Tin-based bismuth-rich flux-cored wire, welding spot and preparation method and application of tin-based bismuth-rich flux-cored wire

Abstract

The invention belongs to the technical field of flux-cored wires, and particularly relates to a tin-based bismuth-rich flux-cored wire, a welding spot and a preparation method and application of the tin-based bismuth-rich flux-cored wire. According to the tin-based bismuth-rich flux-cored wire, the Sn-based alloy with the low Bi content is adopted for preparing the welding wire skin, the flux core is formed by the scaling powder and the Sn-Bi system alloy, distribution of Sn and Bi elements in the tin-based bismuth-rich flux-cored wire is optimized through the combination, the wire breaking phenomenon is effectively prevented, and therefore preparation of high-quality welding spots is facilitated. In the welding spot preparation process, uniform distribution of welding flux is achieved through the low-Bi welding wire skin and Sn-Bi series alloy in the flux core under the combined action of ultrasonic cavitation and mechanical oscillation. The distribution promotes the fast reaction of the Sn-Bi series alloy in the flux core and the low-Bi welding wire skin, the phenomenon of uneven stress is effectively restrained, and therefore the problems of welding spot segregation, the large melting range, insufficient welding and the like are avoided. In addition, the preparation method of the tin-based bismuth-rich flux-cored wire is simple and easy to implement, and automatic production is easy to realize, so that the application range of the tin-based bismuth-rich flux-cored wire is expanded.

Description

Technical Field

[0001] The present invention belongs to the technical field of flux-cored welding wires, and more specifically, relates to a tin-based bismuth-rich flux-cored welding wire, a welding spot, and a preparation method and application thereof. Background Art

[0002] Tin-lead solder (SnPb solder) is a traditional soft solder in the electronic packaging industry. It is made of Sn and Pb mixed in a certain proportion. It is widely used due to its good weldability, fluidity, electrical conductivity, thermal conductivity and excellent mechanical properties. However, Pb, as a toxic element, has potential hazards to human health and the environment. Therefore, researchers are looking for new solders that can replace it.

[0003] SnBi alloy is one of the alternatives that has attracted much attention. It has the advantages of low price, low melting point and good wettability. By adding other alloying elements, such as SnBi alloy, its performance can be further improved to meet the needs of different application scenarios. However, SnBi alloy also has some problems. The Bi element is brittle and brittle, and has poor plasticity. Especially when the Bi content is too high, its plasticity will drop sharply and the thermal expansion coefficient will also increase, which leads to the phenomenon of wire breakage when the welding wire is prepared by conventional extrusion and drawing. In order to solve this problem, researchers have adopted a variety of methods, such as introducing other metal elements, increasing the cooling rate during casting, semi-solid forming technology, etc., to refine the primary structure or eutectic structure of Bi and improve the plasticity of the cast alloy. For example, Chinese patent application CN102528335A successfully prepared an anti-oxidation solder wire of a difficult-to-draw SnBi brittle solder material using a large deformation extrusion-near-final forming-online anti-oxidation treatment process. However, when using this welding wire, the segregation problem of the Bi-rich phase will inevitably occur, resulting in cold welding in the local area of ​​the solder joint. In addition, the production line equipment needs to be repurchased, and the process parameters need to be adjusted, otherwise problems such as wire breakage and solder joint cracks are prone to occur. Chinese patent application CN107552995A discloses a tin-bismuth cored welding wire, which does not segregate and has excellent welding performance. When casting and extruding the rod, the patent uses semi-solid ultrasonic stirring for fine grain treatment, so that its plasticity and strength are greatly improved compared to solid tin-bismuth welding wire. However, this method also has some problems in actual production, such as the ultrasonic working head needs to be immersed in the high-temperature alloy molten pool for a long time to work continuously and will be severely corroded, and the ultrasonic action range is limited, resulting in non-uniform plasticity of the welding wire, and there is still a probability of wire breakage. Summary of the invention

[0004] The technical problem to be solved by the present invention is to overcome the defects and shortcomings of SnBi series welding wires in the prior art, such as wire breakage and segregation, cold welding and cracking of the prepared solder joints, and to provide a tin-based bismuth-rich flux-cored welding wire.

[0005] Another object of the present invention is to provide a method for preparing the above-mentioned tin-based bismuth-rich flux-cored welding wire.

[0006] Another object of the present invention is to provide a solder joint.

[0007] Another object of the present invention is to provide the application of the above-mentioned tin-based bismuth-rich flux-cored welding wire or the above-mentioned welding spot in the field of electronic packaging.

[0008] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0009] The present invention protects a tin-based bismuth-rich flux-cored welding wire, which comprises a flux core and a welding wire skin, wherein the flux core is filled inside the welding wire skin; wherein the flux core comprises a rosin-based flux and a Sn-Bi alloy, wherein the content of Bi in the Sn-Bi alloy is ≥10%; the welding wire skin is a Sn-based alloy containing a small amount of Bi or no Bi; and the content of Bi in the Sn-based alloy containing a small amount of Bi is ≤5%.

[0010] The tin-based bismuth-rich flux-cored welding wire of the present invention uses a Sn-based alloy containing a small amount of Bi or no Bi as the welding wire skin, and a bismuth-rich alloy and a rosin-based flux as the flux core. Such a combination makes the distribution of Sn and Bi in the tin-based bismuth-rich flux-cored welding wire more uniform, effectively avoiding the occurrence of wire breakage, and further improving the overall quality of the welding wire. In addition, the diameter of the welding wire can reach 2 mm or less, ensuring the fineness and stability of the process. In addition, the welding wire is lead-free, showing a high degree of friendliness to human health and the natural environment, and conforming to the green and environmentally friendly concept of sustainable development.

[0011] Furthermore, the content of Sn in the tin-based bismuth-rich flux-cored welding wire is ≥50%.

[0012] Furthermore, the rosin-based soldering flux includes hydrogenated rosin, tetrahydrofurfuryl alcohol, ethylene glycol, diethylene glycol, succinic acid, citric acid, adipic acid, hydrogenated castor oil, benzotriazole, triethanolamine, ethylphenyl polyethylene glycol, and nonylphenol polyoxyethylene ether.

[0013] Furthermore, the mass ratio of the hydrogenated rosin, tetrahydrofurfuryl alcohol, ethylene glycol, diethylene glycol, succinic acid, citric acid, adipic acid, hydrogenated castor oil, benzotriazole, triethanolamine, ethylphenyl polyethylene glycol, and nonylphenol polyoxyethylene ether is (30-45): (13.5-25): (6.5-19): (15-24.5): (0.5-2): (0.5-1): (1-3.5): (0.5-2): (1.5-2): (0.5-2.5): (0.5-1.5): (0.5-1.5).

[0014] Furthermore, based on the weight of the tin-based bismuth-rich flux-cored welding wire, the weight of the rosin-based flux is 2% to 10%.

[0015] The present invention provides a method for preparing a tin-based bismuth-rich flux-cored welding wire, comprising the following steps:

[0016] S1. Melting the alloy according to the alloy ratio to prepare a billet;

[0017] S2. The rosin flux is melted and then evenly added to the Sn-Bi alloy to prepare a flux core;

[0018] S3. Extrude the rod blank obtained in step S1 to form a welding wire skin, and inject the flux core obtained in step S2 into the welding wire skin while extruding to fill it to form a flux cored coarse tin wire, and draw and shape the obtained flux cored coarse tin wire to obtain a tin-based bismuth-rich flux cored welding wire.

[0019] The present invention uses a tin-based alloy containing a small amount of Bi or no Bi, and uses this alloy as the outer skin of the welding wire for extrusion and drawing. First, the core material is filled with a rosin pen during extrusion, and the core material includes a molten rosin additive and a Bi-rich Sn-Bi alloy. Then, the diameter is reduced in a multi-pass drawing process to obtain a tin-based bismuth-rich flux-cored welding wire of the required diameter. Since the tin-based alloy containing a small amount of Bi or no Bi is extruded in a heated state, its plasticity is significantly increased compared to the tin-based alloy with high Bi, so the preparation process is easy to implement. The preparation method of the welding wire is simple and easy, and can achieve low-cost and high-efficiency automated production on traditional equipment without damaging the equipment, thereby expanding the application range of tin-based bismuth-rich flux-cored welding wire in more fields.

[0020] Furthermore, in step S1, the smelting also includes heat preservation, stirring, standing, slag removal and pouring.

[0021] Furthermore, the heat preservation is heat preservation in the smelting furnace for 10 to 30 minutes.

[0022] Furthermore, the stirring time is 5 to 10 minutes.

[0023] Furthermore, the standing time is 5 to 10 minutes.

[0024] Furthermore, the slag removal is to screen and filter the product after standing to remove impurities such as metal debris, dust, etc. on the surface and inside of the welding wire.

[0025] Furthermore, the pouring is pouring the product after slag removal into a forming mold.

[0026] Specifically, in step S1, the smelting includes keeping the product in a smelting furnace for 10 to 30 minutes, stirring it for 5 to 10 minutes, and then letting the stirred product stand for 5 to 10 minutes, then screening and filtering the standing product to remove metal debris, dust and other impurities on the surface and inside of the welding wire, and pouring the product after slag removal into a molding mold.

[0027] Further, as a preferred mode, in step S2, the Sn-Bi alloy is Sn-Bi alloy powder. The powdered Sn-Bi alloy is more conducive to its rapid reaction with the low-Bi welding wire skin during welding, thereby achieving uniform distribution of the solder.

[0028] Furthermore, in step S2, the Sn-Bi alloy includes one or more of Sn-90Bi, Sn-60Bi-0.5Cu, and Sn-40Bi. Taking Sn-60Bi-0.5Cu as an example, the value 60 indicates that the mass proportion of Bi in the Sn-60Bi-0.5Cu alloy is 60%, and the value 0.5 indicates that the mass proportion of Cu in the Sn-60Bi-0.5Cu alloy is 0.5%, and the meanings of the numbers in other alloys are similar.

[0029] Furthermore, in step S3, the specific preparation method of the welding wire skin includes the following steps:

[0030] According to the alloy ratio, weigh the raw materials and melt the alloy, inject it into the molding die to get the rod billet, inject it into the extrusion die, extrude it, and form the welding wire skin.

[0031] Furthermore, the alloy includes a Sn-based alloy containing a small amount of Bi or no Bi.

[0032] Furthermore, the Sn-based alloy containing a small amount of Bi includes one or more of Sn-5Bi, Sn-5Bi-X, Sn-1Bi, Sn-1Bi-X, Sn-2Bi, Sn-2Bi-X, Sn-3Bi, Sn-3Bi-X, Sn-4Bi, Sn-4Bi-X, Sn-0.5Bi, Sn-0.5Bi-X, wherein X represents other metal elements. Taking Sn-5Bi as an example, the value 5 indicates that the mass proportion of Bi in the Sn-5Bi alloy is 5%, and the meanings of the numbers in other alloys are similar.

[0033] Furthermore, the Sn-based alloy not containing Bi includes one or more of Sn, Sn-5Cu, Sn-10Cu, Sn-10Sb, and Sn-40In. Taking Sn-10Cu as an example, the value 10 indicates that the mass proportion of Cu in the Sn-10Cu alloy is 10%, and the meanings of the numbers in other alloys are similar.

[0034] Furthermore, the conditions of the extrusion die are: extrusion force 150-200 kg / cm 2 , wire delivery speed 1.5~3m / min.

[0035] Furthermore, in step S3, the diameter of the thick tin wire is 5 to 12 mm.

[0036] Further, in step S3, the drawing includes large drawing, medium drawing and small drawing.

[0037] Furthermore, the diameter of the thick tin wire after drawing is 2.5-6 mm.

[0038] Furthermore, the diameter of the thick tin wire after medium drawing is 1.25-2.5 mm.

[0039] Furthermore, the diameter of the thick tin wire after small drawing is 0.2-2 mm.

[0040] The present invention protects a welding spot, which is mainly formed by ultrasonic brazing using the above-mentioned tin-based bismuth-rich flux-cored welding wire as welding material.

[0041] The solder joint of the present invention exhibits outstanding performance advantages. During the welding process, ultrasonic oscillation is applied to the Bi-rich Sn-Bi alloy and the low Bi-phase welding wire skin of the welding wire core. The ultrasonic oscillation step introduced in the welding process makes full use of the cavitation effect of ultrasound. When the metal vapor bubble collapses, it releases strong pressure and high temperature, so that the local welding material melts rapidly at a lower temperature. This method not only effectively reduces the welding temperature, reduces energy consumption and thermal effects on the material, but also achieves uniform distribution of solder components through the stirring effect of ultrasound, which promotes the rapid response of Bi-rich Sn-Bi alloy and low Bi-phase welding wire skin. Due to the uniform distribution of solder components, the uneven stress distribution caused by the large expansion coefficient of Bi-rich alloy is effectively suppressed, thereby avoiding the problems of solder joint segregation, cold welding and excessive melting range. The above advantages work together to make the solder joint have higher strength and reliability, can withstand greater mechanical stress and thermal stress, and significantly improve the overall performance of the solder joint.

[0042] Furthermore, the structure of the welding point includes a stacked structure arranged in sequence: a first shell layer, a connecting layer and a second shell layer, the main component of the connecting layer is the above-mentioned tin-based bismuth-rich flux-cored welding wire, and the first shell layer and the second shell layer are both base materials.

[0043] Furthermore, the parent material refers to the metal workpiece to be welded, which can be a metal material of various shapes.

[0044] Furthermore, the parent material includes one or more of a copper substrate, an aluminum alloy plate, and a stainless steel plate.

[0045] In the welding process, the present invention applies ultrasonic vibration to the Bi-rich Sn-Bi alloy and the low-Bi welding wire skin in the welding wire.

[0046] Furthermore, the vibration frequency of the ultrasonic welding is 18-20 KHz.

[0047] Furthermore, the vibration power of the ultrasonic welding is 200-500W.

[0048] Furthermore, the applied pressure of the ultrasonic welding is 0.2-0.5 MPa.

[0049] Furthermore, the welding temperature of the ultrasonic brazing is 150-280°C.

[0050] Furthermore, the ultrasonic welding time is 10 to 30 seconds.

[0051] The present invention protects the application of the above-mentioned tin-based bismuth-rich flux-cored welding wire or the above-mentioned welding spot in the field of electronic packaging.

[0052] Compared with the prior art, the present invention has the following beneficial effects:

[0053] The present invention adopts a Sn-based alloy with a low Bi content to prepare the welding wire skin, and uses a flux and a Sn-Bi alloy to form a flux core. This combination optimizes the distribution of Sn and Bi elements in the tin-based bismuth-rich flux-cored welding wire, effectively prevents wire breakage, and is conducive to the preparation of high-quality solder joints. In the process of preparing solder joints, the low-Bi welding wire skin and the Sn-Bi alloy in the flux core achieve uniform distribution of solder under the combined action of ultrasonic cavitation and mechanical oscillation. This distribution promotes the rapid reaction of the Sn-Bi alloy in the flux core and the low-Bi welding wire skin, effectively suppresses the stress unevenness phenomenon, thereby avoiding the problems of solder joint segregation, large melting range and cold solder joints. In addition, the preparation method of the welding wire is simple and easy, and it is easy to realize automated production, thereby expanding the application range of the tin-based bismuth-rich flux-cored welding wire. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 The present invention is a flow chart for preparing tin-based bismuth-rich flux-cored welding wire and welding spots in Examples 1 to 4.

[0055] Figure 2 This is a flow chart for preparing tin-based flux-cored welding wire in Comparative Examples 1 to 4.

[0056] Figure 3Schematic diagram of the method for preparing the extruded thick welding wire in Examples 1 to 4 and Comparative Examples 1 to 8. (In the schematic diagram, 1 represents a round rod ingot; 2 represents an extrusion die; 3 represents a rosin pressurizing cylinder; 4 represents a rosin melting cylinder; 5 represents melted rosin and high-bismuth tin powder; 6 represents a rosin pen; 7 represents a wire inner diameter forming die; 8 represents a wire outer diameter forming die; 9 represents the first cavity of the extrusion die; 10 represents the second cavity of the extrusion die; 11 represents a flux-cored welding wire.)

[0057] Figure 4 Schematic diagram of preparing solder joints by welding tin-based bismuth-rich flux-cored welding wire in Examples 1 to 4. (In the schematic diagram, 1 represents an ultrasonic generator; 2 represents an ultrasonic welding head; 3 represents a welding pad; 4 represents a flux-cored welding wire; 41 represents a bismuth-free or low-bismuth welding wire skin; 42 represents Sn-Bi alloy powder; 43 represents a rosin-based flux; and 5 represents a heating base.)

[0058] Figure 5 This is the SEM image of the solder joint in Example 1.

[0059] Figure 6 This is the SEM image of the solder joint in Example 2.

[0060] Figure 7 This is the SEM image of the solder joint in Example 3.

[0061] Figure 8 This is the SEM image of the solder joint in Example 4.

[0062] Fig. 9 This is the SEM image of the solder joint in Comparative Example 5.

[0063] Fig.10 This is the SEM image of the solder joint in Comparative Example 6.

[0064] Fig.11 This is the SEM image of the solder joint in Comparative Example 7.

[0065] Fig.12 This is the SEM image of the solder joint in Comparative Example 8.

[0066] Fig.13 It is the Sn-Bi binary phase diagram. DETAILED DESCRIPTION

[0067] The present invention is further described below in conjunction with the accompanying drawings and specific examples, but the examples do not limit the present invention in any form. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0068] Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0069] Example 1 Preparation of a tin-based bismuth-rich flux-cored welding wire and welding spot

[0070] Sn-5Bi is used as the welding wire skin component, Sn-90Bi alloy and rosin flux are used as the core material, and the target solder joint composition is Cu / Sn-52Bi / Cu. The preparation flow chart of the core, cored welding wire and solder joint is shown in Figure 1 As shown, it specifically includes the following steps:

[0071] (1) Preparation of rod blanks: Since metal has a certain burnout rate, according to the Sn-5Bi alloy ratio, 27.2 kg of pure Sn and 2.08 kg of Sn-70Bi alloy were mixed and added into a smelting furnace at 500°C. The mixture was kept warm for 20 min and then stirred for 5 min. The stirred product was allowed to stand for 5 min and then deslagging was performed. The deslagging product was poured into a mold to prepare a Sn-5Bi alloy rod blank. The rod blank had a specification of φ100 mm, a length of 500 mm, and a weight of 29.07 kg.

[0072] (2) Preparation of flux core: 3.42 kg of rosin flux is heated to 100°C in the melting cylinder of the rosin processing mechanism to make the rosin flux molten, and 35.95 kg of Sn-90Bi alloy powder is added and stirred evenly to obtain flux doped with Sn-90Bi alloy powder, i.e., flux core. The ingredients of the rosin flux include: by weight, 30 parts of hydrogenated rosin, 25 parts of tetrahydrofurfuryl alcohol, 12 parts of ethylene glycol, 15 parts of diethylene glycol, 2 parts of succinic acid, 1 part of citric acid, 2.5 parts of adipic acid, 2 parts of hydrogenated castor oil, 1.5 parts of benzotriazole, 1 part of triethanolamine, 1 part of ethylphenyl polyethylene glycol, and 1 part of nonylphenol polyoxyethylene ether. The rosin flux accounts for 5% of the total weight of the welding wire.

[0073] (3) Preparation of tin-based bismuth-rich flux-cored welding wire: The Sn-5Bi alloy rod blank obtained in step (1) is loaded into Figure 3 The extrusion die shown in the figure is used to gradually form the wire skin by extrusion. The Sn-5Bi alloy bar billet temperature is 60°C and the extrusion pressure is 160kg / cm 2 , the wire drawing speed is 1.8m / min, and the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a thick tin wire, the thick tin wire is φ12mm. The obtained thick tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire is gradually reduced in diameter: φ12mm→φ6mm→φ4mm→φ2mm, and a tin-based bismuth-rich flux cored welding wire is obtained.

[0074] (4) Preparation of solder joints: The tin-based bismuth-rich flux-cored welding wire obtained in step 3 is welded by ultrasonic brazing at a welding temperature of 208°C, and the formed Cu / Sn-52Bi / Cu region is the solder joint. The structure of the solder joint includes a stacked structure arranged in sequence: a first shell layer, a connecting layer, and a second shell layer, wherein the main component of the connecting layer is the tin-based bismuth-rich flux-cored welding wire obtained in step (3), and the first shell layer and the second shell layer are both copper substrates. The ultrasonic vibration frequency is 20KHz, the vibration power is 300W, the applied pressure is 0.5MPa, and the vibration time is 20s.

[0075] Example 2 Preparation of a tin-based bismuth-rich flux-cored welding wire and welding spot

[0076] Sn-5Bi-0.5Cu was used as the welding wire skin component, Sn-60Bi-0.5Cu alloy and rosin-based flux were used as core materials, and the composition of the target solder joint was Cu / Sn-30Bi-0.5Cu / Cu.

[0077] (1) Preparation of rod blanks: Since metals have a certain burnout rate, according to the Sn-5Bi-0.5Cu alloy ratio, 24 kg of pure Sn, 3 kg of Sn-5Cu and 3 kg of Sn-50Bi alloy were mixed and added into a smelting furnace at 500°C. The mixture was kept warm for 30 min and then stirred for 10 min. The stirred product was allowed to stand for 10 min and then deslagging was performed. The deslagging product was poured into a mold to prepare a Sn-5Bi-0.5Cu alloy rod blank. The rod blank had a specification of φ80 mm, a length of 500 mm and a weight of 29.12 kg.

[0078] (2) Preparation of flux core: 5.93 kg of rosin flux is heated to 100° C. in a melting tank of a rosin processing mechanism to melt the rosin flux, and 24.26 kg of Sn-60Bi-0.5Cu alloy powder is added and stirred evenly to obtain flux doped with Sn-60Bi-0.5Cu alloy powder, i.e., flux core. The ingredients of the rosin flux include: by weight, 45 parts of hydrogenated rosin, 20 parts of tetrahydrofurfuryl alcohol, 12 parts of ethylene glycol, 13 parts of diethylene glycol, 1 part of succinic acid, 1 part of citric acid, 2 parts of adipic acid, 1 part of hydrogenated castor oil, 1.5 parts of benzotriazole, 2.5 parts of triethanolamine, 1 part of ethylphenyl polyethylene glycol, and 1 part of nonylphenol polyoxyethylene ether. The rosin flux accounts for 10% of the total weight of the welding wire.

[0079] (3) Preparation of tin-based bismuth-rich flux-cored welding wire: The Sn-5Bi-0.5Cu alloy rod obtained in step (1) is charged into Figure 3 The Sn-5Bi-0.5Cu alloy rod blank is heated to 75°C and the extrusion pressure is 150kg / cm2 , the wire drawing speed is 1.5m / min, and the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a thick tin wire, the thick tin wire is φ10mm. The thick tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire is gradually reduced in diameter: φ10mm→φ5mm→φ2.5mm→φ1mm, and a tin-based bismuth-rich flux cored welding wire is obtained.

[0080] (4) Preparation of solder joints: The tin-based bismuth-rich flux-cored welding wire obtained in step (3) is welded by ultrasonic brazing at a welding temperature of 200°C, and the formed Cu / Sn-30Bi-0.5Cu / Cu region is the solder joint. The structure of the solder joint includes a stacked structure arranged in sequence: a first shell layer, a connecting layer, and a second shell layer, wherein the main component of the connecting layer is the tin-based bismuth-rich flux-cored welding wire obtained in step (3), and the first shell layer and the second shell layer are both copper substrates. The ultrasonic vibration frequency is 20KHz, the vibration power is 200W, the applied pressure is 0.2MPa, and the vibration time is 10s.

[0081] Example 3 Preparation of a tin-based bismuth-rich flux-cored welding wire and welding spot

[0082] Sn-5Sb-Cu is used as the solder wire skin component, Sn-40Bi and rosin-based flux are used as the core material. The composition of the target solder joint is Cu / Sn-10Bi-5Sb-Cu / Cu.

[0083] (1) Preparation of rod blanks: Since metal has a certain burnout rate, 12 kg of pure Sn, 3 kg of Sn-10Cu, and 15 kg of Sn-10Sb alloy were mixed according to the ratio of Sn-5Sb-Cu alloy, added into a smelting furnace at 1000°C, kept warm for 25 min, and then stirred for 8 min. The stirred product was allowed to stand for 8 min, and then deslagging was performed. The deslagging product was poured into a mold to prepare a Sn-5Sb-Cu alloy rod blank. The rod blank had a specification of φ100 mm, a length of 500 mm, and a weight of 28.43 kg.

[0084] (2) Preparation of flux core: 3.3 kg of rosin flux is heated to 100°C in the melting tank of the rosin processing mechanism to make the rosin flux molten, and 9.47 kg of Sn-40Bi alloy powder is added and stirred evenly to obtain flux doped with Sn-40Bi alloy, i.e., flux core. The ingredients of the rosin flux include: by weight, 35 parts of hydrogenated rosin, 22 parts of tetrahydrofurfuryl alcohol, 13 parts of ethylene glycol, 13 parts of diethylene glycol, 1.5 parts of succinic acid, 1 part of citric acid, 2 parts of adipic acid, 1.5 parts of hydrogenated castor oil, 1.8 parts of benzotriazole, 1.5 parts of triethanolamine, 1.5 parts of ethylphenyl polyethylene glycol, and 1.5 parts of nonylphenol polyoxyethylene ether. The rosin flux accounts for 8% of the total weight of the welding wire.

[0085] (3) Preparation of tin-based bismuth-rich flux-cored welding wire: The Sn-5Sb-Cu alloy rod blank obtained in step (1) is loaded into Figure 3 The Sn-5Sb-Cu alloy rod blank is heated to 80°C and the extrusion force is 200kg / cm 2 , the wire drawing speed is 1.6m / min, and the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a thick tin wire, the thick tin wire is φ5mm. The thick tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire is gradually reduced in diameter: φ5mm→φ2.5mm→φ1.25mm→φ0.2mm, and a tin-based bismuth-rich flux cored welding wire is obtained.

[0086] (4) Preparation of solder joints: The tin-based bismuth-rich flux-cored welding wire obtained in step (3) is welded by ultrasonic brazing at a welding temperature of 280°C, and the formed Cu / Sn-10Bi-5Sb-Cu / Cu region is the solder joint. The structure of the solder joint includes a stacked structure arranged in sequence: a first shell layer, a connecting layer, and a second shell layer, wherein the main component of the connecting layer is the tin-based bismuth-rich flux-cored welding wire obtained in step (3), and the first shell layer and the second shell layer are both copper substrates. The ultrasonic vibration frequency is 18KHz, the vibration power is 500W, the applied pressure is 0.4MPa, and the vibration time is 30s.

[0087] Example 4 Preparation of a tin-based bismuth-rich flux-cored welding wire and welding spot

[0088] Sn-24In-2Bi is used as the welding wire skin component, Sn-40Bi alloy powder and rosin-based flux are used as the core material. The target solder joint composition is Cu / Sn-30Bi-15In / Cu.

[0089] (1) Preparation of rod blanks: Since metals have a certain burnout rate, 6.2 kg of pure Sn, 17.5 kg of Sn-40In, and 5.8 kg of Sn-10Bi alloy were mixed according to the ratio of Sn-24In-2Bi alloy, added into a smelting furnace at 650°C, kept warm for 20 min, and then stirred for 6 min. The stirred product was allowed to stand for 6 min, and then deslagging was performed. The deslagging product was poured into a mold to prepare Sn-24In-2Bi alloy rod blanks. The rod blanks had a specification of φ100 mm, a length of 500 mm, and a weight of 28.77 kg.

[0090] (2) Preparation of flux core: 4.44 kg of rosin flux is heated to 100°C in the melting cylinder of the rosin processing mechanism to make the rosin flux molten, and 16.11 kg of Sn-40Bi alloy powder is added and stirred evenly to obtain flux doped with Sn-40Bi alloy powder, i.e., flux core. The ingredients of the rosin flux include: by weight, 35 parts of hydrogenated rosin, 25 parts of tetrahydrofurfuryl alcohol, 14 parts of ethylene glycol, 14 parts of diethylene glycol, 2 parts of succinic acid, 1 part of citric acid, 1.5 parts of adipic acid, 1 part of hydrogenated castor oil, 1.5 parts of benzotriazole, 2 parts of triethanolamine, 1.5 parts of ethylphenyl polyethylene glycol, and 1.5 parts of nonylphenol polyoxyethylene ether. The rosin flux accounts for 9% of the total weight of the welding wire.

[0091] (3) Preparation of tin-based bismuth-rich flux-cored welding wire: The Sn-24In-2Bi alloy rod obtained in step (1) is charged into Figure 3 The Sn-24In-2Bi alloy rod blank is heated to 60°C and the extrusion force is 175 kg / cm 2 , the wire drawing speed is 3m / min, and the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a thick tin wire, the thick tin wire is φ5mm. The thick tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire is gradually reduced in diameter: φ5mm→φ2.5mm→φ1.25mm→φ0.2mm, and a tin-based bismuth-rich flux cored welding wire is obtained.

[0092] (4) Preparation of solder joints: The tin-based bismuth-rich flux-cored welding wire obtained in step (3) is welded by ultrasonic brazing at a welding temperature of 150°C, and the formed Cu / Sn-30Bi-15In / Cu region is the solder joint. The solder joint comprises a stacked structure arranged in sequence: a first shell layer, a connecting layer and a second shell layer, wherein the main component of the connecting layer is the tin-based bismuth-rich flux-cored welding wire obtained in step (3), and the first shell layer and the second shell layer are both copper substrates. The ultrasonic vibration frequency is 20KHz, the vibration power is 300W, the applied pressure is 0.4MPa, and the vibration time is 30s.

[0093] Comparative Example 1 Preparation of a Tin-based Flux-cored Welding Wire

[0094] The difference from Example 1 is that in step (2), no Sn-90Bi alloy powder is added to the flux core. Figure 2 As shown, it specifically includes the following steps:

[0095] The solder alloy Sn-52Bi in the target solder joint Cu / Sn-52Bi / Cu is used as the solder wire skin, and the rosin flux is used as the flux core.

[0096] (1) Preparation of rod blanks: Since the metal has a certain burnout rate, according to the Sn-52Bi alloy ratio, 8.7 kg of pure Sn and 25.3 kg of Sn-70Bi alloy were mixed and added into a smelting furnace at 500°C. The mixture was kept warm for 20 min and then stirred for 5 min. The stirred product was allowed to stand for 5 min and then deslagging was performed. The deslagging product was poured into a mold to prepare a Sn-52Bi alloy rod blank. The rod blank had a specification of φ100 mm, a length of 500 mm, and a weight of 33.6 kg.

[0097] (2) Preparation of flux core: Heat 1.77 kg of rosin flux to 100°C in the melting cylinder of the rosin processing mechanism to melt the rosin flux, and stir evenly to obtain the flux core. The ingredients of the rosin flux include: by weight, 30 parts of hydrogenated rosin, 25 parts of tetrahydrofurfuryl alcohol, 12 parts of ethylene glycol, 15 parts of diethylene glycol, 2 parts of succinic acid, 1 part of citric acid, 2.5 parts of adipic acid, 2 parts of hydrogenated castor oil, 1.5 parts of benzotriazole, 1 part of triethanolamine, 1 part of ethylphenyl polyethylene glycol, and 1 part of nonylphenol polyoxyethylene ether. The rosin flux accounts for 5% of the total weight of the welding wire.

[0098] (3) Preparation of tin-based flux-cored welding wire: The Sn-52Bi alloy rod obtained in step (1) is loaded into Figure 3 The extrusion die shown in the figure is used to gradually form the wire skin by extrusion. The Sn-52Bi alloy bar billet temperature is 60°C and the extrusion pressure is 160kg / cm 2 , the wire drawing speed is 1.8m / min, and the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a thick tin wire, the thick tin wire is φ12mm. The thick tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire is gradually reduced in diameter: φ12mm→φ6mm→φ4mm→φ2mm, and a tin-based flux-cored welding wire is obtained.

[0099] Comparative Example 2 Preparation of a Tin-based Flux-cored Welding Wire

[0100] The difference from Example 2 is that in step (2), no Sn-60Bi-0.5Cu alloy powder is added to the core.

[0101] The solder Sn-30Bi-0.5Cu alloy in the target solder joint Cu / Sn-30Bi-0.5Cu / Cu is used as the wire skin, and the rosin flux is used as the flux core.

[0102] (1) Preparation of rod blanks: Since metals have a certain burnout rate, Sn-30Bi-0.5Cu alloy was used as the alloy. 11.2 kg of pure Sn, 1.65 kg of Sn-10Cu and 19.5 kg of Sn-50Bi alloy were mixed and added into a smelting furnace at 900°C. The mixture was kept warm for 30 min and then stirred for 10 min. The stirred product was allowed to stand for 10 min and then deslagging was performed. The deslagging product was poured into a mold to prepare Sn-30Bi-0.5Cu alloy rod blanks. The rod blanks had a specification of φ80 mm, a length of 500 mm and a weight of 31.54 kg.

[0103] (2) Preparation of flux core: Heat 3.5 kg of rosin flux to 100°C in the melting tank of the rosin processing mechanism to melt the rosin flux, and stir evenly to obtain the flux core. The ingredients of the rosin flux include: by weight, 45 parts of hydrogenated rosin, 20 parts of tetrahydrofurfuryl alcohol, 12 parts of ethylene glycol, 13 parts of diethylene glycol, 1 part of succinic acid, 1 part of citric acid, 2 parts of adipic acid, 1 part of hydrogenated castor oil, 1.5 parts of benzotriazole, 2.5 parts of triethanolamine, 1 part of ethylphenyl polyethylene glycol, and 1 part of nonylphenol polyoxyethylene ether. The rosin flux accounts for 10% of the total weight of the welding wire.

[0104] (3) Preparation of tin-based flux-cored welding wire: The Sn-30Bi-0.5Cu alloy rod obtained in step (1) is charged into Figure 3 The Sn-30Bi-0.5Cu alloy rod blank is heated to 75°C and the extrusion force is 150kg / cm 2 , the wire drawing speed is 1.5m / min, and the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a thick tin wire, the thick tin wire is φ10mm. The thick tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire is gradually reduced in diameter: φ10mm→φ5mm→φ2.5mm→φ1mm, and a tin-based flux-cored welding wire is obtained.

[0105] Comparative Example 3 Preparation of a Tin-based Flux-cored Welding Wire

[0106] The difference from Example 3 is that in step (2), no Sn-40Bi alloy powder is added to the core.

[0107] The solder Sn-10Bi-5Sb-Cu alloy in the target solder joint Cu / Sn-10Bi-5Sb-Cu / Cu is used as the wire skin, and the rosin flux is used as the flux core.

[0108] (1) Preparation of rod blanks: Since metals have a certain burnout rate, Sn-10Bi-5Sb-Cu alloys were prepared by mixing 6 kg of pure Sn, 4 kg of Sn-10Cu, 20 kg of Sn-10Sb, and 10 kg of Sn-40Bi alloy, and adding the mixture into a smelting furnace at 1000° C. The mixture was kept warm for 25 min and then stirred for 8 min. The stirred product was allowed to stand for 8 min and then deslagging was performed. The deslagging product was poured into a mold to prepare Sn-10Bi-5Sb-Cu alloy rod blanks. The rod blanks had a specification of φ100 mm, a length of 500 mm, and a weight of 39.53 kg.

[0109] (2) Preparation of flux core: Heat 3.44 kg of rosin flux to 100°C in the melting cylinder of the rosin processing mechanism to melt the rosin flux, and stir evenly to obtain the flux core. The ingredients of the rosin flux include: by weight, 35 parts of hydrogenated rosin, 22 parts of tetrahydrofurfuryl alcohol, 13 parts of ethylene glycol, 13 parts of diethylene glycol, 1.5 parts of succinic acid, 1 part of citric acid, 2 parts of adipic acid, 1.5 parts of hydrogenated castor oil, 1.8 parts of benzotriazole, 1.5 parts of triethanolamine, 1.5 parts of ethylphenyl polyethylene glycol, and 1.5 parts of nonylphenol polyoxyethylene ether. The rosin flux accounts for 8% of the total weight of the welding wire.

[0110] (3) Preparation of tin-based flux-cored welding wire: The Sn-10Bi-5Sb-Cu alloy rod obtained in step (1) is charged into Figure 3 The Sn-10Bi-5Sb-Cu alloy rod blank is heated to 80°C and the extrusion force is 200kg / cm 2 , the wire drawing speed is 1.6m / min, and at the same time, the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a coarse tin wire, the coarse tin wire is φ5mm. The coarse tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire is gradually reduced in diameter: φ5mm→φ2.5mm→φ1.25mm→φ0.2mm, and a tin-based flux-cored welding wire is obtained. Comparative Example 4 Preparation of a tin-based flux-cored welding wire

[0111] The difference from Example 4 is that in step (2), no Sn-40Bi alloy powder is added to the core.

[0112] The solder Sn-30Bi-15In alloy in the target solder joint Cu / Sn-30Bi-15In / Cu is used as the wire skin, and the rosin flux is used as the flux core.

[0113] (1) Preparation of rod blanks: 6.4 kg of pure Sn, 9.6 kg of Sn-40In, and 16 kg of Sn-60Bi alloy were mixed and added into a smelting furnace at 650°C. The mixture was kept warm for 20 min and then stirred for 6 min. The stirred product was allowed to stand for 6 min and then deslagging was performed. The deslagging product was poured into a mold to prepare Sn-30Bi-15In alloy rod blanks. The rod blanks had a specification of φ100 mm, a length of 500 mm, and a weight of 31.43 kg.

[0114] (2) Preparation of flux core: Heat 3.11 kg of rosin flux to 100°C in the melting cylinder of the rosin processing mechanism to melt the rosin flux, and stir evenly to obtain the flux core. The ingredients of the rosin flux include: by weight, 35 parts of hydrogenated rosin, 25 parts of tetrahydrofurfuryl alcohol, 14 parts of ethylene glycol, 14 parts of diethylene glycol, 2 parts of succinic acid, 1 part of citric acid, 1.5 parts of adipic acid, 1 part of hydrogenated castor oil, 1.5 parts of benzotriazole, 2 parts of triethanolamine, 1.5 parts of ethylphenyl polyethylene glycol, and 1.5 parts of nonylphenol polyoxyethylene ether. The rosin flux accounts for 9% of the total weight of the welding wire.

[0115] (3) Preparation of tin-based flux-cored welding wire: The Sn-30Bi-15In alloy bar blank obtained in step (1) is loaded into Figure 3 The Sn-30Bi-15In alloy bar billet is heated to 60°C and the extrusion force is 175 kg / cm 2 , the wire drawing speed is 3m / min, and the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a thick tin wire, the thick tin wire is φ5mm. The thick tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire is gradually reduced in diameter: φ5mm→φ2.5mm→φ1.25mm→φ0.2mm, and a tin-based flux-cored welding wire is obtained.

[0116] Comparative Example 5 Preparation of a tin-based bismuth-rich flux-cored welding wire and welding spot

[0117] The difference from Example 1 is that in step (4), the ultrasonic brazing is changed to conventional pressure brazing, and the applied pressure is 0.5 MPa.

[0118] Sn-5Bi was used as the welding wire skin component, Sn-90Bi alloy and rosin-based flux were used as core materials, and the composition of the target solder joint was Cu / Sn-52Bi / Cu.

[0119] (1) Preparation of rod blanks: Since the metal has a certain burnout rate, according to the Sn-5Bi alloy ratio, 27.2 kg of pure Sn and 2.08 kg of Sn-70Bi master alloy were mixed and added into a smelting furnace at 500°C. The mixture was kept warm for 20 min and then stirred for 5 min. The stirred product was allowed to stand for 5 min and then deslagging was performed. The deslagging product was poured into a mold to prepare a Sn-5Bi alloy rod blank. The rod blank had a specification of φ100 mm, a length of 500 mm, and a weight of 29.07 kg.

[0120] (2) Preparation of flux core: 3.42 kg of rosin flux is heated to 100°C in the melting cylinder of the rosin processing mechanism to make the rosin flux molten, and 35.95 kg of Sn-90Bi alloy powder is added and stirred evenly to obtain flux doped with Sn-90Bi alloy powder, i.e., flux core. The ingredients of the rosin flux include: by weight, 30 parts of hydrogenated rosin, 25 parts of tetrahydrofurfuryl alcohol, 12 parts of ethylene glycol, 15 parts of diethylene glycol, 2 parts of succinic acid, 1 part of citric acid, 2.5 parts of adipic acid, 2 parts of hydrogenated castor oil, 1.5 parts of benzotriazole, 1 part of triethanolamine, 1 part of ethylphenyl polyethylene glycol, and 1 part of nonylphenol polyoxyethylene ether. The rosin flux accounts for 5% of the total weight of the welding wire.

[0121] (3) Preparation of tin-based bismuth-rich flux-cored welding wire: The Sn-5Bi alloy rod blank obtained in step (1) is loaded into Figure 3 The extrusion die shown in the figure is used to gradually form the wire skin by extrusion. The Sn-5Bi alloy bar billet temperature is 60°C and the extrusion pressure is 160kg / cm 2 , the wire drawing speed is 1.8m / min, and the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a thick tin wire, the thick tin wire is φ12mm. The obtained thick tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire is gradually reduced in diameter: φ12mm→φ6mm→φ4mm→φ2mm, and a tin-based bismuth-rich flux cored welding wire is obtained.

[0122] (4) Preparation of solder joints: The tin-based bismuth-rich flux-cored welding wire obtained in step (3) is soldered by conventional pressure brazing at a welding temperature of 250°C, and the formed Cu / Sn-52Bi / Cu region is the solder joint. The structure of the solder joint includes a stacked structure arranged in sequence: a first shell layer, a connecting layer, and a second shell layer, wherein the main component of the connecting layer is the tin-based bismuth-rich flux-cored welding wire obtained in step (3), and the first shell layer and the second shell layer are both copper substrates. The applied pressure is 0.5 MPa.

[0123] Comparative Example 6 Preparation of a tin-based bismuth-rich flux-cored welding wire and welding spot

[0124] The difference from Example 2 is that in step (4), the ultrasonic brazing is changed to conventional pressure brazing, and the applied pressure is 0.2 MPa.

[0125] Sn-5Bi-0.5Cu was used as the welding wire skin component, Sn-60Bi-0.5Cu alloy and rosin-based flux were used as core materials, and the composition of the target solder joint was Cu / Sn-30Bi-0.5Cu / Cu.

[0126] (1) Preparation of rod blanks: Since the metal has a certain burnout rate, according to the Sn-5Bi-0.5Cu alloy ratio, 24 kg of pure Sn, 3 kg of Sn-5Cu and 3 kg of Sn-50Bi master alloy were mixed and added into a smelting furnace at 500°C. The mixture was kept warm for 30 min and then stirred for 10 min. The stirred product was allowed to stand for 10 min and then deslagging was performed. The deslagging product was poured into a mold to prepare a Sn-5Bi-0.5Cu alloy rod blank. The rod blank had a specification of φ80 mm, a length of 500 mm and a weight of 29.12 kg.

[0127] (2) Preparation of flux core: 5.93 kg of rosin flux is heated to 100° C. in a melting tank of a rosin processing mechanism to melt the rosin flux, and 24.26 kg of Sn-60Bi-0.5Cu alloy powder is added and stirred evenly to obtain flux doped with Sn-60Bi-0.5Cu alloy powder, i.e., flux core. The ingredients of the rosin flux include: by weight, 45 parts of hydrogenated rosin, 20 parts of tetrahydrofurfuryl alcohol, 12 parts of ethylene glycol, 13 parts of diethylene glycol, 1 part of succinic acid, 1 part of citric acid, 2 parts of adipic acid, 1 part of hydrogenated castor oil, 1.5 parts of benzotriazole, 2.5 parts of triethanolamine, 1 part of ethylphenyl polyethylene glycol, and 1 part of nonylphenol polyoxyethylene ether. The rosin flux accounts for 10% of the total weight of the welding wire.

[0128] (3) Preparation of tin-based bismuth-rich flux-cored welding wire: The Sn-5Bi-0.5Cu alloy rod blank prepared in step (1) is loaded into Figure 3 The Sn-5Bi-0.5Cu alloy rod blank is heated to 75°C and the extrusion pressure is 150kg / cm 2 , the wire drawing speed is 1.5m / min, and the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a thick tin wire, the thick tin wire is φ10mm. The thick tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire is gradually reduced in diameter: φ10mm→φ5mm→φ2.5mm→φ1.0mm, and a tin-based bismuth-rich flux cored welding wire is obtained.

[0129] (4) Preparation of solder joints: The tin-based bismuth-rich flux-cored welding wire obtained in step (3) is soldered by conventional pressure brazing at a welding temperature of 245°C, and the formed Cu / Sn-30Bi-0.5Cu / Cu region is the solder joint. The solder joint comprises a stacked structure arranged in sequence: a first shell layer, a connecting layer and a second shell layer, wherein the main component of the connecting layer is the tin-based bismuth-rich flux-cored welding wire obtained in step (3), and the first shell layer and the second shell layer are both copper substrates. The applied pressure is 0.2MPa.

[0130] Comparative Example 7 Preparation of a tin-based bismuth-rich flux-cored welding wire and welding spot

[0131] The difference from Example 3 is that in step (4), the ultrasonic brazing is changed to conventional pressure brazing, and the applied pressure is 0.4 MPa.

[0132] Sn-5Sb-Cu is used as the solder wire skin component, Sn-40Bi and rosin-based flux are used as the flux core. The composition of the target solder joint is Cu / Sn-10Bi-5Sb-Cu / Cu.

[0133] (1) Preparation of rod blanks: Since metal has a certain burnout rate, 12 kg of pure Sn, 3 kg of Sn-10Cu and 15 kg of Sn-10Sb were mixed according to the ratio of Sn-5Sb-Cu alloy, added into a smelting furnace at 1000°C, kept warm for 25 min, and then stirred for 8 min. The stirred product was allowed to stand for 8 min and then deslagging was performed. The deslagging product was poured into a mold to prepare Sn-5Sb-Cu alloy rod blanks. The rod blanks had a specification of φ100 mm, a length of 500 mm and a weight of 28.43 kg.

[0134] (2) Preparation of flux core: 3.3 kg of rosin flux is heated to 100°C in the melting tank of the rosin processing mechanism to make the rosin flux molten, and 9.47 kg of Sn-40Bi alloy powder is added and stirred evenly to obtain flux doped with Sn-40Bi alloy, i.e., flux core. The ingredients of the rosin flux include: by weight, 35 parts of hydrogenated rosin, 22 parts of tetrahydrofurfuryl alcohol, 13 parts of ethylene glycol, 13 parts of diethylene glycol, 1.5 parts of succinic acid, 1 part of citric acid, 2 parts of adipic acid, 1.5 parts of hydrogenated castor oil, 1.8 parts of benzotriazole, 1.5 parts of triethanolamine, 1.5 parts of ethylphenyl polyethylene glycol, and 1.5 parts of nonylphenol polyoxyethylene ether. The rosin flux accounts for 8% of the total weight of the welding wire.

[0135] (3) Preparation of tin-based bismuth-rich flux-cored welding wire: The Sn-5Sb-Cu alloy rod blank obtained in step (1) is loaded into Figure 3The Sn-5Sb-Cu alloy rod blank is heated to 80°C and the extrusion force is 200kg / cm 2 , the wire drawing speed is 1.6m / min, and the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a thick tin wire, the thick tin wire is φ5mm. The thick tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire gradually reduces in diameter: φ5mm→φ2.5mm→φ1.25mm→φ0.2mm. Thus, a tin-based bismuth-rich flux cored welding wire is obtained.

[0136] (4) Preparation of solder joints: The tin-based bismuth-rich flux-cored welding wire obtained in step (3) is soldered by conventional pressure brazing at a welding temperature of 310°C. The formed Cu / Sn-10Bi-5Sb-Cu / Cu region is the solder joint. The solder joint comprises a stacked structure arranged in sequence: a first shell layer, a connecting layer and a second shell layer. The main component of the connecting layer is the tin-based bismuth-rich flux-cored welding wire obtained in step (3). The first shell layer and the second shell layer are both copper substrates. The applied pressure is 0.4 MPa.

[0137] Comparative Example 8 Preparation of a tin-based bismuth-rich flux-cored welding wire and welding spot

[0138] The difference from Example 4 is that in step (4), the ultrasonic brazing is changed to conventional pressure brazing, and the applied pressure is 0.4 MPa.

[0139] Sn-24In-2Bi is used as the solder wire skin component, Sn-40Bi alloy powder and rosin-based flux are used as the flux core. The target solder joint composition is Cu / Sn-30Bi-15In / Cu.

[0140] (1) Preparation of rod blanks: Since metals have a certain burnout rate, 6.2 kg of pure Sn, 17.5 kg of Sn-40In, and 5.8 kg of Sn-10Bi were mixed according to the ratio of Sn-24In-2Bi alloy, added into a smelting furnace at 650°C, kept warm for 20 min, and then stirred for 6 min. The stirred product was allowed to stand for 6 min, and then deslagging was performed. The deslagging product was poured into a mold to prepare Sn-24In-2Bi alloy rod blanks. The rod blanks had a specification of φ100 mm, a length of 500 mm, and a weight of 28.77 kg.

[0141] (2) Preparation of flux core: 4.44 kg of rosin flux is heated to 100°C in the melting cylinder of the rosin processing mechanism to make the rosin flux molten, and 16.11 kg of Sn-40Bi alloy powder is added and stirred evenly to obtain flux doped with Sn-40Bi alloy powder, i.e., flux core. The ingredients of the rosin flux include: by weight, 35 parts of hydrogenated rosin, 25 parts of tetrahydrofurfuryl alcohol, 14 parts of ethylene glycol, 14 parts of diethylene glycol, 2 parts of succinic acid, 1 part of citric acid, 1.5 parts of adipic acid, 1 part of hydrogenated castor oil, 1.5 parts of benzotriazole, 2 parts of triethanolamine, 1.5 parts of ethylphenyl polyethylene glycol, and 1.5 parts of nonylphenol polyoxyethylene ether. The rosin flux accounts for 9% of the total weight of the welding wire.

[0142] (3) Preparation of tin-based bismuth-rich flux-cored welding wire: The Sn-24In-2Bi alloy rod obtained in step (1) is charged into Figure 3 The Sn-24In-2Bi alloy rod blank is heated to 60°C and the extrusion force is 175 kg / cm 2 , the wire drawing speed is 3m / min, and the flux core obtained in step (2) is injected through a rosin pen and filled into the inside of the welding wire skin, finally forming a thick tin wire, the thick tin wire is φ5mm. The thick tin wire is subjected to three processes of large drawing, medium drawing and small drawing, and the welding wire is gradually reduced in diameter: φ5mm→φ2.5mm→φ1.25mm→φ0.2mm, and a tin-based bismuth-rich flux cored welding wire is obtained.

[0143] (4) Preparation of solder joints: The tin-based bismuth-rich flux-cored welding wire obtained in step (3) is soldered by conventional pressure brazing at a welding temperature of 180°C, and the formed Cu / Sn-30Bi-15In / Cu region is the solder joint. The solder joint comprises a stacked structure arranged in sequence: a first shell layer, a connecting layer, and a second shell layer, wherein the main component of the connecting layer is the tin-based bismuth-rich flux-cored welding wire obtained in step (3), and the first shell layer and the second shell layer are both copper substrates. The applied pressure is 0.4 MPa.

[0144] Experimental Example: Characterization of the performance of tin-based bismuth-rich flux-cored welding wire and solder joints

[0145] 1. Schematic diagram of preparing welding wire mold

[0146] from Figure 3It can be seen that the round rod ingot is a rod blank 1, with a specification of φ80~φ100mm, and a composition of a low-Bi or Bi-free Sn-based alloy. Due to the low Bi content, its plasticity is good. Under a certain pressure, it passes through the first cavity 9 and the second cavity 10 of the extrusion die 2, and then passes through the wire inner diameter forming die 7 and the wire outer diameter forming die 8 to form a φ5~φ12mm thick tin wire, and then through the drawing process to reduce the diameter to obtain a flux-cored welding wire 11 with a diameter of 0.2~2mm. The pipeline of the rosin pen 6 is filled with molten rosin flux and bismuth-rich tin powder 5, and the rosin melting cylinder 4 is used to heat the rosin flux to melt, and after adding bismuth-rich tin powder, enter the rosin pressurizing cylinder 3 together, and the pressurized rosin flux and bismuth-rich tin powder are injected into the core of the wire, and the flux-cored welding wire is formed after condensation, wherein the rosin flux accounts for 2%~10% of the weight of the tin-based bismuth-rich flux-cored welding wire.

[0147] 2. Schematic diagram of ultrasonic brazing of welding points

[0148] from Figure 4 It can be seen that pressure can be applied on the ultrasonic generator 1 and the ultrasonic welding work head 2 to compress the welding wire 4 and the copper-based pad 3, and the heating base 5 adopts an electric heating wire heating method to provide the required heat source for welding. The flux-cored welding wire 4 is composed of bismuth-free or low-bismuth metal skin 41, Sn-Bi alloy powder 42 and rosin-based flux 43. Under the combined effect of extrusion force, ultrasonic vibration and temperature provided by the heating base, the welding wire and the copper-based pad are tightly stacked to form a sandwich-like structure, that is, a welding point.

[0149] 3. SEM characterization of solder joints, Sn-Bi binary phase diagram

[0150] The microscopic morphology of the solder joints was analyzed using a German Sigma500 scanning electron microscope. The solder joints showed a stacked structure design, which consisted of the first shell layer (copper substrate), the connecting layer (tin-based bismuth-rich flux-cored welding wire) and the second shell layer (copper substrate). Figures 5 to 8 The morphologies of the solder joints prepared in Examples 1 to 4 are shown respectively, all showing a typical structure with a copper substrate on the top and bottom and a solder (tin-based bismuth-rich flux-cored welding wire) in the middle, and a metallurgical bonding layer (IMC) is formed between the copper and solder interfaces. The observation results show that no cracks or segregation occur inside the solder and in the metallurgical bonding layer, indicating that Examples 1 to 4 have successfully prepared high-quality solder joints. This is due to the use of Bi-free or low-Bi Sn alloy as the rod blank. Its good plasticity promotes the rapid reaction and uniform mixing of the high-Bi alloy powder in the flux core and the Sn in the outer skin of the welding wire during the ultrasonic-assisted heating and pressurizing brazing process, thereby significantly improving the wetting performance of the welding wire, effectively reducing the formation of stress and cracks, and enhancing the connection strength of the solder joint.

[0151] Figures 9 to 12The morphologies of solder joints prepared in Comparative Examples 5 to 8 are shown respectively. In Comparative Example 5, continuous cracks appeared in the IMC near the solder, which was due to the segregation phenomenon caused by the local concentration of the Bi phase. According to the Sn-Bi binary phase diagram ( Fig.13 ), the solid-liquid phase temperature (145-172°C) of Sn-52Bi alloy is close to the eutectic point, and its coarse Bi phase structure is not ultrasonically shattered and shows high brittleness, which in turn causes long cracks in the IMC, forming a cold solder joint, and large cracks also appear inside the solder. Because comparative example 6 did not use ultrasonic assisted brazing, obvious element segregation appeared in the solder joint, and the color of the solder layer was uneven, reflecting the high concentration of Bi phase in the local area. The wide solid-liquid phase temperature range (140-185°C) of the Sn-30Bi alloy exacerbated this process. As the temperature dropped, the Bi concentration gradually increased, and finally a cold solder joint occurred near the copper substrate due to the segregation of Bi. The solder structure of comparative example 7 showed a large number of crack textures because the Sn-20Bi alloy in the welding wire core failed to fully react with the Sn-5Sb-Cu of the welding wire skin metal in time. Since the thermal expansion coefficient of the high Bi solder is greater than that of Sn-5Sb-Cu, excessive stress is generated during the cooling process, resulting in cracking. Comparative Example 8 also did not apply ultrasonic assisted brazing, resulting in insufficient reaction between the Sn-40Bi alloy powder in the flux core and the Sn-24In-2Bi solder skin, and cracks appeared near the IMC of the solder joint. The results show that the solder joints of Comparative Examples 5 to 8 all had problems such as cracks, segregation, and cold solder joints, indicating that the welding wires in these comparative examples failed to weld successfully. In Bi-rich solders, ultrasonic brazing technology must be used to ensure that the high Bi alloy and Sn are completely and evenly mixed, thereby reducing the brittleness of the organization and effectively avoiding the occurrence of defects such as segregation, cracking, and cold solder joints.

[0152] Fig.13The Sn-Bi binary phase diagram has a eutectic composition of Sn-58Bi and a melting point of 138°C. For the Sn-30Bi alloy, if it is slowly cooled from point C (composition Sn-30Bi) in the phase diagram during solidification, when the temperature drops to point O (192°C), the solid phase begins to precipitate. At this time, the concentration of Bi in the solid phase corresponds to the lower Bi content area in the phase diagram (such as point A, composition Sn-10Bi). As the temperature continues to drop, the concentration of Bi in the solid phase gradually increases. When the temperature is lower than point O, the concentration of Bi in the solid phase will exceed the original Bi concentration in the alloy (30%), and may approach or reach a higher Bi content point (such as point B, composition Sn-40Bi). This phenomenon of Bi concentration gradually increasing with decreasing temperature will lead to severe segregation during brazing in an equilibrium state: the solid phase precipitated at high temperature contains less Bi, while the solid phase precipitated at low temperature has a higher Bi content. To solve this problem, ultrasonic assisted brazing technology is used, which uses instantaneous high temperature and high pressure to break the equilibrium solidification conditions, increase the supercooling of the solder, promote the uniform distribution of solder components and refinement of the structure, thereby significantly improving the strength of the solder joint.

[0153] 4. Solder joint performance

[0154] Table 1 Welding wire forming results prepared in Examples 1 to 4 and Comparative Examples 1 to 4

[0155]

[0156] Table 2 Performance of solder joints prepared in Examples 1 to 4 and Comparative Examples 5 to 8

[0157]

[0158] (1) Shear strength

[0159] At room temperature, the welded samples were sheared using an Instron force analyzer, and the shear speed was 300 μm / s. The maximum shear force was recorded and the shear strength was calculated. The greater the shear strength, the better the shear resistance. As can be seen from Tables 1 to 2, in Examples 1 to 4 of the present invention, ultrasonic brazing technology is used to promote the accelerated mixing of the Bi-rich alloy in the flux core and the metal of the welding wire skin. In this process, the Bi-rich phase can solidify rapidly and release energy in a local area, thereby effectively causing the high melting point weld skin to melt. In addition, ultrasonic cavitation also has the ability to effectively break the instantaneously solidified coarse and long Bi phase, thereby improving the shear strength of the weld. In contrast, Comparative Examples 1 to 4 directly used Bi-rich alloy as the extruded rod blank, but failed to successfully form a welding wire; while Comparative Examples 5 to 8 used a conventional pressure brazing method, and the shear strength of the weld obtained was much lower than that of the corresponding embodiment (using ultrasonic brazing), which significantly reduced the connection reliability of the weld and the mechanical properties of the overall structure.

[0160] (2) Soldering temperature

[0161] The welding wire was placed on a copper substrate for heating, and the temperature at the beginning and end of melting of the welding wire was tested, wherein the temperature at the end of melting was the welding temperature. It can be clearly seen from the data in Table 2 that the welding temperatures of Examples 1 to 4 were all lower than those of Comparative Examples 5 to 8, and this phenomenon was attributed to the ultrasonic cavitation effect generated during the welding process. When the metal vapor bubble collapses under the action of ultrasound, it releases huge pressure and instantaneous high temperature, which causes the local welded material to melt, thereby significantly reducing the welding temperature.

[0162] (3) Welding range

[0163] The welding wire is placed on a copper substrate for heating, and the temperature at the beginning and end of melting of the welding wire is tested, where the temperature difference before and after melting is the welding melting range. From the data in Table 2, it can be concluded that under the same welding wire composition, the use of ultrasonic energy-assisted brazing can significantly reduce the welding forming temperature and simultaneously reduce the melting range of the solder. This is mainly due to the stirring effect of the ultrasonic sound field and the cavitation effect generated in the liquid solder joint. These effects cause the solder micro-area to melt rapidly during the welding process, and cause frequent heat exchange between the high melting point and low melting point solder metals, thereby effectively reducing the melting range.

[0164] In summary, the present invention adopts Sn-based alloy with low Bi content to prepare welding wire skin, and uses flux and Sn-Bi alloy to form flux core. This combination optimizes the distribution of Sn and Bi elements in tin-based bismuth-rich flux core welding wire, effectively prevents wire breakage, and is conducive to the preparation of high-quality solder joints. In the process of solder joint preparation, the low Bi welding wire skin and the Sn-Bi alloy in the flux core achieve uniform distribution of solder under the combined action of ultrasonic cavitation and mechanical oscillation. This distribution promotes the rapid reaction of the Sn-Bi alloy in the flux core and the low Bi welding wire skin, effectively suppresses the stress unevenness phenomenon, thereby avoiding the problems of solder joint segregation, large melting range and cold solder joints.

[0165] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A tin-based bismuth-rich flux-cored welding wire, characterized in that: The tin-based bismuth-rich flux-cored welding wire includes a flux core and a welding wire skin, wherein the flux core is filled inside the welding wire skin; wherein the flux core includes a rosin-based flux and a Sn-Bi alloy, wherein the Bi content in the Sn-Bi alloy is ≥10%; the welding wire skin is a Sn-based alloy containing a small amount of Bi or no Bi; and the Bi content in the Sn-based alloy containing a small amount of Bi is ≤5%.

2. The tin-based bismuth-rich flux-cored welding wire according to claim 1, characterized in that: The content of Sn in the tin-based bismuth-rich flux-cored welding wire is ≥50%.

3. The tin-based bismuth-rich flux-cored welding wire according to claim 1, characterized in that: The ingredients of the rosin soldering flux include hydrogenated rosin, tetrahydrofurfuryl alcohol, ethylene glycol, diethylene glycol, succinic acid, citric acid, adipic acid, hydrogenated castor oil, benzotriazole, triethanolamine, ethylphenyl polyethylene glycol, and nonylphenol polyoxyethylene ether.

4. The tin-based bismuth-rich flux-cored welding wire according to claim 1, characterized in that: The weight of the rosin soldering flux is 2% to 10% based on the weight of the tin-based bismuth-rich flux-cored welding wire.

5. The method for preparing the tin-based bismuth-rich flux-cored welding wire according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1. Melting the alloy according to the alloy ratio to prepare a billet; S2. The rosin flux is melted and then evenly added to the Sn-Bi alloy to prepare a flux core; S3. Extrude the rod blank obtained in step S1 to form a welding wire skin, and inject the flux core obtained in step S2 into the welding wire skin while extruding to fill it to form a flux cored coarse tin wire, and draw and shape the obtained flux cored coarse tin wire to obtain a tin-based bismuth-rich flux cored welding wire.

6. The preparation method according to claim 5, characterized in that: The specific preparation method of the welding wire skin comprises the following steps: according to the alloy ratio, weighing the raw materials to melt the alloy, injecting it into a molding die to obtain a rod blank, injecting the obtained rod blank into an extrusion die, and extruding it to form the welding wire skin.

7. A solder joint, characterized in that: The welding spot is mainly formed by ultrasonic brazing using the tin-based bismuth-rich flux-cored welding wire as described in any one of claims 1 to 4 as welding material.

8. The solder joint according to claim 7, characterized in that: The structure of the welding point includes a stacked structure arranged in sequence: a first shell layer, a connecting layer and a second shell layer, the main component of the connecting layer is the tin-based bismuth-rich flux-cored welding wire according to any one of claims 1 to 4, and the first shell layer and the second shell layer are both base materials.

9. The solder joint according to claim 8, characterized in that: The parent material includes one or more of a copper substrate, an aluminum alloy plate, and a stainless steel plate.

10. Use of the tin-based bismuth-rich flux-cored welding wire according to any one of claims 1 to 4 or the solder joint according to any one of claims 7 to 9 in the field of electronic packaging.

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