Cyanide-free gold plating solution applied to wafer and preparation method thereof

Abstract

The application relates to the technical field of chemical plating, and particularly discloses a cyanide-free electroplating gold plating solution applied to a wafer and a preparation method thereof. The cyanide-free electroplating gold plating solution comprises a gold salt, a main complexing agent, an auxiliary complexing agent, a crystal grain adjusting agent, an organic phosphonic acid additive, a buffer and deionized water. The crystal grain adjusting agent can adjust the grain size and structure of the plating layer, so that the plating layer is more compact and uniform. The organic phosphonic acid additive can replace the position of a coordination agent which is prone to cause inclusions, effectively controls the interface adsorption of a cathode, is favorable to improving the inclusions in the plating layer, and improves the plating layer quality.

Description

Technical Field

[0001] This invention relates to the field of electroless plating technology, and more particularly to a cyanide-free electroplating gold plating solution for wafers and its preparation method. Background Technology

[0002] Gold electroplating is widely used in the preparation of metal coatings for critical components such as wafer leads, interconnects, and electronic packaging due to gold's excellent conductivity, corrosion resistance, and chemical stability. With the rapid development of chips towards higher density and miniaturization, higher performance requirements are being placed on the gold plating layers on wafers. As the core material of this process, optimizing the performance of the gold plating solution has become a key focus of industry research and development.

[0003] Currently, to overcome the toxicity and environmental drawbacks of cyanide systems, cyanide-free electroplating systems are commonly used in wafer electroplating gold plating processes. Existing cyanide-free systems often employ sulfites, citrates, etc., as complexing agents, whose complexation constants with gold ions are much lower than those of cyanides. This results in unstable gold ion release rates during electroplating, affecting the performance of the plating solution and the coating. Summary of the Invention

[0004] To improve the stability of the plating solution and the quality of the plating layer, this application provides a cyanide-free electroplating gold plating solution for wafers and its preparation method.

[0005] Firstly, this application provides a cyanide-free electroplating gold plating solution, which adopts the following technical solution: A cyanide-free gold plating solution comprises the following components: Gold salt 0.5~1.5g / L, main complexing agent 20~30g / L, auxiliary complexing agent 5~8g / L, crystallization modifier 20~45mg / L, organophosphonic acid additive 0.1~0.2g / L, buffer 10~20g / L, balance deionized water.

[0006] By adopting the above technical solution, gold salt provides the gold ions required for electroplating and is the basic material for forming the gold plating layer. The main complexing agent can form a stable complex with gold ions, improving the stability of gold ions in the plating solution and avoiding the release of gold ions too quickly or too slowly. The auxiliary complexing agent further enhances the complexing effect and works synergistically with the main complexing agent to make the release of gold ions more stable, thus improving the problem of unstable gold ion release rate caused by the low complexing constant of the existing cyanide-free system. The grain size adjuster can adjust the grain size and structure of the plating layer, making the plating layer more dense and uniform. The organophosphonic acid additive can improve the problem of inclusions in the plating layer.

[0007] Preferably, the preparation method of the grain-modifying agent includes the following steps: Histidine was dissolved in water to obtain a histidine solution; polyethyleneimine was dissolved in water to obtain a PEI solution; at room temperature, the histidine solution was added to the PEI solution, an activator solution was added, and the mixture was stirred for 4-6 hours. After the reaction was completed, post-treatment was performed, and the mixture was dried to obtain a grain size modifier.

[0008] By employing the above technical solution, under the action of the activator, the carboxyl group on histidine is activated, forming a highly reactive activated ester intermediate. The primary or secondary amine groups on the PEI chain act as nucleophiles, attacking the activated ester intermediate and ultimately forming an amide bond. Histidine is then covalently grafted onto the PEI backbone, forming a grain modifier with functional adsorption centers such as imidazole and amino groups in its molecular structure. PEI itself can adsorb onto the coating surface through its amino groups. The introduction of histidine enhances the adsorption strength and selectivity of the entire molecule on the gold surface, more effectively suppressing excessively rapid and irregular grain growth, forming a uniform equiaxed crystal structure, and achieving finer grain control.

[0009] Preferably, the mass ratio of polyethyleneimine, histidine and activator is 1:(1~1.2):(1.2~1.5).

[0010] By adopting the above technical solution, the grain adjuster prepared according to the above mass ratio makes the grain distribution uniform, and the coating is almost free of voids or abnormal crystals, which meets the requirements of wafer electroplating for coating quality.

[0011] Preferably, the activator is 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide.

[0012] Preferably, the organophosphonic acid additive includes one or both of hydroxyethylidene diphosphonic acid and aminotrimethylene phosphonic acid.

[0013] By adopting the above technical solution, the organophosphoric acid additive further refines the grain size, accelerates the nucleation rate, improves the density of the coating, and eliminates impurities such as oxides between coating layers. Furthermore, the organophosphonic acid additive reacts with Au... + The complexation stability constant satisfies both the suppression of Au + The coordination requirements for hydrolysis and disproportionation are not strong enough to prevent the ligand from detaching during gold ion reduction; therefore, Au + Reduced to Au 0 When the gold layer is deposited to form a coating, the dissociated organophosphonic acid molecules, because they are not reduced, will diffuse back into the plating solution and will not be encapsulated by the gold layer. At the same time, organophosphonic acid additives can replace the positions of ligands that easily cause inclusions (such as high-molecular grain adjusters and sulfur elements in sulfur-containing systems), effectively controlling the interfacial adsorption of the cathode, which is beneficial to improving the inclusion situation in the coating and improving the coating quality.

[0014] Preferably, the main complexing agent includes one or both of ethylenediaminetetraacetic acid (EDTA) and disodium EDTA.

[0015] Preferably, the auxiliary complexing agent includes one or both of 5,5-dimethylhydantoin and 1,5,5-trimethylhydantoin.

[0016] By adopting the above technical solution, EDTA or its disodium salt, as the main complexing agent, can form a stable complex with gold ions, which significantly improves the cathodic polarization. Together with the auxiliary complexing agent containing cyclic imide groups, the formation rate of crystal nuclei is much greater than the growth rate of grains, thereby obtaining a coating with smaller and denser grains, which can effectively reduce the porosity of the coating.

[0017] Preferably, the buffer comprises disodium hydrogen phosphate and sodium dihydrogen phosphate in a mass ratio of 1:(0.5~0.7).

[0018] By adopting the above technical solution, disodium hydrogen phosphate is weakly alkaline and sodium dihydrogen phosphate is weakly acidic. The two work together to adjust the pH value of the gold plating solution, so that the pH value of the plating solution is stabilized within a suitable range. This avoids the impact of large fluctuations in pH value on the stability of the plating solution and the quality of the plating layer, thereby ensuring that the cyanide-free electroplating gold plating solution can work continuously and stably during the electroplating process, which helps to improve the density and uniformity of the plating layer.

[0019] Preferably, the gold salt includes one or more of sodium chloroaurate, sodium gold sulfite, and sodium gold thiosulfate.

[0020] By adopting the above technical solution, the gold salts are all cyanide-free compounds, which meet the requirements of environmental protection and safety. The resulting gold plating layer has a large thickness, fine and dense crystals, good adhesion and coverage, and meets the requirements of wafer gold plating.

[0021] Secondly, this application provides a method for preparing a cyanide-free electroplating gold plating solution, which adopts the following technical solution: A method for preparing a cyanide-free gold plating solution includes the following steps: At 50-60℃, gold salt is added to deionized water and stirred until no solid is formed to form a gold solution. The main complexing agent and auxiliary complexing agent are added to the gold solution and stirred for 1-2 hours. Then, grain adjuster, organophosphonic acid additive and buffer are added and stirred for another 1-2 hours. The solution is then brought to a final volume to obtain a cyanide-free electroplating gold plating solution.

[0022] Preferably, the pH of the resulting gold plating solution is 7.5~8.

[0023] By adopting the above technical solution, the cyanide-free electroplating gold plating solution obtained by the above preparation method has good stability and the components are fully mixed and reacted. When applied to wafer electroplating, it can effectively improve the coating quality, enhance electrical performance, and improve product performance and lifespan.

[0024] This application has the following beneficial effects: 1. Gold salts provide the gold ions required for electroplating and are the basic material for forming the gold plating layer. The main complexing agent can form a stable complex with gold ions, improving the stability of gold ions in the plating solution and preventing the release of gold ions too quickly or too slowly. The auxiliary complexing agent further enhances the complexing effect and works synergistically with the main complexing agent to make the release of gold ions more stable, thus improving the problem of unstable gold ion release rate caused by the low complexing constant of the existing cyanide-free system. Grain adjustment agents can adjust the grain size and structure of the plating layer, making the plating layer more dense and uniform. Organophosphonic acid additives can improve the problem of inclusions in the plating layer.

[0025] 2. Under the action of the activator, the carboxyl group on histidine is activated, forming a highly reactive activated ester intermediate. The primary or secondary amine groups on the PEI chain act as nucleophiles to attack the activated ester intermediate, ultimately forming an amide bond. Histidine is then covalently grafted onto the PEI backbone, forming a grain modifier with functional adsorption centers such as imidazole and amino groups in its molecular structure. PEI itself can adsorb onto the coating surface through its amino groups. The introduction of histidine enhances the adsorption strength and selectivity of the entire molecule on the gold surface, more effectively suppressing excessively rapid and irregular grain growth, forming a uniform equiaxed crystal structure, and achieving finer grain control. Detailed Implementation

[0026] The present application will be further described in detail below with reference to the embodiments.

[0027] Preparation Example 1 The preparation method of the grain size modifier includes the following steps: Weigh out polyethyleneimine, histidine, and activator in a mass ratio of 1:1:1.2. The polyethyleneimine used is branched polyethyleneimine with a molecular weight of 25,000. The activator used is 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide. Dissolve the activator in water to obtain an activator solution with a mass ratio of activator to water of 1:10.

[0028] Histidine was dissolved in water at a mass ratio of 1:10 to obtain a histidine solution; polyethyleneimine was dissolved in water at a mass ratio of 1:25 to obtain a PEI solution; the histidine solution was slowly added dropwise to the PEI solution at room temperature, an activator solution was added, and the reaction was stirred for 4 hours. After the reaction was completed, post-treatment was performed, and the product was dried to obtain a grain size modifier.

[0029] The post-processing steps are as follows: After the reaction is completed, transfer the reaction solution to a dialysis bag, use ultrapure water as the dialysis fluid, and dialyze at 4°C for 2-3 days, changing the dialysis fluid 2-3 times a day, and collect the final solution in the dialysis bag.

[0030] Preparation Example 2 The preparation method of the grain size modifier includes the following steps: Weigh out polyethyleneimine, histidine, and activator in a mass ratio of 1:1.1:1.3. The polyethyleneimine used is branched polyethyleneimine with a molecular weight of 25,000. The activator used is 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide. Dissolve the activator in water to obtain an activator solution with a mass ratio of activator to water of 1:10.

[0031] Histidine was dissolved in water at a mass ratio of 1:10 to obtain a histidine solution; polyethyleneimine was dissolved in water at a mass ratio of 1:25 to obtain a PEI solution; the histidine solution was slowly added dropwise to the PEI solution at room temperature, an activator solution was added, and the reaction was stirred for 5 hours. After the reaction was completed, post-treatment was performed, and the product was dried to obtain a grain size modifier.

[0032] The post-processing steps are as follows: After the reaction is completed, transfer the reaction solution to a dialysis bag, use ultrapure water as the dialysis fluid, and dialyze at 4°C for 2-3 days, changing the dialysis fluid 2-3 times a day, and collect the final solution in the dialysis bag.

[0033] Preparation Example 3 The preparation method of the grain size modifier includes the following steps: Weigh out polyethyleneimine, histidine, and activator in a mass ratio of 1:1.2:1.5. The polyethyleneimine used is branched polyethyleneimine with a molecular weight of 25,000. The activator used is 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide. Dissolve the activator in water to obtain an activator solution with a mass ratio of activator to water of 1:10.

[0034] Histidine was dissolved in water at a mass ratio of 1:10 to obtain a histidine solution; polyethyleneimine was dissolved in water at a mass ratio of 1:25 to obtain a PEI solution; the histidine solution was slowly added dropwise to the PEI solution at room temperature, an activator solution was added, and the reaction was stirred for 6 hours. After the reaction was completed, post-treatment was performed, and the product was dried to obtain a grain size modifier.

[0035] The post-processing steps are as follows: After the reaction is completed, transfer the reaction solution to a dialysis bag, use ultrapure water as the dialysis fluid, and dialyze at 4°C for 2-3 days, changing the dialysis fluid 2-3 times a day, and collect the final solution in the dialysis bag.

[0036] Preparation Example 4 The difference between this preparation example and preparation example 2 is that histidine is replaced with serine by mass.

[0037] Example 1 A cyanide-free gold plating solution, comprising: Sodium chloroaurate 0.5 g / L, ethylenediaminetetraacetic acid 20 g / L, 1,5,5-trimethylhydantoin 5 g / L, grain size adjuster 20 mg / L (prepared in Preparation Example 1), hydroxyethylidene diphosphonic acid 0.1 g / L, buffer 10 g / L, and the balance was brought to 50 L with deionized water.

[0038] The buffer is selected from disodium hydrogen phosphate and sodium dihydrogen phosphate in a mass ratio of approximately 1:0.5.

[0039] The preparation method of the cyanide-free electroplating gold plating solution in this embodiment includes the following steps: According to the above raw material composition ratio, sodium chloroaurate was added to 30L of deionized water at 50℃ and stirred until no solid was formed to form a gold solution. Ethylenediaminetetraacetic acid and 1,5,5-trimethylhydantoin were added to the gold solution and stirred at 150rpm for 1h. Grain adjuster, hydroxyethylidene diphosphonic acid, disodium hydrogen phosphate and sodium dihydrogen phosphate were added and stirring was continued for 1h. The pH was adjusted to 7.5 and the solution was brought to a final volume to obtain a cyanide-free electroplating gold plating solution.

[0040] Example 2 A cyanide-free gold plating solution, comprising: Sodium gold sulfite 1 g / L, disodium ethylenediaminetetraacetate 25 g / L, 5,5-dimethylhydantoin 6.5 g / L, grain size adjuster 35 mg / L (prepared in Example 2), hydroxyethylidene diphosphonic acid 0.15 g / L, buffer 15 g / L, and the balance was brought to 50 L with deionized water.

[0041] The buffer is selected from disodium hydrogen phosphate and sodium dihydrogen phosphate in a mass ratio of approximately 1:0.6.

[0042] The preparation method of the cyanide-free electroplating gold plating solution in this embodiment includes the following steps: According to the above raw material ratio, sodium gold sulfite was added to 30L of deionized water at 55℃ and stirred until no solids were found to form a gold solution. Disodium ethylenediaminetetraacetate and 5,5-dimethylhydantoin were added to the gold solution and stirred at 180 rpm for 1.5 hours. Grain adjuster, hydroxyethylidene diphosphonic acid, disodium hydrogen phosphate and sodium dihydrogen phosphate were added and stirring was continued for 1.5 hours. The pH was adjusted to 7.8, and the remaining volume was brought to 50L with deionized water to obtain a cyanide-free electroplating gold plating solution.

[0043] Example 3 A cyanide-free gold plating solution, comprising: Sodium gold thiosulfate 1.5 g / L, disodium ethylenediaminetetraacetate 30 g / L, 5,5-dimethylhydantoin 8 g / L, grain size adjuster 45 mg / L (prepared in Example 3), aminotrimethylenephosphonic acid 0.2 g / L, buffer 20 g / L, and the balance was brought to 50 L with deionized water.

[0044] The buffer is selected from disodium hydrogen phosphate and sodium dihydrogen phosphate in a mass ratio of approximately 1:0.7.

[0045] The preparation method of the cyanide-free electroplating gold plating solution in this embodiment includes the following steps: According to the above raw material ratio, sodium gold thiosulfate was added to 30L of deionized water at 60℃ and stirred until no solids were found to form a gold solution. Disodium ethylenediaminetetraacetate and 5,5-dimethylhydantoin were added to the gold solution and stirred at 200 rpm for 2 hours. Grain conditioner, aminotrimethylenephosphonic acid, disodium hydrogen phosphate and sodium dihydrogen phosphate were added and stirring was continued for 2 hours. The pH was adjusted to 8.0, and the remaining volume was brought to 50L with deionized water to obtain a cyanide-free gold plating solution.

[0046] Example 4 The difference between this embodiment and Example 2 is that the grain modifier prepared in Example 4 is used.

[0047] Example 5 The difference between this embodiment and Embodiment 2 is that sodium dihydrogen phosphate is replaced by citric acid.

[0048] Comparative Example 1 A cyanide-free electroplating gold plating solution differs from Example 2 in that the grain size adjuster is replaced by polyethyleneimine.

[0049] Comparative Example 2 A cyanide-free gold plating solution, which differs from Example 2 in that it does not contain a grain size adjuster.

[0050] Comparative Example 3 A cyanide-free gold plating solution differs from Example 2 in that it does not contain hydroxyethylidene diphosphonic acid.

[0051] Comparative Example 4 A cyanide-free gold plating solution differs from Example 2 in that it replaces disodium ethylenediaminetetraacetate by mass with 5,5-dimethylhydantoin.

[0052] Comparative Example 5 A cyanide-free gold plating solution differs from Example 2 in that 5,5-dimethylhydantoin is replaced by disodium ethylenediaminetetraacetate.

[0053] Comparative Example 6 An electroplating gold plating solution, which differs from Example 2 in that sodium ethylenediaminetetraacetate is replaced by sodium cyanide in equal quantities. Performance testing

[0054] Sample preparation: The parts to be plated were placed in electroplating baths containing the gold plating solutions of each embodiment and comparative example, respectively. The anode was a platinum mesh, and the cathode was the part to be plated. After electroplating, gold-plated parts were obtained. The electroplating parameters during the process were: temperature 55℃, current density 1A / dm³. 2 The electroplating time was 20 minutes. The electroplating performance of the gold-plated parts in each embodiment and comparative example was tested. The part to be plated was a 4-inch wafer. Before electroplating, a nanoscale (50nm) gold film was deposited on the wafer using magnetron sputtering technology as a seed layer for subsequent gold plating.

[0055] 1. Plating solution stability test: 30 mL of the gold plating solution provided in each example and comparative example was placed in a test tube and placed in an oven at 60°C for 96 hours. The stability level was determined by observing whether there was turbidity or gold precipitation. The judgment criteria are as follows: Grade 1: No turbidity, no gold precipitation; Level 2: Slightly turbid, no gold precipitation; Level 3: Obvious turbidity, slight gold precipitation; Level 4: Obvious turbidity and obvious gold precipitation.

[0056] 2. Coating roughness test: Measure the surface Ra value using a profilometer or roughness meter.

[0057] 3. Porosity Test: The porosity of the substrate after electroplating was determined using the filter paper method. The number of characteristic spots per unit area was used as the evaluation index.

[0058] 4. Coating adhesion test: Draw 100 small squares on a 1cm×1cm coating surface. When drawing the squares, the knife should reach the substrate. Use 3M tape to stick to the surface and then quickly tear it off to test the adhesion. If there is no coating peeling off after the 3M tape is torn off, and no gold coating adheres to the tape surface, it indicates that the coating has good adhesion.

[0059] 5. Uniformity Coefficient: Ten samples were randomly selected from each embodiment and comparative example. The thickness of the gold plating layer on each sample was measured. The average thickness μ of the gold plating layer in the same group of samples was calculated, and the standard deviation α of the gold plating layer thickness in the same group of samples was also calculated. The uniformity coefficient CV of the gold plating layer in each group of samples was then calculated and recorded in Table 2 below. The uniformity coefficient of the gold plating layer = (α / μ)*100%. A smaller uniformity coefficient indicates better uniformity of the gold plating layer. In actual production, a uniformity coefficient CV of less than 8% is considered acceptable.

[0060] Table 1 stability Ra (μm) <![CDATA[Porosity (number / cm 2 )]]> bonding force Uniformity coefficient CV / % Example 1 Level 1 0.120 0.50 No shedding 6.54 Example 2 Level 1 0.115 0.45 No shedding 6.23 Example 3 Level 1 0.118 0.47 No shedding 6.38 Example 4 Level 2 0.132 0.83 Shedding 8.45 Example 5 Level 2 0.126 0.78 Shedding 8.02 Comparative Example 1 Level 2 0.140 1.12 Shedding 9.04 Comparative Example 2 Level 3 0.162 1.79 Shedding 12.32 Comparative Example 3 Level 3 0.160 1.65 Shedding 11.56 Comparative Example 4 Level 2 0.147 1.23 Shedding 9.78 Comparative Example 5 Level 2 0.154 1.35 Shedding 10.13 Comparative Example 6 Level 1 0.122 0.66 No shedding 7.48

[0061] Based on the comparison between Examples 2 and 4 and the data in Table 1, it can be seen that the grain refiner molecule in Example 4 does not contain an imidazole group. Although the hydroxyl group of serine also has a certain coordination ability, its strength and characteristics are different, and it may not be able to achieve the same grain refinement and leveling effect. In contrast, this application introduces histidine to enhance the adsorption strength and selectivity of the entire molecule on the gold surface, more effectively suppressing the excessively rapid and irregular growth of grains, achieving finer grain control, and forming a high-quality gold plating layer.

[0062] Based on the comparison between Example 2 and Example 5, and the data in Table 1, it can be seen that in Example 5, sodium dihydrogen phosphate in the buffer was replaced with citric acid. Since citric acid contains hydroxyl groups in its molecular structure and has a certain reducing property, it may react with Au. 3+ A redox reaction occurs, producing gold elemental precipitate, which affects the stability of the plating solution.

[0063] Based on the comparison between Example 2 and Comparative Example 1, and the data in Table 1, it can be seen that the grain modifier prepared in this application, combining the properties of histidine and polyethyleneimine, can better optimize the performance of the gold plating solution and make the gold ion release rate more stable. However, Comparative Example 1, which only uses polyethyleneimine, may result in the inability to effectively control the growth of gold grains during the electroplating process, thereby affecting the performance of the coating.

[0064] Based on the comparison between Example 2 and Comparative Example 2 and the data in Table 1, it can be seen that Comparative Example 2 lacks a grain adjuster. Gold ions may preferentially deposit rapidly on existing, high-energy crystal nuclei, forming coarse, loose columnar or dendritic structures. There will be a large number of gaps and pores between the grains, resulting in a significant increase in the porosity of the coating and an uneven coating surface.

[0065] Based on the comparison between Example 2 and Comparative Example 3 and the data in Table 1, it can be seen that Comparative Example 3 lacks organophosphonic acid additives, which prevents the S element, which is a grain modifier of polymers or a sulfur-containing system, from detaching from the metal surface and thus becomes trapped in the coating. This results in an insufficiently dense coating and affects the adhesion of the coating, leading to a decrease in coating quality.

[0066] Based on the comparison of Example 2 and Comparative Examples 4-5 and the data in Table 1, it can be seen that the complexing ability of a single complexing agent is relatively limited. In Example 2 of this application, the main complexing agent and the auxiliary complexing agent work synergistically, and both coordinate with gold ions simultaneously. Compared with them acting as complexing agents alone, the system stability is greatly improved, the coating performance is improved, and the problem of unstable gold ion release rate caused by the low complexing constant of the existing cyanide-free system is effectively solved.

[0067] Based on the comparison between Example 2 and Comparative Example 6, and the data in Table 1, it can be seen that the coating quality obtained by the cyanide-free electroplating gold plating solution used in this application can reach or even exceed that of the cyanide gold plating system. The coating is smooth, the grain size distribution is uniform, which meets the requirements of wafer electroplating for the quality of the gold plating layer, and has little impact on the environment.

[0068] 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 specific 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 cyanide-free gold plating solution, characterized in that, Includes the following components: Gold salt 0.5~1.5g / L, main complexing agent 20~30g / L, auxiliary complexing agent 5~8g / L, crystallization modifier 20~45mg / L, organophosphonic acid additive 0.1~0.2g / L, buffer 10~20g / L, balance deionized water.

2. The cyanide-free gold plating solution according to claim 1, characterized in that, The preparation method of the grain-modifying agent includes the following steps: Histidine was dissolved in water to obtain a histidine solution; polyethyleneimine was dissolved in water to obtain a PEI solution; at room temperature, the histidine solution was added to the PEI solution, an activator solution was added, and the mixture was stirred for 4-6 hours. After the reaction was completed, post-treatment was performed, and the mixture was dried to obtain a grain size modifier.

3. The cyanide-free gold plating solution according to claim 2, characterized in that, The mass ratio of polyethyleneimine, histidine and activator is 1:(1~1.2):(1.2~1.5).

4. The cyanide-free gold plating solution according to claim 1, characterized in that, The organophosphonic acid additive includes one or both of hydroxyethylidene diphosphonic acid and aminotrimethylene phosphonic acid.

5. The cyanide-free gold plating solution according to claim 1, characterized in that, The primary complexing agent includes one or both of ethylenediaminetetraacetic acid (EDTA) and disodium EDTA.

6. The cyanide-free gold plating solution according to claim 1, characterized in that, The auxiliary complexing agent includes one or both of 5,5-dimethylhydantoin and 1,5,5-trimethylhydantoin.

7. The cyanide-free gold plating solution according to claim 1, characterized in that, The buffer comprises disodium hydrogen phosphate and sodium dihydrogen phosphate in a mass ratio of 1:(0.5~0.7).

8. The cyanide-free gold plating solution according to claim 1, characterized in that, The gold salt includes one or more of sodium chloroaurate, sodium gold sulfite, and sodium gold thiosulfate.

9. A method for preparing a cyanide-free electroplating gold plating solution according to any one of claims 1 to 8, characterized in that, Includes the following steps: At 50-60℃, gold salt is added to deionized water and stirred until no solid is formed to form a gold solution. The main complexing agent and auxiliary complexing agent are added to the gold solution and stirred for 1-2 hours. Then, grain adjuster, organophosphonic acid additive and buffer are added and stirred for another 1-2 hours. The solution is then brought to a final volume to obtain a cyanide-free electroplating gold plating solution.

10. The method for preparing a cyanide-free electroplating gold plating solution according to claim 9, characterized in that, The pH of the gold plating solution is 7.5~8.