Self-repairing gradient protective Sn-Cu lead-free solder based on in-situ interface construction and preparation method thereof

Abstract

The application discloses a self-repairing gradient protective Sn-Cu lead-free solder based on in-situ interface construction and a preparation method thereof, and relates to the technical field of electronic packaging lead-free solder. The Sn-Cu lead-free solder raw material composition comprises, in percentage by mass, 0.6-0.8% of Cu, 0.02-0.3% of Mg, 0.01-0.15% of a Ti precursor, and the balance of Sn and inevitable impurities. A multi-scale gradient protective structure is constructed through electromagnetic auxiliary smelting, in-situ reaction by controlling oxygen partial pressure, ultrasonic-electromagnetic composite field treatment, directional solidification and multi-stage heat treatment. Under the synergistic effect of ultrasonic cavitation and electromagnetic flow field, an oxygen diffusion barrier layer is formed towards the grain boundary and intermetallic compound interface area. The Mg element migrates to the solder surface layer to form a corrosion-resistant enrichment layer, thereby forming a gradient synergistic protective structure. The structure can effectively inhibit the diffusion of oxygen and the invasion of corrosion medium, improve the oxidation resistance, corrosion resistance and interface stability of the solder, and is suitable for the manufacture of high-reliability electronic packaging and high-temperature service electronic devices.

Description

Technical Field

[0001] This invention relates to a self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction and its preparation method, belonging to the field of electronic packaging lead-free solder technology. Background Technology

[0002] Sn-Cu-based lead-free solders have become one of the most promising lead-free solders in the electronic packaging field due to their excellent mechanical properties, cost advantages, and compatibility with packaging processes. However, these solders still face the following technical bottlenecks in actual service: Solder is prone to oxidation during high-temperature welding and long-term use, forming a loose SnO2 oxide layer on its surface. This oxide layer cannot effectively prevent further oxygen diffusion, leading to continuous oxidation inside the solder and affecting the reliability of the solder joint. Secondly, in humid and salt spray corrosive environments, Sn-Cu solder has poor corrosion resistance and is prone to electrochemical corrosion, causing solder joint failure. In addition, the intermetallic compounds (such as Cu6Sn5) in the solder have high interfacial energies, making it easy for oxygen and corrosive media to diffuse along grain boundaries and phase interfaces, further exacerbating solder deterioration.

[0003] To address these issues, related technologies primarily enhance the oxidation and corrosion resistance of solders by adding trace alloying elements or nanoparticles. Mg, due to its high chemical reactivity, preferentially oxidizes on the solder surface to form a dense and stable MgO protective layer, exhibiting self-healing properties and significantly improving the solder's oxidation and corrosion resistance. Nano-TiO2 particles, as a common oxide reinforcing phase, can pin grain boundaries and inhibit the diffusion channels of oxygen and corrosive media. However, existing modification schemes often only optimize a single property (oxidation resistance or corrosion resistance), lacking a systematic design for synergistic improvement of both. For example, while adding Mg alone can form a surface oxide film, oxygen can still diffuse inward along grain boundaries and phase interfaces; while adding TiO2 nanoparticles alone can pin grain boundaries, the solder surface lacks a rapidly self-healing passivation layer. Furthermore, the addition of Mg and Ti elements to the alloy system easily leads to local segregation or coarse phase precipitation, which can actually reduce the mechanical properties of the solder. Traditional manufacturing processes (such as ordinary casting and single-stage aging treatment) are difficult to construct multi-layered and gradient protective structures in solder, and cannot achieve the synergistic protective effect of surface self-healing oxide film, interface oxygen diffusion barrier layer and internal tough matrix.

[0004] Therefore, developing a solder and its preparation method that can simultaneously improve the oxidation resistance and corrosion resistance of Sn-Cu lead-free solder and possess a gradient protective structure has significant engineering application value. This invention utilizes a synergistic mechanism of in-situ generation, directional migration, and gradient construction. Under micro-oxygen conditions, Mg is preferentially oxidized to form a self-healing MgO oxide layer, while the Ti precursor is decomposed in situ to generate nano-TiO2 particles. Under the action of an ultrasonic-electromagnetic composite field, these particles directionally migrate and agglomerate at grain boundaries and intermetallic compound interfaces, forming an oxygen diffusion barrier layer. This constructs a complete gradient synergistic protective structure on the solder surface, at the interface, and within the solder region. Summary of the Invention

[0005] To address the shortcomings of related technologies, this invention provides a self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction and its preparation method. The method of this invention has the advantages of preparing Sn-Cu lead-free solder with excellent oxidation resistance, corrosion resistance and interface stability, and solves the problem that Sn-Cu lead-free solder cannot form a multi-layered, gradient protection structure.

[0006] One of the objectives of this invention is to provide a self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction. The raw material composition of the Sn-Cu lead-free solder includes, by mass percentage: 0.6~0.8% Cu, 0.02~0.3% Mg, 0.01~0.15% Ti precursor, with the balance being Sn and unavoidable impurities.

[0007] Preferably, the Ti precursor is one or more of tetrabutyl titanate, isopropyl titanate, or an organotitanium complex (Tyzor® AA-75 titanate chelate, DuPont).

[0008] More preferably, the Ti precursor is tetrabutyl titanate.

[0009] In the Sn-Cu lead-free solder prepared by this invention, the Ti precursor decomposes in situ in the melt to generate nano-TiO2 particles; Mg preferentially forms a MgO self-healing oxide layer under micro-oxygen conditions, giving the solder surface anti-oxidation and self-healing capabilities; during solidification and subsequent heat treatment, the nano-TiO2 particles agglomerate towards the grain boundaries and intermetallic compound interface regions to form an oxygen diffusion barrier layer; the average particle size of the in-situ generated TiO2 particles is 10~100nm, the Mg content in the surface region of the solder is higher than that in the internal region, the formed MgO self-healing oxide layer is located on the surface of the solder, and the TiO2 particles form an oxygen diffusion barrier layer at the intermetallic compound interface.

[0010] Another objective of this invention is to provide a method for preparing a self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction, specifically including the following steps: (1) Matrix melting: Sn and Cu raw materials are melted in an inert atmosphere to obtain Sn-Cu melt.

[0011] (2) Introduction of Mg raw material and Ti precursor: Mg raw material is added to Sn-Cu melt and electromagnetic assisted melting is carried out. Then, Ti precursor solution is atomized and sprayed into the melt to obtain composite melt.

[0012] (3) Micro-oxygen in-situ reaction: The composite melt undergoes an in-situ reaction under a micro-oxygen melting atmosphere to obtain the melt after the in-situ reaction.

[0013] (4) Ultrasonic-electromagnetic composite field treatment: The melt after in-situ reaction is subjected to ultrasonic-electromagnetic composite field treatment to obtain the melt after composite field treatment.

[0014] (5) Directional solidification: The molten metal after composite field treatment is cast and directionally solidified to obtain directional solidified solder.

[0015] (6) Two-stage heat treatment: The solder after directional solidification is subjected to low-temperature pretreatment and high-temperature treatment in sequence, and then cooled to obtain self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction.

[0016] More preferably, the inert atmosphere in step (1) is an argon atmosphere.

[0017] More preferably, in step (1), the melting temperature of Sn and Cu raw materials is 420~480℃ and the melting time is 20~40min.

[0018] Preferably, the conditions for electromagnetic assisted melting in step (2) are: applying low-frequency electromagnetic stirring with a frequency of 20~80Hz, a magnetic induction intensity of 0.05~0.3T, and a stirring time of 10~20min.

[0019] Preferably, the solvent of the Ti precursor solution in step (2) is an organic solvent, the content of Ti precursor in the Ti precursor solution is 2~10% by mass percentage, and the atomization spray flow rate of the Ti precursor solution is 1~3 mL / min.

[0020] More preferably, the organic solvent in step (2) is one of anhydrous ethanol and isopropanol.

[0021] Preferably, the oxygen content in the micro-oxygen smelting atmosphere in step (3) is 0.05~0.5% by volume; and the in-situ reaction time is 5~15 min.

[0022] More preferably, in step (3), the residual atmosphere in the micro-oxygen melting atmosphere is argon in addition to oxygen.

[0023] Preferably, the conditions for the ultrasonic-electromagnetic composite field treatment in step (4) are: ultrasonic frequency of 20~40kHz, ultrasonic power of 800~1500W, electromagnetic field strength of 0.05~0.5T, and treatment time of 10~20min.

[0024] Preferably, the conditions for directional solidification in step (5) are: a directional solidification temperature gradient of 30~100℃ / mm and a solidification rate of 10~100μm / s.

[0025] More preferably, the mold used for casting in step (5) is a water-cooled copper mold.

[0026] Preferably, the conditions for the low-temperature pretreatment in step (6) are: heat preservation at 120~150℃ for 1~2 hours.

[0027] Preferably, the conditions for the high-temperature treatment in step (6) are: heat preservation at 180~220℃ for 0.5~1h.

[0028] In step (3) of this invention, the micro-oxygen in-situ reaction preferentially oxidizes Mg, forming a precursor of MgO self-healing oxide layer in situ on the surface and inside of the melt. The Ti precursor decomposes instantaneously at the high temperature of the melt, generating nano-TiO2 particles in situ.

[0029] The ultrasonic-electromagnetic composite field treatment in step (4) of this invention utilizes the synergistic effect of ultrasonic cavitation and electromagnetic migration to promote the directional migration and segregation of in-situ generated nano-TiO2 particles to grain boundaries and intermetallic compound interface regions, while further driving Mg elements to enrich the melt surface.

[0030] The directional solidification in step (5) of this invention promotes the significant enrichment of Mg elements in the solder surface layer, forming a composition gradient; at the same time, nano-TiO2 particles are pushed and pinned to the solid-liquid interface and grain boundaries, forming an oxygen diffusion barrier layer; in addition, rapid directional solidification can refine the Sn matrix grains and obtain an internal toughness layer, thereby constructing a gradient protection structure of surface MgO self-healing layer, interface TiO2 oxygen barrier layer, and internal Sn matrix toughness layer.

[0031] The two-stage heat treatment in step (6) of this invention achieves the enrichment of Mg on the surface of the solder and the segregation of TiO2 at the interface through the atomic migration and second phase in different temperature zones and stages. This avoids the mutual interference of competitive diffusion between Mg and TiO2 in single-stage aging and realizes multi-stage interface regulation of enriching Mg first and then pinning TiO2. This is the basis for the solder to obtain the final gradient functional structure. The low-temperature pretreatment uses low temperature to promote the diffusion and segregation of Mg atoms at the surface grain boundary and free surface, further strengthening the compactness and continuity of the MgO self-healing layer. The high-temperature treatment activates the migration and pinning of nano-TiO2 particles to the solid-liquid interface and grain boundary region, forming a stable oxygen diffusion barrier layer, while promoting the homogenization of the matrix structure.

[0032] Mechanism of the invention: This invention constructs a Sn-Cu solder interface structure through a synergistic mechanism of in-situ generation, directional migration, and gradient construction. Under micro-oxygen conditions, Mg preferentially oxidizes to form a self-healing MgO oxide layer, improving the solder surface's oxidation resistance and corrosion resistance. The Ti precursor decomposes in-situ in the melt to generate nano-TiO2 particles, which undergo directional migration under the synergistic effect of ultrasonic cavitation and electromagnetic flow. During directional solidification and subsequent heat treatment, these particles segregate towards grain boundaries and intermetallic compound interface regions, forming an oxygen diffusion barrier layer. Simultaneously, under the influence of a temperature gradient, Mg enriches in the solder surface to form a corrosion-resistant layer. This results in a gradient synergistic protective structure in the solder surface, interface, and internal regions. This structure effectively inhibits oxygen diffusion and corrosive media intrusion, improving the solder's oxidation resistance, corrosion resistance, and interface stability. A flowchart of the specific preparation method is shown below. Figure 1 As shown.

[0033] The beneficial effects of this invention are: (1) Constructing a multi-scale gradient protection structure to achieve synergistic improvement in oxidation resistance and corrosion resistance. Through in-situ generation, directional migration, and gradient construction mechanisms, a MgO self-healing oxide layer is formed on the surface of the solder (endowing it with oxidation resistance and self-healing ability), nano-TiO2 particles segregate at grain boundaries and intermetallic compound interfaces to form an oxygen diffusion barrier layer (inhibiting the diffusion of oxygen and corrosive media along grain boundaries), and a fine-grained Sn matrix is ​​retained inside (maintaining good toughness). The synergistic effect of these three elements overcomes the limitations of traditional single modification of solder.

[0034] (2) Significantly improves high-temperature oxidation resistance and electrochemical corrosion resistance. After oxidation in air at 250℃ for 100h, the weight gain of the solder of the present invention is reduced by more than 60% compared with ordinary Sn-0.7Cu solder; the corrosion current density in 3.5% NaCl solution is reduced by an order of magnitude, the self-corrosion potential is significantly positively shifted, and the pitting corrosion time in salt spray test is extended by more than 2 times.

[0035] (3) Maintaining good mechanical properties and solderability. Nano-TiO2 particles refine the Sn matrix grains, the MgO surface layer is well bonded to the matrix, and the intermetallic compound layer at the solder joint interface is uniform and dense. The shear strength of the solder joint is increased by 15%~25%, and the melting point, wettability and flowability of the solder meet the standard requirements for lead-free solders in electronic packaging.

[0036] (4) The process has good synergy and is easy to industrialize. The micro-oxygen in-situ reaction realizes the controllable generation of MgO and TiO2, avoiding the agglomeration problem of externally added nanoparticles; the ultrasonic-electromagnetic composite field promotes the directional agglomeration of TiO2 and the enrichment of Mg on the surface; directional solidification strengthens the composition gradient; and the two-stage heat treatment realizes the step-by-step control of enriching Mg first and then pinning TiO2. The process links are closely connected and can be implemented on existing solder production lines. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the preparation method of the present invention. Detailed Implementation

[0038] To better illustrate the purpose, technical solution, and advantages of this invention, the invention will be further described below with reference to specific embodiments. In the embodiments and comparative examples of this invention, unless otherwise specified, all chemical reagents used were commercially available analytical grade reagents.

[0039] Example 1 A method for preparing self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction, specifically including the following steps: (1) Matrix melting: Weigh the raw materials of Sn-Cu lead-free solder according to the proportion. The composition of Sn-Cu lead-free solder includes, by mass percentage: 0.7% Cu, 0.16% Mg, 0.08% Ti precursor (tetrabutyl titanate), and the balance is Sn and unavoidable impurities with an impurity content ≤0.05%. Under an argon atmosphere, put the Sn and Cu raw materials into a graphite crucible, heat to 450℃, and hold for 30 min to obtain Sn-Cu melt.

[0040] (2) Introduction of Mg raw material and Ti precursor: Mg raw material is added to Sn-Cu melt, and low-frequency electromagnetic stirring is applied at a frequency of 50Hz, a magnetic induction intensity of 0.15T, and a stirring time of 10min to promote the uniform distribution of Mg in the melt. Then, a Ti precursor solution is prepared by tetrabutyl titanate and anhydrous ethanol. The content of tetrabutyl titanate in the Ti precursor solution is 5% by mass percentage. The Ti precursor solution is introduced into the melt by atomization spraying, and the spray flow rate of the Ti precursor solution is 2mL / min to obtain a composite melt.

[0041] (3) Micro-oxygen in-situ reaction: The composite melt is reacted in-situ for 10 min under a micro-oxygen melting atmosphere, wherein the oxygen content in the micro-oxygen melting atmosphere is 0.2% by volume percentage and the remainder is argon. The melt after in-situ reaction is obtained. Under this condition, Mg is preferentially oxidized to form the precursor of MgO self-repairing oxide layer. The Ti precursor is instantaneously decomposed at high temperature of the melt to generate nano TiO2 particles in-situ with an average particle size of about 45 nm.

[0042] (4) Ultrasonic-electromagnetic composite field treatment: An ultrasonic-electromagnetic composite field with an ultrasonic frequency of 25kHz, an ultrasonic power of 1000W, and an electromagnetic field induction intensity of 0.3T is applied to the melt after in-situ reaction for 15 minutes. By utilizing the synergistic effect of ultrasonic cavitation and electromagnetic migration, the nano-TiO2 particles are promoted to migrate and agglomerate towards the grain boundary and intermetallic compound interface region, while driving Mg elements to accumulate on the surface of the melt, thus obtaining the melt after composite field treatment.

[0043] (5) Directional solidification: The molten metal after composite field treatment is poured into a water-cooled copper mold for directional solidification. The directional solidification temperature gradient is 60℃ / mm and the solidification rate is 50μm / s to obtain the solder after directional solidification. This step promotes the significant enrichment of Mg to the surface of the solder to form a composition gradient. At the same time, the nano-TiO2 particles are pinned to the solid-liquid interface and grain boundaries, refining the Sn matrix grains.

[0044] (6) Two-stage heat treatment: The solder after directional solidification is first subjected to a low-temperature pretreatment at 130℃ for 1.5h to promote the diffusion and segregation of Mg atoms to the surface grain boundaries and free surfaces, and to enhance the density of the MgO self-healing layer. Then, it is subjected to a high-temperature treatment at 200℃ for 0.8h to activate the migration and pinning of nano-TiO2 particles to the interface and grain boundary regions, forming a stable oxygen diffusion barrier layer. After cooling, a self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction is obtained.

[0045] Performance Testing: The obtained solder underwent high-temperature oxidation weight gain testing (oxidation in air at 250℃ for 100h), electrochemical corrosion testing (3.5% NaCl solution, Tafel polarization curve), and interfacial shear strength testing (solder joint Φ=0.5mm, before aging). Compared with ordinary Sn-0.7Cu solder, the weight gain of the Sn-Cu lead-free solder prepared in this embodiment after oxidation in air at 250℃ for 100h was found to be 0.86 mg / cm³. 2 The corrosion current density is 0.87 μA / cm. 2 The solder shear strength is 34.2 MPa, and the weight gain of ordinary Sn-0.7Cu solder after oxidation in air at 250℃ for 100 hours is 2.85 mg / cm³. 2 The corrosion current density is 12.4 μA / cm. 2 The solder shear strength is 28.3 MPa. The Sn-Cu lead-free solder prepared by the method of this invention has significantly reduced high-temperature oxidation weight gain and corrosion current density, significantly improved solder joint shear strength, formed a dense and continuous MgO self-healing layer on the solder surface, and uniformly distributed TiO2 particles at grain boundaries and interfaces.

[0046] Example 2 A method for preparing self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction, specifically including the following steps: (1) Matrix melting: Weigh the raw materials of Sn-Cu lead-free solder according to the proportion. The composition of Sn-Cu lead-free solder includes, by mass percentage: 0.64% Cu, 0.22% Mg, 0.04% Ti precursor (tetrabutyl titanate), with the balance being Sn and unavoidable impurities, the impurity content being ≤0.05%. Under an argon atmosphere, Sn and Cu raw materials are placed in a graphite crucible, heated to 435℃, and held for 35 min to obtain Sn-Cu melt.

[0047] (2) Introduction of Mg raw material and Ti precursor: Mg raw material is added to Sn-Cu melt, and low-frequency electromagnetic stirring is applied at a frequency of 70Hz, a magnetic induction intensity of 0.25T, and a stirring time of 20min to promote the uniform distribution of Mg in the melt. Then, a Ti precursor solution is prepared by tetrabutyl titanate and anhydrous ethanol. The content of tetrabutyl titanate in the Ti precursor solution is 3% by mass percentage. The Ti precursor solution is introduced into the melt by atomization spraying, and the spray flow rate of the Ti precursor solution is 1.5mL / min to obtain a composite melt.

[0048] (3) Micro-oxygen in-situ reaction: The composite melt is reacted in-situ for 8 minutes under a micro-oxygen melting atmosphere, wherein the oxygen content in the micro-oxygen melting atmosphere is 0.4% by volume percentage and the remainder is argon. The melt after in-situ reaction is obtained. Under this condition, Mg is preferentially oxidized to form the precursor of MgO self-repairing oxide layer. The Ti precursor is instantaneously decomposed at high temperature of the melt to generate nano-TiO2 particles in-situ with an average particle size of about 70 nm.

[0049] (4) Ultrasonic-electromagnetic composite field treatment: An ultrasonic-electromagnetic composite field with an ultrasonic frequency of 35kHz, an ultrasonic power of 1200W, and an electromagnetic field induction intensity of 0.45T is applied to the melt after in-situ reaction for 12 minutes. By utilizing the synergistic effect of ultrasonic cavitation and electromagnetic migration, the nano-TiO2 particles are promoted to migrate and aggregate towards the grain boundary and intermetallic compound interface region, while driving Mg elements to accumulate on the surface of the melt, thus obtaining the melt after composite field treatment.

[0050] (5) Directional solidification: The molten metal after composite field treatment is poured into a water-cooled copper mold for directional solidification. The directional solidification temperature gradient is 85℃ / mm and the solidification rate is 25μm / s to obtain the solder after directional solidification. This step promotes the significant enrichment of Mg to the surface of the solder to form a composition gradient. At the same time, the nano-TiO2 particles are pinned to the solid-liquid interface and grain boundaries, refining the Sn matrix grains.

[0051] (6) Two-stage heat treatment: The solder after directional solidification is first subjected to a low-temperature pretreatment at 140℃ for 1.2h to promote the diffusion and segregation of Mg atoms to the surface grain boundaries and free surfaces, and to enhance the density of the MgO self-healing layer. Then, it is subjected to a high-temperature treatment at 210℃ for 0.6h to activate the migration and pinning of nano-TiO2 particles to the interface and grain boundary regions, forming a stable oxygen diffusion barrier layer. After cooling, a self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction is obtained.

[0052] Performance Testing: The obtained solder underwent high-temperature oxidation weight gain testing (oxidation in air at 250℃ for 100h), electrochemical corrosion testing (3.5% NaCl solution, Tafel polarization curve), and interfacial shear strength testing (solder joint Φ=0.5mm, before aging). Compared with ordinary Sn-0.7Cu solder, the weight gain of the Sn-Cu lead-free solder prepared in this embodiment after oxidation in air at 250℃ for 100h was found to be 0.97 mg / cm³. 2 The corrosion current density is 1.12 μA / cm. 2 The solder shear strength is 33.7 MPa, and the weight gain of ordinary Sn-0.7Cu solder after oxidation in air at 250℃ for 100 hours is 2.85 mg / cm³. 2 The corrosion current density is 12.4 μA / cm. 2 The solder shear strength is 28.3 MPa. The Sn-Cu lead-free solder prepared by the method of this invention has significantly reduced high-temperature oxidation weight gain and corrosion current density, significantly improved solder joint shear strength, continuous and dense MgO self-repairing layer on the solder surface, and uniform distribution of TiO2 particles at grain boundaries and interfaces.

[0053] Example 3 A method for preparing self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction, specifically including the following steps: (1) Matrix melting: Weigh the raw materials of Sn-Cu lead-free solder according to the proportion. The composition of Sn-Cu lead-free solder includes, by mass percentage: 0.76% Cu, 0.08% Mg, 0.12% Ti precursor (tetrabutyl titanate), with the balance being Sn and unavoidable impurities, the impurity content being ≤0.05%. Under an argon atmosphere, put the Sn and Cu raw materials into a graphite crucible, heat to 465℃, and hold for 25 min to obtain Sn-Cu melt.

[0054] (2) Introduction of Mg raw material and Ti precursor: Mg raw material is added to Sn-Cu melt, and low-frequency electromagnetic stirring is applied at a frequency of 30Hz, a magnetic induction intensity of 0.08T, and a stirring time of 15min to promote the uniform distribution of Mg in the melt. Then, a Ti precursor solution is prepared using tetrabutyl titanate and isopropanol. The content of tetrabutyl titanate in the Ti precursor solution is 8% by mass percentage. The Ti precursor solution is introduced into the melt by atomization spraying, and the spray flow rate of the Ti precursor solution is 2.5mL / min to obtain a composite melt.

[0055] (3) Micro-oxygen in-situ reaction: The composite melt is reacted in-situ for 12 minutes under a micro-oxygen melting atmosphere, wherein the oxygen content in the micro-oxygen melting atmosphere is 0.1% by volume percentage and the remainder is argon. The melt after in-situ reaction is obtained. Under this condition, Mg is preferentially oxidized to form the precursor of MgO self-repairing oxide layer. The Ti precursor is instantaneously decomposed at high temperature of the melt to generate nano TiO2 particles in-situ with an average particle size of about 25 nm.

[0056] (4) Ultrasonic-electromagnetic composite field treatment: An ultrasonic-electromagnetic composite field with an ultrasonic frequency of 30kHz, an ultrasonic power of 900W, and an electromagnetic field induction intensity of 0.15T is applied to the melt after in-situ reaction for 18 minutes. By utilizing the synergistic effect of ultrasonic cavitation and electromagnetic migration, the nano-TiO2 particles are promoted to migrate and agglomerate towards the grain boundary and intermetallic compound interface region, while driving Mg elements to accumulate on the surface of the melt, thus obtaining the melt after composite field treatment.

[0057] (5) Directional solidification: The molten metal after composite field treatment is poured into a water-cooled copper mold for directional solidification. The directional solidification temperature gradient is 45℃ / mm and the solidification rate is 75μm / s to obtain the solder after directional solidification. This step promotes the significant enrichment of Mg to the surface of the solder to form a composition gradient. At the same time, the nano-TiO2 particles are pinned to the solid-liquid interface and grain boundaries, refining the Sn matrix grains.

[0058] (6) Two-stage heat treatment: The solder after directional solidification is first subjected to a low-temperature pretreatment at 125℃ for 1.8h to promote the diffusion and segregation of Mg atoms to the surface grain boundaries and free surfaces, and to enhance the compactness of the MgO self-healing layer. Then, it is subjected to a high-temperature treatment at 190℃ for 0.9h to activate the migration and pinning of nano-TiO2 particles to the interface and grain boundary regions, forming a stable oxygen diffusion barrier layer. After cooling, a self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction is obtained.

[0059] Performance testing: The obtained solder underwent high-temperature oxidation weight gain testing (oxidation in air at 250℃ for 100h), electrochemical corrosion testing (3.5% NaCl solution, Tafel polarization curve), and interfacial shear strength testing (solder joint Φ=0.5mm, before aging). Compared with ordinary Sn-0.7Cu solder (before improvement), the Sn-Cu lead-free solder prepared in this embodiment showed a weight gain of 0.77 mg / cm³ after oxidation in air at 250℃ for 100h. 2 The corrosion current density is 0.62 μA / cm. 2 The solder shear strength is 35.1 MPa, and the weight gain of ordinary Sn-0.7Cu solder after oxidation in air at 250℃ for 100 hours is 2.85 mg / cm³. 2 The corrosion current density is 12.4 μA / cm. 2 The solder shear strength is 28.3 MPa. The Sn-Cu lead-free solder prepared by the method of this invention has significantly reduced high-temperature oxidation weight gain and corrosion current density, significantly improved solder joint shear strength, uniform and dense MgO self-repair layer on the solder surface, and fine distribution of TiO2 particles at grain boundaries and interfaces.

[0060] Comparative Example 1 A method for preparing Sn-Cu lead-free solder specifically includes the following steps: (1) Matrix melting: Weigh the raw materials of Sn-Cu lead-free solder according to the proportion. The composition of Sn-Cu lead-free solder includes 0.7% Cu, 0.16% Mg, and the balance is Sn and unavoidable impurities with an impurity content of ≤0.05% by mass percentage. Under an argon atmosphere, put the Sn and Cu raw materials into a graphite crucible, heat to 450℃, and hold for 30 min to obtain Sn-Cu melt.

[0061] (2) Introduction of Mg raw material: Add Mg raw material to Sn-Cu melt and apply low-frequency electromagnetic stirring at a frequency of 50Hz, a magnetic induction intensity of 0.15T and a stirring time of 10min to promote the uniform distribution of Mg in the melt and obtain composite melt.

[0062] (3) Micro-oxygen in-situ reaction: The composite melt is reacted in-situ for 10 min under a micro-oxygen melting atmosphere, wherein the oxygen content in the micro-oxygen melting atmosphere is 0.2% by volume percentage, and the remainder is argon gas, to obtain the melt after in-situ reaction.

[0063] (4) Ultrasonic-electromagnetic composite field treatment: An ultrasonic-electromagnetic composite field with an ultrasonic frequency of 25kHz, an ultrasonic power of 1000W, and an electromagnetic field induction intensity of 0.3T is applied to the melt after in-situ reaction for 15 minutes to obtain the melt after composite field treatment.

[0064] (5) Directional solidification: The molten metal after composite field treatment is poured into a water-cooled copper mold for directional solidification. The directional solidification temperature gradient is 60℃ / mm and the solidification rate is 50μm / s to obtain the directional solidified solder.

[0065] (6) Two-stage heat treatment: The solder after directional solidification is first subjected to low temperature pretreatment at 130℃ for 1.5h, and then subjected to high temperature treatment at 200℃ for 0.8h, and then cooled to obtain Sn-Cu lead-free solder.

[0066] Performance testing: The obtained solder underwent high-temperature oxidation weight gain testing (oxidation in air at 250℃ for 100h), electrochemical corrosion testing (3.5% NaCl solution, Tafel polarization curve), and interfacial shear strength testing (solder joint Φ=0.5mm, before aging). The tests showed that the Sn-Cu lead-free solder prepared in this comparative example had a weight gain of 1.56 mg / cm³ after oxidization in air at 250℃ for 100h. 2 The corrosion current density is 3.85 μA / cm. 2 The solder shear strength is 29.8 MPa.

[0067] The only difference between Comparative Example 1 and Example 1 is the absence of a Ti precursor (i.e., no in-situ generation of nano-TiO2 particles). The results show that while the oxidation weight gain and corrosion current density of Comparative Example 1 are superior to ordinary Sn-0.7Cu solder, they are significantly inferior to Example 1, and the solder joint shear strength is also lower than that of Example 1. This is because the Mg element in Comparative Example 1 can form a self-healing MgO oxide layer, which inhibits surface oxidation and corrosion; however, due to the lack of pinning and agglomeration of TiO2 particles at grain boundaries and intermetallic compound interfaces, an effective oxygen diffusion barrier layer cannot be formed, and oxygen and corrosive media can still diffuse inward along grain boundaries and phase interfaces, resulting in limited performance improvement. This comparative example demonstrates that the addition of a Ti precursor and the in-situ generation of TiO2 particles are key elements in constructing the complete gradient protection structure of this invention.

[0068] Comparative Example 2 A method for preparing Sn-Cu lead-free solder specifically includes the following steps: (1) Matrix melting: Weigh the raw materials of Sn-Cu lead-free solder according to the proportion. The composition of Sn-Cu lead-free solder includes 0.7% Cu, 0.08% Ti precursor (tetrabutyl titanate) by mass percentage, and the balance is Sn and unavoidable impurities with impurity content ≤0.05%. Under argon atmosphere, Sn and Cu raw materials are placed in a graphite crucible, heated to 450℃, and held for 30 min to obtain Sn-Cu melt.

[0069] (2) Introduction of Ti precursor: Ti precursor solution was prepared by using tetrabutyl titanate and anhydrous ethanol. The content of tetrabutyl titanate in the Ti precursor solution was 5% by mass percentage. The Ti precursor solution was introduced into Sn-Cu melt by atomization spraying. The spray flow rate of the Ti precursor solution was 2 mL / min to obtain composite melt.

[0070] (3) Micro-oxygen in-situ reaction: The composite melt is reacted in-situ for 10 min under a micro-oxygen melting atmosphere, wherein the oxygen content in the micro-oxygen melting atmosphere is 0.2% by volume percentage, and the remainder is argon gas, to obtain the melt after in-situ reaction.

[0071] (4) Ultrasonic-electromagnetic composite field treatment: An ultrasonic-electromagnetic composite field with an ultrasonic frequency of 25kHz, an ultrasonic power of 1000W, and an electromagnetic field induction intensity of 0.3T is applied to the melt after in-situ reaction for 15 minutes to obtain the melt after composite field treatment.

[0072] (5) Directional solidification: The molten metal after composite field treatment is poured into a water-cooled copper mold for directional solidification. The directional solidification temperature gradient is 60℃ / mm and the solidification rate is 50μm / s to obtain the directional solidified solder.

[0073] (6) Two-stage heat treatment: The solder after directional solidification is first subjected to low temperature pretreatment at 130℃ for 1.5h, and then subjected to high temperature treatment at 200℃ for 0.8h, and then cooled to obtain Sn-Cu lead-free solder.

[0074] Performance testing: The obtained solder underwent high-temperature oxidation weight gain testing (oxidation in air at 250℃ for 100h), electrochemical corrosion testing (3.5% NaCl solution, Tafel polarization curve), and interfacial shear strength testing (solder joint Φ=0.5mm, before aging). The tests showed that the Sn-Cu lead-free solder prepared in this comparative example had a weight gain of 1.21 mg / cm³ after oxidization in air at 250℃ for 100h. 2 The corrosion current density is 2.85 μA / cm. 2 The shear strength of the solder is 33.1 MPa.

[0075] The only difference between Comparative Example 2 and Example 1 is the absence of Mg. The results show that while the oxidation weight gain and corrosion current density of Comparative Example 2 are better than those of ordinary Sn-0.7Cu solder, they are significantly worse than those of Example 1, and the solder joint shear strength is slightly lower than that of Example 1. This is because the TiO2 particles in Comparative Example 2 can agglomerate at grain boundaries and intermetallic compound interfaces to form an oxygen diffusion barrier layer, effectively inhibiting oxygen diffusion inward along grain boundaries and providing pinning reinforcement to the interface. However, due to the lack of Mg, a dense MgO self-healing oxide layer cannot be formed on the solder surface, resulting in insufficient surface oxidation and corrosion resistance, making it easier for oxygen and corrosive media to penetrate from the surface. This comparative example demonstrates that the addition of Mg and the in-situ formation of a MgO self-healing layer are key elements in constructing the complete gradient protection structure of this invention and achieving excellent surface protection performance.

[0076] Comparative Example 3 A method for preparing Sn-Cu lead-free solder specifically includes the following steps: (1) Matrix melting: Weigh the raw materials of Sn-Cu lead-free solder according to the proportion. The composition of Sn-Cu lead-free solder includes, by mass percentage: 0.7% Cu, 0.16% Mg, 0.08% Ti precursor (tetrabutyl titanate), and the balance is Sn and unavoidable impurities with an impurity content ≤0.05%. Under an argon atmosphere, put the Sn and Cu raw materials into a graphite crucible, heat to 450℃, and hold for 30 min to obtain Sn-Cu melt.

[0077] (2) Introduction of Mg raw material and Ti precursor: Mg raw material is added to Sn-Cu melt, and low-frequency electromagnetic stirring is applied at a frequency of 50Hz, a magnetic induction intensity of 0.15T, and a stirring time of 10min to promote the uniform distribution of Mg in the melt. Then, a Ti precursor solution is prepared by tetrabutyl titanate and anhydrous ethanol. The content of tetrabutyl titanate in the Ti precursor solution is 5% by mass percentage. The Ti precursor solution is introduced into the melt by atomization spraying, and the spray flow rate of the Ti precursor solution is 2mL / min to obtain a composite melt.

[0078] (3) Micro-oxygen in-situ reaction: The composite melt is reacted in-situ for 10 min under a micro-oxygen melting atmosphere, wherein the oxygen content in the micro-oxygen melting atmosphere is 0.2% by volume percentage and the remainder is argon. The melt after in-situ reaction is obtained. Under this condition, Mg is preferentially oxidized to form the precursor of MgO self-repairing oxide layer. The Ti precursor is instantaneously decomposed at high temperature of the melt to generate nano TiO2 particles in-situ with an average particle size of about 45 nm.

[0079] (4) Ultrasonic-electromagnetic composite field treatment: An ultrasonic-electromagnetic composite field with an ultrasonic frequency of 25kHz, an ultrasonic power of 1000W, and an electromagnetic field induction intensity of 0.3T is applied to the melt after in-situ reaction for 15 minutes. By utilizing the synergistic effect of ultrasonic cavitation and electromagnetic migration, the nano-TiO2 particles are promoted to migrate and agglomerate towards the grain boundary and intermetallic compound interface region, while driving Mg elements to accumulate on the surface of the melt, thus obtaining the melt after composite field treatment.

[0080] (5) Directional solidification: The molten metal after composite field treatment is poured into a water-cooled copper mold for directional solidification. The directional solidification temperature gradient is 60℃ / mm and the solidification rate is 50μm / s to obtain the solder after directional solidification. This step promotes the significant enrichment of Mg to the surface of the solder to form a composition gradient. At the same time, the nano-TiO2 particles are pinned to the solid-liquid interface and grain boundaries, refining the Sn matrix grains.

[0081] (6) Single-stage aging heat treatment: The solder after directional solidification is subjected to single-stage aging heat treatment at 165℃ for 2.3h, and then cooled with the furnace to obtain Sn-Cu lead-free solder.

[0082] Performance testing: The obtained solder underwent high-temperature oxidation weight gain testing (oxidation in air at 250℃ for 100h), electrochemical corrosion testing (3.5% NaCl solution, Tafel polarization curve), and interfacial shear strength testing (solder joint Φ=0.5mm, before aging). The tests showed that the Sn-Cu lead-free solder prepared in this comparative example had a weight gain of 1.15 mg / cm³ after oxidization in air at 250℃ for 100h. 2 The corrosion current density is 1.95 μA / cm. 2 The solder shear strength is 32.3 MPa.

[0083] The only difference between Comparative Example 3 and Example 1 is the heat treatment: Comparative Example 3 uses single-stage aging (165℃ / 2.3h), while Example 1 uses two-stage heat treatment (130℃ / 1.5h + 200℃ / 0.8h). The results show that the oxidation weight gain rate and corrosion current density of Comparative Example 3 are higher than those of Example 1, while the weld shear strength is lower. This is because single-stage aging cannot achieve the stepwise control of enriching Mg first and then pinning TiO2. Mg atoms and TiO2 particles compete for diffusion at the same temperature, resulting in insufficient enrichment of Mg to the surface and incomplete segregation of TiO2 to the interface, leading to defects in the protective structure. In contrast, the two-stage heat treatment preferentially promotes Mg segregation at low temperatures and drives TiO2 migration at high temperatures, making the gradient structure more complete and continuous. This comparative example demonstrates that the two-stage heat treatment proposed in this invention plays a crucial role in obtaining excellent oxidation resistance, corrosion resistance, and mechanical properties.

[0084] Comparative Example 4 A method for preparing Sn-Cu lead-free solder specifically includes the following steps: (1) Matrix melting: Weigh the raw materials of Sn-Cu lead-free solder according to the proportion. The composition of Sn-Cu lead-free solder includes, by mass percentage: 0.7% Cu, 0.16% Mg, 0.08% Ti precursor (tetrabutyl titanate), and the balance is Sn and unavoidable impurities with an impurity content ≤0.05%. Under an argon atmosphere, put the Sn and Cu raw materials into a graphite crucible, heat to 450℃, and hold for 30 min to obtain Sn-Cu melt.

[0085] (2) Introduction of Mg raw material and Ti precursor: Mg raw material is added to Sn-Cu melt, and low-frequency electromagnetic stirring is applied at a frequency of 50Hz, a magnetic induction intensity of 0.15T, and a stirring time of 10min to promote the uniform distribution of Mg in the melt. Then, a Ti precursor solution is prepared by tetrabutyl titanate and anhydrous ethanol. The content of tetrabutyl titanate in the Ti precursor solution is 5% by mass percentage. The Ti precursor solution is introduced into the melt by atomization spraying, and the spray flow rate of the Ti precursor solution is 2mL / min to obtain a composite melt.

[0086] (3) Micro-oxygen in-situ reaction: The composite melt is reacted in-situ for 10 min under a micro-oxygen melting atmosphere, wherein the oxygen content in the micro-oxygen melting atmosphere is 0.2% by volume percentage and the remainder is argon. The melt after in-situ reaction is obtained. Under this condition, Mg is preferentially oxidized to form the precursor of MgO self-repairing oxide layer. The Ti precursor is instantaneously decomposed at high temperature of the melt to generate nano TiO2 particles in-situ with an average particle size of about 45 nm.

[0087] (4) Directional solidification: The molten material after in-situ reaction is cast into a water-cooled copper mold for directional solidification. The directional solidification temperature gradient is 60℃ / mm and the solidification rate is 50μm / s to obtain the solder after directional solidification. This step promotes the significant enrichment of Mg to the surface of the solder to form a composition gradient. At the same time, the nano-TiO2 particles are pinned to the solid-liquid interface and grain boundaries, refining the Sn matrix grains.

[0088] (5) Two-stage heat treatment: The solder after directional solidification is first subjected to a low-temperature pretreatment at 130℃ for 1.5h to promote the diffusion and segregation of Mg atoms to the surface grain boundaries and free surface, and to strengthen the compactness of the MgO self-repairing layer. Then, it is subjected to a high-temperature treatment at 200℃ for 0.8h to activate the migration and pinning of nano TiO2 particles to the interface and grain boundary region, forming a stable oxygen diffusion barrier layer. After cooling, Sn-Cu lead-free solder is obtained.

[0089] Performance testing: The obtained solder underwent high-temperature oxidation weight gain testing (oxidation in air at 250℃ for 100h), electrochemical corrosion testing (3.5% NaCl solution, Tafel polarization curve), and interfacial shear strength testing (solder joint Φ=0.5mm, before aging). The tests showed that the Sn-Cu lead-free solder prepared in this comparative example had a weight gain of 1.32 mg / cm³ after oxidization in air at 250℃ for 100h. 2 The corrosion current density is 2.41 μA / cm. 2 The shear strength of the solder is 31.8 MPa.

[0090] The only difference between Comparative Example 4 and Example 1 is the absence of ultrasonic-electromagnetic composite field treatment. The results show that the oxidation weight gain and corrosion current density of Comparative Example 4 are significantly higher than those of Example 1, while the weld shear strength is lower. This is because the lack of ultrasonic-electromagnetic composite field treatment results in the absence of the synergistic effect of ultrasonic cavitation and electromagnetic migration, leading to uneven dispersion of nano-TiO2 particles in the melt. These particles cannot effectively migrate and agglomerate towards grain boundaries and intermetallic compound interfaces, and the enrichment effect of Mg on the melt surface is significantly weakened. Simultaneously, the lack of a uniform particle distribution makes it difficult to construct a complete gradient protection structure through subsequent directional solidification and two-stage heat treatment. This comparative example demonstrates that ultrasonic-electromagnetic composite field treatment plays a crucial role in promoting the directional agglomeration of TiO2 particles and the enrichment of Mg on the surface, and in constructing a complete gradient protection structure.

[0091] Comparative Example 5 A method for preparing Sn-Cu lead-free solder specifically includes the following steps: (1) Matrix melting: Weigh the raw materials of Sn-Cu lead-free solder according to the proportion. The composition of Sn-Cu lead-free solder includes, by mass percentage: 0.7% Cu, 0.16% Mg, 0.08% Ti precursor (tetrabutyl titanate), and the balance is Sn and unavoidable impurities with an impurity content ≤0.05%. Under an argon atmosphere, put the Sn and Cu raw materials into a graphite crucible, heat to 450℃, and hold for 30 min to obtain Sn-Cu melt.

[0092] (2) Introduction of Mg raw material and Ti precursor: Mg raw material is added to Sn-Cu melt, and low-frequency electromagnetic stirring is applied at a frequency of 50Hz, a magnetic induction intensity of 0.15T, and a stirring time of 10min to promote the uniform distribution of Mg in the melt. Then, a Ti precursor solution is prepared by tetrabutyl titanate and anhydrous ethanol. The content of tetrabutyl titanate in the Ti precursor solution is 5% by mass percentage. The Ti precursor solution is introduced into the melt by atomization spraying, and the spray flow rate of the Ti precursor solution is 2mL / min to obtain a composite melt.

[0093] (3) Micro-oxygen in-situ reaction: The composite melt is reacted in-situ for 10 min under a micro-oxygen melting atmosphere, wherein the oxygen content in the micro-oxygen melting atmosphere is 0.2% by volume percentage and the remainder is argon. The melt after in-situ reaction is obtained. Under this condition, Mg is preferentially oxidized to form the precursor of MgO self-repairing oxide layer. The Ti precursor is instantaneously decomposed at high temperature of the melt to generate nano TiO2 particles in-situ with an average particle size of about 45 nm.

[0094] (4) Ultrasonic-electromagnetic composite field treatment: An ultrasonic-electromagnetic composite field with an ultrasonic frequency of 25kHz, an ultrasonic power of 1000W, and an electromagnetic field induction intensity of 0.3T is applied to the melt after in-situ reaction for 15 minutes. By utilizing the synergistic effect of ultrasonic cavitation and electromagnetic migration, the nano-TiO2 particles are promoted to migrate and agglomerate towards the grain boundary and intermetallic compound interface region, while driving Mg elements to accumulate on the surface of the melt, thus obtaining the melt after composite field treatment.

[0095] (5) Casting: The molten metal after composite field treatment is cast into a water-cooled copper mold, and then naturally cooled and solidified to obtain solidified solder.

[0096] (6) Two-stage heat treatment: The solidified solder is first pretreated at 130℃ for 1.5h to promote the diffusion and segregation of Mg atoms to the surface grain boundary and free surface, and to strengthen the compactness of the MgO self-healing layer. Then, it is pretreated at 200℃ for 0.8h to activate the migration and pinning of nano TiO2 particles to the interface and grain boundary region, forming a stable oxygen diffusion barrier layer. After cooling, the self-healing gradient protection Sn-Cu lead-free solder based on the in-situ interface is obtained.

[0097] Performance testing: The obtained solder underwent high-temperature oxidation weight gain testing (oxidation in air at 250℃ for 100h), electrochemical corrosion testing (3.5% NaCl solution, Tafel polarization curve), and interfacial shear strength testing (solder joint Φ=0.5mm, before aging). The tests showed that the Sn-Cu lead-free solder prepared in this comparative example had a weight gain of 1.26 mg / cm³ after oxidization in air at 250℃ for 100h. 2 The corrosion current density is 2.23 μA / cm. 2 The shear strength of the solder is 32.0 MPa.

[0098] The only difference between Comparative Example 5 and Example 1 is the absence of directional solidification treatment. The results show that the oxidation weight gain and corrosion current density of Comparative Example 5 are significantly higher than those of Example 1, while the solder joint shear strength is lower. This is because the lack of directional solidification treatment prevents the formation of a significant enrichment of Mg elements on the solder surface driven by a directional temperature gradient, resulting in a discontinuous and insufficiently thick MgO self-healing layer. Simultaneously, the nano-TiO2 particles cannot be effectively moved and pinned at the solid-liquid interface and grain boundaries, resulting in poor particle distribution uniformity and difficulty in forming a continuous oxygen diffusion barrier layer. Furthermore, under natural cooling conditions, the Sn matrix grains coarsen, reducing internal toughness. This comparative example demonstrates that directional solidification treatment plays a crucial role in promoting Mg surface enrichment, TiO2 interface pinning and grain refinement, and constructing a complete gradient protection structure.

[0099] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction, characterized in that, The raw material composition of the Sn-Cu lead-free solder, by mass percentage, includes: 0.6~0.8% Cu, 0.02~0.3% Mg, 0.01~0.15% Ti precursor, with the balance being Sn and unavoidable impurities.

2. The self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction according to claim 1, characterized in that, The Ti precursor is one or more of tetrabutyl titanate, isopropyl titanate, or an organotitanium complex.

3. The method for preparing self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction as described in claim 1, characterized in that, Specifically, the following steps are included: (1) Matrix melting: Sn and Cu raw materials are melted in an inert atmosphere to obtain Sn-Cu melt; (2) Introduction of Mg raw material and Ti precursor: Mg raw material is added to Sn-Cu melt and electromagnetic assisted melting is carried out. Then Ti precursor solution is atomized and sprayed into the melt to obtain composite melt. (3) Micro-oxygen in-situ reaction: The composite melt undergoes an in-situ reaction under a micro-oxygen smelting atmosphere to obtain the melt after the in-situ reaction; (4) Ultrasonic-electromagnetic composite field treatment: The melt after in-situ reaction is subjected to ultrasonic-electromagnetic composite field treatment to obtain the melt after composite field treatment; (5) Directional solidification: The molten metal after composite field treatment is cast and directionally solidified to obtain directionally solidified solder; (6) Two-stage heat treatment: The solder after directional solidification is subjected to low-temperature pretreatment and high-temperature treatment in sequence, and then cooled to obtain self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction.

4. The method for preparing self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction according to claim 3, characterized in that, The conditions for electromagnetic assisted melting in step (2) are: applying low-frequency electromagnetic stirring with a frequency of 20~80Hz, a magnetic induction intensity of 0.05~0.3T, and a stirring time of 10~20min.

5. The method for preparing self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction according to claim 3, characterized in that, In step (2), the solvent of the Ti precursor solution is an organic solvent, and the content of Ti precursor in the Ti precursor solution is 2-10% by mass percentage; the atomization spray flow rate of the Ti precursor solution is 1-3 mL / min.

6. The method for preparing self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction according to claim 3, characterized in that, In step (3), the oxygen content in the micro-oxygen melting atmosphere is 0.05-0.5% by volume; the in-situ reaction time is 5-15 min.

7. The method for preparing self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction according to claim 3, characterized in that, The conditions for the ultrasonic-electromagnetic composite field treatment in step (4) are: ultrasonic frequency of 20~40kHz, ultrasonic power of 800~1500W, electromagnetic field strength of 0.05~0.5T, and treatment time of 10~20min.

8. The method for preparing self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction according to claim 3, characterized in that, The conditions for directional solidification in step (5) are: directional solidification temperature gradient of 30~100℃ / mm and solidification rate of 10~100μm / s.

9. The method for preparing self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction according to claim 3, characterized in that, The conditions for the low-temperature pretreatment in step (6) are: keep warm at 120~150℃ for 1~2 hours.

10. The method for preparing self-healing gradient protection Sn-Cu lead-free solder based on in-situ interface construction according to claim 3, characterized in that, The conditions for the high-temperature treatment in step (6) are: heat preservation at 180~220℃ for 0.5~1h.

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