High-strength lead-free solder for power electronics and method for producing same

Abstract

The application provides a high-strength lead-free solder for power electronics and a preparation method thereof, the high-strength lead-free solder for power electronics comprises the following elements in percentage by weight: Bi 2.5-3.5%, Sb 2.5-3.5%, B 0.001-0.05%, and the rest is Sn and inevitable impurities. The lead-free solder in the application belongs to a high-strength lead-free soft solder, the solder structure has no segregation of Bi, and contains the interface stabilizing element B, has excellent matrix structure and interface structure stability, and thus has high mechanical strength and excellent heat cycle resistance and other reliability. Moreover, compared with the traditional Sn-Ag-Cu solder, the alloy material is low in cost and easy to obtain.

Description

Technical Field

[0001] This invention relates to the field of high-strength, medium-temperature, environmentally friendly solder technology, specifically to a high-strength lead-free solder for power electronics and its preparation method. Background Technology

[0002] New electronic components, integrated circuits, and display devices used in electronic information equipment are developing towards higher frequencies, higher speeds, more multifunctionality, higher performance, smaller size, thinner profiles, and higher density, placing more stringent demands on these devices and their connections. As a result, the mechanical, electrical, and thermal loads borne by a single solder joint increase exponentially. Generally, during the service life of electronic products, solder joints are the weakest link in the entire electronic system, especially for power electronic devices, which generate a lot of heat during operation, and the solder joints operate in a more severe environment. This places higher demands on the temperature resistance, structural stability, and reliability of microelectronic interconnect materials.

[0003] Statistics show that 70% of defects in the electronics manufacturing supply chain originate from processes related to solder materials, and up to 60% of quality incidents during product manufacturing and service life stem from defects in solder materials. Currently, practical lead-free solders include Sn-Ag, Sn-Cu, Sn-Bi, Sn-Sb, Sn-Zn, and Sn-Ag-Cu. Among these, Sn-Bi alloys have low melting points and are prone to segregation and embrittlement; Sn-Sb alloys have relatively high melting points; and Sn-Zn alloys have moderate melting points, but oxidation and corrosion problems are difficult to solve. Therefore, Sn-Ag-Cu alloys are more widely used, especially Sn-3.0Ag-0.5Cu. However, in use, Sn-Ag-Cu solder suffers from insufficient resistance to external environments due to its thick and unstable intermetallic compound (IMC) layer, high electrical interference, and inadequate mechanical reliability. This is particularly true after prolonged exposure to high temperatures or extreme temperature fluctuations, which can easily lead to degradation of critical interconnect components such as pins and solder joints.

[0004] Although currently available lead-free solders contain Bi and Sb elements, none of them mention how to control the segregation and growth of Bi that leads to brittleness. Furthermore, the aforementioned alloys lack consideration for the reliability of the solder interface in applications, and the solder interface remains unstable and prone to "Kirkendall" voids during use. The purpose of this invention is to improve the impact resistance and reliability of the solder by adding beneficial components to it.

[0005] In fact, besides adding and selecting beneficial components, avoiding an indiscriminate expansion of the alloy addition range, precise composition design (reasonable component ratio) and process implementation are crucial to effectively prevent segregation and abnormal microstructure coarsening. Especially since failures under harsh conditions often occur at the interface of weld joints, it is even more important to improve the interface between the solder and the welded materials through solder composition design.

[0006] Therefore, this invention is proposed. Summary of the Invention

[0007] The main objective of this invention is to propose a high-strength lead-free solder for power electronics and its preparation method. The lead-free solder of this invention is a high-strength lead-free flexible solder. The solder microstructure exhibits no Bi precipitation segregation and contains the interface-stabilizing element Bi, resulting in excellent matrix and interface microstructure stability. Consequently, it possesses high mechanical strength and excellent reliability, including superior resistance to thermal cycling. Furthermore, compared to traditional Sn-Ag-Cu solder, the alloy material is inexpensive and readily available.

[0008] To achieve the above objectives, the present invention provides the following technical solutions.

[0009] A first aspect of the present invention provides a high-strength lead-free solder for power electronics, comprising the following elements by weight percentage:

[0010] Bi 2.5-3.5%, Sb 2.5-3.5%, B 0.001-0.05%, with the remainder being Sn and unavoidable impurities.

[0011] This invention relates to a Sn-Bi-Sb-B series high-strength lead-free solder for power electronics. It has a moderate melting point, is easy to solder, and can achieve full interconnect joints without changing the process of existing Sn-Ag-Cu lead-free solder. Furthermore, this solder has high strength, stable structure, and excellent resistance to temperature cycling and external impact. Therefore, using this solder for soldering can result in highly reliable power electronic circuits.

[0012] Furthermore, the lead-free solder also contains one or more of the elements Ag, Cu, and Ni.

[0013] Adding elements such as Ag, Cu, and Ni to solder can lower the melting temperature, achieving the same effect as conventional Sn-Ag-Cu solder. In other words, appropriate addition can also reduce the dissolution of the pad metal, improve brazing wettability, and form intermetallic compounds (IMCs) to further enhance the solder strength and crack propagation resistance.

[0014] Furthermore, when the lead-free solder contains Ag, Cu, and Ni elements, their respective contents, expressed as a percentage by weight, are as follows:

[0015] Ag 3.0~4.0%, Cu 0.5~0.7%, Ni 0.03~0.05%.

[0016] A second aspect of the present invention provides a method for preparing a high-strength lead-free solder for power electronics, comprising the following steps:

[0017] Bi-Sb master alloys were prepared using a melt superheating method.

[0018] Sn-B master alloy was prepared by mechanical alloying.

[0019] Pure Sn, the Bi-Sb master alloy, and the Sn-B master alloy are mixed in a certain alloy ratio and melted below the melt transition temperature of the Bi-Sb master alloy, and then cast to obtain the lead-free solder.

[0020] Furthermore, during smelting, Ag is introduced as a Sn-Ag master alloy.

[0021] Preferably, during smelting, Cu element is introduced as a Sn-Cu master alloy.

[0022] Preferably, during smelting, Ni is introduced as a Sn-Ni master alloy.

[0023] Furthermore, the preparation of the Bi-Sb master alloy includes mixing Bi ingots and Sb ingots, heating them to the melt structure transformation temperature of the Bi-Sb master alloy, holding the mixture at that temperature, cooling it down for casting, and then cooling and solidifying to obtain the Bi-Sb master alloy.

[0024] Preferably, the melt structure transformation temperature of the Bi-Sb master alloy is 1150±100℃.

[0025] Preferably, the heat preservation time is 20 to 30 minutes.

[0026] Furthermore, the preparation of the Sn-B master alloy includes mixing Sn powder and B powder by mechanical alloying, ball milling, and pressing into blocks to obtain the Sn-B master alloy.

[0027] Preferably, the ball-to-material ratio in the ball mill is 1:5 to 1:20; the ball milling time is ≥30 min, preferably 30 min to 2 h.

[0028] Preferably, the weight percentage of boron in the Sn-B master alloy is 3-5 wt%.

[0029] Furthermore, the mixing and melting temperature of the pure Sn, the Bi-Sb master alloy, and the Sn-B master alloy is 300-500℃, and the mixture is kept at this temperature and stirred for 10-20 minutes; during the melting process, the surface is covered with an antioxidant.

[0030] Preferably, pure Sn, Bi-Sb master alloy, and Sn-B master alloy are added sequentially during smelting.

[0031] Furthermore, the preparation method also includes converting the lead-free solder into any one of the following forms: powder, paste, ball, sheet, wire, strip, or tape.

[0032] The third aspect of the present invention provides a solder joint or weld seam formed by using the high-strength lead-free solder for power electronics provided in the first aspect of the present invention or the preparation method provided in the second aspect of the present invention.

[0033] This invention utilizes materials calculations to innovatively design alloy compositions by comprehensively considering three factors: atomic radius of elements, bond energy between different elements, and surface tension. This avoids component segregation and element agglomeration in the alloy, and through a specific process, poorly miscible elements are uniformly dispersed and added to the alloy melt. Specifically, the Sn-Bi-Sb-B solder alloy design utilizes the Bi-Sb interaction to improve the solder's temperature resistance and high / low temperature cycling resistance; and utilizes the small-atom B interface to enrich elements, reducing stress cracking and toughening the welding interface.

[0034] As is well known, Bi can significantly improve the strength of Sn and lower its melting point, promoting wetting. However, Bi is prone to segregation and growth, leading to solder embrittlement and the appearance of low-melting-point phases. Bi and Sb are infinitely soluble in each other. Utilizing the irreversible nature of the "melt structure transformation" in Bi-Sb alloys, where Sb atoms replace Bi lattice atoms, can suppress Bi segregation and growth in the material microstructure. To induce the melt structure transformation in the Bi-Sb alloy melt, Bi and Sb are preferentially heated and eutecticly melted above the melt structure transformation temperature to prepare a Bi-Sb master alloy. Then, at a relatively low liquid temperature, the Bi-Sb master alloy is added to the molten tin to ensure short-range order of Bi-Sb, thereby avoiding Bi-Bi bonding. On the other hand, calculations and experimental verification show that boron (B), as a small atom additive element in this alloy system, has the effect of enriching the Cu substrate side interface. The addition of an appropriate amount of B can improve the element distribution and interface layer structure between the solder and the substrate, reduce the residual stress after the interface reaction, compensate for the "Kirkendall" void phenomenon caused by the different diffusion rates between Cu and Sn, and inhibit the transition fusion of the interface between the solder and the substrate and the growth of IMC between Sn and Cu, thereby improving the reliability of the brazing interface.

[0035] Compared with the prior art, the present invention achieves the following technical effects:

[0036] The Sn-Bi-Sb-B series high-strength lead-free solder for power electronics provided by this invention has a microstructure without segregation or obvious Bi-rich phase, and a melting temperature between approximately 215 and 232°C. Bi is added to the alloy as a Bi-Sb master alloy, which allows the Bi-Sb short-range ordered state to be maintained during subsequent low-temperature melting. B is added as a Sn-B master alloy, which allows it to be easily and effectively dispersed uniformly into the alloy melt during subsequent melting, thereby obtaining a high-strength lead-free solder with the designed composition. This is also an innovation in the solder alloy preparation method of this invention.

[0037] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and in order to make the above and other objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described below. Attached Figure Description

[0038] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. In the drawings:

[0039] Figure 1 Differential scanning calorimetry (DSC) curves of the Sn-Bi3-Sb3-B0.03 solder alloy prepared in Example 1;

[0040] Figure 2 Differential scanning calorimetry (DSC) curves of the Sn-Bi3.2-Sb3-B0.012-Ag3.36-Cu0.62-Ni0.045 solder alloy prepared in Example 16;

[0041] Figure 3 A visual bar chart comparing the alloy strength of the high-strength lead-free solder prepared in the embodiments of the present invention with that of the commercially available solder in the comparative example.

[0042] Figure 4 This is a comparison chart of high- and low-temperature cycling tests between the high-strength lead-free solder prepared in the embodiments of the present invention and the commercially available solder in the comparative example. Detailed Implementation

[0043] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0044] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the invention, are intended to cover non-exclusive inclusion.

[0045] In the description of the embodiments of this invention, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this invention, "multiple" means two or more, unless otherwise explicitly defined.

[0046] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0047] In the description of the embodiments of this invention, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0048] In the description of the embodiments of the present invention, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0049] In the description of the embodiments of the present invention, unless otherwise explicitly specified and limited, the technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of the present invention according to the specific circumstances.

[0050] With growing awareness of environmental protection and restrictions imposed by regulations such as the EU RoHS, electronic components and interconnect solders are gradually becoming lead-free. However, driven by the development of 5G infrastructure and automotive electrification, two engines of power device development, the increasing density of electronic circuits has multiplied the mechanical, electrical, and thermal loads borne by each solder joint. Traditional Sn-Ag-Cu lead-free solders can hardly ensure high reliability of interconnects, making it urgent to develop a lead-free solder that can withstand high loads.

[0051] To address the aforementioned problems, a first aspect of the present invention provides a high-strength lead-free solder for power electronics, the lead-free solder comprising the following elements by weight percentage:

[0052] Bi 2.5-3.5%, Sb 2.5-3.5%, B 0.001-0.05%, with the remainder being Sn and unavoidable impurities.

[0053] In embodiments of the present invention, the weight percentage of Bi element is one of 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3.0%, 3.1%, 3.2%, 3.3%, 3.4%, and 3.5%, or any value satisfying the above range.

[0054] In embodiments of the present invention, the weight percentage of Sb element is one of 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3.0%, 3.1%, 3.2%, 3.3%, 3.4%, and 3.5%, or any value satisfying the above range.

[0055] In embodiments of the present invention, the weight percentage of element B is one of 0.001%, 0.002%, 0.004%, 0.006%, 0.008%, 0.01%, 0.02%, 0.03%, 0.04%, and 0.05%, or any value satisfying the above range.

[0056] To improve soldering properties and lower melting point, the high-strength lead-free solder for power electronics in this invention also contains one or more of the following improving elements: Ag, Cu, Ni, etc.

[0057] In some embodiments of the present invention, when the lead-free solder contains Ag, Cu, and Ni elements, their respective contents by weight percentage are: Ag 3.0-4.0%, Cu 0.5-0.7%, and Ni 0.03-0.05%.

[0058] For example, the weight percentage of Ag element is one of 3.0%, 3.1%, 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, 3.7%, 3.8%, 3.9%, 4.0% or any value that satisfies the above range.

[0059] For example, the weight percentage of Cu is one of 0.5%, 0.6%, 0.7% or any value within the range mentioned above.

[0060] For example, the weight percentage of Ni is one of 0.03%, 0.04%, 0.05% or any value that meets the above range.

[0061] The second aspect of this invention provides a method for preparing a high-strength lead-free solder for power electronics, the key being the ability to suppress Bi segregation and growth in the high-strength solder alloy and to introduce poorly miscible B into the high-strength tin-based alloy. Specifically, Bi and Sb elements are added as a Bi-Sb master alloy, and B element is added as a Sn-B master alloy with a high degree of alloying.

[0062] The preparation method of the high-strength lead-free solder for power electronics in this invention is carried out according to the following steps.

[0063] 1) Preparation of Bi-Sb master alloy.

[0064] In an embodiment of the present invention, a Bi-Sb master alloy is prepared by a melt superheating method.

[0065] Bi ingots and Sb ingots are weighed according to the design ratio, then heated to the melt structure transformation temperature of the Bi-Sb master alloy, held at the temperature for a certain time, cooled and cast, and then cooled and solidified to obtain the Bi-Sb master alloy.

[0066] In some embodiments of the present invention, the melt structure transformation temperature of the Bi-Sb master alloy is 1150±100℃.

[0067] By utilizing the irreversibility of the "melt structure transformation" of Bi-Sb alloy, which causes Sb atoms to replace Bi lattice atoms, the segregation and growth of Bi in the material structure can be suppressed. In order to induce the melt structure transformation of Bi-Sb alloy melt, Bi-Sb master alloy is prepared by heating Bi ingots and Sb ingots to the melt transformation temperature.

[0068] For example, the melt structure transformation temperature of the Bi-Sb master alloy can be one of 1050℃, 1070℃, 1100℃, 1123℃, 1125℃, 1150℃, 1170℃, 1200℃, and 1250℃, or any value that satisfies the above range.

[0069] In some embodiments of the present invention, the heat preservation time is 20 to 30 minutes, which can be set according to the actual situation.

[0070] 2) Preparation of Sn-B master alloy.

[0071] In some embodiments of the present invention, a high-B-concentration Sn-B master alloy is prepared by mechanical alloying.

[0072] In some embodiments of the present invention, the weight percentage of B element in the Sn-B master alloy is 3 to 5 wt%.

[0073] In an embodiment of the present invention, Sn powder and B powder are mixed and ball-milled in a mechanical alloying manner according to a design ratio, and then pressed into blocks to obtain Sn-B intermediate alloy.

[0074] In some embodiments of the present invention, the ball-to-material ratio in the mixed powder ball mill is 1:5 to 1:20.

[0075] In some embodiments of the present invention, the ball milling time is ≥30 min, for example, it can be 30 min to 2 h, and can be set according to actual needs. It should be noted that organic solvents, such as alcohol or paraffin, can be added as lubricants during the ball milling process.

[0076] 3) The prepared Bi-Sb master alloy, Sn-B master alloy and pure Sn are melted in a melting furnace according to a certain alloy ratio. The melting temperature is below the melt transformation temperature of Bi-Sb master alloy and above the melting temperature of pure Sn. During the melting process, the surface is covered with rosin or KCL-LiCl mixed molten salt to prevent oxidation. The mixture is kept at the temperature and stirred for 10 to 20 minutes. The surface covering and oxide slag are removed and the mixture is poured into a mold to make Sn-Bi-Sb-B solder alloy ingot for later use in the preparation of solder powder, solder paste, solder balls, solder sheets, wires, strips and other forms of solder.

[0077] In embodiments of the present invention, the melting temperature is in the range of 300 to 500°C.

[0078] For example, the melting temperature can be one of 300°C, 350°C, 400°C, 450°C, 500°C or any value that meets the above range.

[0079] It is worth mentioning that during smelting, pure Sn, Bi-Sb master alloy, and Sn-B master alloy are added in sequence to ensure short-range order of Bi-Sb, thereby avoiding Bi-Bi bonding.

[0080] In some embodiments of the present invention, pure Sn is heated to a melting temperature of 300-500°C, and then the corresponding Bi-Sb master alloy and Sn-B master alloy are added. The mixture is held at this temperature for 10-20 minutes to remove surface oxide slag, and then poured into a mold to prepare Sn-Bi-Sb-B solder alloy ingots for later use.

[0081] In an embodiment of the present invention, when it is necessary to further add Ag, Cu, Ni and other improving elements to the Sn-Bi-Sb-B series high-strength lead-free solder for power electronics, Sn-Ag, Sn-Cu, Sn-Ni and other intermediate alloys are added in the corresponding proportions according to the composition design in step 3) above.

[0082] The third aspect of the present invention provides a solder joint or weld seam formed by using the high-strength lead-free solder for power electronics provided in the first aspect of the present invention or the preparation method provided in the second aspect of the present invention.

[0083] In embodiments of the present invention, the solder joint, weld seam, or bonding surface may be formed by welding methods such as solder paste reflow, wave soldering, or hot melting with the substrate being welded.

[0084] The substrate to be soldered includes bare Cu, Cu-OSP treated material, tin plating, Ni plating, Ni-Ag plating, or Ni-Au plating treated material.

[0085] Solder forms include preformed solder sheets, solder strips, solder wires, solder balls, and solder powder.

[0086] The following detailed description of the high-strength lead-free solder for power electronics and its preparation method in this invention will be provided through specific embodiments.

[0087] Example 1

[0088] Example 1 provides a Sn-Bi-Sb-B series high-strength lead-free solder alloy for power electronics. By weight percentage, this lead-free solder alloy comprises: Bi 3.0%, Sb 3.0%, B 0.03%, with the remainder being Sn. This lead-free solder alloy has a melting temperature of 225.1–231.1°C and a strength of 76 MPa.

[0089] The method for preparing this lead-free solder alloy includes the following steps:

[0090] Step 1) Bi ingots with a purity of 99.99% and Sb ingots with a purity of 99.95% are added to a vacuum induction melting furnace at a weight ratio of 1:1. The furnace is evacuated, filled with nitrogen, heated to about 1100°C, held for 20 minutes, and then cooled to 550-650°C for casting to obtain Bi-50Sb master alloy.

[0091] Step 2) Pure Sn powder with a purity of 99.99% and B powder with a purity of 99.99% are added to a ball mill jar at a weight ratio of 19:1. Grinding balls and alcohol are added, the jar is sealed after air is removed, and the powders are ball milled for 2 hours. The mixture is then poured out, filtered, and vacuum dried, and finally pressed into Sn-5B intermediate alloy ingots.

[0092] Step 3) The prepared Bi-50Sb master alloy, Sn-5B master alloy ingot, and pure Sn are added to the melting furnace in an alloy ratio of 93.4:6:0.6 and in the order of pure Sn, Bi-50Sb master alloy, and Sn-5B master alloy ingot. The mixture is heated to 300-500℃ for melting. During the melting process, the surface is covered with rosin to prevent oxidation. The mixture is kept warm and stirred for 20 minutes. The surface covering and oxide slag are removed, and the mixture is poured into a mold to form Sn-Bi3-Sb3-B0.03 solder alloy ingot blank.

[0093] The methods for preparing lead-free solder alloys in Examples 2 to 5 are the same as in Example 1, except that the weight ratio of pure Bi and pure Sb is different in step 1), while the Sn-B master alloy ingot in step 2) is still the Sn-5B master alloy ingot prepared in the same way. Similarly, in step 3), the ratio of Bi-Sb master alloy, Sn-5B master alloy and pure Sn is also based on the final designed composition. Wherein:

[0094] In Example 2, the weight ratio of pure Bi to pure Sb is: Bi:Sb = 5:7;

[0095] In Example 3, the weight ratio of pure Bi to pure Sb is: Bi:Sb = 7:5;

[0096] In Example 4, the weight ratio of pure Bi to pure Sb is: Bi:Sb = 6:7;

[0097] In Example 5, the weight ratio of pure Bi to pure Sb is Bi:Sb = 7:6.

[0098] Example 6

[0099] Example 6 provides a Sn-Bi3-Sb3-B0.03-Ag3 series high-strength lead-free solder alloy for power electronics. By weight percentage, this lead-free solder alloy comprises: Bi 3.0%, Sb 3.0%, B 0.03%, Ag 3.0%, with the remainder being Sn. This lead-free solder alloy has a melting temperature of 218.2–224.2°C and a strength of 84 MPa.

[0100] The method for preparing lead-free solder alloy in Example 6 is the same as in Example 1, except that a commercially available Sn-20Ag master alloy was added in step 3).

[0101] The methods for preparing solder alloys in Examples 7 to 16 are the same as in Example 1, except that:

[0102] The Bi-Sb master alloys prepared in step 1) have different compositions. In step 3), in addition to the different alloy ratios, one or more combinations of commercially available Sn-20Ag master alloys, Sn-10Cu master alloys and Sn-5Ni master alloys were added according to the design ratio.

[0103] Table 1 shows the detailed composition of the solder alloys used in Examples 1 to 16.

[0104] In this invention, commercially available lead-free solders were used as comparative examples 1-3 to compare their material properties with those of the high-strength lead-free solders for power electronics in this invention. The results are detailed in Table 1 and the appendix. Figure 1-4 .

[0105] Comparative Example 1

[0106] Commercially available Sn-Ag3-Cu0.5 lead-free solder is used, with a melting point of 217–219℃.

[0107] Comparative Example 2

[0108] Commercially available Sn-Ag3.8-Cu0.7 lead-free solder is used, with a melting point of 217-218℃.

[0109] Comparative Example 3

[0110] Commercially available Sn-Ag4 lead-free solder is used, with a melting point of 221–225℃.

[0111] Test Experiment

[0112] 1. Melting point measurement:

[0113] Melting point tests were conducted using a STA409PC differential scanning calorimeter (TAInstrument) at a heating rate of 10℃ / min. The sample mass was 30mg. The numerical values ​​were automatically calculated by the software, and the peak temperature of the DSC curve was recorded as the melting point value of the solder alloy.

[0114] 2. Strength test:

[0115] Tensile samples were prepared and their strength was tested in accordance with Japanese Industrial Standard JIS Z 3198.

[0116] 3. High and low temperature cycling test:

[0117] Test conditions refer to IPC9701: low temperature -55℃, high temperature 150℃, and heat preservation time 30min.

[0118] Table 1. Comparison of melting point and strength of solder alloys prepared in Examples 1-16 with commercially available solders in Comparative Examples 1-3.

[0119]

[0120]

[0121] Combined with Table 1 and Appendix Figure 1-4 The data shows that the lead-free solder prepared in this invention has a melting temperature of approximately 215–232°C, high strength, stable structure, and a dense IMC (intermetallic compound) layer at the interface after soldering. It has the ability to withstand harsh temperature cycling from -55°C to 150°C, ensuring the high reliability of the alloy solder joints under long-term thermal and force shocks. It is suitable for the field of soft soldering technology for power electronic products.

[0122] The Sn-Bi-Sb-B series high-strength lead-free solder for power electronics provided in this invention includes solder paste, as well as various forms of electrode-filling materials such as solder bars, solder wires, preformed sheets / balls / pillars.

[0123] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. The present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A high-strength lead-free solder for power electronics, characterized by, The lead-free solder comprises the following elements in the following weight percentages: Bi 2.5~3.5%, Sb 2.5~3.5%, B 0.001~0.05%, and the rest being Sn and inevitable impurities; The preparation method of the high-strength lead-free solder for power electronics comprises the following steps: Bi-Sb intermediate alloy is prepared by a melt high superheat method; Sn-B intermediate alloy is prepared by a mechanical alloying method; Pure Sn, the Bi-Sb intermediate alloy and the Sn-B intermediate alloy are mixed according to a certain alloy ratio and are smelted below the melt transition temperature of the Bi-Sb intermediate alloy, and then the lead-free solder is obtained by pouring.

2. The high-strength lead-free solder for power electronics according to claim 1, wherein The lead-free solder further comprises one or more of Ag, Cu and Ni elements.

3. The high-strength lead-free solder for power electronics according to claim 2, wherein When the lead-free solder contains Ag, Cu and Ni elements, the content of each of Ag, Cu and Ni is respectively 3.0~4.0% in terms of weight percentage. The preparation method comprises the following steps:

4. A method of producing the high-strength lead-free solder for power electronics as claimed in any one of claims 1 to 3, characterized by, Bi-Sb intermediate alloy is prepared by a melt high superheat method; Sn-B intermediate alloy is prepared by a mechanical alloying method; Pure Sn, the Bi-Sb intermediate alloy and the Sn-B intermediate alloy are mixed according to a certain alloy ratio and are smelted below the melt transition temperature of the Bi-Sb intermediate alloy, and then the lead-free solder is obtained by pouring. During smelting, Ag is introduced in the form of Sn-Ag intermediate alloy.

5. The method of producing a high-strength lead-free solder for power electronics according to claim 4, wherein the Sn-Ag-Cu based solder alloy contains 0.1 to 0.5 mass% of Ag and 0.01 to 0.1 mass% of Cu. During smelting, Cu is introduced in the form of Sn-Cu intermediate alloy.

6. The method of producing a high-strength lead-free solder for power electronics according to claim 4, wherein the Sn-Ag-Cu based solder alloy contains 0.1 to 0.5 mass% of Ag and 0.01 to 0.1 mass% of Cu. During smelting, Ni is introduced in the form of Sn-Ni intermediate alloy.

7. The method of producing a high-strength lead-free solder for power electronics according to claim 4, wherein the Sn-Ag-Cu based solder alloy contains 0.1 to 0.5 mass% of Ag and 0.01 to 0.1 mass% of Cu. The preparation of the Bi-Sb intermediate alloy comprises mixing Bi ingot and Sb ingot, heating to the melt structure transition temperature of the Bi-Sb intermediate alloy, keeping warm, then cooling, pouring and cooling and solidifying to obtain the Bi-Sb intermediate alloy.

8. The method of producing a high-strength lead-free solder for power electronics according to claim 4, wherein the Sn-Ag-Cu based solder alloy contains 0.1 to 0.5 mass% of Ag and 0.01 to 0.1 mass% of Cu. The melt structure transition temperature of the Bi-Sb intermediate alloy is 1150±100℃.

9. The method of producing a high-strength lead-free solder for power electronics according to claim 8, wherein the Sn-Ag-Cu based solder alloy contains 0.1 to 0.5 mass% of Ag and 0.01 to 0.1 mass% of Cu. The keeping warm time is 20~30min.

10. The method of producing a high-strength lead-free solder for power electronics according to claim 8, wherein the Sn-Ag-Cu based solder alloy contains 0.1 to 0.5 mass% of Ag and 0.01 to 0.1 mass% of Cu. The preparation of the Sn-B intermediate alloy comprises mixing Sn powder and B powder by a mechanical alloying method, ball milling and pressing into a block to obtain the Sn-B intermediate alloy.

11. The method of producing a high-strength lead-free solder for power electronics according to claim 4, wherein the Sn-Ag-Cu based solder alloy contains 0.1 to 0.5 mass% of Ag and 0.01 to 0.1 mass% of Cu. The ball-to-material ratio of the ball milling is 1:5~1:20, and the ball milling time is ≥30min.

12. The method of producing a high-strength lead-free solder for power electronics according to claim 11, wherein The ball milling time is 30min~2h.

13. The method of producing a high-strength lead-free solder for power electronics according to claim 12, wherein The weight percentage of B in the Sn-B intermediate alloy is 3~5wt%.

14. The method of producing a high-strength lead-free solder for power electronics according to claim 4, wherein the Sn-Ag-Cu based solder alloy contains 0.1 to 0.5 mass% of Ag and 0.01 to 0.1 mass% of Cu. The mixing and smelting temperature of the pure Sn, the Bi-Sb intermediate alloy and the Sn-B intermediate alloy is 300~500℃, and the keeping warm and stirring time is 10~20min; the surface is covered with an antioxidant during smelting.

15. The method of producing a high-strength lead-free solder for power electronics according to claim 4, wherein the Sn-Ag-Cu based solder alloy contains 0.1 to 0.5 mass% of Ag and 0.01 to 0.1 mass% of Cu. The pure Sn, the Bi-Sb intermediate alloy and the Sn-B intermediate alloy are sequentially added in order during smelting.

16. The method of producing a high-strength lead-free solder for power electronics according to claim 4, wherein the Sn-Ag-Cu based solder alloy contains 0.1 to 0.5 mass% of Ag and 0.01 to 0.1 mass% of Cu. The preparation method further comprises converting the lead-free solder into any one of the following forms: powder, paste, ball, sheet, wire, strip and ribbon.

17. The method of producing a high-strength lead-free solder for power electronics according to claim 4, wherein the Sn-Ag-Cu based solder alloy contains 0.1 to 0.5 mass% of Ag and 0.01 to 0.1 mass% of Cu. 18.A solder joint or seam formed by the high-strength lead-free solder for power electronics according to any one of claims 1-3 or prepared by the preparation method according to any one of claims 4-17. ​

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