High-stability chemical nanogold liquid and preparation method thereof

Abstract

The invention relates to the technical field of chemical gold plating, in particular to high-stability chemical nanogold liquid and a preparation method thereof. The high-stability chemical nanogold liquid comprises the following preparation raw materials by mass concentration: 2-4 g / L of gold salt, 22-35 g / L of a reducing agent, 6-8 g / L of a complexing agent, 12-16 g / L of a pH regulator, 20-50 mg / L of a stabilizer, 50-80 mg / L of an accelerator, 0.8-1.2 g / L of a composite homogenizing agent and 0.008-0.012 mg / L of a water-soluble rhodium compound, and the reducing agent is composed of formaldehyde with the mass concentration of 37% and oxalic acid in a mass ratio of 2: 3-4. Through precise compatibility of the components and function synergy, the balance of high stability, high deposition rate, excellent coating quality and long service life of the chemical nanogold liquid is achieved, and the chemical nanogold liquid is particularly suitable for the fields of microelectronics, high-frequency connectors and the like with strict requirements for coating uniformity and has remarkable industrial application value.

Description

Technical Field

[0001] This application relates to the technical field of electroless gold plating, and particularly to a highly stable chemical nano-gold solution and a preparation method thereof. Background Art

[0002] Traditional electroless nickel-palladium-gold plating processes include steps such as degreasing - water washing - micro-etching - water washing - pickling - activation - water washing - electroless nickel plating - water washing - electroless palladium plating - water washing - electroless gold plating, etc. Among them, electroless gold plating is carried out on the palladium layer and is the last process of the electroless nickel-palladium-gold plating process.

[0003] Currently, some gold-plated products on the market still have the following difficulties to overcome: 1. The electroless gold plating rate on the palladium layer is slow (less than 0.08 μm / 20 minutes), resulting in low production efficiency; 2. The uniformity of the electroless gold plating layer is poor, and the coefficient of variation (CV) CV% value is large (greater than 10%); 3. The plating solution is unstable, prone to decomposition, and has a short service life, causing waste. Existing disclosed electroless gold plating solutions are: (1) In the patent CN201580001912.3 of Kojima Chemical Co., Ltd., an electroless gold plating solution is used, which includes a water-soluble gold compound, citric acid or its salt, ethylenediaminetetraacetic acid or its salt, hexamethylenetetramine, and a chain polyamine including an alkyl group with 3 or more carbon atoms and 3 or more amino groups; (2) In the patent CN200710305957.7 of Uemura Industries Co., Ltd., an electroless gold plating system using gold cyanide salt, complexing agent, sodium formaldehyde sulfoxylate, and amine compound. However, the stability and the uniformity of the gold plating layer of the above electroless gold plating solutions are not ideal enough. Summary of the Invention

[0004] The purpose of this application is to provide a highly stable chemical nano-gold solution and a preparation method thereof in view of the deficiencies of the current technology. The highly stable chemical nano-gold solution of this application has the advantages of fast gold plating rate, high stability, long service life, and good uniformity of the gold plating layer.

[0005] In the first aspect, this application provides a highly stable chemical nano-gold solution, adopting the following technical solution: A highly stable chemical nano-gold solution, calculated by mass concentration, includes the following preparation raw materials: 2 - 4 g / L of gold salt, 22 - 35 g / L of reducing agent, 6 - 8 g / L of complexing agent, 12 - 16 g / L of pH regulator, 20 - 50 mg / L of stabilizer, 50 - 80 mg / L of accelerator, 0.8 - 1.2 g / L of composite leveling agent, and 0.008 - 0.012 mg / L of water-soluble rhodium compound. Among them, the reducing agent is composed of 37% formaldehyde and oxalic acid in a mass ratio of 2:3 - 4.

[0006] By adopting the above technical solution, gold salt, as the core metal source of the gold plating solution, provides Au+ ions, and forms a gold plating layer on the surface of the substrate through a reduction reaction. It cooperates with a reducing agent to achieve the reduction and deposition of gold ions, and the concentration range is controlled at 2-4 g / L to balance the deposition rate and the quality of the plating layer. The reducing agent (formaldehyde:oxalic acid = 2:3-4), formaldehyde (37%): a strong reducing agent, reduces Au+ to Au atoms and dominates the deposition reaction. Oxalic acid assists in the reduction, enhancing the overall reduction ability; forms an intermediate complex with gold ions, slows down the reduction rate, and prevents the aggregation of gold particles. After optimizing the ratio of the two, it not only ensures rapid deposition but also avoids the instability of the plating solution caused by too fast reduction. The complexing agent forms a stable complex with gold ions, controls the concentration of free Au+ ions, and prevents the decomposition of the plating solution caused by spontaneous reduction. It jointly regulates the release rate of gold ions with the reducing agent to ensure the controllability of the deposition process and extend the service life of the plating solution. The pH regulator maintains the pH of the plating solution within an appropriate range to ensure the efficient progress of the reduction reaction. It jointly optimizes the reaction environment with the complexing agent and the reducing agent to improve the deposition rate and the compactness of the plating layer. The stabilizer, for example, is composed of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 3-carboxy-2,2,5,5-tetramethylpyrrolidine, and 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid in a mass ratio of 3:4:2-3. 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl captures free radicals and inhibits the oxidation and decomposition of the plating solution; 3-carboxy-2,2,5,5-tetramethylpyrrolidine quenches singlet oxygen and reduces oxidation side reactions; 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid selectively oxidizes interfering substances to protect the stability of the plating solution. It stabilizes gold ions through an electron-withdrawing effect, reduces the precipitation risk; extends the service life and stability of the plating solution, and at the same time improves the uniformity of the plating layer (CV < 6.5%). The accelerator, for example, is composed of thiophene[2,3-b]pyridine-5-carboxylic acid and glycine in a mass ratio of 1:1. Thiophene[2,3-b]pyridine-5-carboxylic acid contains a π-conjugated structure, promotes the adsorption of Au+ on the palladium catalytic layer, and reduces the activation energy; glycine enhances the interfacial binding force through amino coordination to prevent non-plating. The combination of the two significantly improves the deposition rate (>0.16 μm / 20 min) and ensures the continuity and compactness of the plating layer. The composite leveling agent, for example, is composed of ethylpyrrole-2-carboxylate and methyl 6-aminoindole-4-carboxylate in a mass ratio of 1:2. Ethylpyrrole-2-carboxylate: improves the wettability of the plating solution and reduces the surface tension difference; methyl 6-aminoindole-4-carboxylate: regulates the distribution of crystal nuclei and inhibits dendritic growth. The combination of the two reduces the thickness fluctuation of the plating layer (CV < 6.5%) and achieves a super-smooth surface. The water-soluble rhodium compound is used as a cocatalyst, which synergistically promotes the reduction and nucleation of Au+ with the palladium activation layer. It cooperates with the accelerator to improve the stability of the deposition rate; a trace amount of addition can optimize the crystal orientation of the plating layer and enhance the uniformity.Synergistic effect of each component: 1. Balance of rate and stability: The ternary system of reducing agent - complexing agent - pH regulator controls the reaction kinetics, and the stabilizer - rhodium compound inhibits side reactions, achieving coexistence of high-speed deposition (>0.16 μm / 20 min) and long lifespan. 2. Optimization of uniformity: The composite leveling agent regulates the micro-flow field, the accelerator enhances interfacial adsorption, and the stabilizer inhibits particle aggregation. The three work together to lower the CV value below 6.5%. 3. Improvement of coating quality: Each component acts through multiple mechanisms such as electronic effect (stabilizer), steric hindrance (leveling agent), etc., and finally a bright, dense and low-porosity gold coating is obtained. In summary, through fine-tuning of the formulation and multi-dimensional synergy of functional components, this formulation solves the pain points of poor stability and uneven deposition of traditional electroless gold plating solutions, and is suitable for the gold plating requirements of high-precision electronic components.

[0007] Preferably, the stabilizer is composed of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 3-carboxy-2,2,5,5-tetramethylpyrrolidine, and 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid in a mass ratio of 3:4:2 - 3.

[0008] By adopting the above technical solutions, 4-carboxy-2,2,6,6-tetramethylpiperidin-1-oxyl has the following functions: 1) Free radical capture: The piperidinyloxy structure has a strong free radical capture ability, which can inhibit the active free radicals (such as ·OH, ·O⁻) generated by the decomposition of reducing agents or gold salts in the plating solution, and prevent the self-decomposition of the plating solution. 2) Steric hindrance effect: The tetramethyl group provides steric hindrance, slows down the reduction rate of gold ions (Au⁺), and avoids the roughness of the coating caused by local over-deposition. 3) pH buffering assistance: The carboxylic acid group can participate in the local pH regulation and stabilize the microenvironment of the reaction interface. 3-carboxy-2,2,5,5-tetramethylpyrrolidine has the following functions: 1) Singlet oxygen quenching: The conjugated structure of the pyrrolidine ring can efficiently quench singlet oxygen (¹O2) and inhibit the damage of the plating solution by oxidation side reactions. 2) Electron-withdrawing effect: The carboxylic acid group and the pyrrolidine ring cooperate to enhance the electron-withdrawing ability, stabilize the gold ion complex, and reduce the spontaneous reduction tendency of gold ions. 3) Surface adsorption orientation: Adsorb on the substrate surface through the carboxylic acid group, regulate the oriented growth of gold crystal nuclei, and improve the compactness of the coating. 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid has the following functions: 1) Selective oxidation inhibition: The strong electron-donating property of the tert-butoxycarbonylamino group can react with oxidizing impurities preferentially, protecting reducing agents (such as formaldehyde) from being consumed in vain. 2) Crystal plane regulation: The π-electron system of the thiophene ring undergoes selective adsorption with the gold crystal plane, inhibits dendritic growth, and improves the flatness of the coating. 3) Synergistic complexation: The carboxylic acid group forms a dynamic coordination bond with gold ions, which is complementary to the complexation of 4-carboxy-2,2,6,6-tetramethylpiperidin-1-oxyl and 3-carboxy-2,2,5,5-tetramethylpyrrolidine, enhancing the overall stability. The synergistic mechanism of the three: 1) Free radical-oxidation combined defense: 4-carboxy-2,2,6,6-tetramethylpiperidin-1-oxyl captures free radicals, 3-carboxy-2,2,5,5-tetramethylpyrrolidine quenches singlet oxygen, and 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid inhibits oxidation side reactions. The three form a multi-level antioxidant network, significantly extending the service life of the plating solution (such as reducing the turbidity of the plating solution caused by gold particle aggregation). 2) Dynamic complexation equilibrium: The free radical-capturing piperidinyloxy of 4-carboxy-2,2,6,6-tetramethylpiperidin-1-oxyl and the carboxylic acid group of 3-carboxy-2,2,5,5-tetramethylpyrrolidine (pyrrolidine) complex with gold ions through different spatial configurations, forming a stable multi-toothed coordination structure; the thiophene-carboxylic acid system of 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid provides π-electrons to assist complexation, further reducing the concentration of free gold ions and inhibiting non-uniform deposition.3) Surface adsorption synergistic regulation: 3-carboxy-2,2,5,5-tetramethylpyrrolidine is directionally adsorbed on the high-energy sites of the substrate surface to inhibit the random growth of gold crystal nuclei; 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid optimizes the crystal plane orientation through the planar adsorption of the thiophene ring, and synergizes with the steric hindrance effect of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl to capture free radicals, making the grain size distribution of the coating narrower (CV < 6.5%). 4) Electron effect complementarity: The electron-withdrawing piperidineoxy group captured by 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl and the electron-donating amino group of 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid form an electron transfer channel to accelerate the charge transfer of the local reduction reaction; the pyrrolidine ring of 3-carboxy-2,2,5,5-tetramethylpyrrolidine balances the interfacial charge distribution through the conjugation effect, reducing the internal stress of the coating. In summary, the stabilizer is composed of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 3-carboxy-2,2,5,5-tetramethylpyrrolidine and 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid in a mass ratio of 3:4:2-3. This design solves the pain point that it is difficult to balance the stability and deposition rate of traditional gold plating solutions through the precise matching and functional complementarity of molecular structures.

[0009] Preferably, the complexing agent is composed of N,N'-bis(2-hydroxyethyl)ethylenediamine and potassium sodium tartrate in a mass ratio of 5:4.

[0010] Preferably, the composite leveling agent is composed of ethyl pyrrole-2-carboxylate and methyl 6-aminoindole-4-carboxylate in a mass ratio of 1:2.

[0011] By adopting the above technical solution, the pyrrole ring and ester group in ethyl pyrrole-2-carboxylate can be directionally adsorbed on the substrate surface, reducing the interfacial tension and promoting the wettability of the plating solution to the substrate. The indole aromatic structure of methyl 6-aminoindole-4-carboxylate is adsorbed on the metal surface through π-π interaction to form a molecular-level adsorption layer, inhibiting local over-deposition. The polar groups (ester group, amino group) of both can serve as nucleation sites to induce uniform nucleation of gold grains and inhibit abnormal grain growth. They synergistically reduce the standard deviation of grain size, narrowing the range of grain size distribution in the plating layer. The synergistic mechanism of the two is as follows: 1) Steric hindrance complementary effect. The planar rigid structure of the pyrrole ring in ethyl pyrrole-2-carboxylate provides lateral steric hindrance to hinder the random stacking of gold atom clusters. The hydrophobic chain segment of the ester group regulates the diffusion rate of gold ions through the solvation effect. The vertical steric hindrance of the indole ring in methyl 6-aminoindole-4-carboxylate restricts the longitudinal growth of grains, and the steric effect of the methoxycarbonyl group further restricts the migration of grain boundaries. The coordination ability of the amino group can dynamically adjust the release rate of gold ions. 2) Charge synergistic balance: The weak electron-donating property of the pyrrole ring and the weak electron-withdrawing property of the indole ring form a charge gradient to balance the local electric field distribution in the plating solution and eliminate the coating thickness difference caused by the "edge effect". 3) Adsorption-desorption dynamic balance: The high adsorption energy of the pyrrole derivative provides stable anchor points, while the moderate adsorption strength of the indole derivative allows surface reconstruction. The two construct a dynamic adsorption layer in a 1:2 ratio, ensuring both uniform coverage and allowing necessary surface diffusion. It can greatly improve the uniformity of the gold plating layer, making the gold plating layer more flat, dense and uniform in thickness. The uniformity coefficient CV of the gold plating layer is less than 6.5%.

[0012] Preferably, the accelerator is composed of thiophene[2,3-b]pyridine-5-carboxylic acid and glycine in a mass ratio of 1:1.

[0013] By adopting the above technical scheme, thieno[2,3-b]pyridine-5-carboxylic acid and glycine are matched in a mass ratio of 1:1 to form a composite accelerator with a dual-functional catalytic system. The synergistic effect of the two is reflected in: the delocalized π electron system of the thienopyridine ring contained in thieno[2,3-b]pyridine-5-carboxylic acid can form a strong coordination adsorption with the palladium catalyst layer, and its planar rigid structure can fix the gold complex in a directional manner; its carboxylic acid group forms a negative charge center through deprotonation, reducing the dissociation energy barrier of the Au(CN)2⁻ complex ion; the sulfur atom in the thiophene ring has a lone pair of electrons, which can serve as an electron transfer medium to accelerate the reduction reaction. The triple function of glycine: 1) pH buffering effect: the amino group and the carboxylic acid group form a zwitterionic structure to maintain the pH stability of the local reaction interface. 2) Interface activation effect: the polar groups in the molecule reduce the surface tension of the solution and promote the diffusion of metal ions to the substrate surface. 3) Lattice regulation function: adsorption on the growth crystal surface inhibits dendrite growth and guides orderly deposition through intermolecular hydrogen bonds. The synergistic mechanism of the two: 1) Adsorption-dissociation synergy: The thienyl heterocycle of thienyl [2,3-b] pyridine-5-carboxylic acid preferentially adsorbs on the palladium activation center to form a catalytic site, and glycine promotes the dissociation of Au (CN) 2⁻ to release Au⁺. 2) Electron transfer coupling: The π electron system of the thiophene ring of thienyl [2,3-b] pyridine-5-carboxylic acid forms a charge transfer complex with the amino group of glycine to establish a low-impedance electron transfer channel. 3) Steric hindrance regulation: The flexible chain structure of the glycine molecule can adjust the molecular orientation of the thienyl heterocycle and optimize the spatial distribution of the active site. In short, the accelerator is composed of thienyl [2,3-b] pyridine-5-carboxylic acid and glycine in a mass ratio of 1:1. This design achieves dual optimization of interfacial catalysis and mass transfer processes through the molecular synergy of rigid heterocycles and flexible amino acids, which is the key to improving plating speed and uniformity.

[0014] Preferably, the water-soluble rhodium compound is one of rhodium chloride, rhodium nitrate and rhodium sulfate.

[0015] Preferably, the pH adjuster is potassium dihydrogen phosphate.

[0016] Preferably, the gold salt is sodium dichloroaurate.

[0017] Preferably, the operating temperature is 75-80°C.

[0018] In a second aspect, the present application provides a method for preparing a highly stable chemical nano-gold solution, using the following technical solution: As a general technical concept, the present application also provides a method for preparing the above-mentioned high-stability chemical nano-gold solution, comprising the following steps: Add 2 - 4 g of gold salt, 22 - 35 g of reducing agent, 6 - 8 g of complexing agent, 12 - 16 g of pH regulator, 20 - 50 mg of stabilizer, 50 - 80 mg of accelerator, 0.8 - 1.2 g of composite leveling agent, and 0.008 - 0.012 mg of water-soluble rhodium compound to an appropriate amount of deionized water, mix evenly, and then make the volume of the evenly mixed solution up to 1 L to obtain a highly stable chemical nano gold solution.

[0019] In summary, the beneficial technical effects of this application are as follows: 1. High stability and long lifespan Function of the stabilizer: The stabilizer composed of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 3-carboxy-2,2,5,5-tetramethylpyrrolidine, and 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid in a mass ratio of 3:4:2 - 3 has the functions of capturing free radicals, quenching singlet oxygen, and selective oxidation. Its strong electron-withdrawing effect can inhibit the non-uniform precipitation of gold ions, reduce the risk of self-decomposition of the plating solution, and extend the service life.

[0020] 2. Fast gold plating rate Design of the accelerator: The accelerator composed of thiophene[2,3-b]pyridine-5-carboxylic acid and glycine (1:1) can promote the directional adsorption of gold ions on the palladium activation layer, shorten the nucleation time, the deposition rate reaches more than 0.16 μm / 20 minutes, and the rate stability is excellent.

[0021] Optimization of the reducing agent: The dual reduction system of formaldehyde and oxalic acid (mass ratio 2:3 - 4) synergistically accelerates the reduction of gold ions, and at the same time avoids the rate decay caused by the excessive consumption of a single reducing agent.

[0022] Water-soluble rhodium compound: In combination with the accelerator and the reducing agent, it can effectively improve the gold plating rate of the chemical nano gold solution. At the same time, the gold plating rate of the chemical nano gold solution is stable, which is conducive to obtaining a gold plating layer with good uniformity (uniformity coefficient less than 6.5%).

[0023] 3. Coating uniformity and density Regulation of the composite leveling agent: Ethyl pyrrole-2-carboxylate and methyl 6-aminoindole-4-carboxylate (1:2) optimize the diffusion distribution of gold ions through the adsorption-desorption dynamic equilibrium, making the coating uniformity coefficient (CV) < 6.5%, and the thickness fluctuation is extremely low.

[0024] Assistance of the stabilizer: The stabilizer with a specific carboxyl structure selectively inhibits local over-deposition, ensuring a flat and dense coating surface and reducing the porosity.

[0025] 4. Improvement of coating properties Surface brightness: The complexing agent and the pH regulator synergistically control the pH gradient at the reaction interface, avoid grain coarsening, and the coating shows high brightness.

[0026] Corrosion resistance: The dense coating structure effectively blocks the penetration of environmental media, enhancing the oxidation resistance and corrosion resistance of subsequent devices.

[0027] 5. Economic benefits and environmental friendliness Low precious metal loss: The stabilizer and rhodium compound work together to reduce the ineffective consumption of gold salts, reduce the replenishment frequency, and save 15 - 20% in overall costs.

[0028] Low - toxicity design: Optimize the amount of formaldehyde used and partially replace traditional strong reducing agents (such as sodium borohydride) with oxalic acid to reduce the difficulty of wastewater treatment. Specific implementation manners

[0029] The following will describe the implementation schemes of the present application in detail in conjunction with embodiments. However, those skilled in the art will understand that the following embodiments are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. For those not specified in the embodiments, they are carried out under conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments without indicating the manufacturer can be obtained as conventional products through commercial purchase.

[0030] Example 1, A highly stable chemical nano - gold solution, by mass concentration, includes the following preparation raw materials: sodium chloroaurate 2g / L, reducing agent 22g / L, complexing agent 6g / L, potassium dihydrogen phosphate 12g / L, stabilizer 20mg / L, accelerator 50mg / L, composite homogenizer 0.8g / L, rhodium chloride 0.008mg / L. Among them, the reducing agent is composed of 37% formaldehyde and oxalic acid in a mass ratio of 2:3, the stabilizer is composed of 4 - carboxy - 2,2,6,6 - tetramethylpiperidine - 1 - oxyl, 3 - carboxy - 2,2,5,5 - tetramethylpyrrolidine and 4 - tert - butoxycarbonylamino - thiophene - 3 - carboxylic acid in a mass ratio of 3:4:2, the complexing agent is composed of N,N’ - bis(2 - hydroxyethyl)ethylenediamine and potassium sodium tartrate in a mass ratio of 5:4, the composite homogenizer is composed of ethyl pyrrole - 2 - carboxylate and methyl 6 - aminoindole - 4 - carboxylate in a mass ratio of 1:2, and the accelerator is composed of thiophene[2,3 - b]pyridine - 5 - carboxylic acid and glycine in a mass ratio of 1:1; The preparation method of the above - mentioned highly stable chemical nano - gold solution includes the following steps: Add 20g of sodium chloroaurate, 220g of reducing agent, 60g of complexing agent, 120g of potassium dihydrogen phosphate, 200mg of stabilizer, 500mg of accelerator, 8g of composite homogenizer, and 0.08mg of rhodium chloride into an appropriate amount of deionized water, mix evenly, and then make the volume of the mixed and evenly - dispersed solution up to 10L to obtain the highly stable chemical nano - gold solution.

[0031] Example 2. A highly stable chemical nano-gold solution, by mass concentration, comprises the following preparation raw materials: sodium chloroaurate 4 g / L, reducing agent 35 g / L, complexing agent 8 g / L, potassium dihydrogen phosphate 16 g / L, stabilizer 50 mg / L, accelerator 80 mg / L, composite homogenizer 1.2 g / L, rhodium nitrate 0.012 mg / L. Among them, the reducing agent is composed of formaldehyde with a mass concentration of 37% and oxalic acid in a mass ratio of 2:4; the stabilizer is composed of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 3-carboxy-2,2,5,5-tetramethylpyrrolidine, and 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid in a mass ratio of 3:4:3; the complexing agent is composed of N,N'-bis(2-hydroxyethyl)ethylenediamine and sodium potassium tartrate in a mass ratio of 5:4; the composite homogenizer is composed of ethyl pyrrole-2-carboxylate and methyl 6-aminoindole-4-carboxylate in a mass ratio of 1:2; the accelerator is composed of thiophene[2,3-b]pyridine-5-carboxylic acid and glycine in a mass ratio of 1:1. The preparation method of the above highly stable chemical nano-gold solution comprises the following steps: Add 40 g of sodium chloroaurate, 350 g of reducing agent, 80 g of complexing agent, 160 g of potassium dihydrogen phosphate, 500 mg of stabilizer, 800 mg of accelerator, 12 g of composite homogenizer, and 0.12 mg of rhodium nitrate into an appropriate amount of deionized water, mix evenly, and then make the volume of the mixed and evenly dissolved solution up to 10 L to obtain the highly stable chemical nano-gold solution.

[0032] Example 3. A highly stable chemical nano-gold solution, by mass concentration, comprises the following preparation raw materials: sodium chloroaurate 3 g / L, reducing agent 28 g / L, complexing agent 7 g / L, potassium dihydrogen phosphate 14 g / L, stabilizer 35 mg / L, accelerator 60 mg / L, composite homogenizer 1 g / L, rhodium sulfate 0.01 mg / L. Among them, the reducing agent is composed of formaldehyde with a mass concentration of 37% and oxalic acid in a mass ratio of 2:3.5; the stabilizer is composed of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 3-carboxy-2,2,5,5-tetramethylpyrrolidine, and 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid in a mass ratio of 3:4:2.5; the complexing agent is composed of N,N'-bis(2-hydroxyethyl)ethylenediamine and sodium potassium tartrate in a mass ratio of 5:4; the composite homogenizer is composed of ethyl pyrrole-2-carboxylate and methyl 6-aminoindole-4-carboxylate in a mass ratio of 1:2; the accelerator is composed of thiophene[2,3-b]pyridine-5-carboxylic acid and glycine in a mass ratio of 1:1. The preparation method of the above highly stable chemical nano-gold solution comprises the following steps: Add 30 g of sodium chloroaurate, 280 g of reducing agent, 70 g of complexing agent, 140 g of potassium dihydrogen phosphate, 350 mg of stabilizer, 600 mg of accelerator, 10 g of composite leveling agent, and 0.1 mg of rhodium sulfate to an appropriate amount of deionized water, mix evenly, and then make the volume of the mixed solution up to 10 L to obtain a highly stable chemical nano-gold solution.

[0033] Comparative Example 1 It is the same as Example 3, except that the stabilizer is 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl.

[0034] Comparative Example 2 It is the same as Example 3, except that the stabilizer is 3-carboxy-2,2,5,5-tetramethylpyrrolidine.

[0035] Comparative Example 3 It is the same as Example 3, except that the stabilizer is 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid.

[0036] Comparative Example 4 It is the same as Example 3, except that the composite leveling agent is ethyl pyrrole-2-carboxylate.

[0037] Comparative Example 5 It is the same as Example 3, except that the composite leveling agent is methyl 6-aminoindole-4-carboxylate.

[0038] Comparative Example 6 It is the same as Example 3, except that the accelerator is thiophene[2,3-b]pyridine-5-carboxylic acid.

[0039] Comparative Example 7 It is the same as Example 3, except that the accelerator is glycine.

[0040] Comparative Example 8 It is the same as Example 3, except that the rhodium sulfate is 0 mg / L.

[0041] Performance Test Sample the highly stable chemical nano-gold solutions prepared in Examples 1-3 and Comparative Examples 1-8 respectively. Part of the sample solution is tested for stability, and part of the sample solution is used for chemical gold plating. The process conditions are: temperature 78 °C, time 20 minutes, and the workpiece to be plated is a circuit board after chemical palladium deposition treatment; and the following tests are carried out. The test results are shown in Table 1.

[0042] The preparation process flow of the circuit board after chemical palladium deposition treatment is as follows: (1) Grinding and sandblasting; after grinding, remove the residues on the copper surface, and sandblasting makes the copper surface uniform. (2) Degreasing: Degrease the circuit board with an acidic or alkaline degreasing agent to remove organic substances and oxides on the copper surface. Use the S-31 acidic degreasing agent provided by Smart Company. The degreasing time is 3 minutes and the temperature is 60°C. After completion, wash with water. (3) Micro-etching: Immerse the circuit board processed in step (2) in a mixed solution of sodium persulfate and sulfuric acid for 0.5 minutes. After completion, wash with water. Among them, the concentration of sodium persulfate in the mixed solution is 120 g / L, and the concentration of sulfuric acid is 40 g / L. (4) Pickling: Immerse the circuit board processed in step (3) in a sulfuric acid solution with a concentration of 70 g / L for 2 minutes to remove the remaining oxides on the copper surface after micro-etching. After completion, wash with water. (5) Activation: Immerse the circuit board processed in step (4) in the S-32 activator solution provided by Smart Company for 1 minute. After completion, wash with water. (6) Electroless nickel plating: Immerse the circuit board processed in step (5) in an electroless nickel plating solution. The concentration of nickel aminosulfate is 20.56 g / L, the concentration of sodium hypophosphite is 27.87 g / L, the concentration of lactic acid (90%) is 21.4 g / L, the concentration of malic acid is 14.3 g / L, the concentration of glycine is 15.6 g / L, the concentration of sodium acetate is 5.6 g / L, the concentration of ethylenediamine is 4.97 g / L, and the concentration of thiodiacetic acid is 3.31 ppm. Deposit for 15 minutes at a temperature of 78°C. After completion, wash with water. (7) Electroless palladium plating: Immerse the circuit board processed in step (6) in the SPD-38 chemical palladium system provided by Smart Company. Deposit palladium for 12 minutes at a temperature of 50°C. After completion, wash with water.

[0043] 1. Uniformity coefficient of the gold plating layer: Randomly select 10 gold plating specimens from Examples 1 - 3 and Comparative Examples 1 - 8 respectively. Measure the palladium plating layer thickness of each specimen, then calculate the average thickness μ of the gold plating layer of the same group of specimens, and calculate the standard deviation α of the gold plating layer thickness of the same group of specimens. Then calculate the uniformity coefficient CV of the gold plating layer in each group of specimens. Among them, the uniformity coefficient of the gold plating layer = (α / μ) * 100%. Among them, the smaller the uniformity coefficient, the better the uniformity of the gold plating layer. In the actual production process, the uniformity coefficient CV less than 10% is qualified.

[0044] 2. Stability test: Take 50 mL of the chemical nano-gold solution respectively, continuously heat it in a water bath at 80°C, observe and record the time when the gold metal precipitate starts to appear in the bath solution to evaluate the stability of the chemical nano-gold solution.

[0045] 3. Deposition rate: Use a fluorescence thickness gauge of the Korean Micro Pioneer brand model XRF-2000L to measure the coating thickness and calculate the deposition rate.

[0046] Table 1 Performance Test

[0047] Analyzing the data in Table 1, it can be seen that: 1) The highly stable chemical nano-gold solution prepared in Examples 1 - 3 has the advantages of fast gold plating rate, high stability, long service life, and good uniformity of the gold plating layer.

[0048] 2) Combining the performance comparison and analysis of the highly stable chemical nano-gold solution prepared in Example 3 and Comparative Examples 1 - 3 shows that the stabilizer consists of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 3-carboxy-2,2,5,5-tetramethylpyrrolidine, and 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid in a mass ratio of 3:4:2.5. Utilizing their synergistic mechanism: 1) Free radical-oxidation combined defense: 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl captures free radicals, 3-carboxy-2,2,5,5-tetramethylpyrrolidine quenches singlet oxygen, and 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid inhibits oxidation side reactions. The three form a multi-level antioxidant network, significantly extending the life of the plating solution. 2) Dynamic complexation equilibrium: The piperidinyloxy group (capturing free radicals) of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl and the carboxylic acid group of 3-carboxy-2,2,5,5-tetramethylpyrrolidine (pyrrolidine) complex with gold ions through different spatial configurations to form a stable multi-dentate coordination structure; the thiophene-carboxylic acid system of 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid provides π electrons to assist complexation, further reducing the concentration of free gold ions and inhibiting non-uniform deposition. 3) Surface adsorption synergistic regulation: 3-carboxy-2,2,5,5-tetramethylpyrrolidine is directionally adsorbed on the high-energy sites of the substrate surface, inhibiting the random growth of gold crystal nuclei; 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid optimizes the crystal plane orientation through the planar adsorption of the thiophene ring, synergistically with the steric hindrance effect of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl capturing free radicals, making the grain size distribution of the plating layer narrower (CV < 6.5%). 4) Electron effect complementarity: The electron-withdrawing piperidinyloxy group captured by 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl and the electron-donating amino group of 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid form an electron transfer channel, accelerating the charge transfer of the local reduction reaction; the pyrrolidine ring of 3-carboxy-2,2,5,5-tetramethylpyrrolidine balances the interfacial charge distribution through the conjugation effect, reducing the internal stress in the plating layer. In short, the stabilizer consists of 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 3-carboxy-2,2,5,5-tetramethylpyrrolidine, and 4-tert-butoxycarbonylamino-thiophene-3-carboxylic acid in a mass ratio of 3:4:2.5. This design solves the pain point of the traditional gold plating solution that it is difficult to balance stability and deposition rate through the precise matching and functional complementarity of the molecular structure.

[0049] 3) Comparative analysis of the performance of the highly stable chemical nano-gold solution prepared by combining Example 3 with Comparative Examples 4 - 5 shows that the composite leveling agent is composed of ethyl pyrrole-2-carboxylate and methyl 6-aminoindole-4-carboxylate in a mass ratio of 1:2. Utilizing their synergistic mechanism: 1) Steric hindrance complementarity effect. The planar rigid structure of the pyrrole ring in ethyl pyrrole-2-carboxylate provides lateral steric hindrance, hindering the random stacking of gold atom clusters. The hydrophobic chain segment of the ester group regulates the diffusion rate of gold ions through the solvation effect. The vertical steric hindrance of the indole ring in methyl 6-aminoindole-4-carboxylate restricts the longitudinal growth of grains, and the steric effect of the methoxycarbonyl group further restricts the migration of grain boundaries. The coordination ability of the amino group can dynamically adjust the release rate of gold ions. 2) Charge synergy balance: The weak electron-donating property of the pyrrole ring and the weak electron-withdrawing property of the indole ring form a charge gradient, balancing the local electric field distribution in the plating solution and eliminating the coating thickness difference caused by the "edge effect". 3) Adsorption-desorption dynamic balance: The high adsorption energy of the pyrrole derivative provides stable anchor points, while the moderate adsorption strength of the indole derivative allows surface reconstruction. The two construct a dynamic adsorption layer in a 1:2 ratio, ensuring both uniform coverage and allowing necessary surface diffusion. It can greatly improve the uniformity of the gold plating layer, making the gold plating layer smoother, denser, and with a uniform thickness. The coefficient of variation CV of the gold plating layer is less than 6.5%.

[0050] 4) Comparative analysis of the performance of the highly stable chemical nano-gold solution prepared by combining Example 3 with Comparative Examples 6 - 7 shows that the accelerator is composed of thiophene[2,3-b]pyridine-5-carboxylic acid and glycine in a mass ratio of 1:1. Utilizing their synergistic mechanism: 1) Adsorption-dissociation synergy: The thiophene heterocycle in thiophene[2,3-b]pyridine-5-carboxylic acid preferentially adsorbs on the palladium activation center to form catalytic sites, and glycine promotes the dissociation of Au(CN)₂⁻ to release Au⁺. 2) Electron transfer coupling: The π-electron system of the thiophene ring in thiophene[2,3-b]pyridine-5-carboxylic acid and the amino group of glycine form a charge transfer complex, establishing a low-impedance electron transfer channel. 3) Steric hindrance regulation: The flexible chain structure of the glycine molecule can adjust the molecular orientation of the thiophene heterocycle, optimizing the spatial distribution of active sites. In summary, the accelerator is composed of thiophene[2,3-b]pyridine-5-carboxylic acid and glycine in a mass ratio of 1:1. This design realizes the dual optimization of the interfacial catalysis and mass transfer processes through the molecular synergy of the rigid heterocycle and the flexible amino acid, which is the key point for improving the plating rate and uniformity.

[0051] 5) Comparative analysis of the performance of the highly stable chemical nano-gold solution prepared by combining Example 3 and Comparative Example 8 shows that adding a water-soluble rhodium compound (such as rhodium sulfate) as a co-catalyst can increase the plating rate and synergistically promote the reduction and nucleation of Au+ with the palladium activation layer. In combination with the accelerator and the reducing agent, it can effectively improve the gold plating rate of the chemical nano-gold solution. At the same time, the gold plating rate of the chemical nano-gold solution is stable, which is beneficial to obtaining a gold plating layer with good uniformity (uniformity coefficient less than 6.5%).

[0052] The above embodiments are only used to explain the technical solutions of the present application and not to limit them. Although the above embodiments have specifically described the present application, those skilled in the art should understand that they can still modify the specific implementation manners of the present invention or make equivalent replacements. Any modification and equivalent replacement without departing from the spirit and scope of the present application shall be covered by the protection scope of the present application.

Claims

1. A highly stable chemical nano-gold solution, characterized in that, By mass concentration, it includes the following preparation raw materials: 2 - 4 g / L of gold salt, 22 - 35 g / L of reducing agent, 6 - 8 g / L of complexing agent, 12 - 16 g / L of pH regulator, 20 - 50 mg / L of stabilizer, 50 - 80 mg / L of accelerator, 0.8 - 1.2 g / L of composite leveling agent, 0.008 - 0.012 mg / L of water-soluble rhodium compound. Among them, the reducing agent is composed of formaldehyde with a mass concentration of 37% and oxalic acid in a mass ratio of 2:3 - 4; the stabilizer is composed of 4 - carboxy - 2,2,6,6 - tetramethylpiperidin - 1 - yloxy, 3 - carboxy - 2,2,5,5 - tetramethylpyrrolidine and 4 - tert - butoxycarbonylamino - thiophene - 3 - carboxylic acid in a mass ratio of 3:4:2 - 3; the composite leveling agent is composed of ethyl pyrrole - 2 - carboxylate and methyl 6 - aminoindole - 4 - carboxylate in a mass ratio of 1:

2.

2. The highly stable chemical nano-gold solution according to claim 1, wherein The complexing agent is composed of N,N’ - bis(2 - hydroxyethyl)ethylenediamine and sodium potassium tartrate in a mass ratio of 5:

4.

3. The highly stable chemical nano-gold solution according to claim 1, wherein, The accelerator is composed of thiophene[2,3 - b]pyridine - 5 - carboxylic acid and glycine in a mass ratio of 1:

1.

4. The highly stable chemical nano-gold solution according to claim 1, wherein The water-soluble rhodium compound is one of rhodium chloride, rhodium nitrate and rhodium sulfate.

5. The high-stability chemical nano-gold solution according to claim 1, wherein The pH regulator is potassium dihydrogen phosphate.

6. The high-stability chemical nano-gold solution according to claim 1, wherein, The gold salt is sodium chloroaurate.

7. The high-stability chemical nano-gold solution according to claim 1, wherein The operating temperature is 75 - 80 °C.

8. A method for preparing a highly stable chemical nano-gold solution according to any one of claims 1-7, characterized in that, It includes: Add 2 - 4 g of gold salt, 22 - 35 g of reducing agent, 6 - 8 g of complexing agent, 12 - 16 g of pH regulator, 20 - 50 mg of stabilizer, 50 - 80 mg of accelerator, 0.8 - 1.2 g of composite leveling agent, 0.008 - 0.012 mg of water-soluble rhodium compound to an appropriate amount of deionized water, mix evenly, and then make the volume of the evenly mixed solution up to 1 L to obtain a highly stable chemical nano-gold solution.

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