Welded sheet, welded assembly, and method for manufacturing the same

Abstract

The present invention relates to a welded sheet, a welded assembly, and a method for manufacturing the same. The welded sheet comprises a copper substrate having opposing first and second surfaces, and at least one nanometal layer deposited on at least one of the first and second surfaces of the copper substrate, and the yield strength of the welded sheet is 100 MPa or less. The welded assembly comprises a first substrate, a second substrate, and a welded portion located between the first and second substrates and connecting them, the welded portion comprising a copper substrate, a first weld interface on the first side of the copper substrate, and a second weld interface on the second side of the copper substrate facing the first side, the first and second weld interfaces being formed by joining the nanometal on the opposite side of the copper substrate to the first and second substrates, and the shear strength of the welded assembly is 40 MPa or more.

Description

Technical Field

[0001] The present invention relates to a welding piece that can be used for mounting a high-power member to be welded, and includes a copper substrate and a nano metal layer deposited on at least one outer surface of the copper substrate. The present invention also relates to a welded assembly obtained by welding with a welding sheet, and a corresponding manufacturing method of the welding sheet and the welded assembly.

Background Art

[0002] Currently, with the progress of diversification and miniaturization of functions, the mounting of high-power members is facing major challenges. According to the US Department of Energy, the energy per unit volume of in-vehicle high-power members is predicted to increase from 18 kW / L in 2020 to 100 kW / L in 2025. The increase in power density directly causes an increase in the operating temperature of high-power members. To meet the increase in power density, high-power members have already shifted from the first-generation silicon-based semiconductors to the third-generation wide-bandgap silicon carbide semiconductors. Along with this, the operating temperature has also improved from 100°C to 175°C and is expected to further improve in the future. This poses a problem for conventional solder because conventional solder cannot function stably at high operating temperatures, and the increase in the power density of high-power members is limited. For example, the tin-based solder paste with a melting point of 250°C is currently used for device mounting. This type of conventional solder can only withstand a device operating temperature of less than 150°C. When the temperature exceeds this, the reliability of the interconnection by the tin-based solder paste is significantly reduced, causing early failure and short circuit of the device. Therefore, new high-temperature-resistant solders mainly composed of nano silver and nano copper are being introduced into the mainstream market in place of conventional solders.

[0003] CN110640354A describes a preformed solder comprising a preformed metal substrate and a nanoparticle film coated on the outer surface of the substrate. The nanoparticle film mainly comprises nanometal particles, is coated on the preformed surface after pre-sintering, and the melting point of the nanometal particles is lower than that of the metal substrate. Because the nanoparticle film itself has a sparse structure and uniform pores, it can more effectively compensate for the flatness and surface roughness of the material to be welded (e.g., a tip, a copper-clad ceramic substrate, or the surface of a heat sink). This allows for more uniform pressure distribution to each weld area during the welding process, and internal stresses generated during the welding process or drying period can be absorbed by the pores without accumulating within the nanoparticle film. However, it should be noted that, due to the inherent sparseness of the nanometal particle structure, excess pores still remain in the final weld. This not only causes a decrease in thermal conductivity and electrical conductivity but is also detrimental to the shear strength of the welded assembly. In addition, the nanoparticle layer cannot be elastically deformed after pre-drying, resulting in reduced adhesion to the substrate. This makes secondary shaping of the solder impossible, making it inconvenient to use.

[0004] WO2022 / 127776A1 describes a method for performing low-temperature welding using a nanocopper layer. This method involves depositing a nanocopper layer on the surface of one or more structures to be welded. Here, the thickness of the nanocopper layer is 10 nm to 10 μm, and the grain size is 5 to 500 nm. Compared to CN110640354A, the plating layer is denser and free of holes, eliminating pore defects in the nanometal particle structure, and improving electrical and thermal conductivity. However, the inventors have found that obtaining high shear strength requires a very high degree of flatness of the components to be welded.

[0005] In light of the above, there is still a need to improve the solder in order to increase the shear strength of welded assemblies. [Overview of the project]

[0006] This invention has been made in view of the above-mentioned problems existing in the prior art.

[0007] A first aspect of the present invention provides a welding sheet. When the porosity of the surface plating layer is low and the requirement for surface flatness of the members to be welded is not high, the welding sheet allows the final welded assembly to have a high effective weld area and shear strength, thereby improving the reliability of the weld.

[0008] The welded sheet of the present invention is A copper substrate having a first surface and a second surface facing each other, At least one nanometal layer deposited on at least one of the first surface and the second surface of the copper substrate, Includes, The yield strength of the welded sheet is 100 MPa or less.

[0009] The welding surface of a component to be welded (e.g., a tip) is usually wavy, i.e., relatively rough, unless special treatment is applied. The inventors found that when a welding sheet is made by depositing a dense nano-copper layer onto a general substrate, the shear strength of the welded assembly using this sheet is insufficient. The inventors investigated and found that this is because the dense nano-copper layer cannot deform synchronously, depending on particle displacement and hole deformation, as nanoparticle layers do. As a result, the wavy surface of the component to be welded cannot be compensated for, resulting in obvious non-contact areas and, i.e., a reduction in the effective welding area. Therefore, only partial welding can be achieved between the components to be welded, resulting in low shear strength.

[0010] To resolve this issue, the flatness of the surface to be welded can be improved by wet polishing. This effectively increases the welding area and improves the shear strength. However, this operation is not only costly but also causes some damage to the parts to be welded, making it not a preferred method for improving the shear strength of the welded assembly.

[0011] Therefore, the object of the present invention is to provide a solution to improve the shear strength of a welded assembly without applying any special treatment (e.g., wet polishing) to the weld surface of the member to be welded. In this invention, a copper substrate is used as the substrate for the weld sheet. By modeling the surface morphology of the member to be welded (e.g., a tip), the yield strength corresponding to the copper substrate reaching its yield point at the pressure (15-40 MPa) applied to the hot press welding is calculated. Here, when the member to be welded is placed horizontally, the vertical difference between the highest and lowest points on its outer surface can reach a maximum of 200 nm (i.e., it is a rough surface). The calculated yield strength is used as a criterion for selecting the copper substrate so that the weld sheet can be at least partially plastically deformable during the hot press welding period. Through theoretical research and experimental verification by the inventors, it has been found that the yield strength of the weld sheet should not exceed 100 MPa. At this time, during hot press welding, the weld sheet comes into contact with the convex portion on the tip surface. The contact area is small and the pressure is high, so it can be easily reached by the yield strength of the weld sheet. This allows the weld sheet to undergo localized plastic deformation around the convex portion, thereby enabling the indentation on the tip surface to come into contact with the deformed weld piece, and thus diffusion welding becomes possible. This achieves the objective of increasing the effective welding area, thereby improving the shear strength of the final welded assembly. If the yield strength of the weld sheet exceeds 100 MPa, the weld sheet cannot undergo plastic deformation, or only undergoes slight plastic deformation, making it difficult to compensate for large differences in the surface flatness of the members to be welded. As a result, the effective welding area cannot be significantly increased, and the shear strength is low. Therefore, in this invention, the yield strength of the weld sheet is required to be 100 MPa or less. Preferably, the yield strength of the weld sheet is in the range of 18 to 100 MPa, more preferably in the range of 18 to 67.5 MPa, more preferably in the range of 20 to 46.5 MPa, and even more preferably in the range of 23 to 36 MPa.

[0012] Preferably, the average crystal grain size of the copper substrate is in the range of 30 to 140 μm, more preferably in the range of 50 to 140 μm, more preferably in the range of 75 to 122.5 μm, and more preferably in the range of 90 to 115 μm.

[0013] Preferably, the elongation of the welded sheet is 11.5% or more, preferably 23% or more, preferably in the range of 27.5% to 36%, and more preferably in the range of 29% to 34%.

[0014] Preferably, the welding sheet is in the form of a thin flake, which allows it to be cut to a desired size or shape by secondary cutting. Compared to a nanoparticle layer that is dried after coating, cutting and bending do not cause peeling of the nanometal layer on the welding sheet, thus improving the integrity of the welding sheet and expanding the range of applications for the welding sheet.

[0015] Preferably, the average grain size of the nanometal layer is in the range of 5 to 500 nm, and more preferably in the range of 5 to 100 nm. This achieves the objective of low-temperature welding and promotes welding performance between the interfaces to be welded. Preferably, the average thickness of the nanometal layer is 100 nm to 10 μm, and more preferably 500 nm to 5 μm. Preferably, the metal of the nanometal layer can be selected from copper, gold, nickel, silver, platinum, and titanium. Preferably, the nanometal layer is a nanocopper layer.

[0016] A second aspect of the present invention is: Step A provides a copper substrate having opposing first and second surfaces, Step B involves annealing the copper substrate under vacuum to obtain a soft copper substrate having an average crystal grain size in the range of 30 to 140 μm. Step C involves depositing at least one nanometal layer on at least one of the first and second surfaces of the soft copper substrate to obtain the welded sheet, including, The present invention provides a method for manufacturing welded sheets.

[0017] The process involved in this method is simple and does not require complex or expensive equipment.

[0018] Preferably, the copper substrate is a general industrial-grade electroplated copper foil or rolled copper foil, and its yield strength is typically in the range of 140 to 250 MPa. In the present invention, a soft copper foil is first obtained by annealing the copper foil to sufficiently recrystallize it to an average crystal grain size of 30 to 140 μm. Since the yield strength of this soft copper foil is 100 MPa or less, the yield strength of the resulting weld piece is 100 MPa or less.

[0019] As is well known, at the same annealing temperature, the longer the holding time, the larger the final average grain size. At the same holding time, the higher the annealing temperature, the larger the final average grain size. Therefore, various heat treatment conditions can be expected based on the target average grain size of 30 to 140 μm. Preferably, in step B, the vacuum level is 1.0 × 10⁻⁶ -3 The material is Torr, the annealing temperature is 400-850°C, and the holding time is 2-16 hours, preferably 500-800°C and 6-12 hours. At low annealing temperatures, such as 400°C, if the annealing time is too short, it is insufficient to increase the average grain size to 30 μm, so a minimum holding time of 2 hours is required. On the other hand, at high annealing temperatures, such as 850°C, if the annealing time is too long, the average grain size will exceed the desired range, which is detrimental to the final shear strength of the device, so a holding time of 16 hours or less is required. This is also advantageous for shortening the overall work process and improving the productivity of the production line.

[0020] Preferably, a cleaning step is performed before step C. In this cleaning step, the soft copper substrate is washed for 30 seconds with a 1:1 volume fraction mixture of dilute hydrochloric acid and deionized water to remove the surface oxide layer, followed by ultrasonic cleaning in deionized water, and then drying, for example by blowing with a nitrogen gun at room temperature.

[0021] Preferably, the deposition of the nano-metal layer on the copper substrate is carried out by methods such as electroplating, vapor deposition, vapor phase deposition (e.g., magnetron sputtering), ion plating, etc., whereby a nano-metal layer with relatively uniform thickness and grain size is obtained. The thickness and grain size of the nano-metal layer are selected so as not to significantly change the yield strength of the entire welded sheet after being deposited on the copper substrate. Thereby, the yield strength of the entire welded sheet is dominated by the copper substrate, ensuring that it does not exceed 100 MPa. Therefore, when attempting to use a relatively thick nano-metal layer, for example, a 10-μm nano-metal layer, in order to ensure that the yield strength of the entire welded sheet obtained after depositing the nano-metal layer is 100 MPa or less, a copper substrate with a relatively large thickness and low yield strength (i.e., a large grain size) should be selected.

[0022] Preferably, the average thickness of the nano-metal layer is 100 nm to 10 μm, more preferably 500 nm to 5 μm. Preferably, the average grain size of the nano-metal layer is within the range of 5 to 500 nm, preferably within the range of 5 to 100 nm. Thereby, the purpose of low-temperature welding is achieved, and the welding performance between the planned welding interfaces is promoted.

[0023] Preferably, the average thickness of the copper substrate is at least 10 times the average thickness of the nano-metal layer, preferably 50 to 600 times, more preferably 100 to 400 times, and the average grain size of the copper substrate is at least 60 times, preferably at least 100 times, the average grain size of the nano-metal layer. Thereby, it is ensured that the yield strength of the welded sheet is dominated by the copper substrate.

[0024] Preferably, the metal of the nano-metal layer can be selected from copper, gold, nickel, silver, platinum, and titanium. Preferably, the nano-metal layer is a nano-copper layer.

[0025] Preferably, the nano copper layer is formed by electroplating deposition as follows. The electrolytic solution contains 100 - 150 g / L of anhydrous copper sulfate, 80 - 150 g / L of concentrated sulfuric acid with a mass fraction of 98%, 50 - 150 mg / L of sodium chloride, 350 - 480 mg / L of PEG20000, and 20 - 40 mg / L of Janus green B. Preferably, the current density of the electroplating deposition is 0.05 - 0.15 A / cm 2 and the deposition time is 10 - 300 seconds.

[0026] The third aspect of the present invention is a first substrate, a second substrate, a welding part located between the first substrate and the second substrate and connecting the two, including the welding part includes a copper substrate, a first welding interface on the first side of the copper substrate, and a second welding interface on the second side opposite to the first side of the copper substrate. The first welding interface and the second welding interface are respectively formed by joining the nano metal on the opposite side of the copper substrate with the first substrate and the second substrate. The shear strength of the welding assembly is 40 MPa or more. in the welding assembly.

[0027] Preferably, the average grain size of the copper substrate is within the range of 30 - 140 μm, preferably within the range of 50 - 140 μm, preferably within the range of 75 - 122.5 μm, and also preferably within the range of 92 - 115 μm.

[0028] Preferably, the first substrate and / or the second substrate can be members such as chips, copper - clad ceramic substrates, heat sinks, etc.

[0029] Preferably, the effective welding area of the first welding interface and / or the second welding interface is 67.5% or more, preferably 76% or more, preferably 86% or more, preferably 92.5% or more of the total interface area.

[0030] Preferably, the shear strength of the welded assembly is 45 MPa or more, more preferably 50 MPa or more, and even more preferably 55 MPa or more.

[0031] A fourth aspect of the present invention is: Step A provides a first substrate having a first outer surface, a second substrate having a second outer surface, and a welding sheet. Step B, in which a first nanometal layer of the welding sheet and the first outer surface of the first substrate are provided to be in surface contact, and a second nanometal layer of the welding sheet and the second outer surface of the second substrate are provided to be in surface contact, thereby forming a laminated structure, Step C involves hot-press welding the aforementioned laminated structure to weld the first substrate, the second substrate, and the welding sheet together, thereby obtaining a welded assembly with a shear strength of 40 MPa or more. Includes, The welding sheet comprises a copper substrate having opposing first and second surfaces, a first nanometal layer on the first surface of the copper substrate, and a second nanometal layer on the second surface facing the first surface of the copper substrate, wherein the yield strength of the welding sheet is 100 MPa or less. It lies in the production method of welded assemblies.

[0032] Preferably, in step C, the hot press welding is performed as follows: in a helium gas protected environment, pressurized from above and below the laminated structure at a temperature of 250 to 350°C, with a pressure of 15 to 40 MPa and a welding time of 10 to 30 minutes. More preferably, the welding temperature is 250 to 300°C, or the pressure is 20 to 30 MPa, or the welding time is 10 to 20 minutes.

[0033] Preferably, when the first substrate and / or the second substrate is placed horizontally, the vertical difference between the highest and lowest points on the first outer surface and the second outer surface can reach a maximum of 200 nm, eliminating the need for additional surface treatment to obtain high flatness.

[0034] The description of the weld sheet and preferred technical methods according to the first aspect of the present invention are applicable to the second, third, and fourth aspects of the present invention.

[0035] Based on the above, the present invention achieves the following technical effects compared to the conventional technology: 1) The present invention controls the yield strength of a weld sheet by softening a copper substrate. This allows the weld sheet to increase the effective welding area of ​​the weld interface without requiring a planar treatment (e.g., wet polishing) on ​​rough weld surfaces (i.e., where the vertical difference between the highest and lowest points when the weld surface is placed horizontally can reach up to 200 nm). As a result, the shear strength after welding increases, improving welding reliability. Therefore, the weld sheet of the present invention achieves relatively high shear strength while increasing tolerance to the roughness of the surface to be welded. 2) Compared to welding sheets made of nanocopper particle film, the surface nanometal layer of the present invention has a higher density, improving the thermal conductivity and electrical conductivity of the welded assembly, and 3) Because the welding sheet of the present invention exhibits superior adhesion between the copper substrate and the nanometal layer thereon, secondary processing (e.g., cutting) of the welding sheet allows for obtaining sizes and shapes suitable for specific applications, making it adaptable to desired application scenarios, and preventing peeling of the nanometal layer, unlike nanocopper particle films. [Brief explanation of the drawing]

[0036] Illustrative embodiments of the present invention will be described below with reference to the attached drawings. In the drawings: Figure 1 shows the change in height perpendicular to the weld surface of an exemplary tip, as measured by atomic force microscopy, where y is the vertical height and x is the horizontal length. Figure 2 shows a comparison of the morphology of the chip model and the weld sheet model before and after pressurization in welding simulation 1. Figure 3 shows a comparison of the morphology of the chip model and the weld sheet model before and after pressurization in welding simulation 2. Figure 4 shows a comparison of the morphology of the chip model and the weld sheet model before and after pressurization in welding simulation 3. Figure 5 shows a comparison of the morphology of the chip model and the weld sheet model before and after pressurization in welding simulation 4. Figure 6 shows a scanning electron microscope (SEM) image of the surface morphology of the soft copper substrate of Sample 1 of Example S1. Figure 7 shows a scanning acoustic microscope image of Sample 1 of Example S1 after welding of the welding sheet and the tip. Figure 8 shows an SEM image of the surface morphology of the soft copper substrate of Sample 1 in Example S2. Figure 9 shows an SEM cross-sectional view of the welded sheet of sample 1 in Example S2, where the W-Pt layer is a protective layer for focused ion beam sample preparation and is not included in the original structure. Figure 10 shows an SEM image of the surface morphology of the nanocopper layer of the welded sheet of Sample 1 in Example S2. Figure 11 shows a scanning acoustic microscope image of Sample 1 of Example S2 after welding of the welding sheet and tip. Figure 12 shows an SEM image of the surface morphology of the soft copper substrate of Sample 1 in Example S3. Figure 13 shows a scanning acoustic microscope image of Sample 1 of Example S3 after welding of the welding sheet and the tip. Figure 14 shows an SEM image of the surface morphology of the soft copper substrate of Sample 1 of Example S4. Figure 15 shows a scanning acoustic microscope image of Sample 1 of Example S4 after welding of the welding sheet and the tip. Figure 16 shows an SEM image of the surface morphology of the soft copper substrate of Sample 1 in Example S5. Figure 17 shows a scanning acoustic microscope image of Sample 1 of Example S5 after welding of the welding sheet and the tip. Figure 18 shows an SEM image of the surface morphology of the soft copper substrate of Sample 1 of Example S6. Figure 19 shows a scanning acoustic microscope image of Sample 1 of Example S6 after welding of the welding sheet and the tip. Figure 20 shows an SEM image of the surface morphology of the soft copper substrate of Sample 1 in Example S7. Figure 21 shows a scanning acoustic microscope image of Sample 1 of Example S7 after welding of the welding sheet and the tip. Figure 22 shows an SEM image of the surface morphology of the copper substrate of Sample 1 of Comparative Example C1. Figure 23 shows a scanning acoustic microscope image of Sample 1 of Comparative Example C1 after welding of the weld sheet and tip. Figure 24 shows an SEM image of the surface morphology of the copper substrate of Sample 1 of Comparative Example C2. Figure 25 shows a scanning acoustic microscope image of the weld sheet and tip after welding of Sample 1 of Comparative Example C2, and Figure 26 shows the relationship between the average yield strength of the weld sheet and the average shear strength of the welded assembly (a), the average effective weld area ratio (b), the average grain size of the copper substrate (c), and the average elongation of the weld sheet (d). [Modes for carrying out the invention]

[0037] The present invention will be described in more detail below with reference to exemplary embodiments. The following embodiments and experimental data are for illustrative purposes only, and it will be apparent to those skilled in the art that the present invention is not limited to these embodiments and experimental data. Features or elements described in combination with or in relation to one embodiment apply to all embodiments, provided there is no feature conflict. One or more features or elements from one embodiment may be incorporated into or combined with any other embodiment disclosed herein, and such features or elements extracted from one embodiment may be included to supplement or substitute for one or more features or elements of the other embodiments. Features or combinations of features of embodiments disclosed herein may be extracted independently of other features of that embodiment. Alternatively, features or combinations of features of an embodiment may be omitted from this embodiment.

[0038] Those skilled in the art will understand that any range or value within each interval specified herein applies to the present invention. For example, the yield strength of the weld sheet being in the range of 18 to 100 MPa means that it can take any range or any specific numerical value within that range. For example, the yield strength may be in any range such as 18 to 95 MPa, 21 to 90 MPa, 28 to 90 MPa, 20 to 80 MPa, 22 to 70 MPa, 19 to 60 MPa, 20 to 55 MPa, 22 to 64 MPa, 25 to 42 MPa, 25 to 60 MPa, 28 to 82 MPa, or 23 to 58 MPa. Alternatively, the yield strength may be any numerical value such as 23 MPa, 25 MPa, 26 MPa, 30 MPa, 35 MPa, 40 MPa, 42 MPa, 48 MPa, 55 MPa, 60 MPa, 65 MPa, 70 MPa, 76 MPa, 80 MPa, 88 MPa, or 94 MPa.

[0039] In this specification, yield strength, tensile strength, and elongation have meanings well known in the art. Specifically, yield strength refers to the limit of the elastic behavior of a sample and the onset of plastic deformation, i.e., the stress value when the sample is deformed by 0.2%. Tensile strength is the stress value when the sample undergoes maximum uniform plastic deformation. Elongation is the deformation capacity of a material before it breaks under force, and refers to the percentage ratio of the total deformation ΔL of the gauge length section after tensile fracture of the sample to the original gauge length L.

[0040] In this specification, the shear failure test method is as follows: The welded assembly consists of a first welded substrate and a weld sheet, with dimensions of 10 × 10 mm at the weld interface between them. The unwelded side of the weld sheet is fixed to the test stage with resin, and a movable mechanical component is brought into contact with the side of the first substrate, applying a shear force parallel to the welded assembly at a moving speed of 1 mm / min. Five samples are tested for each type of welded assembly, and the average value is taken.

[0041] In this specification, the effective weld area ratio is measured as follows: Acoustic scanning detection is performed on a welded assembly having a weld area of ​​10 × 10 mm. Of these, the dark areas are the effective welded areas, and the light areas are the separated areas, i.e., areas where welding was not possible. The effective weld area ratio is obtained by the ratio of the effective welded area to the total area.

[0042] This invention controls the yield strength of the welding sheet, allowing the welding sheet to undergo localized yield deformation during the welding process and adhere closely to the welding surface of the member to be welded. This improves the effective welding area and increases the welding strength, thus eliminating the problem of reduced shear strength caused by the dense metal layer being unable to cope with the surface roughness of the member to be welded.

[0043] Specifically, to solve the above problem, the present invention estimated the required yield strength for the weld sheet through simulation. The inventors used a common tip as an example of a component to be welded, and measured the roughness of the weld surface of different tips using an atomic force probe. As a result, a periodic, wavy undulation was found perpendicular to the weld surface of the tip, with a roughness Ra of 20-52 nm and a vertical difference of 56-150 nm between the highest and lowest points. Figure 1 shows the height undulation perpendicular to the weld surface of an exemplary tip. To quantify such undulation, the vertical values ​​of the wavy undulation shown in Figure 1 can be simplified and simulated into the following curve: TIFF2026509748000002.tif1763Equation 1 Here, y represents the surface undulation in the thickness direction of the tip (i.e., the direction perpendicular to the weld interface), and x represents the length along the planned weld interface.

[0044] To simulate hot press welding, the measured maximum roughness is used as the experimental baseline, i.e., Ra = 52 nm, with a vertical difference of approximately 150 nm between the highest and lowest points. To improve the fault tolerance of the product, the vertical difference between the highest and lowest points in the simulation experiment is 200 nm, i.e., a roughness greater than Ra = 52 nm, with a period length of 10 μm. The following curve is obtained: TIFF2026509748000003.tif1564 Equation 2 The curve above is used to simulate the undulation of the surface on which the tip is to be welded.

[0045] The welding simulation steps are as follows: 1) Using the modeling software SolidWorks, a chip model and a weld sheet model are constructed. The planned welding surface of the chip model is rough, meaning the vertical difference between the highest and lowest points is 200 nm. The weld sheet model includes a copper substrate and a nano-copper layer provided on the copper substrate. In the simulation, it is assumed by default that diffusion will occur and welding will be achieved if the chip model and the weld sheet model can be brought into close contact. Therefore, the simulation considers the yield strength of the weld sheet as a whole. 2) The initial state before hot pressing is simulated by bringing the nano-copper layer of the weld sheet model into contact with the planned welding surface of the chip model. At this time, since the planned welding surface is rough, only the protrusions of the planned welding surface come into contact with the nano-copper layer. Therefore, an unjoined region exists between the two. In each figure, the unjoined region is shown in white. 3) In the initial state, the software simulates applying pressure in the vertical direction using the finite element method, and calculates the stress and strain near the welding interface between the weld sheet model and the tip model under the action of vertical pressure. This determines the yield strength range required to generate sufficient plastic deformation to adapt to the surface roughness of the tip at equivalent pressure.

[0046] In all simulations, the material mechanics parameters used for the chip model were preset metal material parameters from the database, while the weld sheet model used copper material parameters whose yield strength and tensile strength were adjusted by annealing. The pressure range used in the welding simulations is derived from the commonly used copper sintering welding pressure, i.e., 15–40 MPa.

[0047] Simulation 1: The yield strength of the weld sheet model is 258 MPa. Static mechanical analysis is performed by restricting the position of the tip's top while applying a pressure equivalent to 15 MPa to the bottom of the weld sheet model in the direction shown in Figure 2 (i.e., perpendicular to the welding surface). Figure 2 shows a comparison of the morphology of the weld interface before and after pressurization. In the left figure, a white area is observed between the nanocopper layer of the weld sheet model and the welding surface of the tip model, indicating a region where there is no joint between them. In the right figure, a white area is also shown in the same location, indicating that the region that was not joined before pressurization remains unjoined after pressurization. The right figure also shows the stress distribution in the system after pressurization. As can be seen from the comparison between the rightmost grayscale bar and the gray level of the weld sheet region, the stress in the contact region between the welding surface of the tip and the weld sheet at the weld interface is 9-13.6 MPa, which is much lower than the yield strength of the weld sheet model (258 MPa). Therefore, the weld sheet model cannot plastically deform to compensate for the waviness on the tip surface. This confirmed that, in order for the weld sheet to yield and adhere to the intended welding surface of the tip under a welding pressure of 15 MPa, the yield strength of the weld sheet must be at least less than 13.6 MPa.

[0048] Simulation 2: The yield strength of the weld sheet model is 258 MPa. A static mechanical analysis is performed by restricting the position of the tip's top while applying a pressure equivalent to 40 MPa to the bottom of the weld sheet model in the direction shown in the figure (i.e., perpendicular to the surface to be welded). Figure 3 shows a comparison of the morphology of the weld interface before and after pressurization. Similar to Figure 2, there is no clear change in the unjoined portion in the figure before and after pressurization. Furthermore, the stress distribution in the system after pressurization in the right figure shows that the stress in the contact area between the surface to be welded of the tip and the weld sheet at the weld interface is 76.8 to 152 MPa, which is much lower than the yield strength of the weld sheet model (258 MPa). Therefore, the weld sheet cannot plastically deform to compensate for the change in roughness on the tip. Similarly, it was confirmed that in order for the weld sheet to yield and adhere to the surface to be welded of the tip under a welding pressure of 40 MPa, the yield strength of the weld sheet must be less than 152 MPa.

[0049] Simulation 3: The yield strength of the weld sheet model is 18 MPa. A static mechanical analysis is performed by restricting the position of the tip's top while applying a pressure equivalent to 15 MPa to the bottom of the weld sheet model in the direction shown in the figure (i.e., perpendicular to the surface to be welded). The morphology of the weld interface before and after pressurization is shown in Figure 4. Compared to the left figure showing the state before pressurization, the right figure showing the state after pressurization shows a significant reduction in the unjointed area, which accounts for less than 10% of the entire weld interface, indicating that the weld sheet model has deformed significantly at the weld interface. The stress distribution in the system after pressurization in the right figure shows that the stress in the contact area between the surface to be welded of the tip and the weld sheet at the weld interface is 15-28 MPa, which exceeds the yield strength of the weld sheet (18 MPa). This theoretically proves that a weld sheet with a yield strength of 18 MPa can plastically deform at a pressure of 15 MPa and adhere tightly to the tip.

[0050] Simulation 4: The yield strength of the weld sheet model is 100 MPa. A static mechanical analysis is performed by restricting the position of the tip while applying a pressure equivalent to 40 MPa to the bottom of the weld sheet model in the direction shown in the figure (i.e., perpendicular to the welding surface) and the position of the tip. The morphology of the weld interface before and after pressurization is shown in Figure 5. Compared to the left figure showing the state before pressurization, the right figure showing the state after pressurization shows a significant reduction in the unjointed area, which accounts for less than 10% of the entire weld interface, indicating that the weld sheet model has deformed significantly at the weld interface. The stress distribution in the system after pressurization in the right figure shows that the stress in the contact area between the welding surface of the tip and the weld sheet at the weld interface is 84-146 MPa, which exceeds the yield strength of the weld sheet (100 MPa). This theoretically proves that a weld sheet with a yield strength of 100 MPa can plastically deform at a pressure of 40 MPa and adhere tightly to the tip.

[0051] Based on the above simulation, the inventors have found the following: By controlling the yield strength of the weld sheet to 100 MPa or less, the weld sheet can locally yield at different sintering pressures within the range of 15 to 40 MPa, thereby at least partially compensating for the roughness of the tip. This increases the effective welding area and solves the problem of reduced shear strength caused by poor adhesion between the weld sheet and the tip's intended welding surface. Preferably, the entire weld sheet maintains a constant strength, thereby ensuring welding strength along with an increase in the effective welding area. This is because the shear strength of the welding area is influenced by the tensile strength of the weld sheet, and the tensile strength of the material usually has a positive correlation with the yield strength. Therefore, a weld sheet with a relatively high yield strength should be used, as long as it ensures that the weld sheet can locally yield and adhere to the tip. Accordingly, in the present invention, it is preferably required that the yield strength of the weld sheet be 18 MPa or higher.

[0052] The following describes improvements to welded assemblies using the welding sheet of the present invention, with reference to specific examples.

[0053] The copper substrate of the weld sheets in each of Examples S1 to S7 and Comparative Example C1 was taken from commercially available rolled copper foil, with a thickness of 100 μm, a yield strength of 156 MPa, a tensile strength of 350 MPa, and an elongation of 7.8%. The copper substrate of the weld sheet in Comparative Example C2 was commercially available rolled copper foil, with a thickness of 100 μm, a yield strength of 202 MPa, a tensile strength of 380 MPa, and an elongation of 5.4%.

[0054] Fabrication of welding sheets 1. Heat Treatment Step: The copper substrates of each example S1 to S7 were heat-treated according to the parameters shown in Table 1 to obtain soft copper substrates. Samples were prepared from the obtained soft copper substrates and photographed with a scanning electron microscope (see Figures 6, 8, 12, 14, 16, 18, and 20). The average grain size was measured according to a method known in the art and is shown in Table 1. No heat treatment was performed on the copper substrates of Comparative Examples C1 to C2. The surface morphology of both is shown in Figures 22 and 24. Similarly, the average grain size of Comparative Examples C1 to C2 was measured according to a method known in the art and is shown in Table 1. For each example and comparative example, the yield strength was tested on five samples and the average value was taken. The results are shown in Table 2. Table 1 Heat treatment conditions for copper substrates S1-S7 and C1-C2 TIFF2026509748000004.tif68147

[0055] 2. Deposition Step: Schematically, an electroplating method is employed to deposit nano-copper layers on two surfaces: soft copper substrates S1-S7 and copper substrates C1-C2. Using Sample 1 of Example S2 as an example, the schematic electrolytic solution used contained 120 g / L anhydrous copper sulfate, 110 g / L 98% mass fraction concentrated sulfuric acid, 100 mg / L sodium chloride, 420 mg / L PEG20000, and 25 mg / L Janus Green B. The current density for electroplating deposition was 0.08 A / cm². 2The deposition time is 30 seconds. As shown in Figure 9, the thickness of the obtained single-sided nanocopper layer is approximately 300 nm, and as shown in Figure 10, the average grain size is approximately 30 nm. The nanocopper layers of each sample S1-S7 and C1-C2 are obtained by deposition on the corresponding copper substrate by the same process as sample 1 of S2, so the explanation will not be repeated here. Selectively, the nanocopper layer may be deposited on only one side of the soft copper substrate / copper substrate. Selectively, at least two nanometal layers may be deposited on at least one surface of the soft copper substrate, and the metal of the nanometal layer may be selected from copper, gold, nickel, silver, platinum, and titanium.

[0056] After deposition, weld sheets of grades S1-S7 and C1-C2 were obtained, respectively. For each example and comparative example, five samples were tested and the average values ​​were taken. The corresponding average yield strength, average tensile strength, and average elongation are shown in Table 2. Compared to the average yield strength of the copper substrate, the average yield strength of the weld sheets was only slightly higher; that is, the average yield strength of the weld sheets is controlled by the copper substrate. Thus, by controlling the yield strength of the copper substrate, the desired yield strength of the weld sheets can be obtained.

[0057] Each of the weld sheets S1 to S7 comprises a copper substrate having opposing first and second surfaces, and two nano-copper layers deposited on the first and second surfaces of the copper substrate, respectively, and the yield strength of the weld sheet is 100 MPa or less (see Table 2 for details). Similarly, each of the weld sheets C1 to C2 comprises a copper substrate having opposing first and second surfaces, and two nano-copper layers deposited on the first and second surfaces of the copper substrate, respectively, and the yield strengths of the weld sheets are 157 MPa and 203 MPa, respectively. Table 2 Performance of copper substrates and weld sheets for S1-S7 and C1-C2 TIFF2026509748000005.tif74147

[0058] Preferably, before deposition, the surfaces of the soft copper substrates S1-S7 and the copper substrates of Comparative Examples C1-C2 are washed with dilute hydrochloric acid of HCl:H2O=1:1 to remove any existing oxide films, etc. As can be understood, the nanometal layer can be deposited on the surface of the copper substrate using other methods such as vapor deposition, gas phase deposition (e.g., magnetron sputtering), or ion plating. As can be understood further, the present invention is not limited to nanocopper layers, and the deposited layer may be a nanogold layer, nanosilver layer, nanonickel layer, nanotitanium layer, or nanoplatinum layer.

[0059] Fabrication of welded assemblies Welding step: First, the nano-copper layers of the welding sheets S1-S7 and C1-C2 are placed opposite the weld surfaces of the two tips, respectively. To obtain high flatness, no additional treatment is applied to the weld surfaces, so that the vertical difference between the highest and lowest points on the weld surface can reach a maximum of 200 nm. Then, hot press welding is performed in a direction perpendicular to the weld surfaces, with a welding vacuum of 1 × 10⁻⁶. -3 The welding temperature is Torr (e.g., in a helium gas protected environment), the welding temperature is 280°C, the welding time is 15 mins, and the welding pressure is 25 MPa. Thus, a welded assembly according to the present invention is obtained. As can be understood, the welding parameters are not limited to these. For example, the welding temperature may be 250-350°C, the pressure 15-40 MPa, and the welding time 10-30 minutes. More preferably, the welding temperature is 250-300°C, or the pressure 20-30 MPa, or the welding time 10-20 minutes.

[0060] Each welded assembly S1 to S7 includes a first tip, a second tip, and a weld located between the first tip and the second tip and connecting them, wherein the weld includes a copper substrate, a first weld interface on the first side of the copper substrate, and a second weld interface on the second side opposite the first side of the copper substrate, where the first and second weld interfaces are formed by joining the nanometal on the opposite side of the copper substrate to the first and second tips, respectively, and the shear strength of the welded assembly is 40 MPa or more (see Table 3). As can be understood, the tips in the welded assembly can be replaced by other members, such as copper-clad ceramic substrates or heat sinks.

[0061] To test the effective weld area between the welding sheet and the member to be welded, and the shear strength of the welded assembly, a single tip and a welding sheet (hereinafter referred to as a single-sided welded assembly) were welded according to the method described above. In the following, the test samples for effective weld area ratio and shear strength will be single-sided welded assemblies (i.e., only one weld interface exists) to avoid the inability to accurately identify unwelded areas on each side due to the overlap of weld areas in double-sided welds. As should be understood, the measurement results for single-sided welded assemblies are applied to each weld interface of double-sided welded assemblies.

[0062] Effective Weld Area Ratio: Using Sample 1 of S1 as an example, the welding position (welding interface 10 × 10 mm) of the single-sided welded assembly obtained from it is detected by acoustic scanning. Figure 7 shows the scanning result of Sample 1 of S1, where the blocks indicate the welding position. Within the blocks, the dark areas are the effective welded areas, and the areas surrounded by irregular dashed edges are separated areas, i.e., unwelded (non-adherent) areas. The effective weld area ratio calculated from the proportion of the unwelded area to the total welded area is 94.8%, which is shown in Table 3. Similarly, the effective weld area ratio was measured for each of the five single-sided welded assemblies of S1 to S7 and C1 to C2, and the average value was calculated and shown in Table 3.

[0063] Shear strength test: Five samples of single-sided welded assemblies, S1-S7 and C1-C2, were tested according to the shear strength test method described herein, and the average shear strength was obtained and shown in Table 3. Table 3 Effective weld area ratio and shear strength of single-sided welded assemblies AS1-AS7 and AC1-AC2 welded with S1-S7 and C1-C2. TIFF2026509748000006.tif168113

[0064] The S1-S7 welding sheets of the present invention have a yield strength in the range of 18-100 MPa. In contrast, the yield strengths of the C1 and C2 welding sheets are 157 MPa and 203 MPa, respectively. The average effective welding area ratio of the welded assemblies AS1-AS7 obtained using the S1-S7 welding sheets is 69% or more (AS7), which is at least 44% better than AC1 and more than 70% better than AC2. Furthermore, the average shear strength is 40.2 MPa or more, which is at least 28% better than AC1 and more than twice that of AC2. As can be seen from the above, the present invention effectively solves the problem of a small effective welding area and low shear strength at the welding interface of a welded assembly when no additional treatment is applied to the surface to be welded in order to obtain high flatness, by controlling the yield strength of the welding sheet.

[0065] For AS1 to AS7, as the yield strength increases, the final shear strength of the welded assembly first increases to 57.3 MPa, and then gradually decreases to 40.2 MPa. The effective weld area ratio showed a similar trend. As shown in Figure 26, the relationship between the average yield strength of the weld sheet, the average shear strength of the welded assembly, the average effective weld area ratio, the average grain size of the copper substrate of the weld sheet, and the average elongation of the weld sheet was fitted based on the experimental data. The degree of fit R of each curve is... 2 All values ​​are close to 1, indicating a high degree of fit. Below, we will analyze the fitting curves and each example in combination.

[0066] Overall, by controlling the yield strength of the weld sheet to 18-100 MPa, the resulting welded assembly has a shear strength of 40 MPa or more and an effective weld area ratio of 67.5% or more. Correspondingly, the present invention controls the yield strength of the weld sheet by controlling the yield strength of the copper substrate of the weld sheet. Preferably, the yield strength of the copper substrate can be controlled by controlling the crystal grain size of the copper substrate. In this case, in order to control the yield strength of the weld sheet to 18-100 MPa, the average crystal grain size of the copper substrate is in the range of 30-140 μm. Correspondingly, the elongation of the weld sheet is 11.5% or more.

[0067] By comparing Figures 17, 19, and 21 (AS5-AS7) with Figures 7, 11, 13, and 15 (AS1-AS4), it can be seen that the unwelded area in AS5-AS7 is larger than that in AS1-AS4, and is mainly located at the edges of the weld interface. The reason for this is as follows: The weld sheet used in AS5-AS7 has a relatively high yield strength and can undergo localized plastic deformation during the welding process. However, compared to the uniform force in the central region, the non-uniform force at the edges makes it difficult for the weld sheet, which has a relatively high yield strength, to undergo appropriate plastic deformation. As a result, it cannot adequately adhere to the rough surface of the tip in those areas. Unwelded areas at the edges are more likely to become the starting point of failure than unwelded areas not located at the edges. Therefore, in order to keep the non-welded area as far away as possible from the edge of the weld interface and to achieve a shear strength of 45 MPa or more, it is preferable to set the yield strength of the weld sheet to 18 to 67.5 MPa by referring to the fitting curve, which results in an effective weld area ratio of 76% or more. Accordingly, in order to control the yield strength of the weld sheet to 18 to 67.5 MPa, it is preferable that the average crystal grain size of the copper substrate is in the range of 50 to 140 μm and the elongation of the weld sheet is 23% or more.

[0068] Comparing AS1 (Figure 7) and AS4 (Figure 15), the welded area of ​​AS1 is significantly larger than that of AS4 (an improvement of approximately 12.4%), but the shear strength of both is almost the same. The reason for this is as follows: From Figure 7, it can be observed that the unwelded area of ​​AS1 is elongated and crescent-shaped, extending continuously along the diagonal of the welded area. In contrast, as seen in Figure 15, the unwelded area of ​​AS4 is scattered, and no continuous elongated unwelded area is formed. Therefore, in the same shear test, the unwelded area of ​​AS1, which extends continuously along the diagonal of the welded area, weakens the effect of increasing the welded area, and as a result, the final shear strength obtained is almost the same as that of AS4. In elongated crescent-shaped unwelded areas, the yield strength of the welded sheet is relatively low, so excessive deformation and ultimately fracture occur in the localized area, and the presence of cracks is unfavorable to the shear strength and also threatens the reliability of subsequent use. Therefore, in order to further avoid continuous unwelded regions such as long crescent shapes while improving the shear strength to 50 MPa or more, it is preferable to set the yield strength of the welded sheet to 20 to 46.5 MPa by referring to the fitting curve, which results in an effective weld area ratio of 86% or more. Accordingly, it is preferable to control the average crystal grain size of the copper substrate within the range of 75 to 122.5 μm to obtain a yield strength of 20 to 46.5 MPa for the welded sheet, and its elongation is 27.5 to 36%.

[0069] The effective weld area ratio and shear strength of AS2 and AS3 are clearly higher than those of other embodiments, representing a more preferred embodiment in the present invention. Compared to AS2, AS3 has a similar effective weld area ratio to AS2, but its shear strength is improved by approximately 4.6%. The superiority of AS3's shear strength over AS2 is due to the following: The yield strength of the weld sheet used in AS3 increases from 24.3 MPa for the S2 weld sheet to 31.1 MPa. This allows for the simultaneous improvement of tensile strength by increasing yield strength while satisfying the local yield deformation requirements at the current sintering pressure (25 MPa). This increases the strength of the weld sheet, and consequently, the shear strength of the welded area. In other words, when a similar effective weld area can be achieved, a higher shear strength can be obtained by selecting a relatively high yield strength, i.e., a relatively high tensile strength. For this reason, in order to further improve the shear strength to 55 MPa or higher, it is preferable to set the yield strength of the weld sheet to 23-36 MPa by referring to the fitting curve. This results in an effective weld area ratio of 92.5% or more. Preferably, by controlling the average crystal grain size of the copper substrate within the range of 90 to 115 μm, a yield strength of 23 to 36 MPa for the welded sheet and an elongation of 29 to 34% are obtained.

[0070] In view of the above, the present invention enables local deformation of the weld sheet during the welding process by controlling the yield strength of the weld sheet. This improves the adhesion between the weld sheet and the weld surface of the member to be welded, which does not undergo additional planarization treatment. As a result, the effective welding area of ​​the weld interface is increased, and the shear strength and work stability of the resulting welded assembly are improved.

[0071] The embodiments and experimental data described above are intended to illustrate the present invention. It will be apparent to those skilled in the art that the present invention is not limited to these embodiments and that various modifications are possible without departing from the scope of protection of the invention.

Claims

1. A copper substrate having a first surface and a second surface facing each other, At least one nanometal layer deposited on at least one of the first surface and the second surface of the copper substrate, Includes, The yield strength of the welded sheet is 100 MPa or less. Welding sheet.

2. The yield strength of the welded sheet is in the range of 18 to 100 MPa, preferably in the range of 18 to 67.5 MPa, preferably in the range of 20 to 46.5 MPa, and also preferably in the range of 23 to 36 MPa. The welding sheet according to claim 1.

3. The average crystal grain size of the copper substrate is in the range of 30 to 140 μm, preferably in the range of 50 to 140 μm, preferably in the range of 75 to 122.5 μm, and also preferably in the range of 92 to 115 μm. The welding sheet according to claim 1 or 2.

4. The elongation of the welded sheet is 11.5% or more, preferably 23% or more, preferably in the range of 27.5% to 36%, and also preferably in the range of 29% to 34%. The welding sheet according to claim 1 or 2.

5. The average crystal grain size of the nanometal layer is in the range of 5 to 500 nm, preferably in the range of 5 to 100 nm, and the average thickness of the nanometal layer is in the range of 100 nm to 10 μm, preferably in the range of 500 nm to 5 μm. The welding sheet according to claim 1 or 2.

6. The metal of the nanometal layer can be selected from copper, gold, nickel, silver, platinum, and titanium, and at least one of the first surface and the second surface has at least two stacked nanometal layers. The welding sheet according to claim 1 or 2.

7. Step A provides a copper substrate having a first surface and a second surface facing each other, Step B involves annealing the copper substrate under vacuum to obtain a soft copper substrate having an average crystal grain size in the range of 30 to 140 μm. Step C involves depositing at least one nanometal layer on at least one of the first surface and the second surface of the soft copper substrate to obtain the welded sheet, including, A method for manufacturing welded sheets.

8. The yield strength of the soft copper substrate is 100 MPa or less, and the yield strength of the weld sheet is in the range of 18 to 100 MPa, preferably in the range of 18 to 67.5 MPa, preferably in the range of 20 to 46.5 MPa, and also preferably in the range of 23 to 36 MPa, and the average crystal grain size of the copper substrate is in the range of 50 to 140 μm, preferably in the range of 75 to 122.5 μm, and also preferably in the range of 92 to 115 μm. The method according to claim 7.

9. In step B, the vacuum level is 1.0 × 10⁻⁶ －３ The material is Torr, the annealing temperature is 400 to 850°C, the holding time is 2 to 16 hours, and preferably the annealing temperature is 500 to 800°C and the holding time is 6 to 12 hours. The method according to claim 7 or 8.

10. The average crystal grain size of the nanometal layer is in the range of 5 to 500 nm, preferably in the range of 5 to 100 nm. The method according to claim 7 or 8.

11. The method includes depositing at least two stacked nanometal layers on at least one of the first surface and the second surface, wherein the metal of the nanometal layer can be selected from copper, gold, nickel, silver, platinum, and titanium. The method according to claim 7 or 8.

12. The aforementioned nanometal layer is a nanocopper layer, and is deposited by electroplating as follows: the electrolytic solution contains 100-150 g / L of anhydrous copper sulfate, 80-150 g / L of 98% mass fraction concentrated sulfuric acid, 50-150 mg / L of sodium chloride, 350-480 mg / L of PEG20000, and 20-40 mg / L of Janus Green B, and the current density for electroplating deposition is 0.05-0.15 A / cm². ２ The deposition time is 10 to 300 seconds. The method according to claim 7 or 8.

13. The first substrate and The second circuit board, A welded portion located between the first substrate and the second substrate, connecting the two, Includes, The welded portion includes a copper substrate, a first weld interface on the first side of the copper substrate, and a second weld interface on the second side of the copper substrate facing the first side, wherein the first and second weld interfaces are formed by joining the nanometal on the opposite side of the copper substrate to the first and second substrates, respectively. The shear strength of the welded assembly is 40 MPa or more. Welded assembly.

14. The average crystal grain size of the copper substrate is in the range of 30 to 140 μm, preferably in the range of 50 to 140 μm, preferably in the range of 75 to 122.5 μm, and also preferably in the range of 92 to 115 μm. The welded assembly according to claim 13.

15. The effective welding area of ​​the first and / or second welding interface is 67.5% or more, preferably 76% or more, and more preferably 86% or more of the total interface area. The welded assembly according to claim 13 or 14.

16. The shear strength of the welded assembly is 45 MPa or more, preferably 50 MPa or more, and more preferably 55 MPa or more. The welded assembly according to claim 13 or 14.

17. Step A provides a first substrate having a first outer surface, a second substrate having a second outer surface, and a welding sheet. Step B, in which a laminated structure is formed, wherein the first nanometal layer of the welding sheet and the first outer surface of the first substrate are in surface contact, and the second nanometal layer of the welding sheet and the second outer surface of the second substrate are in surface contact, Step C involves hot-press welding a laminated structure to weld a first substrate, a second substrate, and a welding sheet together to obtain a welded assembly with a shear strength of 40 MPa or more. Includes, The welding sheet comprises a copper substrate having opposing first and second surfaces, a first nanometal layer on the first surface of the copper substrate, and a second nanometal layer on the second surface of the copper substrate, wherein the yield strength of the welding sheet is 100 MPa or less. A method for producing welded assemblies.

18. In step C, hot press welding is performed as follows: in a helium gas protected environment, pressurization is applied from above and below the laminated structure at a temperature of 250-350°C, with a pressure of 15-40 MPa and a holding time of 10-30 minutes. The method according to claim 17.

19. When the first substrate and / or the second substrate is placed horizontally, the vertical difference between the highest and lowest points on the first outer surface and the second outer surface does not exceed 200 nm. The method according to claim 17.

20. The yield strength of the weld sheet is in the range of 18 to 100 MPa, preferably in the range of 18 to 67.5 MPa, preferably in the range of 20 to 46.5 MPa, and also preferably in the range of 23 to 36 MPa. The average crystal grain size of the copper substrate is in the range of 30 to 140 μm, preferably in the range of 50 to 140 μm, preferably in the range of 75 to 122.5 μm, and also preferably in the range of 92 to 115 μm. The method according to any one of claims 17 to 19.

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