Composite silver-copper brazing filler metal, soldering paste and preparation method and brazing method of composite silver-copper brazing filler metal and soldering paste

Abstract

The invention provides composite silver-copper brazing filler metal, soldering paste and a preparation method and a brazing method of the composite silver-copper brazing filler metal and the soldering paste, and belongs to the field of brazing. The composite silver-copper brazing filler metal comprises first metal powder and second metal powder, the first metal powder is silver-copper alloy powder, and the mass fraction of Ag is 70-80 wt%; the second metal powder has no flowability or has flowability lower than that of the first metal powder in a brazing temperature zone, has intermiscibility or reaction activity higher than that of the first metal powder on a copper-based base material, and has a mass fraction of less than 15 wt%; wherein the second metal powder is configured to form a network framework with communicating pores in the process of heating to the brazing temperature, and the network framework at least partially wraps a silver-copper melt formed by melting the first metal powder. According to the composite silver-copper brazing filler metal, the soldering paste and the preparation method and the brazing method of the composite silver-copper brazing filler metal, the soldering paste, the preparation method and the brazing method of the composite silver-copper brazing filler metal and the soldering paste, brazing seams with tidy boundaries and low void ratio can be obtained on the premise of not sacrificing the silver-copper system strength and heat conduction and electric conduction advantages.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of brazing, and particularly relates to a composite silver-copper brazing filler metal, a solder paste, a preparation method thereof and a brazing method. BACKGROUND

[0002] Silver-copper-based brazing filler metals are widely used in the brazing connection of copper and copper alloy and other metal materials due to their high electrical conductivity, high thermal conductivity and good mechanical strength. In the fields of vapor chamber, vacuum device, heat sink and electrical contact, silver-copper eutectic or near-eutectic systems (commonly Ag-Cu or Ag-Cu-X with a small amount of Sn, Ni and other micro-alloying elements) can form a low-viscosity liquid phase in the range of about 740-820℃.

[0003] However, under the conditions of continuous mesh belt furnace and other automatic processes, especially to achieve high reliability connection of precision devices such as vapor chamber, the high wetting and low viscosity characteristics of traditional silver-copper brazing filler metals also bring a series of engineering problems: first, the melt is prone to excessive spreading and flooding on the surface of the copper base material, it is difficult to stably control the spreading coefficient and the overflow width, the solder loss causes the defects of virtual welding of the welding point or the welding seam, the reliability decreases, the edge morphology of the welding seam is irregular, and even the components are displaced and the adjacent welding points are bridged and short-circuited; second, for small welding areas or slit structures, the transverse diffusion of the melt is often coupled with capillary penetration, and the forming defects of local material shortage / edge accumulation coexist. SUMMARY

[0004] The purpose of the present disclosure is to provide a composite silver-copper brazing filler metal, a solder paste, a preparation method thereof and a brazing method, which can inhibit solder overflow while maintaining high strength and high electrical conductivity.

[0005] To achieve the above-mentioned purpose, the technical solutions provided by the present disclosure are as follows:

[0006] In a first aspect, the present disclosure provides a composite silver-copper brazing filler metal, which comprises a first metal powder and a second metal powder, the first metal powder is a silver-copper alloy powder, the mass fraction of Ag is 70-80wt%, the second metal powder has no flowability or lower flowability than the first metal powder in the brazing temperature range, and has higher solubility or reactivity to the copper base material than the first metal powder, the mass fraction is less than 15wt%, wherein the second metal powder is configured to form a network skeleton with interconnected pores during heating to the brazing temperature, and the network skeleton at least partially wraps the silver-copper melt formed by the melting of the first metal powder.

[0007] In one or more embodiments, the median particle size D 50 of the first metal powder is 15-45μm, and the median particle size D 50 of the second metal powder is 0.5-10μm.

[0008] In one or more embodiments, the second metal powder comprises at least one of Ni, Sn, P, Zn.

[0009] In one or more embodiments, the second metal powder is a Ni-P alloy powder, wherein the mass fraction of P is 8-14wt%.

[0010] In one or more embodiments, the second metal powder is a Cu-Sn alloy powder, wherein the mass fraction of Sn is 10-35wt%.

[0011] In one or more embodiments, the second metal powder is a Ni-Cu-P alloy powder, wherein the content of Ni is 70-90wt%, the content of Cu is 5-20wt%, and the content of P is 5-12wt%.

[0012] In one or more embodiments, the second metal powder is a Ag-Cu-Zn alloy powder, wherein the content of Ag is 10-20wt%, the content of Cu is 40-60wt%, and the content of Zn is 30-40wt%.

[0013] In one or more embodiments, the second metal powder is a Cu-Sn@Ni core-shell powder, wherein the core is a Cu-Sn alloy powder and the content of Cu is 40-70wt% and the content of Sn is 30-60wt%, and the shell is a Ni layer with a thickness of 50-80nm.

[0014] In one or more embodiments, the second metal powder is a Ni-Sn short fiber with a length of 5-10μm and an equivalent diameter of 0.5-1μm, wherein the content of Ni is 25-40wt% and the content of Sn is 60-75wt%.

[0015] In a second aspect, the present disclosure provides a composite silver-copper brazing paste, which comprises the aforementioned composite silver-copper brazing filler metal and an organic carrier, the content of the composite silver-copper brazing filler metal being 85-91wt%; the organic carrier comprising a resin 2-4wt%, an alcohol ether solvent 7-11wt%, and a thixotropic agent 0.1-0.5wt%; the dynamic viscosity of the composite silver-copper brazing paste at 25℃ and a shear rate of 10s -1 -1 is 80-180Pa·s, the thixotropic index is 3-8, and the total amount of halogen is ≤50ppm.

[0016] In one or more embodiments, the resin is ethyl cellulose, epoxy resin, phenolic resin, etc., the alcohol ether solvent is tripropylene glycol methyl ether, tripropylene glycol butyl ether, dipropylene glycol methyl ether, dipropylene glycol butyl ether, diethylene glycol, glycerol, etc., and the thixotropic agent is hydrogenated castor oil, etc.

[0017] In a third aspect, the present disclosure provides a preparation method of a composite silver-copper brazing paste, which comprises: mixing a first metal powder with a particle size of 15-45μm and a second metal powder to obtain a composite silver-copper brazing filler metal; and mixing the composite silver-copper brazing filler metal with an organic carrier to obtain the composite silver-copper brazing paste. 50 The second metal powder is the same as defined in the first aspect.50 The second metal powder with a particle size of 0.5-10 μm is uniformly mixed, and then mixed with an organic carrier, ground, planet-stirred and vacuum defoamed to form the solder paste.

[0018] In a fourth aspect, the disclosure provides a continuous soldering method using the composite silver-copper solder paste as described above, which comprises: dispensing or screen printing the composite silver-copper solder paste on a copper or copper alloy substrate; and sequentially experiencing a pre-drying at 150-220°C, an organic removal at 280-450°C, a contact reaction triggering platform at 500-750°C, and a peak zone at 780-820°C with a holding time of 300-1200 s in an inert atmosphere or a reducing atmosphere in a mesh belt type continuous soldering furnace.

[0019] The composite silver-copper solder, the solder paste, the preparation method and the soldering method provided by the disclosure can inhibit lateral flow during soldering by introducing a second metal powder with no flow or lower flow and higher reactivity or solubility to copper into the silver-copper alloy powder, so that a network skeleton with pores is formed during heating, and at least partial pinning of the front is achieved after the appearance of the silver-copper melt, thereby obtaining a solder joint with neat boundaries and low porosity without sacrificing the strength and the advantages of thermal and electrical conductivity of the silver-copper system. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the disclosure or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments described in the disclosure, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0021] Figure 1 The boundary solder joint image after soldering in Example 1 of the disclosure;

[0022] Figure 2 The boundary solder joint image after soldering in Example 2 of the disclosure;

[0023] Figure 3 The boundary solder joint image after soldering in Example 3 of the disclosure;

[0024] Figure 4 The boundary solder joint image after soldering in Example 4 of the disclosure;

[0025] Figure 5 The boundary solder joint image after soldering in Example 5 of the disclosure;

[0026] Figure 6 The boundary solder joint image after soldering in Example 6 of the disclosure;

[0027] Figure 7The boundary weld image after brazing in Example 1 of the present disclosure;

[0028] Figure 8 The boundary weld image after brazing in Example 2 of the present disclosure. DETAILED DESCRIPTION

[0029] In order to make the person skilled in the art better understand the technical solutions in the present disclosure, the technical solutions in the embodiments of the present disclosure will be described clearly and completely in the following with reference to the drawings in the embodiments of the present disclosure. Obviously, the described embodiments are only part of the embodiments of the present disclosure, rather than all the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative labor should belong to the scope of protection of the present disclosure.

[0030] It should be noted that in the following description, the "%" and "parts" representing the amount are weight basis unless otherwise specified. Unless otherwise indicated, all numbers expressing features, quantities, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the description and the claims are approximations. Those skilled in the art can obtain the required desired properties using the teachings disclosed herein, and by adjusting the numerical parameters, as needed. The use of numerical ranges includes all numbers within the range and any range subsumed therein, for example, 1 to 5 includes 1, 1.2, 1.4, 1.55, 2, 2.75, 3, 3.80, 4, and 5, and the like.

[0031] The silver-copper system has excellent wetting and thermal and electrical conductivity characteristics, but in continuous production, the melt often shows problems such as excessive spreading, unstable boundary, uneven slit filling, and sensitivity to process window. Existing improvements are mostly focused on composition fine-tuning or geometric flow limiting, the former is easy to introduce new brittle structure and performance trade-off, and the latter increases assembly complexity and beat burden, which is difficult to balance the unity of high strength, low overflow and high consistency.

[0032] To suppress the above problems, three paths can be taken: one is to reduce the liquidus or change the wetting angle by adjusting the formula, such as introducing elements such as Zn, Sn, Mn, Ni, etc. in Ag-Cu, or using Ag-coated copper, Sn-coated copper, etc. surface engineered powder, in order to balance wetting and strength. But this kind of chemical composition, interface energy regulation often accompanied by post-weld intermetallic layer thickening, continuous brittle phase or decrease of electrical conductivity, and the improvement of the geometric stability of the spreading boundary is limited. The second is to use geometric flow restriction and process constraints, such as using brazing ring, coating preform, shielding sheet or setting flow stop groove / barrier coating on the surface of the base material. This method is restricted by part tolerance, assembly alignment and additional preparation process, which is not conducive to high rhythm, batch production, and has poor repeatability on complex curved surface or multiple distributed welding sites. The third is to introduce refractory particles or ceramic reinforced phase (such as TiC, Al2O3, W / WB, etc.) to form a composite filler, to increase the equivalent viscosity of the melt or refine the structure, but often brings the side effects of reduced wettability, uneven interface reaction, increased porosity and increased joint resistance, and the adaptability to high thermal conductivity substrates such as copper is also uncertain.

[0033] The present disclosure starts from the synergistic regulation of material morphology and interface reaction, and proposes a general idea of constructing a network skeleton at the powder scale and realizing pre-staking at the interface scale. The core idea is: in the silver-copper powder-based filler system, a second metal powder with lower flowability in the brazing temperature range and more active reaction with the copper-based base material is introduced, which first connects between particles and forms a solid-phase network with connected pores during the heating process; when the main silver-copper particles melt, the melt is controlled to diffuse and fill locally under the constraint and guidance of the network, and the rapid interface reaction between the network and the base material forms a staking effect near the three-phase line, thereby stabilizing the wetting front and forming the boundary. Through this double mechanism of skeleton flow restriction and interface staking, the transverse free spreading of the melt is inhibited, but the in-plane connected pores provide a channel for the in-gap infiltration, so that flow restriction and filling are no longer contradictory.

[0034] Unlike the traditional path that only relies on surface energy or external geometric flow restriction, the present disclosure embeds flow restriction into the microstructure of the filler itself and in-situ stakes the wetting front with interface reaction. This concept is universal for various silver consumption levels, different substrates and various automatic coating methods, and can improve the controllability of spreading, the consistency and stability of weld filling without sacrificing the inherent strength and thermal and electrical conductivity of the silver-copper system.

[0035] The composite silver-copper brazing filler metal in the embodiment of the present disclosure comprises a first metal powder and a second metal powder, the first metal powder is a silver-copper alloy powder, the mass fraction of Ag is 70-80wt%, the second metal powder has no flowability or lower flowability than the first metal powder in the brazing temperature range, and has higher reactivity or solubility to the copper-based base material than the first metal powder, the mass fraction is less than 15wt%, wherein the second metal powder is configured to form a network skeleton with connected pores during heating to the brazing temperature, and the network skeleton at least partially wraps the silver-copper melt formed by melting of the first metal powder.

[0036] The first metal powder is a silver-copper alloy powder, the mass fraction of Ag is 70-80wt%, for forming the silver-copper melt mainly bearing in the brazing temperature range and providing the joint with thermal conductivity, electrical conductivity and mechanical strength. The second metal powder has lower flowability than the first metal powder in the same temperature range and has higher reactivity to the copper-based base material, and the mass fraction is limited to less than 15wt%. During heating to the brazing temperature, the second metal powder is overlapped and bonded between the first metal powders, gradually evolving into a network skeleton with connected pores, and at least partially wrapping the silver-copper melt formed by melting of the first metal powder in the subsequent reflow stage. The network skeleton geometrically limits and viscously damps the lateral diffusion of the melt, while retaining the in-plane connected pores, ensuring that the silver-copper melt can infiltrate and fill the joint gap, thereby suppressing the spreading while maintaining the necessary wetting and bonding.

[0037] The formation of the network skeleton is derived from two intrinsic properties of the second metal powder: one is the relatively high solid phase stability or delayed melting behavior, which enables it to have solid or semi-solid support capability in the peak temperature zone; the other is the high reactivity to the copper-based base material, which generates a reaction layer with the base material during heating, realizing the pinning of the wetting front. For example, when Ni-P alloy is selected as the second metal powder, short-range rearrangement and necking occur between the powders after reaching the brazing temperature zone, and the skeleton is networked on a microscopic scale; when the silver-copper melt appears and contacts the copper-based base material, P migrates to the interface and reacts with Cu to form Cu3P and other reaction phases, Ni and Cu are mutually soluble, and further induce a more stable bite on the copper-based material side. For example, when Ni-Sn or Cu-Sn system is selected as the second metal powder, (Cu, Ni)6Sn5 or Cu6Sn5 belts can be preferentially grown on the interface side during the heating stage, the belt-shaped phase anchors and buffers the melt front, weakening the randomness of the edge free spreading.

[0038] The mass fraction of the second metal powder is limited to less than 15 wt%, which can ensure that the network skeleton provides sufficient current-limiting and boundary stability, and on the other hand, avoid excessive use of the second metal powder to cause excessive wrapping and channel blockage, thereby affecting the wetting and filling of the first metal powder melt, and causing the joint strength to decrease and the porosity to increase. By reasonably limiting the mass fraction of the second metal powder, the network skeleton can form a surrounding but not closed wrapping interface for the silver-copper melt, which can suppress the out-of-plane overflow and invalid overflow, and maintain capillary penetration in the joint.

[0039] In an exemplary embodiment, the median particle size D 50 of the first metal powder is 15-45 μm, and the median particle size D 50 of the second metal powder is 0.5-10 μm.

[0040] The median particle size D 50 of the first metal powder is set to 15-45 μm, which can obtain stable stacking and repeatable transfer rate in screen printing and jet dispensing, and avoid excessive specific surface area, intensified oxidation and abnormal viscosity caused by excessively fine powder; on the other hand, the silver-copper melt formed at the size has sufficient volume potential and capillary driving force, which can smoothly cross the gap and complete wetting, and at the same time, will not cause rough stacking and difficult-to-eliminate pores like oversized particles.

[0041] The median particle size D 50 of the second metal powder is limited to 0.5-10 μm, which can make it preferentially lap joint, neck joint and build a network skeleton with pores between the gaps of the first metal powder during heating. The particle size range of 0.5-10 μm can ensure high enough interface activity on the one hand, which facilitates the reaction with the copper-based base material and forms pinning to the wetting front, and on the other hand, avoids the problems of agglomeration, residual increase and storage stability decrease commonly seen in nano-sized powder; if the particle size is greater than 10 μm, it is difficult to form a surrounding structure around the first metal powder at a microscale, and the connectivity and current-limiting capacity of the skeleton are attenuated.

[0042] In an exemplary embodiment, the second metal powder comprises at least one of Ni, Sn, P and Zn. The second metal powder can be a Ni-P alloy powder, wherein the mass fraction of P is 8-14 wt%. The second metal powder can be a Cu-Sn alloy powder, wherein the mass fraction of Sn is 10-35 wt%. The second metal powder can also be a Ni-Cu-P alloy powder, wherein the content of Ni is 70-90 wt%, the content of Cu is 5-20 wt%, and the content of P is 5-12 wt%. The second metal powder is an Ag-Cu-Zn alloy powder, wherein the content of Ag is 10-20 wt%, the content of Cu is 40-60 wt%, and the content of Zn is 30-40 wt%.

[0043] The second metal powder is defined as an alloy system containing at least one of Ni, Sn, P, and Zn, aiming to meet two types of properties at the same time: one is to present no flowability or lower flowability relative to silver-copper alloy powder, so as to maintain a support state of solid or delayed semi-solid in the reflow stage, and the other is to have higher solubility or reactivity with the copper-based base material, so as to generate a reaction layer in the heating process, thereby producing a pinning effect on the wetting front, cooperating with the network skeleton formed by the inter-particle lap joint, and realizing the constraint on the lateral diffusion of the silver-copper melt without blocking the infiltration of the weld.

[0044] The window (P is 8-14wt%) of the Ni-P alloy powder faces the comprehensive balance of the delayed melting support and the interface activity. The Ni-P in the range is mostly in nanocrystalline or near-amorphous structure, has low flowability and high viscosity in the reflow temperature zone, and the particles can be continuously connected by micro-necking in the middle of the heating process; when the silver-copper melt contacts the copper-based base material, the Ni and P migrate to the interface in a short distance, which promotes the rapid generation of compounds such as Cu3P and CuNi solid solution, and produces a stable pinning effect on the wetting front. For example, the silver-copper alloy powder D 50 about 25 μm is matched with the Ni-P alloy powder D 50 about 2.5 μm, about 5wt% of the Ni-P alloy powder is added into the metal phase, which can establish a network skeleton and induce interface nucleation in the stage of 500-750°C, and after entering the peak zone, the silver-copper melt completes the ordered diffusion along the connected pores of the skeleton, and the outer edge retraction and invalid overflow are obviously suppressed, and the continuous thin reaction zone is often found in the post-weld fracture, and the joint strength and resistance repeatability are good.

[0045] The window (Sn is 10-35wt%) of the Cu-Sn alloy powder emphasizes the interface reaction and morphology control driven by Sn. The Sn content is limited in the above range, which can ensure that the alloy powder still has low flowability before and after reflow to form a skeleton support, and can selectively generate reaction phases such as Cu6Sn 5、 Cu3Sn on the copper interface, anchor the front, and reduce the risk of random spreading and overflow of the boundary. The particle size of Cu-Sn is preferably one order of magnitude lower than that of silver-copper particles (first metal powder), so as to ensure that a surrounding but not closed pore network is formed around the silver-copper particles in the microscale. For example, when Sn in Cu-Sn is about 20wt%, under the same coating and temperature curve conditions, the lap joint overflow and boundary overflow are significantly reduced compared with the system without the second metal powder.

[0046] The window for Ni-Cu-P alloy powder (Ni content 70~90wt%, Cu content 5~20wt%, P content can achieve coupling advantages between Ni-P and Cu-Sn alloy powders) is as follows: High Ni content ensures that the particles have delayed melting characteristics and skeletal load-bearing capacity during the reflow stage; an appropriate amount of Cu improves the wetting compatibility with silver-copper melt and mitigates the risk of brittleness in the pure Ni-P interface layer; P maintains the activation of interface chemistry, making it possible to form Cu-containing phosphides or composite reaction layers on the copper substrate side, thus stabilizing the skeletal structure and improving the toughness of the interface reaction layer, making it suitable for scenarios that are more sensitive to strength, electrical resistance and long-term reliability.

[0047] In one exemplary embodiment, the second metal powder can be Cu-Sn@Ni core-shell powder, wherein the core is Cu-Sn alloy powder with a Cu content of 40~70wt% and a Sn content of 30~60wt%, and the shell is a Ni layer with a thickness of 50~80nm.

[0048] The Cu-Sn core can provide an active Sn source to the copper substrate at 500-750℃, easily inducing the formation of a thin, discontinuous Cu6Sn5 reaction band near the triple phase line. The outer Ni layer exhibits dense coating during pre-drying and organic removal stages, reducing early spreading caused by premature Sn efflux. Simultaneously, interfacial exchange and localized rupture occur at the contact reaction trigger plateau temperature, allowing Sn to participate in the leading-edge reaction through slow release. In the peak region, the Ni layer partially dissolves into the reaction layer, forming (Cu,Ni)6Sn5 through the solid solution and substitution effect of Ni on the Cu6Sn5 phase. This solid solution / substitution reduces the diffusion flux of Cu and Sn in the reaction layer, increases the activation energy of the interfacial reaction, and alters the interfacial energy and stress state to inhibit Cu3Sn continuity and improve the discontinuity and stability of the reaction layer. Therefore, the core-shell structure enhances the effectiveness of framework formation and the controllability of interfacial reactions. Cu-Sn@Ni core-shell powder can be used to prepare Cu-Sn cores through gas atomization, and after surface activation, Ni shells can be obtained by chemical Ni plating or physical vapor deposition.

[0049] In one exemplary embodiment, the second metal powder may be a Ni-Sn short fiber with a length of 5-10 μm and an equivalent diameter of 0.5-1 μm, wherein the Ni content is 25-40 wt% and the Sn content is 60-75 wt%.

[0050] Ni-Sn short fibers, with a low volume fraction, can establish a connected network across the gaps between silver and copper particles, reducing the percolation threshold and lateral free flow space. In the 500–750°C range, Ni-Sn short fibers first form necking connections, creating inter-particle bridging and providing geometric confinement and viscous damping at the melt front, maintaining delayed melting or semi-solid support even at the peak temperature. Combined with the mesh belt furnace temperature range, Ni-Sn short fibers, together with the copper substrate, promote a discontinuous reaction layer in the trigger plateau region. At the peak temperature, they create confined diffusion of the silver-copper melt, exhibiting a spreading coefficient lower than that of an equal-volume spherical fine powder system, a convergent overflow width with neat boundaries, dense visible welds, few pores, and no dark areas of incomplete welds. Ni-Sn short fibers with the target size distribution can be prepared using a template electrodeposition-exfoliation method or a rapid solidification-shear defibrillation method.

[0051] This disclosure also provides a composite silver-copper solder paste, comprising the aforementioned composite silver-copper solder and an organic carrier, wherein the composite silver-copper solder content is 85-91 wt%; the organic carrier comprises 2-4 wt% resin, 7-11 wt% alcohol ether solvent, and 0.1-0.5 wt% thixotropic agent; the composite silver-copper solder paste is tested at 25°C and a shear rate of 10 s. -1 The dynamic viscosity is 80~180 Pa·s, and the thixotropic index is 0.5 / 10s. -1 The concentration of halogens is 3~8, and the total halogen content is ≤50ppm.

[0052] Composite silver-copper solder paste uses composite silver-copper solder and an organic carrier as its two-phase base, with a metallic phase content set at 85-91 wt%. This aims to ensure the metallic continuity and load-bearing capacity of the weld after soldering, while providing sufficient rheological controllability for coating and storage. Too low a metallic phase ratio will result in insufficient metal skeleton after forming, increased shrinkage, and higher porosity; too high a ratio will lead to excessive yield stress in the paste, poor transfer rate and resilience, making it difficult to achieve stable screen printing and jet dispensing.

[0053] To achieve a balance between stable forming boundaries and post-weld compactness, the organic carrier utilizes a window of 2-4 wt% resin, 7-11 wt% alcohol ether solvent, and 0.1-0.5 wt% thixotropic agent. The resin provides particle adhesion and film strength, the alcohol ether solvent maintains open time and edge retention through moderate evaporation rate and good solubility, and the thixotropic agent establishes a shear-sensitive three-dimensional weak structure, allowing the slurry to decrease viscosity under shear stress and recover rapidly after shear release, preventing collapse, stringing, and edge burrs. Rheological properties were determined at 25°C and a shear rate of 10 s⁻¹. -1 The dynamic viscosity was controlled at 80~180 Pa·s, and the thixotropic index was 0.5 / 10 s⁻¹. -1The index is set between 3 and 8 to characterize the magnitude and speed of shear thinning and rapid recovery. A low index will cause spreading after printing or paste application, while a high index will cause poor material supply and line breakage during the printing or paste application process. The trade-off between these two ranges directly affects solder joint overflow, line width maintenance, and thickness uniformity. The total halogen content is limited to ≤50ppm to reduce the risk of corrosion and electromigration caused by residual halides after reflow soldering.

[0054] In practice, ethyl cellulose can be used as the film-forming resin, combined with tripropylene glycol methyl ether or dipropylene glycol methyl ether as the main solvent, and hydrogenated castor oil as the thixotropic modifier. Uniformly dispersed solder paste can be obtained through planetary mixing and three-roll milling.

[0055] This disclosure also provides a method for preparing a composite silver-copper solder paste, which includes: applying D... 50 The first metal powder with a diameter of 15~45μm and D 50 The second metal powder, with a particle size of 0.5~10μm, is mixed evenly; then it is mixed with an organic carrier, ground, planetarily stirred and vacuum degassed to form solder paste.

[0056] Two metal powders can be added in batches at a predetermined mass ratio to a mixing container protected by an inert gas. A combination of low-shear rolling and short-range mechanical stirring is used to ensure that fine particles fully fill the pores between coarse particles. The purpose of dry mixing is to establish initial spatial randomness, laying the foundation for uniform wetting and stable dispersion in the wet mixing stage.

[0057] In the wet stage, the mixed composite metal powder and pre-prepared organic carrier are kneaded in a planetary or double planetary mixer. To suppress agglomeration and bubble entrainment, the mixture is first pre-wetted in the order of metal powder-resin and thixotropic agent solution-partial solvent, ensuring that the resin and thixotropic agent are fully dispersed in the system and form a continuous film on the particle surface. Then, the remaining solvent is gradually added to allow the yield stress and viscosity of the system to rise smoothly within the processing window. The temperature is controlled at 20~30℃, and the rotation speed is increased in two stages: the low-speed stage eliminates powder lumps and dry spots, and the high-speed stage establishes a uniform slurry and breaks up aggregates. After kneading, a three-roll mill is used to ensure that fine particles are uniformly embedded in the organic phase network between coarse particles.

[0058] Vacuum degassing is used to remove microbubbles introduced during the mixing and grinding stages, as well as dissolved gases released from the solvent-resin system. The ground slurry is maintained at ≤50 Pa for 10-20 minutes, with intermittent low-speed stirring to assist the bubbles in rising and breaking. During degassing, the material temperature is controlled to not exceed 30℃ to avoid premature solvent evaporation, which could lead to viscosity drift and surface skin formation. After degassing, the slurry is allowed to stand and reflux in a sealed environment to allow the thixotropic network to fully recover.

[0059] For example, select silver-copper alloy powder D 50 Approximately 25 μm was selected as the first metal powder, and phosphorus-containing nickel alloy powder D was chosen.50 Approximately 2.5 μm of metal powder was used as the second metal powder. The two were first rolled together in an oxygen-free environment for 20 minutes to obtain a dry, homogeneous mixture. Then, resin, a thixotropic agent, and approximately 50% of the total resin volume of solvent were added to pre-wet the metal powder. The remaining solvent was then added, and the mixture was planetarily mixed for 10 minutes before being transferred to a three-roll mill. Two gaps of 50 μm and 30 μm were used to complete particle deagglomeration and homogenization. Finally, the mixture was degassed under a vacuum of 30 Pa for 15 minutes, with the material temperature controlled at approximately 28 °C. The resulting composite silver-copper brazing paste was tested at 25 °C for 10 seconds. -1 The lower dynamic viscosity is approximately 120 Pa·s, and the thixotropic index is (0.5 / 10 s⁻¹). -1 Approximately 4.5.

[0060] This disclosure also provides a continuous brazing method for composite silver-copper solder paste, which includes: applying or screen printing the composite silver-copper solder paste onto a copper or copper alloy substrate; and sequentially subjecting the substrate to a temperature range profile in a mesh belt continuous brazing furnace under an inert or reducing atmosphere, including pre-drying at 150-220°C, removing organics at 280-450°C, reaching a contact reaction triggering plateau at 500-750°C, and holding at a peak temperature of 780-820°C for 300-1200s.

[0061] Specifically, the composite silver-copper brazing paste is first dotted or screen-printed onto the copper or copper alloy soldering area, preferably in conjunction with plasma or micro-etching pretreatment to obtain a clean surface and a stable wetting angle. After entering the mesh belt continuous brazing furnace, the alcohol and ether solvents are first dispersed in the pre-drying zone at 150–220°C. The hydrogenated castor oil thixotropic network and ethyl cellulose form a film at this stage to prevent premature spreading and edge collapse. Subsequently, the resin skeleton is pyrolyzed and the carrier is removed in the organic removal zone at 280–450°C. The atmosphere should preferably be N2 or N2+H2 (3–5 vol%).

[0062] After organic removal, the material enters a contact reaction triggering platform at 500-750℃. The second metal powder undergoes a rapid interfacial reaction with the copper substrate, generating a reaction band that pins the wetting front in situ. Simultaneously, necking between fine particles increases the connectivity of the network skeleton, establishing a surrounding but not closed coating interface that restricts lateral free spread while preserving directional wetting within the gaps. Subsequently, the material enters the peak region, holding at 780-820℃ for 300-1200s. The first metal powder transforms into a silver-copper melt and achieves controlled reflow and densification along the network skeleton pores. The second metal powder maintains solid or delayed-melting semi-solid viscous support in the peak region, stabilizing the melt boundary and preventing overflow convergence. After the peak region, a slow cooling at 3°C / s is implemented to refine the microstructure, inhibit the continuity of coarse lamellar intermetallic compounds, and reduce edge shrinkage caused by thermal stress.

[0063] The present disclosure will be further described below with reference to specific embodiments.

[0064] To verify the effect of the composite silver-copper solder paste disclosed herein on suppressing spread behavior in continuous brazing, a polished copper sheet (30mm×60mm×0.075mm) was selected as the substrate. A continuous dispensing machine with a 23-needle (0.33mm inner diameter) was used to dispense the paste at 4-6 psi pressure. The atmosphere of the mesh belt continuous brazing furnace was N2+H2 (5vol%). The spread coefficient of the solder was tested through a spread experiment according to GB / T 11364 208. For butt welding samples, a 50mm length of paste was dispensed, and a substrate of the same size and material was placed on top to form a sandwich structure. A slight pressure of 50g was applied to promote adhesion. After welding, the cross-section was cut to evaluate the overflow of the solder at the weld boundary. The temperature profile was uniformly set as follows: 200℃ pre-drying for 300s, 450℃ organic removal for 600s, 750℃ contact reaction triggering for 300s, peak temperature of 800℃ and holding for 900s, with a cooling rate of 3℃ / s. For each sample group, the weld spread after brazing was recorded.

[0065] Example 1

[0066] Silver-copper alloy powder (Ag=72wt%) was selected. 50 =25μm as the first metallic powder, Ni-P alloy powder (P=12wt%) D 50 =2.5μm as the second metal powder, with Ni-P accounting for 5wt% in the metal phase and a total metal content of 88.5wt%. The organic carrier consists of 3.0wt% ethyl cellulose, 8.7wt% tripropylene glycol methyl ether, and 0.3wt% hydrogenated castor oil. After planetary mixing, three-roll milling, and vacuum degassing, printing and brazing are completed. The boundary weld image after brazing is shown below. Figure 1 As shown.

[0067] Example 2

[0068] Silver-copper alloy powder (Ag=72wt%) D 50 =22μm is the first metal powder, Cu-Sn alloy powder (Sn=25wt%) D 50 =1.8μm is the second metal powder, with Cu-Sn accounting for 4wt% of the metal phase and a total metal content of 87.9wt%. The organic carrier, paste preparation process, and brazing temperature range are the same as in Example 1. The boundary weld image after brazing is shown below. Figure 2 As shown.

[0069] Example 3

[0070] Using silver-copper alloy powder (Ag=72wt%) D 50 =20μm is the first metal powder, and the second metal powder is Ni-Cu-P alloy powder (Ni=80wt%, Cu=10wt%, P=10wt%). 50=2.0μm, Ni-Cu-P content in the metallic phase is 6wt%, and the total metal content is 88.2wt%. Other conditions are the same as in Example 1. The boundary weld image after brazing is as follows. Figure 3 As shown.

[0071] Example 4

[0072] Using silver-copper alloy powder (Ag=72wt%) D 50 =30μm is the first metal powder, and the second metal powder is Ag-Cu-Zn alloy powder (Ag=18wt%, Cu=47wt%, Zn=35wt%). 50 =3.0μm, Ag-Cu-Zn content in the metallic phase is 6wt%, and the total metal content is 88.2wt%. Other conditions are the same as in Example 1. The boundary weld image after brazing is as follows. Figure 4 As shown.

[0073] Example 5

[0074] Using silver-copper alloy powder (Ag=72wt%) D 50 =18μm is the first metal powder; the second metal powder is Cu-Sn@Ni core-shell powder (core is Cu=50wt%, Sn=50wt%, shell is a Ni layer with a thickness of 60nm) D 50 =1.8μm, Cu-Sn@Ni core-shell powder content in the metallic phase is 5wt%. Other conditions are the same as in Example 1. The boundary weld image after brazing is as follows. Figure 5 As shown.

[0075] Example 6

[0076] Using silver-copper alloy powder (Ag=72wt%) D 50 =22μm is the first metal powder; the second metal powder is Ni-Sn short fiber (Ni=27wt%, Sn=73wt%, length approximately 8μm, equivalent diameter 0.6μm, aspect ratio ≈13), with Ni-Sn short fiber accounting for 3wt% of the metal phase. Other conditions are the same as in Example 1. The boundary weld image after brazing is as follows. Figure 6 As shown.

[0077] Comparative Example 1

[0078] Traditional silver-copper solder paste is used, without the addition of a second metal powder. The metallic phase is Ag-Cu alloy powder (Ag=72wt%, D...). 50 =22μm), total metal content 88.5wt%, organic carrier and temperature range are the same as in Example 2. Image of the boundary weld after brazing is shown below. Figure 7 As shown.

[0079] Comparative Example 2

[0080] Solder paste was prepared using a silver-copper-tin alloy powder obtained by gas shielded gas atomization, without the addition of a second metal powder. The metallic phase was Ag-Cu-Sn alloy powder (Ag=72wt%, Cu=27wt%, Sn=1wt%, D...). 50 =25μm), total metal content 88.5wt%, organic carrier and temperature range are the same as in Example 2. The boundary weld image after brazing is shown below. Figure 8 As shown.

[0081] The performance test results of each embodiment and comparative example are shown in the table below:

[0082]

[0083] Spread factor: The percentage of the difference between the diameter (mm) of a sphere of equal test solder volume and the measured maximum height (mm) of the solder after spread, expressed as the diameter of the sphere of equal test solder volume. It indicates the solder's wetting ability on the substrate.

[0084] Porosity: The volume fraction of porosity / unfilled area in the weld, calculated by non-destructive X-ray imaging combined with ImageJ software.

[0085] Single lap shear strength: Two copper substrates are welded together with a fixed lap size to form a sample. The sample is then subjected to constant speed loading in a universal testing machine, and the maximum load is taken as the effective lap area.

[0086] Intermetallic compound thickness: The average thickness of the reaction layer (such as Cu6Sn5, Cu3Sn, Cu3P, etc.) between the weld and the copper substrate. The average value is obtained by measuring multiple points using cross-sectional SEM and phase identification is performed using EDS.

[0087] In summary, the composite silver-copper brazing filler metal, solder paste, preparation method, and brazing method provided in this disclosure introduce a second metal powder with lower fluidity and higher reactivity with copper in addition to the silver-copper alloy powder. This allows the powder to form a porous network skeleton during heating and achieve at least partial encirclement and frontal pinning after the silver-copper melt appears. This suppresses lateral flow during the brazing process, thereby obtaining a brazing seam with neat boundaries and low porosity without sacrificing the strength and thermal and electrical conductivity advantages of the silver-copper system.

[0088] 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.

[0089] 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 composite silver-copper brazing filler metal, characterized in that, Including the first metal powder and the second metal powder, The first metallic powder is a silver-copper alloy powder, in which the mass fraction of Ag is 70~80 wt%; The second metal powder has no flowability or lower flowability than the first metal powder in the brazing temperature range, and its reactivity or compatibility with the copper-based substrate is higher than that of the first metal powder, with a mass fraction of less than 15 wt%. The second metal powder is configured to form a network skeleton with interconnected pores during the heating process to the brazing temperature, and the network skeleton at least partially encapsulates the silver-copper melt formed by the melting of the first metal powder.

2. The composite silver-copper brazing filler metal according to claim 1, characterized in that, The median particle size D of the first metal powder 50 The median particle size D of the second metal powder is 15~45μm. 50 The range is 0.5~10μm.

3. The composite silver-copper brazing filler metal according to claim 1, characterized in that, The second metal powder contains at least one of Ni, Sn, P, and Zn.

4. The composite silver-copper brazing filler metal according to claim 3, characterized in that, The second metal powder is a Ni-P alloy powder, wherein the mass fraction of P is 8~14wt%; or the second metal powder is a Cu-Sn alloy powder, wherein the mass fraction of Sn is 10~35wt%.

5. The composite silver-copper brazing filler metal according to claim 3, characterized in that, The second metal powder is a Ni-Cu-P alloy powder, wherein the Ni content is 70~90wt%, the Cu content is 5~20wt%, and the P content is 5~12wt%; or the second metal powder is an Ag-Cu-Zn alloy powder, wherein the Ag content is 10~20wt%, the Cu content is 40~60wt%, and the Zn content is 30~40wt%.

6. The composite silver-copper brazing filler metal according to claim 3, characterized in that, The second metal powder is Cu-Sn@Ni core-shell powder, wherein the core is Cu-Sn alloy powder with a Cu content of 40~70wt% and a Sn content of 30~60wt%, and the shell is a Ni layer with a thickness of 50~80nm.

7. The composite silver-copper brazing filler metal according to claim 3, characterized in that, The second metal powder is a Ni-Sn short fiber with a length of 5~10μm and an equivalent diameter of 0.5~1μm, wherein the Ni content is 25~40wt% and the Sn content is 60~75wt%.

8. A composite silver-copper solder paste, characterized in that, The composite silver-copper solder as described in any one of claims 1 to 7 and an organic carrier are included, wherein the composite silver-copper solder content is 85-91 wt%; the organic carrier comprises 2-4 wt% resin, 7-11 wt% alcohol ether solvent, and 0.1-0.5 wt% thixotropic agent; the composite silver-copper solder paste is tested at 25°C and a shear rate of 10 s. -1 The dynamic viscosity is 80~180 Pa·s, the thixotropic index is 3~8, and the total halogen content is ≤50ppm.

9. The composite silver-copper solder paste according to claim 7, characterized in that, The resin is ethyl cellulose, epoxy resin, or phenolic resin; the alcohol ether solvent is tripropylene glycol methyl ether, tripropylene glycol butyl ether, dipropylene glycol methyl ether, dipropylene glycol butyl ether, diethylene glycol, or glycerin; and the thixotropic agent is hydrogenated castor oil.

10. A method for preparing the composite silver-copper solder paste according to claim 8 or 9, characterized in that, include: D 50 The first metal powder with a diameter of 15~45μm and D 50 The second metal powder, with a particle size of 0.5~10μm, is mixed evenly; then it is mixed with an organic carrier, ground, planetarily stirred and vacuum degassed to form solder paste.

11. A continuous brazing method using the composite silver-copper brazing paste of claim 8 or 9, characterized in that, include: Apply composite silver-copper solder paste by dotting or screen printing onto a copper or copper alloy substrate; The temperature profile of the brazing furnace is as follows: pre-drying at 150~220℃, organic removal at 280~450℃, contact reaction triggering plateau at 500~750℃, and holding at the peak zone of 780~820℃ for 300~1200s in an inert or reducing atmosphere.

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