Chemical tin plating solution suitable for fine-pitch high-density interconnection and preparation method of chemical tin plating solution

Abstract

The invention discloses a chemical tin plating solution suitable for fine-pitch high-density interconnection and a preparation method of the chemical tin plating solution, and belongs to the technical field of chemical tin plating. The chemical tin plating solution suitable for fine-pitch high-density interconnection comprises the following components: 12-20 g / L of stannous mono-sulphate, 35-45 mL / L of sulfuric acid, 90-110 g / L of a complexing agent, 30-45 g / L of a reducing agent, 1.2-1.8 g / L of a stabilizer, 1.5-3 g / L of a dispersing agent, 0.8-1.2 g / L of a surfactant, a pH regulator and water. The invention also provides a preparation method of the composition. Compared with the prior art, the dispersing agent is obtained through condensation and sulfonation of the ketone-containing monomer and the formaldehyde, and wettability is improved after sulfonation, so that the blind hole filling problem of fine-pitch electroplating can be solved, and the tin plating solution can form a uniform and stable tin protection layer in a tiny bonding pad and dense wiring environment.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electroless tin plating, and particularly relates to an electroless tin plating solution suitable for fine-pitch high-density interconnection and a preparation method thereof. BACKGROUND

[0002] The electroless tin plating solution is a kind of aqueous solution system that realizes uniform deposition of tin on the surface of a catalytically active substrate through a self-catalytic oxidation-reduction reaction without external current. The core principle is that Sn2 + ions in the solution are reduced to Sn 0 atoms on the surface of the substrate at the same time as the reducing agent in the solution is oxidized, thereby driving the continuous deposition of tin atoms on the substrate. First, the stably complexed Sn2 + ions are adsorbed on the catalytically active surface after activation treatment and receive electrons to be reduced to tin atoms and deposited; second, the reducing agent is oxidized on the surface to provide the electrons required for the reduction of tin ions. This self-catalytic mechanism enables the electroless tin plating to form a pure tin coating with uniform thickness, compactness and low porosity on the surface of a complex geometric shape. The electroless tin plating solution technology is an important method to realize uniform tin deposition on the surface of a material, and is particularly suitable for microelectronic interconnection, packaging and precision component manufacturing, and other occasions with extremely high requirements for uniformity and reliability of the coating.

[0003] CN106939417A discloses a kind of electroless tin plating formula for printed circuit board, and its component concentration is as follows: stannous ions 10-30g / L, thiourea 70-130g / L, methyl sulfonic acid 30-70g / L, citric acid 30-60g / L, imino disuccinic acid tetrasodium 20-50g / L, carbohydrazide 10-30g / L, surfactant 10-50mg / L, bismuth acetate 0.05-0.15g / L and polyamino polyether-based methylidene phosphonic acid 20-40g / L. After mixing the component solutions uniformly, the electroless tin plating solution is prepared, and the solution has the characteristics of stable tin deposition rate, stannous ions not easy to be oxidized to tetravalent tin, and good solution stability. The presence of a variety of complexing agents in the invention formula effectively inhibits the oxidation of stannous ions to tetravalent tin, the reducing agent can remove the influence of dissolved oxygen in the plating solution on the oxidation of stannous ions, and the addition of bismuth acetate forms a tin-bismuth alloy coating, which can effectively inhibit the growth of tin whiskers.

[0004] The patent CN110331392B relates to the technical field of electroless tin plating, and specifically discloses an electroless tin plating solution and a preparation method thereof. The electroless tin plating solution comprises sulfuric acid, tin salt, complexing agent, multifunctional additive, reducing agent and functional additive, and the content of each component is as follows: sulfuric acid 33-40 mL·L-1, tin salt 10-15 g·L-1, complexing agent 85-95 g·L-1, multifunctional additive 0.5-1.2 g·L-1, reducing agent 25-40 g·L-1, and functional additive 1-2 g·L-1. The electroless tin plating solution ensures excellent stability, improves the deposition rate of the plating layer, and achieves a good balance between the stability of the plating solution and the deposition efficiency.

[0005] Electroless tin plating technology has been applied in many fields due to its unique advantages, but it also has certain limitations. Its main advantages include the high uniformity of the deposited plating layer thickness on complex geometric surfaces, the compactness and low porosity of the formed tin layer, which can provide excellent corrosion protection. The process does not require external power supply equipment, simplifying the equipment configuration, improving the processing flexibility, and obtaining a pure tin layer with good electrical conductivity, weldability, and compatibility with electronic components. The limitations are that the deposition rate is usually lower than that of electroplating tin, the production efficiency is relatively low, and the plating solution itself has stability challenges. Sn2+ + is easily oxidized, and the plating solution has a tendency to self-decompose, limiting its service life. SUMMARY

[0006] In view of the above-mentioned defects of the prior art, the technical problem to be solved by the present application is to provide an electroless tin plating solution suitable for fine pitch and high density interconnection and a preparation method thereof.

[0007] With the development of electronic products towards miniaturization and high performance, the lines on PCBs are becoming thinner, the pad pitch is becoming smaller, and the component density is becoming higher. This trend poses extremely stringent requirements on surface treatment technology. On tiny pads and extremely narrow pitches, even a small difference in plating layer thickness can lead to disastrous consequences. This requires the tin plating solution to form an ultra-thin, uniform, tin whisker-free, and highly reliable tin protective layer in a tiny pad and dense wiring environment, thereby ensuring the yield and long-term reliability of the final electronic product.

[0008] In order to improve the penetration and uniformity of the tin plating solution in fine pitch and high density interconnection scenarios, the present application obtains a dispersant by condensing a ketone monomer with formaldehyde and sulfonating. The wettability is improved after sulfonation, thereby solving the problem of blind hole filling in fine pitch electroplating, and enabling the tin plating solution to form a uniform and stable tin protective layer in a tiny pad and dense wiring environment.

[0009] Therefore, the present application provides an electroless tin plating solution suitable for fine pitch and high density interconnection. Sulfuric acid maintains an acidic environment, inhibits Sn 2+Oxidative hydrolysis, ensure plating solution stability. Stannous sulfate provides tin ion main source, under the action of reducing agent, deposited as metal tin plating. Potassium sodium tartrate, citric acid as a complexing agent, tartrate strong chelate Sn2 + Anti-precipitation, citric acid buffer pH fluctuation, and control the deposition rate. Sodium hypophosphite as a reducing agent, by oxidation reaction to provide electrons, Sn2 + Propyl gallate adsorption active site inhibition from the decomposition, polyethylene imine dynamic coordination Sn2 + Reduce the concentration of free ions, thereby extending the life of the plating solution. By containing ketone monomer and formaldehyde condensation, sulfonation to get dispersant, zwitterionic micelles improve wettability, electrostatic repulsion to prevent particle agglomeration, improve dispersibility. Fatty alcohol polyoxyethylene ether reduces the surface tension, optimizes the micro zone plating uniformity, reduces the edge thickening. The chemical tin plating solution of the application solves the difficulty of fine pitch plating by inhibiting decomposition and improving dispersion, realizes the reliability of high-density interconnection deposition.

[0010] To achieve the above object, the application provides a kind of chemical tin plating solution suitable for fine pitch high-density interconnection, comprising the following components: stannous sulfate 12-20 g / L, sulfuric acid 35-45 mL / L, complexing agent 90-110 g / L, reducing agent 30-45 g / L, stabilizer 1.2-1.8 g / L, dispersant 1.5-3 g / L, surfactant 0.8-1.2 g / L, pH regulator and water.

[0011] The preparation method of the dispersant comprises the following steps:

[0012] The ketone monomer and formaldehyde are condensed and sulfonated to obtain the stabilizer.

[0013] Further, the preparation method of the dispersant comprises the following steps:

[0014] Ethyl acetoacetate and formaldehyde aqueous solution are mixed, stirred at 35-45 DEG C for 10-15 min, sodium pyrosulfite is slowly added, the temperature is increased to 70-80 DEG C, 50wt% NaOH aqueous solution is used to adjust pH to 7-8, continue to stir for 3-5 h, after cooling, dilute to solid content 30-35wt%, filter, and the mass ratio of ethyl acetoacetate, formaldehyde aqueous solution and sodium pyrosulfite is 1:1.8-2:2-2.2.

[0015] Preferably, the preparation method of the dispersant comprises the following steps:

[0016] The acetylpicinic acid is mixed with formaldehyde water solution, and is pre-reacted for 20-40 minutes at 50-60 DEG C, ammonium sulfite is added, and is reacted for 4-6 hours at 85-90 DEG C, sulfuric acid is used to adjust pH to 3-4, and is dehydrated under reduced pressure until solid content reaches 30-35 wt%, and then the product is obtained; the mass ratio of acetylpicinic acid, formaldehyde water solution and ammonium sulfite is 1:1.5-1.8:1.5-1.7.

[0017] More preferably, the preparation method of the dispersant comprises the following steps:

[0018] The diacetone propenamide is mixed with formaldehyde water solution and dimethylamine, and is reacted for 1-3 hours at 55-65 DEG C, ammonia water is used to adjust pH to alkaline, then sodium bisulfite is added, and is reacted for 2-4 hours at 70-90 DEG C, and then the product is obtained after the temperature is lowered to 30-40 DEG C, and glacial acetic acid is used to adjust pH to 6-7; the mass ratio of diacetone propenamide, formaldehyde water solution, dimethylamine and sodium bisulfite is 1:1.4-1.5:0.8-1:0.7-0.8.

[0019] Further, the complexing agent is a mixture of potassium sodium tartrate and citric acid.

[0020] Further, the reducing agent is sodium hypophosphite.

[0021] Further, the stabilizing agent is a mixture of propyl gallate and polyethylene imine.

[0022] Further, the surfactant is fatty alcohol polyoxyethylene ether.

[0023] Further, the pH adjusting agent is ammonia water.

[0024] A preparation method of a chemical tin plating solution suitable for fine-pitch high-density interconnection, comprising the following steps:

[0025] After sulfuric acid is diluted to 10 wt% with water, stannous sulfate is added, then a complexing agent is added, and stirring is carried out at 50-65 DEG C until it is clear, then a reducing agent, a surfactant, a stabilizing agent and a dispersant are added, and after ultrasonic dispersion, the rest of water is added, and after mixing, a pH adjusting agent is added, and pH is adjusted to 4-4.5.

[0026] The beneficial effects of the present application are as follows:

[0027] 1. Compared with the prior art, the dispersant is obtained by condensation of a ketone monomer with formaldehyde and sulfonation, and the wettability is improved after sulfonation, so that the problem of blind hole filling in fine-pitch electroplating can be solved, and the tin plating solution can form a uniform and stable tin protective layer in a small pad and dense wiring environment.

[0028] 2. The present application maintains an acidic environment by sulfuric acid, and inhibits Sn2 +Oxidative hydrolysis ensures the stability of the plating bath. Stannous sulfate provides the main source of tin ions, which are deposited as a metallic tin plating layer under the action of a reducing agent. Potassium sodium tartrate and citric acid act as a complexing agent, with tartrate ions strongly chelating Sn22-. + To prevent precipitation, citric acid buffers pH fluctuations and controls the deposition rate. Sodium hypophosphite acts as a reducing agent, donating electrons through an oxidation reaction to reduce Sn. 2+ Propyl gallate adsorption active sites inhibit autodecomposition, and polyethyleneimine dynamically coordinates Sn2. + Reducing the concentration of free ions extends the life of the plating solution. A dispersant is obtained through the condensation and sulfonation of ketone-containing monomers with formaldehyde; zwitterionic micelles improve wettability, and electrostatic repulsion prevents particle agglomeration, thus improving dispersibility. Fatty alcohol polyoxyethylene ether reduces surface tension, optimizes the uniformity of the micro-area plating layer, and reduces edge thickening. This invention's chemical tin plating solution addresses the challenges of fine-pitch plating by inhibiting decomposition and enhancing dispersion, achieving reliable deposition of high-density interconnects. Detailed Implementation

[0029] Fatty alcohol polyoxyethylene ether, AEO-9, is derived from BASF.

[0030] Formaldehyde aqueous solution, 37 wt%.

[0031] Dimethylamine, 33 wt%.

[0032] Sulfuric acid, 98 wt%.

[0033] Ammonia, 10 wt%.

[0034] Sodium salt of 2-naphthalenesulfonic acid formaldehyde polymer, CAS No.: 36290-04-7, sourced from Nanxing Chemical.

[0035] Polyethyleneimine, model: G-35, sourced from Wuhan Bolaite Chemical Co., Ltd. Example 1

[0036] A method for preparing a chemical tin plating solution suitable for fine-pitch, high-density interconnects includes the following steps:

[0037] Dilute 40 mL of 98% sulfuric acid to 10%, then add 15 g of stannous sulfate, followed by 80 g of potassium sodium tartrate and 20 g of citric acid. Stir at 60 °C until clear. Continue to add 35 g of sodium hypophosphite, 1 g of fatty alcohol polyoxyethylene ether, 0.375 g of propyl gallate, 1.125 g of polyethyleneimine, and 2 g of dispersant. After ultrasonic dispersion, add the remaining water to make up to 1 L. Mix well, then add ammonia water to adjust the pH to 4.2.

[0038] The method for preparing the dispersant includes the following steps:

[0039] Ethyl acetoacetate and formaldehyde aqueous solution were mixed, stirred at 40℃ for 10 min, sodium metabisulfite was slowly added, the temperature was raised to 75℃, 50wt% NaOH aqueous solution was used to adjust the pH to 7.5, stirring was continued for 4h, after cooling, it was diluted to solid content of 35%, and then filtered to obtain the product; the mass ratio of ethyl acetoacetate, formaldehyde aqueous solution and sodium metabisulfite was 1:1.9:2.1.

[0040] Example 2

[0041] A preparation method of a tin electroless plating solution suitable for fine-pitch high-density interconnection, comprising the following steps:

[0042] After 40 mL of 98% sulfuric acid was diluted to 10%, 15 g of stannous sulfate was added, then 80 g of potassium sodium tartrate and 20 g of citric acid were added, stirring was continued at 60℃ until it was clear, 35 g of sodium hypophosphite, 1 g of fatty alcohol polyoxyethylene ether, 0.375 g of propyl gallate, 1.125 g of polyethyleneimine and 2 g of dispersant were added, and then the remaining water was added to make up to 1 L after ultrasonic dispersion, and then ammonia water was added to adjust the pH to 4.2.

[0043] The preparation method of the dispersant comprises the following steps:

[0044] Levulinic acid and formaldehyde aqueous solution were mixed, pre-reacted at 55℃ for 30 min, ammonium sulfite was added, and then the reaction was carried out at 85℃ for 5h, 10% sulfuric acid was used to adjust the pH to 3.5, and then dehydration was carried out under reduced pressure until the solid content reached 35%; the mass ratio of levulinic acid, formaldehyde aqueous solution and ammonium sulfite was 1:1.7:1.6.

[0045] Example 3

[0046] A preparation method of a tin electroless plating solution suitable for fine-pitch high-density interconnection, comprising the following steps:

[0047] After 40 mL of 98% sulfuric acid was diluted to 10%, 15 g of stannous sulfate was added, then 80 g of potassium sodium tartrate and 20 g of citric acid were added, stirring was continued at 60℃ until it was clear, 35 g of sodium hypophosphite, 1 g of fatty alcohol polyoxyethylene ether, 0.375 g of propyl gallate, 1.125 g of polyethyleneimine and 2 g of dispersant were added, and then the remaining water was added to make up to 1 L after ultrasonic dispersion, and then 10% ammonia water was added to adjust the pH to 4.2.

[0048] The preparation method of the dispersant comprises the following steps:

[0049] Dissolve diacetone acrylamide in water, then add dimethylamine, and react at 60℃ for 2h. Add sodium bisulfite after adjusting pH to alkaline with 10% ammonia water, and react at 80℃ for 3h. Add glacial acetic acid to adjust pH to 6.5 at 35℃. The mass ratio of diacetone acrylamide, water solution of formaldehyde, dimethylamine, and sodium bisulfite is 1:1.45:0.9:0.75.

[0050] Comparative Example 1

[0051] A preparation method of a chemical tin plating solution suitable for fine-pitch high-density interconnection, comprising the following steps:

[0052] Dilute 40mL of 98% sulfuric acid to 10%, then add stannous sulfate 15g, and then add potassium sodium tartrate 80g and citric acid 20g. Stir at 60℃ until clear. Continue to add sodium hypophosphite 35g, fatty alcohol polyoxyethylene ether 1g, propyl gallate 0.375g, and polyethyleneimine 1.125g. Disperse uniformly by ultrasonic, then add the remaining water to make up to 1L. Mix uniformly, then add ammonia water to adjust pH to 4.2.

[0053] Comparative Example 2

[0054] A preparation method of a chemical tin plating solution suitable for fine-pitch high-density interconnection, comprising the following steps:

[0055] Dilute 40mL of 98% sulfuric acid to 10%, then add stannous sulfate 15g, and then add potassium sodium tartrate 80g and citric acid 20g. Stir at 60℃ until clear. Continue to add sodium hypophosphite 35g, fatty alcohol polyoxyethylene ether 1g, propyl gallate 0.375g, polyethyleneimine 1.125g, and sodium lignosulfonate 2g. Disperse uniformly by ultrasonic, then add the remaining water to make up to 1L. Mix uniformly, then add ammonia water to adjust pH to 4.2.

[0056] Comparative Example 3

[0057] A preparation method of a chemical tin plating solution suitable for fine-pitch high-density interconnection, comprising the following steps:

[0058] Dilute 40mL of 98% sulfuric acid to 10%, then add stannous sulfate 15g, and then add potassium sodium tartrate 80g and citric acid 20g. Stir at 60℃ until clear. Continue to add sodium hypophosphite 35g, fatty alcohol polyoxyethylene ether 1g, propyl gallate 0.375g, polyethyleneimine 1.125g, and 2-naphthalenesulfonic acid formaldehyde polymer sodium salt 2g. Disperse uniformly by ultrasonic, then add the remaining water to make up to 1L. Mix uniformly, then add ammonia water to adjust pH to 4.2.

[0059] Test Example 1

[0060] The plated part is subjected to oil removal, water washing, micro-etching, activation, heating, and then immersed in a chemical tin plating solution, and soaked at 70℃ for 3h, and then subjected to water washing, blow drying, the thickness of Sn plating layer is detected, 15 points are tested, and an average value is taken, and a deposition rate is calculated according to the thickness.

[0061] Table 1

[0062] Experimental protocol Deposition rate / pm / h Example 1 10.8 Example 2 11.0 Example 3 12.1 Control 1 7.3 Control 2 8.6 Control 3 9.0

[0063] Since even a slight difference in plating thickness may lead to disastrous consequences in the environment of fine pitch and high density interconnection, it is required that the tin plating solution can form a uniform tin protective layer in the environment of fine pads and dense wiring. The dispersant does not directly participate in the deposition reaction, but indirectly regulates the deposition rate by affecting mass transfer and surface state.

[0064] As can be seen from the comparison of the comparative example 1 and other examples, when no dispersant is added, the deposition rate is obviously slower, the dispersant can improve the permeability of the plating solution, thereby ensuring uniform wetting everywhere, the dispersant can also inhibit particle agglomeration, thereby improving thickness uniformity, optimizing interface wetting, thereby reducing edge effect, and thus achieving fast and good deposition effect. In the example 1, the beta-keto ester cooperates with the sulfonic acid group to improve the deep plating capacity, in the example 2, the carboxylic acid group further improves the wettability, so that the coverage is higher, and the deposition rate is faster than that of the comparative examples 2-3. In the example 3, the zwitterion cooperates to make the coverage more comprehensive, and therefore the deposition rate is best.

[0065] Test example 2

[0066] The parts to be plated are respectively immersed in the chemical tin plating solutions prepared in different groups, and are chemically plated for 5h at a temperature of 85℃, the time when the plating solution becomes turbid during use of the plating solution is observed, and the surface condition of the plated parts after 3 times of 250℃ reflow soldering is observed as a stability index.

[0067] Table 2

[0068]

[0069]

[0070] The prepared chemical tin plating solution of the present application maintains an acidic environment by sulfuric acid, inhibits Sn 2+ oxidation and hydrolysis, and ensures the stability of the plating solution. Stannous sulfate provides a main source of tin ions, and is deposited into a metal tin plating layer under the action of a reducing agent. Potassium sodium tartrate and citric acid act as a composite complexing agent, tartrate strongly chelates Sn + to prevent precipitation, citric acid buffers pH fluctuation and controls deposition rate. Sodium hypophosphite acts as a reducing agent, provides electrons through oxidation reaction, and reduces Sn + . Propyl gallate adsorbs active sites to inhibit self-decomposition, and polyethyleneimine dynamically coordinates Sn2+ The free ion concentration is reduced, thereby prolonging the plating solution life. The dispersant is obtained by condensation of ketone monomer with formaldehyde and sulfonation, the zwitterionic micelles improve wettability, the electrostatic repulsion prevents particle agglomeration, and the dispersibility is improved. The fatty alcohol polyoxyethylene ether reduces the surface tension, optimizes the micro-zone plating layer uniformity, and reduces the edge thickening. The electroless tin plating solution of the present application solves the difficulty of fine-pitch plating by inhibiting decomposition and improving dispersion, and realizes reliable deposition of high-density interconnection.

[0071] Compared with the control example, the dispersant is obtained by condensation of ketone monomer with formaldehyde and sulfonation, and the wettability is improved after sulfonation, thereby solving the blind hole filling problem of fine-pitch electroplating, and the plating layer does not oxidize even after multiple reflow soldering. In Example 1, the β-keto ester weakly complexes Sn2 + , which can reduce dendrites, and the carboxylic acid group in Example 2 does not complex Sn2 + , and has higher stability than lignin sulfonate sodium or 2-naphthalenesulfonic acid formaldehyde polymer sodium salt, and the tin plating solution and the plating layer have better stability. In Example 3, the quaternary ammonium-sulfonic acid zwitterion constructs a directional adsorption channel, so the tin plating solution is less likely to become turbid, and the blind hole coverage rate is significantly reduced, the coverage rate and wettability after tin plating are very good, and there are no holes and uncovered areas, so it has better high-temperature stability.

[0072] The above describes the preferred embodiments of the present application in detail. It should be understood that those skilled in the art can make many modifications and changes without creative labor based on the concept of the present application. Therefore, any technical solution obtained by logical analysis, reasoning or limited experiment based on the prior art according to the concept of the present application shall be within the protection scope defined by the claims.

Claims

1. A chemical tin plating solution suitable for fine-pitch, high-density interconnects, characterized in that, It includes the following components: stannous sulfate 12-20 g / L, sulfuric acid 35-45 mL / L, complexing agent 90-110 g / L, reducing agent 30-45 g / L, stabilizer 1.2-1.8 g / L, dispersant 1.5-3 g / L, surfactant 0.8-1.2 g / L, pH adjuster, and water; The method for preparing the dispersant includes the following steps: The stabilizer is obtained by condensing and sulfonating ketone monomers with formaldehyde and then performing post-treatment.

2. The chemical tin plating solution suitable for fine-pitch high-density interconnects as described in claim 1, characterized in that, The complexing agent is a mixture of potassium sodium tartrate and citric acid.

3. The chemical tin plating solution suitable for fine-pitch high-density interconnects as described in claim 1, characterized in that, The reducing agent is sodium hypophosphite.

4. The chemical tin plating solution suitable for fine-pitch high-density interconnects as described in claim 1, characterized in that, The stabilizer is a mixture of propyl gallate and polyethyleneimine.

5. The chemical tin plating solution suitable for fine-pitch high-density interconnects as described in claim 1, characterized in that, The surfactant is a fatty alcohol polyoxyethylene ether.

6. The chemical tin plating solution suitable for fine-pitch high-density interconnects as described in claim 1, characterized in that, The pH adjuster is ammonia.

7. The chemical tin plating solution for fine-pitch, high-density interconnects as described in claim 1, characterized in that, The method for preparing the dispersant includes the following steps: Ethyl acetoacetate was mixed with formaldehyde aqueous solution and stirred at 35-45℃ for 10-15 min. Sodium metabisulfite was slowly added, and the temperature was raised to 70-80℃. The pH was adjusted to 7-8 with 50wt% NaOH aqueous solution, and stirring was continued for 3-5 h. After cooling, the mixture was diluted to a solid content of 30-35wt% and filtered to obtain the final product. The mass ratio of ethyl acetoacetate to formaldehyde aqueous solution and sodium metabisulfite was 1:1.8-2:2-2.

2.

8. The chemical tin plating solution suitable for fine-pitch high-density interconnects as described in claim 1, characterized in that, The method for preparing the dispersant includes the following steps: Mix levulinic acid with formaldehyde aqueous solution, pre-react at 50-60℃ for 20-40 min, add ammonium sulfite, react at 85-90℃ for 4-6 h, adjust pH to 3-4 with sulfuric acid, and dehydrate under reduced pressure until solid content reaches 30-35 wt% to obtain the product; the mass ratio of levulinic acid to formaldehyde aqueous solution and ammonium sulfite is 1:1.5-1.8:1.5-1.

7.

9. A method for preparing the chemical tin plating solution suitable for fine-pitch high-density interconnects as described in any one of claims 1 to 8, characterized in that, Includes the following steps: Sulfuric acid is diluted with water to 10 wt%, then stannous sulfate is added, followed by a complexing agent. The mixture is stirred at 50–65 °C until clear. Then, a reducing agent, surfactant, stabilizer, and dispersant are added. After ultrasonic dispersion, the remaining water is added and mixed thoroughly. Finally, a pH adjuster is added to adjust the pH to 4–4.5, thus obtaining the chemical tin plating solution suitable for fine-pitch high-density interconnects.

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