Co-sn3-sn3ag composite electroplating solution, preparation method and application thereof

Abstract

This invention provides a CoSn3-Sn3Ag composite electroplating solution, its preparation method, and its application. The composite electroplating solution comprises CoSn3 nanocrystals, Sn(CH3SO3)2, Ag salt, additives, and a solvent. The concentration of the CoSn3 nanocrystals is 1-12 g / L, the concentration of Sn(CH3SO3)2 is 20-30 g / L, and the molar ratio of Sn(CH3SO3)2 to Ag salt is (25-35):1. The additives include dispersants, surfactants, complexing agents, grain refiners, rectifying agents, and pH adjusters. By adding CoSn3 nanocrystals to the electroplating solution, the wettability of the tin cap is effectively improved, and surface tension is reduced to enhance boiling, thereby improving the welding quality of the bumped Sn cap, enhancing the reliability of the Sn cap during service, and improving the overall mechanical properties of the tin-silver eutectic joint.

Description

Technical Field

[0001] This invention belongs to the field of electroplating technology, and particularly relates to a CoSn3-Sn3Ag composite electroplating solution, its preparation method, and its application. Background Technology

[0002] With the rapid development of the global semiconductor industry, integrated circuits (ICs) not only require astronomical connections between components, but also place greater emphasis on systematization, high integration, and high reliability. For decades, the development of ICs has followed Moore's Law, primarily driven by the reduction of transistor size. However, today, transistor feature sizes are approaching physical limits, and internal wiring density is exploding, leading to a significant increase in interconnect latency. Physical limits and interconnect bottlenecks make it increasingly difficult to further shrink transistor size to improve IC performance. Therefore, three-dimensional integrated circuits (3D-ICs) have attracted widespread attention and research as a solution to these problems. By vertically integrating ICs in a package, 3D-IC technology can provide solutions with short interconnect lengths, low power consumption, and the ability to achieve heterogeneous integration. Vertical bonding is key to chip or wafer stacking in 3D-ICs, and the industry continues to explore vertical bonding solutions. For example, DDR4 and Wide I / O memory require small-pitch I / O arrangements to achieve high bandwidth and low latency, but due to severe parasitic capacitance problems with wire bonding, the industry has generally shifted to copper pillar bump technology. Other 3D-IC and 2.5D packages (such as image sensors that require high-density I / O) are also forced to adopt copper pillar bumps due to the surge in the number of interconnects per unit area.

[0003] Compared to C4 (controlled collapse chip connection) solder joints, smaller copper pillar bumps offer several advantages: 1) The flat sides and high aspect ratio of copper pillar bumps facilitate ultra-fine pitch and high density; 2) The larger inter-pillar space reduces noise; 3) Copper pillar bumps possess excellent thermal and electrical properties; 4) Unlike C4 solder joints which may collapse during reflow soldering leading to solder bridging, copper pillar bumps avoid this problem. The type of solder used in copper pillar technology is flexible and can be adjusted as needed to meet specific requirements. In summary, copper pillars not only provide finer and more uniform pitch but also improve the quality of the bumps.

[0004] Unlike traditional C4 solder joints, the reduction in solder joint size leads to fundamental differences in interfacial reactions during the connection process, interfacial evolution during service, and micromechanical behavior. As the solder joint size decreases, the number of grains within the joint decreases dramatically, with the joint consisting of only a few or even a single grain. Due to the significant reduction in solder content, smaller bump diameter, and increased power density in the copper pillar bumps, the current density carried by the copper pillar bumps can easily reach 1×10⁻⁶.4 A / cm 2 Exceeding the electromigration threshold causes directional movement of atoms within the solder, leading to serious reliability issues for electronic components. Therefore, the Sn3Ag solder currently used on the top of copper pillar bumps cannot meet the stringent reliability requirements of future applications. Summary of the Invention

[0005] To address the above technical problems, this invention discloses a CoSn3-Sn3Ag composite electroplating solution, its preparation method, and its applications. The CoSn3 nanocrystals in the electroplating solution effectively improve the wettability of the tin cap, reduce surface tension to enhance boiling, and promote early bubble removal, which helps reduce micro-defects such as pores and improves the soldering quality of the bumped Sn cap. Furthermore, the CoSn3 nanocrystals completely dissolve during the Sn cap reflow process, with Co atoms dissolving in Sn. Simultaneously, the dissolution of Co atoms in Sn reduces the solubility of Cu atoms in Sn, promoting the formation of numerous Cu6Sn5 nanoparticles in the Sn cap. These Cu6Sn5 nanoparticles play a role in solid solution strengthening, further improving the reliability of the Sn cap during service.

[0006] The technical solution adopted by this invention is as follows:

[0007] A CoSn3-Sn3Ag composite electroplating solution comprises CoSn3 nanocrystals, Sn(CH3SO3)2, Ag salt, additives, and solvent. The concentration of the CoSn3 nanocrystals is 1-12 g / L, the concentration of Sn(CH3SO3)2 is 20-30 g / L, and the molar ratio of Sn(CH3SO3)2 to Ag salt is (25-35):1. The Ag salt is at least one of AgCH3SO3 or AgCl. The additives include dispersants, surfactants, coordinating agents, grain refiners, rectifying agents, and pH adjusters.

[0008] Using this technical solution, the copper pillar bump tin caps prepared with this electroplating solution exhibit significantly reduced surface tension between the solder liquid and the copper substrate due to the presence of CoSn3 nanocrystals. This promotes boiling, facilitating early bubble removal, reducing hole defects, improving bump soldering quality, and enhancing the overall performance and reliability of soldering applications. Furthermore, the addition of CoSn3 nanocrystals greatly influences the microstructure of the intermetallic compounds formed after reflow, producing novel and complex morphological features (such as six-fold cyclic twins), improving the overall mechanical properties of the tin-silver eutectic joint, and enhancing tensile properties.

[0009] As a further improvement of the present invention, the pH adjuster includes at least one of CH3SO3H and HCl; further, the amount of pH adjuster used is to adjust the pH value of the electroplating solution to 3.0-4.0. Further, the pH value of the electroplating solution is adjusted to 3.8.

[0010] As a further improvement of the present invention, the dispersant includes HS(CH2). 11 OH. Further, the concentration of the dispersant is 1-5 g / L.

[0011] As a further improvement of the present invention, the surfactant includes C 18 H 29 NaO3S, C5H 11 N5S. Further, the concentration of the surfactant is 0.5-5 g / L. Further, the concentration of the surfactant is 1 g / L. Further, the C... 18 H 29 NaO3S, C5H 11 The mass ratio of N5S is 1:0.5-1.5. Further, the C... 18 H 29 NaO3S, C5H 11 The mass ratio of N5S is 1:1.

[0012] As a further improvement of the present invention, the coordinating agent includes C 10 H 14 At least one of N2Na2O8 and C6H9NO6. Further, the concentration of the ligand is 20-50 g / L. Further, C... 10 H 14 The mass ratio of N2Na2O8 to C6H9NO6 is 1:1-2. More preferably, the C... 10 H 14 The mass ratio of N2Na2O8 to C6H9NO6 is 1:1.6.

[0013] As a further improvement of the present invention, the grain refiner includes HSCH2CH(NH2)CO2H. This technical solution uses cysteine ​​as a grain refiner, which improves the grain structure by influencing the surface bilayer structure and electrode kinetics, allowing nanoparticles to adsorb in the high-energy surface region, effectively suppressing the growth of the most active tin sites, and resulting in a better coating morphology.

[0014] Furthermore, the concentration of the grain refiner is 15-25 g / L. More preferably, the concentration of the grain refiner is 20 g / L.

[0015] As a further improvement of the present invention, the rectifying agent comprises C7H4NNaO3S. Further, the concentration of the rectifying agent is 0.5-5 g / L. More preferably, the concentration of the rectifying agent is 1 g / L.

[0016] As a further improvement of the present invention, the solvent comprises an aqueous solution of polyethylene glycol. Further, the concentration of the polyethylene glycol is 3-8 g / L. Further, the concentration of the polyethylene glycol is 5 g / L.

[0017] As a further improvement of the present invention, the CoSn3 nanocrystals have a particle size of 10-200 nm.

[0018] As a further improvement of the present invention, the CoSn3 nanocrystals are prepared by the following steps:

[0019] Step S11: Add anhydrous cobalt chloride and stannous chloride dihydrate to ethylene glycol to obtain solution A; wherein, stannous chloride dihydrate should be in appropriate excess to prevent tin ion hydrolysis in subsequent processes;

[0020] Step S12: Hydroxypropyl methylcellulose (HPMC), polyvinyl alcohol (PVA), hydroxypropyl cellulose (HPC), and sodium borohydride (NaBH4) are added to an aqueous ethanol solution and mixed thoroughly to obtain solution B;

[0021] While solution B is being stirred, solution A is added dropwise to solution B to obtain a mixed solution;

[0022] Step S13: Place the obtained mixed solution in a microwave oven, add any ionic liquid, heat in the microwave oven for 10 minutes, and then acid wash and centrifuge to obtain CoSn3 nanocrystals; or perform a hydrothermal reaction on the obtained mixed solution, and then acid wash and centrifuge to obtain CoSn3 nanocrystals.

[0023] Furthermore, in step S11, the mass ratio of ethylene glycol, anhydrous cobalt chloride, and stannous chloride dihydrate is 50:(3-5):(7-10).

[0024] Furthermore, in step S12, the mass ratio of hydroxypropyl methylcellulose, polyvinyl alcohol, hydroxypropyl cellulose, and sodium borohydride is (0.5-2):(1-5):(4-8):100.

[0025] As a further improvement of the present invention, in step S12, the stirring state of solution B is that solution B is incorporated into the ultrasonic disperser while magnetic stirring is performed, with a stirring speed of 150-250 rpm / min, an ultrasonic power of 450-500 W, and a frequency of 18-22 kHz. Further, in step S12, the stirring speed is 200 rpm / min, the ultrasonic power is 480 W, and the frequency is 20 kHz.

[0026] As a further improvement of the present invention, in step S13, the pickling includes sequentially ultrasonic cleaning with dilute hydrochloric acid and anhydrous ethanol for 3 minutes each, followed by cleaning with deionized water.

[0027] This invention discloses a method for preparing the CoSn3-Sn3Ag composite electroplating solution as described above, comprising the following steps:

[0028] Step S1: Prepare CoSn3 nanocrystals;

[0029] Step S2: Dissolve Sn(CH3SO3)2 and AgCH3SO3 in deionized water and stir until completely dissolved to obtain a precursor solution.

[0030] Step S3: Take CoSn3 nanocrystal particles, disperse them in a solvent, add a dispersant and a surfactant, add the precursor solution, and stir to mix.

[0031] In step S4, the complexing agent, grain refiner, and rectifier are added to the solution obtained in step S3, mixed evenly, and the pH value is adjusted to 3.0-4.0 by adding a pH adjuster to obtain the CoSn3-Sn3Ag composite electroplating solution.

[0032] Furthermore, in step S4, a pH adjuster is added to adjust the pH value to 3.8.

[0033] As a further improvement of the present invention, step S1 includes the following sub-steps:

[0034] Step S11: Add anhydrous cobalt chloride and stannous chloride dihydrate to ethylene glycol to obtain solution A; wherein, stannous chloride dihydrate should be in appropriate excess to prevent tin ion hydrolysis in subsequent processes;

[0035] Step S12: Hydroxypropyl methylcellulose (HPMC), polyvinyl alcohol (PVA), hydroxypropyl cellulose (HPC), and sodium borohydride (NaBH4) are added to an aqueous ethanol solution and mixed thoroughly to obtain solution B;

[0036] While solution B is being stirred, solution A is added dropwise to solution B to obtain a mixed solution;

[0037] Step S13: Place the obtained mixed solution in a microwave oven, add any ionic liquid, heat in the microwave oven for 10 minutes, and then acid wash and centrifuge to obtain CoSn3 nanocrystals; or perform a hydrothermal reaction on the obtained mixed solution, and then acid wash and centrifuge to obtain CoSn3 nanocrystals.

[0038] Furthermore, in step S11, the mass ratio of ethylene glycol, anhydrous cobalt chloride, and stannous chloride dihydrate is 50:(3-5):(7-10).

[0039] Furthermore, in step S12, the mass ratio of hydroxypropyl methylcellulose, polyvinyl alcohol, hydroxypropyl cellulose, and sodium borohydride is (0.5-2):(1-5):(4-8):100.

[0040] As a further improvement of the present invention, in step S12, the stirring state of solution B is that solution B is incorporated into the ultrasonic disperser while magnetic stirring is performed, with a stirring speed of 150-250 rpm / min, an ultrasonic power of 450-500 W, and a frequency of 18-22 kHz. Further, in step S12, the stirring speed is 200 rpm / min, the ultrasonic power is 480 W, and the frequency is 20 kHz.

[0041] As a further improvement of the present invention, in step S13, the pickling includes sequentially ultrasonic cleaning with dilute hydrochloric acid and anhydrous ethanol for 3 minutes each, followed by cleaning with deionized water.

[0042] This invention also discloses the application of the CoSn3-Sn3Ag composite electroplating solution described above, used to prepare copper pillar bump tin caps. The copper pillar bump tin caps obtained using this technique can form connections on a Cu-coated substrate after reflowing at 250°C for 1 minute. Before reflow, the micro-solder joints have a large number of Co-enriched areas; after reflow, Co atoms diffuse uniformly throughout the interface. Instead, uniformly distributed Cu6Sn5 nanoparticles appear in the micro-solder joints. This is because the diffusion of Co atoms in Sn reduces the solubility of Cu atoms in Sn, and the remaining Sn after the diffusion of Co atoms in CoSn3 easily combines with Cu atoms to form Cu6Sn5 nanoparticles.

[0043] As a further improvement of the present invention, the preparation of the copper pillar bump tin cap includes: placing the copper pillar-plated wafer on the cathode, using a tin plate as the sacrificial anode, and electroplating with the CoSn3-Sn3Ag composite electroplating solution as described above to obtain the copper pillar bump tin cap.

[0044] Using this technical solution, after removing the adhesive from the wafer, the copper pillar bump tin caps obtained are reflowed at 250°C for 5 minutes, and the CoSn3 nanocrystals in the coating basically disappear.

[0045] Furthermore, electroplating is performed under a DC power supply; furthermore, the current density is 1-2 A / dm³. 2 Furthermore, the current density is 1.6 A / dm³. 2 The electroplating time is about 1 minute, which can obtain copper pillar bump tin caps of about 30 micrometers.

[0046] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0047] First, by adopting the technical solution of the present invention, adding CoSn3 nanoparticles to the electroplating solution can improve the wetting performance of the solder. The CoSn3 nanoparticles exist in the solder alloy in a dispersed state, which weakens the attraction between the internal atoms of the molten solder alloy and the surface atoms. The liquid atoms are more likely to overcome their own attraction and tend to the surface of the liquid than the atoms without added nanoparticles, thereby obtaining better wetting performance.

[0048] Secondly, using the technical solution of this invention, the CoSn3 nanocrystals contained in the electroplating solution exhibit rapid elemental diffusion after the reflow process. Simultaneously, the nanoparticles transform from an initial spherical crystalline state to a dispersed distribution mode. When CoSn3 exists in nanoscale form, it undergoes gradual pyrolysis at approximately 250°C in the presence of liquid tin. During this process, the interatomic spacing of Sn atoms in the liquid Sn environment increases, providing favorable conditions for the diffusion of Co. Therefore, the spherical structure of CoSn3 disappears. This decrease in free energy due to the negative surface enthalpy promotes the dynamic coupling of surface element diffusion, causing Co to decouple from tin at the interface. The reduction in free energy leads to the ordering of the surface composition, which in turn leads to a decrease in system entropy. This coupling effect hinders the collision and aggregation of CoSn3 particles under thermal influence. With prolonged reflow time, cobalt is uniformly distributed throughout the tin interface. This uniform distribution is beneficial for the subsequent nucleation behavior of tin and Cu6Sn5. The uniformly distributed Co enhances the nucleation process, contributing to improved overall performance and reliability in welding applications.

[0049] Third, the addition of CoSn3 to the electroplating solution significantly affects the microstructure of the intermetallic compounds formed after reflow, resulting in novel and complex morphological characteristics. Co diffusion not only reduces the undercooling required for tin nucleation but also promotes the formation of six-fold cyclic twins in the tin-silver eutectic with lower undercooling. The mechanical response of the cyclic twins is highly advantageous for the tin-silver eutectic joint because all segments within the cyclic twin share a common crystal axis. Therefore, this set of crystals exhibits monocrystalline behavior along the twin axis but polycrystalline behavior in other directions, improving the overall mechanical properties of the tin-silver eutectic joint. Attached Figure Description

[0050] Figure 1 These are transmission electron microscope (TEM) images of CoSn3 nanocrystals obtained in the embodiments of the present invention; wherein (a) and (b) are at different magnifications.

[0051] Figure 2This is a schematic diagram of electroplating using a composite electroplating solution in an embodiment of the present invention; wherein, (a) is a schematic diagram of composite electroplating, (b) is a transmission electron microscope (TEM) image of nucleated tin grains and CoSn3 nanoparticles on a copper pillar, (c) is a TEM image of tin grains and CoSn3 nanoparticles during the growth process on a copper pillar, (d) is a schematic diagram of the composite electroplating process, (e) is a SEM image of composite electroplating before reflow, (f) is a SEM image of composite electroplating after reflow, (g) is a schematic diagram of joint reflow; (h1)-(h6) are TEM images of the sample of Example 1 at different magnifications.

[0052] Figure 3 This is a schematic diagram of tin-cobalt diffusion in the copper pillar bumps according to an embodiment of the present invention; wherein, A is the energy spectrum of the tin layer after composite deposition; B is the energy spectrum of the elemental distribution at the Sn / Cu6Sn5 interface after reflow, wherein Cu atoms form atomic clusters in the original tin layer, and Co atoms diffuse to the entire interface; C is the distribution of Co atoms at the interface before reflow; D is the distribution of Co atoms at the interface after reflow; E is the distribution of copper atoms at the interface after reflow; F is an electron micrograph of the copper pillar bump interface after reflow; G is a TEM image of the Sn / Cu6Sn5 interface; H is the distribution of elements in the interface after reflow; and I is an electron microscope image of the copper pillar bumps bonded to the copper substrate.

[0053] Figure 4 These are contact angle analysis diagrams of solder pastes with different contents of CoSn3 nanocrystal particles at 250°C and copper substrates according to embodiments of the present invention; wherein (a) is a blank sample, i.e., a schematic diagram of the contact angle of solder paste without added CoSn3 nanocrystals at 250°C and copper substrates; (b) is a schematic diagram of the contact angle of solder paste containing 0.3wt.% CoSn3 nanocrystals at 250°C and copper substrates; (c) is a schematic diagram of the contact angle of solder paste containing 0.6wt.% CoSn3 nanocrystals at 250°C and copper substrates; (d) is a schematic diagram of the contact angle of solder paste containing 0.9wt.% CoSn3 nanocrystals at 250°C and copper substrates; (e) is a schematic diagram of the contact angle of solder paste containing 1.2wt.% CoSn3 nanocrystals at 250°C and copper substrates; and (f) is a comparison of the contact angles of different amounts of added CoSn3 nanocrystals.

[0054] Figure 5 These are transmission electron microscope (TEM) images of the composite plating layer before and after reflow in an embodiment of the present invention; wherein (a), (b) and (c) are TEM images of the composite plating layer before reflow at different magnifications with respect to CoSn3 nanocrystals, and (d), (e) and (f) are TEM images of the composite plating layer after reflow at different magnifications with respect to CoSn3 nanocrystals.

[0055] Figure 6This is a schematic diagram of tin-cobalt diffusion in a copper pillar bump according to an embodiment of the present invention; wherein, A is the energy spectrum of the tin layer after composite deposition, B is the elemental distribution energy spectrum of the Sn / Cu6Sn5 interface after reflow, wherein Cu atoms form atomic clusters in the tin layer and Co atoms diffuse to the entire interface, C is the distribution of Co atoms before reflow, D is the distribution of Co atoms after reflow, E is the distribution of copper atoms after reflow, F is the electron micrograph of the copper pillar bump interface after reflow, G is the TEM image of the Sn / Cu6Sn5 interface, and H is the elemental distribution in the interface after reflow.

[0056] Figure 7 This is a comparison of the EBSD results of the solder joints after reflow soldering of copper pillar bumps and solder caps obtained by electroplating solutions containing 0.3 wt.% CoSn3 nanocrystals and those without CoSn3 nanocrystals according to an embodiment of the present invention; wherein, (A) is the EBSD result of the solder joint without CoSn3 nanocrystals, and (B) is the EBSD result of the solder joint containing 0.3 wt.% CoSn3 nanocrystals.

[0057] Figure 8 This is a comparison of the interface of the solder joints after reflow soldering of copper pillar bumps and solder caps obtained by electroplating solution containing 0.3 wt.% CoSn3 nanocrystals and the interface of the solder joints after reflow soldering of copper pillar bumps and solder caps obtained by electroplating solution without CoSn3 nanocrystals, according to an embodiment of the present invention. A is a TEM image of the interface of the solder joint obtained by containing 0.3 wt.% CoSn3, BC are highly overlapping distribution maps of copper and cobalt elements, respectively; D is a TEM image of the interface of the solder joint without CoSn3 nanoparticles, E is the distribution of copper elements on the connector obtained without CoSn3, and F is a comparison of the Cu6Sn5 grain size of the two. The left side is the solder joint containing 0.3 wt.% CoSn3, and the right side is the solder joint without CoSn3.

[0058] Figure 9 These are SEM images of solder joints after reflow soldering with copper pillar bump tin caps obtained in an embodiment of the present invention. A and B are SEM images of solder joints containing 0.3 wt.% CoSn3 nanocrystals at different magnifications. C is the faceted shape of Cu6Sn5 IMC. D is the faceted shape of Cu6Sn5 IMC. E is a solder joint without CoSn3 nanocrystals. F is the interface of a solder joint without CoSn3 nanocrystals. G and H are Cu6Sn5 with scalloped edges. I is the EDX spectrum of the solder joints after reflow soldering with copper pillar bump tin caps obtained from an electroplating solution containing CoSn3 nanocrystals. J1 is the XRD result of the solder joints after reflow soldering with copper pillar bump tin caps obtained from an electroplating solution without CoSn3 nanocrystals. J2 is the XRD result of the solder joints containing CoSn3 nanocrystals.

[0059] Figure 10The following are electron microscopic observation results of the solder joint interface after aging the copper pillar bump tin cap at 150°C for 10 days according to an embodiment of the present invention. Among them, A and B are electron microscopic images of the solder joint interface without CoSn3 at different magnifications, C is an electron microscopic image of another solder joint interface without CoSn3, and DI is the result of the solder joint interface containing 0.3wt.% CoSn3 nanocrystals at different locations.

[0060] Figure 11 The mechanical properties of the joint after reflow soldering of the copper pillar bump tin cap obtained by electroplating solution with different contents of CoSn3 nanoparticles in the embodiment of the present invention are shown in (a) as shear strength and (b) as Vickers hardness. Detailed Implementation

[0061] The preferred embodiments of the present invention will be described in further detail below.

[0062] A composite Sn3Ag electroplating solution containing CoSn3 nanocrystals, wherein 1L of the electroplating solution comprises 3g CoSn3 nanocrystals, methanesulfonic acid CH3SO3H (to adjust the pH of the electroplating solution to 3.8), 25.2g stannous methanesulfonate Sn(CH3SO3)2, 0.6g silver methanesulfonate AgCH3SO3, and 1g sodium alkylbenzene sulfonate C. 18 H 29 NaO3S, 1g sodium saccharin (C7H4NNaO3S), 2g 11-mercapto-1-undecyl alcohol (HS(CH2)) 11 OH), 5g polyethylene glycol (HO(CHCHO)nH), 10g disodium ethylenediaminetetraacetate (EDTA-2Na, C 10 H 14 N2Na2O8), 16g N-triacetic acid (NTA, C6H9NO6), 20g L-cysteine ​​(HSCH2CH(NH2)CO2H).

[0063] The CoSn3 nanocrystals can be synthesized using microwave and hydrothermal methods, with the specific steps including:

[0064] The first step involves adding anhydrous cobalt chloride and stannous chloride dihydrate to an ethylene glycol solution to obtain solution A. Stannous chloride dihydrate should be added in appropriate excess to prevent tin ion hydrolysis in subsequent processes. The mass ratio is 50:(3-5):(7-10).

[0065] The second step involves adding hydroxypropyl methylcellulose (HPMC), polyvinyl alcohol (PVA), hydroxypropyl cellulose (HPC), and sodium borohydride (NaBH4) to a deionized water-ethanol mixture, followed by sonication and stirring to obtain solution B. The mass ratio of the above-mentioned drugs is (0.5-2):(1-5):(4-8):100:200.

[0066] Place solution B into an ultrasonic disperser and simultaneously perform magnetic stirring at a speed of 200 rpm / min, an ultrasonic power of 480 W, and a frequency of 20 kHz. Add solution A dropwise to solution B using a rubber-tipped dropper.

[0067] The third step is to place the crucible containing the mixed solution into a microwave oven. Any ionic liquid can be added. After heating in the microwave oven for 10 minutes, acid washing and centrifugation will yield CoSn3 nanoparticles with a diameter of approximately 20 nm.

[0068] Fourth step, as an alternative to the third step, involves placing the crucible containing the mixed solution obtained in the second step into a hydrothermal synthesis reactor. After two days, the solution is removed, acid-washed, and centrifuged to obtain CoSn3 nanoparticles with a diameter of approximately 20 nm.

[0069] As a preferred technical solution of the present invention, in the fourth step, the pickling process is carried out by ultrasonic cleaning with 15% dilute hydrochloric acid and anhydrous ethanol for 3 minutes each, followed by cleaning with deionized water and placing it in deionized water solution for later use.

[0070] The composite Sn3Ag electroplating solution containing CoSn3 nanocrystals was prepared using the following steps:

[0071] 1) Take a Sn(CH3SO3)2 and AgCH3SO3 solution with a molar ratio of 97:3, dissolve it in deionized water, and stir it evenly in an ultrasonic environment until it is completely dissolved, as the precursor solution.

[0072] 2) Disperse CoSn3 nanoparticles in a polyethylene glycol aqueous solution, and add a dispersant and surfactant: HS(CH2). 11 OH and C 18 H 29 NaO3S, C5H 11 N5S. After adding the precursor solution obtained in step 5, ultrasonically stir for 5 minutes.

[0073] 3) Take 1g of the complexing agent C 10 H 14 N2Na2O8, 16g C6H9NO6, 20g HSCH2CH(NH2)CO2H as a grain refiner, and C7H4NNaO3S as a rectifier.

[0074] After adding the solution from step 2), ultrasonically stir for 30 minutes to mix evenly to obtain the CoSn3 nanocrystalline composite electroplating solution. Make up the volume to 1L and adjust the pH value to 3.8 using CH3SO3H.

[0075] This invention provides an electroplating method, specifically, as follows: Figure 2As shown in a), the copper-plated wafer is placed at the cathode, and the tin plate is used as the sacrificial anode. The current density is set to 1.5 A / dm² under a DC power supply. 2 Electroplating for about 1 minute yields copper pillar bump caps of approximately 30 micrometers. After removing the adhesive from the wafer, the resulting caps are reflowed at 250°C for 5 minutes, revealing that the CoSn3 nanocrystals in the plating have essentially disappeared.

[0076] The following detailed description is based on specific embodiments.

[0077] Example 1

[0078] First, add 3.2 g of anhydrous cobalt chloride and 8 g of stannous chloride dihydrate to 50 mL of ethylene glycol solution to obtain solution A. Stannous chloride dihydrate should be added in excess to prevent tin ion hydrolysis in subsequent processes.

[0079] In the second step, 1g of hydroxypropyl methylcellulose (HPMC), 3g of polyvinyl alcohol (PVA), 1g of hydroxypropyl cellulose (HPC), and 5.5g of sodium borohydride (NaBH4) were added to 100ml of deionized water and mixed with 200ml of ethanol solution. The mixture was then sonicated and stirred to obtain solution B. Solution B was placed in an ultrasonic disperser and magnetically stirred at a speed of 200rpm / min, an ultrasonic power of 480W, and a frequency of 20kHz. Solution A was then added dropwise to solution B using a rubber-tipped dropper.

[0080] The third step involves placing the crucible containing the mixed solution into a microwave oven. Any ionic liquid can be added. Microwave heating for 10 minutes, followed by acid washing and centrifugation, yields CoSn3 nanoparticles with a diameter of approximately 20 nm, exhibiting the following morphology: Figure 1 As shown.

[0081] Fourth step, as an alternative to the third step, involves placing the crucible containing the mixed solution obtained in the second step into a hydrothermal synthesis reactor. After two days, the solution is removed, acid-washed, and centrifuged to obtain CoSn3 nanoparticles with a diameter of approximately 20 nm.

[0082] As a preferred technical solution of the present invention, in the fourth step, the pickling process is carried out by ultrasonic cleaning with 15% dilute hydrochloric acid and anhydrous ethanol for 3 minutes each, followed by cleaning with deionized water and placing it in deionized water solution for later use.

[0083] Fifth step, take 25.2g of Sn(CH3SO3)2 and 0.6g of AgCH3SO3 solution, dissolve them in 800mL of deionized water, and stir evenly in an ultrasonic environment until completely dissolved to obtain the precursor solution.

[0084] Step 6: Disperse 3g of CoSn3 nanoparticles in 50ml of polyethylene glycol aqueous solution, and add dispersant and surfactant: 2g HS(CH2).11 OH and 1gC 18 H 29 NaO3S, 1gC5H 11 N5S. After adding the precursor solution obtained in step 5, ultrasonically stir for 5 minutes.

[0085] Step 7: Take 10g of the complexing agent C 10 H 14 N2Na2O8, 16g C6H9NO6, 20g HSCH2CH(NH2)CO2H as a grain refiner, and 30g C7H4NNaO3S as a rectifier.

[0086] After adding the solution obtained in step 6, ultrasonically stir for 30 minutes to mix evenly to obtain the CoSn3 nanocrystalline composite electroplating solution. Make up the volume with water to 1L and adjust the pH value to 3.8 with CH3SO3H.

[0087] The copper pillar bump tin caps were fabricated using the obtained CoSn3 nanocrystalline composite electroplating solution, such as... Figure 2 As shown, it specifically includes:

[0088] A silicon wafer with a redistribution layer is coated with adhesive and then copper pillars are deposited by electrodeposition. A layer of nickel is plated on the copper pillars, and then the wafer is placed in the aforementioned composite electroplating solution with an addition of 1.5 A / dm³. 2 At a given current density, the wafer was electroplated for 1 minute, and then removed to obtain copper pillar bump caps of approximately 30 micrometers. After removing the adhesive from the wafer, the resulting caps were reflowed at 250°C for 5 minutes, after which the CoSn3 nanocrystals in the plating layer essentially disappeared.

[0089] By introducing cysteine ​​into the composite electroplating solution system, the grain structure was improved by influencing the surface bilayer structure and electrode kinetics. Notably, the nanoparticles adsorbed in the high-energy surface region effectively suppressed the growth of the most active tin sites, thus achieving a favorable coating morphology, such as... Figure 2 As shown in f).

[0090] The relationship between the amount of CoSn3 nanoparticles in the final tin plating layer and the concentration of nanoparticles in the plating bath is as follows:

[0091]

[0092] C—the concentration of CoSn3 nanoparticles in the electroplating solution;

[0093] α—Concentration of nanoparticles in the coating;

[0094] ρ—Sn in the plating solution 2+ The concentration;

[0095] i0—current density;

[0096] η—overpotential;

[0097] n, W—relative atomic mass and valence state of tin;

[0098] Sn in electroplating solution 2+ The concentration was confirmed by titration with a standard iodide solution. Accurately transfer a volume of the electroplating solution into an Erlenmeyer flask, add a 1:1 hydrochloric acid solution, and pipette 1 mL of 1% starch solution. Then, use 0.1 mol·L⁻¹ hydrochloric acid solution to titrate the solution. -1 Titrate the iodine standard solution until it turns deep blue. If the endpoint color does not change within 1 minute, record the volume of iodine standard solution used. Perform the titration three times in parallel.

[0099]

[0100] C I —Concentration of standard iodine solution

[0101] V I —Volume of standard iodine solution

[0102] V Sn —Electroplating solution volume

[0103] The obtained copper pillar bump tin caps were soldered onto the Cu-coated substrate using a reflow process at 250°C for 1 minute. A schematic diagram of tin-cobalt diffusion in the copper pillar bumps is shown below. Figure 3 As shown, the micro-solder joints before reflow have a large number of Co-enriched areas, while after reflow, Co atoms diffuse uniformly throughout the interface. Instead, uniformly distributed Cu6Sn5 nanoparticles appear in the micro-solder joints. This is because the diffusion of Co atoms in Sn reduces the solubility of Cu atoms in Sn, and the remaining Sn after the diffusion of Co atoms from CoSn3 easily combines with Cu atoms to form Cu6Sn5 nanoparticles.

[0104] Example 2

[0105] Since the wetting angle of the coating cannot be measured, this embodiment uses Sn3Ag0.5Cu solder paste mixed with nanoparticles of different masses for the purpose of facilitating the experiment.

[0106] In this embodiment, CoSn3 nanocrystals prepared in Example 1 were added to SAC305 solder paste at different amounts (0.3 wt.%, 0.6 wt.%, 0.7 wt.%, and 1.2 wt.%), coated onto a copper substrate, and a blank sample (without CoSn3 nanocrystals) was prepared. Contact angle was tested at 250°C, and the results are as follows: Figure 4As shown, the contact angle decreases rapidly with the increase of nanoparticles. This indicates that adding CoSn3 nanoparticles can improve the wetting properties of solder. CoSn3 nanoparticles exist in a dispersed state in the solder alloy, which weakens the attraction between the internal atoms of the molten solder alloy and the surface atoms. Liquid atoms are more likely to overcome their own attraction and tend to move towards the surface of the liquid than atoms without added nanoparticles, thus achieving better wetting properties.

[0107] It is evident that nanoparticles can significantly reduce the surface tension of liquids, promote boiling, and thus facilitate early bubble departure, helping to reduce pore defects and improve the welding quality of protrusions. The microfluidic layer is a liquid layer trapped between the heated surface and the continuously growing bubbles. The thinner the microfluidic layer, the faster the liquid evaporates, promoting bubble growth and thus increasing thermal conductivity. In nanofluids, the orderly accumulation of nanoparticles in the meniscus region of the microfluidic layer helps increase the extended separation pressure (i.e., fluid wettability), while the reduction in surface tension also leads to increased wettability of the fluid on specific solid surfaces.

[0108] Example 3

[0109] CoSn3 nanoparticles have a unique feature: Co is a fast-diffusion element in Sn, requiring only 0.45 eV for diffusion activation.

[0110] Based on Example 1, the copper pillar bump tin cap obtained by electroplating was subjected to a reflow experiment at 250°C. Transmission electron microscope images of the composite coating before and after reflow are shown below. Figure 5 As shown, CoSn3 nanocrystals exhibit rapid element diffusion after the reflux process, and at the same time, the nanoparticles change from the initial spherical crystalline state to a diffuse distribution mode.

[0111] A schematic diagram of tin-cobalt diffusion in copper pillar bumps is shown below. Figure 6 As shown, according to the Co-Sn phase diagram, the CoSn3 phase decomposes at 345℃. However, when CoSn3 exists in nanoscale form, it undergoes gradual pyrolysis at around 250℃ in the presence of liquid tin. During this process, the interatomic spacing of Sn in the liquid Sn environment increases, providing favorable conditions for the diffusion of Co. Therefore, the spherical structure of CoSn3 disappears, and this decrease in free energy due to negative surface enthalpy promotes the dynamic coupling of surface element diffusion, causing Co to decouple and dissociate from tin at the interface. The decrease in free energy leads to the ordering of the surface composition, which in turn leads to a decrease in system entropy. This coupling effect hinders the collision and aggregation of CoSn3 particles under thermal influence. With prolonged reflow time, cobalt becomes uniformly distributed across the entire tin interface. This uniform distribution is beneficial for the subsequent nucleation behavior of tin with Cu6Sn5. The uniformly distributed Co enhances the nucleation process, contributing to improved overall performance and reliability in welding applications.

[0112] Example 4

[0113] The addition of CoSn3 significantly affects the microstructure of the intermetallic compound formed after reflow, resulting in novel and complex morphological features. Firstly, it affects the Sn crystal:

[0114] Based on Example 1, the solder joints of copper pillar bump tin cap reflow soldering obtained using an electroplating solution without CoSn3 nanoparticles and the solder joints of copper pillar bump tin cap reflow soldering obtained using an electroplating solution containing 0.3 wt.% CoSn3 nanocrystal particles were analyzed. The electron backscatter diffraction (EBSD) results are as follows: Figure 7 As shown, there is a significant difference between samples with and without CoSn3 nanoparticles. Solder joint samples without CoSn3 nanoparticles did not exhibit six-fold cyclic twinning, while solder joint samples containing 0.3 wt.% CoSn3 nanoparticles did. The reason for this is that the diffusion of Co not only reduces the undercooling required for tin nucleation but also promotes the formation of six-fold cyclic twins in the tin-silver eutectic with lower undercooling. Typically, in tin-based solders, twinning occurs on two different crystal planes, namely {1 01} and {3 01}. 0 1} plane. In our experimental observations, cyclic twinning only occurs on the {101} plane, which lays the foundation for our subsequent discussion.

[0115] The mechanical response of cyclic twins is highly favorable for tin-silver eutectic joints because all segments within a cyclic twin share a common crystal axis. Therefore, this set of crystals behaves as a monocrystalline crystal along the twin axis and as a polycrystalline crystal in other directions, thus improving the overall mechanical properties of the tin-silver eutectic joint. EBSD results typically show only one set of six tin twins in a given tin-magnesium sample, consistent with multiple twinning events occurring during nucleation. Although the tin grain morphology may differ between samples, the consistent orientation of the tin grains indicates the presence of a specific type of hexagonal twin nucleation. To fill the space, the average angle of all twin portions must be 60°. Mismatches are required between the boundaries of the twin portions, or a low-angle grain boundary system is needed within the grains between the twin interfaces.

[0116] During growth, the slow growth direction faces the adjacent twin and remains perpendicular to the fast growth direction. This specific orientation ensures that competition between adjacent twins is minimized. Secondary dendrites extending from the main [0 0 1] branch fill the gaps between the fast-growing twins, causing the secondary dendrites of adjacent twins to interpenetrate, thus forming irregular interfaces, which is also a common phenomenon. The occurrence of hexagonal ring twinning events is related to tin nucleation in the subhexagonal / pseudohexagonal structure. Impurities such as copper or silver can promote nucleation events, with cobalt playing a particularly significant role. However, this hexagonal crystal structure becomes unstable once it grows, which is why irregular interfaces are observed. The presence of Co is beneficial for forming more stable and energy-efficient twin interfaces during nucleation.

[0117] Example 5

[0118] The addition of CoSn3 significantly affects the microstructure of the intermetallic compound formed after reflow, resulting in novel and complex morphological features. The effect on Cu6Sn5 is analyzed below:

[0119] The influence of CoSn3 on the nucleation and growth behavior of Cu6Sn5 is much more significant. Notably, the morphology of the intermetallic compound (IMC) formed by SA-Co / Cu differs significantly from the typical fan-shaped Cu6Sn5 formed between Sn-3Ag and Cu. The IMC exhibits a more complex morphology, and its unique two-phase structure is more pronounced.

[0120] Specifically, based on Example 1, further analysis of the SA-Co / Cu interface revealed more tin-rich "island" phases in the IMC region, such as... Figure 8 Used. For example Figure 9 As shown, a comprehensive top-down analysis of the overall morphological evolution of the IMC is presented. Clearly, the morphology of the IMC is non-uniform, with the external region exhibiting an elongated and multifaceted shape. Figure 9 This is an enlarged view of the IMC grains in the outer region, clearly showing the formation of the prism shape.

[0121] The addition of CoSn3 significantly influences the microstructure of the intermetallic compound (IMC), resulting in novel and complex morphological features. The prismatic shape and the presence of tin-rich "islands" observed in the IMC region indicate the complexity of the interfacial reactions involved in SA-Co / Cu. The growth of the intermetallic compound primarily stems from two kinetic processes: interfacial reaction processes and aging. Therefore, the balance between these two fluxes determines the growth kinetics of the intermetallic compound. Unlike fan-shaped grains, the growth of faceted grains is mainly influenced by the interfacial reaction flux. Specifically, when the intermetallic compound adopts a prismatic (faceted grain) morphology, the interfacial reaction process is mainly dominated by the precipitation of additional copper atoms from tin. That is, due to the presence of Co in Sn, the solubility of Cu atoms is reduced, leading to easier precipitation of Cu atoms and resulting in the growth of Cu6Sn5 along its length as faceted grains. Therefore, the rapid diffusion channels of copper atoms between grains remain intact and do not decrease with the growth of the compound.

[0122] The results of scanning electron microscopy of the aged samples are as follows: Figure 10 As shown, after 10 days of aging, the intermetallic compounds in the CoSn3-doped sample were significantly less than those in the undoped sample. Furthermore, the Cu6Sn5 atoms at the interface of the undoped sample exhibited a continuous growth trend, which negatively impacts the mechanical properties of the joint. In contrast, the Cu6Sn5 atoms at the CoSn3-doped joints displayed an island-like shape. This is because the presence of Co atoms in tin reduces the solubility of Cu atoms, making Cu6Sn5 nucleation easier. However, the planar morphology of the intermetallic compounds did not reduce copper flux, despite the inhibition of Cu6Sn5 growth by the presence of Co. Instead, during growth, copper atoms preferred to nucleate new Cu6Sn5 atoms rather than attach to existing Cu6Sn5 atoms.

[0123] Although CoSn3 nanoparticles tend to disappear after reflow and are distributed only as Co in the copper-tin system, their presence enhances Cu6Sn5 nucleation and leads to the precipitation of a large number of Cu6Sn5 nanoparticles in the interfacial system, thereby effectively improving the mechanical properties of the joint. Furthermore, the presence of Co unexpectedly inhibits Cu3Sn development, thus reducing the number of Kirkendall voids during aging.

[0124] This embodiment also includes a shearing experiment on the joints of copper pillar bump tin caps obtained by reflow soldering using electroplating solutions with different amounts of CoSn3. The results are as follows: Figure 11As shown, it is noteworthy that the CoSn3 joints with a doping mass ratio of 0.3 wt.% exhibited the highest strength. These joints showed a 25% increase in shear strength compared to undoped CoSn3 joints. However, further increasing the CoSn3 doping amount did not improve the strength. Instead, the fracture morphology indicated that the dissolution of more Co in the tin led to easier precipitation of Cu6Sn5, resulting in Cu6Sn5 aggregates within the joint, which negatively impacted the joint's mechanical properties. The distribution of Cu6Sn5 nanoparticles promoted the refinement of the eutectic microstructure while hindering grain boundary movement, thus improving tensile properties. The presence of finer, more dispersed intermetallic compounds also impeded dislocation movement and effectively stabilized grain boundaries, thereby improving creep resistance through diffusion strengthening.

[0125] The electroplating solution of this invention is mainly used for copper pillar bumps in electronic packaging, and can also be used for general copper substrate modification.

[0126] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A CoSn3-Sn3Ag composite electroplating solution, characterized in that: Its components are CoSn3 nanocrystals, Sn(CH3SO3)2, Ag salt, additives and solvent, wherein the concentration of CoSn3 nanocrystals is 1-12 g / L, the concentration of Sn(CH3SO3)2 is 20-30 g / L, the molar ratio of Sn(CH3SO3)2 to Ag salt is (25-35):1, and the Ag salt is AgCH3SO3; the additives include dispersants, surfactants, coordinating agents, grain refiners, rectifying agents and pH adjusters.

2. The CoSn3-Sn3Ag composite electroplating solution according to claim 1, characterized in that: The pH adjuster includes at least one of CH3SO3H and HCl; The dispersant includes HS(CH2). 11 OH; The surfactant includes C 18 H 29 NaO3S, C5H 11 At least one of N5S; the C 18 H 29 NaO3S is sodium alkylbenzene sulfonate; The ligand includes C 10 H 14 At least one of N2Na2O8 and C6H9NO6; wherein C6H9NO6 is hypozinotriacetic acid, C 10 H 14 N2Na2O8 is disodium ethylenediaminetetraacetate; The grain refiner includes HSCH2CH(NH2)CO2H; The rectifying agent includes C7H4NNaO3S; the C7H4NNaO3S is sodium saccharin. The solvent includes an aqueous solution of polyethylene glycol; The CoSn3 nanocrystals have a particle size of 10-200 nm.

3. The CoSn3-Sn3Ag composite electroplating solution according to claim 1, characterized in that: The CoSn3 nanocrystals were prepared using the following steps: Step S11: Anhydrous cobalt chloride and stannous chloride dihydrate are added to ethylene glycol to obtain solution A; wherein the mass ratio of ethylene glycol, anhydrous cobalt chloride and stannous chloride dihydrate is 50:(3-5):(7-10). Step S12: Add hydroxypropyl methylcellulose, polyvinyl alcohol, hydroxypropyl cellulose and sodium borohydride to an aqueous ethanol solution and mix thoroughly to obtain solution B; wherein the mass ratio of hydroxypropyl methylcellulose, polyvinyl alcohol, hydroxypropyl cellulose and sodium borohydride is (0.5-2):(1-5):(4-8):100; While keeping solution B stirred, solution A is added dropwise to solution B to obtain a mixed solution; Step S13: Place the obtained mixed solution in a microwave oven, heat for 10 minutes, and then wash and centrifuge to obtain CoSn3 nanocrystals; or perform a hydrothermal reaction on the obtained mixed solution, and then wash and centrifuge to obtain CoSn3 nanocrystals.

4. The CoSn3-Sn3Ag composite electroplating solution according to claim 3, characterized in that: In step S12, maintaining the stirring state of solution B involves adding solution B into an ultrasonic disperser while simultaneously performing magnetic stirring at a speed of 150-250 rpm, an ultrasonic power of 450-500 W, and a frequency of 18-22 kHz. In step S13, the pickling includes sequentially ultrasonic cleaning with dilute hydrochloric acid and anhydrous ethanol for 3 minutes each, followed by cleaning with deionized water.

5. The CoSn3-Sn3Ag composite electroplating solution according to any one of claims 1 to 4, characterized in that: In the composite electroplating solution, the concentration of the dispersant is 1-5 g / L, the concentration of the surfactant is 0.5-5 g / L, the concentration of the ligand is 20-50 g / L, the concentration of the grain refiner is 15-25 g / L, the concentration of the rectifying agent is 0.5-5 g / L, and the amount of pH adjuster is used to adjust the pH value of the electroplating solution to 3.0-4.

0.

6. The method for preparing the CoSn3-Sn3Ag composite electroplating solution according to any one of claims 1 to 5, characterized in that, Includes the following steps: Step S1: Prepare CoSn3 nanocrystals; Step S2: Dissolve Sn(CH3SO3)2 and AgCH3SO3 in deionized water and stir until completely dissolved to obtain a precursor solution. Step S3: Take CoSn3 nanocrystal particles, disperse them in a solvent, add a dispersant and a surfactant, add the precursor solution, and stir to mix. In step S4, the complexing agent, grain refiner, and rectifier are added to the solution obtained in step S3, mixed evenly, and the pH value is adjusted to 3.5-4.0 by adding a pH adjuster to obtain the CoSn3-Sn3Ag composite electroplating solution.

7. The method for preparing the CoSn3-Sn3Ag composite electroplating solution according to claim 6, characterized in that, Step S1 includes the following sub-steps: Step S11: Anhydrous cobalt chloride and stannous chloride dihydrate are added to ethylene glycol to obtain solution A; wherein the mass ratio of ethylene glycol, anhydrous cobalt chloride and stannous chloride dihydrate is 50:(3-5):(7-10). Step S12: Hydroxypropyl methylcellulose (HPMC), polyvinyl alcohol (PVA), hydroxypropyl cellulose (HPC), and sodium borohydride (NaBH4) are added to an aqueous ethanol solution and mixed evenly to obtain solution B; wherein the mass ratio of hydroxypropyl methylcellulose, polyvinyl alcohol, hydroxypropyl cellulose, and sodium borohydride is (0.5-2):(1-5):(4-8):100; While solution B is being stirred, solution A is added dropwise to solution B to obtain a mixed solution; Step S13: Place the obtained mixed solution in a microwave oven, heat for 10 minutes, and then wash and centrifuge to obtain CoSn3 nanocrystals; or perform a hydrothermal reaction on the obtained mixed solution, and then wash and centrifuge to obtain CoSn3 nanocrystals.

8. The method for preparing the CoSn3-Sn3Ag composite electroplating solution according to claim 7, characterized in that, In step S12, the stirring state of solution B is to add solution B into the ultrasonic disperser and simultaneously perform magnetic stirring at a stirring speed of 150-250 rpm, an ultrasonic power of 450-500 W, and a frequency of 18-22 kHz. In step S13, the pickling includes sequentially ultrasonic cleaning with dilute hydrochloric acid and anhydrous ethanol for 3 minutes each, followed by cleaning with deionized water.

9. The application of the CoSn3-Sn3Ag composite electroplating solution as described in any one of claims 1 to 5, characterized in that: Used to prepare copper pillar bump tin caps.

10. The application of the CoSn3-Sn3Ag composite electroplating solution according to claim 9, characterized in that: The copper-plated wafer is placed at the cathode, and a tin plate is used as the sacrificial anode. Electroplating is performed using the CoSn3-Sn3Ag composite electroplating solution as described in any one of claims 1 to 5 to obtain copper pillar bump tin caps.

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