High-performance low-silver lead-free solder and preparation method thereof

Abstract

This invention relates to the field of electronic packaging materials technology, specifically disclosing a high-performance low-silver lead-free solder and its preparation method. The solder comprises the following components by mass percentage: 0.1-1% Ag, 0.01-1% Cu, 0.1-3.8% Bi, 0.002-0.3% Ni, 0.02-2% In, 0.001-0.5% Co, 0.001-0.5% Ce, with the balance being Sn; optionally, at least one of Ga, Ge, and P may be added at 0.05-0.2%. The composition of the solder must simultaneously satisfy the following two relationships: 0.15≤(Ni+Co) / In≤3.5 (1); (Ag-0.5)²+(Bi-2)² / 4≤1 (2), where each element symbol represents its mass percentage. This invention controls the mass ratio of Ni, Co, and In using equation (1) to promote the formation of a (Cu,Ni,Co)6(Sn,In)5 tough interface layer; and uses equation (2) to synergistically limit the content of Ag and Bi within the optimal elliptical range, ensuring strength while avoiding brittleness. This solder significantly reduces silver content while possessing excellent wettability, mechanical properties, and thermal fatigue resistance, making it suitable for the electronic packaging field.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging soldering technology for electronic and electrical products, specifically to a high-performance low-silver lead-free solder and its preparation method. Background Technology

[0002] In recent years, with increasing environmental awareness, lead-free electronic packaging has become a global consensus. Among the many lead-free solder systems, Sn-Ag-Cu alloys are widely recognized as the most ideal alternative to traditional Sn-Pb solders due to their excellent comprehensive performance. One of the current clear research directions in the industry is the development of low-silver (Ag<1.0%) and even ultra-low-silver (Ag<0.5%) Sn-Ag-Cu solders. However, simply reducing the silver content is a double-edged sword: while it reduces costs and decreases the internal Ag3Sn phase, it also introduces new technical bottlenecks, specifically uncontrolled interfacial reactions. The reduction in silver content weakens its inhibitory effect on the dissolution of copper (Cu), leading to intensified interfacial reactions between the solder and the copper substrate during reflow soldering and subsequent service, which can form excessively thick Cu6Sn5 and Cu3Sn intermetallic compound layers. The intermetallic compound (IMC) layer is not only a weak point in stress, but its excessive growth will also consume the plastic solder matrix of the solder joint itself, directly leading to a decrease in the connection strength of the solder joint, an increase in resistance, and the initiation of cracks under thermal cycling loads, ultimately causing early failure.

[0003] Therefore, existing low-silver Sn-Ag-Cu solder technology has significant shortcomings in the precise control of interfacial IMC. Developing a novel low-silver solder capable of actively suppressing excessive interfacial IMC growth has become a key technological bottleneck in improving the long-term reliability of next-generation electronic products. Summary of the Invention

[0004] To address the core technical bottleneck of "interface stability" caused by the reduced silver content in existing low-silver / ultra-low-silver Sn-Ag-Cu solders, this invention provides a high-performance low-silver lead-free solder. Based on quantitative synergistic design of components, it can precisely control the interface reaction and actively suppress the excessive growth of intermetallic compounds (IMC) at the solder joint interface, thus preventing it from becoming a weak link in reliability.

[0005] In a first aspect, the present invention provides a high-performance low-silver lead-free solder, employing the following technical solution:

[0006] A high-performance low-silver lead-free solder comprises the following components and their mass percentages: 0.1-1% Ag, 0.01-1% Cu, 0.1-3.8% Bi, 0.002-0.3% Ni, 0.02-2% In, 0.001-0.5% Co, 0.001-0.5% Ce, with the balance being Sn, and the mass percentages of the components satisfy the following equations (1) and (2):

[0007] 0.15≤(Ni+Co) / In≤3.5 (1) Formula;

[0008] (Ag-0.5)²+(Bi-2)² / 4≤1 (2) Formula.

[0009] By adopting the above technical solution, this invention adds 0.1-1% Ag to the Sn-Ag-Cu solder. Ag in the alloy mainly reacts with Sn to form fine Ag3Sn intermetallic compounds. These dispersed strengthening phases effectively improve the strength and hardness of the alloy. Simultaneously, Ag also improves the electrical and thermal conductivity of the alloy. However, excessive Ag leads to coarsening of the Ag3Sn phase, which reduces the alloy's plasticity, and also increases raw material costs. Therefore, considering the overall solution of this invention, the optimal addition amount is 0.1%-1%.

[0010] This invention incorporates 0.01-1% Cu into the Sn-Ag-Cu solder. Cu primarily forms Cu6Sn5 intermetallic compounds in the alloy, which are dispersed in the matrix and act as a second-phase reinforcement. Furthermore, Cu can form a good metallurgical bond with the substrate; however, excessively high Cu content will increase the alloy's melting point and deteriorate its processing performance. Therefore, considering the overall design of this invention, the optimal addition amount is 0.01-1%.

[0011] This invention incorporates 0.1-3.8% Bi into Sn-Ag-Cu solder. Bi exhibits significant solid solution strengthening in the Sn matrix, effectively improving the alloy's strength and hardness. Simultaneously, the addition of Bi significantly lowers the alloy's melting point, improving welding process performance. However, excessive Bi can cause brittleness, necessitating strict control of its content. Therefore, considering the overall design of this invention, the optimal addition amount is 0.1-3.8%.

[0012] This invention incorporates 0.002-0.3% Ni into Sn-Ag-Cu solder. The addition of Ni effectively inhibits excessive dissolution of Cu into the solder, refines the interfacial IMC layer by forming the (Cu,Ni)6Sn5 phase, and improves interfacial bonding strength. Ni also promotes grain refinement, improves the overall mechanical properties of the alloy, and increases the melting point. Research indicates that the optimal addition amount is 0.002-0.3%.

[0013] This invention incorporates 0.02-2% In into Sn-Ag-Cu solder. In effectively lowers the alloy's melting point and improves wetting properties. More importantly, In can dissolve into the IMC layer to form a (Cu,Ni)6(Sn,In)5 composite phase. This phase exhibits better toughness and significantly enhances interfacial reliability. Studies have shown that the optimal addition amount is 0.02-2%.

[0014] This invention incorporates 0.001-0.5% Co into Sn-Ag-Cu solder. Co, as a grain refiner, effectively hinders grain boundary movement and suppresses grain growth. Co can also form fine intermetallic compounds with Sn, further improving the alloy's strength, and it also increases the melting point. Studies have shown that the optimal addition amount is 0.001-0.5%.

[0015] This invention incorporates 0.001-0.5% Ce into Sn-Ag-Cu solder. Ce is a potent rare-earth modifier that effectively purifies the alloy melt and removes impurity elements. Ce also promotes nucleation, refines grains, inhibits the growth of interfacial IMC, and forms a protective film on the alloy surface, improving oxidation resistance. Studies have shown that its optimal addition amount is 0.001-0.5%.

[0016] In this invention, the balance is Sn, but it is not excluded that the balance may include unavoidable impurities.

[0017] It is worth noting that, through in-depth research, this invention has discovered that, under the constraint of a silver content of no more than 1%, solder performance is dominated by two core mechanisms: one is "interface behavior regulation" centered on the interaction of Ni, Co, and In; the other is "matrix strengthening synergy" centered on the Ag and Bi ratio. Regarding interface regulation, this invention found that since Ni and Co are both transition elements with similar atomic radii and chemical properties, they can both dissolve into the interface Cu6Sn5 phase during reflow soldering, forming (Cu,Ni,Co)6(Sn,In)5. This multi-component solid solution has a higher lattice distortion energy and a lower interface energy compared to binary Cu6Sn5, thus significantly refining the IMC grains and inhibiting their excessive growth and coarsening. In atoms can dissolve into the Sn matrix and the Sn-side IMC. Its addition reduces the overall surface energy of the solder and alters atomic diffusion kinetics. More importantly, we found that an appropriate amount of In can promote the more effective participation of Ni and Co elements in the interface reaction, acting as an "activator" and "carrier." However, excessive In can excessively lower the melting point and potentially weaken the interface. This invention discovered that Ni and Co have a synergistic effect in inhibiting IMC growth, and this effect is quantitatively dependent on In, with the mass percentage needing to satisfy: 0.15 ≤ (Ni+Co) / In ≤ 3.5. When (Ni+Co) / In < 0.15, it means that the total amount of interface modifying elements (Ni+Co) is severely insufficient relative to In. At this point, the "regulatory" effect of In is dominant, but there is a lack of sufficient Ni / Co to form a stable (Cu,Ni,Co)6(Sn,In)5 reinforcing phase, resulting in an excessively thin interface IMC layer, reduced mechanical bonding strength of the solder joint, and a loose structure with poor mechanical properties, leading to weak shear and tensile resistance of the solder joint. When (Ni+Co) / In > 3.5, it means that the content of Ni and Co is relatively too high. Excess Ni and Co not only consume the regulating effect of In, but may also react with Sn to form brittle independent phases such as Ni3Sn4 and CoSn3. These hard particles will rupture the solder matrix, leading to a sharp decrease in plasticity and brittle solder joints. Simultaneously, an excessively high Ni / Co ratio will unnecessarily increase the alloy's melting point. When 0.15 ≤ (Ni+Co) / In ≤ 3.5, the "inhibition of IMC growth" effect of Ni and Co and the "optimization of diffusion and synergy" effect of In achieve a precise balance. Experiments show that at this point, a continuous, dense, and moderately thick (Cu,Ni,Co)6(Sn,In)5 interface layer can be formed. This layer bonds firmly to the copper substrate and effectively blocks the interdiffusion of Sn and Cu atoms, thus obtaining solder joints with high strength and excellent reliability. Based on this, this invention innovatively proposes the concept of an "interface synergistic regulation factor" and quantifies it as (Ni+Co) / In.

[0018] Furthermore, the mass percentages of Bi and Ag satisfy the relationship (Ag-0.5)²+(Bi-2)² / 4≤1. Ag provides second-phase reinforcement by forming the Ag3Sn phase, with the optimal effect achieved at an Ag content of 0.5% within the low silver content range. When the Ag content deviates from 0.5%, it is necessary to compensate by adjusting the Bi content to prevent high costs or brittleness due to excessive Ag and decreased plasticity caused by excessive Bi. The addition of Bi has the effects of lowering the melting point, increasing wettability, and solid solution strengthening, with the optimal solid solution strengthening effect of the solder at a content of 2%. Therefore, when Bi and Ag satisfy the relationship (Ag-0.5)²+(Bi-2)² / 4≤1, and their contents are maintained within the elliptical optimal range centered at (0.5% Ag, 2% Bi), the synergistic effect of Bi and Ag is optimal, resulting in the best comprehensive properties of the solder, including mechanical strength and toughness. Based on this, the present invention innovatively proposes the concept of "material strengthening factor" and quantifies it as (Ag-0.5)²+(Bi-2)² / 4.

[0019] The high-performance low-silver lead-free solder of this invention possesses excellent overall performance, specifically exhibiting good wetting and spreading ability, melting point, mechanical strength, and creep resistance even in low-silver environments. This meets the higher reliability requirements of miniaturization and high-density packaging for micro-solder joints, solving the common problem of interface stability and overall performance imbalance in existing technologies. Therefore, the high-performance low-silver lead-free solder of this invention is a novel low-silver solder that can actively suppress excessive interfacial IMC growth while simultaneously ensuring excellent wettability, mechanical properties, and process feasibility.

[0020] Preferably, the high-performance low-silver lead-free solder comprises the following components and their mass percentages: 0.25-0.81% Ag, 0.1-0.7% Cu, 0.69-2.1% Bi, 0.007-0.15% Ni, 0.05-1% In, 0.045-0.1% Co, 0.05-0.2% Ce, with the balance being Sn.

[0021] Preferably, it also includes at least one of Ga, Ge and P in a mass percentage of 0.05-0.2%.

[0022] By adopting the above technical solution, Ga, Ge, and P are used as auxiliary elements to further optimize wettability and oxidation resistance. This invention adds 0-0.2% Ga, Ge, or P to the Sn-Ag-Cu solder. Ga can further lower the alloy's melting point and improve wettability; Ge and P are effective oxidation-resistant elements, which can reduce the formation of oxide slag during the welding process.

[0023] Secondly, the present invention provides a method for preparing a high-performance low-silver lead-free solder, which adopts the following technical solution:

[0024] The method for preparing a high-performance low-silver lead-free solder as described above includes the following steps in sequence according to the processing steps:

[0025] S1: Ni, Co, Ce, P and a portion of Sn are melted in a vacuum environment at 550-650℃ to form Sn-1Ni, Sn-1Co, Sn-5Ce and Sn-2P master alloys, respectively.

[0026] S2: Under a protective atmosphere of high-purity argon, heat the remaining Sn to 430-470℃ until it is completely melted; add Ag, Cu, Bi and In elements, stirring for 10-20 minutes after each addition; then add Sn-1Ni and Sn-1Co master alloys, and stir for another 15-20 minutes until they are completely dissolved and evenly distributed to form a homogeneous base melt;

[0027] S3: Reduce the temperature of the base melt to 390-410℃, add the pre-made Sn-5Ce, Sn-2P master alloy and Ga, Ge elements, and ultrasonically vibrate for 5-10 minutes, then electromagnetically stir for 10-20 minutes to completely dissolve the master alloy and elements, and achieve nanoscale dispersion of Ce, Ga, Ge and P elements to obtain high-performance low-silver lead-free solder.

[0028] By adopting the above technical solution, in S1, due to the high melting points of Ni, Co, and Ce, direct addition to the Sn melt easily leads to segregation and oxidation. Preparing them into intermediate alloys before adding them to Sn ensures uniform dissolution. Secondly, Ce is highly susceptible to oxidation; preparing Sn-5Ce improves its stability after addition and controls the yield of active elements. This is a prerequisite for precise control of the formulation composition and effective element utilization. If the above components are added unevenly, excessively high local Ni / Co concentrations may promote abnormal IMC growth, leading to the opposite effect.

[0029] In S3, due to the reactive or volatile properties of Ce, P, Ga, and Ge, their addition at lower temperatures can significantly reduce burn-off and ensure yield. Secondly, through low-temperature processing, ultrasonic vibration, electromagnetic stirring, and intermediate alloying, the aim is to ensure these trace active elements exist as fine, dispersed particles or solid solutions, rather than aggregates. Nano-dispersion of elements like Ce and P can strongly pin grain boundaries, refine grains, and potentially inhibit lateral growth and coarsening of the IMC layer during subsequent welding, thereby simultaneously improving solder strength (grain refinement) and toughness. This is the core bridge connecting the preparation process and the final performance, and also the most critical step in the preparation process.

[0030] Preferably, it also includes S4: holding the high-performance low-silver lead-free solder at 340-360°C for 50-60 minutes, then pouring it into a metal mold preheated to 150-200°C under argon protection, and cooling it to obtain an alloy ingot.

[0031] Preferably, in step S4, the alloy ingot is subjected to thermomechanical processing to further process it into solder pillars, solder bars, solder wires, solder balls, solder powder, or preformed solder sheets.

[0032] Compared with the prior art, the present invention has the following advantages and technical effects:

[0033] 1. Precise control of interface reaction: By controlling the mass percentages of Ni, Co, and In to satisfy the formula 0.15 ≤ (Ni+Co) / In ≤ 3.5, an intermetallic compound layer (Cu,Ni,Co)6(Sn,In)5 is formed at the interface. This IMC layer has excellent toughness and thermal stability, effectively suppressing the excessive growth of interfacial IMC, controlling the thickness of the IMC layer after reflow soldering to below 3μm, and significantly improving the thermal fatigue resistance of the solder joint.

[0034] 2. Optimal balance of comprehensive performance: Equation (2) limits the content of Ag and Bi to an elliptical region centered at (0.5%, 2%), achieving the optimal ratio of melting point reducing strengthening elements to second-phase strengthening elements. This design enables the solder to maintain excellent comprehensive performance under low silver conditions, with a wetting and spreading rate greater than 79%, tensile strength exceeding 48 MPa, and shear strength exceeding 33 MPa.

[0035] 3. Stable and reliable preparation process: The use of intermediate alloy form and closed-loop feedback adjustment process ensures accurate addition of active elements and precise control of composition, solving the technical problem of difficulty in ensuring the uniformity of composition of multi-element micro-alloyed solder, and providing a reliable guarantee for industrial production. Attached Figure Description

[0036] Figure 1 A photograph of the interface microstructure of the high-performance low-silver lead-free solder prepared in Example 1 of the present invention after soldering on a copper pad. Detailed Implementation

[0037] The present invention will be further described below with reference to specific embodiments, but the implementation and protection scope of the present invention are not limited thereto.

[0038] The following specific examples illustrate the process, and all raw materials used in the examples were purchased from the market.

[0039] Example 1

[0040] A high-performance low-silver lead-free solder comprises the following components and their mass percentages: Ag: 0.3%, Cu: 0.7%, Bi: 3.8%, Ni: 0.002%, In: 0.02%, Co: 0.05%, Ce: 0.001%, Sn: 95.127%. Its processing steps include the following:

[0041] S1: Raw material and intermediate alloy preparation: Prepare Sn, Ag, Cu, Bi and In metals with a purity of not less than 99.9%, and melt 0.2g Ni with 19.8g Sn, 5g Co with 495g Sn, and 0.1g Ce with 1.9g Sn in a vacuum induction furnace at 550℃ to prepare Sn-1Ni, Sn-1Co and Sn-5Ce intermediate alloys respectively;

[0042] S2: Melting of the base solder alloy: Under a protective atmosphere of high-purity argon, heat 899.6g of the remaining Sn to 430℃ in a melting furnace until it is completely melted; add 3gAg, 7gCu, 38gBi, and 0.2gIn in sequence, stirring for 10 minutes after each addition. Then, add 2gSn-1Ni and 50gSn-1Co master alloys, i.e., add Ni in the form of Sn-1Ni master alloy and Co in the form of Sn-1Co master alloy, and stir for another 15 minutes until it is completely dissolved and evenly distributed to form a homogeneous base melt.

[0043] S3: Addition and dispersion of active element Ce: The temperature of the base melt is reduced to the active element addition window of 390℃, 0.2g of pre-made Sn-5Ce master alloy is added, and ultrasonic vibration is performed for 6 minutes, followed by electromagnetic stirring for 10 minutes to completely dissolve the master alloy and achieve nanoscale dispersion of Ce element, thus obtaining high-performance low-silver lead-free solder melt.

[0044] S4: Structure stabilization and molding: The high-performance low-silver lead-free solder melt with adjusted composition is kept at 340℃ for 50 minutes to promote the homogenization of the melt and allow impurities to float to the surface; under argon protection, it is poured into a metal mold preheated to 150℃ and cooled to obtain an alloy ingot, which yields 1000g of high-performance low-silver lead-free solder.

[0045] Examples 2-5

[0046] Examples 2-5 all disclose a high-performance low-silver lead-free solder, which differs from Example 1 in the mass percentage of the components and the preparation conditions, as detailed in Tables 1 and 2.

[0047] Example 6

[0048] Example 6 discloses a high-performance low-silver lead-free solder, which differs from Example 1 in its composition and mass percentage, as detailed in Table 1. The processing steps are as follows:

[0049] S1: Raw material and intermediate alloy preparation: Prepare Sn, Ag, Cu, Bi and In metals with a purity of not less than 99.9%, and melt 0.2g Ni with 19.8g Sn, 5g Co with 495g Sn, and 0.1g Ce with 1.9g Sn in a vacuum induction furnace at 550℃ to prepare Sn-1Ni, Sn-1Co and Sn-5Ce intermediate alloys respectively;

[0050] S2: Melting of the base solder alloy: Under a protective atmosphere of high-purity argon, heat 897.6g of the remaining Sn in a melting furnace to 430℃ until it is completely melted; add 3gAg, 7gCu, 38gBi, and 0.2gIn in sequence, stirring for 10 minutes after each addition. Then, add 2gSn-1Ni and 50gSn-1Co master alloys, i.e., add Ni in the form of Sn-1Ni master alloy and Co in the form of Sn-1Co master alloy, and stir for another 15 minutes until it is completely dissolved and evenly distributed to form a homogeneous base melt.

[0051] S3: Addition and dispersion of active elements Ce and Ge: The temperature of the base melt is reduced to the active element addition window of 390℃. 0.2g of pre-made Sn-5Ce master alloy and 2g of elemental Ge are added and ultrasonically vibrated for 6 minutes, followed by electromagnetic stirring for 10 minutes to completely dissolve the master alloy and achieve nanoscale dispersion of Ce and Ge elements, resulting in a high-performance low-silver lead-free solder melt.

[0052] S4: Structure stabilization and molding: The high-performance low-silver lead-free solder melt with adjusted composition is kept at 340℃ for 50 minutes to promote the homogenization of the melt and allow impurities to float to the surface; under argon protection, it is poured into a metal mold preheated to 150℃ and cooled to obtain an alloy ingot, which yields 1000g of high-performance low-silver lead-free solder.

[0053] Example 7

[0054] Example 7 discloses a high-performance low-silver lead-free solder, which differs from Example 1 in its composition and mass percentage, as detailed in Table 1. The processing steps are as follows:

[0055] S1: Raw material and intermediate alloy preparation: Prepare Sn, Ag, Cu, Bi and In metals with a purity of not less than 99.9%, and melt 0.2g Ni with 19.8g Sn, 5g Co with 495g Sn, and 0.1g Ce with 1.9g Sn in a vacuum induction furnace at 550℃ to prepare Sn-1Ni, Sn-1Co and Sn-5Ce intermediate alloys respectively;

[0056] S2: Melting of the base solder alloy: Under a protective atmosphere of high-purity argon, heat 898.6g of the remaining Sn in a melting furnace to 430℃ until it is completely melted; add 3gAg, 7gCu, 38gBi, and 0.2gIn in sequence, stirring for 10 minutes after each addition. Then, add 2gSn-1Ni and 50gSn-1Co master alloys, i.e., add Ni in the form of Sn-1Ni master alloy and Co in the form of Sn-1Co master alloy, and stir for another 15 minutes until it is completely dissolved and evenly distributed to form a homogeneous base melt.

[0057] S3: Addition and dispersion of active elements Ce, Ga and Ge: The active element addition window is lowered to 390℃ of the base melt temperature. 0.2g Sn-5Ce master alloy, 0.5g Ga and 0.5g Ge are added and ultrasonically vibrated for 6 minutes, followed by electromagnetic stirring for 10 minutes to completely dissolve the master alloy and achieve nanoscale dispersion of Ce, Ga and Ge elements, resulting in a high-performance low-silver lead-free solder melt.

[0058] S4: Structure stabilization and molding: The high-performance low-silver lead-free solder melt with adjusted composition is kept at 340℃ for 50 minutes to promote the homogenization of the melt and allow impurities to float to the surface; under argon protection, it is poured into a metal mold preheated to 150℃ and cooled to obtain an alloy ingot, which yields 1000g of high-performance low-silver lead-free solder.

[0059] Example 8

[0060] Example 8 discloses a high-performance low-silver lead-free solder, which differs from Example 1 in its composition and mass percentage, as detailed in Table 1. The processing steps are as follows:

[0061] S1: Raw material and intermediate alloy preparation: Prepare Sn, Ag, Cu, Bi and In metals with a purity of not less than 99.9%. Melt 0.2g Ni with 19.8g Sn, 5g Co with 495g Sn, 0.1g Ce with 1.9g Sn, and 5g P with 245g Sn in a vacuum induction furnace at 550℃ to prepare Sn-1Ni, Sn-1Co, Sn-5Ce and Sn-2P intermediate alloys respectively.

[0062] S2: Melting of the base solder alloy: Under a protective atmosphere of high-purity argon, 874.6g of the remaining Sn is heated to 430℃ in a melting furnace until it is completely melted; 3g of Ag, 7g of Cu, 38g of Bi, and 0.2g of In are added sequentially, and stirred for 10 minutes after each addition. Then, 2g of Sn-1Ni and 50g of Sn-1Co master alloys are added, i.e., Ni is added in the form of Sn-1Ni master alloy and Co is added in the form of Sn-1Co master alloy. Stir for another 15 minutes until it is completely dissolved and evenly distributed to form a homogeneous base melt.

[0063] S3: Addition and dispersion of active elements Ce and P: The temperature of the base melt is reduced to the active element addition window of 390℃. 0.2g of pre-made Sn-5Ce and 25g of Sn-2P master alloy are added and ultrasonically vibrated for 6 minutes. Then, the mixture is electromagnetically stirred for 10 minutes to completely dissolve the master alloy and achieve nanoscale dispersion of Ce and P elements, resulting in a high-performance low-silver lead-free solder melt.

[0064] S4: Structure stabilization and molding: The high-performance low-silver lead-free solder melt with adjusted composition is kept at 340℃ for 50 minutes to promote the homogenization of the melt and allow impurities to float to the surface; under argon protection, it is poured into a metal mold preheated to 150℃ and cooled to obtain an alloy ingot, which yields 1000g of high-performance low-silver lead-free solder.

[0065] Comparative Example 1

[0066] The raw material components and their mass percentages are as follows: Ag: 0.3%, Cu: 0.7%, Sn: 99%.

[0067] Add 990g of Sn raw material to a melting furnace and melt it at 450℃. Then add 3g of Ag and 7g of Cu, stir and keep warm for 2 hours to obtain 1000g of Sn-0.3Ag-0.7Cu lead-free solder.

[0068] Comparative Example 2

[0069] The raw material components and their mass percentages are as follows: Ag1: 3.0%, Cu: 0.5%, Sn: 96.5%.

[0070] 965g of Sn raw material was added to a melting furnace and melted at 450℃. Then, 30g of Ag and 5g of Cu were added, and the mixture was stirred and kept warm for 2 hours to obtain 1000g of Sn-3.0Ag-0.5Cu lead-free solder.

[0071] Comparative Examples 3-8

[0072] Comparative Examples 3-8 all disclose a high-performance low-silver lead-free solder, which differs from Example 1 in the mass percentage of the components, as shown in Table 1.

[0073] Comparative Example 9

[0074] Comparative Example 9 discloses a high-performance low-silver lead-free solder. The difference from Example 1 is the preparation method. In Comparative Example 9, Ni, Co and Ce are not melted into an intermediate alloy in S1. Instead, Ni and Co are directly added to Sn in S2, and Ce is directly added to Sn in S3.

[0075] Comparative Example 10

[0076] Comparative Example 10 discloses a high-performance low-silver lead-free solder. The difference from Example 1 is the preparation method. In Comparative Example 10, after adding the intermediate alloy in S3, ordinary mechanical stirring is used instead of ultrasonic vibration and electromagnetic stirring, and the stirring time is 22 minutes.

[0077] Comparative Example 11

[0078] Comparative Example 11 discloses a high-performance low-silver lead-free solder, which differs from Example 1 in the preparation method. In Comparative Example 11, the ultrasonic vibration time in S3 is 4 minutes and the electromagnetic stirring time is 8 minutes.

[0079] Table 1. Components and their mass percentages, and values ​​of the two equations for each embodiment and comparative example.

[0080] Ingredients / % Examples / Comparative Examples Ag Cu Bi Ni In Co Ce Ga Ge P Sn (Ni+Co) / In (Result rounded to two decimal places) (Ag-0.5)²+(Bi-2)² / 4 (The result is rounded to two decimal places) Example 1 0.3 0.7 3.8 0.002 0.02 0.05 0.001 0 0 0 95.127 2.6 0.85 Example 2 0.25 1 1.85 0.3 2 0.001 0.5 0 0 0 94.099 0.15 0.07 Example 3 0.1 0.1 0.69 0.15 1 0.1 0.1 0 0 0 97.76 0.25 0.59 Example 4 1 0.5 2.1 0.06 0.16 0.5 0.2 0 0 0 95.48 3.5 0.25 Example 5 0.81 0.01 0.1 0.007 0.05 0.045 0.05 0 0 0 98.928 1.04 1.00 Example 6 0.3 0.7 3.8 0.002 0.02 0.05 0.001 0 0.2 0 94.927 2.6 0.85 Example 7 0.3 0.7 3.8 0.002 0.02 0.05 0.001 0.05 0.05 0 95.027 2.6 0.85 Example 8 0.3 0.7 3.8 0.002 0.02 0.05 0.001 0 0 0.05 95.077 2.6 0.85 Comparative Example 1 0.3 0.7 0 0 0 0 0 0 0 0 99 / / Comparative Example 2 3 0.5 0 0 0 0 0 0 0 0 96.5 / / Comparative Example 3 0.3 0.7 3.8 0.002 0.04 0.4 0.02 0 0 0 94.738 10.05 0.85 Comparative Example 4 0.3 0.7 3.8 0.1 1.5 0.003 0.001 0 0 0 93.596 0.07 0.85 Comparative Example 5 1 0.7 3.8 0.002 0.02 0.05 0.001 0 0 0 94.427 2.6 1.06 Comparative Example 6 0.08 0.7 3.8 0.002 0.01 0.002 0.001 0 0 0 95.405 0.4 0.99 Comparative Example 7 0.3 0.7 3.8 0.002 2.05 0.8 0.001 0 0 0 92.347 0.39 0.85 Comparative Example 8 0.065 0.7 3.8 0.41 2.4 0.05 0.001 0 0 0 92.574 0.19 1.00 Comparative Example 9 0.3 0.7 3.8 0.002 0.02 0.05 0.001 0 0 0 95.127 2.6 0.85 Comparative Example 10 0.3 0.7 3.8 0.002 0.02 0.05 0.001 0 0 0 95.127 2.6 0.85 Comparative Example 11 0.3 0.7 3.8 0.002 0.02 0.05 0.001 0 0 0 95.127 2.6 0.85

[0081] Table 2 Preparation conditions for each example and comparative example

[0082] Example / Comparative Example Preparation Conditions Example 1 Example 2 Example 3 Example 4 Example 5 S1 Melting temperature / °C 550 650 600 580 650 S2 heating temperature / °C 430 470 450 460 450 S2 elemental stirring time / min 10 20 15 14 10 S2 alloy stirring time / min 15 20 18 18 20 S3 cooling temperature / °C 390 410 400 410 390 S3 Ultrasonic Time / min 6 7 10 5 8 S3 stirring time / min 10 15 18 13 20 S4 Insulation Temperature / °C 340 360 350 355 360 S4 heat preservation time / min 50 60 55 55 50 S4 preheating temperature / °C 150 200 180 170 180

[0083] Performance testing

[0084] The present invention tested the various properties of the high-performance low-silver lead-free solders of each embodiment and comparative example under the same test conditions. The test methods are as follows, and the test results are shown in Table 3.

[0085] Method for testing the thickness of the IMC layer at the interface after reflow soldering

[0086] Solder was soldered to a pure copper substrate under a standard reflow profile, then cold-mounted with epoxy resin, and polished to a mirror finish using graded sandpaper and diamond polishing solution. A mild etching solution of 2% HNO3 + 3% HCl + 98% C2H5OH (volume ratio) in nitric acid and hydrochloric acid was used for several seconds to clearly visualize the IMC layer. Field emission scanning electron microscopy in backscattered electron mode revealed a significant difference in contrast between the IMC layer and the solder / substrate. Twenty different locations were randomly selected at the interface, uniformly covering the thick, thin, and transition areas of the IMC layer. The thickness (µm) of the IMC layer was directly measured using image analysis software, and the average value was taken as the test result.

[0087] Melting temperature test method

[0088] Before the test, a small piece of solder with a mass of about 20 mg was accurately measured using a balance of 0.01%. Then, it was placed in an ultrasonic instrument and cleaned of grease and dirt on its surface with anhydrous ethanol. Finally, it was placed in a DSCQ200 instrument under nitrogen atmosphere protection to test the melting range. The test conditions were a heating range of 45-260℃ and a heating rate of 5℃ / min.

[0089] Expansion Rate Testing Method

[0090] Spread rate quantifies the wetting and spreading ability of solder, reflecting its soldering processability. The spread rate test method for the examples and comparative examples follows the "Solder Ball Spread Rate Test Method" in JIS Z 3198. Solder is prepared into 0.3g solder balls, which are then soldered onto a Cu sheet. After soldering, the maximum spread diameter (D) and height (H) of the solder joint are measured using an optical microscope and image analysis software. The spread rate is calculated as: Spread Rate (%) = [(DH) / D] × 100%, or the spread area can be directly used for comparison.

[0091] Tensile strength test method

[0092] The prepared solder was cast and processed into dog bone-shaped tensile specimens with a length of 100 mm and a diameter of 6 mm. Tensile tests were performed on a universal testing machine at room temperature and with a force loading mode of 1 N / min. The stress-strain curves were recorded, and the tensile strength (MPa) and elongation after fracture (%) were directly read. The average value of 10 valid specimens was taken as the tensile strength test result.

[0093] Shear strength test method

[0094] The prepared solder was used to form solder joints on a Cu plate. Shear strength was tested on a push-pull force tester. A shear rate of 0.2 mm / s was applied from the side of the solder joint until it broke. The maximum shear force was recorded and the fracture mode was observed.

[0095] Table 3 Performance test results for each embodiment and comparative example

[0096] Test Project Examples / Comparative Examples IMC thickness at the interface after reflow soldering (μm) Melting range (°C) Expansion rate (%) Tensile strength (MPa) Shear strength (MPa) Example 1 2.7 218.1-223.5 82.5 48.2 35.1 Example 2 2.3 216.8-228.1 79.6 51.5 37.8 Example 3 2.4 217.2-224.7 82.3 53.8 39.5 Example 4 2.9 219.6-226.4 79.1 55.5 33.9 Example 5 2.6 217.3-225.2 80.6 49.7 36.2 Example 6 2.8 218.3-223.6 84.5 49.1 35.5 Example 7 2.7 216.0-221.8 84.1 48.8 36.0 Example 8 2.8 217.5-223.2 83.9 48.6 34.9 Comparative Example 1 3.3 217.2-227.5 73.5 38.4 28.7 Comparative Example 2 4.5 217.4-221.8 75.0 45.0 34.9 Comparative Example 3 3.2 217.8-224.2 80.9 40.0 35.8 Comparative Example 4 2.2 218.6-225.9 83.5 42.6 32.4 Comparative Example 5 2.7 218.4-226.3 78.8 41.7 34.9 Comparative Example 6 2.8 221.2-225.6 76.2 43.5 31.0 Comparative Example 7 2.6 219.6-224.9 84.5 50.5 36.9 Comparative Example 8 2.7 219.2-225.5 85.6 48.5 34.9 Comparative Example 9 3.0 218.3-224.2 82.1 49.0 35.5 Comparative Example 10 3.1 217.5-223.0 81.9 48.6 34.6 Comparative Example 11 3.0 217.7-223.6 82.5 47.7 35.4

[0097] As shown in Table 3, and in conjunction with Examples 1-5, the high-performance low-silver lead-free solder of the present invention can achieve an interface IMC thickness as low as below 3 μm after reflow soldering. Figure 1 As shown, Figure 1The microstructure of the interface after soldering the high-performance low-silver lead-free solder prepared in Example 1 onto copper pads is clearly visible. A continuous, dense, and moderately thick (Cu,Ni,Co)6(Sn,In)5 interface thin layer is formed at the solder interface. This indicates that the high-performance low-silver lead-free solder of the present invention, through quantitative synergistic design of its components, possesses the ability to precisely control the interface reaction and actively suppress excessive growth of the interfacial IMC at the solder joint interface, maintaining the interface IMC thickness below 3 μm after reflow soldering, exhibiting excellent interface stability. Furthermore, a large number of dispersed Ag3Sn phases (larger particles) are present in the solder region near the solder joint interface; these reinforcing phases can significantly improve the mechanical strength of the solder. Meanwhile, the high-performance low-silver lead-free solder of the present invention has a melting range of 216-230℃, a wetting spread rate of over 79%, a tensile strength greater than 48MPa, and a shear strength greater than 33MPa. It takes into account excellent melting ability, wetting spread ability, mechanical properties and process feasibility. Therefore, the high-performance low-silver lead-free solder of the present invention not only effectively solves the problem of excessive growth of IMC, but also solves the problem of interface stability and overall performance imbalance that is common in the prior art.

[0098] As can be seen from Examples 1 and 6-8, Examples 6-8 added at least one auxiliary element Ga, Ge and P to the basis of Example 1, and the spread rate was improved to varying degrees. This is because the addition of antioxidant elements can reduce oxidation and improve wetting, thereby improving the spread rate.

[0099] As can be seen from Examples 1-5 and Comparative Examples 1-2, Comparative Examples 1 and 2 are Sn-0.3Ag-0.7Cu low-silver lead-free solder and Sn-3.0Ag-0.5Cu high-silver lead-free solder, respectively, which are commonly used in the industry. Under the same experimental conditions, compared with conventional silver-containing lead-free solders, the high-performance low-silver lead-free solder of the present invention has a thinner interfacial IMC thickness after reflow soldering, resulting in better performance. By comparing Examples 1-5 with Comparative Example 1, it can be clearly seen that the high-performance low-silver lead-free solder of the present invention is superior to Sn-0.3Ag-0.7Cu low-silver lead-free solder in almost all aspects. Furthermore, compared with Comparative Examples 1-5 and Comparative Example 2, the mechanical properties of the high-performance low-silver lead-free solder of the present invention are close to those of Sn-3.0Ag-0.5Cu high-silver lead-free solder. However, the Ag content of the high-performance low-silver lead-free solder of the present invention is even lower, less than 1%, and its mechanical properties are not weakened even in a low-silver environment. Such lead-free solder not only has a thin IMC thickness, but also has low cost, high wettability, good solderability, and high reliability, and has extremely high cost performance.

[0100] As can be seen from Example 1 and Comparative Examples 3-4, all of them used appropriate amounts of various raw materials. However, the (Ni+Co) / In value of Comparative Example 3 was 10.05, and the (Ni+Co) / In value of Comparative Example 4 was 0.07, both of which were outside the range of 0.15-3.5. The "interface behavior regulation" with Ni, Co, and In interaction as the core is the most important reason affecting the thickness test results of the interface IMC layer after reflow soldering. When (Ni+Co) / In < 0.15, it means that the total amount of interface modifying elements (Ni+Co) is seriously insufficient relative to In. At this time, the "regulatory" effect of In is dominant, but there is a lack of sufficient Ni / Co to form a stable (Cu,Ni,Co)6(Sn,In)5 reinforcing phase, resulting in an excessively thin interface IMC layer, which reduces the mechanical bonding strength of the solder joint, and the structure is loose with poor mechanical properties, resulting in weak shear and tensile resistance of the solder joint. When (Ni+Co) / In > 3.5, it means that the content of Ni and Co is relatively high, resulting in a thicker interfacial IMC layer. Furthermore, the excess Ni and Co not only consume the regulating effect of In, but may also react separately with Sn to form brittle independent phases such as Ni3Sn4 and CoSn3. These hard particles can cleave the solder matrix, leading to a sharp decrease in plasticity and brittle solder joints. Therefore, the interfacial IMC thickness of Comparative Example 3 after reflow soldering (3.2 μm) is thicker than that of Example 1, while the interfacial IMC thickness of Comparative Example 4 after reflow soldering (2.2 μm) is thinner than that of Example 1. The above demonstrates that under low silver conditions, Co and Ni must meet specific ratios to synergize with In and effectively regulate the interface. This not only proves that Ni, Co, and In have a synergistic effect rather than being a simple additive relationship, but also shows that the overall performance of the solder can only be improved if the various raw materials are in appropriate amounts and meet the relationship (Ag-0.5)²+(Bi-2)² / 4≤1 and the relationship 0.15≤(Ni+Co) / In≤3.5.

[0101] Combining Example 1 and Comparative Example 5, it can be seen that Comparative Example 5 uses appropriate amounts of various raw materials, but the value of (Ag-0.5)²+(Bi-2)² / 4 in Comparative Example 5 is 1.06, which is greater than 1. Testing showed that the overall test results of Comparative Example 5 were not as good as those of Example 1, and its cost was also higher. In Example 1, Bi and Ag satisfy the relationship (Ag-0.5)²+(Bi-2)² / 4≤1, and their contents are maintained within the optimal elliptical range centered at (0.5% Ag, 2% Bi). The synergistic effect of Bi and Ag is optimal, resulting in the best comprehensive performance of the solder, including mechanical strength and toughness. This not only proves that Ag and Bi have a synergistic effect, rather than a simple additive effect, but also demonstrates that appropriate amounts of various raw materials, conforming to the relationship 0.15≤(Ni+Co) / In≤3.5, and conforming to the relationship (Ag-0.5)²+(Bi-2)² / 4≤1 are necessary to improve the comprehensive performance of the solder.

[0102] Combining Example 1 and Comparative Examples 6-8, it can be seen that Comparative Examples 6-8 all reduced, increased, or simultaneously reduced and increased the amount of individual raw materials, but all maintained the values ​​of the two relationships within the set range. Due to the changes in different raw materials, Comparative Examples 6-8 had different performance effects: Comparative Example 6 reduced the strength and hardness of the solder alloy, increased the melting point, and reduced wettability; the overall performance of Comparative Examples 7 and 8 was comparable to that of Example 1, but since Comparative Examples 7 and 8 used more expensive In, it was not conducive to cost reduction, which is inconsistent with the low-cost feature of this invention. The above test results show that there is a synergistic effect between Ni, Co, and In, and a synergistic effect between Ag and Bi. However, this synergistic effect is based on the premise that the raw materials are in appropriate amounts. Therefore, it proves that even if the various raw materials meet the relationships 0.15≤(Ni+Co) / In≤3.5 and (Ag-0.5)²+(Bi-2)² / 4≤1, they still need to be in appropriate amounts to improve the overall performance of the solder.

[0103] As can be seen from Example 1 and Comparative Examples 3-8, the high-performance low-silver lead-free solder of the present invention requires not only that the mass percentage of various raw materials be within a specific range, but also that the values ​​of the two relational formulas be within a specific range. Only when all three conditions are met simultaneously can the various raw materials work together synergistically to ensure that the solder achieves optimal overall performance. If any condition is not met, it will affect the overall performance of the solder.

[0104] As can be seen from Example 1 and Comparative Example 9, Comparative Example 9 did not first prepare Ni, Co, and Ce into Sn-1Ni, Sn-1Co, and Sn-5Ce master alloys respectively during the preparation process. Instead, Ni, Co, and Ce were directly added to Sn in elemental form. Due to their high melting point or reactive properties, Ni, Co, and Ce are not easily dissolved in Sn when added directly in elemental form, resulting in uneven addition and excessively high local Ni / Co concentrations. This, in turn, promotes abnormal IMC growth, resulting in an IMC thickness of up to 3 μm. Therefore, it demonstrates that first preparing Ni, Co, and Ce into Sn-1Ni, Sn-1Co, and Sn-5Ce master alloys respectively is a crucial and essential step in preparing the high-performance low-silver lead-free solder of this invention, ensuring the ideal interface IMC thickness.

[0105] Combining Example 1 and Comparative Example 10, it can be seen that Comparative Example 10 uses ordinary mechanical stirring (such as paddle stirrer) instead of ultrasonic vibration and electromagnetic stirring in S3, and the stirring time is 22 minutes. Compared with Example 1, Example 1 first ultrasonically vibrates for 6 minutes and then electromagnetically stirs for 10 minutes in S3. Although the stirring time of Comparative Example 10 is 6 minutes longer than that of Example 1, the solder performance of Comparative Example 10 is worse, specifically, the interface IMC thickness after reflow soldering is 3.1 μm. This is because the intensity of mechanical stirring is far less than that of ultrasonic vibration plus electromagnetic stirring. Active or volatile elements such as Ce, P, Ga, and Ge need to be dispersed in the form of fine, dispersed particles or solid solutions to suppress the lateral growth and coarsening of the IMC layer. However, mechanical stirring alone cannot ensure the achievement of nanoscale dispersion. Even if the mechanical stirring time is longer, it cannot replace ultrasonic vibration plus electromagnetic stirring. Therefore, ultrasonic vibration plus electromagnetic stirring is the core bridge connecting the preparation process and the final performance, and it is also one of the most critical steps in the preparation process.

[0106] As can be seen from Example 1 and Comparative Example 11, Comparative Example 11 reduced the ultrasonic vibration time and electromagnetic stirring time in S3 by 2 minutes each, which resulted in an increase of 3 μm in the thickness of the interface IMC after reflow soldering. This shows that the ultrasonic vibration and electromagnetic stirring time are important factors affecting the thickness of IMC. If the ultrasonic vibration and electromagnetic stirring time are insufficient, some raw materials cannot be completely dissolved and evenly distributed, which will weaken the inhibitory effect on the excessive growth of IMC and make the IMC layer thicker.

[0107] As can be seen from Example 1 and Comparative Examples 9-11, the influence of the process on the performance of the high-performance low-silver lead-free solder of the present invention is no less than the influence of the formulation components on the performance of the high-performance low-silver lead-free solder. Only when the preparation method and preparation conditions are closely matched with the elements of the formulation components can the two play their maximum role and achieve the preparation of low-cost, high-performance low-silver lead-free solder.

[0108] As can be seen from Comparative Examples 1-2 and 3-11, although the overall performance of Comparative Example 3-11 is not as good as that of Examples 1-8, it is still better than that of Comparative Example 1-2. This shows that the high-performance low-silver lead-free solder of the present invention is a new type of low-silver solder compared with the prior art. It has excellent overall performance and helps to improve the long-term reliability of next-generation electronic products.

[0109] In summary, the high-performance low-silver lead-free solder of the present invention, by defining a "material strengthening factor", namely (Ag-0.5)²+(Bi-2)² / 4, and an "interface synergistic regulation factor", namely (Ni+Co) / In, aims to solve the technical problem of "how to ensure the stability of the solder joint interface and the overall mechanical properties at extremely low silver content through the precise regulation of multi-element micro-alloying". The present invention can maintain a balance between "interface stability" and "comprehensive performance" while reducing the silver content. Specifically, the present invention has the following excellent effects: (1) Precisely regulates the interface reaction and actively inhibits the excessive growth of intermetallic compounds (IMC) at the solder joint interface, avoiding it from becoming a weak link in reliability. (2) Breaks through the performance imbalance dilemma and simultaneously achieves excellent wettability, high mechanical strength and good creep resistance under the premise of low silver. (3) Provides an industrially feasible preparation scheme to ensure that the above performance improvement can be achieved stably and repeatedly, fundamentally overcoming the defects of the prior art that are one-sided. The high-performance low-silver lead-free solder alloy ingot of the present invention is subjected to thermomechanical processing to further process it into solder pillars, solder bars, solder wires, solder balls, solder powder and preformed solder sheets, etc., which are suitable for wave soldering, reflow soldering, dip soldering and manual soldering processes in the field of electronic packaging soft soldering, and have a wide range of applications.

[0110] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A high performance low silver lead-free solder, characterized by: It includes the following components and their mass percentages: 0.1-1% Ag, 0.01-1% Cu, 0.1-3.8% Bi, 0.002-0.3% Ni, 0.02-2% In, 0.001-0.5% Co, 0.001-0.5% Ce, with the balance being Sn, and the mass percentages of the components satisfy the following equations (1) and (2): 0.15≤(Ni+Co) / In≤3.5 (1) Formula; (Ag-0.5)²+(Bi-2)² / 4≤1 (2) Formula.

2. The high performance low silver lead-free solder according to claim 1, characterized in that: The high-performance low-silver lead-free solder comprises the following components and their mass percentages: 0.25-0.81% Ag, 0.1-0.7% Cu, 0.69-2.1% Bi, 0.007-0.15% Ni, 0.05-1% In, 0.045-0.1% Co, 0.05-0.2% Ce, with the balance being Sn.

3. The high performance low silver lead-free solder of claim 1, wherein: It also includes at least one of Ga, Ge and P in a mass percentage of 0.05-0.2%.

4. The method of producing a high performance low silver lead-free solder according to claim 3, characterized by: The processing steps are as follows: S1: Ni, Co, Ce, P and a portion of Sn are melted together in a vacuum environment at 550-650℃ to form Sn-1Ni, Sn-1Co, Sn-5Ce and Sn-2P master alloys, respectively. S2: Under a protective atmosphere of high-purity argon, heat the remaining Sn to 430-470℃ until it is completely melted; add Ag, Cu, Bi and In elements, stirring for 10-20 minutes after each addition; then add Sn-1Ni and Sn-1Co master alloys, and stir for another 15-20 minutes until they are completely dissolved and evenly distributed to form a homogeneous base melt; S3: Reduce the temperature of the base melt to 390-410℃, add the pre-made Sn-5Ce, Sn-2P master alloy and Ga, Ge elements, and ultrasonically vibrate for 5-10 minutes, then electromagnetically stir for 10-20 minutes to completely dissolve the master alloy and elements, and achieve nanoscale dispersion of Ce, Ga, Ge and P elements to obtain high-performance low-silver lead-free solder.

5. The method for preparing high-performance low-silver lead-free solder according to claim 4, characterized in that: It also includes S4: holding the high-performance low-silver lead-free solder at 340-360℃ for 50-60 minutes, then pouring it into a metal mold preheated to 150-200℃ under argon protection, and cooling it to obtain an alloy ingot.

6. The method for preparing high-performance low-silver lead-free solder according to claim 5, characterized in that: In step S4, the alloy ingot is subjected to thermomechanical processing to further process it into solder wire, solder powder, or preformed solder sheet.

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