Ni-reinforced Sn-20Bi lead-free solder and method for preparing the same

Abstract

A Ni-reinforced Sn-20Bi-based lead-free solder and its preparation method are disclosed, belonging to the field of high wettability and low-temperature electronic packaging. The mass percentages of each component element are Bi = 20 wt.%, Ni mass percentages are 0 wt.%, 0.3 wt.%, 0.5 wt.%, 0.8 wt.%, 1.0 wt.%, and 2.0 wt.%, respectively, with the remainder being Sn. The Sn-20Bi-based solder has a melting point close to that of Sn-Pb-based solder, and is highly regarded for its compatibility with existing processes and equipment, high strength, abundant resources, and low price. The addition of Ni element changes the thickness and morphology of the IMCs layer, replaces some of the Cu positions in Cu6Sn5, and forms (Cu,Ni)6Sn5 with the same crystal structure, thereby improving the reliability of the solder joint interface. To address the issue that Sn-Bi solder joints suffer from significant Bi segregation on the IMCs layer due to Sn consumption, resulting in brittle Bi bands and reduced solder joint performance, the traditional Cu substrate is replaced with a CuSn7 substrate. This replenishes the Sn matrix consumed during the formation of the IMCs layer, reduces Bi segregation, and improves solder joint reliability.

Description

Technical Field

[0001] This invention relates to the field of low-temperature electronic packaging materials technology, and in particular to a Ni-reinforced Sn-20Bi-based lead-free solder and its preparation method. Background Technology

[0002] The rise of emerging industries such as 5G networks, artificial intelligence (AI), and autonomous driving has led to a surge in demand for integrated packaging. Consequently, the demand for solder is increasing dramatically. Traditional lead-tin solders possess excellent soldering and service performance, making them widely used in electronic packaging. However, lead is toxic and poses a serious threat to human health, and international regulations have already prohibited its use. This underscores the urgent need to develop new lead-free solders for electronic packaging. The mainstream lead-free solder systems are Sn-Ag-Cu, Sn-58Bi, Sn-9Zn, and Sn-Cu lead-free solder pastes. However, these mainstream lead-free solder systems suffer from high costs, poor wettability, severe segregation at the solder interface, and issues such as solder joint detachment and cold solder joints. Therefore, developing non-toxic lead-free solders with good wettability on the substrate and excellent mechanical properties at the interface has become an inevitable trend.

[0003] Based on the aforementioned issues, Sn-20Bi alloys, with their excellent wettability, low cost, and non-toxicity, have attracted widespread attention from researchers. SnBi alloys exhibit good joint strength, creep resistance, wettability, fracture resistance, and solderability comparable to SnPb alloys. Sn-Bi alloys are a popular research focus as potential alternatives to lead-free solders in the microelectronics industry. Sn-58Bi alloys are widely used in low-temperature solders, but due to their higher Bi content, the growth of intermetallic compounds in the Sn-Bi / Cu joint of Sn-58Bi eutectic alloys has a more significant impact on mechanical properties and impact reliability. Bi segregation at the interface leads to poor mechanical properties and difficulty in processing. The defects caused by Bi segregation still result in a certain gap compared to traditional Sn-Pb alloys. Reducing the Bi content in the alloy can effectively reduce the defects caused by Bi phase segregation. Therefore, Sn-20Bi alloys, with their better wettability and mechanical properties, are chosen. The melting point range of the Sn-20Bi alloy is close to that of the traditional Sn-37Pb alloy. Compared to the Sn-58Bi alloy, the Sn-20Bi system, due to its reduced Bi content, significantly reduces Bi segregation at the Cu interface while maintaining good wettability. The Sn-20Bi alloy's melting point range is similar to that of the traditional Sn-37Pb alloy. Therefore, Sn-20Bi alloy can be used for equipment encapsulation without altering existing equipment. The addition of Ni can significantly refine the Bi phase in the Sn-20Bi alloy, improving its mechanical, physical, and corrosion resistance properties. Replacing the traditional Cu substrate with a CuSn7 substrate significantly reduces Bi segregation and improves interface reliability. Summary of the Invention

[0004] The purpose of this invention is to provide a Ni-reinforced Sn-20Bi-based lead-free solder and its preparation method. The solder alloy comprises Sn, Bi, and Ni, with the following percentages: Bi = 20 wt.%, Ni = 0 wt.%, 0.3 wt.%, 0.5 wt.%, 0.8 wt.%, 1.0 wt.%, and 2.0 wt.%, with the balance being Sn. This invention replaces the traditional Cu substrate with a CuSn7 substrate and addresses the shortcomings of existing technologies by providing a Ni-reinforced Sn-20Bi-based lead-free solder and its preparation method.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] According to one aspect of the present invention, a Ni-reinforced Sn-20Bi-based lead-free solder is provided.

[0007] A Ni-reinforced Sn-20Bi-based lead-free solder, comprising Sn, Bi, and Ni, wherein the components are specifically in the following mass percentage: Bi = 20 wt.%;

[0008] Ni = 0-2.0 wt.%;

[0009] The remainder is Sn.

[0010] Furthermore, the ternary Sn-20Bi-xNi solder is soldered onto a CuSn7 substrate.

[0011] According to another aspect of the present invention, a method for preparing the above-described solder is provided.

[0012] A method for preparing a Ni-reinforced Sn-20Bi-based lead-free solder includes the following steps:

[0013] (1) According to the mass percentage of the ternary Sn-20Bi-xNi lead-free solder, weigh Sn, Bi and Ni powder with a purity of 99.99%, and then ball mill the three powders to prepare a Sn-20Bi-xNi ternary solder alloy with uniform composition.

[0014] (2) The ternary Sn-20Bi-xNi lead-free solder after ball milling and mixing in step (1) is cold-pressed into shape by a tablet press.

[0015] (3) Add molten salt that is isolated from air to the block ternary Sn-20Bi-xNi lead-free solder powder that was cold-pressed in step (2), put it into a muffle furnace for melting, and stir it evenly.

[0016] (4) Add a degassing agent to the molten metal after step (3), stir and pour it into a mold;

[0017] (5) Take the block solder formed by melting in step (4) into an appropriate size, weld it to the substrate, and perform welding by reflow soldering.

[0018] Furthermore, in step (1), the elemental particles are Sn powder and Bi powder with a particle size of 30 μm and nano-sized Ni powder, wherein:

[0019] The ball milling mixing conditions are as follows: each component powder and steel balls are placed into the ball mill jar at a mass ratio of 1:10, and argon gas is introduced as a protective gas.

[0020] The ball mill operating parameters are: speed 110 r / min, 5 min rotation, 10 min stop, 20 cycles.

[0021] Furthermore, in step (2), the parameters for the cold pressing of the tablet press are set as follows: pressure setting: 30MPa, holding time: 20min.

[0022] Furthermore, the molten salt composition and ratio added to the muffle furnace in step (3) to isolate air is: LiCl:KCl = 1:1.3, wherein the temperature of the muffle furnace is controlled at 600℃, the melting time is 60min, and the furnace is stirred once every 15min.

[0023] Furthermore, in step (5), the mass of the bulk solder is 0.2g ± 0.05g;

[0024] The flux is H2O:ZnCl2:NH4Cl = 50:45:5.

[0025] Furthermore, step (5) specifically includes the following steps:

[0026] Place the sample into a crucible filled with flux and wait for the sample in the crucible to melt.

[0027] During the cooling process, the sample was rapidly shaken until it became a full and round sphere, and then welded to a 20mm×20mm×1.2mm glossy CuSn7 substrate.

[0028] Place the small ball in the middle of the substrate and drip in an appropriate amount of flux;

[0029] The wetting test was conducted in a reflow oven, where the peak reflow temperature was 255℃ and the reflow time was 353s.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] (1) The present invention prepares Sn-20Bi-xNi alloy by ball milling and powder mixing and muffle furnace melting. This preparation method can ensure the uniformity of the solder alloy structure and the addition of Ni element significantly refines the microstructure of the solder matrix.

[0032] (2) The addition of Ni element does not significantly reduce the melting point of Sn-20Bi solder alloy, which can effectively reduce the damage to the base material caused by large temperature changes during welding. At the same time, the melting point is close to that of traditional Sn37Pb alloy. Sn-20Bi alloy can be used to encapsulate equipment without changing the existing equipment.

[0033] (3) Compared to traditional Cu substrates, CuSn7 exhibits excellent wettability. The solderability of Sn-20Bi-xNi alloy is measured by the size of the wetting angle. Compared to traditional Cu substrates, CuSn7 exhibits excellent wettability.

[0034] (4) When Sn-20Bi-xNi alloy is welded to CuSn7 and traditional Cu substrate, the interface reliability of Sn-20Bi-xNi / CuSn7 is significantly higher than that of Sn-20Bi-xNi / CuSn7. Attached Figure Description

[0035] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0036] Figure 1 The images are SEM microstructure images of Sn-20Bi-xNi (x = 0, 0.3, 0.5, 0.8, 1.0 and 2.0 wt.%) from this invention.

[0037] Figure 2 DSC images of Sn-20Bi-xNi (x = 0, 0.3, 0.5, 0.8, 1.0 and 2.0 wt.%) in this invention;

[0038] Figure 3 SEM wetting angle images of Sn-20Bi-xNi (x = 0, 0.3, 0.5, 0.8, 1.0 and 2.0 wt.%) used in this invention;

[0039] Figure 4 Comparative SEM images showing Bi segregation when Sn-20Bi-xNi (x = 0, 0.3, 0.5, 0.8, 1.0 and 2.0 wt.%) is soldered onto a Cu substrate / CuSn7 substrate in this invention;

[0040] Figure 5 Images showing the shear strength of Sn-20Bi-xNi (x = 0, 0.3, 0.5, 0.8, 1.0 and 2.0 wt.%) soldered onto Cu and CuSn7 substrates in this invention. Detailed Implementation

[0041] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0042] An embodiment of the present invention provides a Ni-reinforced Sn-20Bi-based lead-free solder.

[0043] A Ni-reinforced Sn-20Bi-based lead-free solder, comprising Sn, Bi, and Ni, wherein the components are specifically in the following mass percentage: Bi = 20 wt.%;

[0044] Ni = 0-2.0 wt.%;

[0045] The remainder is Sn.

[0046] In this embodiment, the ternary Sn-20Bi-xNi solder is soldered onto a CuSn7 substrate.

[0047] According to another embodiment of the present invention, a method for preparing the above-mentioned solder is provided.

[0048] A method for preparing a Ni-reinforced Sn-20Bi-based lead-free solder includes the following steps:

[0049] (1) According to the mass percentage of the ternary Sn-20Bi-xNi lead-free solder, weigh Sn, Bi and Ni powder with a purity of 99.99%, and then ball mill the three powders to prepare a Sn-20Bi-xNi ternary solder alloy with uniform composition.

[0050] (2) The ternary Sn-20Bi-xNi lead-free solder after ball milling and mixing in step (1) is cold-pressed into shape by a tablet press.

[0051] (3) Add molten salt that is isolated from air to the block ternary Sn-20Bi-xNi lead-free solder powder that was cold-pressed in step (2), put it into a muffle furnace for melting, and stir it evenly.

[0052] (4) Add a degassing agent to the molten metal after step (3), stir and pour it into a mold;

[0053] (5) Take the block solder formed by melting in step (4) into an appropriate size, weld it to the substrate, and perform welding by reflow soldering.

[0054] In a specific embodiment of this application, the elemental particles in step (1) are Sn powder and Bi powder with a particle size of 30 μm and nano-sized Ni powder, wherein:

[0055] The ball milling mixing conditions are as follows: each component powder and steel balls are placed into the ball mill jar at a mass ratio of 1:10, and argon gas is introduced as a protective gas.

[0056] The ball mill operating parameters are: speed 110 r / min, 5 min rotation, 10 min stop, 20 cycles;

[0057] In step (2), the parameters for cold pressing of the tablet press are set as follows: pressure setting: 30MPa, holding time: 20min;

[0058] In step (3), the molten salt added to the muffle furnace to isolate air has the following composition and ratio: LiCl:KCl = 1:1.3, wherein the temperature of the muffle furnace is controlled at 600℃, the melting time is 60min, and the furnace is stirred once every 15min.

[0059] In step (5), the mass of the bulk solder is 0.2g ± 0.05g;

[0060] The flux is H2O:ZnCl2:NH4Cl = 50:45:5;

[0061] Step (5) further includes the following steps:

[0062] Place the sample into a crucible filled with flux and wait for the sample in the crucible to melt.

[0063] During the cooling process, the sample was rapidly shaken until it became a full and round sphere, and then welded to a 20mm×20mm×1.2mm glossy CuSn7 substrate.

[0064] Place the small ball in the middle of the substrate and drip in an appropriate amount of flux;

[0065] The wetting test was conducted in a reflow oven, where the peak reflow temperature was 255℃ and the reflow time was 353s.

[0066] To better understand the technical solution of the present invention, the present invention has been described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited to the listed embodiments.

[0067] Example 1: A Ni-reinforced Sn-20Bi-based lead-free solder, with a mass percentage of Ni = 0.3 wt.%, Bi = 20 wt.%, and the balance being Sn, was prepared into a bulk solder using the steps described above.

[0068] As can be seen from the attached figures, the Bi phase in the solder is significantly refined compared to Sn-20Bi. The melting point of Sn-20Bi-xNi (x=0.3) is increased to 202.5℃, and the wetting angles of Sn-20Bi-0.3Ni / Cu and Sn-20Bi-0.3Ni / CuSn7 are 28.0° and 20.8°, respectively. The shear strengths of Sn-20Bi-0.3Ni / Cu and Sn-20Bi-0.3Ni / CuSn7 are 49MPa and 69MPa, respectively.

[0069] Example 2: A Ni-reinforced Sn-20Bi-based lead-free solder, with a mass percentage of Ni = 0.8 wt.%, Bi = 20 wt.%, and the balance being Sn, is prepared into a bulk solder using the steps described above.

[0070] Performance testing showed that the Bi phase in the ternary Sn-20Bi-xNi (x=0.8) solder was significantly refined compared to Sn-20Bi. The addition of Ni significantly refined the microstructure of the solder matrix. This phenomenon is attributed to the increased nucleation sites resulting from the addition of Ni during the solidification and crystallization process. Fine Ni particles are distributed at grain boundaries, becoming heterogeneous nucleation sites for the β-Sn matrix and the primary Bi phase. The fine Bi and Sn phases formed in the solder microstructure gradually tended towards a uniform distribution.

[0071] The ternary Sn-20Bi-xNi (x=0.8) alloy has a similar melting point to the Sn-20Bi alloy, but is 2.5℃ higher. Adding a trace amount of Ni increases the melting point to 202.5℃. The addition of Ni forms the Ni3Sn4 intermetallic compound, further raising the melting point. As the Ni content continues to increase, the melting point does not change significantly. Therefore, the addition of Ni does not alter the melting behavior of Sn-20Bi. The wetting angles of Sn-20Bi-0.8Ni / Cu and Sn-20Bi-0.8Ni / CuSn7 are 33.4° and 25.7°, respectively.

[0072] The addition of Ni increases non-uniform nucleation at the interface, promoting the growth of the intermetallic compound layer and leading to increased thickness. This increased thickness hinders solder flow on the substrate, causing the wetting angle to increase with increasing Ni content. Another important reason is the morphology of the intermetallic compound layer. The interface morphology between Sn-20Bi alloy and Cu and CuSn7 substrates is relatively smooth and scalloped. However, after adding Ni, the IMCs layer morphology transforms into a serrated shape, significantly increasing the roughness of the intermetallic compound layer and reducing the flow of molten solder on the substrate.

[0073] The shear strengths of Sn-20Bi-0.8Ni / Cu and Sn-20Bi-0.8Ni / CuSn7 are 56 MPa and 72 MPa, respectively.

[0074] Example 3. A ternary Sn-20Bi-xNi (x=2) lead-free solder, with a mass percentage of Ni=2wt.%, Bi=20wt.%, and the balance being Sn, was prepared as a bulk solder using the steps described above. Performance testing showed that the Bi phase in the ternary Sn-20Bi-xNi (x=2.0wt.%) solder was significantly refined compared to Sn-20Bi, and the melting point of Sn-20Bi-2Ni was increased to 202.5℃. The wetting angles of Sn-20Bi-2Ni / Cu and Sn-20Bi-2Ni / CuSn7 were 40.6° and 38.5°, respectively. The shear strengths of Sn-20Bi-2Ni / Cu and Sn-20Bi-2Ni / CuSn7 were 50MPa and 63MPa, respectively.

[0075] Comparative Example 1: Sn-20Bi-xNi, x=0 has a coarse Bi phase in its microstructure, a melting point of 200℃, and wetting angles of Sn-20Bi / Cu and Sn-20Bi / CuSn7 of 27.5° and 18.1°, respectively. The shear strengths of Sn-20Bi / Cu and Sn-20Bi / CuSn7 are 48MPa and 62MPa, respectively.

[0076] As can be seen from the above, Examples 1-3 and Comparative Example 1 show that the product of the present invention has excellent performance, among which Example 2 has the best performance.

[0077] The solder described in this invention has a melting point similar to that of traditional Sn37Pb solder. It can be used to solder mainstream substrates and welding equipment, exhibiting advantages such as good wettability, low cost, excellent mechanical properties, and high weld shear strength. This solder meets the requirements of the electronic packaging industry and can be used for soldering surface mount components and light-emitting elements.

[0078] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A Ni-reinforced Sn-20Bi based lead-free solder comprising Sn, Bi and Ni, characterized in that, The components are as follows by mass percentage: Bi = 20 wt.%; Ni = 0.3-2.0 wt.%; balance Sn; Ternary Sn-20Bi-xNi solder is soldered on CuSn7 substrate. The addition of Ni element increases the non-uniform nucleation at the interface, promotes the growth of the intermetallic compound layer at the interface, and increases the thickness. After the addition of Ni element, the morphology of the IMCs layer changes to serrated, resulting in a significant increase in the roughness of the intermetallic compound layer and reducing the flow of molten solder on the substrate; Includes the following steps: (1) According to the mass percentage of the ternary Sn-20Bi-xNi lead-free solder, weigh Sn, Bi and Ni powder with a purity of 99.99%, and then ball mill the three powders to prepare a Sn-20Bi-xNi ternary solder alloy with uniform composition. (2) The ternary Sn-20Bi-xNi lead-free solder after ball milling and mixing in step (1) is cold-pressed into shape by a tablet press. (3) Add air-isolated molten salt to the block ternary Sn-20Bi-xNi lead-free solder powder that was cold-pressed in step (2), put it into a muffle furnace for melting, and stir it evenly. The composition and ratio of the molten salt are: LiCl:KCl=1:1.3; (4) Add a degassing agent to the molten metal after step (3), stir and pour it into a mold; (5) Take the block solder formed by melting in step (4) and take an appropriate size to weld it to the substrate. Weld it by reflow soldering. Place the sample in a crucible filled with flux and wait for the sample in the crucible to melt. During the cooling process, shake it quickly until the sample becomes a full and round ball. Place the ball in the middle of the substrate and drip an appropriate amount of flux. The flux is H2O:ZnCl2:NH4Cl=50:45:

5.

2. The Ni-reinforced Sn-20Bi-based lead-free solder according to claim 1, characterized in that, In step (1), the elemental particles in each group are Sn powder and Bi powder with a particle size of 30 μm and nano-sized Ni powder, wherein: The ball milling mixing conditions are as follows: each component powder and steel balls are placed into the ball mill jar at a mass ratio of 1:10, and argon gas is introduced as a protective gas. The ball mill operating parameters are: speed 110 r / min, 5 min rotation, 10 min stop, 20 cycles.

3. The Ni-reinforced Sn-20Bi-based lead-free solder according to claim 2, characterized in that, In step (2), the parameters for the tablet press cold pressing process are set as follows: pressure setting: 30MPa, holding time: 20min.

4. The Ni-reinforced Sn-20Bi-based lead-free solder according to claim 1, characterized in that, In step (3), it is added to the muffle furnace. The temperature of the muffle furnace is controlled at 600℃ and the melting time is 60min. It is stirred once every 15min.

5. The Ni-reinforced Sn-20Bi-based lead-free solder according to claim 1, characterized in that, In step (5), the mass of the bulk solder is 0.2g ± 0.05g.

6. The Ni-reinforced Sn-20Bi-based lead-free solder according to claim 5, characterized in that, Step (5) Solder to a bright CuSn7 substrate of 20mm×20mm×1.2mm; The wetting test was conducted in a reflow oven, where the peak reflow temperature was 255℃ and the reflow time was 353s.

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