WO2026018783A1 - Bonding composition, method for producing bonded structure, and method for temporarily securing body to be bonded - Google Patents

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

The present invention relates to a bonding composition, a method for manufacturing a bonded structure, and a method for temporarily fixing objects to be bonded.

In recent years, semiconductor elements (semiconductor chips) known as power devices have come into widespread use as power conversion and control devices, such as inverters. When mounted, semiconductor elements are joined to a substrate, such as a circuit board or ceramic substrate, or to a wiring body, such as a lead frame. Meanwhile, solder has traditionally been widely used to mount semiconductor elements. Furthermore, with growing environmental awareness in recent years, there has been a demand for the use of lead-free solder.

However, due to the nature of power devices, which control high currents, they generate a lot of heat during operation. Lead-free solder, on the other hand, has poor heat resistance, making it insufficient for mounting power devices.

In response to this, it has been proposed to use a paste-like bonding composition containing metal powder such as copper powder instead of solder. When using a bonding composition to bond a pair of objects 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 create a laminate. When the laminate is then heated, the metal powder in the bonding composition sinters to form a bonding layer made of a sintered body, and the objects to be bonded are bonded together via this bonding layer.

It has also been proposed to heat treat the laminate under pressure when joining the objects 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.

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 in sintering the copper particles together (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 when heating (paragraphs [0042] to [0048]).

International Publication No. 2020/032161 International Publication No. 2021/193150

Thus, although pressure joining techniques using joining compositions containing metal powders such as copper powder have been known for some time, there is still room for improvement in these conventional techniques.

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

The process of creating the laminate and the bonding process of heating the laminate are usually carried out in separate equipment, so the laminate needs to be transported between the two processes, during which time external forces such as vibrations can be applied to the laminate. External forces are also applied to the laminate during the bonding process. However, in the laminate, one of the objects to be bonded (the semiconductor element) is simply mounted on the coating and is not sufficiently fixed. As a result, external forces applied during transport or bonding can cause the mounted object to shift position (chip rotation) or fall off (see Figure 1). If the object to be bonded shifts position or falls off, it becomes difficult to perform pressure bonding in the subsequent process, which leads to reduced productivity.

In order to avoid these problems, one possible approach is to lower the drying temperature during coating formation, leaving the solvent in the coating. If the solvent remains in the coating, the tackiness (adhesion) between the coating and the object to be bonded will improve when the object is placed on top of the coating, which is expected to prevent misalignment and detachment.

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, causing the coating to squeeze out from between the bonded objects (a phenomenon known as squeeze-out). If squeeze-out occurs, a high-strength bonding layer cannot be formed. Furthermore, even if a bonding layer is formed, the large amount of solvent contained can cause pores in the bonding layer, which can lead to bonding failure. Bonded structures used to mount devices that generate a lot of heat, such as power devices, are required to have excellent bonding reliability because they are repeatedly subjected to thermal stress caused by the heat generated by the device. Therefore, bonding failure is undesirable.

Another possible measure is to apply a temporary fixing agent (tacking agent) to the coating before the bonded objects are mounted, thereby preventing displacement or detachment of the bonded objects. 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, while 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 poses the problem of complicating the manufacturing process and increasing manufacturing costs.

As such, there is a trade-off between improving tackiness and improving bond strength, and it has been difficult to simultaneously improve both (tackiness and bond strength) while maintaining high productivity using simple methods.

The inventors conducted extensive research in light of these problems. As a result, they developed a bonding composition that contains, in addition to copper powder, a liquid organic substance with a specific boiling point and viscosity, and a specific carboxylic acid. They then discovered that if this bonding composition is used to bond objects to be bonded, 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 using a simple method.

The present invention was completed based on this knowledge, and aims to provide a bonding composition that improves the tackiness between the coating film and the object to be bonded, and that enables a bonded structure with high bonding strength to be obtained using a simple method. The present invention also aims to provide a method for manufacturing a bonded structure using the bonding composition, and a method for temporarily fixing objects to be bonded.

The present invention encompasses 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."

(1) A bonding composition comprising (A) copper powder, (B) a solvent, and (C) a carboxylic acid,
the solvent contains a liquid organic substance having a boiling point of 230°C or higher and 300°C or lower and a viscosity of 100 mPa s or higher at 25°C and a shear rate of 10 s ^-1 ;
The bonding composition, wherein the carboxylic acid includes a polycarboxylic acid having 3 or more carbon atoms.

(2) The bonding composition of (1) above, wherein the polycarboxylic acid is either or both of a dicarboxylic acid and a tricarboxylic acid.

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

(4) A bonding composition according to any one of (1) to (3) above, 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.

(5) A method for manufacturing a bonded structure in which a first bonded body and a second bonded body are bonded via a bonding layer, comprising:
A step of applying the bonding composition according to any one of (1) to (4) above to one surface of a 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;
a step of temporarily fixing a second object to be joined 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; and a step of heating the laminate under pressure to sinter the copper powder in the coating film, thereby forming a bonding layer, thereby bonding the first object to be joined and the second object to be joined together.

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

The present invention provides a bonding composition that improves the tackiness between a coating film and a bonded object, and enables a bonded structure with high bonding strength to be obtained using a simple method. The present invention also provides a method for manufacturing a bonded structure and a method for temporarily fixing bonded objects using the bonding composition.

1A to 1C are diagrams illustrating a manufacturing process of a conventional joined structure. 1A to 1C are diagrams illustrating a manufacturing process of the joined structure of the present embodiment.

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 suitable aspects can be adopted as long as technical consistency can be achieved. For example, one preferred numerical range can be combined with another preferred numerical range.

<<1. Bonding composition >>
The bonding composition of this embodiment (hereinafter sometimes simply referred to as the "composition") contains (A) copper powder, (B) a solvent, and (C) a carboxylic acid. The solvent contains a liquid organic substance having a boiling point of 230°C or higher and 300°C or lower and a viscosity of 100 mPa·s or higher at 25°C and a shear rate of 10 s ^-1 . The carboxylic acid contains a polycarboxylic acid having 3 or more carbon atoms. Such a composition allows a bonded structure having excellent tackiness and high bonding strength to be obtained through a simple bonding process.

<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.

The copper powder may contain elemental copper, with the remainder being unavoidable impurities. The unavoidable impurities may be, for example, oxides that are unavoidably formed on the surfaces of the copper particles that make up 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 50% by mass or more of copper, with the remainder being other elements. An example of such copper powder is copper alloy powder. To enhance 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.

The shape of the copper particles constituting the copper powder is not particularly limited. For example, they may have various shapes, such as spherical, flake (scale-like), polyhedral, dendritic (branch-like), and columnar. However, from the viewpoint of improving particle packing and forming a bonding layer with high strength (bonding strength), the copper powder preferably contains spherical copper particles. Whether copper particles are spherical or not 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, from which the circularity coefficient 4πS/ ^L2 is calculated. The circularity coefficients of multiple copper particles are calculated, and the average value is calculated. Copper particles are defined as spherical when the arithmetic mean value of the circularity coefficients is 0.85 or more.

Preferably, the copper powder content 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. Note that the content is the total amount of copper powder contained in the composition. In other words, when the copper powder contains first copper powder and second copper powder, as described below, it is the total amount of the first copper powder and the second copper powder.

The average particle size (D50) of the copper powder is preferably 0.03 μm or more, more preferably 0.05 μm or more. By appropriately increasing the particle size 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 size (D50) of the copper powder is preferably 20 μm or less, more preferably 10 μm or less. By appropriately decreasing the particle size of the copper powder, the sinterability of the copper powder can be improved. From the perspective of achieving both dispersibility and sinterability, D50 is preferably 0.03 μm or more and 20 μm or less, and more preferably 0.05 μm or more and 10 μm or less.

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) the amount of the bonding composition, and the mixture is thoroughly stirred. This is followed by repeated washing procedures to remove only the supernatant liquid while leaving the solids (cake). The resulting cake is left to dry thoroughly at room temperature, 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 visible in the SEM image are randomly selected and their particle diameters (Heywood diameters) are measured. The particle volume for 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 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).

The particle diameter D50 of the copper powder may also be determined by measuring the particle diameter of the copper powder used when preparing the bonding composition. When the copper particles that make up 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), and 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 is determined. Then, assuming that the copper particles are truly spherical, the volume is calculated from the obtained particle diameter. The 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 taken as D50.

More preferably, the copper powder comprises copper 1 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 copper 2 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 this particle size distribution. If the copper particles constituting the copper powder are not spherical, i.e., flake-shaped (scale-shaped), 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 product is used as the ultrasonic homogenizer. The particle size distribution on a volume basis is then 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. The laser diffraction/scattering particle size distribution analyzer used is the MT-3300EXII manufactured by Microtrackbell, Inc., or an equivalent model.

The D50 of the first copper powder is preferably 0.05 μm or more, from the viewpoint of preventing aggregation in the composition and achieving good particle dispersibility. The D50 of the first copper 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 ensuring sufficient sinterability of the copper powder. The D50 of the second copper 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 second copper 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.

The proportion of copper 1 powder relative to the total mass of copper 1 powder and copper 2 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. Furthermore, the proportion of copper 2 powder relative to the total mass of copper 1 powder and copper 2 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. Setting the blending ratio within the above ranges increases the packing of the particles, making it possible to sufficiently increase the strength of the bonding layer.

The copper powder may be unsurface-treated (surface-untreated copper powder). Alternatively, it may be surface-treated (surface-treated copper powder) to the extent that the effects of this embodiment are not impaired. Examples of surface-treated copper powder include copper powder 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-based coupling agent, an aluminate-based coupling agent, or the like.

<Solvent>
The (B) solvent has the function of imparting an appropriate viscosity and thus 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.

The solvent of this embodiment contains a liquid organic substance having a boiling point of 230°C or higher and 300°C or lower and a viscosity of 100 mPa·s or higher at 25°C and a shear rate of 10 s ^-1 (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 enhance the tackiness between the coating film and the bonded body when manufacturing a bonded structure. That is, because this liquid organic substance has a relatively high boiling point, 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 (e.g., 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.

If the boiling point of the liquid organic substance at 1 atmosphere is less than 230°C, excessive evaporation of the liquid organic substance will occur when the coating film dries. As a result, no liquid organic substance will remain in the dried coating film, potentially making it difficult to achieve improved tackiness. Furthermore, the high temperatures applied in the subsequent pressure bonding process may alter the liquid organic substance, potentially impairing the 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, potentially impairing its shape retention. Furthermore, the large amount of remaining liquid composition may inhibit the sintering of the copper powder, potentially forming pores in the bonding layer (sintered body) that reduce strength. From the perspective of achieving a balanced improvement in tackiness, shape retention, and bonding strength, the boiling point of the liquid organic substance is preferably between 235°C and 290°C, and more preferably between 240°C and 285°C.

If the viscosity of the liquid organic material is less than 100 mPa·s, the viscosity will be too low, making it difficult to achieve the effect of improving tackiness. Furthermore, the shape retention of the coating film will be reduced, which may result in the coating film squeezing out during the subsequent pressure bonding process. There is no upper limit to the viscosity of the liquid organic material. However, from the perspective of improving the coatability of the bonding composition, a viscosity of 10,000 mPa·s or less is preferred. From the perspective 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.

The high-boiling-point, high-viscosity liquid organic substance that meets 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 high-boiling-point, high-viscosity liquid organic substance alone, or may contain a combination of two or more high-boiling-point, high-viscosity liquid organic substances.

For the purpose of improving 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, for example, have a boiling point of less than 230°C or a viscosity of less than 100 mPa·s at 25°C and a shear rate of 10 s ^-1 . In either case, from the viewpoint of improving shape retention and bonding strength, the boiling point of the other solvent components is preferably 300°C or less.

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 does not have to contain any other solvent components.

The amount of solvent in the bonding composition is not limited. However, from the perspective of achieving a balanced improvement in the application properties of the bonding composition and the tackiness and shape retention of the coating film, the total content of 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 solvent is the amount of high-boiling-point, high-viscosity liquid organic substance; when the solvent contains other solvent components, the total amount of high-boiling-point, high-viscosity liquid organic substance and the other solvent components is the amount of solvent.

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

<Carboxylic Acid>
The bonding composition of this embodiment includes (C) a carboxylic acid. The carboxylic acid includes 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 a 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.

Polycarboxylic acids (polycarboxylic acids) are organic acids containing multiple carboxyl groups (-COOH) in one molecule. In the bonding composition, the hydrogen (H) contained in 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, they are thought to form stable adsorption structures with the copper particles. The copper particles that form stable adsorption structures with the high-carbon polycarboxylic acids then 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 sintered copper powder 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 the copper particles and are therefore thought to be insufficient in improving bonding strength.

The number of carboxy groups contained in each molecule of the polycarboxylic acid 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 perspective 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 a deterioration in fillet strength and reduce reliability. Here, a fillet refers to a portion of the bonding composition coating film formed during the production of a bonded structure where the bonded object is not placed. To avoid this problem, a carbon number of 20 or less is preferred.

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). Furthermore, the bonding composition may contain one high-carbon polycarboxylic acid alone, or may contain two or more high-carbon polycarboxylic acids in combination.

Table 1 below shows examples of the structural formulas of several carboxylic acids, including high-carbon polycarboxylic acids. 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, the main component of MMA-10R.

From the perspective 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 high, there is a problem in that the storage stability of the bonding composition is impaired. From the perspective 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.

<Other ingredients>
The bonding composition may contain other components in addition to (A) copper powder, (B) solvent, and (C) carboxylic acid, as long as the effects of this embodiment are not impaired. Examples of such 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 such other components is preferably 15% by mass or less relative to the copper powder. The amount of such 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.

<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 level, 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.

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

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. In other words, the bonding composition is preferably a pressure bonding composition.

<<2. Method for producing bonding composition>>
The bonding composition of this embodiment may be produced by any method as long as it satisfies the above-described requirements. However, it is preferably produced by mixing (A) copper powder, (B) a solvent, (C) a carboxylic acid, and, as necessary, other additives. Copper powder produced by various methods, such as wet reduction, atomization, and electrolytic reduction, can be used. Spherical particles are easily obtained using wet reduction or atomization. Dendritic or columnar particles are easily obtained using electrolytic reduction. Flake-shaped particles can be obtained by applying a mechanical external force to spherical particles to cause plastic deformation. When using copper powder containing fine copper (I) powder and coarse copper (II) powder, the copper (I) powder and the copper (II) powder are prepared and mixed separately. At least a portion of the solvent is a liquid organic substance having a boiling point of 230°C or higher and 300°C or lower and a viscosity of 100 mPa·s or higher at 25°C and a shear rate of 10 s ^-1 . At least a part of the carboxylic acid is a polycarboxylic acid having at least 3 carbon atoms. Mixing may be carried out using a known mixer such as a roll mill.

<<3. Method for manufacturing bonded structure>>
The method for manufacturing a bonded structure of this embodiment includes the following steps: a step of applying the above-described bonding composition 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 bond the first bonded body and the second bonded body via the formed bonding layer (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.

<Joined 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 together. 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, ceramic substrates with metal plates attached, 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.

Specific examples of bonding structures include a semiconductor device in which a semiconductor element mounting substrate is used as the first bonded object, a semiconductor element is used as the second bonded object, and the two are bonded together with a sintered bonding composition. Other examples include a bonded body in which a semiconductor element mounting substrate is used as the first bonded object, a heat sink is used as the second bonded object, and the two are bonded together with a sintered bonding composition, and a bonded body in which a semiconductor element is used as the first bonded object, and a metal clip is used as the second bonded object, and the two are bonded together with a sintered bonding composition.

<Coating process>
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.

From the perspective of increasing the strength of the bonding layer, it is preferable that the thickness of the wet film be relatively large. On the other hand, from the perspective of preventing cracks from occurring in the coating film after solvent removal, it is preferable that the thickness of the wet film be relatively small. From the perspective of improving 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 this 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 wet films are formed on both the first bonded body and the second bonded body.

<Solvent Removal Step>
In the solvent removal process, 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, thereby suppressing deformation of the coating film even when pressure is applied to the coating film in the subsequent bonding process. Furthermore, if the coating film used in the bonding process contains an excessive amount of solvent, pores may form during the heating process during bonding, potentially reducing 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 natural drying, hot air drying, infrared irradiation, hot plate drying, or other means that utilize solvent evaporation. However, it is preferable to heat-dry the wet film to promote solvent evaporation and removal.

When heating and drying a wet film, it is desirable to adjust the heating temperature and heating time to fall within an appropriate range. Controlling the heating temperature and heating time prevents excessive solvent 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 matter remains in the coating film, it is possible to fully utilize the improved tackiness (adhesion) provided by the high-boiling-point, high-viscosity liquid organic matter.

From the viewpoint of achieving a balanced improvement in tackiness, shape retention, and bonding strength, 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 inert gas atmosphere such as nitrogen gas or argon gas, in the air, or in a reduced pressure atmosphere.

In order to achieve a balanced improvement in tackiness, shape retention, and bonding strength, the solvent content in the coating film obtained after the solvent removal process is preferably 0.01% by mass or more and 10% by mass or less, and more preferably 0.1% by mass or more and 9% by mass or less, of the total amount of copper powder.

<Lamination process>
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.

When applying pressure, it is preferable to apply pressure so that the second object to be joined is in close contact with the coating. For example, pressure is applied perpendicular to the coating 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 damage to the components, 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. The pressure application time is preferably 0.01 seconds or more, and 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.

Temporary fixing may be performed at room temperature or while heating. When temporary fixing is performed under heating, excessively high heating temperatures are preferably avoided in order to maintain tackiness by leaving the high-boiling-point, high-viscosity liquid organic substance in the coating film. The heating temperature for the first bonded object and 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 for the second bonded object 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.

<Joining process>
In the bonding process, 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 formed bonding layer. 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. 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.).

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, and 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, and 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 in, for example, air, an inert gas, or a reducing atmosphere, and is preferably performed in an inert gas or a reducing atmosphere.

In this way, a bonded structure can be obtained. The bonded structure of this embodiment has a high strength bonding layer. A bonded structure with these 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.

<<4. Method for temporarily fixing objects to be joined>>
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 (a coating step), a step of removing at least a part of the solvent from the wet film to obtain a coating film (a 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 (a temporary fixing step). Details of the coating step, the solvent removal step, and the temporary fixing step are as described in the method for manufacturing a bonded structure.

The temporary fixing method of this embodiment can improve the tackiness between the coating film and the object to be joined. Furthermore, a joined structure with high joining strength can be obtained using a simple method.

The present invention will be described in more detail using the following examples. However, the present invention is not limited to the following examples.

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

Next, the raw materials were weighed out so that the ratio of octylene glycol (OG), polyethylene glycol (PEG300), and glutaric acid to the copper powder (spherical copper powder and flaky copper powder) was 26% by mass, 1.9% by mass, and 0.3% by mass. The weighed raw materials (spherical copper powder, flaky copper powder, OG, PEG300, and glutaric acid) were then mixed, and the resulting mixture was pre-kneaded with a spatula and then made into a paste using a planetary 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 during the paste-making process. The resulting paste was then dispersed and mixed using a three-roll mill to prepare a paste-like joining composition.

(2) Fabrication of the bonded structure
A substrate (20 mm × 20 mm, thickness 2 mm) made of copper and ceramic laminated 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 × 6 mm, thickness 100 μm) to form a rectangular wet coating film (application step). The wet coating film was dried at 120°C for 30 minutes in a nitrogen atmosphere with an oxygen ( _O2 ) concentration of 3% or less to partially remove the solvent and obtain a coating film (solvent removal step).

Assuming a model component for a semiconductor power device, an Ag-plated SiC chip (5 mm x 5 mm x 190 μm) was prepared as the second bonded object. The SiC chip was then adsorbed onto the adsorption collet of a chip mounter, which had been heated to 170°C. Then, while maintaining that temperature (170°C), the SiC chip was placed in the center of the coating so that the Ag-plated surface was in contact with the coating, and at the same time, the SiC chip was pressed into the coating. When 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, and a laminate was produced (lamination process).

The resulting laminate was then transported to a firing furnace, where it was pressurized to 25 MPa in a nitrogen atmosphere and heated to 300°C. It was then held at that temperature (300°C) for 5 minutes to fire the coating and form a bonding layer, yielding a bonded structure (bonding process).

[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).

[Example 3]
Instead of glutaric acid, which is a carboxylic acid, the same amount of pimelic acid was used. Except for this, a bonding composition and a bonded structure were prepared in the same manner as in Example 1. As shown in Table 1 above, pimelic acid is a dicarboxylic acid having 7 carbon atoms.

[Example 4]
Instead of glutaric acid, which is a carboxylic acid, the same amount of MMA-10R was used. Except for this, 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.

[Example 5]
As the copper powder, 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 in a mass ratio of 70:30. Octylene glycol (OG) was also prepared as the solvent, and glutaric acid and citric acid were also prepared as carboxylic acids. Furthermore, 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 having 5 carbon atoms, and citric acid is a tricarboxylic acid having 6 carbon atoms.

Next, the raw materials were weighed out so that the copper powder (spherical copper powder and flaky copper powder) contained 36.6% by mass of octylene glycol (OG), 1.9% by mass of polyethylene glycol (PEG300), 0.3% by mass of glutaric acid, and 0.1% by mass of citric acid. The weighed raw materials (spherical copper powder, flaky copper powder, OG, PEG300, glutaric acid, and citric acid) were mixed together, and the resulting mixture was pre-mixed with a spatula and then made into a paste using a planetary 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 joining composition.

[Example 6]
As the copper powder, 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 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.

Next, the raw materials were weighed out so that the ratio of octylene glycol (OG), polyethylene glycol (PEG300), and citric acid to the copper powder (spherical copper powder and flaky copper powder) was 27.7% by mass, 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 then mixed, and the resulting mixture was pre-kneaded with a spatula and then made into a paste using a planetary 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 during the paste-making process. The resulting paste was then dispersed and mixed using a three-roll mill to prepare a paste-like joining composition.

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

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

[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 in the same manner as in Example 1.

[Comparative Example 4]
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 in place of the solvent octylene glycol (OG).

[Comparative Example 5]
As the copper powder, 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 in a mass ratio of 70:30. Furthermore, 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 six carbon atoms.

Next, the raw materials were weighed out so that the copper powder (spherical copper powder and flaky copper powder) contained 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. The weighed raw materials (spherical copper powder, flaky copper powder, HG, 1.3-butanediol, PEG300, and citric acid) were mixed together, and the resulting mixture was pre-mixed with a spatula and then made into a paste using a planetary 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 joining composition.

[Comparative Example 6]
As the copper powder, 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 in 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.

Next, the raw materials were weighed out so that the ratios of hexylene glycol (HG), BTG, polyethylene glycol (PEG300), and citric acid were 11.2% by mass, 16.4% by mass, 1.9% by mass, and 0.3% by mass, respectively, of the copper powder (spherical copper powder and flaky copper powder). The weighed raw materials (spherical copper powder, flaky copper powder, HG, BTG, PEG300, and citric acid) were mixed together, and the resulting mixture was pre-mixed with a spatula and then made into a paste using a planetary 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 during the paste-making process. The resulting paste was then dispersed and mixed using a three-roll mill to prepare a paste-like joining composition.

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

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

- Measurement mode: Shear rate dependency measurement - Sensor: Parallel type (φ60 mm)
-Measurement temperature: 25℃
- Gap: 0.300 mm
-Shear rate: 0.05 to 120.01 ^s-1
- Measurement time: 2 minutes

<Tackiness>
The tackiness (adhesion) of the laminate obtained in the lamination process (tack 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.

〇: The tip did not fall off in all six attempts.
×: The tip fell off at least once.

<Joining strength>
To confirm the bonding strength, the shear strength of the bonded structures 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) is a value defined as the breaking load (N)/the base area of the SiC chip (mm ² ). The measurement was performed under the following conditions.

-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 a 6 mm square printed coating film portion.)

(3) Evaluation Results <Solvent Viscosity>
The viscosities of the solvents used in Examples 1 to 6 and Comparative Examples 1 to 6 at a shear rate of 10 s ⁻¹ are shown in Table 2 below, along with the structural formulas and boiling points of the solvents.

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.

<Tackiness, bonding strength>
The evaluation results of tackiness and bonding strength are summarized in Table 3 below.

When the bonding compositions of Examples 1 to 6, which contain high-boiling-point, high-viscosity solvents (OG, MPD) and polycarboxylic acids with three 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 structure was also relatively high, at 40 MPa or more. In particular, Examples 1 and 2, which used glutaric acid as the polycarboxylic acid, had an extremely high bond strength of 50 MPa or more.

In contrast, the bonding compositions of Comparative Examples 1 and 4-6, 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. As a result, chip slippage occurred and bonding was not possible. Furthermore, the bonding compositions of Comparative Examples 1-3, which did not contain a polycarboxylic acid, produced bonded structures with a bond strength of less than 40 MPa.

From the above results, it can be seen that the bonding composition of this embodiment improves the tackiness between the coating film and the object to be bonded, and also makes it possible to obtain a bonded structure with high bonding strength using a simple method.

Claims (6)

A bonding composition comprising (A) copper powder, (B) a solvent, and (C) a carboxylic acid,
the solvent contains a liquid organic substance having a boiling point of 230°C or higher and 300°C or lower and a viscosity of 100 mPa s or higher at 25°C and a shear rate of 10 s ^-1 ;
The bonding composition, wherein the carboxylic acid includes a polycarboxylic acid having 3 or more carbon atoms. The bonding composition according to claim 1, wherein the polycarboxylic acid is either or both of a dicarboxylic acid and a tricarboxylic acid. The 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. 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. A method for manufacturing a bonded structure in which a first bonded body and a second bonded body are bonded via a bonding layer, comprising:
A step of applying the bonding composition according to claim 1 or 2 to one surface of a 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;
a step of temporarily fixing a second object to be joined 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; and a step of heating the laminate under pressure to sinter the copper powder in the coating film, thereby forming a bonding layer, thereby bonding the first object to be joined and the second object to be joined together. A method for temporarily fixing a first object to be bonded and a second object to be bonded, comprising:
A step of applying the bonding composition according to claim 1 or 2 to one surface of a first object to be bonded to form a wet film;
a step of removing at least a part of the solvent from the wet film to obtain a coating film; and a step of temporarily fixing the first and second bodies to be joined together via the coating film by applying pressure.