 5 carbon atoms, and citric acid is a tricarboxylic acid having 6 carbon atoms.

[0087] Next, the raw materials were weighed so that the ratio of octylene glycol (OG), polyethylene glycol (PEG300), glutaric acid, and citric acid to the copper powder (spherical copper powder and flaky copper powder) was 36.6% by mass, 1.9% by mass, 0.3% by mass, and 0.1% by mass. The weighed raw materials (spherical copper powder, flaky copper powder, OG, PEG300, glutaric acid, and citric acid) were mixed, and the resulting mixture was pre-kneaded with a spatula and then formed into a paste using a centrifugal vacuum mixer (Thinky Corporation, ARE-500). Two cycles of stirring mode (1000 rpm x 1 minute) and degassing mode (2000 rpm x 30 seconds) were performed. The resulting paste was then dispersed and mixed using a three-roll mill to prepare a paste-like bonding composition.

[0088] Example 6: A copper powder was prepared by mixing a copper powder (spherical copper powder) consisting of spherical copper particles with an average particle diameter (D50) of 0.16 μm and a copper powder (flat copper powder) consisting of flat copper particles with an average particle diameter (D50) of 4.5 μm in a mass ratio of 70:30. Octylene glycol (OG) was also prepared as the solvent, and citric acid was also prepared as the carboxylic acid. Furthermore, polyethylene glycol (PEG300) with a number average molecular weight of 300 was also prepared as another component. As shown in Table 1 above, citric acid is a tricarboxylic acid having 6 carbon atoms.

[0089] Next, the raw materials were weighed so that the ratio of octylene glycol (OG), polyethylene glycol (PEG300), and citric acid was 27.7% by mass relative to the copper powder (spherical copper powder and flaky copper powder), 1.9% by mass, and 0.3% by mass. The weighed raw materials (spherical copper powder, flaky copper powder, OG, PEG300, and citric acid) were mixed, and the resulting mixture was pre-kneaded with a spatula and then paste-formed using a centrifugal vacuum mixer (Thinky Corporation, ARE-500). Two cycles of stirring mode (1000 rpm x 1 minute) and degassing mode (2000 rpm x 30 seconds) were performed. The resulting paste was then dispersed and mixed using a three-roll mill to prepare a paste-like bonding composition.

[0090] Comparative Example 1: A bonding composition and a bonded structure were prepared in the same manner as in Example 1 except that the same amount of hexylene glycol (HG) was used instead of the solvent octylene glycol (OG). Furthermore, no carboxylic acid (glutaric acid) was added.

[0091] Comparative Example 2 A bonding composition and a bonded structure were produced in the same manner as in Example 1, except that no carboxylic acid (glutaric acid) was added.

[0092] Comparative Example 3: Instead of the solvent octylene glycol (OG), the same amount of 3-methyl-1,5-pentanediol (MPD) was used. Furthermore, no carboxylic acid (glutaric acid) was added. A bonding composition and a bonded structure were prepared using the same procedures as in Example 1.

[0093] Comparative Example 4 A bonding composition and a bonded structure were produced in the same manner as in Example 1, except that the same amount of hexylene glycol (HG) was used instead of octylene glycol (OG) as the solvent.

[0094] Comparative Example 5 A copper powder was prepared by mixing a copper powder (spherical copper powder) consisting of spherical copper particles with an average particle diameter (D50) of 0.16 μm and a copper powder (flat copper powder) consisting of flat copper particles with an average particle diameter (D50) of 4.5 μm in a mass ratio of 70:30. Hexylene glycol (HG) and 1,3-butanediol (boiling point 207°C, viscosity 88 mPa·S) were prepared as solvents, and citric acid was prepared as a carboxylic acid. Furthermore, polyethylene glycol (PEG300) with a number average molecular weight of 300 was prepared as another component. As shown in Table 1 above, citric acid is a tricarboxylic acid having 6 carbon atoms.

[0095] Next, raw materials were weighed out so that the ratios of hexylene glycol (HG), 11.2% by mass of hexylene glycol (HG), 16.4% by mass of 1.3-butanediol, 1.9% by mass of polyethylene glycol (PEG300), and 0.3% by mass of citric acid relative to the copper powder (spherical copper powder and flaky copper powder) were 11.2% by mass, 1.3-butanediol, PEG300, and citric acid. The weighed raw materials (spherical copper powder, flaky copper powder, HG, 1.3-butanediol, PEG300, and citric acid) were mixed, and the resulting mixture was pre-kneaded with a spatula and then formed into a paste using a centrifugal vacuum mixer (Thinky Corporation, ARE-500). Two cycles of stirring mode (1000 rpm x 1 minute) and degassing mode (2000 rpm x 30 seconds) were performed. The resulting paste was then dispersed and mixed using a three-roll mill to prepare a paste-like bonding composition.

[0096] Comparative Example 6: A copper powder (spherical copper powder) consisting of spherical copper particles with an average particle diameter (D50) of 0.16 μm and a copper powder (flat copper powder) consisting of flat copper particles with an average particle diameter (D50) of 4.5 μm were mixed at a mass ratio of 70:30. Hexylene glycol (HG) and triethylene glycol monobutyl ether (BTG, boiling point 278°C, viscosity 7.7 mPa·S) were prepared as solvents, and citric acid was prepared as a carboxylic acid. Furthermore, polyethylene glycol (PEG300) with a number average molecular weight of 300 was prepared as another component. As shown in Table 1 above, citric acid is a tricarboxylic acid having 6 carbon atoms.

[0097] Next, the raw materials were weighed so that the ratio of hexylene glycol (HG) to copper powder (spherical copper powder and flaky copper powder) was 11.2% by mass, BTG was 16.4% by mass, polyethylene glycol (PEG300) was 1.9% by mass, and citric acid was 0.3% by mass. The weighed raw materials (spherical copper powder, flaky copper powder, HG, BTG, PEG300, citric acid) were mixed, and the resulting mixture was pre-kneaded with a spatula and then paste-formed using a rotation / revolution vacuum mixer (Thinky Corporation, ARE-500). During pasting, two cycles of processing were performed, each consisting of a stirring mode (1000 rpm x 1 minute) and a degassing mode (2000 rpm x 30 seconds). The resulting paste was then dispersed and mixed using a three-roll mill to prepare a paste-like bonding composition.

[0098] (2) Evaluation The samples obtained in Examples 1 to 6 and Comparative Examples 1 to 6 were evaluated for various properties as follows.

[0099] <Solvent Viscosity> The viscosity of the solvent used in preparing the bonding composition was measured using a rheometer (Thermo Scientific, Rheometer MARS III). The measurement was performed under the following conditions.

[0100] - Measurement mode: Shear rate dependency measurement - Sensor: Parallel type (φ60 mm) - Measurement temperature: 25°C - Gap: 0.300 mm - Shear rate: 0.05 to 120.01 s －１ - Measurement time: 2 minutes

[0101] <Tackiness> The tackiness (adhesion) of the laminate obtained in the lamination process (tackiness process) was evaluated. Specifically, the laminate was stood vertically so that the bonding layer of the laminate included a vertical axis, and after leaving it in that state for 5 seconds, the laminate was returned to a horizontal position. This operation was repeated six times, and the presence or absence of detachment of SiC chips contained in the laminate was checked, and the results were ranked according to the following criteria.

[0102] ◯: The tip did not fall off in any of the six attempts. ×: The tip fell off at least once.

[0103] <Bonding strength> To confirm the bonding strength, the shear strength of the bonded structure was measured. Specifically, the shear strength was measured for five bonded structures made from the same bonding composition, and the minimum, maximum, and average values ​​were calculated. The shear strength (MPa) was calculated by dividing the breaking load (N) by the base area of ​​the SiC chip (mm ２ The measurement was carried out under the following conditions.

[0104] - Measuring device: Condor Sigma manufactured by XYZTEC - Load cell: 200 kgf - Shear tool: Width 6.0 mm, thickness 2.0 mm, shaft 1 / 4 inch (model number TOS663060) - Shear speed: 50 μm / s - Shear height: 0.02 mm (the zero point was the 6 mm square printed coating film portion.)

[0105] (3) Evaluation Results <Solvent Viscosity> The shear rate of the solvents used in Examples 1 to 6 and Comparative Examples 1 to 6 was 10 s －１ The viscosity at 2000 kJ / ml is shown in Table 2 below, along with the structural formula and boiling point of the solvent.

[0106] Octylene glycol (OG) and 3-methyl-1,5-pentanediol (MPD) had relatively high boiling points and viscosities, while hexylene glycol (HG), 1,3-butanediol, and triethylene glycol monobutyl ether (BTG) had low viscosities.

[0107]

[0108] <Tackiness and Bonding Strength> The evaluation results of tackiness and bonding strength are summarized in Table 3 below.

[0109] When the bonding compositions of Examples 1 to 6 containing high-boiling-point, high-viscosity solvents (OG, MPD) and polycarboxylic acids having 3 or more carbon atoms (glutaric acid, pimelic acid, MMA-10R) were used, the tackiness of the coating film was good (◯). The bond strength (shear strength) of the bonded structures was relatively high, at 40 MPa or more. In particular, in Examples 1 and 2, which used glutaric acid as the polycarboxylic acid, the bond strength was very high, at 50 MPa or more.

[0110] In contrast, the bonding compositions of Comparative Examples 1 and 4 to 6, which contained a low-viscosity solvent (HG), produced coatings with poor tackiness (×). In particular, Comparative Examples 5 and 6 did not exhibit tackiness during bonding. Therefore, chip slippage occurred, and bonding was not possible. Furthermore, the bonding compositions of Comparative Examples 1 to 3, which did not contain a polycarboxylic acid, produced bonded structures with bond strengths of less than 40 MPa.

[0111]

[0112] From the above results, it can be seen that the bonding composition of the present embodiment improves the tackiness between the coating film and the object to be bonded, and also makes it possible to obtain a bonded structure having high bonding strength by a simple method.

Claims

1. A bonding composition comprising (A) copper powder, (B) a solvent, and (C) a carboxylic acid, wherein the solvent has a boiling point of 230°C or higher and 300°C or lower and is soluble in water at 25°C and a shear rate of 10 s －１ a liquid organic substance having a viscosity of 100 mPa·s or more under the above conditions, wherein the carboxylic acid includes a polycarboxylic acid having 3 or more carbon atoms.

2. The bonding composition according to claim 1, wherein the polycarboxylic acid is either or both of a dicarboxylic acid and a tricarboxylic acid.

3. A bonding composition according to claim 1 or 2, wherein the content of the polycarboxylic acid is 0.1 mass % or more based on the total amount of the copper powder.

4. The bonding composition according to claim 1 or 2, wherein the content of the liquid organic substance is 30 mass % or more based on the total amount of the solvent.

5. A method for manufacturing a bonded structure in which a first object to be bonded and a second object to be bonded are bonded via a bonding layer, the method comprising the steps of: applying the bonding composition according to claim 1 or 2 to one surface of the first object to be bonded to form a wet film; removing at least a portion of the solvent from the wet film to obtain a coating film; temporarily fixing the second object to be bonded to the coating film by applying pressure to obtain a laminate in which the first object to be bonded, the coating film, and the second object to be bonded are laminated in this order; and heating the laminate under pressure to sinter the copper powder in the coating film, thereby forming a bonding layer to bond the first object to the second object to be bonded.

6. A method for temporarily fixing a first object to be joined and a second object to be joined, comprising the steps of: applying the bonding composition according to claim 1 or 2 to one surface of the first object to be joined to form a wet film; removing at least a portion of the solvent from the wet film to obtain a coating film; and temporarily fixing the first and second objects to be joined via the coating film by applying pressure.

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