A method for preparing copper-phosphorus-silver based solder strip

Abstract

This invention proposes a method for preparing copper-phosphorus-silver-based brazing filler metal strips, comprising: S1, performing subsolid-phase isothermal spheroidization pretreatment on a continuously cast slab of copper-phosphorus-silver-based brazing filler metal; S2, heating the pretreated slab to a rolling temperature, wherein the rolling temperature is ≥ the measured flow point temperature of the copper-phosphorus-silver-based brazing filler metal and ≤ the liquidus temperature of the brazing filler metal; S3, performing multi-pass rolling on the heated slab, with a total reduction rate ≥ 85%, to obtain a strip; S4, cooling the rolled strip to room temperature. This invention first performs a "subsolid-phase isothermal spheroidization pretreatment" on the continuously cast slab of copper-phosphorus-silver-based brazing filler metal, followed by multi-pass rolling in a "semi-solid warm rolling" range. This utilizes the pre-existing micro-molten liquid phase at the grain boundaries to reduce deformation resistance and coordinate grain slip, thereby completely eliminating cracks.

Description

Technical Field

[0001] This invention relates to the field of brazing materials technology, and specifically to a method for preparing copper-phosphorus-silver brazing filler metal strips. Background Technology

[0002] Copper-phosphorus silver-based brazing filler metals (such as BCu80AgP and BCu89PAg) occupy an important position in the field of precision brazing due to their excellent overall performance. Compared with silver-based brazing filler metals, they have the advantage of lower cost. Their core advantages include good wettability, flowability, electrical and thermal conductivity, and a certain degree of self-fluxing effect (phosphorus can lower the melting point and improve flowability). These properties make them particularly suitable for applications with extremely high requirements for sealing and reliability, such as large-area, narrow-gap sealing brazing of copper tubes and copper plates in liquid cooling plates (such as IGBT heat sink substrates and power battery cold plates).

[0003] Currently, high-quality copper-phosphorus-silver brazing filler metal strips (typically ≤0.3 mm thick) are mainly produced via the traditional "melting-casting-rolling" process. However, this process faces significant challenges, stemming from the inherent brittleness of the material.

[0004] Causes of brittle phases: During the alloy melting and solidification process, phosphorus (P) and copper (Cu) readily form a large number of hard and brittle intermetallic compounds, mainly... These brittle phases are dispersed throughout the as-cast structure, severely deteriorating the material's plasticity.

[0005] Problems in the rolling process: To obtain thin strip, continuous casting slabs need to be rolled with large deformation. Due to the extremely poor plasticity of the slab, stress easily concentrates in the brittle zone during conventional cold or hot rolling, leading to edge cracks, core cracks, and even strip breakage. Although plasticity can be restored by adding multiple intermediate annealing processes, this not only complicates the process and reduces efficiency, but also may cause surface oxidation due to repeated heating, affecting yield and product quality. Traditional smelting, extrusion / continuous casting methods suffer from difficulties in composition control and secondary processing and shaping.

[0006] It is worth noting that copper-phosphorus-silver brazing filler metals possess a characteristic that is extremely advantageous for the brazing process: they can begin to flow under capillary action (existing a "flow point") even before they are fully melted (i.e., below the liquidus temperature). This characteristic is crucial for achieving uniform filling of large-area brazing seams in liquid-cooled plates.

[0007] However, existing rolling processes have completely failed to utilize this characteristic. On the contrary, in order to avoid high-temperature oxidation and grain coarsening, traditional processes choose to deform at lower temperatures (cold rolling or low-temperature hot rolling), which precisely induces and exacerbates the cracking problem caused by brittle phases.

[0008] Meanwhile, in the field of metal processing, semi-solid processing technology has proven to be an effective means of handling difficult-to-deform materials and improving formability. This technology utilizes the characteristic that the deformation resistance of materials is significantly reduced in the semi-solid temperature range between the solid and liquid phase lines for forming. This technology has been successfully applied in fields such as aluminum alloys and magnesium alloys, enabling efficient and near-net-shape forming. However, research and technology on rolling (i.e., semi-solid casting-rolling or semi-solid rolling) in the unique semi-solid temperature range of solidified continuous casting slabs such as copper-phosphorus-silver brazing filler metals are currently lacking. Summary of the Invention

[0009] In order to solve the technical problems existing in the background art, the present invention proposes a method for preparing copper-phosphorus-silver brazing filler metal strips.

[0010] The present invention proposes a method for preparing copper-phosphorus-silver based solder strips, comprising the following steps: S1. Perform subsolid-phase isothermal spheroidization pretreatment on the continuous casting slab of copper-phosphorus-silver brazing filler metal. S2. Heat the pretreated slab to the rolling temperature T_R, wherein T_R satisfies: T_F ≤ T_R ≤ T_L -5°C, where T_F is the measured flow point temperature of the copper-phosphorus-silver brazing filler metal (the flow point temperature is the lowest temperature at which the brazing filler metal begins to spread under capillary action), and T_L is the liquidus temperature of the copper-phosphorus-silver brazing filler metal. S3. The heated slab is rolled in multiple passes in the constant temperature T_R range, with a total reduction rate of ≥85%, to obtain a thin strip; S4. Cool the rolled strip to room temperature at a cooling rate of 20~120°C / s.

[0011] Preferably, in step S1, the temperature of the subsolid-line isothermal spheroidization pretreatment is 0.75T_S~0.95T_S, where T_S is the solidus temperature of the copper-phosphorus-silver solder, and the holding time is 2~6 hours.

[0012] Preferably, in step S2, the flow point T_F and liquidus temperature T_L are obtained by cutting a sample from the pretreated slab and measuring it using differential scanning calorimetry (DSC) or wetting equilibrium method.

[0013] Preferably, in step S3, during the rolling process, a high-frequency induction with a frequency of 80kHz and a power of 30-50W is applied to the roll inlet side.

[0014] Preferably, in step S4, the cooling adopts zoned controlled cooling, specifically: the thin strip is passed sequentially through a first controlled cooling zone and a second controlled cooling zone; the first controlled cooling zone uses air mist cooling at a cooling rate of 30~60°C / s to cool the thin strip to 300~400°C; the second controlled cooling zone uses water spray cooling at a cooling rate of 60~120°C / s to cool the thin strip to room temperature; the total cooling time of the two controlled cooling zones is ≤5 seconds.

[0015] Preferably, the process further includes step S5: performing surface micro-etching activation treatment on the obtained thin strip, specifically by using 5-10% dilute sulfuric acid and 0.3-0.8%... The mixed solution was treated for 30-60 seconds.

[0016] Preferably, the chemical composition of the copper-phosphorus-silver brazing filler metal is as follows by weight percentage: Ag 2~20%, P 4~8%, with the balance being Cu and unavoidable impurities.

[0017] Preferably, the copper-phosphorus-silver brazing filler metal also contains at least one rare earth element selected from Ce and La, with an addition amount of 0.03~0.08 wt.%, and trace amounts of In.

[0018] Preferably, the final thickness of the thin strip is 0.05~0.3mm.

[0019] This invention first performs "subsolid-phase isothermal spheroidization pretreatment" on continuously cast slabs of copper-phosphorus-silver brazing filler metal. This step optimizes the as-cast microstructure and lays the foundation for subsequent deformation. Subsequently, the pretreated slab is placed in a narrow semi-solid temperature zone between its "flow point" and the liquidus line for isothermal rolling. Under this specific temperature range, a 5-15% volume fraction of micro-molten liquid phase forms at the grain boundaries. This micro-molten liquid phase plays multiple roles during rolling deformation, including lubricating grain boundaries (reducing frictional resistance between grains), coordinating slip (promoting relative sliding and rotation between grains, coordinating deformation), and buffering stress (absorbing and dispersing local stress concentrations, preventing their accumulation in brittle environments). Through the combined effect of these mechanisms, this method fundamentally changes the deformation mode of the material, transforming brittle grain boundary fracture, which easily leads to cracking, into grain slip coordinated by the liquid phase. This effectively avoids the initiation of defects such as edge cracks and core cracks, achieving high-yield forming of brittle brazing filler metal strips. Attached Figure Description

[0020] Figure 1 This is a flowchart of a method for preparing copper-phosphorus-silver solder strip. Detailed Implementation

[0021] Implementation 1: Basic Methods Reference Figure 1 This embodiment presents a method for preparing copper-phosphorus-silver solder strips, comprising the following steps: S1, Subsolid-phase isothermal spheroidization pretreatment The continuously cast slab of copper-phosphorus-silver brazing filler metal is heated to temperature T_pre under a protective atmosphere, satisfying 0.75T_S ≤ T_pre ≤ 0.95T_S, where T_S is the solidus temperature. This heating is carried out for 2–6 hours to allow the network structure in the as-cast copper-phosphorus-silver brazing filler metal to develop. The brittle phase spheroidizes and disperses along the grain boundaries, improving the initial plasticity of the slab (from <2% to 5~8%), thus providing a basis for a uniform distribution of liquid phase at grain boundaries for subsequent semi-solid warm rolling.

[0022] S2. Confirm rolling temperature and heat. First, samples were cut from the pretreated slab, and the flow point temperature T_F and liquidus temperature T_L of the batch of copper-phosphorus-silver solder were measured by differential scanning calorimetry (DSC) or wetting equilibrium method.

[0023] The pretreated slab is then rapidly heated to the rolling temperature T_R in a protective atmosphere, wherein T_R satisfies: T_F≤ T_R≤ T_L - 5°C, that is, the material is already in a semi-solid state when rolling begins.

[0024] S3, constant temperature multi-pass rolling Multi-pass rolling is performed under isothermal conditions, with a total reduction rate ≥85%, a reduction rate of 8~15% per pass, and a rolling speed of 0.1~0.5 m / s. This ensures that the entire slab remains in a stable semi-solid state throughout the rolling process, with the liquid phase continuously present at the grain boundaries. During the actual rolling process, a multi-zone independent temperature control system maintains the slab rolling temperature within the range of T_R ± 5°C, and no intermediate annealing is performed between passes.

[0025] As a further priority of this step, during the rolling process, a high-frequency induction with a frequency of 80kHz and a power of 30-50W is applied to the roll inlet side to promote the uniform distribution of the grain boundary micro-molten liquid phase.

[0026] S4, Rapid Cooling After rolling, the strip is rapidly cooled to room temperature using differentiated cooling rates of 20~120°C / s in controlled cooling zones to obtain a finished strip with a grain size ≤10μm. The specific steps are as follows: the strip is passed sequentially through a first controlled cooling zone and a second controlled cooling zone; the first controlled cooling zone uses air mist cooling at a rate of 30~60°C / s to cool the strip to 300~400°C; the second controlled cooling zone uses water spray cooling at a rate of 60~120°C / s to cool the strip to room temperature, resulting in a copper-phosphorus-silver brazing filler strip with a final thickness of 0.05~0.3mm. The total cooling time for both controlled cooling stages is ≤5 seconds.

[0027] S5. Perform surface micro-etching activation treatment on the obtained thin strip (this step is optional). This step is used to remove the surface oxide film, ensuring that the initial wetting angle of the solder during brazing on the liquid-cooled plate is ≤20°. The specific method is to use 5~10% dilute sulfuric acid and 0.3~0.8%... The mixed solution was treated for 30-60 seconds.

[0028] Example 2: Preparation of BAg15CuP solder strip This embodiment uses a typical BAg15CuP solder as an example. Its chemical composition, by weight percentage, is: Ag 14.5-15.5%, P 4.8-5.2%, with the balance being Cu and unavoidable impurities. The solidus temperature (T_S) of this alloy is approximately 645°C, and the liquidus temperature (T_L) is approximately 800°C.

[0029] Step S1: Subsolid-phase isothermal spheroidization pretreatment A BAg15CuP brazing filler metal slab (approximately 20 mm thick) was prepared using conventional smelting and continuous casting processes. Based on the solidus temperature T_S (645°C) of this alloy, its subsolidus temperature range was calculated to be 0.75T_S~0.95T_S, or approximately 484°C~613°C. The slab was placed in a protective atmosphere furnace and heated to 580°C, then held for 4 hours. This process aims to reduce the coarse, continuous network brittleness in the as-cast microstructure. The phase undergoes spheroidization and partial dissolution, which initially improves the plasticity of the alloy and lays the microstructure foundation for subsequent large deformation rolling.

[0030] Step S2: Determine the rolling temperature and heat. Small samples were cut from the pretreated slab and tested using differential scanning calorimetry (DSC) at a heating rate of 8°C / min. The measured flow point temperature T_F of the alloy was 660°C and the liquidus temperature T_L was 795°C.

[0031] The rolling temperature T_R is selected as T_R = 680°C. The pretreated slab is placed in a heating furnace with multi-zone independent temperature control and heated to 680°C under a protective atmosphere (such as nitrogen or argon), and held at this temperature to ensure uniform temperature. During the rolling process, the temperature control system ensures that the slab temperature remains stable within the range of 680 ± 5°C.

[0032] Step S3: Constant Temperature Multi-Pass Rolling The uniformly heated slab is conveyed to the rolling mill in a protective atmosphere channel. To promote the uniform distribution of trace liquid phase at the grain boundaries (generated due to the temperature being higher than the flow point T_F), a high-frequency induction with a frequency of 80kHz and a power of 300W is applied at the roll inlet side.

[0033] Multi-pass rolling was performed at a constant temperature of 680°C. Specific parameters were: initial slab thickness 10 mm, target strip thickness 0.1 mm, and total reduction rate 99%. A total of 15 passes were performed, with an average reduction rate of approximately 13% per pass, and the rolling speed was controlled at 0.3 m / s. The entire rolling process was continuous, with no intermediate annealing between passes.

[0034] Step S4: Rapid Cooling of Partitions The 0.1 mm thick strip obtained by rolling is immediately put into the cooling system for zoned controlled cooling in order to fix the fine grain structure formed by rolling and suppress the growth of brittle phases.

[0035] First controlled cooling zone: adopts air mist cooling method, with a cooling rate of about 50°C / s, to rapidly cool the strip from the rolling temperature of 680°C to 350°C.

[0036] The second cooling zone uses water spray cooling at a rate of approximately 100°C / s to rapidly cool the strip from 350°C to room temperature (approximately 25°C).

[0037] The total cooling time for the two stages is approximately 4 seconds, which is much shorter than the cooling time of traditional processes.

[0038] Step S5: Surface micro-etching activation treatment (optional) The resulting thin strip is punched into pre-formed sheets of the required size. To obtain better brazing wetting properties, the sheets are immersed in a solution of 8% dilute sulfuric acid and 0.5%... The solder is treated in a mixed solution for 45 seconds to completely remove the extremely thin oxide film that forms on the surface during processing and cooling. After treatment, it is rinsed with deionized water and dried. This treatment reduces the initial wetting angle of the solder on the copper substrate to below 18°.

[0039] Example 3: Preparation of Improved AgCuP Solder Strip Containing Rare Earth Elements This embodiment optimizes the basic composition of BAg15CuP by adding rare earth elements to further improve performance. The alloy composition by weight percentage is: Ag 15.0%, P 5.0%, Ce 0.05%, In 0.1%, with the balance being Cu.

[0040] Step S1: Subsolid-phase isothermal spheroidization pretreatment The continuously cast slab was pretreated at 600°C for 5 hours. The added Ce and In elements worked synergistically during the pretreatment process to effectively inhibit… Phase coarsening leads to a network structure in the as-cast microstructure. The thickness of the phase is reduced by about 40% compared to the alloy without rare earth addition.

[0041] Step S2: Determine the rolling temperature and heat. Differential scanning calorimetry (DSC) results showed that, due to the addition of rare earth elements and trace amounts of In, the flow point T_F of the alloy decreased to 650°C, and the liquidus T_L was 790°C. A rolling temperature of T_R = 670°C was selected for heating.

[0042] Step S3: Constant Temperature Multi-Pass Rolling Rolling was performed at a constant temperature of 670°C. High-frequency induction rolling (400W power) was used. The total reduction rate was 98%, rolling from 8mm to 0.15mm, with a reduction rate of 10% per pass and a rolling speed of 0.2 m / s. The addition of rare earth elements improved the alloy's plasticity at high temperatures and made the rolling process smoother.

[0043] Step S4: Rapid Cooling of Partitions The system uses aerosol cooling (40°C / s) to cool to 380°C, followed by water spray cooling (80°C / s) to room temperature, with a total cooling time of approximately 5 seconds.

[0044] Compared to the strip prepared in Example 2, the strip prepared in Example 3 exhibits a more than 25% increase in elongation after fracture in room temperature tensile tests, demonstrating significantly improved plasticity. Simultaneously, the lower flow point broadens the semi-solid rolling temperature window, making the process easier to control. Furthermore, the final strip microstructure shows improved... The phase particles are finer and more evenly distributed, resulting in effective refinement of the microstructure.

[0045] Comparative example: Traditional "rolling-annealing" process Traditional methods for preparing BAg15CuP solder sheets typically employ a multi-stage "box-type resistance furnace annealing + rolling" process. For example: first, a 3.0mm thick extruded billet is annealed at 710°C for 60 minutes, then rolled while hot to 1.5mm; next, it is annealed at 700°C for 60 minutes, rolled to 0.5mm; finally, it is annealed at 690°C in a hydrogen-protected furnace and water-quenched, then rolled to 0.05mm. This traditional process has the following drawbacks: 1. Intermittent annealing and rolling result in discontinuous production and low efficiency; 2. The annealing time is long and the surface is easily oxidized in a non-protected furnace, resulting in material loss and a decrease in surface quality; 3. The process essentially involves annealing at the solid recrystallization temperature to restore plasticity, and cannot utilize the semi-solid liquid phase to improve deformation uniformity.

[0046] This invention achieves efficient, high-quality, and safe continuous production through a one-time subsolid phase line pretreatment and continuous isothermal rolling in the semi-solid temperature zone. Furthermore, through precise temperature control and rapid cooling, it obtains superior microstructure and mechanical properties.

[0047] As described above, this invention first subjects the continuously cast slab of copper-phosphorus-silver brazing filler metal to "subsolid-phase isothermal spheroidization pretreatment," followed by isothermal rolling in a narrow semi-solid temperature range (typically only 10~30°C) between the flow point and the liquidus line, supplemented by high-frequency induction and ultrafast cooling. This successfully produces ultrathin copper-phosphorus-silver brazing filler metal strips with a thickness of 0.05-0.3 mm, uniform microstructure, and good plasticity. Adding rare earth elements can further optimize the performance. This method fundamentally avoids crack initiation.

[0048] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a copper-phosphorus-silver based solder strip, characterized in that, Includes the following steps: S1. Perform subsolid isothermal spheroidization pretreatment on the continuous casting slab of copper-phosphorus-silver brazing filler metal. S2. Heat the pretreated slab to the rolling temperature T_R, wherein T_R satisfies: T_F ≤ T_R ≤ T_L - 5°C, where T_F is the measured flow point temperature of the copper-phosphorus-silver brazing filler metal, and T_L is the liquidus temperature of the copper-phosphorus-silver brazing filler metal. S3. The heated slab is rolled in multiple passes in the constant temperature T_R range, with a total reduction rate of ≥85%, to obtain a thin strip; S4. Cool the rolled strip to room temperature at a cooling rate of 20~120°C / s.

2. The method for preparing copper-phosphorus-silver solder strip according to claim 1, characterized in that, In step S1, the temperature of the subsolid-phase isothermal spheroidization pretreatment is 0.75T_S~0.95T_S, where T_S is the solidus temperature of the copper-phosphorus-silver solder, and the holding time is 2~6 hours.

3. The method for preparing copper-phosphorus-silver solder strip according to claim 1, characterized in that, In step S2, the flow point T_F and liquidus temperature T_L are obtained by cutting samples from the pretreated slab and measuring them using differential scanning calorimetry or wetting equilibrium method.

4. The method for preparing copper-phosphorus-silver solder strip according to claim 1, characterized in that, In step S3, during the rolling process, a high-frequency induction with a frequency of 80kHz and a power of 30-50W is applied to the roll inlet side.

5. The method for preparing copper-phosphorus-silver solder strip according to claim 1, characterized in that, In step S4, the cooling adopts zoned controlled cooling, specifically: the thin strip is passed through the first controlled cooling zone and the second controlled cooling zone in sequence; the first controlled cooling zone adopts air mist cooling at a cooling rate of 30~60°C / s to cool the thin strip to 300~400°C; the second controlled cooling zone adopts water spray cooling at a cooling rate of 60~120°C / s to cool the thin strip to room temperature; the total cooling time of the two controlled cooling zones is ≤5 seconds.

6. The method for preparing copper-phosphorus-silver solder strip according to claim 1, characterized in that, It also includes step S5: performing surface micro-etching activation treatment on the obtained thin strip, specifically: treating it with a mixed solution of 5~10% dilute sulfuric acid and 0.3~0.8% H2O2 for 30~60 seconds.

7. The method for preparing copper-phosphorus-silver solder strip according to claim 1, characterized in that, The chemical composition of the copper-phosphorus-silver brazing filler metal, by weight percentage, is: Ag 2~20%, P 4~8%, with the balance being Cu and unavoidable impurities.

8. The method for preparing copper-phosphorus-silver solder strip according to claim 7, characterized in that, The copper-phosphorus-silver brazing filler metal also contains at least one rare earth element selected from Ce and La, with an addition amount of 0.03~0.08 wt.%, and trace amounts of In.

9. The method for preparing copper-phosphorus-silver solder strips according to any one of claims 1-8, characterized in that, The final thickness of the thin strip is 0.05~0.3mm.

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