A tin-bismuth composite solder, a preparation method thereof and a solder strip

Abstract

The application provides a tin-bismuth composite solder, a preparation method thereof and a solder strip, and the tin-bismuth composite solder comprises the following components in percentage by weight: Bi: 57% to 59%, Sb: 0.1% to 0.3%, Cd: 0.05% to 0.15%, Ce: 0.02% to 0.08%, and the rest is Sn. The tin-bismuth composite solder has the dual effects of 'potential regulation + microstructure refinement' through the synergistic effect of the three elements of Sb, Cd and Ce, the potential of the Sn-rich phase is improved, the potential of the Bi-rich phase is reduced, the two-phase potential is bidirectionally gradient-regulated, and then the potential difference is greatly reduced, the solder performance is improved, and the intrinsic corrosion problem of the low-temperature solder strip can be solved by using the tin-bismuth composite solder.

Description

Technical Field

[0001] This invention belongs to the field of solder alloy technology, and relates to a tin-bismuth composite solder, its preparation method, and solder strip. Background Technology

[0002] With the rapid development of industries such as photovoltaics, automotive electronics, and MiniLED, the demand for low-temperature soldering continues to grow. Sn-58Bi eutectic alloy has become the mainstream choice for low-temperature lead-free solders due to its advantages such as low melting point (138℃), soldering temperature of only 180~200℃, effective avoidance of thermal damage to heat-sensitive components, high strength, and low cost.

[0003] However, the inherent corrosion defects of Sn-Bi alloys severely limit their application in high-reliability scenarios: the standard electrode potentials of Sn and Bi differ by as much as 0.446V (Bi... 3+ / Bi is +0.308V vs. SHE, Sn 2+ / Sn is -0.138Vvs.SHE), which forms a microcouple pair consisting of a Bi-rich phase (cathode) and a Sn-rich phase (anode) in the alloy structure. Under the action of the microcouple, the Sn-rich phase dissolves preferentially, resulting in increased porosity of the solder joint, decreased mechanical properties, and shortened service life.

[0004] CN119604042A discloses a high corrosion-resistant low-temperature welding strip and its preparation method. It isolates the corrosive medium by coating the surface of the photovoltaic welding strip with an organic or inorganic protective layer. However, this only protects the outside of the welding strip. During the welding process, the coating layer is easily damaged under the action of high temperature and flux. Furthermore, the alloy inside the weld joint still has the risk of Bi phase segregation and microgalvanic corrosion, which cannot solve the intrinsic corrosion problem. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a tin-bismuth composite solder, its preparation method, and solder ribbon. The tin-bismuth composite solder of the present invention achieves a dual effect of "potential regulation + microstructure refinement" through the synergistic action of three elements, Sb, Cd, and Ce, thereby increasing the potential of the Sn-rich phase and decreasing the potential of the Bi-rich phase, realizing bidirectional gradient regulation of the two-phase potential, and thus significantly reducing the potential difference and improving the solder performance. The use of the tin-bismuth composite solder can solve the intrinsic corrosion problem of low-temperature solder ribbons.

[0006] To achieve this objective, the present invention adopts the following technical solution: In a first aspect, the present invention provides a tin-bismuth composite solder, wherein the tin-bismuth composite solder comprises the following components in weight percentage: Bi: 57%~59%, Sb: 0.1%~0.3%, Cd: 0.05%~0.15%, Ce: 0.02%~0.08%, and the remainder being Sn.

[0007] Preferably, the melting point of the tin-bismuth composite solder is 138℃~142℃.

[0008] Preferably, the Sn-rich phase potential in the tin-bismuth composite solder is -0.35V to -0.45V.

[0009] Preferably, the Bi-rich phase potential in the tin-bismuth composite solder is -0.28V to -0.32V.

[0010] Preferably, the potential difference between the Sn-rich phase and the Bi-rich phase in the tin-bismuth composite solder is 0.1V~0.15V.

[0011] In a second aspect, the present invention provides a method for preparing a tin-bismuth composite solder as described in the first aspect, the method comprising the following steps: After mixing and melting Sn and Bi sources, Sb source is added for the first stirring and melting treatment, Cd source is added for the second stirring and melting treatment, Ce source is added for the third stirring and melting treatment, and then the mixture is allowed to stand and degas to obtain a mixed molten liquid. The mixed molten liquid was sequentially cooled and annealed to obtain the tin-bismuth composite solder.

[0012] Preferably, the Sn source comprises Sn blocks with a purity ≥ 99.99%.

[0013] Preferably, the Bi source comprises Bi blocks with a purity of ≥99.99%.

[0014] Preferably, the Sn source and Bi source are subjected to acid washing and anhydrous ethanol washing in sequence before mixing.

[0015] Preferably, the Sb source comprises Sb particles with a purity of ≥99.99%.

[0016] Preferably, the median particle size D50 of the Sb particles is 3 mm to 5 mm.

[0017] Preferably, the Cd source comprises Cd particles with a purity of ≥99.99%.

[0018] Preferably, the median particle size D50 of the Cd particles is 3 mm to 5 mm.

[0019] Preferably, the Ce source comprises a Sn-Ce alloy.

[0020] Preferably, the median grain size D50 of the Sn-Ce alloy is 10 mm to 20 mm.

[0021] Preferably, the temperature at which the mixture is melted is 280°C to 320°C.

[0022] Preferably, the heating rate of the mixed melting is 8°C / min to 12°C / min.

[0023] Preferably, the holding time for the mixed melting is 8 min to 12 min.

[0024] Preferably, electromagnetic stirring is performed independently during the first, second, and third stirring-melting processes.

[0025] Preferably, the electromagnetic stirring speed is 200 rpm to 400 rpm.

[0026] Preferably, the temperature of the first stirring and melting treatment is 390°C to 410°C.

[0027] Preferably, the heating rate of the first stirring and melting treatment is 4℃ / min to 6℃ / min.

[0028] Preferably, the holding time for the first stirring and melting treatment is 8 min to 12 min.

[0029] Preferably, the temperature of the second stirring and melting treatment is 390°C to 410°C.

[0030] Preferably, the atmosphere for the second stirring and melting treatment includes nitrogen and / or argon.

[0031] Preferably, the holding time for the second stirring and melting treatment is 4 min to 6 min.

[0032] Preferably, the temperature of the third stirring and melting treatment is 415℃~425℃.

[0033] Preferably, the heating rate of the third stirring and melting treatment is 2℃ / min to 4℃ / min.

[0034] Preferably, the holding time for the third stirring and melting treatment is 2 min to 4 min.

[0035] Preferably, the temperature of the static degassing treatment is 415℃~425℃.

[0036] Preferably, argon gas is introduced during the static degassing process.

[0037] Preferably, during the static degassing process, the argon flow rate is 0.4 L / min to 0.6 L / min.

[0038] Preferably, the time for the static degassing treatment is 4 min to 6 min.

[0039] Preferably, the mixed molten liquid is injected into a graphite mold at 120°C to 180°C before cooling.

[0040] Preferably, the cooling rate is 15℃ / s to 25℃ / s.

[0041] Preferably, the annealing temperature is 80℃~120℃.

[0042] Preferably, the annealing process takes 1.5 to 2.5 hours.

[0043] Thirdly, the present invention provides a solder strip made by rolling a tin-bismuth composite solder as described in the first aspect.

[0044] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0045] Compared with the prior art, the present invention has the following beneficial effects: This invention employs a gradient potential control strategy to simultaneously adjust the electrode potentials of the Sn-rich and Bi-rich phases, significantly reducing the potential difference between the two phases from 0.4V to below 0.15V. This reduces the microcouple corrosion driving force by nearly 70%, the corrosion rate by over 65%, and the Bi phase segregation rate from 35% to below 5%. The solder melting point remains at 138~142℃, fully compatible with traditional Sn-58Bi solder welding processes. The cost increase is only 0.5 RMB / kg~0.8 RMB / kg, far lower than modification schemes using precious metals such as Ag and In. After 1000 hours of damp heat aging at 85℃ / 85%RH, the shear strength retention rate of the solder joint reaches over 85%. Detailed Implementation

[0046] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0047] The scope of this invention can be defined by lower and upper limits. The selected lower and upper limits define the boundaries of a specific range. The range defined in this way can be defined by the inclusion or exclusion of endpoints. Any endpoint can be independently selected for inclusion or exclusion, and all lower and upper limits can be arbitrarily combined to form new ranges. That is, any lower limit can be combined with any upper limit to form an effective range. For example, if the ranges of 60~120 and 80~110 are listed for specific parameters, it should be understood that the ranges of 60~110 and 80~120 also fall within the scope of this invention. In addition, if the minimum range values ​​1 and 2 are listed, and the maximum range values ​​3, 4 and 5 are also listed, then all ranges of 1~3, 1~4, 1~5, 2~3, 2~4 and 2~5 fall within the scope of this invention. In this invention, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0~5" means that all real numbers between 0 and 5 have been fully listed in this document, and "0~5" is only a shortened representation of this set of numerical combinations. When a parameter is expressed as an integer ≥2, it is equivalent to listing positive integers that meet the requirements, such as 2, 3, 4, 5, 6, 7, 8, 9, 10, etc. When a parameter is expressed as an integer selected from "2~10", it is equivalent to listing any integer among 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0048] In this invention, "a combination of at least two" refers to a quantity greater than or equal to 2 unless otherwise specified. For example, "any one or a combination of at least two" means that any one of the listed items can be selected, or a combination of at least two of the listed items formed in a manner that does not conflict and enables the implementation of this invention. In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" cover any one of two or more related listed items, as well as any and all combinations of the related listed items. The arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" means a set consisting of A, B, and combinations of A and B, where "containing A and / or B" can be understood, depending on the context of the statement, as containing A, containing B, or simultaneously containing both A and B. In this invention, "optional" means that the corresponding feature, component, step or solution is not necessary, that is, it is selected from either "with" or "without". If there are multiple "optional" limitations in a technical solution, unless otherwise specified and there is no technical conflict or mutual constraint, each "optional" limitation is independent and does not affect the others.

[0049] In this invention, technical features or solutions described using open-ended terms such as "comprising" or "including" do not exclude additional non-conflicting elements beyond the listed elements unless otherwise specified. They are considered to disclose both closed-ended features or solutions consisting solely of the listed elements and open-ended features or solutions that may include additional non-conflicting elements beyond the listed elements. For example, if A includes a1, a2, and a3, unless otherwise specified, this means that A can consist only of a1, a2, and a3, or it can include other non-conflicting elements based on a1, a2, and a3. This corresponds to the disclosure of technical solutions such as "A consists of a1, a2, and a3," "A is selected from a1, a2, and a3," and "A not only includes a1, a2, and a3, but may also include other non-conflicting elements." All embodiments and optional embodiments of this invention, unless otherwise specified and without technical conflict, can be combined to form new technical solutions, and such combinations fall within the scope of this invention. The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various locations throughout the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art will understand, explicitly and implicitly, that the embodiments described in this invention can be combined with other embodiments that do not conflict with the technology. The ordinal numbers "first," "second," "third," and "fourth," etc., used in the expressions "first aspect," "second aspect," "third aspect," and "fourth aspect" in this invention are for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly specifying the importance or quantity of the indicated technical features. They serve only as a non-exhaustive enumeration and do not constitute a closed limitation on quantity.

[0050] In this invention, the order in which the steps are written in the methods described in each embodiment does not imply a strict execution order. The actual execution order of each step should be determined according to its function and possible internal logic. Unless otherwise specified, all steps of this invention can be executed in the order in which they are written or in any order that does not conflict with the technology.

[0051] In a first aspect, the present invention provides a tin-bismuth composite solder in some specific embodiments, the tin-bismuth composite solder comprising the following components by weight percentage: Bi: 57%~59%, for example: 57%, 57.5%, 58%, 58.5% or 59%, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable; Sb: 0.1%~0.3%, for example: 0.1%, 0.15%, 0.2%, 0.25% or 0.3%, etc., not limited to the listed values, and the weight percentage is not limited to the listed values. Other unlisted values ​​within the range also apply, Cd: 0.05%~0.15%, such as: 0.05%, 0.08%, 0.1%, 0.12% or 0.15%, etc., not limited to the listed values, other unlisted values ​​within this range also apply, Ce: 0.02%~0.08%, such as: 0.02%, 0.04%, 0.05%, 0.06% or 0.08%, etc., not limited to the listed values, other unlisted values ​​within this range also apply, the rest is Sn.

[0052] In the tin-bismuth composite solder of this invention, Sb is doped into the Sn-rich phase to increase the potential of the Sn-rich phase, while Cd is doped into the Bi-rich phase to decrease the potential of the Bi-rich phase, thus achieving synergistic regulation of the potentials of the two phases and significantly reducing the potential difference between them. Furthermore, by adding trace amounts of Ce to refine the Bi phase, the microgalvanic corrosion effect is further suppressed.

[0053] In the tin-bismuth composite solder of this invention, the addition of 0.1 wt% Sb can increase the potential of the Sn-rich phase by approximately 0.05 V, achieving precise control of the Sn-rich phase potential. Adding more than 0.3 wt% will generate SbSn metallic compounds, leading to increased solder brittleness and a decrease in elongation of approximately 15%. In the tin-bismuth composite solder, the addition of 0.05 wt% Cd can decrease the potential of the Bi-rich phase by approximately 0.06 V, forming a synergistic potential control effect with Sb. Adding more than 0.15 wt% will cause the solder melting point to rise above 145°C, compromising compatibility with low-temperature processes. In the tin-bismuth composite solder of this invention, Ce precipitates at the Bi phase grain boundaries in the form of CeSn3 intermetallic compound, pinning the grain boundaries and inhibiting Bi phase coarsening, reducing the Bi phase segregation rate from 35% to below 5%. Adding more than 0.08 wt% will form large rare earth inclusions, leading to a decrease in the mechanical properties of the solder joint, with a reduction in shear strength of more than 10%.

[0054] In some embodiments, the melting point of the tin-bismuth composite solder is 138°C to 142°C, for example: 138°C, 139°C, 140°C, 141°C or 142°C, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0055] The tin-bismuth composite solder of the present invention has a melting point of 138℃~142℃, which is fully compatible with the traditional Sn-58Bi solder welding process, and the cost increases by only 0.5 yuan / kg~0.8 yuan / kg, which is far lower than the modification scheme of precious metals such as Ag and In.

[0056] In some embodiments, the Sn-rich phase potential in the tin-bismuth composite solder is -0.35V to -0.45V, for example: -0.35V, -0.38V, -0.4V, -0.42V or -0.45V, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0057] In some embodiments, the Bi-rich phase potential in the tin-bismuth composite solder is -0.28V to -0.32V, for example: -0.28V, -0.29V, -0.3V, -0.31V or -0.32V, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0058] In some embodiments, the potential difference between the Sn-rich phase and the Bi-rich phase in the tin-bismuth composite solder is 0.1V to 0.15V, for example: 0.1V, 0.11V, 0.12V, 0.13V, 0.14V or 0.15V, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0059] Secondly, in some specific embodiments, the present invention provides a method for preparing a tin-bismuth composite solder as described in the first aspect, the method comprising the following steps: After mixing and melting Sn and Bi sources, Sb source is added for the first stirring and melting treatment, Cd source is added for the second stirring and melting treatment, Ce source is added for the third stirring and melting treatment, and then the mixture is allowed to stand and degas to obtain a mixed molten liquid. The mixed molten liquid was sequentially cooled and annealed to obtain the tin-bismuth composite solder.

[0060] In some embodiments, the Sn source comprises Sn blocks with a purity ≥ 99.99%.

[0061] In some embodiments, the Bi source comprises Bi blocks with a purity ≥ 99.99%.

[0062] In some embodiments, the Sn source and Bi source are sequentially acid-washed and washed with anhydrous ethanol before being mixed.

[0063] In this invention, the oxide film on the surface of Sn source and Bi source is removed by acid washing in advance, and then the surface is cleaned with anhydrous ethanol and dried for later use.

[0064] In some embodiments, the Sb source comprises Sb particles with a purity ≥ 99.99%.

[0065] In some embodiments, the median particle size D50 of the Sb particles is 3mm to 5mm, for example: 3mm, 3.5mm, 4mm, 4.5mm or 5mm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0066] In some embodiments, the Cd source comprises Cd particles with a purity ≥ 99.99%.

[0067] In some embodiments, the median particle size D50 of the Cd particles is 3mm to 5mm, for example: 3mm, 3.5mm, 4mm, 4.5mm or 5mm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0068] The present invention uses Sb and Cd particles with a median particle size D50 of 3mm to 5mm to avoid oxidation and burn-off during the smelting process.

[0069] In some embodiments, the Ce source comprises a Sn-Ce alloy.

[0070] In some embodiments, the median grain size D50 of the Sn-Ce alloy is 10mm to 20mm, for example: 10mm, 12mm, 15mm, 18mm or 20mm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0071] This invention uses Sn-Ce alloy with a median particle size D50 of 10mm~20mm as Ce source, which can reduce Ce oxidation loss.

[0072] In some embodiments, the temperature of the mixture melting is 280°C to 320°C, for example: 280°C, 290°C, 300°C, 310°C or 320°C, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0073] In some embodiments, the heating rate of the mixed melting is 8°C / min to 12°C / min, for example: 8°C / min, 9°C / min, 10°C / min, 11°C / min or 12°C / min, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0074] In some embodiments, the holding time for the mixing and melting is 8 min to 12 min, for example: 8 min, 9 min, 10 min, 11 min or 12 min, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0075] In some embodiments, electromagnetic stirring is performed independently during the first, second, and third stirring-melting processes.

[0076] In some embodiments, the speed of the electromagnetic stirring process is 200 rpm to 400 rpm, for example: 200 rpm, 250 rpm, 300 rpm, 350 rpm or 400 rpm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0077] In some embodiments, the temperature of the first stirring and melting treatment is 390°C to 410°C, for example: 390°C, 395°C, 400°C, 405°C or 410°C, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0078] In some embodiments, the heating rate of the first stirring and melting treatment is 4°C / min to 6°C / min, for example: 4°C / min, 4.5°C / min, 5°C / min, 5.5°C / min or 6°C / min, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0079] In some embodiments, the holding time of the first stirring and melting treatment is 8 min to 12 min, for example: 8 min, 9 min, 10 min, 11 min or 12 min, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0080] In some embodiments, the temperature of the second stirring and melting treatment is 390°C to 410°C, for example: 390°C, 395°C, 400°C, 405°C or 410°C, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0081] The boiling point of Cd is 765℃. The present invention performs a second stirring and melting treatment at 390℃~410℃ to avoid high-temperature volatilization of Cd. At this temperature, the volatilization rate of Cd is <0.5%.

[0082] In some embodiments, the atmosphere of the second stirring melt treatment includes nitrogen and / or argon.

[0083] In some embodiments, the holding time of the second stirring and melting treatment is 4 min to 6 min, for example: 4 min, 4.5 min, 5 min, 5.5 min or 6 min, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0084] In some embodiments, the temperature of the third stirring and melting treatment is 415°C to 425°C, for example: 415°C, 418°C, 420°C, 422°C or 425°C, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0085] In some embodiments, the heating rate of the third stirring and melting treatment is 2℃ / min to 4℃ / min, for example: 2℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min or 4℃ / min, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0086] In some embodiments, the holding time of the third stirring and melting treatment is 2 min to 4 min, for example: 2 min, 2.5 min, 3 min, 3.5 min or 4 min, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0087] In some embodiments, the temperature of the static degassing treatment is 415℃~425℃, for example: 415℃, 418℃, 420℃, 422℃ or 425℃, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0088] In some embodiments, argon gas is introduced during the static degassing process.

[0089] In some embodiments, during the static degassing process, the argon flow rate is 0.4 L / min to 0.6 L / min, for example: 0.4 L / min, 0.45 L / min, 0.5 L / min, 0.55 L / min or 0.6 L / min, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0090] In some embodiments, the settling and degassing time is 4 min to 6 min, for example: 4 min, 4.5 min, 5 min, 5.5 min or 6 min, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0091] In some embodiments, the mixed molten liquid is injected into a graphite mold at a temperature of 120°C to 180°C before cooling, for example: 120°C, 130°C, 150°C, 160°C or 180°C, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0092] In some embodiments, the cooling rate is 15℃ / s to 25℃ / s, for example: 15℃ / s, 18℃ / s, 20℃ / s, 22℃ / s or 25℃ / s, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0093] The present invention controls the cooling rate within the above-mentioned range, which can avoid macroscopic segregation of the Bi phase.

[0094] In some embodiments, the annealing temperature is 80°C to 120°C, for example: 80°C, 90°C, 100°C, 110°C or 120°C, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0095] This invention eliminates casting internal stress through annealing treatment, promoting the uniform distribution of metal elements in tin-bismuth composite solder.

[0096] In some embodiments, the annealing time is 1.5h to 2.5h, for example: 1.5h, 1.8h, 2h, 2.2h or 2.5h, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0097] Thirdly, in some specific embodiments, the present invention provides a solder strip formed by rolling a tin-bismuth composite solder as described in the first aspect.

[0098] The welding strip described in this invention is produced by a single-roller spinning method, with a copper roller speed of 1000 rpm and a roller surface temperature of 25°C, resulting in a welding strip with a thickness of 0.1 mm to 0.3 mm, and the average size of the Bi phase is controlled below 1 μm.

[0099] The raw materials used in the tin-bismuth composite solders described in the embodiments and comparative examples of this invention are as follows: All raw materials have a purity of ≥99.99%. Among them, pure Sn blocks and pure Bi blocks are pre-washed with 5% dilute hydrochloric acid for 5 minutes to remove the surface oxide film, then ultrasonically cleaned with anhydrous ethanol for 3 minutes, and dried for later use. Pure Sb metal particles with a median particle size D50 of 4 mm, pure Cd metal particles with a median particle size D50 of 4 mm, and Sn-20wt%Ce master alloy with a median particle size D50 of 15 mm were used as Ce sources.

[0100] It is prepared by the following method: 2. Gradient feeding and smelting process (key temperature control and timing of feeding) Melting temperature profile and charging sequence: Pure Sn and pure Bi bulk materials were mixed and heated to 300°C at a rate of 10°C / min and held for 10 min until completely melted. Sb particles were added and the temperature was increased to 400°C at a rate of 5°C / min. The mixture was then stirred electromagnetically at 300 rpm for 10 min until completely dissolved. Cd particles were added and the mixture was stirred at 400°C under argon protection for 5 min. Sn-Ce master alloy was added and the temperature was increased to 420°C at a rate of 3°C / min. The mixture was stirred for 3 min and then allowed to stand at 420°C for 5 min to remove gas. During this period, the argon flow rate was controlled at 0.5 L / min to obtain a mixed molten liquid. The mixed molten liquid was poured into a high-purity graphite mold preheated to 150°C, the cooling rate was controlled at 20°C / s, and the ingot was annealed for 2 hours under argon protection at 100°C to obtain the tin-bismuth composite solder.

[0101] The melting point tests of the solders described in the embodiments and comparative examples of this invention were all performed using differential scanning calorimetry (DSC) at a heating rate of 10°C / min under argon protection. Potential tests were performed using a three-electrode system, with a saturated calomel electrode as the reference electrode and a platinum electrode as the auxiliary electrode. The test medium was a 3.5 wt% NaCl solution.

[0102] Example 1 This embodiment provides a tin-bismuth composite solder, which comprises the following components by weight percentage: Bi: 57.8%, Sb: 0.18%, Cd: 0.15%, Ce: 0.04%, with the remainder being Sn. The tin-bismuth composite solder has a melting point of 141.8℃, and the Sn-rich phase potential (vs. SCE) in the tin-bismuth composite solder is -0.43V, the Bi-rich phase potential (vs. SCE) is -0.32V, and the potential difference between the Sn-rich and Bi-rich phases is 0.11V.

[0103] Example 2 This embodiment provides a tin-bismuth composite solder, which comprises the following components by weight percentage: Bi: 58.5%, Sb: 0.3%, Cd: 0.12%, Ce: 0.06%, with the remainder being Sn. The melting point of the tin-bismuth composite solder is 140.5℃. The Sn-rich phase potential (vs. SCE) in the tin-bismuth composite solder is -0.39V, the Bi-rich phase potential (vs. SCE) is -0.29V, and the potential difference between the Sn-rich phase and the Bi-rich phase is 0.10V.

[0104] Example 3 This embodiment provides a tin-bismuth composite solder, which comprises the following components by weight percentage: Bi: 57.5%, Sb: 0.15%, Cd: 0.08%, Ce: 0.03%, with the remainder being Sn. The tin-bismuth composite solder has a melting point of 138.8℃. The Sn-rich phase potential (vs. SCE) in the tin-bismuth composite solder is -0.45V, the Bi-rich phase potential (vs. SCE) is -0.31V, and the potential difference between the Sn-rich and Bi-rich phases is 0.14V.

[0105] Example 4 This embodiment provides a tin-bismuth composite solder, which comprises the following components by weight percentage: Bi: 58%, Sb: 0.2%, Cd: 0.1%, Ce: 0.05%, with the remainder being Sn. The tin-bismuth composite solder has a melting point of 139.2℃. The Sn-rich phase potential (vs. SCE) in the tin-bismuth composite solder is -0.42V, the Bi-rich phase potential (vs. SCE) is -0.30V, and the potential difference between the Sn-rich and Bi-rich phases is 0.12V.

[0106] Comparative Example 1 The only difference between this comparative example and Example 1 is that the mass percentage of Cd in the tin-bismuth composite solder is 0.2% (using Sn to adjust the total amount). All other conditions and parameters are exactly the same as in Example 1. The melting point of the tin-bismuth composite solder is 146.7℃, and the potential difference between the Sn-rich phase and the Bi-rich phase is 0.08V.

[0107] Comparative Example 2 The only difference between this comparative example and Example 1 is that the mass percentage of Cd in the tin-bismuth composite solder is 0.02% (using Sn to adjust the total amount). All other conditions and parameters are exactly the same as in Example 1. The melting point of the tin-bismuth composite solder is 139°C, and the potential difference between the Sn-rich phase and the Bi-rich phase is 0.36V.

[0108] Comparative Example 3 The only difference between this comparative example and Example 1 is that the mass percentage of Sb in the tin-bismuth composite solder is 0.4% (using Sn to adjust the total amount). All other conditions and parameters are exactly the same as in Example 1. The melting point of the tin-bismuth composite solder is 143.2℃, and the potential difference between the Sn-rich phase and the Bi-rich phase is 0.09V.

[0109] Comparative Example 4 The only difference between this comparative example and Example 1 is that the mass percentage of Sb in the tin-bismuth composite solder is 0.05% (using Sn to adjust the total amount). All other conditions and parameters are exactly the same as in Example 1. The melting point of the tin-bismuth composite solder is 138.7℃, and the potential difference between the Sn-rich phase and the Bi-rich phase is 0.25V, indicating insufficient potential control.

[0110] Comparative Example 5 The only difference between this comparative example and Example 1 is that the mass percentage of Ce in the tin-bismuth composite solder is 0.1% (using Sn to adjust the total amount). All other conditions and parameters are exactly the same as in Example 1. The melting point of the tin-bismuth composite solder is 139.5℃, and the potential difference between the Sn-rich phase and the Bi-rich phase is 0.12V.

[0111] Comparative Example 6 The only difference between this comparative example and Example 1 is that the mass percentage of Ce in the tin-bismuth composite solder is 0.01% (using Sn to adjust the total amount). All other conditions and parameters are exactly the same as in Example 1. The melting point of the tin-bismuth composite solder is 138.8℃, and the potential difference between the Sn-rich phase and the Bi-rich phase is 0.12V.

[0112] Comparative Example 7 This comparative example uses pure Sn-58Bi eutectic solder without any added alloying elements. The pure Sn-58Bi eutectic solder has a melting point of 138.5℃ and a potential difference of 0.41V between the Sn-rich phase and the Bi-rich phase.

[0113] Performance testing: The solders described in the examples and comparative examples were tested. Corrosion performance was tested using potentiodynamic polarization curves at a scan rate of 1 mV / s. Mechanical properties were tested using a universal testing machine at a tensile rate of 1 mm / min. The damp heat aging test conditions were: aging at 85℃ / 85%RH for 1000 h, followed by testing the shear strength retention rate. The solders described in the examples and comparative examples were then processed into solder strips using a single-roller spinning method, with the copper roller rotating at 1000 rpm and the roller surface temperature at 25℃, resulting in solder strips with a thickness of 0.2 mm. The average size of the Bi phase in the solder strips was tested.

[0114] The test results are shown in Table 1: Table 1 As shown in Table 1, based on Examples 1 to 4, the corrosion current density of the tin-bismuth composite solder of the present invention can reach 2.3 × 10⁻⁶. -6 A / cm 2 Within this range, the room temperature shear strength of the solder strip can reach over 45 MPa, the shear strength retention rate can reach over 94%, the elongation can reach over 39%, and the average size of the Bi phase is controlled between 0.9 μm and 1.1 μm. Compared with the prior art, the potential difference between the two phases of the tin-bismuth composite solder of the present invention is significantly reduced, the corrosion resistance is significantly improved, the risk of microgalvanic corrosion is effectively controlled, the mechanical properties are well maintained, and the long-term service reliability is excellent.

[0115] Comparing Example 1 and Comparative Examples 1 and 2, it can be seen that the Cd content in the tin-bismuth composite solder of the present invention affects its properties. When the mass ratio of Cd in the tin-bismuth composite solder is controlled at 0.05% to 0.15%, the performance of the tin-bismuth composite solder is better. If the mass ratio of Cd is too high, the welding temperature requirement is above 220°C when the potential difference between the two phases is 0.08V. Excessive Cd causes the melting point to rise by more than 8°C, which cannot meet the requirements of low-temperature welding process. The heat damage rate during the welding of heat-sensitive components reaches 15%. If the mass ratio of Cd is too low, the potential control is insufficient.

[0116] As can be seen from the comparison between Example 1 and Comparative Examples 3-4, the Sb content in the tin-bismuth composite solder of the present invention affects its properties. When the mass ratio of Sb in the tin-bismuth composite solder is controlled at 0.1% to 0.3%, the performance of the tin-bismuth composite solder is better. If the mass ratio of Sb is too high, excessive Sb will generate SbSn hard and brittle phases, the elongation will decrease significantly, and cracks will easily occur during the soldering process. If the mass ratio of Sb is too low, the potential control is insufficient.

[0117] As can be seen from the comparison between Example 1 and Comparative Examples 5-6, the Ce content in the tin-bismuth composite solder of the present invention affects its properties. When the mass percentage of Ce in the tin-bismuth composite solder is controlled at 0.02% to 0.08%, the performance of the tin-bismuth composite solder is better. If the mass percentage of Ce is too high, it will lead to a decrease in structural performance, such as yield strength and tensile strength, and large impurities are easily formed. If the mass percentage of Ce is too low, the potential control is insufficient.

[0118] Comparing Example 1 and Comparative Example 7, it can be seen that in the tin-bismuth composite solder of the present invention, Sb element is doped into the Sn-rich phase to increase the potential of the Sn-rich phase, while Cd element is doped into the Bi-rich phase to decrease the potential of the Bi-rich phase, thereby achieving synergistic regulation of the potentials of the two phases and significantly reducing the potential difference between the two phases. By adding trace amounts of Ce element to refine the Bi phase, the micro-galvanic corrosion effect is further suppressed. In the tin-bismuth composite solder of the present invention, the three elements work synergistically to achieve the dual effect of "potential regulation + microstructure refinement", reducing the potential difference of the tin-bismuth solder from 0.4V to 0.1~0.15V, and reducing the driving force of micro-galvanic corrosion by 70%.

[0119] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A tin-bismuth composite solder, characterized in that, The tin-bismuth composite solder comprises the following components by weight percentage: Bi: 57%~59%, Sb: 0.1%~0.3%, Cd: 0.05%~0.15%, Ce: 0.02%~0.08%, with the remainder being Sn.

2. The tin-bismuth composite solder as described in claim 1, characterized in that, The melting point of the tin-bismuth composite solder is 138℃~142℃.

3. The tin-bismuth composite solder as described in claim 1 or 2, characterized in that, The Sn-rich phase potential in the tin-bismuth composite solder is -0.35V to -0.45V; Preferably, the Bi-rich phase potential in the tin-bismuth composite solder is -0.28V to -0.32V; Preferably, the potential difference between the Sn-rich phase and the Bi-rich phase in the tin-bismuth composite solder is 0.1V~0.15V.

4. A method for preparing a tin-bismuth composite solder as described in any one of claims 1-3, characterized in that, The preparation method includes the following steps: After mixing and melting Sn and Bi sources, Sb source is added for the first stirring and melting treatment, Cd source is added for the second stirring and melting treatment, Ce source is added for the third stirring and melting treatment, and then the mixture is allowed to stand and degas to obtain a mixed molten liquid. The mixed molten liquid was sequentially cooled and annealed to obtain the tin-bismuth composite solder.

5. The preparation method according to claim 4, characterized in that, The Sn source comprises Sn blocks with a purity ≥ 99.99%; Preferably, the Bi source comprises Bi blocks with a purity ≥ 99.99%; Preferably, the Sn source and Bi source are subjected to acid washing and anhydrous ethanol washing in sequence before mixing; Preferably, the Sb source comprises Sb particles with a purity ≥ 99.99%; Preferably, the median particle size D50 of the Sb particles is 3 mm to 5 mm; Preferably, the Cd source comprises Cd particles with a purity ≥ 99.99%; Preferably, the median particle size D50 of the Cd particles is 3 mm to 5 mm; Preferably, the Ce source comprises a Sn-Ce alloy; Preferably, the median grain size D50 of the Sn-Ce alloy is 10 mm to 20 mm.

6. The preparation method according to claim 4 or 5, characterized in that, The temperature at which the mixture is melted is 280℃~320℃; Preferably, the heating rate of the mixed melting is 8°C / min to 12°C / min; Preferably, the holding time for the mixed melting is 8 min to 12 min.

7. The preparation method according to any one of claims 4-6, characterized in that, Electromagnetic stirring is performed independently during the first, second, and third stirring-melting processes. Preferably, the electromagnetic stirring speed is 200 rpm to 400 rpm; Preferably, the temperature of the first stirring and melting treatment is 390℃~410℃; Preferably, the heating rate of the first stirring and melting treatment is 4°C / min to 6°C / min; Preferably, the holding time for the first stirring and melting treatment is 8 min to 12 min; Preferably, the temperature of the second stirring and melting treatment is 390°C to 410°C; Preferably, the atmosphere for the second stirring and melting treatment includes nitrogen and / or argon; Preferably, the holding time for the second stirring and melting treatment is 4 min to 6 min; Preferably, the temperature of the third stirring and melting treatment is 415℃~425℃; Preferably, the heating rate of the third stirring and melting treatment is 2℃ / min to 4℃ / min; Preferably, the holding time for the third stirring and melting treatment is 2 min to 4 min.

8. The preparation method according to any one of claims 4-7, characterized in that, The temperature for the static degassing treatment is 415℃~425℃; Preferably, argon gas is introduced during the static degassing process; Preferably, during the static degassing process, the argon flow rate is 0.4 L / min to 0.6 L / min; Preferably, the time for the static degassing treatment is 4 min to 6 min.

9. The preparation method according to any one of claims 4-8, characterized in that, Before cooling, the mixed molten liquid is injected into a graphite mold at 120℃~180℃; Preferably, the cooling rate is 15℃ / s to 25℃ / s; Preferably, the annealing temperature is 80℃~120℃; Preferably, the annealing process takes 1.5 to 2.5 hours.

10. A welding strip, characterized in that, The solder strip is formed by rolling the tin-bismuth composite solder as described in any one of claims 1-3.

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