JP7786202B2 - Joining sheet and method for manufacturing joined body - Google Patents

Description

The present invention relates to a bonding sheet and a method for manufacturing a bonded structure.

When assembling or mounting electronic components, a bonding material may be used to bond two or more components. For example, Patent Document 1 describes a bonding sheet formed by mixing copper particles with a solvent with a boiling point of 150°C or higher and pressing the mixture at room temperature.

Japanese Patent Application Laid-Open No. 2021-116463

Such bonding sheets are required to improve the bonding strength between components.

The present invention was made in consideration of the above, and aims to provide a bonding sheet that can improve the bonding strength between components, and a method for manufacturing a bonded structure.

To solve the above-mentioned problems and achieve the objectives, the bonding sheet disclosed herein is made of copper, has a Young's modulus of 15 GPa or less, and a filling rate of 45% or more.

It is preferable that the Young's modulus of the bonding sheet be 1 GPa or more and 10 GPa or less.

It is preferable that the joining sheet have a filling rate of 45% or more and 60% or less.

It is preferable that the bonding sheet have a thermal conductivity of 1 W/mK or more and 20 W/mK or less.

The joining sheet preferably contains a copper sintered body.

The bonding sheet preferably contains copper particles and a solvent with a boiling point of 150°C or higher.

In order to solve the above-mentioned problems and achieve the objectives, the method for manufacturing a bonded body according to the present disclosure includes the steps of placing the bonding sheet on a first member, placing a second member on the bonding sheet to obtain a laminate in which the bonding sheet is placed between the first member and the second member, and heating the laminate to produce a bonded body in which the first member and the second member are bonded.

This invention can improve the bonding strength between components.

FIG. 1 is a schematic diagram of a bonding sheet according to this embodiment. FIG. 2 is a schematic partially enlarged view of the bonding sheet. FIG. 3 is a flowchart illustrating the method for manufacturing the bonding sheet according to this embodiment. FIG. 4 is a schematic diagram for explaining the method for manufacturing the bonded body. FIG. 5 is a schematic diagram of a bonding sheet according to the second embodiment. FIG. 6 is a flowchart illustrating a method for manufacturing a bonding sheet according to the second embodiment. FIG. 7 is a schematic diagram for explaining the method for manufacturing the bonded body. FIG. 8 is a table showing the properties and evaluation results of the bonding sheets of the examples.

The present invention will now be described in detail with reference to the drawings. The present invention is not limited to the following modes for carrying out the invention (hereinafter referred to as "embodiments"). The components in the following embodiments include those that would be easily conceivable to a person skilled in the art, those that are substantially the same, and those that are within the scope of what is known as equivalents. Furthermore, the components disclosed in the following embodiments can be combined as appropriate.

(First embodiment)
(bonding sheet)
FIG. 1 is a schematic diagram of a bonding sheet according to this embodiment. The bonding sheet 10 according to this embodiment is a sheet-like member for bonding components together. The bonding sheet 10 is made of copper. In this embodiment, the bonding sheet 10 is a copper sintered body, and as shown in FIG. 1 , it has a structure in which multiple copper particles 12 are bonded by sintering. The bonding sheet 10 preferably undergoes further sintering when heated at 150°C or higher and 300°C or lower. Here, "progression of sintering" refers to an increase in the packing ratio as the bonding between the copper particles 12 progresses further. Because the bonding sheet 10 includes a sintered body, the components can be appropriately bonded together by advancing the sintering. Note that the bonding sheet 10 of the first embodiment may include a copper sintered body, and may also include some unsintered copper particles.

The bonding sheet 10 has a filling rate of 45% or more, preferably 45% to 60%, more preferably 45% to 55%, and even more preferably 45% to 50%. By having the filling rate within this range, the surface area can be increased to allow sintering to proceed appropriately, thereby allowing members to be bonded appropriately.
The filling rate is the ratio of bulk density to true density (bulk density/true density). Here, true density refers to the density of the material (here, copper) itself when it is assumed that there are no pores or cracks in the sintered body, i.e., the theoretical density. Also, bulk density refers to the value obtained by dividing the weight of the bonding sheet 10 by the volume calculated from the external dimensions of the bonding sheet 10. The volume calculated from the external dimensions refers to the total volume of the bonding sheet 10, including the pores.
The true density can be, for example, the literature value for the density of copper (e.g., 8.96 g/cm ³ ). The bulk density can be measured using a balance, a coordinate measuring machine, and a micrometer. For example, the area and thickness of the bonding sheet 10 are measured using the coordinate measuring machine and the micrometer, and the value obtained by multiplying the area and thickness is used as the volume calculated from the external dimensions. Then, the weight of the bonding sheet 10 is measured using a balance, and the value obtained by dividing the measured weight by the volume calculated from the external dimensions is used as the bulk density.
As will be described later, the bonding sheet 10 may have a copper sintered body impregnated with resin. In this case, the filling rate refers to the filling rate of the copper sintered body excluding the resin. Hereinafter, the characteristics of the bonding sheet 10 other than the filling rate are specified, but they may also refer to the characteristics of the copper sintered body excluding the resin.

The bonding sheet 10 has a surface 10a which is a main surface on one side and a surface 10b which is a main surface on the other side.
The arithmetic mean roughness Ra of the bonding sheet 10 is preferably 1 μm or more and 40 μm or less, more preferably 1 μm or more and 35 μm or less, and even more preferably 1 μm or more and 30 μm or less. The arithmetic mean roughness Ra of at least one of the surfaces 10a and 10b of the bonding sheet 10 may be within the above range. When the arithmetic mean roughness Ra (surface roughness) is within this range, further sintering can be properly progressed, and the members can be properly bonded to each other.
The arithmetic mean roughness Ra can be measured in accordance with the provisions of JIS B 0601:2001.

2 is a schematic partial enlarged view of the bonding sheet. The bonding sheet 10 preferably has an average particle size (D) of 100 nm to 200 nm, more preferably 120 nm to 180 nm, and even more preferably 140 nm to 160 nm. By having the average particle size within this range, the surface area of the bonding sheet 10 can be kept large, allowing further sintering to proceed appropriately.
In the bonding sheet 10, the copper particles 12 are bonded together by sintering, and therefore the average particle size of the copper particles 12 can be said to be the average value of the particle sizes of the copper particles 12 separated by the interfaces between the bonded copper particles 12 in the bonding sheet 10. For example, the BET diameter calculated based on the BET specific surface area may be used as the average particle size of the copper particles 12. In this case, the amount of nitrogen gas adsorption of the bonding sheet 10 was measured using a specific surface area measuring device (Quantachrome Instruments, QUANTACHROME AUTOSORB-1), and the specific surface area of the bonding sheet 10 was obtained by the BET method. Using the obtained specific surface area S (m ² /g) and the density ρ (g/cm ³ ) of the copper particles, the BET diameter may be calculated according to the following formula, and used as the average particle size of the copper particles 12.
BET diameter (nm) = 6000/(ρ (g/cm ³ )×S (m ² /g))

The bonding sheet 10 has a Young's modulus of 15 GPa or less, preferably 1 GPa to 15 GPa, more preferably 1 GPa to 10 GPa, and even more preferably 1 GPa to 8 GPa. When the Young's modulus is in this range, the bonding sheet 10 can deform appropriately to follow the members during bonding, and the members can be bonded appropriately.
Young's modulus can be measured using a Picodentor HM500 (manufactured by Fisher Instruments, analysis software WIN-HCU ver.7.0), which is a device that complies with the ISO 14577 standard for nanoindentation. Nanoindentation is a method for calculating hardness and Young's modulus from the load and indentation depth applied to a sample.
In this embodiment, the Young's modulus of the bonding sheet 10 is measured five times at five randomly selected locations on the upper surface of the bonding sheet 10 using the device and analysis software, and the average value of the indentation Young's modulus obtained is taken as the Young's modulus of the bonding sheet 10. The measurement conditions were a Vickers indenter terminal, an indentation depth of 2 μm, an indentation speed of 0.067 μm/sec, and a measurement temperature of 25° C., and the measurement was performed with the bonding sheet 10 placed on a 0.4 mm thick silicon wafer that was larger than the bonding sheet 10. The Young's modulus hereinafter may also refer to values measured using the same method.

The bonding sheet 10 preferably has a thermal conductivity of 10 W/mK to 20 W/mK, more preferably 15 W/mK to 55 W/mK, and even more preferably 20 W/mK to 50 W/mK. When the thermal conductivity is in this range, heat from the members to be bonded can be suitably transferred.
The thermal conductivity can be calculated, for example, by converting the resistivity. Specifically, the sheet resistance at 25°C of a 10 mm x 10 mm bonded sheet is measured using a Loresta (MCP-250T, manufactured by Mitsubishi Yuka Co., Ltd.) by the four-probe method, and the resistivity is calculated by multiplying the sheet thickness by the four-probe method. The thermal conductivity at 25°C is calculated by converting the resistivity using the Wiedemann-Franz law.

The bonding sheet 10 preferably has a thickness W of 0.1 mm to 1 mm, more preferably 0.2 mm to 0.7 mm, and even more preferably 0.3 mm to 0.5 mm. The thickness W refers to the distance in the thickness direction Z between the point on the surface 10a that protrudes most from the surface 10a and the point on the surface 10b that protrudes most from the surface 10b, as shown in FIG.
Moreover, the projected area of the bonding sheet 10 when projected in the direction Z is preferably 10,000 mm ² or less, more preferably 6,400 mm ² or less, and even more preferably 3,600 mm ² or less.
By using a bonding sheet 10 having a thickness W and a projected area within this range, members can be bonded appropriately.

The large area bonding index D1 is defined as the value (thickness W/filling rate) obtained by dividing the thickness W of the bonding sheet 10 by the filling rate (%) of the bonding sheet 10. In this case, the large area bonding index D1 is preferably 2.0 or more and 8.3 or less, and more preferably 2.2 or more and 5.0 or less.
When the large area bonding index D1 is in this range, the members can be bonded appropriately to each other.

The bonding sheet 10 may be a copper sintered body impregnated with resin. That is, the bonding sheet 10 may have at least some of its internal pores filled with resin. Examples of resins used here include epoxy resin and silicone resin. By impregnating the bonding sheet 10 with resin, appropriate functions can be imparted to the bonding sheet 10.

(Method for manufacturing bonding sheet)
FIG. 3 is a flowchart illustrating the method for manufacturing the bonding sheet according to this embodiment.

(Preparation of copper particles)
3, in this manufacturing method, first, copper particles 12A are prepared (step S10). The copper particles 12A here are copper particles that are the raw material of the bonding sheet 10, and can also be said to be copper powder before pre-sintering. In other words, by pre-sintering the copper particles 12A, the bonding sheet 10 in which the copper particles 12 are bonded by pre-sintering is manufactured. The copper particles before pre-sintering can be said to be the copper particles 12A, and the copper particles after pre-sintering can be said to be the copper particles 12.

The copper particles 12A preferably have a BET diameter of 50 nm or more and 300 nm or less. 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 12A are true spheres or cubes. Specifically, the BET diameter can be determined by the method described in the Examples below.

If the BET diameter of the copper particles 12A is 50 nm or greater, it is difficult to form strong agglomerates. This allows the surfaces of the copper particles 12 after pre-sintering to be uniformly coated with the solvent 20 described below. On the other hand, if the BET diameter of the copper particles 12A is 300 nm or less, the reaction area is large and the sintering property by heating is high, making it possible to form a strong bonding layer. The BET diameter of the copper particles 12A is preferably in the range of 80 nm to 200 nm, and particularly preferably in the range of 80 nm to 170 nm.

The BET specific surface area of the copper particles 12A is preferably in the range of 2.0 m ² /g to 8.0 m ² /g, more preferably in the range of 3.5 m ² /g to 8.0 ^{m 2} /g, and particularly preferably in the range of 4.0 m ² /g to 8.0 m ² /g. The shape of the copper particles 12A is not limited to spherical, but may be acicular or flat plate-like.

The surfaces of the copper particles 12A are preferably coated with an organic protective film, which is a film of an organic substance. By being coated with the organic protective film, oxidation of the copper particles 12A is suppressed, making it even less likely that a decrease in sinterability due to oxidation of the copper particles 12A will occur. Note that the organic protective film coating the copper particles 12A is not formed by the solvent 20, and can be said to be not derived from the solvent 20. It can also be said that the organic protective film coating the copper particles 12A is not a copper oxide film formed by the oxidation of copper.

The fact that copper particles 12A are coated with an organic protective film can be confirmed by analyzing the surface of copper particles 12A using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Therefore, in this embodiment, copper particles 12A preferably have a ratio of the amount of C 3 H 3 O ₃ ^- ions to the amount of Cu ⁺ ions detected by surface analysis using time-of-flight secondary ion mass spectrometry (C ₃ H ₃ O ₃ ^- /Cu ⁺ ratio) of 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 or more and 0.2 or less. Note that the "surface of copper particles 12A" in this analysis does not refer to the surface of copper particles 12A after the organic protective film has been removed from the copper particles 12A, but refers to the surface of copper particles 12A including the coating organic protective film (i.e., the surface of the organic protective film).

Copper particles 12A may _be subjected to surface analysis using time-of ^- flight secondary ion mass spectrometry to detect _C3H4O2- ions and _C5 or larger ions. The ratio of the amount of _C3H4O2- ions detected to the amount of Cu ⁺ ions detected ( _{C3H4O2- / Cu} ^{+ ratio} ) is preferably 0.001 or ^greater . Furthermore, the ratio of the amount of _{C5 or larger} ions detected to the amount of Cu ⁺ ions detected ( _{C5 or} larger ions/Cu ⁺ ratio) is preferably less than 0.005.

The _C3H3O3- ions ^, _C3H4O2- ions _, and _{C5 or} higher ions detected by time-of-flight _secondary ion mass spectrometry originate from ^the organic protective film coating the surface of the copper particles 12A. Therefore, when _{the C3H3O3-} ^/ _Cu ⁺ ratio and _{the C3H4O2-} /Cu+ ratio are each 0.001 or more, the surface of the copper particles 12A _{is less} likely to oxidize and the copper particles 12A are less likely to aggregate. Furthermore, when _the ^{C3H3O3- /} Cu ⁺ ratio and _{the C3H4O2-} /Cu ⁺ ratio are _{0.2 or} less ^, oxidation and aggregation ^of the copper particles 12A can be suppressed without excessively reducing the sinterability of the copper particles 12A, and further _, the generation of decomposition gas of the organic protective film during heating can be suppressed, thereby forming a bonding layer with fewer voids. To further improve the oxidation resistance of copper particles 12A during storage and the sinterability at low temperatures, the _C3H3O3- ^/ _Cu ⁺ ratio and the _C3H4O2- /Cu ⁺ ratio are preferably in the range of 0.08 to _0.16 . Furthermore, if _{the C5 or} larger ^ion /Cu ⁺ ratio is 0.005 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 _C5 or larger ion/Cu ⁺ ratio is preferably less than 0.003.

The organic protective film is preferably derived from citric acid. A method for producing copper particles 12A coated with a citric acid-derived organic protective film is described below. The coating amount of the organic protective film on the copper particles 12A is preferably in the range of 0.5% to 2.0% by mass, more preferably 0.8% to 1.8% by mass, and even more preferably 0.8% to 1.5% by mass, relative to 100% by mass of the copper particles. A coating amount of the organic protective film of 0.5% by mass or more allows the copper particles 12A to be uniformly coated with the organic protective film, thereby more reliably suppressing oxidation of the copper particles 12A. Furthermore, a coating amount of the organic protective film of 2.0% by mass or less prevents 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 upon heating. The coating amount of the organic protective film can be measured using a commercially available device. For example, the coating amount can be measured using a differential thermobalance TG8120-SL (manufactured by RIGAKU Corporation). In this case, for example, copper particles from which moisture has been removed by freeze-drying are used as the sample. Measurements are made in nitrogen (G2 grade) gas to prevent oxidation of the copper particles, and the temperature is raised at a rate of 10°C/min. The weight loss rate when heated from 250°C to 300°C can be defined as the coating amount of the organic protective film. In other words, coating amount = (sample weight after measurement) / (sample weight before measurement) x 100 (wt%). Measurements can be performed three times using copper particles from the same lot, and the arithmetic mean value can be used as the coating amount.

When copper particles 12A are heated at 300°C for 30 minutes in an inert gas atmosphere such as argon gas, it is preferable that at least 50% by mass of the organic protective film decomposes. The citric acid-derived organic protective film generates carbon dioxide gas, nitrogen gas, acetone vapor, and water vapor upon decomposition.

Copper particles 12A coated with a citric acid-derived organic protective film can be produced, for example, as follows. First, an aqueous dispersion of copper citrate is prepared, and a pH adjuster is added to this copper citrate aqueous dispersion to adjust the pH to 2.0 or more and 7.5 or less. Next, in an inert gas atmosphere, 1.0 to 1.2 equivalents of a hydrazine compound capable of reducing copper ions are added as a reducing agent to this pH-adjusted copper citrate aqueous dispersion 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 to 2.5 hours. This reduces the copper ions eluted from the copper citrate to produce copper particles 12A, and also forms an organic protective film derived from citric acid on the surface of the copper particles 12A.

An 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% to 40% by weight, 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 facilitates mild pH adjustment. The pH of the aqueous dispersion of copper citrate is set to 2.0 or higher to accelerate the elution rate of copper ions eluted from the copper citrate, thereby facilitating the rapid production of copper particles and achieving the desired fine copper particles 12A. The pH is set to 7.5 or lower to prevent the eluted copper ions from converting to copper(II) hydroxide, thereby increasing the yield of copper particles 12A. Setting the pH to 7.5 or lower also prevents the reducing power of the hydrazine compound from becoming excessively high, making it easier to obtain the desired copper particles 12A. The pH of the copper citrate aqueous dispersion is preferably adjusted to a range of 4 to 6.

The reduction of copper citrate with a hydrazine compound is carried out in an inert gas atmosphere to prevent oxidation of copper ions dissolved in the liquid. Examples of inert gases include nitrogen gas and argon gas. Hydrazine compounds have the advantages of not leaving any residue after the reduction reaction when reducing copper citrate under acidic conditions, being relatively safe, and being easy to handle. Examples of such hydrazine compounds 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 could 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 serves as a reducing agent, is added to and mixed with an acidic solution with a pH of less than 7, and copper particles 12A are produced in the resulting mixture. As a result, components derived from citric acid produced from copper citrate quickly coat the surfaces of the copper particles 12A, thereby inhibiting dissolution of the copper particles 12A. After adjusting the pH, the aqueous dispersion of copper citrate is preferably kept at a temperature of 50°C or higher and 70°C or lower to facilitate 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 maintained for 1.5 to 2.5 hours to generate copper particles 12A and to form an organic protective film on the surface of the generated copper particles 12A. Maintaining the heated solution in an inert gas atmosphere is intended to prevent oxidation of the generated copper particles 12A. The starting material, copper citrate, typically contains approximately 35% by weight of copper. Adding a hydrazine compound reducing agent to a copper citrate aqueous dispersion containing this amount of copper, heating it to the above temperature, and maintaining it for the above time ensures a balanced generation of copper particles 12A and the formation of an organic protective film on the surface of the copper particles 12A. This results in copper particles 12A with an organic protective film coverage of 0.5% to 2.0% by weight of copper particles (100% by weight). If the heating temperature is below 60°C and the holding time is less than 1.5 hours, the copper citrate may not be completely reduced, the rate at which the copper particles 12A are produced may be too slow, and the amount of organic protective film coating the copper particles 12A may be excessive. If the heating temperature exceeds 80°C and the holding time exceeds 2.5 hours, the rate at which the copper particles 12A are produced may be too fast, and the amount of organic protective film coating the copper particles 12A 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 12A produced in the mixed solution are separated from the mixed solution in an inert gas atmosphere using, for example, a centrifuge, and then dried using freeze-drying or vacuum drying to obtain copper particles 12A whose surfaces are coated with an organic protective film. Because the copper particles 12A are coated with an organic protective film, they are resistant to oxidation even when stored in the air until they are used as the bonding sheet 10.

(Filling of copper particles into a mold)
Next, as shown in Fig. 3, the prepared copper particles 12A are filled into a mold (step S12). The shape and material of the mold into which the copper particles 12A are filled may be arbitrary.

(Pre-sintering of copper particles)
Next, the copper particles 12A filled in the mold are pressurized at a predetermined temperature to pre-sinter the copper particles 12A, thereby producing a bonding sheet 10 (step S14). In this step, the copper particles 12A filled in the mold are pressurized at a predetermined pressure and held at a predetermined temperature for a predetermined time, thereby pre-sintering the copper particles 12A and producing a bonding sheet 10. The predetermined pressure here is preferably 1 MPa to 30 MPa, more preferably 1 MPa to 20 MPa, and even more preferably 1 MPa to 10 MPa. The predetermined temperature here is preferably 40°C to 250°C, more preferably 40°C to 200°C, and even more preferably 40°C to 150°C. The predetermined time here (the time held at the predetermined pressure and temperature) is preferably 1 minute to 30 minutes, more preferably 2 minutes to 25 minutes, and even more preferably 3 minutes to 15 minutes.
By pre-sintering the copper particles 12A under such conditions, it is possible to appropriately manufacture the bonding sheet 10 that can be appropriately bonded by appropriately deforming during bonding.

Note that the bonding sheet 10 is not limited to being manufactured by the above method, and any method for manufacturing the bonding sheet 10 may be used.

(Method for manufacturing a bonded body)
Next, a method for manufacturing a bonded structure 100 by bonding members together using the bonding sheet 10 will be described. FIG. 4 is a schematic diagram for explaining a method for manufacturing a bonded structure. In this embodiment, the bonded structure 100 is manufactured by bonding a first member A and a second member B using the bonding sheet 10 as a bonding layer. The first member A and the second member B may be any members. For example, one of the first member A and the second base member B may be a substrate, and the other may be an electronic component. That is, a semiconductor module in which a substrate and an electronic component are bonded by a bonding layer may be manufactured as the bonded structure 100. The substrate is not particularly limited, but examples thereof include an oxygen-free copper plate, a copper-molybdenum plate, a high-heat-dissipation insulating substrate (e.g., DCB (Direct Copper Bond)), and a substrate for mounting a semiconductor element such as an LED (Light Emitting Diode) package. Examples of electronic components include semiconductor elements such as IGBTs (Insulated Gate Bipolar Transistors), diodes, Schottky barrier diodes, MOS-FETs (Metal Oxide Semiconductor Field Effect Transistors), thyristors, logic, sensors, analog integrated circuits, LEDs, semiconductor lasers, and oscillators.

In this manufacturing method, as shown in step S20 of Figure 4, the bonding sheet 10 is placed on the surface of the first member A. In the example of Figure 4, the bonding sheet 10 is placed on the first member A so that the surface 10b of the bonding sheet 10 contacts the surface of the first member A.

Next, as shown in step S22, a solvent 20 is applied to the surface 10a of the bonding sheet 10 (the surface not in contact with the first component A). The solvent 20 acts as a binder for the copper particles 12. The solvent 20 is an organic solvent. Note that the addition of the solvent 20 is not essential.

The boiling point of solvent 20 is preferably 150°C or higher, and preferably 200°C or lower. The boiling point of solvent 20 is more preferably 150°C or higher and 300°C or lower, and even more preferably 200°C or higher and 250°C or lower. Furthermore, solvent 20 preferably has a molecular weight in the range of 100 to 1000, more preferably 200 to 800, and particularly preferably 200 to 600. Furthermore, the solvent is preferably a compound having a reducing group at its terminal. The reducing group is preferably a hydroxyl group. Furthermore, solvent 20 preferably has a dielectric constant in the range of 4 to 80, more preferably 10 to 45, and even more preferably 20 to 40. The dielectric constant may be measured using a liquid dielectric constant meter (Model 871, manufactured by Nippon Luft Co., Ltd.).

For example, diol compounds and triol compounds can be used as the solvent 20. Examples of diol compounds include ethylene glycol, diethylene glycol, and polyethylene glycol. Examples of triol compounds include glycerin, butanetriol, and polyoxypropylenetriol. These organic solvents and polymer solvents may be used alone or in combination of two or more.

The solvent 20 is preferably added in an amount of 0.5% to 10% by mass, more preferably 1% to 8%, and even more preferably 2% to 5% by mass, relative to the bonding sheet 10. By keeping the amount of solvent 20 added within this range, proper bonding properties can be maintained.

As shown in step S24, the solvent 20 penetrates into the pores of the bonding sheet 10 and fills them. In other words, the bonding sheet 10 becomes impregnated with the solvent 20.

Then, as shown in step S26, the second member B is placed on the surface 10a of the bonding sheet 10 impregnated with the solvent 20. In other words, the bonding sheet 10 impregnated with the solvent 20 is placed between the first member A and the second member B.

Then, the laminate in which the bonding sheet 10 is disposed between the first member A and the second member B is heated to further promote sintering of the bonding sheet 10, resulting in a bonded body 100 in which the first member A and the second member B are bonded together with a bonding layer (sintered bonding sheet 10). The heating temperature of the laminate may be, for example, in the range of 150°C to 300°C. The heating time of the laminate may be, for example, in the range of 10 minutes to 1 hour. The laminate is preferably heated in an inert gas atmosphere while applying pressure to the laminate in the stacking direction. Nitrogen gas or argon gas can be used as the inert gas. The pressure applied to the laminate is preferably in the range of 0.5 MPa to 30 MPa.

(effect)
As described above, the bonding sheet 10 according to this embodiment is made of copper, has a Young's modulus of 15 GPa or less, and a packing ratio of 45% or more. The bonding sheet 10 according to this embodiment has such a relatively small Young's modulus that it can appropriately deform to follow the components during bonding, and the packing ratio within this range increases the surface area, allowing sintering to proceed appropriately, so that the components can be appropriately bonded together.

The bonding sheet 10 preferably has a Young's modulus of 1 GPa or more and 10 GPa or less. Having a Young's modulus within this range allows for more appropriate bonding of components.

The bonding sheet 10 preferably has a filling rate of 45% or more and 60% or less. A filling rate within this range allows for more appropriate bonding of components.

The bonding sheet 10 preferably has a thermal conductivity of 10 W/mK or more and 20 W/mK or less. Having a thermal conductivity within this range ensures appropriate heat transfer properties for the bonded structure 100.

The joining sheet 10 preferably contains a copper sintered body. By including a copper sintered body, the joining sheet 10 can properly join components together.

The manufacturing method for the bonded structure 100 according to this embodiment includes the steps of placing the bonding sheet 10 on a first member A, placing a second member B on the bonding sheet 10 to obtain a laminate in which the bonding sheet is placed between the first member and the second member, and heating the laminate to manufacture the bonded structure 100 in which the first member A and the second member B are bonded. According to this manufacturing method, the bonded structure 100 can be manufactured appropriately by using the bonding sheet 10.

Second Embodiment
Next, a second embodiment will be described. The bonding sheet 10A according to the second embodiment differs from the first embodiment in that it does not include a copper sintered body. That is, the bonding sheet according to the present disclosure may include sintered copper particles 12A or may include unsintered copper particles 12A. Descriptions of parts of the second embodiment that are common to the first embodiment will be omitted.

Figure 5 is a schematic diagram of a bonding sheet according to the second embodiment. The bonding sheet 10A according to the second embodiment is made of copper, as in the first embodiment, but is not a sintered copper body. As shown in Figure 5, the bonding sheet 10A contains copper particles 12A and a solvent 20. In other words, the bonding sheet 10A contains copper particles 12A in an unsintered (pre-sintered) state.

The characteristics of the copper particles 12A contained in the joining sheet 10A are similar to those of the copper particles 12A that are the raw material for the joining sheet 10 of the first embodiment before pre-sintering, and therefore will not be described further.

The properties of the solvent 20 contained in the bonding sheet 10A are similar to those of the solvent 20 used to bond members A and B using the bonding sheet 10 in the first embodiment, and therefore will not be described further.

The content ratio of copper particles 12A to solvent 20 in bonding sheet 10A is preferably 99:1 to 90:10 (copper particles:solvent) by mass. That is, bonding sheet 10A preferably has a copper particle 12A content in the range of 90% to 99% by mass, and a solvent 20 content in the range of 1% to 10% by mass. The content ratio of copper particles 12A to solvent 20 is more preferably 99:1 to 92:8 by mass, and even more preferably 98:2 to 95:5 by mass.

The bonding sheet 10A may have the same properties as the bonding sheet 10 of the first embodiment, except that it does not contain a sintered body. That is, for example, the filling rate, Young's modulus, thermal conductivity, thickness W, projected area, and large-area bonding index D1 of the bonding sheet 10A may be in the same range as those of the bonding sheet 10 of the first embodiment. Furthermore, the methods for measuring these properties may also be the same as those of the first embodiment.

(Method for manufacturing bonding sheet)
FIG. 6 is a flowchart illustrating a method for manufacturing a bonding sheet according to a second embodiment. As shown in FIG. 6 , in this manufacturing method, copper particles 12A and a solvent 20 are prepared (step S30), and then the prepared copper particles 12A and the solvent 20 are mixed (step S32). In the step of mixing the copper particles 12A and the solvent 20, the copper particles 12A and the solvent 20 may be mixed so that the content ratio of the copper particles 12A to the solvent 20 is preferably 99:1 to 90:10 (copper particles:solvent), more preferably 99:1 to 92:8, and even more preferably 98:2 to 95:5 by mass. The copper particles 12A and the solvent 20 may be mixed by any method, but for example, a rotation-revolution mixer or a planetary mixer may be used.

Next, a bonding sheet 10A is formed using a mixture of copper particles 12A and solvent 20 (step S34). In this process, the mixture of copper particles 12A and solvent 20 is pressed at a predetermined pressure for a predetermined time at room temperature to form a sheet, thereby obtaining a bonding sheet 10A. Room temperature may refer to room temperature, but is not limited thereto, and may be a temperature at which sintering of the copper particles 12A does not proceed (for example, 100 ° C or less). The predetermined pressure here is preferably 1 MPa or more and 30 MPa or less, more preferably 5 MPa or more and 25 MPa or less, and even more preferably 10 MPa or more and 20 MPa or less. The predetermined time here (the time held at the predetermined pressure) is preferably 1 minute or more and 30 minutes or less, more preferably 2 minutes or more and 25 minutes or less, and even more preferably 3 minutes or more and 20 minutes or less.
By molding the bonding sheet 10A under these conditions, a bonding sheet 10 that can be properly deformed during bonding can be manufactured. The mixture can be molded into a sheet by a rolling method using a pressure roller or a pressing method using a mold. For example, the mixture may be molded into a sheet and then cut into a predetermined shape to produce the bonding sheet 10A.

Note that the bonding sheet 10A is not limited to being manufactured by the above method, and any manufacturing method for the bonding sheet 10A may be used.

(Method for manufacturing a bonded body)
Next, a method for manufacturing a bonded body 100A by bonding members together using the bonding sheet 10A will be described. Fig. 7 is a schematic diagram for explaining the manufacturing method of the bonded body. In this embodiment, the bonded body 100A is manufactured by bonding a first member A and a second member B using the bonding sheet 10A as a bonding layer.

The bonded body 100A can be produced by placing a bonding sheet 10A between a first member A and a second member B to obtain a laminate, and then heating the resulting laminate to sinter the copper particles 12A of the bonding sheet 10A to form a bonding layer. The heating temperature for the laminate is, for example, in the range of 150°C to 300°C. The heating time for the laminate is, for example, in the range of 10 minutes to 1 hour. The laminate is preferably heated in an inert gas atmosphere while being pressurized in the stacking direction. Nitrogen gas or argon gas can be used as the inert gas. The pressurization pressure for the laminate is preferably in the range of 0.5 MPa to 30 MPa.

As explained above, the bonding sheet 10A according to the second embodiment contains copper particles 12A and a solvent 20 having a boiling point of 150°C or higher. The bonding sheet 10A according to the second embodiment is a molded body of copper particles 12A and solvent 20 that have not been pre-sintered, and has a Young's modulus of 15 GPa or less and a filling rate of 45% or more. This allows the sheet to deform appropriately to follow the components during bonding, and the increased surface area allows sintering to proceed appropriately, thereby enabling the components to be bonded appropriately.

(Example)
Next, examples will be described. Fig. 8 is a table showing the properties and evaluation results of the bonding sheets of the respective examples.

Example 1
(Preparation of copper particles)
In Example 1, 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 was added as a pH adjuster to the resulting aqueous dispersion, adjusting the pH of the copper citrate dispersion to 5. The resulting aqueous dispersion of copper citrate was then heated to 50°C, and while maintaining the temperature, an aqueous solution of hydrazine monohydrate (diluted 2-fold) was added all at once as a copper ion reducing agent under a nitrogen gas atmosphere, followed by stirring and mixing using a stirring blade. The amount of hydrazine monohydrate aqueous solution added was 1.2 times the amount required to reduce all copper ions. The resulting mixture was heated to 70°C under a nitrogen gas atmosphere and maintained at that temperature for 2 hours to produce copper particles. The produced copper particles were collected using a centrifuge. The collected copper particles were dried by a reduced pressure drying method to prepare copper particles.

A specific surface area measuring device (Quantachrome Instruments, QUANTACHROME AUTOSORB-1) was used. After removing the adsorbed gas in advance at a degassing temperature of 50°C for 60 minutes, the amount of nitrogen gas adsorbed by the copper particles was measured, and the specific surface area of the copper particles was determined by the BET method. Using the obtained specific surface area S (m ² /g) and the density ρ (g/cm ³ ) of the copper particles, the BET diameter was calculated according to the following formula.
BET diameter (nm) = 6000/(ρ (g/cm ³ )×S (m ² /g))

(Production of bonding sheet)
An aluminum frame with an outer dimension of 50 mm square x 30 mm and an inner dimension of 30 mm square x 30 mm was prepared as a mold for filling the prepared copper particles. 1.5 g of copper particles was filled into this aluminum frame, uniformly leveled with a 30 mm square x 20 mm aluminum block, and then PEG (polyethylene glycol) was added as a solvent. The mixture was hot-pressed in air at 20 MPa and 40 ° C for 15 minutes to produce a bonding sheet. A CYPT-50kN hot press machine (manufactured by Shinto Kogyo Co., Ltd.) was used. After the hot press treatment, the mixture was cooled to room temperature, and the 30 mm square x 0.4 mm bonding sheet sandwiched between the aluminum blocks was peeled off from the aluminum block to obtain the desired bonding sheet.

(Characteristics of the bonding sheet)
The Young's modulus of the bonding sheet was measured using a Picodentor. The Young's modulus was measured using the method described in this embodiment.
The filling rate of the bonding sheet was also measured. The filling rate was measured as follows: The volume calculated from the external dimensions of the bonding sheet was measured using a vernier caliper and a micrometer. Specifically, five points on each of the two horizontal sides of the bonding sheet were randomly measured, and the product of the average values was used as the area of the bonding sheet. Furthermore, the thickness of the bonding sheet was randomly measured at 10 points using a micrometer, and the average value was used as the thickness of the bonding sheet. The product of the area and the thickness was used as the volume calculated from the external dimensions of the bonding sheet. Next, the weight of the bonding sheet was measured. The weight of the bonding sheet was then divided by the volume calculated from the external dimensions of the bonding sheet 10 to calculate the bulk density. The volume calculated from the external dimensions refers to the total volume of the bonding sheet 10, including the pores. The true density was set to 8.96 g/cm ³ , and the ratio of the bulk density to the true density was calculated as the filling rate.
The thermal conductivity of the bonding sheet was also measured. Specifically, the sheet resistance at 25°C of a 10mm x 10mm bonding sheet was measured using a Loresta (MCP-250T, manufactured by Mitsubishi Yuka Co., Ltd.) by a four-probe method, and the resistivity was calculated by multiplying the sheet thickness. The thermal conductivity at 25°C was then calculated by converting the resistivity using the Wiedemann-Franz law.
The measured values of Young's modulus, packing ratio, and thermal conductivity are shown in FIG.
In Fig. 8, the bonding sheet in which at least a part of the copper particles is sintered, i.e., the bonding sheet containing the copper sintered body, is designated as a sintered body "◯". In Fig. 5, the bonding sheet in which all the copper particles are not sintered, i.e., the bonding sheet not containing the copper sintered body, is designated as a sintered body "X". Also, Fig. 8 shows the type of solvent added to the bonding sheet. As shown in Fig. 8, the bonding sheet of Example 1 does not contain a sintered body, but contains copper particles and PEG as a solvent with a boiling point of 150 ° C or higher.

(Example 2-8)
In Examples 2-8, bonding sheets were produced in the same manner as in Example 1, except that at least one of the heating temperature, applied pressure, and pressure time was changed as shown in FIG. 8. Measured values of Young's modulus, filling rate, and thermal conductivity, whether or not the bonding sheet contains a sintered body, and the solvent added to the bonding sheet are shown in FIG. 8. Note that, as in the second embodiment, Examples 1 and 2 are bonding sheets that do not contain a sintered body but contain copper particles and a solvent having a boiling point of 150°C or higher, while Example 3-8 is a bonding sheet that contains a sintered body as in the first embodiment, and the bonding sheet of Example 3-8 does not contain a solvent having a boiling point of 150°C or higher.

(Comparative Example 1-2)
In Comparative Example 1-2, a bonding sheet was produced in the same manner as in Example 1, except that at least one of the heating temperature, applied pressure, and pressure time was changed as shown in Figure 8. The measured values of Young's modulus, filling rate, and thermal conductivity, whether or not the sintered body was included, and the solvent added to the bonding sheet are shown in Figure 8.

(evaluation)
A bonded structure was produced by bonding members together using the bonding sheet of each example, and the shear strength of the bonded structure was evaluated. The bonding sheet of each example was cut using a commercially available cutter knife to prepare a bonding sheet piece (2.5 mm square x 500 μm thick). The above bonding 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, polyethylene glycol was applied as a solvent to the bonding sheet piece at 0.05 g per 0.95 g of sheet piece weight, and then a 2.5 mm square x 1 mm thick oxygen-free copper dummy element was placed on it. Thus, a laminate was obtained in which the oxygen-free copper substrate and the oxygen-free copper dummy element were stacked via the bonding sheet piece. The obtained laminate was held for 15 minutes using a die bonder (HTB-MM, manufactured by Alpha Design Co., Ltd.) under conditions of a nitrogen gas atmosphere, a pressurized pressure of 5 MPa, and a temperature of 250°C, thereby producing 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 bonded assembly (Sample A) was measured using a method in accordance with JIS Z 3198-7 (Lead-Free Solder Testing Methods - Part 7: Shear Test Method for Solder Joints of Chip Components). Specifically, a load was applied to an oxygen-free copper dummy element using a tool from a bond tester (Nordson DAGE, SERIES 4000), and the load (maximum shear load) at which the oxygen-free copper dummy element peeled from the copper bonding 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 to Newtons and divided by the area of the copper bonding layer (2.5 mm × 2.5 mm) to obtain the shear strength (unit: MPa). Seven bonded assembly specimens were fabricated, and the shear strength of each was measured. The results are shown in Figure 8.

In this evaluation, a shear strength of 40 MPa or more was rated as ◯ (pass), and a shear strength of less than 40 MPa was rated as × (fail). As shown in Figure 8, when the bonding sheet of the example, which has a Young's modulus of 15 GPa or less and a filling rate of 45% or more, was used, the shear strength of the bonded body was high, and it can be seen that the bonding strength was improved. On the other hand, when the bonding sheet of the comparative example, which does not satisfy at least one of the Young's modulus of 15 GPa or less and the filling rate of 45% or more, was used, the shear strength of the bonded body was low, and it can be seen that the bonding strength was not improved.

(option evaluation)
As an optional evaluation, the density of the bonding layer in the bonded structure manufactured using the bonding sheet was measured. The bonding sheet was cut using a commercially available cutter knife to prepare bonding sheet pieces (10 mm square x 500 μm thick). The above bonding 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, polyethylene glycol was applied as a solvent to the bonding sheet piece at 0.05 g per 0.95 g of sheet piece weight, and then a 10 mm square x 1 mm thick oxygen-free copper dummy element was placed on it. In this way, a laminate was obtained in which the oxygen-free copper substrate and the oxygen-free copper dummy element were stacked via the bonding sheet piece. The obtained laminate was held for 15 minutes using a die bonder (HTB-MM, manufactured by Alpha Design Co., Ltd.) under conditions of a nitrogen gas atmosphere, a pressurized pressure of 5 MPa, and a temperature of 250°C, thereby producing 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 resulting bonded body (Sample B) was measured for an ultrasonic flaw detection image using an ultrasonic flaw detector (FINE-SAT, manufactured by Hitachi High-Technologies Corporation). The obtained ultrasonic flaw detection image was binarized using image processing software (ImageJ, manufactured by the National Institutes of Health, USA) to separate the image into voids (cavities) and the bonded body (copper particle sintered body), and the void fraction was calculated using the following formula.
Void rate (%) = (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 results are shown in Figure 8.

Although the embodiments of the present invention have been described above, the embodiments are not limited to the content of these embodiments. Furthermore, the components described above include those that would be easily imagined by a person skilled in the art, those that are substantially the same, and those that are within the scope of what is known as equivalents. Furthermore, the components described above can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the components can be made without departing from the spirit of the embodiments described above.

10, 10A Bonding sheet 12, 12A Copper particles 20 Solvent

Claims (7)

A copper pre-sintered body, wherein the copper particles have an average particle size of 100 nm or more and 200 nm or less, a Young's modulus of 15 GPa or less, and a filling rate of 45% or more.
Joining sheet. The joining sheet according to claim 1, having a Young's modulus of 1 GPa or more and 10 GPa or less. The joining sheet according to claim 1 or 2, having a filling rate of 45% or more and 60% or less. The joining sheet according to any one of claims 1 to 3, having a thermal conductivity of 1 W/mK or more and 20 W/mK or less. The joining sheet according to any one of claims 1 to 4, comprising a copper sintered body. The bonding sheet according to any one of claims 1 to 4, comprising copper particles and a solvent having a boiling point of 150°C or higher. A step of placing the bonding sheet according to any one of claims 1 to 6 on a first member;
applying an organic solvent to a surface of the bonding sheet;
A step of arranging a second member on the bonding sheet to obtain a laminate in which the bonding sheet is arranged between a first member and a second member;
a step of manufacturing a bonded body in which the first member and the second member are bonded by heating the laminate;
Including,
A method for manufacturing a bonded body.