A SnAgCuBiIn-based lead-free solder alloy, its design method and preparation method

Abstract

The present invention discloses a SnAgCuBiIn-based lead-free solder alloy, a design method thereof and a preparation method thereof. The solder alloy contains 1.0 - 5.5% Ag, 0.5 - 1.0% Cu, 0.5 - 5.0% Bi, 0.4 - 4.0% In, 0 - 1.0% Ti, 0 - 0.5% Ni, and the balance is Sn. The design method of the solder alloy of the present invention uses machine learning to assist in the design of solder components. First, experimental data is collected to obtain an initial data set of solder alloy components and mechanical properties; then the maximum mutual information coefficient and Pearson correlation coefficient are used to screen alloy components with a relatively large correlation with mechanical properties; then the gradient descent tree algorithm is used for modeling and training; the model is verified by the leave-one-out cross-validation method; then the virtual samples are input into the machine learning model to obtain the prediction results; systematic tests are carried out for verification to obtain a solder alloy with excellent performance. The preparation method of the present invention uses a method combining a high-throughput vacuum arc melting furnace and an electromagnetic induction furnace for melting. The present invention significantly improves the mechanical properties and wetting properties of the solder alloy, has high strength, good creep resistance, brazing characteristics and oxidation resistance, and has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of electronic micro-connection and packaging materials, and relates to a high-reliability lead-free solder alloy of the SnAgCu system, and its design and preparation methods. Background Art

[0002] Tin-lead (Sn-Pb) alloy solders have the advantages of excellent welding performance, high reliability, easy preparation, and low price, and have been used in the field of electronic component packaging for more than 50 years. However, due to the fact that Pb and its compounds will pollute the environment and endanger human health, Sn-Pb solders have gradually been prohibited from use, and lead-free solders have become the main development trend in current electronic packaging and micro-connection. With the continuous development of the electronic industry and the miniaturization and multi-functionality of electronic products, it has promoted the development of electronic packaging technology towards the direction of high density, high functionality, and high integration. Since the substrate has become shorter and shorter, the size of solder joints and the pitch of solder joints have become narrower and narrower, while the mechanical, electrical, and thermodynamic loads they bear have become heavier and heavier. Therefore, more severe challenges have been posed to their reliability.

[0003] To solve these problems, it is necessary to develop lead-free solder alloys with low melting points, high strength, and good creep resistance. Among current lead-free solders, Sn-Ag-Cu (SAC) solder alloys have excellent comprehensive properties and have become the standard lead-free solder alloys internationally. For SAC alloys, different countries have different composition standards. The European Union advocates controlling the composition near Sn3.8Ag0.7Cu (SAC387). The compositions recommended by the Japan Electronics and Information Technology Industries Association are Sn3.0Ag0.5Cu (SAC305) and Sn3.5Ag0.7Cu. The Sn-Ag-Cu alloy has an almost eutectic alloy composition, and its melting point range is between 183°C and 232°C. Among them, Ag and Sn form the Ag3Sn intermetallic compound, which is beneficial to improving the mechanical properties and reliability of the solder. Cu in the solder can, to a certain extent, slow down the dissolution and diffusion of substrate Cu into the solder and reduce the thickness of the intermetallic compound interface layer. Taking the high-silver alloy SAC387 as an example, it has good wettability and is widely used, but there are still problems such as low strength, and the need to improve its resistance to thermal fatigue and creep performance, and it cannot meet the use requirements in special and harsh service environments.

[0004] The material R & D concept of the Materials Genome integrates material databases, material calculations, and high-throughput experiments, accelerating the R & D process of new materials and achieving a double reduction in R & D costs and R & D cycles. Applying machine learning (ML) to the R & D of new materials is an important means of Materials Genome R & D technology. By constructing a material data set, training a model in a flexible and highly non-linear form, finding the relationship between different factors and properties in material design, and combining expert domain knowledge, rapid design of new materials can be carried out according to performance requirements. Summary of the Invention

[0005] To solve the problems of the prior art, the purpose of the present invention is to overcome the deficiencies of the existing technologies, and provide a SnAgCuBiIn-based lead-free solder alloy, its design method and preparation method, to optimize the composition range and design and development of high-reliability lead-free solder alloys. Through machine learning methods and combined with expert knowledge, the present invention optimizes a SnAgCu-based lead-free solder alloy. The present invention also solves the problems of uneven alloy composition and difficulty in melting caused by metals with higher melting points, and has a higher preparation efficiency. Multiple alloys with different compositions can be prepared simultaneously in a high-throughput vacuum arc melting furnace to meet the usage requirements of different solder alloys.

[0006] To achieve the above object of the invention, the following technical solutions are adopted:

[0007] A SnAgCuBiIn-based lead-free solder alloy, by weight percentage, contains 1.0 - 5.5% Ag, 0.5 - 1.0% Cu, 0.5 - 5.0% Bi, 0.4 - 4.0% In, 0 - 1.0% Ti, 0 - 0.5% Ni, and the balance is composed of Sn.

[0008] Preferably, the SnAgCuBiIn-based lead-free solder alloy of the present invention, by weight percentage, contains 3.0 - 4.0% Ag, 0.5 - 0.7% Cu, 1.0 - 3.5% Bi, 0.5 - 3.5% In, 0.1 - 0.8% Ti, 0.1 - 0.2% Ni, and the balance is composed of Sn.

[0009] Further preferably, for the composition and content of the SnAgCuBiIn-based lead-free solder alloy of the present invention, by weight percentage, the alloy further contains at least one or more of the following alloy elements: Ce ≤ 0.2%, Ge ≤ 0.2%.

[0010] Further preferably, for the composition and content of the SnAgCuBiIn-based lead-free solder alloy of the present invention, by weight percentage, the alloy further contains at least one or more of the following alloy elements: 0.1 - 0.2% Ce, 0.05 - 0.2% Ge.

[0011] A design method for the SnAgCuBiIn-based lead-free solder alloy of the present invention, which uses machine learning to assist in the design of the lead-free solder composition. The method comprises the following steps:

[0012] (1) Collect experimental data to obtain an initial data set of the lead-free solder alloy composition and mechanical properties;

[0013] (2) Use the Maximal Information Coefficient (MIC) and Pearson correlation coefficient to screen the alloy components with a relatively large correlation with the mechanical properties;

[0014] (3) Take the alloy components as the input and the mechanical properties as the output, and use the Gradient Boosting Decision Tree (GDBT) algorithm for modeling and training; the model is verified by the leave-one-out cross-validation method;

[0015] (4) Design the alloy component combination and step size, construct virtual samples in a permutation and combination manner, and then input the virtual samples into the machine learning model to obtain the prediction results; select points at the edge of the virtual sample space for systematic experimental verification to obtain a solder alloy with excellent performance.

[0016] A preparation method for the SnAgCuBiIn-based lead-free solder alloy of the present invention, which uses a combination of high-throughput vacuum arc melting and induction melting to prepare the alloy. The preparation method includes the following steps:

[0017] a. In a high-throughput vacuum arc melting furnace, use materials containing metals Ti, Ni, Ce, Ge, and Sn with a mass fraction of not less than 99.9% as the alloy component raw materials, and prepare Sn-Ti master alloys, Sn-Ni master alloys, Sn-Ce master alloys, and Sn-Ge master alloys with a mass fraction of Sn-5% Ti, Sn-4% Ni, Sn-7% Ce, and Sn-5% Ge respectively;

[0018] During the preparation of the master alloy, first evacuate the inside of the high-throughput vacuum arc melting furnace. When the vacuum degree is not higher than 1×10 -3 Pa, fill it with protective argon gas to not less than 0.5 atm, load the current to 110 A, and ignite pure Ti metal to consume the remaining oxygen in the furnace; then place the prepared alloy component raw materials in the melting station, load the current to 110 - 120 A, ignite the metal in the melting station until the alloy component raw materials are completely melted. After extinguishing the arc, wait for the alloy to cool completely and then turn it over and remelt and cool it. Repeat the melting 4 times to ensure the uniformity of the alloy composition and obtain the master alloy for standby;

[0019] b. According to the composition of the target SnAgCuBiIn lead-free solder alloy to be prepared, calculate and weigh pure metal raw materials of Sn, Ag, Cu, Bi, and In with a mass fraction of not less than 99.9% and the master alloy prepared in the step a, and perform stock preparation;

[0020] Use a vacuum induction furnace for melting. Before melting, repeat the same operations of vacuum pumping and argon filling as in the step a three times to remove the oxygen in the furnace. Finally, perform melting in an argon environment. When melting the alloy, first put the pure metal raw materials of Sn, Ag, Cu, Bi, and In into the crucible, heat up to not less than 300 °C and keep warm for 15 - 20 minutes; then select Sn-Ti master alloy, Sn-Ni master alloy, Sn-Ce master alloy, and Sn-Ge master alloy according to the composition design requirements and put them into the crucible, heat up to not less than 500 °C, keep warm for 20 - 30 minutes, then cool down to not higher than 300 °C, and pour the alloy liquid into a stainless steel mold for cooling and forming to obtain the finished SnAgCuBiIn lead-free solder alloy.

[0021] Compared with the prior art, the present invention has the following obvious outstanding substantial features and remarkable advantages:

[0022] 1. The present invention guides the design of high-performance lead-free solder alloys by integrating machine learning methods, optimizes the alloy composition, and accelerates the material R & D process;

[0023] 2. The present invention uses high-throughput vacuum arc melting to prepare master alloys, solves the problems of uneven alloy composition and refractoriness caused by metals with higher melting points, improves the reliability of smelting, and has a higher preparation efficiency. Multiple alloys with different compositions can be prepared simultaneously in a high-throughput vacuum arc melting furnace;

[0024] 3. The present invention guides alloy design according to the screening results of the machine learning model, and at the same time combines the expert domain knowledge. A certain amount of Ag, Cu, Bi, In, Ti, and Ni elements are added to the solder alloy, which significantly improves the strength and creep resistance of the solder alloy and reduces the possibility of internal shear fracture of the solder joints under harsh service conditions;

[0025] 4. Experimental verification shows that the addition of Bi and In elements can effectively improve the strength and creep resistance of the solder, and the simultaneous addition of the two elements has excellent synergistic strengthening effect; usually, the addition of excessive Bi elements will cause some Bi elements to precipitate in the form of precipitates during the solidification of the alloy, and the Bi precipitates are prone to coarsening in an environment with continuously changing temperature, thus having an adverse effect on the mechanical properties of the solder alloy; in the present invention, the addition of 2.8% by mass of In can significantly increase the solid solubility of Bi in the Sn matrix, effectively reduce the coarsening of Bi precipitates, and further reduce the adverse effect on the alloy properties; at the same time, the increase in the solid solubility of Bi in the Sn matrix means that the solid solution strengthening effect of Bi will be enhanced, which can further improve the creep resistance of the alloy;

[0026] 5. The addition of trace amounts of Ce and Ge in the present invention can further improve the oxidation and corrosion resistance of the solder alloy, and at the same time can maintain good melting characteristics and wetting properties, which is beneficial to improving the size stability and drop resistance of the solder joints after soldering, thereby enhancing the reliability of the solder joints under harsh service conditions. Brief Description of the Drawings

[0027] Figure 1 is the overall flow chart of the alloy design of the present invention.

[0028] Figure 2 is the result diagram of the maximum mutual information coefficient and Pearson correlation coefficient of the present invention.

[0029] Figure 3 is the trend diagram of the selected point distribution in the virtual space and the mechanical properties varying with the composition content of the present invention.

[0030] Figure 4 is the trend diagram of the mechanical properties of the experimental samples of the present invention varying with the composition content.

[0031] Figure 5 is the differential thermal analysis diagram of the solder alloy of Example 8 of the present invention.

[0032] Figure 6 is the tensile curve diagram of the solder alloy of Example 5 of the present invention.

[0033] Figure 7 is the wetting angle diagram of the solder alloy of Example 3 of the present invention.

[0034] Figure 8 is the metallographic structure diagram of the solder alloy of Example 4 of the present invention.

[0035] Figure 9 is the metallographic structure diagram of the solder alloy of Example 6 of the present invention.

[0036] Figure 10 is the oxidation resistance comparison diagram between the examples of the present invention and the comparative samples. Specific Embodiments

[0037] The object of the present invention is to use machine learning methods to guide alloy design. By adding alloying elements such as Bi, In, Ni, Ti, etc. on the basis of SnAgCu-based lead-free solder alloys, and in addition, including at least one or more elements of Ce, Ge, etc., while ensuring good wettability, the mechanical properties and creep resistance of the solder alloy are improved. The problem to be solved by the present invention is to develop a lead-free solder alloy with good wettability, good solderability, excellent mechanical properties and high reliability, and improve the service performance of welded devices in harsh environments.

[0038] Starting from improving the comprehensive mechanical properties of the solder alloy, the present invention optimizes and improves traditional lead-free tin-based solders. In the field of electronic packaging, the final application effect of lead-free solders is reflected in the reliability of the prepared welded devices. During use, due to the frequent influence of stress, strain and thermal shock on the solder joints, cracks may initiate at the solder alloy itself or at the connection with the substrate, and then fracture and fail. Therefore, improving the comprehensive mechanical properties of the solder alloy is beneficial to reducing the possibility of failure of the solder or solder joints under harsh service conditions, such as high load and alternating thermal stress conditions. Good wettability can enhance the bonding strength between the solder alloy and the substrate, and further improve the reliability of the solder joints. The present invention combines machine learning technology and adds a variety of trace alloying elements to the SnAgCu system simultaneously to achieve the effect of improving the comprehensive mechanical properties of the solder.

[0039] As Figure 1 shown, the present invention first guides the design of lead-free solder alloys through machine learning, and further elaborates on the machine learning method in combination with the accompanying drawings:

[0040] 64 pieces of experimental data were collected as the initial data set. The alloying components involved were Sn, Ag, Cu, Bi, In, Sb, Ti, Ni, Zn and Al, and the component combinations were carried out in mass percentages. The mechanical properties included tensile strength and elongation at break.

[0041] The results of screening alloying elements using the maximum mutual information coefficient and Pearson correlation coefficient are as Figure 2 shown. The components involved were Sn, Bi, In, Sb, Ti, Ni, Zn and Al. From the results, it can be seen that Bi and In have the greatest correlation with the mechanical properties. Therefore, in the subsequent content, the SnAgCuBiIn system is used as the basic system for alloy design.

[0042] The SnAgCuBiIn alloy was arranged and combined with a step size of 0.1%, and a total of 2091 virtual samples were obtained. The virtual samples were input into the model to obtain the prediction results, and selected points were experimentally verified. Figure 3It is a trend chart of the prediction results of the mechanical properties of virtual samples varying with the component content. Figure 4 It is a trend chart of the experimental results of the mechanical properties of verification samples varying with the component content. According to Figure 3 and Figure 4 the results, the component range of the alloy is preferably selected.

[0043] The lead-free solder alloy preferably selected in the embodiment of the present invention is composed of the following elements by weight percentage: 3.0 - 4.0% Ag, 0.5 - 0.7% Cu, 1.0 - 3.5% Bi, 0.5 - 3.5% In. In addition, it contains at least one selected from the following alloy elements: 0.1 - 0.8% Ti, 0.1 - 0.2% Ni, 0.1 - 0.2% Ce, 0.05 - 0.2% Ge.

[0044] Meanwhile, the present invention has also carried out relevant determination and screening on the melting point of the solder. The melting point temperature of the solder is an important parameter index for determining the reflow soldering process. Generally, the reflow soldering peak temperature of the solder is 25°C higher than the melting point of the solder. If the reflow soldering peak temperature is too high, it is easy to cause irreversible damage to the PCB and electronic components. For example, during welding, if the reflow soldering peak temperature exceeds the glass transition temperature of the PCB, it will cause board warping or softening of local materials. On the other hand, taking the most widely used lead-free solder SAC305 as an example, if the melting temperature of the newly prepared lead-free solder is much higher than that of SAC305, it may be necessary to replace the reflow soldering equipment or formulate a new process flow, increasing additional costs. Therefore, the melting temperature of the lead-free solder should be as matched as possible with the working temperature of the current soldering equipment.

[0045] The equipment used for testing the melting point in the following embodiments of the present invention is a synchronous thermal analyzer (Simultaneous Thermal Analyzer, STA - DTA) produced by Netzsch Company. The test sample is 15 mg of solder alloy powder, the heating rate is 5°C / min, and the cooling rate is 10°C / min. The solidus temperature of the solder alloy taken in the present invention is the starting point of the peak of the heating curve, and the liquidus temperature is the temperature at the peak point of the heating curve.

[0046] The embodiment of the present invention has also tested the oxidation performance of the preferably selected solder. The thermogravimetric analysis function in the STA - DTA equipment is used to conduct an antioxidant experiment on the solder. The sample weight is 30 mg, and it is kept at a constant temperature of 280°C for 60 min. The result is expressed as the weight gain percentage of the sample.

[0047] In the embodiments of the present invention, the wetting performance of the selected solder is also tested. The wettability of solder refers to the wetting ability of molten solder on the base material. Solder with good wettability can make the connection between electronic devices and the substrate more reliable and improve the reliability of solder joints. The wetting performance of solder can be measured by indicators such as wetting angle, wetting force, wetting time, and wetting area. The main factors affecting wettability include: the composition of solder and base material, temperature, intermetallic compounds formed at the interface, flux, the surface state of the base material, surface active substances, etc.

[0048] In the embodiments of the present invention, the wetting angle is used to measure the wettability of solder. The size of the copper sheet used in the experiment is 30mm×30mm×1mm. After removing the surface oxide layer and oil stain with sandpaper, it is immersed in acetone for ultrasonic cleaning; the lead-free solder is made into a block of 0.2g with a deviation of ±1%, and then placed in ethanol for ultrasonic cleaning; a flux is selected, the solder is placed in the center of the copper sheet, the flux is applied, and it is placed horizontally in a reflow soldering machine, and the heating temperature curve is set; after cleaning the spreading sample, a wetting angle measuring instrument is used to measure the wetting angle of the spreading sample; 3 parallel samples are measured for each sample, and the average value is taken.

[0049] In the embodiments of the present invention, the creep performance of the selected solder is tested. In the embodiments of the present invention, an iMicro type nanoindentation instrument produced by KLA Corporation is used to test the steady-state creep rate of the solder. The sample size is 5×5×3mm. Before testing, the sample is precisely ground, polished and ultrasonically cleaned to make the sample have better flatness. Experimental parameters: the strain rate is 0.05s -1 , the maximum load is 50mN, the holding time is 5min, the point layout is 3*3, and a total of 9 points are distributed on the entire screen to ensure the uniformity and effectiveness of the data.

[0050] The present invention analyzes the composition of the selected solder, and the alloying elements added in the embodiments have the following functions:

[0051] 1. The function of adding Ag in the embodiments of the present invention is as follows:

[0052] A certain amount of Ag can improve the wettability of the solder alloy. Ag and Sn can form Ag3Sn intermetallic compounds, which are distributed in the Sn matrix in a network form, enhancing the strength of the solder alloy. At the same time, it reduces the liquidus temperature of the solder and narrows the melting range. The Ag content of the solder alloy of the present invention is controlled at 1.0 - 5.5%.

[0053] 2. The function of adding Cu in the embodiments of the present invention is as follows:

[0054] The addition of copper in the solder alloy can inhibit the dissolution of copper elements on the substrate into the solder matrix during the soldering process, thereby inhibiting or reducing the formation of brittle intermetallic compound layers at the interface. At the same time, due to the reduced dissolution of copper elements, the supersaturated state of copper in the solder during the soldering process is also inhibited, thereby reducing the formation of coarse Cu6Sn5 phases and reducing the probability of cracks caused by the coarse Cu6Sn5 phases. At the same time, the Cu element also has a certain effect of refining the grains. The Cu content of the solder alloy of the present invention is controlled at 0.5-1.0%.

[0055] 3. The functions of adding Bi in the embodiments of the present invention are as follows:

[0056] The solid solution of Bi element into the Sn matrix causes the effect of solid solution strengthening, which can significantly enhance the strength of the solder alloy, improve the creep resistance of the solder alloy, reduce the liquidus and solidus temperatures of the solder, and at the same time Bi also reduces the surface tension of the liquid solder, thereby improving the wetting performance of the solder. When the content of Bi in the present invention is less than 1%, the solid solution strengthening effect is not obvious and the solder strength is not greatly improved. When the Bi content exceeds 5.0%, the strength of the solder is greatly improved, but the plasticity drops sharply. The addition of too much Bi element will increase the melting range of the solder, which is not conducive to the soldering process. The Bi content of the solder alloy of the present invention is controlled at 0.5-5.0%, and more preferably 1.0-3.5%.

[0057] 4. The functions of adding In in the embodiments of the present invention are as follows:

[0058] The addition of In can significantly reduce the melting point of the solder; In can be solid-solved into the Sn matrix to play a role in solid solution strengthening. At the same time, In can refine the alloy structure, improve the uniformity of the distribution of strengthening phases in the alloy, and thus improve the mechanical properties and soldering reliability of the solder alloy. When the content of In is less than 0.4%, the solid solution amount of In relative to Sn is small and the solid solution strengthening effect is not significant. In itself is an element prone to oxidation. When the content of In is more than 4.0%, it will reduce the antioxidant ability of the solder alloy, cause the solder alloy to turn yellow, and is prone to generate voids during soldering. The In content of the solder alloy of the present invention is controlled at 0.4-4.0%, and more preferably 0.5-3.5%.

[0059] Under normal circumstances, the addition of Bi element can effectively improve the strength and creep resistance of the solder. In the present invention, when the addition amount of Bi element reaches 3.5%, the precipitation of Bi-containing precipitates is observed under an electron microscope. To a certain extent, the precipitates will hinder the movement of dislocations and improve the alloy strength. On the other hand, in the actual application of the solder, the Bi-containing precipitates in the solder are prone to coarsening in an environment with constantly changing temperatures, thereby having an adverse effect on the mechanical properties of the solder alloy, reducing the plasticity of the alloy, and being prone to stress concentration and crack generation in the solder joints.

[0060] In the present invention, adding 2.8% of In while adding 3.5% of Bi can effectively solve the problem that Bi precipitates are prone to coarsening. At room temperature, the solubility of Bi element in the Sn matrix is certain. During the solidification process of the alloy, the excessive Bi element cannot be completely dissolved into the Sn matrix, and part of Bi precipitates in the form of precipitation phases, distributing at the final solidification points, grain boundaries and other positions. In the present invention, adding 2.8% of In can significantly increase the solubility of Bi in the Sn matrix, effectively reducing the coarsening of Bi precipitates, and further reducing the adverse effects on the alloy properties. The increase in the solubility of Bi in the Sn matrix means that the solution strengthening effect of Bi will be enhanced, which can further improve the creep resistance of the alloy.

[0061] The functions of adding Ti in the embodiments of the present invention are as follows:

[0062] Refine the microstructure of the solder alloy and improve the strength and plasticity of the solder. However, when the content of Ti exceeds 1.0%, coarse intermetallic compound Ti2Sn3 will be formed, reducing the plasticity and toughness of the solder. The content of Ti in the present invention is controlled within 0 - 1.0%.

[0063] The functions of adding Ni in the embodiments of the present invention are as follows:

[0064] Ni can form three intermetallic compounds Ni3Sn, Ni3Sn2, and Ni3Sn4 with Sn, having the effect of precipitation strengthening, and can reduce the dissolution of Cu into the solder, thereby inhibiting the growth of intermetallic compounds at the interface with the copper substrate caused by high - temperature aging, which is beneficial to improving the shear strength and impact resistance of the prepared solder joints. However, when the Ni content is higher than 0.2%, it will increase the melting point of the solder alloy. The Ni content of the solder alloy in the present invention is controlled within 0 - 0.5%.

[0065] The functions of adding Ce and Ge in the embodiments of the present invention are as follows: Adding a small amount of Ce can play a role in refining grains and improving the microstructure. However, adding excessive Ce will form CeSn3, reducing the tensile strength and elongation of the alloy. The content of Ce element in the embodiments of the present invention is controlled within 0 - 0.2%. Ge element has a high - strength oxygen - loving skin effect, promoting the formation of a barrier layer on the surface of the solder during the welding process, which can hinder the further oxidation of the solder and improve the spreading rate of the solder. When the Ge content is too high, a large amount of CeO2 will be generated, increasing the surface tension of the solder, thereby reducing the wettability of the solder. The content of Ge element in the embodiments of the present invention is controlled within 0 - 0.2%.

[0066] Hereinafter, the present invention will be further described in detail. In the following description, the % of the solder alloy composition refers to mass percentage unless otherwise specified.

[0067] The above solution will be further described below in conjunction with specific implementation examples. The preferred embodiments of the present invention are described in detail as follows:

[0068] Example 1

[0069] In this embodiment, a SnAgCuBiIn high-reliability lead-free solder alloy has a weight percentage composition of 5.0% Ag, 0.5% Cu, 2.0% Bi, 0.6% In, and the balance is Sn. The solidus temperature of this lead-free solder is 209.1 °C, the liquidus temperature is 215.3 °C, the tensile strength is 76.7 MPa, the elongation at break is 26.0%, and the wetting angle is 38.2°.

[0070] Example 2

[0071] This embodiment is basically the same as Example 1, except that:

[0072] In this embodiment, a SnAgCuBiIn high-reliability lead-free solder alloy has a weight percentage composition of 3.8% Ag, 0.7% Cu, 0.5% Bi, 0.8% In, and the balance is Sn. The solidus temperature of this lead-free solder is 213.0 °C, the liquidus temperature is 217.5 °C, the tensile strength is 49.2 MPa, the elongation at break is 29.1%, and the wetting angle is 35.3°.

[0073] Example 3

[0074] This embodiment is basically the same as the above embodiments, except that:

[0075] In this embodiment, a SnAgCuBiIn high-reliability lead-free solder alloy has a weight percentage composition of 3.8% Ag, 0.7% Cu, 3.5% Bi, 2.8% In, and the balance is Sn. The solidus temperature of this lead-free solder is 200.1 °C, the liquidus temperature is 207.7 °C, the tensile strength is 85.1 MPa, the elongation at break is 20.7%, and the wetting angle is 28.8°.

[0076] Example 4

[0077] This embodiment is basically the same as the above embodiments, except that:

[0078] In this embodiment, a SnAgCuBiInNi high-reliability lead-free solder alloy has a weight percentage composition of 3.8% Ag, 0.7% Cu, 2.5% Bi, 3.1% In, 0.1% Ni, and the balance is Sn. The solidus temperature of this lead-free solder is 205.9 °C, the liquidus temperature is 215.3 °C, the tensile strength is 80.4 MPa, the elongation at break is 18.2%, and the wetting angle is 34.3°.

[0079] Example 5

[0080] This example is basically the same as the above examples, with the special feature being that:

[0081] In this example, a SnAgCuBiInNi high-reliability lead-free solder alloy, the weight percentage composition of this lead-free solder alloy is 5.5% Ag, 0.7% Cu, 2.0% Bi, 0.4% In, 0.2% Ni, and the rest is Sn. The solidus temperature of this lead-free solder is 209.0 °C, the liquidus temperature is 215.0 °C, the tensile strength is 76.2 MPa, the elongation at break is 24.6%, and the wetting angle is 35.7°.

[0082] Example 6

[0083] This example is basically the same as the above examples, with the special feature being that:

[0084] In this example, a SnAgCuBiInTi high-reliability lead-free solder alloy, the weight percentage composition of this lead-free solder alloy is 3.8% Ag, 0.7% Cu, 3.0% Bi, 1.0% In, 0.2% Ti, and the rest is Sn. The solidus temperature of this lead-free solder is 207.6 °C, the liquidus temperature is 218.3 °C, the tensile strength is 85.7 MPa, the elongation at break is 19.0%, and the wetting angle is 33.6°.

[0085] Example 7

[0086] This example is basically the same as the above examples, with the special feature being that:

[0087] In this example, a SnAgCuBiInTiNi high-reliability lead-free solder alloy, the weight percentage composition of this lead-free solder alloy is 3.8% Ag, 0.7% Cu, 2.5% Bi, 1.8% In, 0.1% Ti, 0.2% Ni, and the rest is Sn. The solidus temperature of this lead-free solder is 207.0 °C, the liquidus temperature is 213.2 °C, the tensile strength is 77.9 MPa, the elongation at break is 17.5%, and the wetting angle is 34.2°.

[0088] Example 8

[0089] This example is basically the same as the above examples, with the special feature being that:

[0090] In this embodiment, a SnAgCuBiInTiCe high-reliability lead-free solder alloy is provided. The weight percentage composition of this lead-free solder alloy is 3.8% Ag, 0.7% Cu, 3.0% Bi, 1.0% In, 0.2% Ti, 0.2% Ce, and the balance is Sn. The solidus temperature of this lead-free solder is 208.0 °C, the liquidus temperature is 213.8 °C, the tensile strength is 79.5 MPa, the elongation at break is 20.0%, and the wetting angle is 35.6°.

[0091] Example 9

[0092] This embodiment is basically the same as the above embodiments, with the special feature being that:

[0093] In this embodiment, a SnAgCuBiInNiCeGe high-reliability lead-free solder alloy is provided. The weight percentage composition of this lead-free solder alloy is 1.0% Ag, 0.5% Cu, 3.0% Bi, 2.0% In, 0.1% Ni, 0.2% Ce, 0.05% Ge, and the balance is Sn. The solidus temperature of this lead-free solder is 195.3 °C, the liquidus temperature is 213.7 °C, the tensile strength is 72.7 MPa, the elongation at break is 22.0%, and the wetting angle is 36.2°.

[0094] Example 10

[0095] This embodiment is basically the same as the above embodiments, with the special feature being that:

[0096] In this embodiment, a SnAgCuBiInNiCe high-reliability lead-free solder alloy is provided. The weight percentage composition of this lead-free solder alloy is 3.8% Ag, 0.7% Cu, 2.5% Bi, 3.1% In, 0.1% Ni, 0.1% Ce, and the balance is Sn. The solidus temperature of this lead-free solder is 203.1 °C, the liquidus temperature is 210.5 °C, the tensile strength is 72.9 MPa, the elongation at break is 20.7%, and the wetting angle is 34.9°.

[0097] Table 1 Performance data table of lead-free solder alloys in Examples 1 - 10

[0098]

[0099]

[0100] Table 1 shows the comparison of the melting characteristics, mechanical properties, and wetting properties of some examples and comparative specimens: The melting temperatures of the alloy examples of the present invention are generally lower than those of SAC387 and SAC305, meeting the basic reflow soldering process temperature requirements; the strength improvement effects of the example alloys compared to SAC387 are 59% - 87%, and compared to SAC305 are 76% - 108%; in terms of wettability, the wetting angles of the example alloys are generally smaller than those of SAC387 and SAC305, indicating that the examples have better wettability, which is beneficial to enhancing the firmness between the solder joints and the substrate and improving the reliability of the solder joints.

[0101] Table 2 shows the comparison of the creep stress indices of the examples and comparative specimens: The larger the creep stress index, the better the creep resistance of the alloy. As can be seen from Table 2, the creep stress indices of the example alloys are generally higher than those of the comparative specimens, and the creep resistance of the example alloys has been improved, which is due to the strengthening effects of the added multi-element alloying elements, such as the solid solution strengthening effects of Bi and In elements, the precipitation strengthening effect of Ni element, and the microstructure refinement strengthening effect of Ti element, etc.

[0102] Table 2. Comparison Table of Creep Properties of Solder Alloys in Some Examples of the Present Invention

[0103]

[0104]

[0105] The test sample is 15 mg of the solder alloy powder prepared in Example 8, with a heating rate of 5 °C / min and a cooling rate of 10 °C / min. The solidus temperature of the solder alloy taken in the present invention is the starting point of the peak of the heating curve, and the liquidus temperature is the temperature at the peak point of the heating curve. Figure 5 It is the differential thermal analysis diagram of the solder alloy in Example 8. From Figure 5 it can be seen that the solidus temperature of Example 8 is 208.0 °C and the liquidus temperature is 213.8 °C.

[0106] Mechanical tests were carried out on the solder alloy in Example 5, Figure 6 which is the tensile curve diagram of the solder alloy in Example 5 of the present invention and the comparison with the examples. From Figure 6 it can be seen that Example 5 has relatively high strength and good toughness, with a tensile strength of 76.2 and an elongation at break of 24.6.

[0107] The wetting performance of the solder in Embodiment 3 of the present invention was also tested. The wetting property of the solder was measured by the wetting angle. The size of the copper sheet used in the experiment was 30 mm × 30 mm × 1 mm. After removing the surface oxide layer and oil stain with sandpaper, it was immersed in acetone for ultrasonic cleaning; the lead-free solder was made into a block of 0.2 g with a deviation of ±1%, and then placed in ethanol for ultrasonic cleaning; a soldering flux was selected, the solder was placed in the center of the copper sheet, and the brazing flux was applied. It was placed horizontally in a reflow soldering machine, and the heating temperature curve was set; after cleaning the spreading sample, the wetting angle of the spreading sample was measured with a wetting angle measuring instrument; 3 specimens were measured for each sample, and the average value was taken. Figure 7 is the wetting angle diagram of the solder alloy in Embodiment 3 of the present invention. From Figure 7 it can be seen that the wetting angle of Embodiment 3 on the copper sheet is 28.8°, and Embodiment 3 has good wetting performance.

[0108] The solders prepared in Embodiment 4 and Embodiment 6 were microscopically observed. Figure 8 is the metallographic structure diagram of the solder alloy in Embodiment 4 of the present invention. Figure 9 is the metallographic structure diagram of the solder alloy in Embodiment 6 of the present invention. From Figure 8 and Figure 9 it can be seen that Embodiment 4 and Embodiment 6 have intermetallic compounds with uniform distribution, and these intermetallic compounds can hinder the movement of dislocations and effectively improve the tensile strength of the solder.

[0109] Figure 10 The comparison of the oxidation resistance performance between the embodiments and the comparative specimens is as follows: Figure 10 In [diagram], A1 is the comparative specimen SnAg3.8Cu0.7, A2 is Embodiment 10: SnAg3.8Cu0.7Bi2.5In3.1Ni0.1Ce0.1, A3 is Embodiment 9: SnAg1.0Cu0.5Bi3.0In2.0Ni0.1Ce0.2Ge0.05, and A4 is Embodiment 6: SnAg3.8Cu0.7Bi3.0In1.0Ti0.2. From Figure 10 it can be seen that within the same oxidation time, the weight gain percentage of the embodiments is significantly less than that of the comparative specimens, indicating that the high-temperature oxidation resistance of the solder alloy of the present invention is superior to that of the comparative specimens.

[0110] In summary, the above embodiments of the present invention are for the design and preparation of a high-reliability lead-free solder alloy. The composition range of the alloy is by weight percentage: 1.0 - 5.5% Ag, 0.5 - 1.0% Cu, 0.5 - 5.0% Bi, 0.4 - 4.0% In, 0 - 1.0% Ti, 0 - 0.5% Ni, and the balance is composed of Sn; in addition, depending on the situation, the alloy contains one or more selected from Ce and Ge, and the balance is Sn. The design method of the high-reliability lead-free solder alloy in the above embodiments of the present invention includes the following steps: collecting relevant experimental data as the initial data set; screening key alloy elements using the maximum mutual information coefficient and Pearson correlation coefficient; using the gradient descent tree algorithm to model with the alloy composition as the input and the mechanical properties as the output; constructing virtual samples and obtaining prediction results through a reliable machine learning model, and optimizing the alloy composition range according to the prediction results. The preparation method of the lead-free solder alloy of the present invention is: using a high-throughput arc melting furnace to prepare intermediate alloys from refractory metals with higher melting points and pure tin metals, and then weighing appropriate amounts of pure Sn, Ag, Cu, Bi, In and various intermediate alloys according to the designed composition and mixing and melting them in an induction melting furnace. The present invention solves the problems of low strength and poor creep resistance of traditional lead-free solders. The present invention guides alloy design through machine learning, combines high-throughput experimental methods, and utilizes multi-element alloying and the interactive strengthening effects of various elements such as Bi and In to significantly improve the mechanical properties and wetting properties of the solder alloy. The finally obtained solder alloy has high strength and creep resistance, and at the same time has good soldering characteristics and oxidation resistance, and has good application prospects.

[0111] The above has described the embodiments of the present invention in conjunction with the accompanying drawings, but the present invention is not limited to the above embodiments, and various changes can be made according to the purpose of the invention of the present invention. Any changes, modifications, substitutions, combinations or simplifications made based on the spirit and principle of the technical solution of the present invention shall be equivalent replacement methods, as long as they meet the invention purpose of the present invention and do not deviate from the technical principle and inventive concept of the present invention, they all belong to the protection scope of the present invention.

Claims

1. A SnAgCuBiIn-based lead-free solder alloy, characterized in that: By weight percentage, it contains 3.8 - 4.0% Ag, 0.5 - 0.7% Cu, 0.5 - 2.5% Bi, 3.1% - 3.5% In, 0.1 - 0.8% Ti, 0.1 - 0.2% Ni, and the balance is composed of Sn; the alloy further contains at least one or more of the following alloy elements: Ce ≤ 0.2%, Ge ≤ 0.2%.

2. The SnAgCuBiIn-based lead-free solder alloy according to claim 1, characterized in that, By weight percentage, the composition and content of the solder alloy, the alloy further contains at least one or more of the following alloy elements: 0.1 - 0.2% Ce, 0.05 - 0.2% Ge.

3. A design method for the SnAgCuBiIn-based lead-free solder alloy according to claim 1, characterized in that, Using machine learning to assist in the design of lead-free solder compositions, the method includes the following steps: (1) Collect experimental data to obtain an initial data set of lead-free solder alloy compositions and mechanical properties; (2) Use the Maximal Information Coefficient (MIC) to screen alloy compositions that have a strong correlation with mechanical properties, and further screen out alloy compositions that have a favorable impact on mechanical properties based on the Pearson correlation coefficient and its positive and negative correlations; (3) Use the gradient boosting tree algorithm to model and train with alloy compositions as inputs and mechanical properties as outputs; The model is verified using the leave-one-out cross-validation method; (4) Design alloy composition combinations and step sizes, construct virtual samples using permutations and combinations, and then input the virtual samples into the machine learning model to obtain prediction results; Select points on the edge of the virtual sample space for systematic experimental verification to obtain a solder alloy with excellent performance.

4. A method for preparing the SnAgCuBiIn-based lead-free solder alloy according to claim 1, characterized in that, The alloy is prepared by combining high-throughput vacuum arc melting and induction melting. The preparation method includes the following steps: a. In a high-throughput vacuum arc melting furnace, materials containing metals Ti, Ni, Ce, Ge, and Sn with a mass fraction of not less than 99.9% are used as alloy component raw materials to prepare Sn-Ti master alloy, Sn-Ni master alloy, Sn-Ce master alloy, and Sn-Ge master alloy with a mass fraction of Sn-5% Ti, Sn-4% Ni, Sn-7% Ce, and Sn-5% Ge respectively; During the preparation of the master alloy, first evacuate the inside of the high-throughput vacuum arc melting furnace, and when the vacuum degree is not higher than 1×10-3 Pa, fill it with protective argon gas to not less than 0.5 atm, load the current to 110 A, and ignite pure Ti metal to consume the remaining oxygen in the furnace; Then place the prepared alloy component raw materials in the melting station, load the current to 110 - 120 A, ignite the metal in the melting station until the alloy component raw materials are completely melted. After extinguishing the arc, wait for the alloy to cool completely and then turn it over and remelt and cool it. Repeat the melting 4 times to ensure the uniformity of the alloy composition and obtain the master alloy for standby; b. According to the composition of the target SnAgCuBiIn-based lead-free solder alloy to be prepared, calculate and weigh pure metal Sn, Ag, Cu, Bi, In raw materials with a mass fraction of not less than 99.9% and the master alloy prepared in step a above for material preparation; Melting is carried out using a vacuum induction furnace. Before melting, the same operations of evacuating and filling with argon as in step a are repeated 3 times to remove the oxygen in the furnace. Finally, melting is carried out in an argon environment. When melting the alloy, first put the pure metal raw materials of Sn, Ag, Cu, Bi, and In into the crucible, heat up to no less than 300 °C and keep warm for 15 - 20 minutes; then select Sn-Ti master alloy, Sn-Ni master alloy, Sn-Ce master alloy, and Sn-Ge master alloy according to the requirements of composition design and put them into the crucible. Heat up to no less than 500 °C, keep warm for 20 - 30 minutes, then cool down to no higher than 300 °C, and pour the alloy liquid into a stainless steel mold to cool and form, obtaining the finished product of SnAgCuBiIn series lead-free solder alloy.

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