JP7589564B2 - Bonding Sheet - Google Patents

Description

The present invention relates to a joining sheet.

When assembling or mounting electronic components, a bonding material is generally used to bond two or more components. A known example of such a bonding material is a paste-like bonding material in which metal particles are dispersed in a solvent. When bonding components using a paste-like bonding material, the bonding material is applied to the surface of one component, the other component is brought into contact with the applied surface, and heated in this state to sinter the metal particles and form a bonding layer, thereby bonding the components. Patent Document 1 describes a silver particle paste.

Also, a sheet-like bonding material in which metal particles are partially sintered is known as a bonding material. When bonding parts using a sheet-like bonding material, the bonding material is placed between the parts and heated in this state to sinter the unsintered parts of the metal particles to form a bonding layer, thereby bonding the parts. Compared to paste-like bonding materials, sheet-like bonding materials have the advantage that voids (air bubbles) are less likely to form in the bonding layer because the solvent does not volatilize when heated. Patent Document 2 describes a sheet-like bonding material in which silver particles are sintered.

Patent No. 6428339 Patent No. 6245933

Silver is useful as a bonding material for electronic components and the like because of its excellent electrical and thermal conductivity. However, silver has the problem of being prone to ion migration and sulfurization. In particular, with the recent trend toward narrower pitches and thinner wiring patterns in electronic components, if ion migration or sulfurization occurs in the bonding layer, short circuits are likely to occur between the pitches of electronic components or between wiring patterns. For this reason, it is considered to use copper as a bonding material. Copper is also cheaper than silver, making it useful for reducing the manufacturing costs of electronic components. However, copper is more easily oxidized than silver. For this reason, bonding sheets made of sintered copper particles have the problem that sintering is difficult due to the oxidation of the copper particles.

This invention was made in consideration of the above-mentioned circumstances, and aims to provide a bonding sheet using copper particles that is less susceptible to deterioration of sinterability due to oxidation of the copper particles, can form a dense bonding layer with few voids, and can bond electronic components and the like with high strength.

In order to solve the above problems, the bonding sheet of the present invention contains copper particles and a solvent having a boiling point of 150°C or higher, the content ratio of the copper particles to the solvent is 99:1 to 90:10 by mass, the BET diameter of the copper particles is in the range of 40 nm to 750 nm, and the solvent has an OH group in its structure.

The bonding sheet of the present invention contains copper particles and a solvent, and the copper particles are coated with the solvent, so that the copper particles are not easily oxidized. In addition, the solvent has a boiling point of 150°C or higher, so that it is not easily volatilized. Therefore, the bonding sheet of the present invention is less likely to suffer from a decrease in sinterability due to oxidation of the copper particles, and has improved shape stability. Furthermore, the copper particles are fine, with a BET diameter in the range of 40 nm to 750 nm, so that the sinterability is high, and the content ratio of the copper particles to the solvent is in the range of 99:1 to 90:10 by mass, and the copper particle content is 90 mass% or more, so that a dense sintered body (bonding layer) of copper particles can be formed by heating. In addition, since the content of the solvent is 10 mass% or less, the amount of volatilized gas and decomposition gas of the solvent generated during heating is small. In addition, since the solvent contains OH groups in its structure, the oxidation of the copper particles can be suitably suppressed. Therefore, according to the bonding sheet of the present invention, a dense bonding layer with few voids can be formed, and electronic components and the like can be bonded with high strength.

In addition, in the bonding sheet of the present invention, it is preferable that the solvent has a hydroxyl group or a carboxyl group. When the solvent has a hydroxyl group, the oxidation of the copper particles can be more suitably suppressed. When the solvent has a carboxyl group, the oxidation of the copper particles can be more suitably suppressed.

In addition, the bonding sheet of the present invention preferably contains a solvent that contains at least one of a diol compound, a triol compound, and a carboxylic acid. In this case, the diol compound, the triol compound, and the carboxylic acid have high adhesion to the copper particles and are therefore unlikely to volatilize for a long period of time. As a result, the deterioration of sinterability due to oxidation of the copper particles can be suppressed for a long period of time, and the shape stability of the bonding sheet is improved for a long period of time.

In addition, in the bonding sheet of the present invention, it is preferable that the surfaces of the copper particles are coated with an organic protective film. By coating the surfaces of the copper particles with an organic protective film, oxidation of the copper particles can be suppressed. In addition, by using a solvent containing an OH group in its structure, the organic protective film can be appropriately removed during sintering, and a decrease in the bonding strength of the bonding layer obtained by sintering the copper particles can be suppressed.

Here, in the bonding sheet of the present invention, it is preferable that the surface of the copper particle has a ratio of the amount of C ₃ H ₃ O ₃ ^- ions to the amount of Cu ⁺ ions detected by analysis using time-of-flight secondary ion mass spectrometry of 0.001 or more.
When the ratio of the detected amount of C ₃ H ₃ O ₃ ^- ions to the detected amount of Cu ⁺ ions on the surface of the copper particles is 0.001 or more, the surfaces of the copper particles are appropriately covered with an organic protective film, and oxidation of the copper particles can be more effectively suppressed.

According to the present invention, it is possible to provide a bonding sheet using copper particles that is less susceptible to a decrease in sinterability due to oxidation of the copper particles, can form a dense bonding layer with few voids, and can bond electronic components and the like with high strength.

FIG. 2 is a schematic cross-sectional view of a bonding sheet according to one embodiment of the present invention. FIG. 2 is a flow diagram showing a method for producing a bonding sheet according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of a bonded body produced using the bonding sheet according to one embodiment of the present invention.

Hereinafter, a bonding sheet according to one embodiment of the present invention will be described with reference to the attached drawings.
The bonding sheet of this embodiment is used as a bonding material for forming a bonded body in which the substrate and the electronic component are bonded via a bonding layer, for example, by placing it between a substrate and an electronic component and heating it in this state to sinter the copper particles and form a bonding layer.

FIG. 1 is a schematic cross-sectional view of a bonding sheet according to one embodiment of the present invention.
1, the bonding sheet 1 contains copper particles 2 and a solvent 3. The ratio of the copper particles 2 to the solvent 3 is 99:1 to 90:10 (=copper particles:solvent) in terms of mass ratio. That is, the bonding sheet 1 has a copper particle 2 content in the range of 90 mass% to 99 mass% and a solvent 3 content in the range of 1 mass% to 10 mass%.

The bonding sheet 1 preferably has a density in the range of 50% to 90%. The density is the proportion of the copper particles 2 spatially occupying the bonding sheet 1. If the density is 60% or more, the adhesion between the copper particles 2 is high, so that it is easier to form a dense bonding layer with few voids. If the density is 90% or less, the surface of the copper particles 2 can be covered with the solvent 3, so that oxidation of the copper particles 2 is less likely to occur, and the decrease in sinterability due to oxidation of the copper particles can be more suppressed. The density is more preferably in the range of 55% to 75%, and particularly preferably in the range of 60% to 70%. The density of the bonding sheet 1 can be calculated by observing the cross section of the bonding sheet 1 with a SEM (scanning electron microscope), binarizing the obtained SEM image, and dividing it into the copper particle 2 portion and the space portion containing the solvent 3. Specifically, it can be calculated by the method described in the examples described later.

There are no particular limitations on the shape or size of the bonding sheet 1. The bonding sheet 1 may be, for example, a circular sheet with a diameter of 1 mm to 50 mm, or a rectangular sheet with one side of 1 mm to 50 mm. There are no particular limitations on the thickness of the bonding sheet 1, but it is preferably in the range of 50 μm to 1000 μm.

The copper particles 2 have a BET diameter in the range of 40 nm to 750 nm. The BET diameter is calculated from the BET specific surface area and true density of the copper particles determined by the BET method, assuming that the copper particles 2 are true spheres or cubes. Specifically, it can be determined by the method described in the examples below.

If the BET diameter of the copper particles 2 is 40 nm or more, it is difficult to form strong aggregates. Therefore, the surface of the copper particles 2 can be uniformly coated with the solvent 3. On the other hand, if the BET diameter of the copper particles 2 is 750 nm or less, the reaction area is large and the sintering property by heating is high, so that a strong bonding layer can be formed. The BET diameter of the copper particles 2 is preferably 50 nm or more and 750 nm or less, more preferably 50 nm or more and 300 nm or less, more preferably 80 nm or more and 200 nm or less, and particularly preferably 80 nm or more and 170 nm or less.

The BET specific surface area of the copper particles 2 is preferably in the range of 2.0 m ² /g or more and 8.0 m ² /g or less, more preferably in the range of 3.5 m ² /g or more and 8.0 m ^{2 /g or less, and particularly preferably in the range of 4.0 m 2 /} g or more and 8.0 m ² /g or less. The shape of the copper particles 2 is not limited to a spherical shape, and may be an acicular shape or a flat plate shape.

The surfaces of the copper particles 2 are preferably coated with an organic protective film, which is a film of an organic material. By being coated with the organic protective film, oxidation of the copper particles 2 is suppressed, and a decrease in sinterability due to oxidation of the copper particles 2 is further unlikely to occur. The organic protective film coating the copper particles 2 is not formed by the solvent 3, and can be said to be not derived from the solvent 3. It can also be said that the organic protective film coating the copper particles 2 is not a copper oxide film formed by the oxidation of copper.

The fact that the copper particles 2 are coated with an organic protective film can be confirmed by analyzing the surface of the copper particles 2 using time-of-flight secondary ion mass spectrometry (TOF-SIMS). For this reason, in this embodiment, the ratio of the amount of C ₃ H ₃ O ₃ ^- ions to the amount of Cu ⁺ ions detected by analyzing the surface of the copper particles 2 using time-of-flight secondary ion mass spectrometry (C ₃ H ₃ O ₃ ^- /Cu ⁺ ratio) is preferably 0.001 or more. It is more preferable that the C ₃ H ₃ O ₃ ^- /Cu ⁺ ratio is in the range of 0.05 to 0.2. Note that the surface of the copper particles 2 in this analysis does not refer to the surface of the copper particles 2 when the organic protective film is removed from the copper particles 2, but refers to the surface of the copper particles 2 including the organic protective film that covers them (i.e., the surface of the organic protective film).

The copper particles 2 may be subjected to surface analysis using time-of-flight secondary ion mass spectrometry to detect C ₃ H ₄ O ₂ ^- ions and C ₅ or more ions. The ratio of the amount of C ₃ H ₄ O ₂ ^- ions to the amount of Cu ⁺ ions (C ₃ H ₄ O ₂ ^- /Cu ⁺ ratio) is preferably 0.001 or more. In addition, the ratio of the amount of C ₅ or more ions to the amount of Cu ⁺ ions (C ₅ or more ions /Cu ⁺ ratio) is preferably less than 0.005.

The C ₃ H ₃ O ₃ ^- ions, C ₃ H ₄ O ₂ ^- ions, and C ₅ or more ions detected by time-of-flight secondary ion mass spectrometry are derived from the organic protective film covering the surface of the copper particles 2. Therefore, when the C ₃ H ₃ O ₃ ^- /Cu ⁺ ratio and the C ₃ H ₄ O ₂ ^- /Cu ⁺ ratio are each 0.001 or more, the surface of the copper particles 2 is less likely to oxidize and the copper particles 2 are less likely to aggregate. Furthermore, when the C ₃ H ₃ O ₃ ^- /Cu ⁺ ratio and the C ₃ H ₄ O ₂ ^- /Cu ⁺ ratio are 0.2 or less, the oxidation and aggregation of the copper particles 2 can be suppressed without excessively reducing the sinterability of the copper particles 2, and further, the generation of decomposition gas of the organic protective film during heating can be suppressed, so that a bonding layer with fewer voids can be formed. In order to further improve the oxidation resistance during storage of the copper particles 2 and further improve the sinterability at low temperatures, the C ₃ H ₃ O ₃ ^- /Cu ⁺ ratio and the C ₃ H ₄ O ₂ ^- /Cu ⁺ ratio are preferably in the range of 0.08 to 0.16. Furthermore, if the C ₅ or more ion/Cu ⁺ ratio is 0.005 times or more, a large amount of organic protective film with a relatively high desorption temperature is present on the particle surface, resulting in insufficient sinterability and a difficult to obtain strong bonding layer. The C ₅ or more ion/Cu ⁺ ratio is preferably less than 0.003 times.

The organic protective film is preferably derived from citric acid. A method for producing copper particles 2 coated with an organic protective film derived from citric acid will be described later. The amount of the organic protective film on the copper particles 2 is preferably in the range of 0.5% by mass to 2.0% by mass, more preferably in the range of 0.8% by mass to 1.8% by mass, and even more preferably in the range of 0.8% by mass to 1.5% by mass, relative to 100% by mass of the copper particles. By having the amount of the organic protective film be 0.5% by mass or more, the copper particles 2 can be uniformly coated with the organic protective film, and the oxidation of the copper particles 2 can be more reliably suppressed. In addition, by having the amount of the organic protective film be 2.0% by mass or less, it is possible to suppress the generation of voids in the sintered body (bonding layer) of the copper particles due to gas generated by decomposition of the organic protective film by heating. The amount of the organic protective film can be measured using a commercially available device. Specifically, it can be measured by the method described in the examples described later.

When the copper particles 2 are heated for 30 minutes at 300°C in an inert gas atmosphere such as argon gas, it is preferable that 50% or more by mass of the organic protective film decomposes. The organic protective film derived from citric acid generates carbon dioxide gas, nitrogen gas, evaporated acetone gas, and water vapor when decomposed.

Copper particles 2 coated with an organic protective film derived from citric acid can be produced, for example, as follows. First, an aqueous dispersion of copper citrate is prepared, and a pH adjuster is added to the aqueous dispersion of copper citrate to adjust the pH to 2.0 or more and 7.5 or less. Next, in an inert gas atmosphere, 1.0 equivalent to 1.2 equivalents of a hydrazine compound capable of reducing copper ions is added as a reducing agent to the aqueous dispersion of copper citrate whose pH has been adjusted, and mixed. The resulting mixture is heated to a temperature of 60° C. to 80° C. in an inert gas atmosphere and held for 1.5 hours to 2.5 hours. As a result, the copper ions eluted from the copper citrate are reduced to produce copper particles 2, and an organic protective film derived from citric acid is formed on the surface of the copper particles 2.

The aqueous dispersion of copper citrate can be prepared by adding powdered copper citrate to pure water such as distilled water or ion-exchanged water to a concentration of 25% by mass or more and 40% by mass or less, and stirring with a stirring blade to uniformly disperse the mixture. Examples of pH adjusters include triammonium citrate, ammonium hydrogen citrate, and citric acid. Among these, triammonium citrate is preferred because it is easy to adjust the pH to a mild level. The reason why the pH of the aqueous dispersion of copper citrate is set to 2.0 or more is to increase the elution rate of copper ions eluted from copper citrate, to rapidly advance the generation of copper particles, and to obtain the target fine copper particles 2. The reason why the pH is set to 7.5 or less is to suppress the eluted copper ions from becoming copper hydroxide (II) and to increase the yield of copper particles 2. The reason why the pH is set to 7.5 or less is to suppress the reduction power of the hydrazine compound from becoming excessively high, and the target copper particles 2 can be easily obtained. It is preferable to adjust the pH of the copper citrate aqueous dispersion to a range of 4 to 6.

The reduction of copper citrate with a hydrazine compound is carried out in an inert gas atmosphere. This is to prevent oxidation of the copper ions dissolved in the liquid. Examples of inert gases include nitrogen gas and argon gas. When reducing copper citrate under acidic conditions, hydrazine compounds have the advantages of not producing residues after the reduction reaction, being relatively safe, and being easy to handle. Examples of this hydrazine compound include hydrazine monohydrate, hydrazine anhydrous, hydrazine hydrochloride, and hydrazine sulfate. Among these hydrazine compounds, hydrazine monohydrate and hydrazine anhydrous are preferred, as they do not contain components that can become impurities such as sulfur and chlorine.

Generally, copper produced in an acidic solution with a pH of less than 7 dissolves. However, in this embodiment, a hydrazine compound, which is a reducing agent, is added and mixed with an acidic solution with a pH of less than 7, and copper particles 2 are produced in the resulting mixed solution. As a result, the components derived from citric acid produced from copper citrate quickly cover the surfaces of the copper particles 2, suppressing the dissolution of the copper particles 2. It is preferable to keep the temperature of the aqueous dispersion of copper citrate after adjusting the pH at 50°C or higher and 70°C or lower to facilitate the progress of the reduction reaction.

The mixed solution containing the hydrazine compound is heated to a temperature of 60°C to 80°C in an inert gas atmosphere and held for 1.5 to 2.5 hours in order to generate copper particles 2 and to form and coat the surfaces of the generated copper particles 2 with an organic protective film. The reason for holding the mixture under heating in an inert gas atmosphere is to prevent oxidation of the generated copper particles 2. The starting material, copper citrate, usually contains about 35% by mass of copper components. By adding a hydrazine compound, which is a reducing agent, to a copper citrate aqueous dispersion containing this amount of copper components, heating it to the above temperature, and holding it for the above time, the generation of copper particles 2 and the generation of an organic protective film on the surface of the copper particles 2 proceed in a well-balanced manner, so that copper particles 2 having a coating amount of the organic protective film in the range of 0.5% by mass to 2.0% by mass can be obtained with respect to 100% by mass of copper particles. If the heating temperature is less than 60°C and the holding time is less than 1.5 hours, the copper citrate is not completely reduced, the rate of production of the copper particles 2 becomes too slow, and the amount of the organic protective film covering the copper particles 2 may become excessive. If the heating temperature exceeds 80°C and the holding time exceeds 2.5 hours, the rate of production of the copper particles 2 becomes too fast, and the amount of the organic protective film covering the copper particles 2 may be too small. The preferred heating temperature is 65°C or higher and 75°C or lower, and the preferred holding time is 2 hours or higher and 2.5 hours or lower.

The copper particles 2 produced in the mixed liquid are separated from the mixed liquid in an inert gas atmosphere, for example, using a centrifuge, and then dried by freeze-drying or vacuum drying to obtain copper particles 2 whose surfaces are coated with an organic protective film. Because the copper particles 2 have their surfaces coated with an organic protective film, they are less likely to oxidize even if stored in the air until they are used as a bonding sheet.

The solvent 3 acts as a binder for the copper particles 2. The solvent 3 also acts as an antioxidant that coats the copper particles 2 and prevents the copper particles 2 from oxidizing.

The boiling point of the solvent 3 is set to be 150°C or higher. Therefore, the solvent 3 is not easily volatilized and is retained in the bonding sheet 1 for a long period of time. The upper limit of the boiling point of the solvent 3 is a temperature lower than the temperature at which the bonding sheet 1 is heated to sinter the copper particles 2. The boiling point of the solvent 3 is preferably 200°C or lower.

The solvent 3 is preferably liquid at room temperature. The solvent 3 preferably has a freezing point of 30°C or higher. When the solvent 3 is liquid at room temperature, the copper particles 2 and the solvent 3 can be easily mixed in the production of the bonding sheet 1.

The solvent 3 preferably has a molecular weight of 1000 or less. When the solvent 3 is a polymer compound, the molecular weight is a number average molecular weight. By setting the molecular weight of the solvent 3 to 1000 or less, it is possible to suppress the solvent from remaining in the bonding layer after heating, thereby suppressing the occurrence of voids. In addition, the solvent 3 is preferably an organic solvent having a molecular weight of 100 to 1000. By using an organic solvent having the above molecular weight as the solvent 3, the surface of the copper particles 2 can be uniformly coated with the solvent 3 by mixing the copper particles 2 and the solvent 3, and the solvent is less likely to flow out during storage of the bonding sheet 1. In addition, since the boiling point of the organic solvent is within an appropriate range, it is possible to suppress the shape change caused by the solvent volatilizing during storage and the sheet drying, and it is possible to suppress the solvent from remaining in the bonding layer after heating. It is more preferable that the molecular weight of the organic solvent is in the range of 200 to 800, and particularly preferably in the range of 200 to 600.

The solvent 3 is a compound having an OH group at the end, and can be said to contain an OH group in the structure. By having an OH group, the oxidation of the copper particles 2 can be suppressed. More specifically, the solvent 3 preferably contains a hydroxyl group as a reducing group. The hydroxyl group is the OH group itself. In addition, the carboxyl group (COOH group) contains an OH group in the structure. By containing a carboxyl group, the solvent 3 can suppress the oxidation of the copper particles. In addition, the solvent 3 may contain both a hydroxyl group and a carboxyl group, and it can be said that it is preferable to contain at least one of a hydroxyl group and a carboxyl group. In order to sinter the copper particles 2, it is necessary to remove the organic protective film on the surface of the copper particles 2 by heating or the like. On the other hand, the copper particles 2 from which the organic protective film has been removed are prone to a decrease in sinterability due to oxidation, and the bonding layer (copper sintered body) obtained by sintering the copper particles 2 is prone to a decrease in bonding strength. By using a solvent containing an OH group, the organic protective film can be appropriately removed during heating for sintering, and the decrease in the bonding strength of the bonding layer obtained by sintering the copper particles 2 can be suppressed, which is preferable. In addition, by using a solvent having an OH group as the solvent 3, oxidation of the copper particles 2 can be suppressed, and the decrease in the bonding strength of the bonding layer obtained by sintering the copper particles 2 can be suppressed. In addition, since solvents having an OH group generally tend to have high boiling points, the bonding sheet 1 using a solvent having a hydroxyl group makes it difficult for the solvent 3 to volatilize, and the shape stability of the sheet is further improved. In addition, since the solvent 3 contains a carboxyl group, oxidation of the copper particle surface can be suppressed, and the decrease in bonding strength can be appropriately suppressed.

The solvent 3 preferably has a dielectric constant of 4 or more and 80 or less, more preferably 10 or more and 45 or less, and even more preferably 20 or more and 40 or less. When the dielectric constant is in this range, the reducing property is appropriately exhibited, and the oxidation of the copper particles 2 can be suppressed. The dielectric constant may be measured with a liquid dielectric constant meter (Model 871, manufactured by Nippon Luft Co., Ltd.).

As the solvent 3, for example, it is preferable to use at least one of monool, diol compound, triol compound, polyol compound, and carboxylic acid, and it is more preferable to use at least one of diol compound, triol compound, and carboxylic acid. Examples of diol compounds include ethylene glycol, diethylene glycol, and polyethylene glycol. Examples of triol compounds include glycerin, butanetriol, and polyoxypropylenetriol. In addition, it is preferable to use monool, polyol, or carboxylic acid as the solvent 3. An example of a polyol compound is "2-ethyl-1,3-hexanediol". Examples of monools include dodecanol, tetradecanol, and hexadecanol. Examples of carboxylic acids include hexanoic acid, octanoic acid, octanoic acid, and decanoic acid. These organic solvents and polymer solvents may be used alone or in combination of two or more. By using such a solvent, the reduction property can be appropriately exhibited, and the oxidation of the copper particles 2 can be suppressed.

Next, a method for producing the bonding sheet 1 of this embodiment will be described.
FIG. 2 is a flow diagram showing a method for producing a bonding sheet according to an embodiment of the present invention.
The bonding sheet of this embodiment can be produced by a method including a mixing step and a molding step, as shown in FIG.

The mixing step S01 is a step of mixing copper particles 2 with a solvent 3. A rotation-revolution mixer or a planetary mixer can be used to mix copper particles 2 with a solvent 3.

The forming step S02 is a step of forming the mixture obtained in the mixing step S01 into a sheet shape.
The mixture can be formed into a sheet by a rolling method using a pressure roller or a pressing method using a mold.

The sheet-like mixture adjusted to the desired thickness in the manner described above can be cut into a desired shape to obtain the joining sheet 1.

Next, a method for producing a bonded body using the bonding sheet of the present embodiment will be described. Fig. 3 is a schematic cross-sectional view of a bonded body produced using the bonding sheet according to one embodiment of the present invention.
3, the bonded body 11 includes a substrate 12, a bonding layer 13, and an electronic component 14. The substrate 12 and the electronic component 14 are bonded to each other via the bonding layer 13.

As the substrate 12, for example, a printed wiring board having an insulating plate and a wiring pattern formed on the insulating plate can be used. There are no particular limitations on the printed wiring board, and a flexible printed wiring board, a rigid printed wiring board, or a rigid flexible printed wiring board can be used.

For example, a semiconductor element, a resistor, a capacitor, or a crystal oscillator can be used as the electronic component 14. Examples of semiconductor elements include SBD (Schottky Barrier Diode), MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor), IGBT (Insulated Gate Bipolar Transistor), LSI (Large Scale Integration), LED chip, and LED-CSP (LED-Chip Size Package).

The bonded body 11 can be manufactured by placing the above-mentioned bonding sheet between the substrate 12 and the electronic component 14 to obtain a laminate, and heating the obtained laminate to sinter the copper particles of the bonding sheet to form the bonding layer 13. The heating temperature of the laminate is, for example, in the range of 150°C to 300°C. The heating time of the laminate is, for example, in the range of 10 minutes to 1 hour. The heating of the laminate is preferably performed in an inert gas atmosphere while pressurizing the laminate in the stacking direction of the laminate. As the inert gas, nitrogen gas or argon gas can be used. The pressurizing pressure of the laminate is preferably in the range of 0.5 MPa to 30 MPa.

The bonding sheet 1 of the present embodiment configured as described above contains copper particles 2 and a solvent 3, and the copper particles 2 are covered with the solvent 3, so that the copper particles 2 are not easily oxidized. In addition, the solvent 3 has a boiling point of 150°C or higher, so that it is not easily volatilized. Therefore, the bonding sheet 1 of the present embodiment is less likely to suffer from a decrease in sinterability due to oxidation of the copper particles 2, and has improved shape stability. In addition, since the solvent 3 has an OH group in its structure, it can suitably suppress the oxidation of the copper particles 2. Furthermore, since the copper particles 2 are fine with a BET diameter in the range of 40 nm or more and 750 nm or less, they have high sinterability, and the content ratio of the copper particles 2 to the solvent 3 is in the range of 99:1 to 90:10 by mass, and the content of the copper particles 2 is 90 mass% or more, a dense sintered body (bonding layer) of the copper particles 2 can be formed by heating. In addition, since the content of the solvent 3 is 10 mass% or less, the amount of volatilized gas and decomposition gas of the solvent generated during heating is small. Therefore, with the bonding sheet 1 of this embodiment, the sintering property is less likely to decrease due to oxidation of the copper particles, and a dense bonding layer with few voids can be formed, allowing electronic components and the like to be bonded with high strength.

In addition, in the bonding sheet 1 of this embodiment, the copper particles 2 have their surfaces covered with an organic protective film. In the bonding sheet 1, the copper particles 2 have their surfaces covered with an organic protective film, so the copper particles are less likely to oxidize. When the copper particles are oxidized, the copper oxide film reduces the bonding strength. Therefore, by making the copper particles less likely to be oxidized by the organic protective film as in this embodiment, the decrease in bonding strength can be suppressed. In addition, the organic protective film can be appropriately removed during sintering by the solvent 3 containing a reducing group, and the decrease in the bonding strength of the bonding layer obtained by sintering the copper particles 2 can be suppressed.

In the bonding sheet 1 of the present embodiment, the ratio of the amount of C ₃ H ₃ O ₃ - ions to the amount of Cu ⁺ ions detected by time-of-flight secondary ion mass spectrometry on the surface of the copper particles is preferably 0.001 or more. By the ratio of the amount of C ₃ H ₃ O ₃ ^- ions to the amount of Cu ⁺ ions being 0.001 or more, ^the surface of the copper particles is appropriately covered with an organic protective film, and the oxidation of the copper particles can be more appropriately suppressed.

In the bonding sheet 1 of this embodiment, when the solvent 3 has a hydroxyl group, the solvent 3 has reducing properties, so oxidation of the copper particles 2 is less likely to occur. In addition, because the solvent 3 has a carboxyl group, the copper particle surface is converted into thermally decomposable copper acetate, so that the decrease in sinterability due to oxidation of the copper particles 2 can be further suppressed.

In addition, in the bonding sheet 1 of this embodiment, when the reducing group of the solvent 3 is a hydroxyl group or a carboxyl group, it has a high affinity for the copper particles, so that oxidation of the copper particles 2 is further suppressed and the solvent 3 is less likely to volatilize. Therefore, the decrease in sinterability due to oxidation of the copper particles 2 can be further suppressed, and the shape stability of the bonding sheet 1 is further improved.

Furthermore, in the bonding sheet 1 of this embodiment, when the solvent 3 contains at least one of a diol compound, a triol compound, and a carboxylic acid, the diol compound, the triol compound, and the carboxylic acid have high adhesion to the copper particles 2 and are therefore unlikely to volatilize for a long period of time. Therefore, the decrease in sinterability due to oxidation of the copper particles can be suppressed for a long period of time, and the shape stability of the bonding sheet is improved for a long period of time.

Although the embodiment of the present invention has been described above, the present invention is not limited to this, and can be modified as appropriate without departing from the technical concept of the invention.
For example, in FIG. 3, a bonded body 11 using the bonding sheet 1 of this embodiment is described in which a substrate 12 and an electronic component 14 are bonded via a bonding layer 13, but the use of the bonding sheet 1 is not limited to this. For example, the bonding sheet 1 can be used when bonding two substrates together. Specifically, it can be used when bonding a base substrate and a substrate (submount substrate) that is relatively smaller in size than the base substrate. It can also be used in a power module when bonding a ceramic circuit substrate incorporating multiple semiconductor elements to a heat sink. The bonding sheet 1 can also be used when bonding an LED element and a submount substrate in an LED device.

[Preparation of copper particles A]
Copper citrate 2.5 hydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and ion-exchanged water were stirred and mixed using a stirring blade to prepare an aqueous dispersion of copper citrate with a concentration of 30% by mass. Next, an aqueous solution of ammonium citrate as a pH adjuster was added to the obtained aqueous dispersion of copper citrate to adjust the pH of the aqueous dispersion of copper citrate to 5. Next, the obtained aqueous dispersion of copper citrate was heated to 50°C, and while maintaining the temperature, an aqueous solution of hydrazine monohydrate (diluted 2 times) as a reducing agent for copper ions was added at one time under a nitrogen gas atmosphere, and the mixture was stirred and mixed using a stirring blade. The amount of the aqueous solution of hydrazine monohydrate added was 1.2 times the amount required to reduce the total amount of copper ions. The obtained mixture was heated to 70°C under a nitrogen gas atmosphere and maintained at that temperature for 2 hours to generate copper particles. The generated copper particles were collected using a centrifuge. The recovered copper particles were dried by a reduced pressure drying method to prepare copper particles A.

[Preparation of copper particles B]
Copper particles B were prepared in the same manner as copper particles A, except that the pH of the copper citrate aqueous dispersion was adjusted to 2.0.

[Preparation of copper particles C]
Copper particles C were prepared in the same manner as copper particles A, except that the pH of the copper citrate aqueous dispersion was adjusted to 7.5.

[Preparation of copper particles D]
Copper particles D were prepared in the same manner as copper particles A, except that the pH of the copper citrate aqueous dispersion was adjusted to 1.7.

[Preparation of copper particles E]
Copper particles E were prepared in the same manner as copper particles A, except that the pH of the copper citrate aqueous dispersion was adjusted to 8.0.

[Preparation of copper particles F]
Copper microparticles F were prepared in the same manner as copper particles A, except that RCV11 (Type-B) manufactured by Dowa Electronics was used.

The BET diameter and coating layer composition of the obtained copper particles A to F were measured using the following method. The results are shown in Table 1 below.

(BET diameter)
The amount of nitrogen gas adsorbed by the copper particles was measured using a specific surface area measuring device (Quantachrome Instruments, QUANTACHROME AUTOSORB-1), and the specific surface area of the copper particles was determined by the BET method. The BET diameter was calculated from the obtained specific surface area S (m ² /g) and the density ρ (g/cm ³ ) of the copper particles according to the following formula.
BET diameter (nm) = 6000/(ρ (g/cm ³ )×S (m ² /g))

(Components of Coating Layer)
A time-of-flight ^secondary ion mass spectrometer (TOF-SIMS: ULVAC PHI, nanoTOFII) was used to detect C ₃ H ₃ O ₃ ^- ions, C ₃ H ₄ O ₂ ^- ions, and ions of C ₅ or more relative to Cu + ions. Specifically, the measurement sample was made by embedding copper powder on the surface of an In foil. The measurement range was a 100 μm square range, the primary ion was Bi ₃ ⁺⁺ (30 kV), and the measurement time was 5 minutes to obtain a TOF-SIMS spectrum. The detected amounts of Cu ⁺ , C ₃ H ₃ O ₃ ^- ions, C ₃ H ₄ O ₂ ^- ions and C ₅ or higher ions were obtained from the obtained TOF-SIMS spectrum, and the detected amount of each ion was divided by the detected amount of Cu ⁺ ions to calculate the C ₃ H ₃ O ₃ ^- /Cu ⁺ ratio, C ₃ H ₄ O ₂ ^- /Cu ⁺ ratio and C ₅ or higher ions/Cu ⁺ ratio.

(Coating amount)
The amount of coating of the copper particles was measured using a differential thermobalance TG8120-SL (manufactured by RIGAKU Corporation). The sample used was copper particles from which moisture had been removed by freeze-drying. In order to suppress oxidation of the copper particles, the measurement was performed in nitrogen (G2 grade) gas, the temperature increase rate was set to 10°C/min, and the weight loss rate when heated from 250°C to 300°C was defined as the amount of coating of the organic protective film. In other words, the amount of coating = (sample weight after measurement) / (sample weight before measurement) x 100 (wt%). The measurement was performed three times for each of the same lot of copper particles, and the arithmetic mean value was taken as the amount of coating.

[Example 1]
The copper particles and polyethylene glycol (molecular weight: 200) as a binder were mixed in a mass ratio of 95:5. The dielectric constant of the binder was measured. The dielectric constant was measured at 22°C using a liquid dielectric constant meter (Model 871, manufactured by Nippon Luft Co., Ltd.).
The mixture was then rolled using a powder rolling machine (2RM-63K, manufactured by Ohno Roll Co., Ltd.) with a pressure roller at a pressure roller gap width of 500 μm to obtain a copper sheet having a thickness of 500 μm. The density of the obtained copper sheet was measured by the following method. In addition, the shear strength and void ratio of a bonded body made using the obtained copper sheet were measured by the following method. The results are shown in Table 2.

(Density)
After sealing the copper sheet with epoxy resin, the copper sheet was cut horizontally to the thickness direction of the copper sheet. The cut surface of the copper sheet was subjected to mechanical polishing and cross-polishing to expose the copper sheet. The cut surface of the copper sheet was then observed at 50,000 times magnification using a SEM (scanning electron microscope). The obtained SEM image was binarized using image processing software (ImageJ, manufactured by the National Institutes of Health, USA) to separate it into a particle portion and a void portion, and the density was calculated using the following formula.
Density (%)=(total area of particle parts/(total area of particle parts+total area of void parts))×100

The density was measured on 10 randomly taken SEM images. The values shown in Table 2 are the average density values calculated from the 10 SEM images.

(Shear strength of joint)
The copper sheet was cut using a commercially available cutter knife to prepare a copper sheet piece (2.5 mm square x 500 μm thick). The above copper sheet piece (2.5 mm square x 500 μm thick) was placed on a 30 mm square x 1 mm thick oxygen-free copper substrate. Next, a 2.5 mm square x 1 mm thick oxygen-free copper dummy element was placed on the copper sheet piece. In this way, a laminate in which the oxygen-free copper substrate and the oxygen-free copper dummy element were laminated via the copper sheet piece was obtained. The obtained laminate was held for 15 minutes under conditions of a nitrogen gas atmosphere, a pressurized pressure of 5 MPa, and a temperature of 250 ° C. using a die bonder (HTB-MM, manufactured by Alpha Design Co., Ltd.) to prepare a bonded body (Sample A) in which a 2.5 mm square oxygen-free copper substrate and an oxygen-free copper dummy element were bonded via a copper bonding layer.

The shear strength of the resulting joint (sample A) was measured by a method conforming to JIS Z 3198-7 (Lead-free solder test method - Part 7: Shear test method for solder joints of chip components). Specifically, a load was applied to the oxygen-free copper dummy element using a tool of a bond tester (Nordson DAGE, SERIES 4000), and the load (maximum shear load) when the oxygen-free copper dummy element peeled off from the copper joint layer was measured. The tool movement speed was 50 μm/sec, and the gap between the tip of the tool and the oxygen-free copper substrate was 50 μm. The obtained maximum shear load was converted into Newton and divided by the area of the copper joint layer (2.5 mm × 2.5 mm) to obtain the shear strength (unit: MPa). Seven joints were produced, and the shear strength of each joint was measured. The values shown in Table 2 are the average shear strengths of the seven joints. A shear strength of 20 MPa or more is good, and one less than 20 MPa is unacceptable.

(Void ratio of bonded body)
The copper sheet was cut using a commercially available cutter knife to prepare a copper sheet piece (10 mm square x 500 μm thick). The above copper sheet piece (10 mm square x 500 μm thick) was placed on a 30 mm square x 1 mm thick oxygen-free copper substrate. Next, a 10 mm square x 1 mm thick oxygen-free copper dummy element was placed on the copper sheet piece. In this way, a laminate in which the oxygen-free copper substrate and the oxygen-free copper dummy element were laminated via the copper sheet piece was obtained. The obtained laminate was held for 15 minutes under conditions of a nitrogen gas atmosphere, a pressurized pressure of 5 MPa, and a temperature of 250 ° C. using a die bonder (HTB-MM, manufactured by Alpha Design Co., Ltd.) to prepare a bonded body (Sample B) in which a 2.5 mm square oxygen-free copper substrate and an oxygen-free copper dummy element were bonded via a copper bonding layer.

The copper bonding layer of the obtained bonded body (sample B) was subjected to ultrasonic inspection using an ultrasonic inspection device (FINE-SAT, Hitachi High-Technologies Corporation). The obtained ultrasonic inspection image was binarized using image processing software (ImageJ, National Institutes of Health, USA) to separate the image into voids (cavities) and the bonded body (copper particle sintered body), and the void ratio was calculated using the following formula.
Void ratio (%) = (total area of voids / area of copper bonding layer (10 mm x 10 mm)) x 100

Seven bonded bodies were produced and the void ratio was measured for each bonded body. The values shown in Table 2 are the average void ratios of the seven bonded bodies. A void ratio of less than 10% is acceptable, and one of 10% or more is unacceptable.

[Inventive Examples 2 to 8, Reference Examples 9 to 12, Inventive Examples 13 to 17, Comparative Examples 1 to 5]
A copper sheet was produced in the same manner as in Example 1 of the present invention, except that the type of copper particles, the type of binder, the boiling point, the number average molecular weight, and the amount of copper particles and binder were changed as shown in Table 2 below. In Table 2, PEG represents polyethylene glycol, DEG represents diethylene glycol, and EG represents ethylene glycol. Then, in the same manner as in Example 1 of the present invention, the density of the obtained copper sheet was measured, and the shear strength and void ratio of the bonded body produced using the obtained copper sheet were measured. The results are shown in Table 2.

The copper sheets of Examples 1 to 8 and 13 to 17 of the present invention, which contain copper particles having a BET diameter in the range of 40 nm or more and 750 nm or less, and a solvent having a boiling point of 150° C. or more and an OH group as a binder, in a mass ratio of 99:1 to 90:10 (copper particles:solvent), all have a high shear strength of 15 MPa or more and a low void ratio of less than 10%, and are unlikely to suffer from a decrease in sinterability due to oxidation of the copper particles. It can be seen that these bonding layers are dense and have few voids, and can bond electronic components and the like with high strength.

In contrast, the bonded body using the copper sheet of Comparative Example 1, which contains polyethylene glycol that is solid at room temperature as a binder, shows a large decrease in shear strength and a large increase in void ratio, and it is clear that a dense bonded layer with few voids cannot be formed. This is thought to be because the polyethylene glycol does not volatilize during heating to produce the bonded body, but remains in the bonded layer. Also, the bonded body using the copper sheet of Comparative Example 2, which contains ethanol with a boiling point of 80°C as a binder, shows an increase in void ratio, and it is clear that a dense bonded layer with few voids cannot be formed. This is thought to be because the ethanol volatilizes all at once during heating to produce the bonded body, causing the pores to become larger.

In Comparative Example 3, in which the content ratio of polyethylene glycol, the binder, exceeds the upper limit of the present invention, the copper sheet obtained by the rolling process adhered with strong adhesion to the surface of the pressure roller of the powder rolling machine. When the copper sheet was peeled off from the pressure roller, the copper sheet was damaged, and the copper sheet of the minimum dimensions required to produce a bonded body could not be recovered, so the shear strength and void ratio of the bonded body could not be evaluated. In Comparative Example 4, in which the content ratio of polyethylene glycol, the binder, is below the lower limit of the present invention, many cracks occurred in the copper sheet obtained by the rolling process. The copper sheet was damaged by these cracks, and the copper sheet of the minimum dimensions required to produce a bonded body could not be recovered, so the shear strength and void ratio of the bonded body could not be evaluated. In Comparative Example 5, in which tetradecane without OH groups was used as a solvent, the shear strength deteriorated, the void ratio increased, and the oxidation of the copper particles could not be suppressed, and the sinterability decreased, so it is understood that a dense bonded layer with few voids cannot be formed.

REFERENCE SIGNS LIST 1 Bonding sheet 2 Copper particles 3 Solvent 11 Bonded body 12 Substrate 13 Bonding layer 14 Electronic component

Claims (5)

A method for producing a solvent having a boiling point of 150° C. or higher, comprising:
The content ratio of the copper particles to the solvent is 99:1 to 90:10 by mass ratio,
The BET diameter of the copper particles is in the range of 40 nm or more and 750 nm or less,
The solvent has an OH group in its structure and a molecular weight of 100 or more and 1000 or less .
Joining sheet. The joining sheet according to claim 1, wherein the solvent has a hydroxyl group or a carboxyl group. The joining sheet according to claim 1 or 2, wherein the solvent contains at least one of a diol compound, a triol compound, and a carboxylic acid. The bonding sheet according to any one of claims 1 to 3, wherein the copper particles have a surface coated with an organic protective film. The bonding sheet according to claim 4, wherein the surface of the copper particle has a ratio of the amount of C ₃ H ₃ O ₃ ^- ions to the amount of Cu ⁺ ions detected by time-of-flight secondary ion mass spectrometry of 0.001 or more.

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