Copper-based paste and method of use and construction structure thereof

Abstract

The invention provides copper-based slurry and a using method and a construction structure thereof, and belongs to the field of welding flux. The copper-based slurry comprises copper-based powder; the copper-based powder comprises the following components in percentage by mass: 10 to 20 weight percent of Ag, 2 to 4 weight percent of P, 15 to 25 weight percent of Sn, 1 to 3 weight percent of Ni and the balance of Cu and inevitable impurities; wherein when the copper-based slurry is used for carrying out thermocompression welding on a copper-steel composite base material at the temperature of 650-750 DEG C, a formed welding interface is provided with alpha-Ag phases which are distributed in a dispersed mode and P-containing intermetallic compound layers which are distributed in an interrupted mode. According to the copper-based slurry and the using method and the construction structure thereof, through collaborative design of the components of Ag, P, Sn, Ni and Cu, cracks can be effectively prevented from rapidly expanding along a welding interface, the bonding strength, the structural stability and the long-term service reliability of a welding joint are improved, and the service life of the welding joint is prolonged. Therefore, the method is particularly suitable for application scenes such as ultra-thin uniform-temperature plates with high requirements on the welding seam strength and the sealing reliability.

Description

Technical Field

[0001] This invention belongs to the field of solder technology, specifically relating to a copper-based paste, its application method, and its structural design. Background Technology

[0002] As electronic products develop towards higher integration, thinner and lighter designs and higher performance, the heat flux density generated by chips, power devices and high-density packaging modules in a limited space is constantly increasing. Heat dissipation components not only need to have high thermal conductivity and heat diffusion capabilities, but also need to take into account structural strength, dimensional thinning adaptability and reliability during long-term use.

[0003] As a commonly used phase change heat dissipation device, vapor chambers can achieve rapid heat dissipation by utilizing the evaporation and condensation of the working fluid, and are therefore widely used in mobile phones, tablets, and other portable electronic devices. Especially in ultra-thin terminal products, the thickness of the vapor chamber is strictly limited, which makes the mechanical properties, deformation resistance, and welding sealing reliability of the vapor chamber shell material important factors affecting product performance and lifespan.

[0004] Existing vapor chamber shells are mostly made of copper, which has good thermal conductivity and compatibility with working fluids, and can meet general heat dissipation requirements under conventional thickness conditions. However, as vapor chambers become increasingly ultra-thin, the mechanical strength of copper alone is relatively insufficient, making it prone to deformation during processing, assembly, or service, thus affecting the stability of the internal cavity and the overall structural reliability. On the other hand, stainless steel has high strength and rigidity, and good resistance to deformation in ultra-thin structures. However, there are compatibility issues between stainless steel and water, the commonly used working fluid in vapor chambers. Long-term use may lead to chemical reactions, potentially adversely affecting the heat dissipation performance and service reliability of the vapor chamber.

[0005] Based on the above, copper-steel composite substrates have gradually become an optional material solution for heat exchanger shells. By forming a copper layer on the surface of steel or stainless steel substrates, such as through copper plating or copper cladding, the high mechanical strength of steel or stainless steel can be utilized, while the copper layer can improve the compatibility with the working fluid interface, thus balancing structural strength and the adaptability of the internal environment of the heat dissipation device. However, in this type of copper-steel composite substrate, especially when the copper layer is thin, the problem of welding reliability becomes even more prominent. For example, in composite substrates represented by copper-plated stainless steel, the copper layer thickness may only be on the order of a few micrometers. The effective material that can participate in metallurgical bonding during the welding process is limited, and the interfacial structure of the weld joint is more easily affected by the solder composition, diffusion behavior, and interfacial reaction, resulting in insufficient weld joint strength and difficulty in meeting the requirements of ultra-thin heat exchangers for sealing and structural reliability. Summary of the Invention

[0006] Traditional Ag-Cu (72wt% Ag content) solder, while possessing good brazing performance, has a high silver content, resulting in high material costs and hindering large-scale applications. Furthermore, this type of solder typically requires welding temperatures exceeding 800℃. These higher welding temperatures not only increase manufacturing energy consumption but also negatively impact the mechanical properties of the stainless steel substrate. Particularly in ultra-thin structures, the high temperatures can further weaken the material's original strength, thereby affecting the service reliability of the heat exchanger shell.

[0007] Furthermore, to lower the melting point of copper-based solders, medium- and low-temperature welding can be achieved by increasing the content of components such as P, Sn, and Zn. However, this low-temperature design also has shortcomings when used in copper-steel composite substrates. Due to the high reactivity of elements such as P, Sn, and Zn, and their strong diffusion and reaction tendencies during welding, when they come into contact with the stainless steel side interface, they easily form a continuous and thick layer of brittle reaction products or brittle intermetallic compounds at the interface. For composite substrates with thin copper layers, the interface reaction is more likely to penetrate or significantly affect the protective effect of the limited copper layer, damaging the continuity and toughness of the weld interface, ultimately leading to problems such as reduced weld strength and insufficient joint reliability.

[0008] It is evident that existing materials for heat exchangers, such as copper, stainless steel, copper-steel composite substrates, and traditional solder systems, still suffer from various problems in ultra-thin heat exchanger applications, including insufficient mechanical strength, poor compatibility with working fluids, high welding temperatures, high costs, and insufficient welding reliability of thin copper composite substrates.

[0009] To address the aforementioned technical problems, the purpose of this disclosure is to provide a copper-based paste, its application method, and its structural design, which is applicable to copper-steel composite substrates such as copper-plated or copper-clad stainless steel, and can improve the toughness and strength of the welding interface.

[0010] To achieve the above objectives, the technical solution provided in this disclosure is as follows:

[0011] In a first aspect, this disclosure provides a copper-based slurry for hot-press welding of copper-steel composite substrates, comprising copper-based powder; the copper-based powder, by mass percentage, comprises the following components: 10-20 wt% Ag, 2-4 wt% P, 15-25 wt% Sn, 1-3 wt% Ni, with the balance being Cu and unavoidable impurities; wherein, when the copper-based slurry is used for hot-press welding of the copper-steel composite substrate at 650-750°C, the resulting welding interface has a diffusely distributed α-Ag phase and a discontinuously distributed P-containing intermetallic compound layer.

[0012] In one or more embodiments, the copper-steel composite substrate includes at least one of copper-plated stainless steel substrate and copper-clad stainless steel substrate, wherein the copper layer thickness in the copper-steel composite substrate is 1~20μm.

[0013] In one or more embodiments, the α-Ag phase is dispersed in the interface region between the solder and the substrate, and is distributed at the breaks in the P-containing intermetallic compound layer.

[0014] In one or more embodiments, the copper-based slurry is prepared by mixing the copper-based powder and an organic carrier.

[0015] In one or more embodiments, the copper-based powder is prepared by gas atomization, and the D50 of the copper-based powder is 20~60μm.

[0016] In one or more embodiments, the total metal content in the copper-based slurry is 80% to 92% based on the total mass of the copper-based slurry; the organic carrier includes cellulose thickener, alcohol ether solvent and thixotropic agent.

[0017] In one or more embodiments, the cellulose thickener includes at least one of hydroxyethyl cellulose and hydroxypropyl methyl cellulose, the alcohol ether solvent includes at least one of tripropylene glycol methyl ether, dipropylene glycol methyl ether, dipropylene glycol butyl ether, diethylene glycol, and glycerin, and the thixotropic agent includes hydrogenated castor oil.

[0018] Secondly, this disclosure provides a hot-press welding method for copper-steel composite substrates, comprising the following steps: applying the aforementioned copper-based slurry to the area to be welded on the copper-steel composite substrate; and performing hot-press welding in a reducing atmosphere or an inert atmosphere at 650~750℃ and 1~20MPa to form a welded joint.

[0019] In one or more embodiments, the heating rate during the hot-press welding process is 5~20℃ / min, and the holding time after reaching the welding temperature is 5~30min.

[0020] Thirdly, this disclosure provides a copper-steel composite substrate welded structure, which includes a first copper-steel composite substrate and a second copper-steel composite substrate welded together, and a weld located between the two; the weld is formed by welding with the aforementioned copper-based slurry; the interface between the weld and the substrate has a diffusely distributed α-Ag phase and a discontinuously distributed P-containing intermetallic compound layer.

[0021] The copper-based slurry, its application method, and structural design disclosed herein, through the synergistic design of the Ag, P, Sn, Ni, and Cu components, enable the slurry to achieve low-temperature hot-press welding at 650-750℃, thereby avoiding the adverse effects of excessively high welding temperatures on the strength preservation of stainless steel substrates caused by traditional solders. Simultaneously, after welding, this copper-based slurry allows for the formation of a dispersed α-Ag phase and a discontinuously distributed P-containing intermetallic compound layer at the interface between the weld and the substrate. The α-Ag phase, being a ductile structure, enhances the interface's load-bearing capacity and crack resistance, while the discontinuously distributed P-containing intermetallic compound layer prevents the formation of a continuous brittle interface. The combined effect of these two components effectively inhibits rapid crack propagation along the weld interface, improving the joint strength, structural stability, and long-term service reliability of the weld joint. Therefore, it is particularly suitable for applications such as ultra-thin homogeneous plates where high weld strength and sealing reliability are required. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this disclosure or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in this disclosure. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 The image shows a scanning electron microscope-energy dispersive spectroscopy (SEM-EDS) image of the weld interface in Example 1.

[0024] Figure 2 The image shows the scanning electron microscope-energy dispersive spectroscopy (SEM-EDS) image of the weld interface in Comparative Example 2. Detailed Implementation

[0025] To enable those skilled in the art to better understand the technical solutions in this disclosure, the technical solutions in the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this disclosure, and not all embodiments. Based on the embodiments in this disclosure, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this disclosure.

[0026] It should be noted that, unless otherwise specified, "%" and "parts" in the following description refer to weight. Unless otherwise indicated, all figures used in this specification and claims to indicate characteristic dimensions, quantities, and physical properties should be understood to be modified by the term "about" in all cases. Therefore, unless stated to the contrary, the numerical parameters listed in the foregoing specification and appended claims are approximations, and those skilled in the art can appropriately modify these approximations to obtain the desired characteristics using the teachings disclosed herein. The use of numerical ranges expressed as endpoints includes all numbers within that range and any range within that range; for example, 1 to 5 includes 1, 1.2, 1.4, 1.55, 2, 2.75, 3, 3.80, 4, and 5, etc.

[0027] As electronic products continue to evolve towards higher power density and thinner designs, vapor chambers not only need excellent heat transfer capabilities but also require sufficient mechanical strength and long-term reliability within a smaller structural space. Therefore, while copper alone offers good thermal conductivity and compatibility with working fluids, its structural strength is increasingly insufficient for ultra-thin designs. High-strength stainless steel, while beneficial for improving the shell's resistance to deformation, may pose compatibility risks during prolonged contact with the working fluid. Thus, employing composite substrates that balance strength and interfacial compatibility has become a crucial direction for the development of ultra-thin vapor chamber materials.

[0028] The inventors discovered in their research that while the application of composite substrates alleviated the inherent limitations of the shell materials, new problems arose at the bonding interface. Especially when the copper layer on the surface of the composite substrate was thin, the formation of the weld joint no longer depended solely on whether the solder could achieve a connection, but rather more on whether the interfacial reaction during welding was controllable, whether the interfacial structure was stable, and whether the welded joint possessed sufficient toughness.

[0029] In existing solder systems, one type relies on high temperatures to achieve bonding, which can negatively impact the performance of high-strength substrates. Another type, while capable of lowering welding temperatures, tends to trigger strong interfacial reactions when interacting with composite substrates, leading to the formation of a brittle, unsustainable continuous layer at the interface, thus compromising the welding reliability of ultrathin composite substrates. Therefore, the key issue is not simply about lowering welding temperatures or increasing solder strength, but rather about achieving a synergistic balance between welding temperature, interfacial reaction, and interfacial microstructure, specifically targeting the composite substrate.

[0030] Based on the above understanding, this disclosure does not follow the traditional approach of solely pursuing a lower melting point or a higher bonding strength. Instead, it takes a holistic approach to designing the copper-based slurry system from the perspective of adapting to the interfacial characteristics of ultra-thin composite substrates. The core idea is to use copper-based powder as a foundation and through the synergistic configuration of multiple functional elements, on the one hand, enable the slurry to achieve stable bonding under relatively mild hot-press welding conditions, mitigating the adverse effects of high temperatures on the composite substrate, especially the high-strength matrix; on the other hand, by controlling the interfacial reaction path and interfacial morphology during the welding process, the weld interface does not form a continuous, single brittle structure, but rather establishes an interfacial region that balances bonding strength and toughness.

[0031] The technical approach disclosed herein is as follows: based on the application requirements of ultra-thin isothermal plates for high-strength and reliable connections, and addressing the high-temperature damage and interface embrittlement problems that easily occur at the thin-layer interface of composite substrates during welding, a copper-based slurry system capable of achieving low-temperature connection and controllable interface morphology is constructed. This system forms a morphology at the welding interface where a tough structure and a controlled reaction layer complement each other, thereby balancing welding temperature, interface strength, and long-term reliability.

[0032] A copper-based slurry for hot-press welding of copper-steel composite substrates according to one embodiment of this disclosure comprises copper-based powder. The copper-based powder comprises the following components by mass percentage: 10-20 wt% Ag, 2-4 wt% P, 15-25 wt% Sn, 1-3 wt% Ni, with the balance being Cu and unavoidable impurities. When the copper-based slurry is used for hot-press welding of the copper-steel composite substrate at 650-750°C, the resulting welding interface has a diffusely distributed α-Ag phase and a discontinuously distributed P-containing intermetallic compound layer.

[0033] In this copper-based slurry, Ag, P, Sn, Ni, and Cu work synergistically, playing a crucial role in the melting, wetting, diffusion, reaction, and solidification evolution during the hot-press welding process. Ag in this slurry system not only improves the solder's microstructure but also forms a dispersed α-Ag phase at the solder-substrate interface after hot-press welding. The α-Ag phase possesses metallic toughness and exists in a discrete, dispersed manner at the weld interface. This microstructure creates multiple tough regions at the weld interface capable of transmitting stress, blunting crack tips, and delaying crack propagation. It can also distribute localized stress concentrations at the interface when the weld joint is subjected to external forces, thermal stress, or cyclic loading, thereby improving weld strength.

[0034] P and Sn primarily function to lower the temperature of the solder. The introduction of P and Sn can significantly reduce the liquidus formation temperature, allowing copper-based slurries to be hot-pressed within the range of 650~750℃. This not only reduces the adverse effects of high temperatures on the microstructure and mechanical properties of stainless steel substrates, but also helps to control warping, collapse, or localized overheating of ultra-thin copper-steel composite substrates during the welding process.

[0035] It should be noted that while phosphorus (P) helps lower the melting point and improve metallurgical reactions during brazing, it is also a highly reactive element. When interacting with elements such as Fe, Cr, and Ni at the stainless steel interface, it easily forms a P-containing intermetallic compound layer, such as (Fe,Cr,Ni)3P. This type of structure typically exhibits high brittleness, and once continuously formed at the weld interface, it easily becomes a site for joint fracture. Therefore, the key to the aforementioned copper-based slurry is not to completely eliminate the P-containing intermetallic compound layer, but to achieve a discontinuous distribution of the P-containing intermetallic compound layer at the interface through the proportions of Ag, Ni, P, Sn, and Cu, thus avoiding the formation of a continuous brittle interface layer.

[0036] If the phosphorus-containing brittle phase extends continuously at the weld interface, cracks can easily propagate rapidly along this path after formation, and the failure mode of the joint will mainly manifest as brittle fracture at the interface. When the aforementioned phosphorus-containing intermetallic compound layer is discontinuously distributed, the brittle region at the interface is divided into multiple discontinuous local regions. Even if a crack initiates in a local brittle region, it is difficult for it to propagate continuously along the interface.

[0037] Furthermore, the dispersed α-Ag phase can be distributed at the breaks in the discontinuous P-containing intermetallic compound layer, which is equivalent to embedding a ductile metallic structure between different brittle regions. This structure weakens the continuity of the brittle phase on the one hand, and provides additional ductile support to the interface on the other, requiring the crack to continuously cross ductile regions during propagation, thereby increasing the fracture resistance.

[0038] The addition of Ni improves the fluidity and spreadability of the solder during hot-press welding, making it easier for the solder to fill the interface under pressure and enhancing the contact between the solder and the copper-steel composite substrate. Simultaneously, Ni participates in interfacial reactions and solidification evolution, influencing the formation and distribution of the P-containing intermetallic compound layer, making the interfacial structure more likely to develop towards a combination of discontinuously distributed reaction layers and diffusely distributed tough phases.

[0039] In specific implementations, for mobile phone heat spreader shells with small areas and narrow weld seams, the aforementioned copper-based paste can be applied to the area to be welded by means of dotting, screen printing or scraping to ensure uniform distribution of solder on the narrow frame; for the edge sealing area of ​​the composite shell with a large area, pre-formed welding sheets or welding strips can also be placed between the two copper-steel composite substrates to complete the overall connection under hot pressing conditions.

[0040] Copper-steel composite substrates can be either copper-plated stainless steel substrates or copper-clad stainless steel substrates. For copper-plated stainless steel substrates, the copper layer is typically formed on the surface of the stainless steel substrate through electroplating, chemical plating, or other deposition methods. The copper layer bonds tightly to the stainless steel substrate, making it suitable for obtaining thinner and more uniform surface copper layers, especially for ultra-thin structural components where thickness control is critical. For copper-clad stainless steel substrates, the copper layer can be deposited on the surface of the stainless steel substrate through methods such as lamination, rolling, or deposition. The resulting copper layer is typically denser and helps maintain a more stable surface condition during subsequent processing.

[0041] In copper-steel composite substrates, the copper layer thickness is typically 1–20 μm. When the copper layer thickness is too small, the amount of material available for metallurgical bonding and buffering of interfacial reactions during welding is limited. If the solder system and interfacial structure are not properly controlled, the interfacial reaction can easily develop directly towards the stainless steel side, leading to a decrease in the strength of the welded joint. When the copper layer thickness is too large, although the interfacial buffering effect is enhanced, the overall advantages of the composite substrate in terms of strength, thickness control, and material cost will be weakened, which is not conducive to the application of ultra-thin devices.

[0042] The aforementioned α-Ag phase is dispersed in the interface region between the solder and the substrate, and is distributed at the breaks in the P-containing intermetallic compound layer. This interface region can be understood as the transition area formed between the weld side and the substrate side after the solder melts and wets, diffuses, and reacts with the copper-steel composite substrate. The α-Ag phase exists in this region in the form of fine particles, short strips, islands, or discrete aggregates.

[0043] Meanwhile, the P-containing intermetallic compound layer does not extend continuously along the interface, but forms discontinuous regions in some locations. The α-Ag phase is further located at the break points of these discontinuous regions, so that the interface structure presents a state in which the brittle phase is divided and the ductile phase is filled in between.

[0044] For copper-steel composite substrates, phosphorus (P) readily reacts with elements such as Fe, Cr, and Ni in the substrate during welding, forming a P-containing intermetallic compound layer at the interface. While this structure reflects a metallurgical bond between the solder and the substrate, it is inherently brittle. If it forms continuously along the interface, cracks can easily propagate rapidly along this continuous brittle path, leading to preferential interfacial fracture of the welded joint under stress.

[0045] When the aforementioned α-Ag phase is dispersed in the interface region and happens to be distributed at the break point of the P-containing intermetallic compound layer, it is equivalent to inserting a transitional structure with metallic toughness between the brittle layers that would otherwise develop continuously. In this way, the interface is no longer dominated by a single brittle layer, but forms a composite interface with alternating brittle and tough structures. Even if a crack initiates at a P-containing intermetallic compound layer, it is difficult to continue along the interface. When it propagates to the break point, it will be blocked, deflected, or passivated by the α-Ag phase, thus improving weld strength.

[0046] In practice, the aforementioned α-Ag phase can be formed through solder composition design and the heating, holding, and pressurizing processes during hot-press welding. For example, after the copper-based slurry melts and spreads on the surface of the copper-plated stainless steel substrate, Ag elements participate in the solidification structure formation with Cu near the interface, ultimately forming a dispersed α-Ag phase in the interface region between the solder and the substrate; P reacts with elements on the stainless steel side during the interface reaction to form a P-containing intermetallic compound layer. When the welding temperature, pressure, and element ratio are within a suitable range, the P-containing intermetallic compound layer will not form a continuous, dense, solid layer, but will exhibit a discontinuous distribution, with the α-Ag phase located between these discontinuous regions.

[0047] In one exemplary embodiment, the copper-based paste is prepared by mixing the copper-based powder and an organic carrier. The copper-based powder here refers to powdered solder particles formed based on the aforementioned copper-based powder composition system. Its purpose is to ensure that the paste retains the compositional design and interfacial structure control effects of the copper-based paste during subsequent heating. The organic carrier primarily imparts suitable dispersibility, adhesion, printability, and thixotropy to the copper-based paste, enabling the copper-based powder to be uniformly suspended and stably adhered to the substrate surface at room temperature. Simultaneously, it gradually volatilizes or decomposes during heating to release the copper-based powder for soldering.

[0048] In a specific embodiment, copper-based powder can be obtained through gas atomization, water atomization, or other powder preparation methods, and then mixed with an organic carrier in a predetermined ratio. After kneading, grinding, and degassing, a copper-based slurry is formed. The organic carrier may include thickening components, solvent components, and rheology modifiers to enable the copper-based slurry to have good shape retention during dotting, scraping, printing, or transfer processes.

[0049] The aforementioned copper-based powder is preferably prepared by gas atomization, with a D50 preferably of 20-60 μm. Copper-based powders formed by gas atomization typically have more regular particle morphology and better compositional uniformity, which helps reduce component segregation between different particles, allowing the components of the aforementioned copper-based slurry to participate more consistently in interfacial wetting and microstructure formation during the melting process. A D50 of 20-60 μm ensures that the copper-based powder will not cause rough slurry coating or localized melting delays due to excessively large particles, nor will excessively fine particles significantly increase the specific surface area, enhance oxidation tendency, and lead to abnormally high slurry viscosity.

[0050] The total metal content in the copper-based paste, based on its total mass, is preferably 80%–92% to balance the paste's formability and the effective metal supply after soldering. When the total metal content is too low, the proportion of organic carrier is too high, resulting in more volatile components during soldering, which can easily lead to weld collapse, increased porosity, or insufficient metal filling after soldering. Conversely, when the total metal content is too high, the copper-based paste may become too dry and hard, hindering application, printing, or scraping. A range of 80%–92% allows the copper-based paste to possess suitable rheological properties at room temperature and provides sufficient solder metal after heating, thus better meeting the hot-press welding requirements of copper-plated or copper-clad stainless steel substrates.

[0051] The aforementioned organic carriers include cellulose thickeners, alcohol ether solvents, and thixotropic agents. The cellulose thickeners can be hydroxyethyl cellulose, hydroxypropyl methyl cellulose, etc.; the alcohol ether solvents can be tripropylene glycol methyl ether, dipropylene glycol methyl ether, dipropylene glycol butyl ether, diethylene glycol, glycerin, etc.; and the thixotropic agent can be hydrogenated castor oil. This combination is mainly used to impart good dispersibility, shape retention, and construction stability to the copper-based slurry. Hydroxyethyl cellulose and similar substances can improve the system's adhesion, allowing the copper-based powder to be uniformly suspended and adhered to the substrate surface; tripropylene glycol methyl ether and similar substances can adjust the slurry's fluidity and open time, facilitating the formation of a smooth paste after application; and hydrogenated castor oil helps establish a thixotropic structure, making the copper-based slurry easy to extrude under shear and maintain its shape after settling.

[0052] This disclosure also provides a hot-press welding method for copper-steel composite substrates, comprising the following steps: applying the aforementioned copper-based slurry to the area to be welded on the copper-steel composite substrate; and performing hot-press welding in a reducing atmosphere or an inert atmosphere at 650~750℃ and 1~20MPa to form a welded joint.

[0053] The copper-steel composite substrate can be a copper-plated stainless steel substrate or a copper-clad stainless steel substrate. The area to be welded can be the edge sealing area, overlap area, or partial connection area where two copper-steel composite substrates need to be bonded together. Before applying the copper-based slurry, the area to be welded can be pre-treated to remove oil, particulate impurities, and surface deposits, thereby improving the adhesion stability of the copper-based slurry to the copper-steel composite substrate and improving the wetting conditions during the subsequent heating process.

[0054] The application method of the copper-based paste can be selected according to the shape, size, and weld distribution of the area to be welded. For example, methods such as dot coating, linear coating, scraping, printing, or transfer coating can be used to continuously distribute the copper-based paste along the area to be welded or according to a predetermined pattern. This step allows the copper-based paste to be pre-placed at the interface in a relatively uniform manner, providing a stable material basis for the melting and spreading of the solder and interface bonding during subsequent hot-press welding. This is particularly beneficial for copper-steel composite substrates with thin copper layers to achieve a more consistent solder supply.

[0055] After the copper-based slurry coating is completed, the mating copper-steel composite substrates can be assembled and hot-pressed at 650~750℃ and 1~20MPa in a reducing or inert atmosphere to form a welded joint. During the hot-pressing process, as the temperature rises, the organic carrier in the copper-based slurry gradually volatilizes or decomposes, while the copper-based powder melts and fills the spaces between the interfaces under pressure. The temperature range of 650~750℃ satisfies the requirements for melting, flowing, and metallurgical bonding of the copper-based powder, while also helping to reduce the adverse effects of excessively high welding temperatures on the strength preservation of the copper-steel composite substrate, especially the stainless steel matrix.

[0056] A pressure of 1-20 MPa can promote tight bonding between the two copper-steel composite substrates, reduce the interfacial gap, promote the full spread of molten solder in the area to be welded, and improve the density of the weld joint. The reducing atmosphere can be a hydrogen-containing atmosphere, and the inert atmosphere can be a nitrogen atmosphere or an argon atmosphere. Such an atmosphere helps to inhibit the oxidation of the interface and solder surface during the welding process, ensuring good wetting and diffusion conditions between the copper-based paste and the copper-steel composite substrate.

[0057] The preferred heating rate during the aforementioned hot-press welding process is 5~20℃ / min, and the preferred holding time after reaching the welding temperature is 5~30min. For copper-steel composite substrates with a thin copper layer, both excessively fast and excessively slow heat input will significantly affect the formation of the weld interface.

[0058] If the heating rate is too low, the overall heating time will be too long. Before the solder reaches full melting, excessive solid-state diffusion and interfacial reactions may occur, easily leading to increased local oxidation, uneven removal of organic carriers, or premature enhancement of interfacial reactions, which is detrimental to obtaining a stable weld structure. If the heating rate is too high, the temperature field between the solder and the substrate will be established too drastically, and volatile components in the slurry may not have enough time to escape smoothly, easily causing local bubbling, spattering, or uneven spreading. Controlling the heating rate between 5~20℃ / min is beneficial for balancing heating efficiency and the stability of the interfacial formation process, allowing the organic carriers in the copper-based slurry to be gradually removed, and enabling the copper-based slurry to complete melting and spreading under relatively stable heat input conditions.

[0059] The holding time after reaching the welding temperature is 5-30 minutes. This is mainly to ensure that the copper-based powder has sufficient time to complete interface wetting, filling, and microstructure evolution in the molten state, while avoiding the adverse effects of prolonged high-temperature exposure on the copper-steel composite substrate. If the holding time is too short, the flow and filling of the molten solder between the interfaces may not be fully completed, resulting in insufficient interfacial diffusion and metallurgical bonding, which can easily lead to local unbonded areas or uneven reaction areas in the weld. If the holding time is too long, the interfacial reaction may be further aggravated, especially for copper-plated stainless steel substrates with thin copper layers, which may cause the interfacial reaction layer to thicken, hindering the improvement of weld joint strength.

[0060] In practice, the heating rate and holding time can be adjusted according to the copper layer thickness of the copper-steel composite substrate, the area to be welded, and the amount of copper-based slurry applied. For example, for copper-plated stainless steel substrates with a copper layer thickness of approximately 5 μm, a heating rate of about 10 °C / min can be used to allow the organic carriers in the slurry to be removed relatively smoothly, and a holding time of 20 min after reaching 650 °C can be used to ensure that the solder spreads fully and forms a relatively uniform interface structure. For copper-clad stainless steel substrates with a relatively thick copper layer or a large welding area, the heating rate and holding time can also be adjusted within the aforementioned range to adapt to the thermal response characteristics of different welding objects.

[0061] This disclosure also provides a copper-steel composite substrate welded structure, which includes a first copper-steel composite substrate and a second copper-steel composite substrate welded together, and a weld located between the two; the weld is formed by welding with the aforementioned copper-based slurry; the interface between the weld and the substrate has a dispersed α-Ag phase and a discontinuously distributed P-containing intermetallic compound layer.

[0062] The first and second copper-steel composite substrates can be copper-plated stainless steel substrates or copper-clad stainless steel substrates, respectively. They can also be copper-steel composite components of different thicknesses and structural forms that can be assembled and mated together. After hot-press welding, a weld is formed between the relatively set connection boundaries. The weld serves to reliably connect the first and second copper-steel composite substrates into one unit and undertakes the functions of maintaining structural airtightness and transmitting loads.

[0063] The weld is formed by welding with the aforementioned copper-based slurry. During the hot-press welding process, the copper-based slurry melts, spreads, and reacts with the first copper-steel composite substrate and the second copper-steel composite substrate at the interface. Finally, an interface with a diffusely distributed α-Ag phase and a discontinuously distributed P-containing intermetallic compound layer is formed between the weld and the substrate. As a result, the welded joint no longer exhibits a single interface morphology dominated by a continuous brittle reaction layer, but rather forms a composite structure in the interface region with ductile and brittle structures alternating with each other.

[0064] In practical implementation, the aforementioned copper-steel composite substrate welding structure can be manifested as a frame-type connection structure, an lap-joint connection structure, or an enclosed and sealed connection structure. For example, the first copper-steel composite substrate can serve as the upper shell component of the heat spreader, and the second copper-steel composite substrate can serve as the lower shell component of the heat spreader. The two are joined together along their peripheral areas and form a continuous and closed weld seam through the aforementioned copper-based slurry. After welding, a sealed cavity is formed inside the shell, and the cavity contains working fluid, thus constituting the shell welding structure of the heat spreader.

[0065] The present disclosure will be further described below with reference to specific embodiments and comparative examples.

[0066] Example 1

[0067] A copper-based slurry was prepared by gas atomization of Ag (20wt%), Cu (55wt%), Sn (20wt%), P (3wt%), and Ni (2wt%) copper-based powder. The slurry was then used to perform hot-press welding on a copper-plated stainless steel substrate with a copper layer thickness of 5μm and a total thickness of 80μm. The welding temperature was 680℃, the welding atmosphere was 5% H2 / N2, the welding pressure was 4MPa, the heating rate was 10℃ / min, and the holding time after reaching the welding temperature was 20min.

[0068] After welding, the samples underwent a 180° tensile test using a mechanical property testing instrument. The maximum force that pulled the weld joint apart was recorded, and the average of the three test results was taken. The samples were then subjected to 500 cycles of high and low temperature cycling from -20°C to 100°C before another tensile test to evaluate the reliability of the solder under long-term service conditions. A maximum pull-out force below 30N macroscopically indicates easy tearing; at this level, the heat spreader poses a risk of working fluid leakage under service conditions, resulting in poor reliability.

[0069] Example 2

[0070] The Ag content of the gas-atomized copper-based powder was adjusted to 15wt%, and the remaining components were adjusted to P 3wt%, Sn 20wt%, Ni 2wt%, with the balance being Cu. Hot pressing welding, pull-out force testing, and high and low temperature cycle reliability testing were performed under the same substrate and process conditions as in Example 1.

[0071] Example 3

[0072] The Ag content of the gas-atomized copper-based powder was adjusted to 10wt%, and the remaining components were adjusted to P 3wt%, Sn 18wt%, Ni 2wt%, with the balance being Cu. Hot pressing welding, pull-out force testing, and high and low temperature cycle reliability testing were performed under the same substrate and process conditions as in Example 1.

[0073] Example 4

[0074] The Ag content of the gas-atomized copper-based powder was adjusted to 18wt%, and the remaining components were adjusted to P 2wt%, Sn 17wt%, Ni 2wt%, with the balance being Cu. Hot pressing welding, pull-out force testing, and high and low temperature cycle reliability testing were performed under the same substrate and process conditions as in Example 1.

[0075] Example 5

[0076] Using the same copper-based paste as in Example 1, the substrate was replaced with a copper-clad stainless steel substrate with a copper layer thickness of approximately 15 μm and a total thickness of 80 μm. Hot pressing welding, pull-out force testing, and high and low temperature cycle reliability testing were performed under the same process conditions as in Example 1.

[0077] Comparative Example 1

[0078] A paste made of traditional Ag (72wt%) and Cu (28wt%) solder was hot-pressed onto a copper-plated stainless steel substrate with a copper layer thickness of about 5μm and a total thickness of 80μm. The welding temperature was 820℃, and the remaining process conditions were the same as in Example 1. Pull-out force test and high and low temperature cycle reliability test were performed.

[0079] Comparative Example 2

[0080] Using industry-standard copper-based powder, Cu (75wt%), Sn (16wt%), P (5wt%), and Ni (4wt%), a copper-based slurry was prepared and then hot-pressed onto a copper-plated stainless steel substrate with a copper layer thickness of approximately 5μm and a total thickness of 80μm. The welding temperature was 680℃, and the remaining process conditions were the same as in Example 1. Pull-out force tests and high and low temperature cycle reliability tests were also performed.

[0081] Comparative Example 3

[0082] A copper-based slurry was prepared by atomizing Ag (5wt%), Cu (70wt%), Sn (20wt%), P (3wt%), and Ni (2wt%) copper-based powder. The slurry was then hot-pressed onto a copper-plated stainless steel substrate with a copper layer thickness of approximately 5μm and a total thickness of 80μm. The welding temperature was 680℃. The remaining process conditions were the same as in Example 1. Pull-out force tests and high and low temperature cycle reliability tests were also performed.

[0083] Comparative Example 4

[0084] A copper-based slurry was prepared by atomizing Ag (20wt%), Cu (52wt%), Sn (20wt%), P (5wt%), and Ni (2wt%) copper-based powder. The slurry was then hot-pressed onto a copper-plated stainless steel substrate with a copper layer thickness of approximately 5μm. The welding temperature was 680℃. The remaining process conditions were the same as in Example 1. Pull-out force tests and high and low temperature cycle reliability tests were also performed.

[0085] Comparative Example 5

[0086] A copper-based slurry was prepared by atomizing Ag (20wt%), Cu (65wt%), Sn (10wt%), P (3wt%), and Ni (2wt%) copper-based powder. The slurry was then hot-pressed onto a copper-plated stainless steel substrate with a copper layer thickness of approximately 5μm and a total thickness of 80μm. The welding temperature was 680℃. The remaining process conditions were the same as in Example 1. Pull-out force tests and high and low temperature cycle reliability tests were also performed.

[0087] Comparative Example 6

[0088] A copper-based slurry was prepared by atomizing Ag (20wt%), Cu (57wt%), Sn (20wt%), and P (3wt%) copper-based powder. The slurry was then hot-pressed onto a copper-plated stainless steel substrate with a copper layer thickness of approximately 5μm and a total thickness of 80μm. The welding temperature was 680℃. The remaining process conditions were the same as in Example 1. Pull-out force tests and high and low temperature cycle reliability tests were also performed.

[0089] Table 1 - Performance Test Table

[0090]

[0091] As shown in Table 1, the pull-out forces of the solder joints in Examples 1 to 5 are significantly higher than those in Comparative Examples 2 to 6, indicating that the copper-based paste of this disclosure has significantly better welding strength on copper-steel composite substrates, especially ultra-thin copper-plated stainless steel substrates, than existing solder systems. Appropriate metal composition ratios are necessary to effectively increase welding strength. Comparative Example 1, with an Ag content as high as 72 wt%, exhibits high solder joint strength, but due to the higher welding temperature, the strength of the copper-plated stainless steel substrate is compromised, showing a significant decrease in strength after 500 high- and low-temperature cycles. Furthermore, the high silver content significantly increases raw material costs, resulting in poor market acceptance. In contrast, Example 1, with a silver content reduced to 20%, shows little difference in mechanical properties, and cyclic testing confirms high long-term reliability. In addition, Comparative Example 2, using an Ag-free Cu-Sn-P-Ni solder, exhibits the lowest solder joint pull-out force, demonstrating the important role of Ag introduction in improving the strength of the welded joint.

[0092] As can be seen from the accompanying drawings, in the embodiment, the interface between the weld and the substrate forms a dispersed α-Ag phase and a discontinuously distributed P-containing intermetallic compound layer, which helps to avoid the formation of a continuous brittle interface. In contrast, in the comparative example, a continuous or near-continuous brittle reaction layer is more likely to form, leading to an increased tendency for interfacial fracture. Therefore, this disclosure, through the synergistic design of Ag, P, Sn, Ni, and Cu, can not only reduce the welding temperature but also significantly improve the strength of the welded joint of the copper-steel composite substrate.

[0093] It will be apparent to those skilled in the art that this disclosure is not limited to the details of the exemplary embodiments described above, and that this disclosure can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of this disclosure is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within this disclosure. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0094] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A copper-based slurry for hot-press welding of copper-steel composite substrates, characterized in that, Including copper-based powder; the copper-based powder, by mass percentage, comprises the following components: 10-20 wt% Ag, 2-4 wt% P, 15-25 wt% Sn, 1-3 wt% Ni, with the balance being Cu and unavoidable impurities; When the copper-based slurry is hot-pressed to weld a copper-steel composite substrate at 650~750℃, the resulting welding interface has a diffusely distributed α-Ag phase and a discontinuously distributed P-containing intermetallic compound layer.

2. The copper-based paste according to claim 1, characterized in that, The copper-steel composite substrate includes at least one of copper-plated stainless steel substrate and copper-clad stainless steel substrate, and the copper layer thickness in the copper-steel composite substrate is 1~20μm.

3. The copper-based paste according to claim 1, characterized in that, The α-Ag phase is dispersed in the interface region between the solder and the substrate, and is also distributed at the breaks in the P-containing intermetallic compound layer.

4. The copper-based paste according to claim 1, characterized in that, The copper-based slurry is prepared by mixing the copper-based powder and an organic carrier.

5. The copper-based paste according to claim 4, characterized in that, The copper-based powder is prepared by gas atomization, and the D50 of the copper-based powder is 20~60μm.

6. The copper-based paste according to claim 4, characterized in that, The total metal content in the copper-based slurry is 80% to 92% based on the total mass of the slurry; the organic carrier includes cellulose thickener, alcohol ether solvent and thixotropic agent.

7. The copper-based paste according to claim 6, characterized in that, The cellulose thickener includes at least one of hydroxyethyl cellulose and hydroxypropyl methyl cellulose, the alcohol ether solvent includes at least one of tripropylene glycol methyl ether, dipropylene glycol methyl ether, dipropylene glycol butyl ether, diethylene glycol, and glycerin, and the thixotropic agent includes hydrogenated castor oil.

8. A hot-press welding method for copper-steel composite substrates, characterized in that, Includes the following steps: The copper-based paste according to any one of claims 1 to 7 is applied to the area to be welded on the copper-steel composite substrate; Hot pressing welding is performed at 650~750℃ and 1~20MPa in a reducing or inert atmosphere to form a welded joint.

9. The hot-press welding method according to claim 8, characterized in that, The heating rate during hot-press welding is 5~20℃ / min, and the holding time after reaching the welding temperature is 5~30min.

10. A copper-steel composite substrate welded structure, characterized in that, The invention comprises a first copper-steel composite substrate and a second copper-steel composite substrate welded together, and a weld between the two; the weld is formed by welding with copper-based slurry according to any one of claims 1 to 7; the interface between the weld and the substrate has a diffusely distributed α-Ag phase and a discontinuously distributed P-containing intermetallic compound layer.

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