A palladium or palladium alloy electroplating solution and a method for preparing the same

Abstract

The application discloses a kind of palladium or palladium alloy electroplating solution and preparation method thereof, belong to palladium electroplating technical field.The electroplating solution includes palladium source 10~25g / L, complexing agent 80~120g / L, buffering agent 30~40g / L, hydrogen inhibiting leveling additive 3~30mg / L.The hydrogen inhibiting leveling additive is prepared by dimethyl oxalate and dimethylamine by single substitution amide esterification, with N-Boc-ethylenediamine condensation, phosphorus pentasulfide sulfurization, Boc protection and with oxalyl chloride, 2-amino naphthalene-1,4-diketone amide condensation.Preparation first prepares base plating solution, then the additive is dissolved in N-methyl pyrrolidone and water to form mother liquor, then dropwise add mixture, filter, deaerate to obtain electroplating solution.The application utilizes dithio oxamide anchoring and naphthoquinone redox synergistic inhibition of hydrogen evolution and removal of adsorbed hydrogen, and forms two-center three-electron dynamic adsorption layer, significantly reduces the hydrogen content and internal stress of plating layer, improves brightness, bonding force and current efficiency.

Description

Technical Field

[0001] This invention relates to the field of palladium electroplating technology, and more particularly to a palladium or palladium alloy electroplating solution and its preparation method. Background Technology

[0002] Palladium plating possesses advantages such as excellent conductivity, high chemical stability, and good corrosion resistance, making it widely used in electronic connectors, microelectronic components, and other fields. However, industrial palladium electroplating systems still commonly suffer from severe hydrogen evolution side reactions. During electroplating, a large amount of hydrogen is evolved at the cathode, which not only reduces the current efficiency of the palladium metal but also allows the evolved hydrogen to easily penetrate into the plating layer, causing hydrogen embrittlement defects. Furthermore, it increases the residual internal stress of the plating layer, leading to peeling and cracking, severely impacting the reliability of the plating during service.

[0003] To improve the appearance and performance of the coating, existing processes often add traditional sulfur-containing brighteners such as saccharin and thiourea. While these substances can improve the brightness of the coating to some extent, their hydrogen suppression ability is limited, making it difficult to effectively inhibit hydrogen evolution side reactions. The resulting coating has a high hydrogen content, large residual internal stress, and poor contact electrical properties. At the same time, most commercially available additives have a narrow applicable pH window and poor compatibility with different palladium plating solution systems, such as acidic and weakly alkaline ones, making it difficult to simultaneously achieve multiple indicators such as brightening and leveling, low hydrogen content, low internal stress, and high current efficiency.

[0004] In existing technologies, naphthoquinone and dithioamide compounds have been reported as electroplating additives, but single-structure molecules often only possess a single function of brightening or hydrogen suppression. There are few technologies related to assembling naphthoquinone, ethylenediamine linking chains, and dithioamides to construct multifunctional synergistic palladium electroplating additives. Therefore, developing a palladium electroplating additive that can simultaneously suppress hydrogen, level and reduce stress, ensure conductivity, and adapt to multiple pH conditions is of practical significance for improving the overall quality of palladium plating layers and meeting the industrial production needs of electronic components. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by providing a palladium or palladium alloy electroplating solution and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: This invention first proposes a palladium or palladium alloy electroplating solution, comprising the following components at the following concentrations: palladium source 10-25 g / L, complexing agent 80-120 g / L, buffer 30-40 g / L, hydrogen inhibition and leveling additive 3-30 mg / L, with the balance being deionized water; the pH value of the electroplating solution is 2.0-9.8. The hydrogen-suppressing and leveling additive is prepared through the following steps: S1. Synthesis of N,N-dimethyloxamic acid methyl ester: Dimethyl oxalate and dimethylamine were mixed in a molar ratio of 1:1, and the mixture was stirred at a low temperature of -10 to 0°C for 3 to 4 hours using dichloromethane or methanol as solvent. After the reaction was completed, the solvent was evaporated and purified to obtain N,N-dimethyloxamic acid methyl ester. Dimethyl oxalate contains two adjacent ester carbonyl groups, each of which can undergo ammonolysis. Dimethylamine acts as a nucleophile, attacking the carbonyl carbon of the ester. The lone pair electrons on the nitrogen atom of dimethylamine attack one of the carbonyl carbonyl carbons of dimethyl oxalate, forming a tetrahedral intermediate. The proton on the nitrogen atom is transferred to the oxonium, resulting in a neutral tetrahedral intermediate. The methoxy group leaves as a leaving group, and the carbonyl group is reconstituted to form N,N-dimethyloxalamidate methyl ester. S2. Preparation of Boc-protected asymmetric oxamide: The N,N-dimethyl oxalamide methyl ester obtained in S1 was mixed with N-Boc-ethylenediamine at a molar ratio of 1:1.2. The mixture was stirred at room temperature for 12-16 h with dichloromethane or methanol as solvent to carry out the ester-amide exchange reaction and purified to obtain the Boc-protected asymmetric oxamide intermediate. The primary amine terminus of N-Boc-ethylenediamine acts as a nucleophile, attacking the carbonyl carbon of the ester in the substrate to form a tetrahedral intermediate. The methoxy group leaves, the carbonyl group is rebuilt, and an amide bond is formed. S3. Preparation of dithiocarbamate intermediate by sulfurization modification: The Boc-protected asymmetric oxalamide intermediate obtained in S2 was mixed with phosphorus pentasulfide at a molar ratio of 1:1.1. Anhydrous toluene was used as solvent and the mixture was refluxed under nitrogen protection for 4-8 hours. After cooling, the mixture was quenched, extracted, dried, and purified by column chromatography to obtain the Boc-protected dithiocarbamate intermediate. Deprotection was then performed to obtain free amino dithiocarbamate. The substrate contains two amide carbonyl groups, both of which are converted to thiocarbonyl groups under the action of phosphorus pentasulfide; deBoc protection is then performed, releasing free amines; ; S4. Acylation condensation: Free aminodithiooxalamide and oxalyl chloride are mixed in a molar ratio of 1:1.1, with anhydrous dichloromethane as solvent and triethylamine as acid-binding agent. The mixture is reacted at 0°C for 2-3 h. Then, 2-aminonaphthalene-1,4-dione with an equal molar amount of free aminodithiooxalamide is added, and the mixture is stirred at room temperature for 8-12 h. After washing, drying, and purification by column chromatography, the hydrogen-suppressing and leveling additive is obtained.

[0007] ; The lone pair of electrons from the nitrogen atom of the free primary amine attacks one of the carbonyl carbonyl groups of oxaloyl chloride, forming a tetrahedral intermediate. The chloride ion leaves, the carbonyl group is re-established, and a monoamide acyl chloride is formed. The amino group of the 2-aminonaphthyl-1,4-dione attacks the carbonyl carbon of the monoamide chloride, the chloride leaves, and a final amide bond is formed. The lone pair of electrons from the amino group in the 2-aminonaphthyl-1,4-dione is partially delocalized to the quinone ring, exhibiting weaker nucleophilicity than aliphatic primary amines. Therefore, a reaction at room temperature for 8–12 hours is required to ensure complete reaction. The electron-withdrawing property of the naphthoquinone ring reduces the nitrogen electron density of the amino group, but amidation can still proceed.

[0008] Preferably, the purification method in S1 is vacuum distillation, with the temperature controlled at ≤30℃ throughout the process to avoid the formation of disubstituted byproducts; the reaction in S1 is carried out in a closed and low-temperature manner, and dimethylamine is added slowly dropwise for a period of not less than 60 minutes.

[0009] Preferably, the quenching method in S3 is low-temperature quenching with a saturated sodium bicarbonate aqueous solution, and the pH value of the system after quenching is controlled at 8.0-9.0; the column chromatography eluent is a mixture of n-hexane and ethyl acetate with a volume ratio of 2:1.

[0010] Preferably, in S3, deprotection specifically includes: placing the Boc-protected dithiocarbamate intermediate in a trifluoroacetic acid / dichloromethane mixed solution, stirring at room temperature for 50-70 min, evaporating the solvent, neutralizing with an alkaline solution, and purifying by extraction to obtain free aminodithiocarbamate; The volume ratio of trifluoroacetic acid to dichloromethane is 1:1; alkaline neutralization is performed using a saturated sodium bicarbonate aqueous solution, and the pH value of the system after neutralization is controlled at 8.0-9.0 to avoid hydrolysis of thioamide bonds due to strong alkalinity.

[0011] Preferably, all raw materials are of electronic grade: the palladium source is any one of palladium aminosulfonate, palladium chloride, and tetraamminepalladium chloride; the complexing agent is aminosulfonic acid; and the buffer is boric acid.

[0012] The present invention also proposes a method for preparing the aforementioned palladium or palladium alloy electroplating solution, comprising the following steps: Step 1: Prepare the base plating solution: Add the complexing agent and buffer to the water in sequence, stir until completely dissolved, then add the palladium source, and continue stirring for 20-30 minutes until the system is homogeneous and transparent. Adjust the pH value of the system to the target range using a pH adjuster to obtain the base electroplating solution. Add the complexing agent aminosulfonic acid, the buffer boric acid, and the palladium source sequentially to deionized water. The aminosulfonic acid dissolves and ionizes. ; Aminosulfonic acid is an amphoteric ion. It partially ionizes in water, providing H⁺ to make the initial solution slightly acidic (pH approximately 1.2–2.0). Boric acid has limited solubility in water, but in acidic solutions containing aminosulfonic acid, it is solubilized by forming hydrogen bonds or weak coordination with sulfonic acid groups. Heating to 40–50°C can accelerate its dissolution and increase its solubility.

[0013] Palladium source dissolution and complexation: Taking palladium aminosulfonate Pd(NH2SO3)2 as an example: ; Free Pd 2+ Immediately complexed by excess aminosulfonic acid: ; Aminosulfonate can pass through oxygen atoms (-SO3) - The complex is composed of a nitrogen atom (-NH2) or a palladium atom. Under acidic conditions, the O coordination mode is dominant; under near-neutral conditions, the contribution of N coordination increases. The actual complex formed is a mixed coordination complex, with coordination numbers typically ranging from 2 to 4.

[0014] The main complexation equilibrium in the system: ; in .

[0015] The concentration of the complexing agent is 80–120 g / L (approximately 0.82–1.24 mol / L), which is much higher than the concentration of palladium ions (10–25 g / L Pd, approximately 0.094–0.235 mol / L), ensuring that palladium ions exist stably in a complexed state, while free Pd... 2+ The concentration is extremely low.

[0016] This causes a significant negative shift in the equilibrium potential of palladium, preventing palladium ions from being reduced and precipitated during the preparation or storage of the plating solution.

[0017] The buffering mechanism of boric acid: ; When hydrogen evolution at the cathode consumes H⁺ (or local OH⁻) - When the pH is elevated, boric acid releases H⁺ to buffer the pH. ; The buffering capacity is strongest in the pH range of 8–10. In addition, boric acid may form a borate adsorption film on the cathode surface, which can act as a co-adsorbent to regulate the metal deposition potential.

[0018] Step 2: Prepare the additive stock solution: Dissolve the hydrogen inhibition and leveling additive in N-methylpyrrolidone, stir until completely dissolved, and then use water to prepare an additive stock solution with a concentration of 10 g / L. Filter in the dark for later use. Step 3, Mixing and Shaping: Slowly add the additive mother liquor to the base electroplating solution according to the target concentration, stirring while adding. After the addition is complete, continue stirring in the dark for 15-30 minutes. Filter through a 0.45μm filter membrane to obtain palladium or palladium alloy electroplating solution.

[0019] The dithiooxazone fragment in the hydrogen-suppressing and leveling additive molecule has two adjacent thiocarbonyl sulfur atoms. Under cathodic polarization conditions, this fragment can undergo single-electron reduction or react with surface-adsorbed hydrogen to form a 2c-3e bond. During electroplating, the sulfur-containing organic matter accepts an electron at the cathode, forming a sulfur free radical anion intermediate. ; The free radical interacts with the adjacent sulfur atom to form a 2c-3e bond.

[0020] The abundant adsorbed hydrogen on the electrode surface can transfer part of the electron cloud to sulfur atoms, forming an SHS triple center bond.

[0021] The delocalized electronic structure of the 2c-3e bond allows it to be reversibly formed and broken between the cathode and anode, acting as an electronic mediator.

[0022] The 2c-3e species possess large dipole moments and electron cloud densities, enabling them to firmly anchor themselves at hydrogen evolution active sites on the palladium surface, blocking the discharge channels of hydrated protons and increasing the hydrogen evolution overpotential. Simultaneously, the formation of SHS bridging bonds indicates that hydrogen atoms have been partially captured by sulfur atoms, reducing their probability of entering the palladium lattice.

[0023] Due to the low energy of the 2C-3E bond, its formation and breakage are rapid and reversible, resulting in a dynamic equilibrium in the adsorption of additives on the electrode surface. This soft adsorption ensures sufficient coverage without forming a dense insulating barrier layer, allowing palladium ions to deposit normally in the valleys, thus achieving micro-leveling without causing significant inclusions.

[0024] The 2c-3e species can also participate in electron transfer between the electrode and palladium ions, reducing the activation energy of metal deposition and improving cathode current efficiency. Simultaneously, its reversible redox behavior can synergistically with the naphthoquinone unit to form an electron buffer system, further consuming H_ads and stabilizing the interface potential.

[0025] Therefore, the hydrogen-suppressing and leveling additive of the present invention, through the special arrangement of sulfur atoms in its molecular structure, can spontaneously generate two-center three-electron active species under electroplating conditions, integrating physical adsorption, chemical hydrogen removal and electronic mediation functions, and achieving multiple regulation of hydrogen evolution, internal stress and surface roughness from a mechanistic perspective. This is an advantage that existing additives that rely solely on simple sulfur-containing groups for adsorption do not possess.

[0026] Preferably, in step one, the stirring temperature is 40-50℃ and the stirring speed is 300-500 r / min; the pH adjuster is dilute sulfuric acid or dilute ammonia, which is selected according to the acid-base system of the plating solution.

[0027] Preferably, in step two, the preparation of the additive mother liquor is carried out in the dark and at room temperature, and heating is prohibited. The mother liquor is prepared and used immediately, and the storage time does not exceed 7 days. The volume ratio of N-methylpyrrolidone to ultrapure water is 1:9.

[0028] Preferably, in step three, the dropping speed is 200-300 r / min, and the entire process is carried out in the dark; after filtration, the plating solution is allowed to stand for 30 minutes to remove bubbles, and is then ready for use if there is no sediment, no scum, and no bubbles.

[0029] On a micro-rough surface, the electric field strength is higher at protrusions than at valleys. The adsorption driving force for additive molecules is stronger at protrusions, forming a denser adsorption film and reducing the palladium deposition rate at protrusions. At valleys, less adsorption occurs, resulting in a relatively higher palladium deposition rate. This difference leads to the surface gradually becoming smoother.

[0030] The adsorption of additives at the liquid / solid interface is a diffusion-controlled consumption process. At protrusions, the diffusion layer is thin, additive replenishment is rapid, and adsorption coverage is high; at valleys, the diffusion path is long, replenishment is slow, and coverage is low. This diffusion difference further enhances the leveling effect.

[0031] Compared with the prior art, the beneficial effects of the present invention are: 1. By employing a synergistic mechanism of dithiooxazone anchoring and naphthoquinone redox scavenging, hydrogen evolution and hydrogen absorption in the coating are suppressed, significantly reducing internal stress and the risk of hydrogen embrittlement. Existing hydrogen-suppressing additives are mostly single-function sulfur-containing organic compounds or surfactants, which can only partially cover hydrogen evolution sites through physical adsorption, lacking the ability to actively remove adsorbed atomic hydrogen, and are prone to desorption failure in high current density regions. In the additive molecule of this invention, the dithiooxazone fragment contains multiple soft basic sulfur atoms, which can form strong chemisorption with the palladium surface, persistently occupying the active sites for hydrogen evolution and increasing the hydrogen evolution overpotential; simultaneously, the naphthoquinone unit has reversible redox activity, capable of capturing adsorbed hydrogen atoms (H_ads) on the cathode surface, converting them into hydroquinone via a semiquinone radical intermediate, achieving catalytic hydrogen removal. This dual hydrogen-suppressing mechanism of physical blocking and chemical scavenging significantly reduces the hydrogen content of the coating, avoiding the formation of β-phase hydrides and lattice expansion, thereby solving the problems of excessive internal stress and hydrogen embrittlement in the palladium coating, with better results than traditional additives that rely solely on adsorption blocking.

[0032] 2. Microscopic leveling is achieved through selective adsorption and adsorption-diffusion control, refining grains and improving brightness while avoiding coating embrittlement caused by excessive inclusions. Existing sulfur-containing brighteners or leveling agents mostly rely on strong adsorption, but their simple molecular structures make it difficult to balance leveling and coating toughness. Even slightly higher concentrations lead to increased sulfur inclusions and higher contact resistance. In this invention, the dithiooxanil unit in the additive molecule provides a strong anchoring effect, enabling the molecule to form a stable adsorption layer on the electrode surface. The oxanil chain and naphthoquinone ring endow the molecule with suitable polarity and steric hindrance, allowing it to preferentially adsorb at microscopic protrusions. Because the electric field is concentrated and diffusion replenishment is fast at protrusions, the adsorption coverage is high, and deposition is suppressed. Less adsorption occurs at valleys, and deposition is relatively faster, thus achieving microscopic leveling. At the same time, the redox buffering effect of naphthoquinone allows the additive to maintain a dynamic adsorption-desorption balance on the electrode surface, avoiding the formation of a dense insulating film. Grain refinement is achieved without significant inclusions, resulting in improved coating brightness and superior ductility and contact resistance performance compared to traditional strong adsorption additives.

[0033] 3. The dithiooxanil fragment spontaneously generates two-center, three-electron active species under electroplating conditions, endowing the additive with dynamic and reversible adsorption and electronic mediation functions. This significantly improves current efficiency and stabilizes the electrode interface, a mechanistic advantage not possessed by existing static adsorption additives. Existing sulfur-containing additives typically only physically adsorb onto the electrode surface in molecular form. Once formed, the adsorption layer tends to be statically dense. While this can suppress hydrogen evolution, it also hinders the normal mass transfer and discharge of palladium ions, leading to decreased current efficiency and plating inclusions. In the additive molecule of this invention, adjacent thiocarbonyl sulfur atoms can undergo single-electron reduction under cathodic polarization, forming a two-center, three-electron bond. This type of bond has a bond order of only 0.5 and a low bond energy, exhibiting rapid and reversible formation-breakage characteristics. This allows the additive to maintain a dynamic adsorption equilibrium on the electrode surface: it can maintain sufficient coverage to block hydrogen evolution active sites without forming a dense insulating barrier layer, allowing palladium ions to be smoothly reduced at the valleys. Thus, while suppressing hydrogen evolution, it avoids excessive hindrance to metal deposition. More importantly, the electron delocalization properties of the 2c-3e species enable it to act as an electronic mediator between the electrode and palladium ions, reducing the activation energy of palladium reduction and improving the cathode current efficiency. At the same time, it works synergistically with the redox pair of the naphthoquinone unit to form an interfacial electronic buffer system, continuously capturing and oxidizing adsorbed hydrogen atoms, thus stabilizing the electrode potential. Attached Figure Description

[0034] Figure 1 The hydrogen nuclear magnetic resonance spectrum of the hydrogen-suppressing and leveling additive designed in this invention. Detailed Implementation

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0036] Preparation Example 1: Synthesis of Hydrogen Suppression and Leveling Additives: S1 synthesis of N,N-dimethyloxalamidate methyl ester: Raw material ratio: dimethyl oxalate 1.0 eq, electronic grade, ≥99.5%; dimethylamine (2M methanol solution) 1.0 eq, anhydrous grade; anhydrous dichloromethane as solvent.

[0037] Procedure: Dimethyl oxalate was dissolved in dichloromethane under nitrogen protection and stirred until the solid dissolved. The mixture was cooled to -10°C in a low-temperature bath, and dimethylamine methanol solution was slowly added dropwise at a rate of 0.17 mL / min for a total addition time of 60 min, with the internal temperature of the system controlled between -10°C and 0°C throughout. After the addition was complete, the reaction was continued at 0°C with stirring for 3.5 h. The solvent was removed by rotary evaporation to obtain the crude product. The crude product was purified by vacuum distillation, and the fraction with a boiling point of 70–72°C at 10 mmHg was collected to obtain colorless liquid N,N-dimethyl oxalamide. S2 preparation of Boc-protected asymmetric oxalamide: Raw material ratio: methyl N,N-dimethyloxamyl ester 1.0 eq; N-Boc-ethylenediamine 1.2 eq, electronic grade, ≥98%; anhydrous dichloromethane as solvent.

[0038] Procedure: N,N-dimethyl oxalamide and N-Boc-ethylenediamine were dissolved in anhydrous dichloromethane and reacted with the solution at 25°C for 14 h. The solvent was removed by rotary evaporation to obtain a white crude solid product. The crude product was purified by silica gel column chromatography using a 2:1 volume ratio of dichloromethane to ethyl acetate as the eluent. The main component spot corresponding to Rf=0.35 was collected. After the solvent in the eluent was evaporated, the eluent was dried under vacuum at 40°C for 2 h to obtain a white solid Boc-protected asymmetric oxalamide.

[0039] S3 sulfurization for the preparation and deprotection of Boc-protected dithiooxanil: S3.1 Sulfidation reaction: Raw material ratio: Boc protected asymmetric oxalamide 1.0 eq; phosphorus pentasulfide (P4S) 10 1.1eq; anhydrous toluene as solvent.

[0040] Procedure: Under a nitrogen atmosphere, Boc-protected asymmetric oxalamide was dissolved in anhydrous toluene and added to phosphorus pentasulfide. The mixture was stirred at room temperature for 10 min. A reflux condenser with an anhydrous calcium chloride drying tube was assembled, and nitrogen was continuously purged. The mixture was heated in an oil bath to approximately 110°C under reflux and stirred for 6 h. The material was then cooled to room temperature. The reaction solution was slowly poured into 200 mL of saturated sodium bicarbonate aqueous solution pre-cooled with ice water. The pH of the system was adjusted to 8.0–9.0, and the mixture was stirred for 30 min to complete the quenching process. The liquid was separated, and all organic phases were combined. The organic phase was dried with anhydrous sodium sulfate, filtered to remove the desiccant, and the solvent was removed by rotary evaporation. The crude product was purified by silica gel column chromatography using a 2:1 (v / v) mixture of n-hexane and ethyl acetate as the eluent. The fraction with Rf = 0.40 was collected. After removing the solvent from the eluent by rotary evaporation, the eluent was dried under vacuum at 40°C for 2 h to obtain an orange-red solid, Boc-protected dithiooxalamide.

[0041] S3.2 De-Boc Protection: Procedure: Boc-protected dithiocarbamate was dissolved in dichloromethane and cooled to 0°C in an ice bath. Trifluoroacetic acid was added, with the system temperature controlled between 0 and 5°C during the dropwise addition. The ice bath was removed, and the mixture was stirred at 25°C for 60 min. Most of the trifluoroacetic acid and dichloromethane were removed by rotary evaporation under reduced pressure, but the material should not be completely evaporated; approximately 1 / 8 of the liquid remained in the system. The remaining material was slowly poured into a saturated sodium bicarbonate aqueous solution cooled in ice water, and the pH was adjusted to 8.5. The mixture was stirred for 15 min. The aqueous phase was extracted three times with dichloromethane, and the organic phases were combined. The organic phase was dried over anhydrous sodium sulfate, filtered, and the solvent was removed by rotary evaporation in a water bath at a temperature not exceeding 30°C to obtain a dark red oily substance, free aminodithiocarbamate.

[0042] Preparation of hydrogen-suppressing and leveling additives by S4 acylation condensation: Raw material ratio: free aminodithiooxalamide 1.0 eq; oxalyl chloride 1.1 eq; 2-aminonaphthalene-1,4-dione 1.0 eq; triethylamine 1.1 eq; anhydrous dichloromethane is used as the reaction solvent.

[0043] Procedure: Dissolve free aminodithiooxalamide in anhydrous dichloromethane, add triethylamine, and cool the system to 0°C. Dissolve oxalyl chloride in anhydrous dichloromethane and add it to the above reaction system. Maintain the internal temperature at 0°C and continue stirring for 2.5 h to obtain the monoacyl chloride intermediate reaction solution. The intermediate is not separated. In a separate container, dissolve 2-aminonaphthalene-1,4-diketone and triethylamine in anhydrous dichloromethane. Under 0°C conditions, slowly add this mixed solution dropwise to the monoacyl chloride intermediate reaction solution. After the addition is complete, remove the cryo-bath and stir the reaction at 25°C for 10 h. Monitor the reaction by TLC, with a dichloromethane / ethyl acetate volume ratio of 3:2, confirming complete reaction. Wash the reactants twice with water and once with saturated brine. Dry the organic phase with anhydrous sodium sulfate, filter to remove the drying agent, and evaporate the solvent to obtain a dark red crude product. The crude product was purified by silica gel column chromatography using a gradient elution method with a dichloromethane / ethyl acetate volume ratio transitioning from 4:1 to 1:1. The main spot fraction with Rf=0.30 was collected. The solvent was removed from the eluent by rotation, and the product was dried under vacuum at 30°C in the dark for 4 hours to obtain a dark red solid hydrogen suppressant and leveling additive.

[0044] A portion of the hydrogen suppression and leveling additive was dissolved in dimethyl sulfoxide and used for detection by 1H NMR. The results are as follows. Figure 1 As shown; Among them, the multiplet at δ7.0–8.5 ppm is attributed to the aromatic and olefinic hydrogens on the 1,4-naphthoquinone structural unit; the multiplet at δ3.4–3.6 ppm is attributed to the methylene hydrogen of the ethylenediamine bridging structure -NH-CH2CH2-NH- in the middle of the molecule; the strong singlet at δ2.4–2.6 ppm is attributed to the methyl hydrogen of N,N-dimethyl-N(CH3)2 on the dithioamide group; the active NH proton peaks of amides and thioamides partially overlap with the aromatic hydrogen region.

[0045] Example 1: The electroplating solution formula, based on a 1L plating solution preparation: Palladium sulfamate concentration (calculated as palladium metal) is 15g / L, equivalent to 30.6g of palladium sulfamate, electronic grade, palladium content 49%; 100g / L sulfamic acid, added as a complexing agent; 35g / L boric acid, added as a buffer; 10mg / L hydrogen-suppressing and leveling additive, added as a 10mg, pre-prepared as a 10g / L stock solution, using 1mL of the stock solution; deionized water to bring the solution to 1L, with a water resistivity ≥18MΩ・cm. The pH of the plating solution is adjusted to 4.0±0.1 using dilute sulfuric acid.

[0046] Electroplating solution preparation: Add 800 mL of deionized water to a 1 L beaker and heat to 45 °C; add 100 g of sulfamic acid and 35 g of boric acid sequentially while stirring at 400 r / min, stirring until completely dissolved; add 30.6 g of palladium sulfamic acid and continue stirring for 30 min until the system is homogeneous and transparent; adjust the pH to 4.0 using dilute sulfuric acid. Preparation of additive stock solution: Under light-protected conditions, dissolve 100 mg of hydrogen-suppressing and leveling additive in 10 mL of N-methylpyrrolidone (NMP), add 90 mL of deionized water after dissolution, mix well, and filter through a 0.45 μm filter membrane to obtain a 10 g / L additive stock solution. Under stirring at 250 r / min, slowly add 1.0 mL of the stock solution dropwise to the base plating solution, taking 5 min to add; after the addition is complete, stir for 20 min in the dark; filter the system through a 0.45 μm filter membrane, and allow to stand for 30 min to remove bubbles, thus obtaining the electroplating solution of Example 1.

[0047] Electroplating operation parameters: The cathode is a 50mm×50mm×0.3mm brass sheet, and the workpiece is pre-plated with copper as a base; the anode is a platinum-titanium plated mesh, and the anode to cathode area ratio is 2:1; the electroplating temperature is 45±1℃; the cathode is moved to stir, and the moving speed is 2m / min; the current density is 0.8A / dm2; the electroplating time is 30min, and the target coating thickness is about 3μm; the pH of the plating solution is maintained at 4.0±0.1, and the pH is finely adjusted by dilute sulfuric acid and dilute ammonia during the electroplating process.

[0048] Example 2: Electroplating solution formulation: Prepare 1L of plating solution as follows: Palladium chloride concentration 18g / L (equivalent to 30.0g of palladium chloride); sulfamic acid 120g / L (equivalent to 120g of palladium chloride); boric acid 40g / L (equivalent to 40g of boric acid); hydrogen inhibitor and leveling additive 20mg / L (equivalent to 20mg of boron); dilute with deionized water to 1L. Adjust the pH of the plating solution to 8.5±0.1 using dilute ammonia.

[0049] Electroplating solution preparation: Add 800 mL of deionized water to a 1 L beaker and heat to 50 °C; add 120 g of sulfamic acid and 40 g of boric acid sequentially while stirring at 450 r / min and stir until dissolved; add 30.0 g of palladium chloride and stir for 30 min until the solution is clear. If the dissolution is slow, the stirring time can be extended appropriately; adjust the pH to 8.5 with dilute ammonia. For additive stock solution preparation, refer to Example 1. Dissolve 200 mg of additive in 20 mL of NMP, add water to prepare 200 mL of 10 g / L stock solution; stir at 300 r / min, slowly add 2.0 mL of stock solution dropwise to the base plating solution over 8 min; stir in the dark for 30 min, filter through a 0.45 μm filter membrane, and allow to stand for 30 min to remove bubbles.

[0050] Electroplating operation parameters: The cathode is a 50mm×50mm×0.3mm copper sheet; the anode is a platinum-plated titanium mesh; the electroplating temperature is 50±1℃; the circulating filter is stirred, and the circulation flow rate is 2L / min; the current density is 1.0A / dm2; the electroplating time is 25min, and the target coating thickness is about 3μm; the pH of the plating solution is controlled at 8.5±0.1.

[0051] Example 3: Electroplating solution formulation: To prepare 1L of plating solution: Palladium tetraammine chloride concentration 25g / L (equivalent to 42.5g of palladium tetraammine chloride); sulfamic acid 80g / L (80g); boric acid 30g / L (30g); hydrogen inhibitor and leveling additive 30mg / L (30mg); deionized water to a final volume of 1L. Adjust the pH of the plating solution to 2.0±0.1 with dilute sulfuric acid.

[0052] Electroplating solution preparation: Add 800 mL of deionized water to a 1 L beaker and heat to 40 °C; add 80 g of aminosulfonic acid and 30 g of boric acid sequentially while stirring at 350 rpm and stirring until dissolved; add 42.5 g of tetraamminepalladium chloride and stir for 25 min until completely dissolved; adjust the pH to 2.0 with dilute sulfuric acid. Additive stock solution preparation: Dissolve 300 mg of additive in 30 mL of NMP and add water to prepare a 300 mL, 10 g / L stock solution; stir at 200 rpm, add 3.0 mL of stock solution dropwise to the plating solution over 10 min; stir in the dark for 15 min, filter through a 0.45 μm filter, and allow to stand for 30 min to remove bubbles.

[0053] Electroplating operation parameters: The cathode is a 50mm×50mm stainless steel sheet, and the workpiece is pre-plated with nickel as a base; the anode is a platinum-titanium plated mesh; the electroplating temperature is 40±1℃; the cathode is moved and stirred at a speed of 1.5m / min; the current density is 0.5A / dm2; the electroplating time is 50min, and the target coating thickness is about 3μm; the pH of the plating solution is maintained at 2.0±0.1.

[0054] The following comparison model was also set: Comparative Example 1: No hydrogen-suppressing leveling additive added: The remaining formulation and preparation procedures of the plating solution are consistent with those in Example 1, and no hydrogen-suppressing or leveling additives are added to the system. To prepare 1L of plating solution: palladium aminosulfonate 15g / L (calculated as palladium), 30.6g added; aminosulfonic acid 100g / L, 100g added; boric acid 35g / L, 35g added; deionized water to a final volume of 1L; adjust the pH of the plating solution to 4.0. The preparation process is the same as in Example 1, omitting the step of adding the additive mother liquor.

[0055] The electroplating operation parameters were exactly the same as in Example 1: electroplating temperature 45℃, current density 0.8A / dm2, and electroplating time 30min.

[0056] Comparative Example 2: Using saccharin as a hydrogen-inhibiting additive: The remaining formulation, preparation, and electroplating operation of the plating solution are the same as in Example 1, except that saccharin is used to replace the hydrogen-suppressing and leveling additive of this invention. The amount of saccharin added is 1 g / L. Saccharin is a commonly used sulfur-containing brightener. To prepare 1 L of plating solution: palladium aminosulfonate 15 g / L (calculated as palladium), 30.6 g added; aminosulfonic acid 100 g / L, 100 g added; boric acid 35 g / L, 35 g added; saccharin 1 g / L, 1 g added; deionized water to a final volume of 1 L; pH of the plating solution = 4.0. Electroplating conditions are the same as in Example 1.

[0057] Comparative Example 3: Using thiourea as a hydrogen-suppressing additive: The remaining conditions are the same as in Example 1, except that thiourea is used instead of the hydrogen-suppressing and leveling additive of this invention, and the amount of thiourea added is 10 mg / L. Prepare 1 L of plating solution: palladium aminosulfonate 15 g / L, 30.6 g added; aminosulfonic acid 100 g / L, 100 g added; boric acid 35 g / L, 35 g added; thiourea 10 mg / L, 10 mg added; dilute to 1 L with deionized water; pH of the plating solution = 4.0. The electroplating operating parameters are the same as in Example 1.

[0058] Comparative Example 4: The additive concentration exceeds the upper limit of this invention (100 mg / L): The remaining formulation, preparation, and electroplating operations are the same as in Example 1, except that the dosage of the hydrogen-suppressing and leveling additive is adjusted to 100 mg / L. To prepare 1 L of plating solution: palladium aminosulfonate 15 g / L (calculated as palladium), 30.6 g added; aminosulfonic acid 100 g / L, 100 g added; boric acid 35 g / L, 35 g added; hydrogen-suppressing and leveling additive 100 mg / L, 100 mg added; deionized water to a final volume of 1 L; adjust the pH of the plating solution to 4.0; during preparation, 10 mL of a 10 g / L additive stock solution is added to the base plating solution. All electroplating operating parameters are the same as in Example 1.

[0059] Performance testing: pH: The pH meter was calibrated with a standard buffer solution before testing, and the test temperature of the plating solution was kept consistent with the electroplating working temperature. During the electroplating process, online sampling was performed, and the pH was measured directly after cooling / maintaining the temperature to the process temperature.

[0060] Gloss level: Gloss meter method, using a 60° geometric measurement angle. Calibration is performed using the instrument's standard calibration plate before testing. The sample to be tested is placed flat, avoiding edges and defect locations. Five different locations on the sample surface are selected for testing, and the arithmetic mean is taken as the gloss level result, in GU.

[0061] Internal stress: Thin sheet bending method (thin sheet method for testing coating internal stress). A thin metal substrate specimen with known thickness and elastic modulus is used. After electroplating, the bending deflection of the specimen is measured. Based on the specimen thickness, coating thickness, and deflection, the Stoney formula is used to calculate the coating internal stress value. The results are labeled as tensile stress / compressive stress, with positive values ​​representing tensile stress and negative values ​​representing compressive stress.

[0062] Hydrogen content: Thermal desorption method. The electroplated palladium-coated sample is sealed and placed in a thermal desorption analyzer. The temperature is programmed to rise, and the hydrogen trapped inside the coating is released by heat. The total amount of hydrogen released is detected by mass spectrometry, and the mass fraction of hydrogen element is calculated in ppm in combination with the mass of the coating sample. Before the test, the sample should be kept away from water vapor to reduce external hydrogen interference.

[0063] Current efficiency: Gravimetric method. The actual deposited mass of the coating is obtained by weighing the difference in mass of the cathode sample before and after electroplating; according to Faraday's law of electrolysis, the theoretical deposited mass of palladium under the same amount of charge is calculated; Current efficiency = Actual coating mass ÷ Theoretical deposited mass × 100%.

[0064] Contact resistance: Four-probe method. A four-probe contact resistance tester is used to apply a specified small positive test current and measure the voltage between the two probes; the contact resistance is calculated and multiplied by the contact area to obtain the area-normalized contact resistance, in mΩ•cm2; the coating surface is cleaned before testing to remove oil and oxide layers, and five points are tested for each sample and the average is taken.

[0065] Table 1. Summary of performance of each experimental group ; Data Analysis: The performance test results of Examples 1-3 and Comparative Examples 1-4 show that, within the suitable addition range of the additives of the present invention, the brightness of the resulting palladium coating can reach 65-92 GU, which is much higher than that of the blank plating solution without additives, and also better than the comparative system with saccharin and thiourea as additives, indicating that the additives of the present invention have good brightening and leveling capabilities. The pH conditions of the plating solution have a certain influence on the performance of the coating. The overall performance is optimal at pH 4, and a high-brightness coating can be obtained. Under weakly alkaline conditions, a semi-bright and fine coating with low residual compressive stress can be obtained. Even under strongly acidic conditions, although the various properties of the coating are somewhat reduced, the overall performance is still better than that of the traditional additive system.

[0066] Hydrogen evolution is a major problem in palladium electroplating, and excessive hydrogen content in the coating can easily lead to hydrogen embrittlement defects. The additive of this invention can effectively suppress hydrogen evolution during the electroplating process. In Examples 1-3, the hydrogen content in the coatings was only 0.5-1.2 ppm; while the hydrogen content in the blank plating solution was as high as 4.6 ppm, and the hydrogen contents in the saccharin and thiourea comparative systems were 3.2 ppm and 4.0 ppm, respectively, showing a significantly inferior hydrogen suppression effect compared to this invention. Simultaneously, the additive of this invention can significantly reduce the residual internal stress of the coating. The internal stress of the coatings in the examples was 15-42 MPa, while the internal stress of the coatings in the blank plating solution was as high as 98 MPa, and the internal stresses in the saccharin and thiourea systems reached 55 MPa and 75 MPa, respectively. High stress easily causes peeling and cracking of the coating. This invention effectively improves the stress state of the coating and enhances its reliability.

[0067] Regarding current efficiency, the plating solution using the additive of this invention achieves a current efficiency of 85.6%–92.5%, significantly higher than the blank plating solution (78.4%) and the comparative systems of saccharin and thiourea (82.1% and 79.0%). The palladium deposition efficiency is also higher, which is beneficial for practical production applications. In terms of electrical performance, the palladium plating layer obtained by this invention has a contact resistance of 1.8–2.5 mΩ•cm², exhibiting dense crystal structure and excellent contact conductivity. The contact resistance of the plating layers in the blank plating solution, saccharin, and thiourea comparative systems is significantly higher.

[0068] Furthermore, experimental results show that the concentration of the additives used in this invention needs to be strictly controlled. When the amount of additives exceeds the appropriate upper limit, such as when the amount added in Comparative Example 4 reaches 100 mg / L, although the brightness and hydrogen content of the coating are still acceptable, the internal stress of the coating increases, the current efficiency decreases significantly, the organic inclusions in the coating intensify, the contact resistance rises sharply, and the overall performance of the coating deteriorates. This indicates that only within the concentration range specified in this invention can a bright palladium coating with low hydrogen content, low internal stress, high current efficiency, and good electrical contact performance be synergistically obtained.

[0069] In summary, the hydrogen-suppressing and leveling additive of this invention overcomes the shortcomings of traditional sulfur-containing additives, such as limited hydrogen suppression effect, high internal stress of the coating, and poor electrical properties. It is applicable to palladium electroplating systems with different pH values, such as acidic and weakly alkaline, and has excellent comprehensive coating performance, showing great promise for industrial application.

[0070] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A palladium or palladium alloy electroplating solution, characterized in that, The components include the following concentrations: palladium source 10-25 g / L, complexing agent 80-120 g / L, buffer 30-40 g / L, hydrogen inhibition and leveling additive 3-30 mg / L, with the balance being deionized water; the pH value of the electroplating solution is 2.0-9.

8. The hydrogen-suppressing and leveling additive is prepared through the following steps: S1. Synthesis of N,N-dimethyloxamic acid methyl ester: Dimethyl oxalate and dimethylamine were mixed in a molar ratio of 1:1, and the mixture was stirred at a low temperature of -10 to 0°C for 3 to 4 hours using dichloromethane or methanol as solvent. After the reaction was completed, the solvent was evaporated and purified to obtain N,N-dimethyloxamic acid methyl ester. S2. Preparation of Boc-protected asymmetric oxamide: The N,N-dimethyl oxalamide methyl ester obtained in S1 was mixed with N-Boc-ethylenediamine at a molar ratio of 1:1.

2. The mixture was stirred at room temperature for 12-16 h with dichloromethane or methanol as solvent to carry out the ester-amide exchange reaction and purified to obtain the Boc-protected asymmetric oxamide intermediate. S3. Preparation of dithiocarbamate intermediate by sulfurization modification: The Boc-protected asymmetric oxalamide intermediate obtained in S2 was mixed with phosphorus pentasulfide at a molar ratio of 1:1.

1. Anhydrous toluene was used as solvent and the mixture was refluxed under nitrogen protection for 4-8 hours. After cooling, the mixture was quenched, extracted, dried, and purified by column chromatography to obtain the Boc-protected dithiocarbamate intermediate. Deprotection was then performed to obtain free amino dithiocarbamate. S4. Acylation condensation: Free aminodithiooxalamide and oxalyl chloride are mixed in a molar ratio of 1:1.1, with anhydrous dichloromethane as solvent and triethylamine as acid-binding agent. The mixture is reacted at 0°C for 2-3 h. Then, 2-aminonaphthalene-1,4-dione with an equal molar amount of free aminodithiooxalamide is added, and the mixture is stirred at room temperature for 8-12 h. After washing, drying, and purification by column chromatography, the hydrogen-suppressing and leveling additive is obtained.

2. The palladium or palladium alloy electroplating solution according to claim 1, characterized in that, The purification method described in S1 is vacuum distillation, with the temperature controlled at ≤30℃ throughout the process to avoid the formation of disubstituted byproducts; the reaction in S1 is carried out in a closed and low-temperature manner, and dimethylamine is added slowly dropwise for no less than 60 minutes.

3. The palladium or palladium alloy electroplating solution according to claim 1, characterized in that, The quenching method described in S3 is low-temperature quenching with a saturated sodium bicarbonate aqueous solution, and the pH value of the system is controlled at 8.0-9.0 after quenching; the column chromatography eluent is a mixture of n-hexane and ethyl acetate with a volume ratio of 2:

1.

4. The palladium or palladium alloy electroplating solution according to claim 1, characterized in that, In S3, deprotection specifically includes: placing the Boc-protected dithiocarbamate intermediate in a trifluoroacetic acid / dichloromethane mixed solution, stirring at room temperature for 50-70 min, evaporating the solvent, neutralizing with an alkaline solution, and purifying by extraction to obtain free aminodithiocarbamate; The volume ratio of trifluoroacetic acid to dichloromethane is 1:1; alkaline neutralization is performed using a saturated sodium bicarbonate aqueous solution, and the pH value of the system after neutralization is controlled at 8.0-9.0 to avoid hydrolysis of thioamide bonds due to strong alkalinity.

5. The palladium or palladium alloy electroplating solution according to claim 1, characterized in that, All raw materials are of electronic grade: the palladium source is any one of palladium aminosulfonate, palladium chloride, and palladium tetraammine chloride; the complexing agent is aminosulfonic acid; and the buffer is boric acid.

6. A method for preparing the palladium or palladium alloy electroplating solution according to any one of claims 1-5, characterized in that, Includes the following steps: Step 1: Prepare the base plating solution: Add the complexing agent and buffer to the water in sequence, stir until completely dissolved, then add the palladium source, and continue stirring for 20-30 minutes until the system is homogeneous and transparent. Adjust the pH value of the system to the target range using a pH adjuster to obtain the base electroplating solution. Step 2: Prepare the additive stock solution: Dissolve the hydrogen inhibition and leveling additive in N-methylpyrrolidone, stir until completely dissolved, and then use water to prepare an additive stock solution with a concentration of 10 g / L. Filter in the dark for later use. Step 3, Mixing and Shaping: Slowly add the additive mother liquor to the base electroplating solution according to the target concentration, stirring while adding. After the addition is complete, continue stirring in the dark for 15-30 minutes. Filter through a 0.45μm filter membrane to obtain palladium or palladium alloy electroplating solution.

7. The method for preparing palladium or palladium alloy electroplating solution according to claim 6, characterized in that, In step one, the stirring temperature is 40-50℃ and the stirring speed is 300-500 r / min; the pH adjuster is dilute sulfuric acid or dilute ammonia, which should be selected according to the acid-base system of the plating solution.

8. The method for preparing palladium or palladium alloy electroplating solution according to claim 6, characterized in that, In step two, the preparation of the additive mother liquor should be carried out in the dark and at room temperature, and heating is prohibited. The mother liquor should be prepared and used immediately, and the storage time should not exceed 7 days. The volume ratio of N-methylpyrrolidone to ultrapure water is 1:

9.

9. The method for preparing palladium or palladium alloy electroplating solution according to claim 6, characterized in that, In step three, the dropping speed is 200-300 r / min, and the entire process is carried out in the dark. After filtration, the plating solution is allowed to stand for 30 minutes to remove bubbles. Once there is no sediment, no scum, and no bubbles, it is ready for use.

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