Bonding composition and bonded structure

Abstract

Provided is a bonding composition which has a bleed-out suppression effect, and with which it is possible to obtain a bonded structure that has a bonded site having a high shear strength. This bonding composition contains a copper powder and a solvent. The solvent includes a first solvent that has a boiling point of 150°C or more and 300°C or less and a surface tension of 30 mN / m or more, and a second solvent that has a surface tension of less than 30 mN / m. The content of the first solvent is 0.1 mass% or more and 20 mass% or less with respect to the copper powder, and the content of the second solvent is 1.5 mass% or more and 20 mass% or less with respect to the copper powder.

Description

Bonding composition and bonded structure

[0001] The present invention relates to a bonding composition and a bonded structure.

[0002] In recent years, semiconductor elements known as power devices have been widely used as power conversion and control devices, such as inverters. These semiconductor elements are mounted (joined) on wiring bodies such as circuit boards, ceramic substrates, or lead frames. 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. During bonding (mounting), the bonding composition is applied to the surface of one of the objects to be bonded (such as a substrate) to form a coating film. Next, the other object to be bonded (such as a power device) is placed on the coating film to form a laminate. When the laminate is heat-treated, the metal powder in the bonding composition sinters to form a bonding site, thereby obtaining a bonded structure in which the objects to be bonded are bonded together.

[0005] Patent Documents disclosing such bonding compositions include Patent Documents 1 and 2. Patent Document 1 discloses a bonding copper paste containing metal particles and a dispersion medium, the metal particles including submicro copper particles and micro copper particles (Claim 1 of Patent Document 1). This bonding copper paste is used to bond a first member and a second member such as a semiconductor element (paragraphs

[0072] to

[0085] of Patent Document 1). Patent Document 2 also discloses a bonding copper paste containing copper particles, a solvent, and an additive consisting of a phosphate ester (Claim 1 of Patent Document 2).

[0006] JP 2022-180400 A JP 2022-128185 A

[0007] As described above, bonding compositions (hereinafter sometimes simply referred to as "compositions") containing metal powders such as copper powder have been proposed in the past, but there is still room for improvement in the conventional compositions.

[0008] As described above, during bonding, a paste-like bonding composition is applied to the surfaces of the objects to be bonded, such as substrates. The application is performed by means of printing or the like. During this process, the solvent contained in the bonding composition may wet and spread on the objects to be bonded (substrates), reaching areas other than the printed area. This phenomenon of the solvent wetting and spreading is called a bleed-out phenomenon.

[0009] For example, as shown in Figures 1 and 2, in the case of bonding a substrate 2 and a semiconductor element 6 as a bonded object, a bonding composition 4 is applied to the substrate 2, and the semiconductor element 6 is placed thereon. Note that Figure 1 is a top view showing the state in which the semiconductor element 6 is placed, and Figure 2 is a side view. At this time, the solvent may separate from the applied bonding composition 4 and spread over the upper surface of the substrate 2. This is the bleed-out phenomenon. In this case, a region (bleed-out region) 8 where the solvent spreads beyond the region (paste region) where the bonding composition 4 is applied is formed. Note that the bleed-out region 8 is a region where the solvent spreads and wets, and is a region other than the paste region.

[0010] The bleed-out phenomenon occurs to some extent. However, if the bleed-out phenomenon occurs significantly, the metal powder (copper powder, etc.) contained in the bonding composition flows out together with the solvent and adheres to the insulating portion of the object to be bonded (substrate, etc.), which may cause problems such as poor insulation. Therefore, in order to prevent such problems, it is important to find a way to suppress the bleed-out phenomenon.

[0011] Furthermore, although it is desirable to increase the shear strength (bonding strength) of the bonded portion between the bonded objects to ensure the reliability of the bonded structure, it has been difficult with conventional techniques to simultaneously suppress bleedout and increase the shear strength.

[0012] The present inventors have conducted extensive research in light of these problems, and as a result have found that by replacing a part of the solvent in a bonding composition containing copper powder and a solvent with a high surface tension solvent having predetermined physical properties, it is possible to suppress bleed-out and obtain a bonded structure having a bonded portion with high shear strength.

[0013] The present invention was completed based on such findings, and an object of the present invention is to provide a bonding composition that has the effect of suppressing bleed-out and that can obtain a bonding structure having a bonding portion with high shear strength.

[0014] The present invention encompasses the following aspects (1) to (7). 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."

[0015] (1) A bonding composition comprising copper powder and a solvent, wherein the solvent comprises a first solvent having a boiling point of 150°C or more and 300°C or less and a surface tension of 30 mN / m or more, and a second solvent having a surface tension of less than 30 mN / m, wherein the content of the first solvent is 0.1% by mass or more and 20% by mass or less relative to the copper powder, and the content of the second solvent is 1.5% by mass or more and 20% by mass or less relative to the copper powder.

[0016] (2) The bonding composition according to (1), wherein the surface tension of the first solvent is 30 mN / m or more and 65 mN / m or less, and the surface tension of the second solvent is 20 mN / m or more and less than 30 mN / m.

[0017] (3) The bonding composition according to (1) or (2), wherein the first solvent is at least one solvent selected from the group consisting of alcohol-based solvents, glycol-based solvents, ketone-based solvents, and ester-based solvents.

[0018] (4) The bonding composition according to any one of (1) to (3) above, wherein the second solvent is a liquid carboxylic acid.

[0019] (5) The bonding composition according to any one of (1) to (4), wherein the content of the copper powder in the bonding composition is 80% by mass or more and 96% by mass or less.

[0020] (6) The bonding composition according to any one of (1) to (5), wherein the paste viscosity of the bonding composition is 10 Pa·s or more and 800 Pa·s or less.

[0021] (7) A joining structure comprising a first body to be joined, a second body to be joined, and a sintered body of the joining composition according to any one of (1) to (6) above, which joins the first body to be joined and the second body to be joined.

[0022] According to the present invention, there is provided a bonding composition that has the effect of suppressing bleed-out and that can obtain a bonded structure having a bonded portion with high shear strength.

[0023] 1A and 1B are diagrams (top view and side view) for explaining the bleed-out phenomenon;

[0024] A specific embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described. Note that the present invention is not limited to the following embodiment, and various modifications are possible within the scope of the present invention.

[0025] <<1. Bonding Composition>> The bonding composition of this embodiment includes copper powder and a solvent. The solvent includes a first solvent having a boiling point of 150°C or higher and 300°C or lower and a surface tension of 30 mN / m or higher, and a second solvent having a surface tension of less than 30 mN / m. The content of the first solvent is 0.1% by mass or higher and 20% by mass or lower with respect to the copper powder. The content of the second solvent is 1.5% by mass or higher and 20% by mass or lower with respect to the copper powder. Details of each component are described below.

[0026] <Copper powder> Copper powder is a powder containing copper as a main component, and serves as a constituent material for a joining portion obtained by firing a joining composition. That is, a sintered body of the copper powder contained in the composition constitutes the joining portion. In this specification, the term "powder" or "powder" refers to an aggregate of a large number of particles. It can also be said that a large number of particles constitute the powder or "powder."

[0027] 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.

[0028] 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 the packing of the particles and obtaining a bonding portion with high shear strength, the copper particles are preferably spherical. 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.

[0029] Preferably, the copper powder content in the bonding composition is 80% by mass or more and 96% by mass or less. By appropriately increasing the amount of copper powder, the conductivity and shear strength of the bonded portion are further increased. Furthermore, by appropriately reducing the amount of copper powder, aggregation of copper particles in the composition is suppressed, thereby improving the coatability of the composition. The content is the total amount of copper powder contained in the bonding composition. That is, when the copper powder includes 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.

[0030] Average particle diameter of copper powder (D ５０) is preferably 0.03 μm or more, more preferably 0.05 μm or more. By appropriately increasing the particle size of the copper powder, it is possible to prevent the particles from agglomerating in the composition and improve the particle dispersibility. On the other hand, the average particle size (D ５０ ) is preferably 20 μm or less, more preferably 10 μm or less. By appropriately reducing the particle size of the copper powder, the sinterability of the copper powder can be improved.

[0031] The average particle diameter of the copper powder (D ５０ ) can be determined as follows. First, 2-propanol is added in an amount (mass) 10 times the amount of the bonding composition and thoroughly stirred, followed by repeated washing to remove only the supernatant liquid while leaving the solid content (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 can be observed are randomly selected from the SEM image and their particle diameters (Heywood diameter) are measured. The particle volume when the particles are spherical is calculated from the Heywood diameter, and the particle size distribution on a volume basis is determined from the obtained data. Next, in the obtained particle size distribution, the particle diameter at which the cumulative volume (cumulative volume) from the smaller particle size side is 50% by volume (cumulative volume 50% diameter) is calculated, and this is defined as the average particle diameter (D ５０ ) is defined as follows.

[0032] More preferably, the copper powder has a cumulative volume 50% diameter (D) in the region of less than 1 μm in the particle size distribution measured by SEM observation. ５０ ) of 0.11 μm or more and less than 1 μm, and a cumulative volume 50% diameter (D ５０ ) preferably contains cupric powder having a size of 1 μm or more and 10 μm or less.

[0033] Cuprous powder D ５０ is preferably 0.11 μm or more from the viewpoint of preventing aggregation in the bonding composition and obtaining good particle dispersibility. ５０ From the viewpoint of ensuring sufficient sinterability of the copper powder, the D of the copper powder is preferably 0.9 μm or less, more preferably 0.8 μm or less, and even more preferably 0.6 μm or less.５０ In order to improve the shear strength of the bonded portion obtained by sintering the copper powder, the diameter of the second copper powder is preferably 1 μm or more. ５０ From the viewpoint of improving the coatability of the composition, the thickness is preferably 10 μm or less, more preferably 8 μm or less, and even more preferably 6 μm or less.

[0034] In addition, the D of the copper (I) powder and the copper (II) powder ５０ can be determined as follows. First, the particle size distribution of the entire copper powder is determined using the above-mentioned method. Next, the obtained particle size distribution is divided into a region where the particle diameter is less than 1 μm (first copper powder region) and a region where the particle diameter is 1 μm or more (second copper powder region). When measuring particle diameter, 50 or more particles are measured in each of the first copper powder region and the second copper powder region. Then, the cumulative volume 50% diameter (D ５０ ) was measured and determined as the particle diameter of the first copper powder. Also, the cumulative volume 50% diameter (D ５０ ) is measured and determined as the particle size of the cupric powder.

[0035] The ratio of the copper first powder to the total mass of the copper first powder and the copper second 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 ratio of the copper second powder to the total mass of the copper first powder and the copper second 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-mentioned ranges improves the particle packing property and makes it possible to sufficiently increase the shear strength of the joint. The masses of the copper first powder and the copper second powder can be calculated as follows. Specifically, the dried product (copper powder) prepared as described above is divided into a region of particle size distribution less than 1 μm and a region of 1 μm or more by SEM observation, and the mass of the copper powder in the former region is defined as the mass of the copper first powder, and the mass of the copper powder in the latter region is defined as the mass of the copper second powder. Specifically, the average particle size of the copper powder is calculated in the region of less than 1 μm and the region of 1 μm or more of the particle size distribution, and then the volume of the copper powder is calculated from the average particle size, and the mass of the copper powder in each region is calculated by multiplying the volume by the density.

[0036] 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.

[0037] The solvent has the function of imparting an appropriate viscosity and thus good coatability to the bonding composition, and also has the effect of uniformly dispersing copper particles in the composition, thereby ensuring good electrical conductivity and shear strength at the bonded portion obtained by firing the composition.

[0038] In this embodiment, a mixed solvent of a first solvent and a second solvent is used as the solvent. The first solvent has a relatively high surface tension of 30 mN / m or more. Using a first solvent with a high surface tension makes it possible to suppress bleed-out. This is thought to be because the high surface tension of the entire solvent (the mixed solvent of the first solvent and the second solvent) reduces the wettability of the bonded bodies, thereby suppressing the spreading of the solvent when the bonding composition is applied to the bonded bodies. In contrast, if the surface tension of the first solvent is less than 30 mN / m, the solvent spreads more easily, resulting in insufficient bleed-out suppression. From the viewpoint of dispersibility of the bonding composition, the surface tension of the first solvent is preferably 30.2 mN / m or more and 65 mN / m or less, and more preferably 30.4 mN / m or more and 50 mN / m or less. Surface tension measurement is performed using the method described in the Examples below or a method equivalent thereto.

[0039] The content of the first solvent in the bonding composition is 0.1% by mass or more and 20% by mass or less relative to the copper powder. Here, the first solvent content is the mass ratio of the first solvent to the copper powder. If the content is less than 0.1% by mass, the effect based on the first solvent, i.e., the effect of suppressing bleed-out, becomes insufficient. If the content exceeds 20% by mass, the total amount of solvent becomes excessively large. The paste viscosity of the bonding composition becomes excessively low, which may also result in the occurrence of bleed-out. Furthermore, the shear strength of the bonded portion obtained by firing the bonding composition may decrease. From the viewpoint of achieving a higher level of shear strength improvement and bleed-out suppression, the content of the first solvent is preferably 0.1% by mass or more and 15% by mass or less relative to the copper powder, and more preferably 0.1% by mass or more and 10% by mass or less.

[0040] The boiling point of the first solvent is 150°C or higher and 300°C or lower. If the boiling point exceeds 300°C, volatilization and removal when the bonding composition is fired will be insufficient. If volatilization and removal are insufficient, organic matter will remain at the resulting bonded portion, which may result in reduced shear strength and reduced conductivity. On the other hand, if the boiling point is lower than 150°C, the composition may bump during firing, causing pores at the bonded portion, which may result in reduced conductivity and shear strength at the bonded portion. From the viewpoint of further increasing the conductivity and shear strength at the bonded portion, the boiling point of the first solvent is preferably 150°C or higher and 290°C or lower, more preferably 150°C or higher and 280°C or lower.

[0041] The second solvent has a relatively low surface tension of less than 30 mN / m. This second solvent improves the wettability of the copper powder and improves the dispersibility of the copper powder. This improves the shear strength of the bonded portion of the bonded structure. On the other hand, while the first solvent has a high surface tension, it is not possible to obtain the desired shear strength of the bonded portion using only the first solvent. Therefore, by using a second solvent with a low surface tension in combination with the first solvent as in the present invention, it is possible to improve the shear strength while maintaining the suppression of bleedout. If the surface tension of the second solvent is 30 mN / m or higher, the effect of improving the shear strength becomes insufficient. From the viewpoint of suppressing bleedout, the surface tension of the second solvent is preferably 20 mN / m or higher and 29.5 mN / m or lower, more preferably 22 mN / m or higher and 29 mN / m or lower.

[0042] The content of the second solvent in the bonding composition is 1.5% by mass or more and 20% by mass or less relative to the copper powder. Here, the second solvent content is the mass ratio of the second solvent to the copper powder. If the content is less than 1.5% by mass, the effect of the second solvent will be insufficient, resulting in insufficient improvement in shear strength. If the content is more than 20% by mass, the total amount of solvent will be excessively large. The paste viscosity of the bonding composition will be excessively low, which may cause bleed-out. Furthermore, the shear strength of the bonded portion obtained by firing the bonding composition may be reduced. From the viewpoint of improving shear strength, the content of the second solvent is preferably 2% by mass or more relative to the copper powder. On the other hand, from the viewpoint of achieving a higher level of improvement in shear strength and suppression of bleed-out, the content of the second solvent is preferably 15% by mass or less relative to the copper powder, more preferably 10% by mass or less.

[0043] The boiling point of the second solvent is not limited. However, a range of 150°C to 300°C is preferred. By setting the boiling point to 300°C or less, it is possible to prevent organic matter from remaining at the bonded portion, thereby making it easier to prevent a decrease in conductivity and shear strength. On the other hand, by setting the boiling point to 150°C or more, it is also easier to prevent a decrease in conductivity and shear strength at the bonded portion. From the viewpoint of further increasing the conductivity and shear strength at the bonded portion, the boiling point is preferably 150°C to 290°C, more preferably 150°C to 280°C.

[0044] Preferably, the surface tension of the first solvent is 30 mN / m or more and 65 mN / m or less, and the surface tension of the second solvent is 20 mN / m or more and less than 30 mN / m, thereby achieving both improved shear strength and suppression of bleed-out at a higher level.

[0045] The types of the first and second solvents are not limited as long as they satisfy the above-mentioned requirements. Examples include alcohols, ketones, esters, ethers, hydrocarbons, and organic acids. Specific examples include alcohols such as propylene glycol, ethylene glycol, hexylene glycol, diethylene glycol, 1,3-butanediol, 1,4-butanediol, dipropylene glycol, tripropylene glycol, and dihydroterpineol; ethers such as ethylene carbitol and butyl carbitol; and organic acids such as carboxylic acids. However, it is preferable that the first and second solvents are mutually compatible. Compatibility can be determined by examining whether phase separation occurs when a mixed solvent of the first and second solvents is allowed to stand.

[0046] Preferably, the first solvent is one or more selected from the group consisting of alcohol-based solvents, glycol-based solvents, ketone-based solvents, and ester-based solvents. Examples of the first solvent include, but are not limited to, one or more selected from the group consisting of cyclohexanol, ethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, glycerin, acetonylacetone, acetophenone, monoacetin, triacetin, diethylene glycol, triethylene glycol, methyl triglycol, Γ-butyrolactone, dipropylene glycol, terpineol, dihydroterpineol, octylene glycol, benzyl alcohol, ethoxyethoxyethanol, butoxyethoxyethanol, tripropylene glycol, isophorone, and butyl carbitol acetate. The first solvent may be a single solvent or a mixed solvent of multiple solvents. When a mixed solvent is used, it is preferable that the solvents contained in the mixed solvent are mutually compatible, and the total amount of the solvents contained in the mixed solvent satisfies the desired range of the amount of the first solvent (0.1% by mass to 20% by mass relative to the copper powder).

[0047] Preferably, the second solvent is a liquid carboxylic acid. The liquid carboxylic acid is a carboxylic acid that is liquid at 1 atmosphere and 20°C. When a carboxylic acid is used as a solvent, the copper particles are surface-etched and activated, and a carboxylic acid copper having a sintering-promoting effect is produced. This improves the sinterability of the copper powder, enabling sufficient increase in the shear strength of the bonded portion. Furthermore, the use of a liquid carboxylic acid allows the copper particles to be uniformly dispersed, resulting in a composition with good coatability.

[0048] Examples of liquid carboxylic acids include branched-chain saturated aliphatic monocarboxylic acids having from 4 to 18 carbon atoms. Preferably, the liquid carboxylic acid is at least one selected from secondary saturated aliphatic monocarboxylic acids and tertiary saturated aliphatic monocarboxylic acids that are liquid at 1 atmosphere and 20°C. Such carboxylic acids are preferably at least one selected from 2,2-dimethylbutyric acid, neodecanoic acid, and octylic acid, and more preferably at least one selected from the group consisting of neodecanoic acid and octylic acid. The use of such carboxylic acids can further increase the shear strength of the bonded portion. The second solvent may be a single solvent or a mixed solvent of multiple solvents. When a mixed solvent is used, it is preferable that the solvents contained in the mixed solvent are mutually compatible. Furthermore, the total amount of the solvents contained in the mixed solvent should satisfy the desired range of the second solvent amount (1.5% by mass to 20% by mass relative to the copper powder).

[0049] Preferably, the total content of the first solvent and the second solvent in the bonding composition is 4% by mass or more and 20% by mass or less, more preferably 4% by mass or more and 15% by mass or less, and particularly preferably 4% by mass or more and 10% by mass or less.

[0050] Preferably, the mixed solvent of the first solvent and the second solvent has a surface tension of 20 mN / m or more and 40 mN / m or less. Here, the mixed solvent is a solvent obtained by mixing the first solvent and the second solvent in the same ratio as in the liquid composition. By setting the surface tension of the mixed solvent within the above range, the effect of suppressing bleed-out can be more significantly exhibited.

[0051] In particular, the following combinations of the first and second solvents are preferred: First solvent: terpineol, second solvent: neodecanoic acid First solvent: dipropylene glycol, second solvent: neodecanoic acid First solvent: octylene glycol, second solvent: neodecanoic acid First solvent: diethylene glycol, second solvent: neodecanoic acid First solvent: terpineol, second solvent: octylic acid First solvent: dipropylene glycol, second solvent: octylic acid First solvent: octylene glycol, second solvent: octylic acid First solvent: diethylene glycol, second solvent: octylic acid

[0052] <Other Components> The bonding composition of this embodiment may contain other additives in addition to the copper powder, the first solvent, and the second solvent. Examples of other additives include a sintering aid, a binder resin, a reducing agent, a surface tension modifier, an antifoaming agent, and a viscosity modifier. However, the bonding composition may be composed only of the copper powder, the first solvent, and the second solvent without containing other additives. Even without containing other additives, the bonding composition can sufficiently exhibit the effects of improving shear strength and suppressing bleed-out.

[0053] <Viscosity> Preferably, the paste viscosity of the bonding composition is 10 Pa·s or more and 800 Pa·s or less. Increasing the viscosity to a certain level can more reliably suppress bleed-out. Furthermore, uniform dispersion of copper particles in the composition is possible, suppressing their sedimentation. Furthermore, the filling of copper particles in the coating film obtained by applying the composition is improved, making it possible to obtain a bonded portion with superior conductivity and shear strength. From the same perspective, the paste viscosity of the bonding composition is preferably 30 Pa·s or more, more preferably 50 Pa·s or more. 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 obtain a bonded portion with particularly superior conductivity and shear strength. From the same perspective, the paste viscosity of the bonding composition is preferably 600 Pa·s or less, more preferably 400 Pa·s or less. Viscosity measurement is performed using the method of the examples described below or a method equivalent thereto.

[0054] By using the bonding composition of this embodiment, the bleed-out phenomenon after application of the composition is unlikely to occur. Therefore, it is possible to prevent problems such as poor insulation caused by bleed-out. Furthermore, the bonded portion formed using this bonding composition has high shear strength (bonding strength). Therefore, it is possible to fabricate a highly reliable bonded structure.

[0055] <<2. Manufacturing Method of Bonding Composition>> The bonding composition of this embodiment is manufactured by mixing copper powder, a first solvent, a second solvent, and, if necessary, other additives. Copper powder manufactured by various methods, such as wet reduction, atomization, or electrolytic reduction, can be used. For example, when using wet reduction or atomization, spherical particles are likely to be obtained. When using electrolytic reduction, dendritic or columnar particles are likely to be obtained. Flake-shaped particles can be obtained, for example, 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 can be prepared separately and then mixed. Mixing can be performed using a known mixing device, such as a roll mill.

[0056] <<3. Method for manufacturing bonded structure>> The method for manufacturing the bonded structure of this embodiment includes a step of applying the above-described bonding composition to the surface of a first body to be bonded to form a coating film, and then placing a second body to be bonded on this coating film to obtain a laminate (coating-laminating step), and a step of heat-treating the obtained laminate to sinter the copper powder contained in the coating film, thereby bonding the first body to be bonded and the second body to be bonded and obtaining a bonded structure in which a bonded portion derived from the bonding composition is formed (heating step).

[0057] <Coating and Laminating Step> In the coating and laminating step, the bonding composition is applied to the surface of the first object to be joined to form a coating film, and then the second object to be joined is placed on this coating film to obtain a laminate. The coating film may be formed by a known coating method, such as screen printing, dispense printing, gravure printing, a reverse coating method, or a doctor blade method. The coating film may be provided over the entire surface of the first object to be joined, or may be provided discontinuously on a portion of the surface. Furthermore, from the viewpoint of more reliably ensuring high shear strength, the coating film may be provided not only on the surface of the first object to be joined but also on the surface of the second object to be joined.

[0058] By placing a second object to be joined on the coating film, a laminate having a first object to be joined, a second object to be joined, and a coating film interposed therebetween is obtained. The first object to be joined and the second object to be joined may be made of the same material or different materials. From the viewpoint of obtaining a joint portion having a stably high shear strength, the thickness of the coating film portion when the first object to be joined and the second object to be joined are overlapped is preferably 1 μm or more and 500 μm or less, and more preferably 5 μm or more and 300 μm or less.

[0059] <Heating Step> In the heating step, the obtained laminate is heat-treated to sinter the copper powder contained in the coating film, thereby joining the first and second objects to be joined and obtaining a bonded structure in which a bonded portion derived from the bonding composition is formed. If necessary, the coating film may be dried at a low temperature before the heat treatment. The bonded portion, which is a copper powder sintered body formed by the heat treatment, mechanically bonds the first and second objects to be joined and also electrically and thermally connects them. Therefore, a bonded structure having a bonded portion with excellent electrical conductivity, shear strength, and thermal conductivity is obtained.

[0060] The heat treatment is preferably carried out in a reducing gas atmosphere such as hydrogen or formic acid, or in an inert gas atmosphere such as nitrogen or argon. The heating temperature is preferably less than 300° C., more preferably 150° C. or higher but lower than 300° C., and may be 180° C. or higher but lower than 300° C., or 200° C. or higher but lower than 300° C. The heating time (firing time), provided that the heating temperature is within the above range, is preferably 20 minutes or longer, more preferably 20 minutes or longer but lower than 120 minutes, and even more preferably 30 minutes or longer but lower than 120 minutes.

[0061] The heat treatment is carried out under no pressure or under pressure. The pressure-free treatment is a treatment carried out without applying any pressure other than the weight of the bonded bodies and atmospheric pressure, while the pressure treatment is a treatment carried out with pressure. When the pressure treatment is carried out, a pressure of preferably 0.001 MPa or more, more preferably 0.001 MPa to 20 MPa, and even more preferably 0.01 MPa to 15 MPa is applied.

[0062] <Joining Structure> The joining structure of this embodiment comprises a first body to be joined, a second body to be joined, and a sintered body of a bonding composition that bonds the first body to be joined and the second body to be joined. Here, the bodies to be joined (first body to be joined, second body to be joined) are objects to be joined, 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 chips. Specific examples include semiconductor elements such as power modules, transmitters, amplifiers, and LED modules, lead frames, ceramic substrates with metal plates attached, substrates for mounting semiconductor elements, metal wiring, blocks, power supply members, heat sinks, water-cooled plates, and metal clips.

[0063] Specific examples of the bonding structure include a semiconductor device in which a semiconductor element mounting substrate is used as the first body to be bonded, a semiconductor chip is used as the second body to be bonded, and the two are bonded together with a sintered body of the bonding composition. Also included are a bonded body in which a semiconductor element mounting substrate 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 chip electrode 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.

[0064] The bonded structure obtained by the method of this embodiment has a bonded portion made of a sintered body of the bonding composition. This composition has good sinterability of copper powder, and the bonded portion made of the sintered body has high conductivity and high shear strength. Bonded structures with these characteristics are 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.

[0065] The present invention will be described in more detail using the following examples, but the present invention is not limited to the following examples.

[0066] (1) Preparation of bonding composition [Example 1] As the copper powder, a mixture of copper I powder (Mitsui Mining & Smelting Co., Ltd., CH-0200L1) and copper II powder (Mitsui Mining & Smelting Co., Ltd., CS-20) in a mass ratio of copper I powder:copper II powder = 3:7 was used. ５０ As will be described later, a paste-like bonding composition was prepared, and then the D of the copper(II) powder was measured by the measurement method described above. ５０ is 0.14 μm, and the D of the cupric powder ５０ The diameter of the copper(I) powder and the copper(II) powder was 2.2 μm. Both the copper(I) powder and the copper(II) powder were spherical powders.

[0067] Terpineol was used as the first solvent, and neodecanoic acid (Versatic 10, manufactured by Hexion) was used as the second solvent. Neodecanoic acid (Versatic 10) is a tertiary saturated aliphatic monocarboxylic acid with a molecular chain that is liquid at 1 atmosphere and 20°C, and has 10 carbon atoms.

[0068] To 95 parts by mass of copper powder, 2 parts by mass of a first solvent (terpineol) and 3 parts by mass of a second solvent (neodecanoic acid) were added. The resulting mixture was pre-mixed with a spatula and then made 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.

[0069] (2) Fabrication of Bonded Structure: A bonding composition was applied by dispense printing to the center of the chip mounting area of ​​a copper lead frame (thickness 2.0 mm) as the first bonded object. Next, the back side of a 3 mm square SiC (thickness 0.2 mm, backside gold-plated) as the second bonded object was placed on the center of the bonding composition to obtain a laminate. The thickness of the bonding composition at this time was 50 μm. Under this condition, the bonded structure was fabricated by firing at 150° C. for 90 minutes in a formic acid atmosphere.

[0070] Example 2 A bonding composition and a bonded structure were produced in the same manner as in Example 1, except that dipropylene glycol was used as the first solvent instead of terpineol.

[0071] Example 3 A bonding composition and a bonded structure were produced in the same manner as in Example 1, except that octylene glycol was used as the first solvent instead of terpineol.

[0072] Example 4 A bonding composition and a bonded structure were produced in the same manner as in Example 1, except that the amount of the first solvent (terpineol) added was changed to 3.5 parts by mass and the amount of the second solvent (neodecanoic acid) added was changed to 1.5 parts by mass.

[0073] [Example 5] A bonding composition and a bonded structure were produced in the same manner as in Example 1, except that the amount of copper powder was changed to 93.5 parts by mass, the amount of the first solvent (terpineol) added was changed to 3.5 parts by mass, and the amount of the second solvent (neodecanoic acid) added was changed to 3 parts by mass.

[0074] Example 6 A bonding composition and a bonded structure were produced in the same manner as in Example 1, except that octylic acid was used as the second solvent instead of neodecanoic acid.

[0075] Comparative Example 1: The first solvent (terpineol) was not added. The amount of the second solvent (neodecanoic acid) added was changed to 5 parts by mass. Otherwise, a bonding composition and a bonded structure were produced in the same manner as in Example 1.

[0076] Comparative Example 2 A bonding composition and a bonded structure were produced in the same manner as in Example 1, except that dodecane was used as the first solvent instead of terpineol.

[0077] Comparative Example 3 A bonding composition and a bonded structure were produced in the same manner as in Example 1, except that 1-hexanol was used as the first solvent instead of terpineol.

[0078] Comparative Example 4 A bonding composition and a bonded structure were produced in the same manner as in Example 1, except that the amount of the first solvent (terpineol) added was changed to 4 parts by mass and the amount of the second solvent (neodecanoic acid) added was changed to 1 part by mass.

[0079] The physical properties (surface tension, boiling point) of the first solvent, second solvent, and mixed solvents thereof used in Examples 1 to 6 and Comparative Examples 1 to 4, as well as the contents of the first solvent and second solvent in the compositions (mass ratio relative to copper powder, mass proportion in the composition) are shown in Table 1 below.

[0080] (3) Evaluation The bonding compositions and bonding structures obtained in Examples 1 to 6 and Comparative Examples 1 to 4 were evaluated for various properties in the following manner.

[0081] <Surface Tension> The surface tension of each of the first and second solvents was measured by the Pt plate method using a surface tensiometer (K20 Easy Dyne, manufactured by Kruss). The measurement was performed in an environment of 25°C. The surface tension of the mixed solvent of the first and second solvents was also measured using the same method. The ratio of the first and second solvents in the mixed solvent was the same as the ratio in the bonding composition.

[0082] <Paste Viscosity> The viscosity of the bonding composition paste was measured. The viscosity measurement was carried out under the following conditions: shear rate 10 s －１ The viscosity value was obtained.

[0083] - Measurement device: Rheometer MARS III (manufactured by Thermo Scientific) - Measurement mode: Shear rate dependency measurement - Sensor: Parallel type (φ20 mm) - Measurement temperature: 25°C - Gap: 0.300 mm - Shear rate: 0.05 to 120.01 s －１ - Measurement time: 2 minutes

[0084] <Bleeding Rate> Laminates were prepared in the same manner as in each Example and Comparative Example, except that the copper lead frame was replaced with a 10 mm square AMB substrate for the bleed-out test. The AMB substrate used was a bonded structure of 0.32 mm thick silicon nitride ceramic and 0.2 mm thick copper plate (oxygen-free copper C1020), which was treated for rust prevention with a rust inhibitor containing benzotriazole as the main component. The arithmetic mean height Sa of the AMB substrate was 0.13 μm. The arithmetic mean height Sa was measured within a 2 mm square area using a white light interferometer (Zygo ZeGage® Pro).

[0085] The laminate was observed from the top surface using an optical microscope, and an optical microscope image was taken. Next, in the obtained optical microscope image (top view image), the area (bonding composition area) of the region where the bonding composition was applied (bonding composition region 4 in FIG. 1 ) and the area (bleed-out area) of the region where the solvent was wetted and spread (bleed-out region 8 in FIG. 1 ) were determined, and the ratio (bleed-out area / bonding composition area×100) was calculated as the bleed-out rate (%).

[0086] <Shear Strength> The shear strength of the bonded structure was measured using a bond tester (Condor Sigma, manufactured by XYZTEC). The shear strength was calculated as a function of the breaking load (unit: N) and the bonded area (unit: mm ２ The breaking load was calculated as the ratio (breaking load / bonding area).

[0087] (4) Evaluation Results The evaluation results obtained are summarized in Table 1 below.

[0088] The example samples (Examples 1 to 6) in which the surface tensions and contents of the first and second solvents satisfied the ranges specified in this embodiment had a bleed-out rate of 22.9% or less and a shear strength of 27 MPa or more at the bonded portion. In particular, Examples 1, 2, and 6 had a low bleed-out rate of 16.3% or less and a high shear strength of 50 MPa or more.

[0089] In contrast, the comparative sample (Comparative Example 1) that did not contain the first solvent had a high bleed-out rate of 24.2%. Furthermore, the comparative samples (Comparative Examples 2 and 3) that contained the first solvent but had low surface tensions had relatively high bleed-out rates of 23.6% or more. Furthermore, the comparative sample (Comparative Example 4) that contained an excessively small amount of the second solvent had a very low shear strength of 8 MPa.

[0090] From the above results, it can be seen that the present embodiment provides a bonding composition that has the effect of suppressing bleed-out and can obtain a bonding structure having a bonding portion with high shear strength.

[0091]

[0092] 2 Object to be bonded 4 Bonding composition (bonding composition region) 6 Semiconductor element 8 Bleed-out (bleed-out region)

Claims

1. A bonding composition comprising copper powder and a solvent, wherein the solvent comprises a first solvent having a boiling point of 150°C or more and 300°C or less and a surface tension of 30 mN / m or more, and a second solvent having a surface tension of less than 30 mN / m, wherein the content of the first solvent is 0.1% by mass or more and 20% by mass or less relative to the copper powder, and the content of the second solvent is 1.5% by mass or more and 20% by mass or less relative to the copper powder.

2. The bonding composition according to claim 1, wherein the surface tension of the first solvent is 30 mN / m or more and 65 mN / m or less, and the surface tension of the second solvent is 20 mN / m or more and less than 30 mN / m.

3. The bonding composition according to claim 1 or 2, wherein the first solvent is at least one solvent selected from the group consisting of alcohol-based solvents, glycol-based solvents, ketone-based solvents, and ester-based solvents.

4. The bonding composition according to claim 1 or 2, wherein the second solvent is a liquid carboxylic acid.

5. A bonding composition according to claim 1 or 2, wherein the content of the copper powder in the bonding composition is 80% by mass or more and 96% by mass or less.

6. The bonding composition according to claim 1 or 2, wherein the paste viscosity of the bonding composition is 10 Pa·s or more and 800 Pa·s or less.

7. A joining structure comprising a first object to be joined, a second object to be joined, and a sintered body of the joining composition according to claim 1 or 2, which joins the first object to be joined and the second object to be joined.

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