Method for improving the oxidation resistance of Sn-Cu lead-free solder by external field induced in-situ reaction

Abstract

This invention discloses a method for improving the oxidation resistance of Sn-Cu lead-free solder through an external field-induced in-situ reaction, belonging to the field of lead-free solder technology for electronic packaging. The solder uses Sn-0.7Cu as a matrix, with trace amounts of Al and nano-CeO2 particles added. Through ultrasonic cavitation dispersion, rapid solidification, magnetic field-assisted isothermal annealing, and directional aging treatment, Al and CeO2 are induced to react in situ at the interface to form a stable Ce-Al-O composite phase. Under external field drive, the nanoparticles and alloying elements are directionally agglomerated, constructing a multi-level gradient anti-oxidation structure in the solder, consisting of a surface self-healing Al2O3 oxide film layer, a sub-surface Ce-Al-O diffusion barrier layer, a grain boundary CeO2 / intermetallic compound interface pinning layer, and an internal fine-grained stabilizing layer. This structure can achieve multi-level synergistic control of oxygen diffusion under high-temperature conditions through surface blocking, grain boundary blocking, and bulk diffusion inhibition. At the same time, Al can continuously diffuse to the surface to form a regenerated oxide film, giving the solder self-healing anti-oxidation ability and inhibiting the coarsening of Cu6Sn5 intermetallic compounds, thus improving the solder's high-temperature welding performance.

Description

Technical Field

[0001] This invention relates to the field of lead-free solder technology for electronic packaging, and specifically to a method for improving the oxidation resistance of Sn-Cu lead-free solder through an external field-induced in-situ reaction. Background Technology

[0002] As electronic packaging evolves towards higher power and higher reliability, solder is prone to oxidation during high-temperature soldering and long-term service, leading to decreased wettability, interface failure, and reduced solder joint reliability. Sn-Cu lead-free solder is widely used due to its low cost and environmental friendliness, but it easily forms a loose SnO2 oxide layer under high-temperature conditions, making it difficult to effectively block oxygen diffusion. Existing technologies typically improve oxidation resistance by adding trace elements or nanoparticles, but adding a single element cannot simultaneously achieve surface oxide film stability, grain boundary diffusion inhibition, and internal structure stability. Most solders still suffer from problems such as rapid oxygen diffusion, discontinuous oxide films, and coarsening of intermetallic compounds.

[0003] To improve the oxidation resistance of tin-based solders, existing technologies mainly employ alloying methods, which improve surface oxidation behavior by adding trace elements. For example, adding trace amounts of Ni, P, and other elements can form a dense oxide layer on the surface, but this is often accompanied by problems such as decreased wettability or precipitation of brittle phases. Adding rare earth elements such as La, Ce, and their oxides can purify grain boundaries and refine the microstructure, but rare earth elements are chemically reactive and easily burned off during the smelting process, and adding them alone has limited effect on improving the continuity of the surface oxide film.

[0004] Currently, tin-based lead-free solders are widely used in the electronic packaging industry due to their environmental friendliness and low melting point. However, during high-temperature soldering and long-term service, the surface of the solder alloy is prone to oxidation, forming a loose oxide layer, which leads to a decrease in solder joint reliability. Therefore, improving the oxidation resistance of tin-based lead-free solders is a current focus of the industry and is also the problem that this invention aims to solve. Therefore, it is necessary to develop a Sn-Cu solder that can form a multi-level gradient anti-oxidation structure through external field control and in-situ reaction, in order to achieve multi-level blocking of oxygen diffusion and improve high-temperature service stability. Summary of the Invention

[0005] This invention regulates the interface structure of Sn-Cu solder through the synergistic effect of Al and nano-CeO2 particles. During the melting process, Al dissolves uniformly in the Sn-Cu melt and preferentially forms a stable Al-O oxide film under subsequent high-temperature conditions, improving the oxidation resistance of the solder surface. The nano-CeO2 particles are uniformly dispersed through ultrasonic cavitation treatment and undergo rapid solidification (cooling rate 10). 3 ~10 6Under conditions of K / s, the solder aggregates at grain boundaries and intermetallic compound interfaces, and further enriches itself during subsequent magnetic field-assisted isothermal annealing and directional aging, forming an oxygen diffusion barrier layer. Through rapid solidification and rapid cooling (50~150℃ / s) after directional aging, a CeO2-enriched layer forms on the solder surface, and a refined internal microstructure is formed, thereby constructing a gradient anti-oxidation structure and improving the solder's oxidation resistance and interfacial stability. The specific processing steps are as follows: (1) Heat pure Sn to 350~500℃ to melt it, add Cu to form Sn-Cu melt, then add Al and stir for 10~20 min to make it uniformly dissolved; slowly add CeO2 nanoparticles into the melt while stirring slightly to obtain a mixed melt.

[0006] (2) Place the mixed melt in an ultrasonic field to make the nano CeO2 particles uniformly dispersed.

[0007] (3) The ultrasonically treated melt is subjected to rapid solidification treatment to obtain a rapid solidification solder ingot.

[0008] (4) The solder ingot is placed in a magnetic field-assisted isothermal treatment device for annealing. The annealed solder is then subjected to directional aging treatment. After aging, it is rapidly cooled to room temperature.

[0009] Preferably, the raw materials of the Sn-Cu lead-free solder of the present invention include: Cu: 0.6~0.8 wt%; Al: 0.01~0.15 wt%; Nano CeO2 particles: 0.01~0.08 wt%; balance Sn and unavoidable impurities.

[0010] Preferably, the CeO2 particles of the present invention have a particle size of 20~80nm.

[0011] Preferably, the ultrasonic power of the ultrasonic cavitation treatment of the present invention is 800~1500 W, the frequency is 20~40 kHz, and the time is 10~20 min.

[0012] Preferably, the rapid solidification cooling rate of the present invention is 10. 3 ~10 6 K / s.

[0013] Preferably, the magnetic field-assisted isothermal annealing temperature of the present invention is 150~220℃, the magnetic field strength is 0.5~10 T to apply a temperature gradient, and the holding time is 60~120 min.

[0014] Preferably, the aging temperature of the present invention is 120~200℃, and the heat preservation time is 30~120 min.

[0015] Preferably, the present invention provides rapid cooling at 50~150℃ / s.

[0016] Another objective of this invention is to provide a Sn-Cu lead-free solder prepared by the method described above, wherein the solder forms a multi-level gradient anti-oxidation structure, including: a surface self-healing Al2O3 oxide film layer; a surface Ce-Al-O composite diffusion barrier layer; a grain boundary CeO2 / intermetallic compound interface pinning layer; and an internal fine-grained stabilizing layer; the gradient structure is induced by magnetic field-assisted isothermal heat treatment, enabling the solder to have self-healing anti-oxidation ability and oxygen diffusion inhibition ability under high temperature conditions; the Ce-Al-O composite phase is a stable oxide phase formed by the in-situ reaction of CeO2 and Al elements during heat treatment; the Al2O3 oxide film can be continuously formed and has regeneration ability under high temperature environment; the CeO2 particles agglomerate towards the grain boundary and Cu6Sn5 interface during rapid solidification and external field heat treatment to form a diffusion barrier layer.

[0017] The principle of this invention is as follows: During rapid solidification, nano-CeO2 particles preferentially segregate towards the grain boundary region, effectively pinning the grain boundaries. Simultaneously, driven by the coupling of the magnetic field and temperature gradient, Al elements undergo directional migration to the surface and react in situ with the segregated CeO2 to generate a Ce-Al-O composite phase with high thermal stability. Ultimately, a gradient multi-level barrier structure is formed inside the material: the outermost layer is a continuous and dense Al2O3 oxide barrier, the next layer is a Ce-Al-O particle reinforcement layer, the grain boundaries are filled with dispersed CeO2 phase, and the interior of the matrix is ​​a fine-grained reinforced structure. This structure achieves efficient stepwise inhibition of oxygen diffusion through a dual mechanism of physical barrier and chemical stabilization.

[0018] Beneficial effects of the present invention (1) Constructing a multi-level gradient antioxidant structure: Constructing a multi-level gradient antioxidant structure consisting of a dense surface oxide film, a subsurface reaction barrier phase, a grain boundary dispersion strengthening phase, and an internal fine-grained structure to achieve a step-by-step barrier to oxygen diffusion from the surface to the interior.

[0019] (2) Formation of in-situ reaction composite phase: Al elements migrate directionally to the surface under the drive of magnetic field and react in-situ with enriched nano CeO2 to generate Ce-Al-O composite reinforced phase with high thermal stability.

[0020] (3) It has a self-healing oxide film: When the Al2O3 oxide film formed on the surface is damaged during high-temperature service, it can achieve dynamic self-healing by utilizing the continuous diffusion of residual Al elements in the matrix, thus giving the material long-term antioxidant capacity.

[0021] (4) Suppressing IMC coarsening: During the rapid solidification process, nano CeO2 particles agglomerate towards the grain boundaries and Cu6Sn5 interface region, forming a diffusely distributed pinned structure, which effectively hinders grain boundary migration and Cu6Sn5 growth coarsening; at the same time, the trace solid solution of Al element in the lattice further stabilizes the interface structure and significantly suppresses the excessive growth of intermetallic compounds during high-temperature aging.

[0022] (5) Improve high temperature stability: Through the inhibition of oxygen diffusion, the pinning of grain boundary migration and the stabilization of interface reaction by the multi-level gradient structure, the structural stability and mechanical property retention of the solder under high temperature aging are synergistically improved. Attached Figure Description

[0023] Figure 1 A schematic diagram of the improved nano-segregation and rapid solidification brazing filler metal preparation method of this invention; Figure 2 Schematic diagram of gradient oxidation structure. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the content described.

[0025] Example 1 This embodiment provides a Sn-Cu lead-free solder with an external field-induced in-situ reaction gradient antioxidant structure, the chemical composition of which is shown in Table 1.

[0026] Table 1 Chemical composition of tin-based solder alloy in Example 1 The specific processing steps are as follows: (1) Smelting: Add pure Sn to a graphite crucible and heat it to 420°C to melt it. Then add pure Cu particles and stir to completely dissolve them to form Sn-Cu melt.

[0027] (2) Add Al: Add pure Al sheets to Sn-Cu melt and mechanically stir at 450℃ for 15 min to dissolve Al uniformly.

[0028] (3) Add CeO2: Slowly add CeO2 nanoparticles with a particle size of 40~60 nm into the melt while stirring gently to prevent particle agglomeration.

[0029] (4) Ultrasonic cavitation treatment: The melt is placed in an ultrasonic field with an ultrasonic power of 1200 W, a frequency of 30 kHz, and a treatment time of 15 min to make the nano CeO2 particles uniformly dispersed.

[0030] (5) Rapid solidification: The ultrasonically treated melt is poured into a water-cooled copper mold, with a cooling rate of approximately 5 × 10⁻⁶.3 K / s, to obtain rapidly solidified solder ingots.

[0031] (6) Magnetic field-assisted isothermal annealing: The solder ingot is placed in a magnetic field-assisted isothermal treatment device, the annealing temperature is set to 200℃, the magnetic field strength is 5 T, and a temperature gradient (temperature difference of about 50℃) is applied along the sample axis, and the holding time is 60 min.

[0032] (7) Directional aging: The annealed solder is subjected to directional aging treatment at 160℃ for 60 min.

[0033] (8) Rapid cooling: After aging, the solder is rapidly cooled to room temperature at a cooling rate of 50℃ / s.

[0034] The Sn-Cu solder prepared by the above process has a microstructure with obvious multi-level gradient characteristics. A continuous and dense Al2O3 oxide film with a thickness of about 50~80 nm is formed on the surface, and the film is uniform and complete. Fine Ce-Al-O composite phase particles are distributed in the subsurface region with a thickness of about 5~10 μm, which are well bonded to the matrix. In the grain boundary region, nano CeO2 particles are dispersed along the grain boundary, forming an obvious grain boundary pinning layer. The grain size inside the matrix is ​​small and uniform, with an average grain size of about 2~5 μm, and no obvious Cu6Sn5 coarsening phenomenon is observed.

[0035] Antioxidant performance: When the solder is kept at 260°C in air for 30 minutes, the thickness of the oxide film on the surface only increases by about 15%, which shows excellent antioxidant ability.

[0036] Self-healing performance: After surface scratches, the area was aged at 200℃ for 1 h, and the scratched area was covered by a newly formed Al2O3 film, indicating that it has good self-healing ability.

[0037] IMC suppression effect: After aging at 200℃ for 100 h, the thickness of the Cu6Sn5 layer at the interface is about 2.8 μm, which is 50% less than that of the unmodified Sn-0.7Cu solder (about 5.6 μm).

[0038] High temperature stability: After aging at 200℃ for 500 h, the solder microstructure showed no obvious coarsening, and the hardness retention rate was >85%.

[0039] This embodiment demonstrates that by optimizing the alloy composition and process parameters, a multi-level gradient anti-oxidation structure consisting of a surface Al2O3 self-healing film, a subsurface Ce-Al-O barrier layer, a grain boundary CeO2 pinning layer, and an internal fine-grained structure was successfully constructed, significantly improving the high-temperature anti-oxidation performance and interface stability of the solder.

[0040] Example 2 This embodiment provides a Sn-Cu lead-free solder with an external field-induced in-situ reaction gradient antioxidant structure, the chemical composition of which is shown in Table 2.

[0041] Table 2 Chemical composition of tin-based solder alloy in Example 2 The specific processing steps are as follows: (1) Smelting: Add pure Sn to a graphite crucible, heat to 380℃ to melt, add pure Cu particles, stir to completely dissolve, and form Sn-Cu melt.

[0042] (2) Add Al: Add pure Al sheets to Sn-Cu melt and mechanically stir at 400℃ for 10 min to dissolve Al uniformly.

[0043] (3) Add CeO2: Slowly add CeO2 nanoparticles with a particle size of 20~40 nm into the melt while stirring gently.

[0044] (4) Ultrasonic cavitation treatment: The melt is ultrasonically treated with a power of 1000 W, a frequency of 30 kHz, and a time of 20 min to make the nano CeO2 particles uniformly dispersed.

[0045] (5) Rapid solidification: The ultrasonically treated melt is cast onto a rotating copper roller for rapid quenching by strip spinning, with a cooling rate of approximately 1×10⁻⁶. 6 K / s, to obtain rapidly solidified solder strips.

[0046] (6) Magnetic field-assisted isothermal annealing: The solder strip is placed in a magnetic field-assisted isothermal treatment device, the annealing temperature is set to 150℃, the magnetic field strength is 0.5 T, and a temperature gradient (temperature difference of about 30℃) is applied along the thickness direction of the sample, and the holding time is 90 min.

[0047] (7) Directional aging: The annealed solder is subjected to directional aging treatment at 120℃ for 120 min.

[0048] (8) Rapid cooling: After aging, the solder is rapidly cooled to room temperature at a cooling rate of 120℃ / s.

[0049] The Sn-Cu solder prepared by the above process has the following microstructural characteristics: a continuous Al2O3 oxide film with a thickness of about 30-50 nm is formed on the surface, and the film is uniform and dense; fine Ce-Al-O composite phase particles with a thickness of about 3-8 μm are distributed in the subsurface layer, with a particle size of about 50-100 nm, and are dispersed; in the grain boundary region, nano CeO2 particles agglomerate along the grain boundary to form a discontinuous pinning layer, which effectively hinders grain boundary migration; the grain size inside the matrix is ​​small, with an average grain size of about 1-3 μm, and the structure is uniform.

[0050] Antioxidant properties: After the solder was kept at 260℃ in air for 60 min, the thickness of the surface oxide film increased by approximately 22%, and the oxidation weight gain was 0.12 mg / cm³. 2 The concentration was significantly lower than that of unmodified Sn-0.7Cu solder (0.35 mg / cm³). 2 ).

[0051] Self-healing performance: After being scratched on the surface and aged at 200℃ for 2 hours, a new Al2O3 film is generated at the scratch, and the scratch depth is reduced by about 60%, demonstrating good self-healing ability.

[0052] IMC suppression effect: After aging at 200℃ for 200 h, the thickness of the Cu6Sn5 layer at the interface is about 3.2 μm, which is 53% less than that of the unmodified Sn-0.7Cu solder (about 6.8 μm).

[0053] High temperature stability: After aging at 200℃ for 300 h, the solder microstructure remained stable, grain growth was not obvious, and the microhardness decreased by only 12%.

[0054] This embodiment demonstrates that a multi-level gradient antioxidant structure can be successfully constructed using lower Al and CeO2 addition amounts, combined with optimized process parameters. This structure achieves excellent antioxidant performance and interface stability while maintaining good wettability, making it suitable for cost-sensitive electronic packaging applications requiring good solderability.

[0055] Example 3 This embodiment provides a Sn-Cu lead-free solder with an external field-induced in-situ reaction gradient antioxidant structure, the chemical composition of which is shown in Table 3.

[0056] Table 3 Chemical composition of tin-based solder alloy in Example 3 The specific processing steps are as follows: (1) Smelting: Add pure Sn to a graphite crucible, heat to 480℃ to melt, add pure Cu particles, stir to completely dissolve, and form Sn-Cu melt.

[0057] (2) Add Al: Add pure Al sheets to Sn-Cu melt and mechanically stir at 500℃ for 18 min to dissolve Al uniformly.

[0058] (3) Add CeO2: Slowly add CeO2 nanoparticles with a particle size of 60~80 nm into the melt while stirring gently.

[0059] (4) Ultrasonic cavitation treatment: The melt is placed in an ultrasonic field with an ultrasonic power of 1500 W, a frequency of 40 kHz, and a treatment time of 10 min to make the nano CeO2 particles uniformly dispersed.

[0060] (5) Rapid solidification: The ultrasonically treated melt is poured into a water-cooled copper mold and combined with a double-roller rapid cooling technology, with a cooling rate of approximately 8 × 10⁻⁶. 5 K / s, to obtain rapidly solidified solder flakes.

[0061] (6) Magnetic field-assisted isothermal annealing: The solder sheet is placed in a magnetic field-assisted isothermal treatment device, the annealing temperature is set to 220℃, the magnetic field strength is 10T, and a temperature gradient (temperature difference of about 70℃) is applied along the length of the sample. The holding time is 120 min.

[0062] (7) Directional aging: The annealed solder is subjected to directional aging treatment at 200℃ for 30 minutes.

[0063] (8) Rapid cooling: After aging, the solder is rapidly cooled to room temperature at a cooling rate of 150℃ / s.

[0064] The Sn-Cu solder prepared by the above process exhibits a significant multi-level gradient microstructure. A continuous, dense Al2O3 oxide film with a thickness of approximately 80–120 nm is formed on the surface layer. This film is uniform, intact, and firmly bonded to the substrate. In the subsurface layer, a large number of Ce-Al-O composite phase particles with a thickness of approximately 8–15 μm are distributed, with particle sizes of approximately 100–200 nm, exhibiting a banded enrichment distribution and forming a dense diffusion barrier layer. Meanwhile, nano-CeO2 particles in the grain boundary region are highly enriched at the grain boundaries, forming a continuous grain boundary pinning network that effectively blocks the rapid diffusion channels of oxygen along the grain boundaries. The grain size inside the substrate is small and uniform, with an average grain size of approximately 1–2 μm, exhibiting a dense structure without obvious defects.

[0065] Antioxidant properties: After the solder was kept at 260℃ in air for 120 min, the thickness of the surface oxide film increased by only about 18%, and the oxidation weight gain was 0.09 mg / cm³. 2 This is significantly lower than that of unmodified Sn-0.7Cu solder (0.48 mg / cm³). 2 (After being kept at 300℃ for 30 min under extreme conditions, the surface still maintains a metallic luster and there is no obvious oxidation or peeling.) Self-healing performance: After surface scratches, the area is aged at 250℃ for 1 h, and the scratches are completely covered by newly formed Al2O3 film, reducing the scratch depth by about 85%, demonstrating excellent dynamic self-healing ability. IMC suppression effect: After aging at 200℃ for 500 h, the thickness of the Cu6Sn5 layer at the interface is about 3.5 μm, which is 57% less than that of the unmodified Sn-0.7Cu solder (about 8.2 μm); after aging at 150℃ for 1000 h, the thickness of the Cu6Sn5 layer is only 4.1 μm, showing excellent long-term interface stability. High temperature stability: After aging at 200℃ for 800 h, the solder microstructure remains stable, grain growth is not obvious, the microhardness decrease rate is only 8%, and the tensile strength retention rate is >90%.

[0066] This embodiment demonstrates that by employing higher Al and CeO2 additions, combined with enhanced external field treatment conditions, a denser multi-level gradient anti-oxidation structure can be constructed. This structure maintains excellent anti-oxidation performance and self-healing capabilities even under extreme high-temperature conditions, significantly extending the high-temperature service life of solders. It is suitable for applications with extremely high reliability requirements, such as high-power-density electronic packaging, automotive electronics, and aerospace.

[0067] Comparative Example 1 This comparative example provides a conventional Sn-0.7Cu solder without added Al and nano CeO2 particles, prepared using a traditional casting process, for performance comparison with the embodiments of the present invention. Its chemical composition is shown in Table 4.

[0068] Table 4 Chemical composition of tin-based solder alloy in Comparative Example 1 The specific processing steps are as follows: (1) Melting: Add pure Sn to a graphite crucible, heat to 400℃ to melt, add pure Cu particles, stir to completely dissolve, and form Sn-Cu melt.

[0069] (2) Casting: The molten material is directly cast into a regular steel mold and allowed to cool naturally to room temperature at a rate of approximately 10°C. 2 K / s.

[0070] (3) Homogenization treatment: The ingot is kept at 180℃ for 60 min for homogenization treatment, and then air-cooled to room temperature.

[0071] The conventional Sn-0.7Cu solder prepared by the above process has the following microstructural characteristics: the naturally formed oxide film on the surface is mainly SnO2, with a thickness of about 200~500 nm. The film is loose and porous, with obvious cracks and peeling. There is no obvious composite phase formation in the subsurface layer, and the structure is a single β-Sn matrix. There are no nanoparticle pinning in the grain boundary region, and the grain boundaries are clearly visible. The grains inside the matrix are coarse, with an average grain size of about 50~100 μm. The structure is uneven and there is obvious dendritic segregation. Intermetallic compounds: coarse Cu6Sn5 particles with a size of about 5~15 μm are distributed in the matrix, in an irregular block shape.

[0072] Antioxidant properties: After the solder was kept at 260℃ in air for 30 min, the oxide film on the surface thickened significantly, with an oxidation weight gain of 0.32 mg / cm³. 2 After being kept at a temperature of 120 min, the oxidative weight gain rate reached 0.85 mg / cm², and obvious oxide scale peeling appeared on the surface.

[0073] Self-healing performance: After surface scratches and aging at 200℃ for 2 hours, there is no obvious oxide film regeneration at the scratches, the scratch depth remains unchanged, and there is no self-healing ability.

[0074] IMC coarsening behavior: After aging at 200℃ for 100 h, the thickness of the Cu6Sn5 layer at the interface increased from the initial 2.5 μm to 6.2 μm, with a coarsening rate of about 0.037 μm / h; after aging for 200 h, the thickness reached 8.5 μm, showing a clear coarsening trend.

[0075] High temperature stability: After aging at 200℃ for 200 h, the matrix grains grew significantly, with the average grain size increasing from 50 μm to 120 μm, and the microhardness decreasing from the initial 15 HV to 9 HV, a decrease rate of 40%.

[0076] Compared with the embodiments of the present invention, Comparative Example 1 shows significant differences in antioxidant performance, self-repair ability, IMC inhibition effect and high temperature stability. This confirms the necessity and advancement of the present invention in constructing a multi-level gradient antioxidant structure by synergistic addition of Al and nano CeO2, combined with ultrasonic cavitation, rapid solidification, magnetic field-assisted isothermal annealing and directional aging treatment.

[0077] Comparative Example 2 This comparative example provides a Sn-Cu-Al solder with added Al but without added nano CeO2 particles. It is processed using the same preparation process as in Example 1, and its composition is shown in Table 5.

[0078] Table 5 Chemical composition of tin-based solder alloy in Comparative Example 2 The specific processing steps are as follows: (1) Smelting: Add pure Sn to a graphite crucible, heat to 420℃ to melt, add pure Cu particles, stir to completely dissolve, and form Sn-Cu melt.

[0079] (2) Add Al: Add pure Al sheets to Sn-Cu melt and mechanically stir at 450℃ for 15 min to dissolve Al uniformly.

[0080] (3) Add CeO2: This comparative example skips this step and does not add nano CeO2 particles.

[0081] (4) Ultrasonic cavitation treatment: The melt is placed in an ultrasonic field with an ultrasonic power of 1200 W, a frequency of 30 kHz, and a treatment time of 15 min.

[0082] (5) Rapid solidification: The ultrasonically treated melt is poured into a water-cooled copper mold, with a cooling rate of approximately 5 × 10⁻⁶. 4 K / s.

[0083] (6) Magnetic field-assisted isothermal annealing: Place the solder ingot in a magnetic field-assisted isothermal treatment device, set the annealing temperature to 200℃, the magnetic field strength to 5 T, apply a temperature gradient (temperature difference of about 50℃), and hold for 60 min.

[0084] (7) Directional aging: The annealed solder is subjected to directional aging treatment at 160℃ for 60 min.

[0085] (8) Rapid cooling: After aging, the solder is rapidly cooled to room temperature at a cooling rate of 100℃ / s.

[0086] The Sn-Cu-Al solder prepared by the above process has the following microstructure characteristics: an Al2O3 oxide film with a thickness of about 40-70 nm is formed on the surface layer, and the film layer is continuous and dense, similar to that in Example 1; however, no Ce-Al-O composite phase is formed in the subsurface layer, and the structure is a single β-Sn matrix with no obvious particle reinforcement layer; there is no nanoparticle segregation in the grain boundary region, the grain boundaries are clear, and no pinning phase exists; the grain size inside the matrix is ​​slightly coarser than that in Example 1, with an average grain size of about 5-10 μm and a relatively uniform structure; the Cu6Sn5 particles in the matrix have a size of about 1-3 μm and are finely dispersed.

[0087] Antioxidant properties: When the solder is kept at 260℃ in air for 30 min, the oxidation weight gain rate is 0.18 mg / cm2; after being kept at 260℃ for 120 min, the oxidation weight gain rate reaches 0.42 mg / cm2, and the oxidation rate is significantly accelerated.

[0088] Self-healing performance: After surface scratches, the surface was aged at 200°C for 1 hour, and a new Al2O3 film was formed covering the scratches, reducing the scratch depth by about 50%, demonstrating a certain self-healing ability, but the repair speed was slightly lower than that in Example 1.

[0089] IMC suppression effect: After aging at 200℃ for 100 h, the thickness of the Cu6Sn5 layer at the interface is about 3.8 μm; after aging for 200 h, the thickness reaches 5.2 μm, and the coarsening rate is between that of Example 1 and Comparative Example 1.

[0090] High temperature stability: After aging at 200℃ for 200 h, the matrix grains grew from the initial 8 μm to 20 μm, and the microhardness decreased by 18%.

[0091] This comparative example confirms the key role of nano-CeO2 particles in this invention: CeO2 not only participates in the in-situ reaction to generate the Ce-Al-O composite phase and constructs the subsurface diffusion barrier layer, but also agglomerates at the grain boundaries to form a pinning network, forming a comprehensive synergistic protection system with Al elements in the "surface-subsurface-grain boundary-interior" manner. Although Comparative Example 2 is superior to Comparative Example 1, which does not add any modifying elements, it fails to achieve the multi-level gradient structure of this invention. It is inferior to Example 1 in terms of long-term high-temperature stability, oxygen diffusion suppression, and IMC coarsening control, which fully demonstrates the necessity and advancement of the synergistic effect of Al and nano-CeO2.

[0092] Comparative Example 3 This comparative example provides a Sn-Cu-CeO2 solder with added nano CeO2 particles but no added Al element. It is processed using the same preparation process as in Example 1, and its composition is shown in Table 6.

[0093] Table 6 Chemical composition of tin-based solder alloy in Comparative Example 3 The specific processing steps are as follows: (1) Smelting: Add pure Sn to a graphite crucible, heat to 420℃ to melt, add pure Cu particles, stir to completely dissolve, and form Sn-Cu melt.

[0094] (2) Add Al: This comparison model skips this step and does not add the Al element.

[0095] (3) Add CeO2: Slowly add CeO2 nanoparticles with a particle size of 40~60 nm into the melt while stirring gently.

[0096] (4) Ultrasonic cavitation treatment: The melt is placed in an ultrasonic field with an ultrasonic power of 1200 W, a frequency of 30 kHz, and a treatment time of 15 min to make the nano CeO2 particles uniformly dispersed.

[0097] (5) Rapid solidification: The ultrasonically treated melt is poured into a water-cooled copper mold, with a cooling rate of approximately 5 × 10⁻⁶. 4 K / s.

[0098] (6) Magnetic field-assisted isothermal annealing: Place the solder ingot in a magnetic field-assisted isothermal treatment device, set the annealing temperature to 200℃, the magnetic field strength to 5 T, apply a temperature gradient (temperature difference of about 50℃), and hold for 60 min.

[0099] (7) Directional aging: The annealed solder is subjected to directional aging treatment at 160℃ for 60 min.

[0100] (8) Rapid cooling: After aging, the solder is rapidly cooled to room temperature at a cooling rate of 100℃ / s.

[0101] The Sn-Cu-CeO2 solder prepared by the above process has the following microstructural characteristics: the naturally formed oxide film on the surface is mainly SnO2, with a thickness of about 150~300 nm. The film is loose and has microcracks; no dense Al2O3 oxide layer is formed; no Ce-Al-O composite phase is formed in the subsurface, but a small amount of CeO2 particles are enriched in the near-surface area and are unevenly distributed; the nano CeO2 particles in the grain boundary region are obviously segregated along the grain boundary, forming a discontinuous grain boundary pinning layer, and some grain boundaries are filled by particles; the grain size inside the matrix is ​​small, with an average grain size of about 3~8 μm, and the uniformity of the structure is better than that of Comparative Example 1; the Cu6Sn5 particles in the matrix are about 1~2 μm in size, and are finely dispersed, with a significant refining effect.

[0102] Antioxidant properties: After the solder was kept at 260℃ in air for 30 min, the oxidation weight gain was 0.28 mg / cm³. 2 After incubation for 120 min, the oxidative weight gain reached 0.68 mg / cm³. 2 The oxidation rate was higher than that of Example 1 but lower than that of Comparative Example 1; Self-healing performance: After surface scratches and aging at 200℃ for 2 hours, there is no obvious oxide film regeneration at the scratches, the scratch depth remains basically unchanged, and there is no self-healing ability. IMC suppression effect: After aging at 200℃ for 100 h, the thickness of the Cu6Sn5 layer at the interface is about 3.5 μm; after aging for 200 h, the thickness reaches 4.8 μm. The IMC coarsening suppression effect is between that of Example 1 and Comparative Example 2. High temperature stability: After aging at 200℃ for 200 h, the matrix grains grew from the initial 5 μm to 12 μm, and the microhardness decreased by 22%.

[0103] This comparative example confirms the key role of Al in this invention: Al is not only the element that forms the dense Al2O3 oxide film on the surface, but also the provider of self-healing function. At the same time, it also participates in the in-situ reaction of Ce-Al-O composite phase. Although Comparative Example 3 achieves grain boundary pinning and microstructure refinement through CeO2, and is superior to Comparative Example 1 in terms of IMC suppression and high temperature stability, it cannot form a complete surface protection and self-healing system due to the lack of the synergistic effect of Al. It is significantly inferior to Example 1 in terms of antioxidant performance, self-healing ability and wettability.

[0104] Combining the results of Comparative Examples 2 and 3, we can conclude that Al and nano CeO2 particles are indispensable in this invention. Al provides surface protection and self-healing capabilities, while CeO2 provides grain boundary pinning and in-situ reaction phases. The synergistic effect of the two is necessary to construct a multi-level gradient antioxidant structure from the surface to the interior, achieving multi-layered step-by-step barrier to oxygen diffusion. Adding either element alone cannot achieve the expected effect of this invention, fully demonstrating the completeness and advancement of the technical solution of this invention.

[0105] Table 7 Comparison of Core Performance between Examples and Comparative Examples Comparative Example 4 This comparative example provides a Sn-Cu-Al-CeO2 solder with added nano CeO2 particles and Al elements, which is prepared using the same process as in Example 1 (without using the magnetic field-temperature gradient coupling process) to verify the key role of the magnetic field-temperature gradient coupling process in the construction of the multi-level gradient antioxidant structure of the present invention. Its composition is shown in Table 7.

[0106] Table 7 Chemical composition of tin-based solder alloy in Comparative Example 4 The specific processing steps are as follows: (1) Smelting: Add pure Sn to a graphite crucible, heat to 420℃ to melt, add pure Cu particles, stir to completely dissolve, and form Sn-Cu melt.

[0107] (2) Add Al: Add pure Al sheets to Sn-Cu melt and mechanically stir at 450℃ for 15 min to dissolve Al uniformly.

[0108] (3) Add CeO2: Slowly add CeO2 nanoparticles with a particle size of 40~60 nm into the melt while stirring gently.

[0109] (4) Ultrasonic cavitation treatment: The melt is placed in an ultrasonic field with an ultrasonic power of 1200 W, a frequency of 30 kHz, and a treatment time of 15 min to make the nano CeO2 particles uniformly dispersed.

[0110] (5) Rapid solidification: The ultrasonically treated melt is poured into a water-cooled copper mold, with a cooling rate of approximately 5 × 10⁻⁶. 4 K / s, to obtain rapidly solidified solder ingots.

[0111] (6) Conventional isothermal annealing: Place the solder ingot in a conventional annealing furnace, set the annealing temperature to 200℃, do not apply a magnetic field or a temperature gradient (i.e., no external field coupling), and hold for 60 min.

[0112] (7) Directional aging: The annealed solder is subjected to directional aging treatment at 160℃ for 60 min.

[0113] (8) Rapid cooling: After aging, the solder is rapidly cooled to room temperature at a cooling rate of 100℃ / s.

[0114] The Sn-Cu-Al-CeO2 solder prepared by the above process has the following microstructure characteristics: an Al2O3 oxide film with a thickness of about 40-60 nm is formed on the surface. The film is continuous but slightly less dense than that in Example 1, and there is local unevenness in thickness. The Ce-Al-O composite phase in the subsurface layer is significantly reduced, with only a small amount of sporadic distribution, and no continuous enriched layer is formed. The thickness is about 1-3 μm, which is much lower than the 5-10 μm in Example 1. The nano-CeO2 particles in the grain boundary region are somewhat agglomerated at the grain boundaries, but the distribution is uneven. Some grain boundaries are not pinned by particles and no continuous network structure is formed. The grain size inside the matrix is ​​about 3-8 μm, which is finer than that in Comparative Example 1 but coarser than that in Example 1, and the uniformity of the structure is generally average. The Cu6Sn5 particles in the matrix are about 1-3 μm in size, and the refining effect is between that in Example 1 and Comparative Example 2.

[0115] Antioxidant properties: After the solder was kept at 260℃ in air for 30 min, the oxidation weight gain was 0.22 mg / cm³. 2 After incubation for 120 min, the oxidative weight gain rate reached 0.48 mg / cm², which was significantly higher than that in Example 1.

[0116] Self-healing performance: After surface scratches and aging at 200°C for 1 hour, a new Al2O3 film is formed at the scratches, and the scratch depth is reduced by about 35%. The repair ability is weaker than that of Example 1 (about 60%).

[0117] IMC suppression effect: After aging at 200℃ for 100 h, the thickness of the Cu6Sn5 layer at the interface is about 3.5 μm; after aging for 200 h, the thickness reaches 4.6 μm.

[0118] High temperature stability: After aging at 200℃ for 200 h, the matrix grains grew from the initial 5 μm to 15 μm, and the microhardness decreased by 25%.

[0119] The comparison between Comparative Example 4 and Example 1 shows that the same alloy composition but different heat treatment processes lead to significant structural differences and performance gaps. This fully demonstrates that the magnetic field-assisted isothermal annealing process is one of the core innovations of the technical solution of this invention, and is a key guarantee for realizing a multi-level gradient anti-oxidation structure and obtaining excellent high-temperature performance.

[0120] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for improving the oxidation resistance of Sn-Cu lead-free solder through an externally induced in-situ reaction, characterized in that: Specifically, the following steps are included: (1) Heat pure Sn to melt, add Cu to form Sn-Cu melt, then add Al and stir to dissolve it evenly; slowly add CeO2 nanoparticles to the melt while stirring gently to obtain a mixed melt; (2) Place the mixed melt in an ultrasonic field to uniformly disperse the nano CeO2 particles; (3) The ultrasonically treated melt is subjected to rapid solidification treatment to obtain a rapid solidification solder ingot; (4) The solder ingot is placed in a magnetic field-assisted isothermal annealing device for annealing, and the annealed solder is subjected to directional aging treatment. After aging, it is rapidly cooled to room temperature.

2. The method for improving the oxidation resistance of Sn-Cu lead-free solder through external field-induced in-situ reaction according to claim 1, characterized in that: The raw materials of the Sn-Cu lead-free solder Includes: Cu: 0.6~0.8 wt% Al: 0.01~0.15 wt% Nano CeO2 particles: 0.01~0.08 wt%; balance Sn and unavoidable impurities.

3. The method for improving the oxidation resistance of Sn-Cu lead-free solder through external field-induced in-situ reaction according to claim 1, characterized in that: The CeO2 particles have a diameter of 20~80nm.

4. The method for improving the oxidation resistance of Sn-Cu lead-free solder through external field-induced in-situ reaction according to claim 1, characterized in that: The ultrasonic power for ultrasonic cavitation treatment is 800~1500 W, the frequency is 20~40 kHz, and the time is 10~20 min.

5. The method for improving the oxidation resistance of Sn-Cu lead-free solder through external field-induced in-situ reaction according to claim 1, characterized in that: The rapid solidification cooling rate is 10 3 ~10 6 K / s.

6. The method for improving the oxidation resistance of Sn-Cu lead-free solder through external field-induced in-situ reaction according to claim 1, characterized in that: The temperature for magnetic field-assisted isothermal annealing is 150-250℃, the magnetic field strength is 0.5-10 T, the holding time is 60-120 min, and the temperature uniformity during annealing is controlled within ±5℃.

7. The method for improving the oxidation resistance of Sn-Cu lead-free solder through external field-induced in-situ reaction according to claim 1, characterized in that: The aging temperature is 120~200℃, and the holding time is 30~120 min.

8. The method for improving the oxidation resistance of Sn-Cu lead-free solder through external field-induced in-situ reaction according to claim 1, characterized in that: Rapid cooling: 50~150℃ / s.

9. Sn-Cu lead-free solder prepared by any one of the methods described in claims 1 to 8.

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