CN113026005B - Chemical plating solution and process applied to chemical nickel-palladium-gold plating layer of flexible circuit board - Google Patents

Abstract

The invention relates to a chemical plating solution and a process applied to a chemical nickel-palladium-gold plating layer of a flexible circuit board, belonging to the technical field of integrated circuits; comprises a chemical nickel plating solution of double reducing agents and a chemical palladium plating solution taking palladium tetraammine sulfate as a main salt, wherein the temperature of the chemical palladium plating is 51-57 ℃, and the pH value is 7.0-7.6. The composite plating layer obtained by the invention has the advantages of compact and flat surface, strong binding force and strong corrosion resistance.

Description

Chemical plating solution and process applied to chemical nickel-palladium-gold plating layer of flexible circuit board

Technical Field

The invention belongs to the technical field of integrated circuits, and particularly relates to a chemical plating solution and a process applied to a chemical nickel-palladium-gold plating layer of a flexible circuit board.

Background

Copper-based circuits and pads of a Printed Circuit Board (PCB) are easily oxidized, so that circuit resistance becomes large to reduce product reliability. At present, the copper circuit of the PCB is protected mainly by a surface treatment technology, and a plated protective layer needs to meet the requirements of different processes and component installation. Along with the integration and miniaturization of electronic devices, more and more coating layers have functional requirements, so that the traditional surface treatment process cannot meet the requirements. The industry demands more and more for the process of 'universal plating' electroless nickel palladium gold (ENEPIG), so the research on the process is very important.

However, in the chemical nickel palladium gold process, nickel corrosion is an unavoidable problem of the process, after chemical nickel deposition, a nickel-phosphorus alloy layer is formed on the surface, wherein the phosphorus content is 6-10%, when nickel and palladium are replaced, phosphorus remains on the surface of nickel, the existence of phosphorus can cause poor bonding force of a palladium layer and a nickel layer, and the later welding strength can be greatly reduced to cause quality risks.

Disclosure of Invention

The invention aims to provide a chemical plating solution and a chemical plating process applied to a chemical nickel-palladium-gold plating layer of a flexible circuit board, so as to solve the technical problems mentioned in the background technology.

In order to realize the purpose, the specific technical scheme of the chemical plating solution and the process applied to the chemical nickel-palladium-gold plating layer of the flexible circuit board is as follows:

the chemical nickel-palladium-gold (ENEPIG) plating process is developed based on an ENIG process, wherein a chemical palladium layer is inserted between a chemical nickel layer and a gold-leaching layer, and the process comprises the following steps: degreasing → microetching → presoaking → activating → chemical nickel → chemical palladium → immersion gold. The composite plating layer has high welding reliability and good routing performance, and the introduced palladium layer can effectively reduce the corrosion of the nickel layer in the gold immersion process.

The chemical property of palladium is very stable, and the application range is wide, the palladium salt has many kinds in electroplating and chemical plating, the invention adopts sulfate, and the palladium tetraammine sulfate is an ideal raw material for electroplating and chemical plating, and has the name of English: tetraamminiperallium (II) Surfate with the molecular formula of Pd (NH)₃)₄SO₄The palladium content was about 39.65%, and the product was a yellow crystalline powder.

The palladium salts commonly used are mainly palladium chloride and palladium sulfate. Wherein, the palladium chloride is insoluble in water, and chloride ions can cause the corrosion of the galvanic cell in the chemical plating solution, so the palladium chloride needs to be dissolved in concentrated hydrochloric acid in the using process. This not only results in a high operational risk factor, but also reduces the useful life of the equipment. In addition, palladium chloride is deliquescent and the solid has poor storage stability. Sulfates do not present galvanic corrosion problems compared to chloride salts, but palladium sulfate decomposes in hot water and is less stable in solution. The ligand containing palladium in the tetraamminepalladium sulfate has higher stability, good water solubility and higher storage stability.

The chemical plating solution applied to the chemical nickel-palladium-gold plating layer of the flexible circuit board comprises a chemical nickel plating solution and a chemical palladium plating solution, wherein the chemical nickel plating solution comprises the following components:

sodium citrate: 20-30 g/L;

double reducing agents: 10-15g/L of sodium hypophosphite and dimethylamino borane: 1-3g/L of a mixture;

amine chloride: 5-15 g/L;

nickel sulfate: 25-35 g/L;

the electroless palladium plating solution comprises the following components:

palladium salt: 0.5-2 g.L of tetraamminepalladium sulfate^-1；

Main complexing agent: ethylenediamine 4-10 g.L^-1And 3-9 g.L of ethylene diamine tetraacetic acid disodium salt^-1A mixture of (a);

auxiliary complexing agent: sodium citrate 5-20 g.L^-1；

Reducing agent: sodium hypophosphite 5-21 g.L^-1；

A stabilizer: 0.5-1.5ppm of 4-dimethylamino pyridine;

pH buffer: sodium dihydrogen phosphate 0.2-1.2 g.L^-1And disodium hydrogen phosphate 0.1-0.5 g.L^-1A mixture of (a).

Further, the electroless nickel plating solution comprises the following components:

sodium citrate: 25 g/L;

sodium hypophosphite: 12 g/L;

amine chloride: 10 g/L;

nickel sulfate: 30 g/L;

dimethylamino borane: 2 g/L.

Further, the electroless palladium plating solution comprises the following components:

palladium salt: tetraammine palladium sulfate 1 g.L^-1；

Main complexing agent: ethylenediamine 4 g.L^-1Disodium ethylene diamine tetraacetate 4 g.L^-1；

Auxiliary complexing agent: sodium citrate 10 g.L^-1；

Reducing agent: sodium hypophosphite 8 g.L^-1；

A stabilizer: 1ppm of 4-dimethylaminopyridine;

pH buffer: sodium dihydrogen phosphate 0.6 g.L^-1Disodium hydrogen phosphate 0.2 g. L^-1。

The invention also provides a chemical plating process applied to the chemical nickel, palladium and gold plating layer of the flexible circuit board, which comprises the following steps in sequence:

carrying out chemical nickel plating on the surface of the matrix to obtain a nickel plating layer;

the electroless nickel plating solution according to any one of claims 1 to 3, wherein the electroless nickel plating solution is an electroless nickel plating solution;

performing chemical palladium plating on the surface of the substrate to obtain a palladium plating layer;

the electroless palladium plating solution for electroless palladium plating is the electroless palladium plating solution of any one of claims 1 to 3;

the temperature of the chemical palladium plating is 51-57 ℃, and the pH value is 7.0-7.6.

The chemical plating solution and the process applied to the chemical nickel-palladium-gold plating layer of the flexible circuit board have the following advantages: the composite plating layer obtained by the invention has the advantages of compact and flat surface, strong binding force and strong corrosion resistance.

Drawings

FIG. 1 is a graph showing the effect of temperature on deposition rate and phosphorus content in example 1.

FIG. 2 is a surface topography of the plating at different temperatures in example 1: (a-c) are 49 ℃, 53 ℃ and 57 ℃.

FIG. 3 is a graph showing the effect of pH on electroless palladium deposition rate and phosphorus content in example 1.

FIG. 4 shows the dual reducing agent and the single reducing agent in example 1

Detailed Description

In order to better understand the purpose, structure and function of the present invention, the following describes the electroless plating solution and process applied to the electroless nickel-palladium-gold plating layer of the flexible circuit board in detail with reference to the attached drawings.

Compared with industrially developed countries, the chemical plating industry in China starts at a later time, the technical strength difference is large. At present, the universal plating layer is in vigorous demand, the reason for restricting the development of the ENEPIG process is that no stable chemical palladium plating solution capable of realizing industrialization exists in China, so that the development of the chemical palladium plating solution capable of realizing industrialization is very important. The invention uses the novel palladium salt of palladium tetraammine sulfate to obtain an activating solution and a chemical palladium plating solution, evaluates the solution by taking the actual production requirement of the industry as guidance, combines the solution with the existing chemical process and gold leaching process, is applied to the ENEPIG and ENEP process, thereby verifying the practical performance of the plating solution and the functionality of the plating layer, and hopes to provide a stable ionic palladium activating solution and chemical palladium plating solution for the industry.

Table 1 is a comparison of the properties of three palladium salts:

table 1.

Example 1:

pure copper or a PCB board is taken as a matrix, and the composition of the pure copper is shown in a table 2:

table 2.

1. Preparing a basic plating solution:

preparation of pretreatment plating solution: placing a clean beaker on a magnetic stirrer, adding 50% of deionized water by volume, and mixing the stock solution according to the proportion of 1: diluting the mixture in 10 proportion in deionized water, stirring the mixture evenly and fixing the volume;

preparing a chemical nickel plating solution: a clean beaker is taken and placed on a magnetic stirrer, 50 percent of deionized water is added, and sodium hypophosphite and dimethylamino borane, 55 mL.L, are sequentially added according to the proportion^-1The nickel sulfate solution is stirred evenly and the volume is constant.

The quality of the chemical nickel-plating layer plays a key role in the chemical nickel-palladium-gold plating layer of the whole flexible circuit board. The content of sodium citrate has great influence on the stability of the nickel preplating solution, and the increase of sodium hypophosphite is beneficial to improving the deposition efficiency of a plating layer.

Under the condition that the concentration of sodium citrate is fixed, the plating speed is obviously improved along with the increase of the concentration of sodium hypophosphite, which indicates that the addition of the sodium hypophosphite is favorable for improving the plating speed, and the reason is mainly that in the pre-plating process, after a layer of thin nickel with autocatalytic activity is attached to the surface through the reduction of dimethylamino borane (DMAB), the sodium hypophosphite starts to reduce nickel ions in a solution, so that the deposition speed is accelerated, therefore, a certain amount of sodium hypophosphite is added into the pre-plating solution, the deposition of nickel on the surface of the pre-plating layer is favorable for improving the subsequent conventional chemical autocatalytic nickel plating speed.

Preparing a chemical palladium plating solution: placing a clean beaker on a magnetic stirrer, adding 50% volume of deionized water, adding a complexing agent, a pH stabilizer and a stabilizer, stirring and dissolving uniformly, then adding a reducing agent, stirring and dissolving uniformly, adjusting the pH to 6-8, finally adding palladium salt, stirring uniformly and fixing the volume.

In the experiment, ethylenediamine and disodium ethylene diamine tetraacetate are used as main complexing agents, sodium citrate is used as an auxiliary complexing agent, and 4-dimethylamino pyridine is used as a stabilizer. If only disodium ethylene diamine tetraacetate is used as a complexing agent, the uniformity of the thickness of a plating layer is poor, and when the pH value is more than 8.5, the stability of the plating solution is reduced, and black insoluble substances are separated out. If the plating solution only adopts sodium citrate as the complexing agent, the stability of the plating solution is poor. When ethylenediamine, disodium ethylene diamine tetraacetate and sodium citrate are used as complexing agents, the stability of the plating solution is good, the binding force of the plating layer is good, and the thickness distribution is uniform.

The stabilizer is a key factor for improving the stability of the plating solution, generally bismuth or bismuth compounds or sulfur-containing nitrogen-containing organic heterocyclic compounds are adopted, 4-dimethylamino pyridine is adopted in the invention, and the nitrogen-containing heterocyclic organic compounds can be adsorbed on the reaction surface to hinder the exchange process of metal ions, reduce the oxidation-reduction speed and play a role in stabilizing the plating solution.

Preparing a chemical gold plating solution: placing a clean beaker on a magnetic stirrer, adding 50% deionized water, and sequentially adding 100m L & L^-1Gold sodium sulfite solution, 160m L. L^-1Complexing agents such as aminotrimethylene phosphonic acid, 100m L. L^-1Adding 20ml L after stirring complexing agents such as anhydrous sodium sulfite and the like uniformly^-1The gold sodium sulfite solution is stirred evenly and the volume is constant.

2. The process flow for preparing the metal coating comprises the following steps:

2.1 pretreatment process flow:

the pretreatment process generally comprises oil removal, microetching, activation and post-soaking. Wherein the activation process directly determines whether the composite plating layer can plate on the surface of the piece to be plated.

The oil removal is to clean the residual oily substances on the surface of the workpiece to be plated in the machining or storage process, slightly bite the surface of the workpiece to be plated, increase the wetting effect of the surface of the workpiece to be plated and the like. The oil stain on the surface of the piece to be plated can not only cause the phenomenon of plating leakage of the piece to be plated, but also bring the oil stain into the subsequent plating solution, destroy the stability of the plating solution and pollute the plating solution, so the oil removal is thorough. The acidic degreasing fluid provided by Shenzhenshua semiconductor material Limited is adopted, and the degreasing fluid comprises an emulsifier OP-10 and the like, so that the acidic degreasing fluid has the advantages of good degreasing effect, no toxicity in solution, low cost and the like. Referring to the product use instruction, the operation temperature is 40 ℃, and the time is 3-5 min.

The microetching is carried out after the oil removing process, aims to remove a loose oxide layer formed on the surface of a sample due to oxidation, can slightly etch the copper surface of a piece to be plated, ensures that oxides on the copper surface are completely removed, simultaneously, micro-coarsens the copper surface, is beneficial to the deposition of a subsequent chemical nickel plating layer, and the microetching not only influences the activation effect of the copper surface but also influences the binding force between the plating layer and a matrix. Currently, there are three systems of microetching solutions that are commonly used. In this embodiment, a microetching solution of sodium persulfate + sulfuric acid system provided by shenzhen chemical semiconductor materials ltd is used. The chemical nickel-phosphorus plating layer obtained after the system is adopted for treatment has good binding force with matrix copper. Referring to the product use instruction, the operation temperature is room temperature, and the time is 1-3 min. The reaction process in the process is as follows:

S₂O₈ ^2-+2H⁺+Cu→Cu²⁺+2HSO_4-

Na₂S₂O₈+H₂SO₄+Cu→CuSO₄+2NaHSO₄

the purpose of the activation is to make the copper surface capable of catalyzing hypophosphite. The clean copper surface is obtained after oil removal and micro-etching treatment, and the activated substrate surface has catalytic activity and can enable the metal ions of the plating layer to grow on the surface. After the activation treatment, the coating is cleaned immediately and placed into the bath solution of the next step, so that the phenomenon of poor binding force between the coating and the substrate or plating jump caused by oxidation and activity loss due to long-time air contact is avoided. The copper surface after oil removal and microetching treatment has a large number of negatively charged hydroxyl groups which can absorb positively charged palladium complex ions and then undergo a displacement reaction, so that a layer of palladium catalytic active center is displaced on the copper surface.

The purpose of post-dipping is to clean and remove the activation solution from the non-plated areas (substrate and solder resist ink). The system has a palladium ion complexing agent, plays an important role in the quality of products, can prevent the occurrence of the phenomenon of diffusion plating after being treated by the working procedure, and can effectively protect the nickel bath. Strong ligand of palladium in post-dip solution, Pd²⁺Re-chelated into negatively charged particles [ R-Pd]The material repels the substrate with the same electric charge and is then removed by washing. Thereby ensuring that no catalytic center is on the surface of the non-plating area and effectively avoiding the occurrence of the diffusion plating phenomenon.

2.2 Process flow for preparing metal coating:

in this embodiment, a copper sheet or a PCB is used as a base material. The process flow of the chemical nickel-palladium plating of the plated part is shown in the table 3, and the process flow of the chemical nickel-palladium-gold plating of the plated part is shown in the table 4. After each treatment operation, the mixture was washed with tap water and deionized water in two steps. And finally, drying the plated piece by cold air for testing.

Table 3.

Table 4.

The AC-1 acidic cleaning agent mainly comprises a surfactant; the main component of the ME-2 microetching agent is sodium persulfate; the main component of the CA-4 palladium activator is palladium sulfate; the main component of the AS-1 anti-seepage plating agent is sodium citrate.

Temperature is an important factor for the electroless palladium plating process. The temperature is studied on the deposition rate and the coating morphology of the chemical palladium platingAnd the effect of composition. First, the pH of the plating solution was fixed at 7 for 20min, and the deposition rate and the phosphorus content of the plating layer were examined as a function of temperature, as shown in Table 5 and FIG. 1. It can be seen that the deposition rate gradually increases from 0.762. mu. m.h with increasing temperature from 49 ℃ to 65 ℃^-1Increased to 1.056 mu m.h^-1And the phosphorus content in the plating layer is reduced from 11% to 4.96%. Too high a temperature can lead to too high and unstable activity of the plating solution, and the phosphorus content of the obtained plating layer is higher at a lower temperature and does not meet the industrial requirements.

The influence of temperature on the chemical deposition process can cause the microscopic morphology of the coating to be different, and the surface morphology of the coating at different temperatures is characterized, as shown in FIG. 2. When the plating temperature is 49 ℃, the compactness of the plating layer is poor. The reason is that when the plating temperature is lower, the nucleation dispersibility is poor, the particle growth process is influenced, the particle dispersibility is poor, the plating layer is not compact, when the temperature is gradually increased from 53 ℃ to 61 ℃, the nucleation dispersibility is good, the particle growth distribution is compact, and the plating layer compactness is good. When the temperature reaches 65 ℃, the grain boundaries are blurred due to the higher temperature, and even part of the grains are fused with each other, because the temperature is too high, so that the grains grow too fast, and the boundaries are blurred and even are fused with each other. The optimal temperature range of the chemical palladium plating solution is considered comprehensively to be 51-57 ℃, and the optimal temperature is 53 ℃.

Table 5.

pH is also an important process condition in electroless palladium plating processes. Sodium hypophosphite has different reducing power under different pH conditions, which not only influences the deposition rate, but also influences the element composition of the prepared plating layer. Therefore, the change conditions of the deposition rate of the chemical palladium plating solution and the phosphorus content of the plating layer under the condition of the pH value of 6-9 are considered. First, the temperature of the fixed plating solution was 53 ℃ and the deposition time was limited to 20 min. The test results are shown in table 6 below and fig. 3. It can be seen from the figure that the plating rate gradually decreases as the pH of the solution increases. This is because ethylenediamine reacts with Pd as the pH increases²⁺Complexation with waterHigher stability, and reduced free Pd in the plating solution^{2 +}And (4) concentration. The phosphorus content of the plating layer gradually increases with the increase of the pH, because the reduction capability of the sodium hypophosphite is improved with the increase of the pH, the palladium-phosphorus codeposition phenomenon is more obvious, and the phosphorus content gradually increases. The higher the pH of the plating solution, the lower the chemical palladium plating speed, the higher the surface uniformity of the plating layer and the higher the stability of the plating solution. The pH value of the chemical palladium plating solution is 7.0-7.6 by comprehensively considering the influence of the pH value.

Table 6.

3. Plating solution maintenance and MTO test:

the pre-plating layer obtained by the alkaline nickel pre-plating formula by using the dimethylamino borane-sodium hypophosphite double reduction system non-palladium activation technology is subjected to chemical nickel plating by using sodium hypophosphite as a reducing agent, so that the nickel plating layer with more excellent performance by using a single sodium hypophosphite reduction system can be obtained. Electrochemical workstation tests show that the nickel plating layer obtained by utilizing the double-reducing agent preplating formula has higher catalytic activity than the nickel plating layer obtained by utilizing a single reducing agent, and shows more excellent corrosion resistance in 3.5% NaCl solution.

Because the breaking of the P-H bond in the hypophosphite on the metal surface is influenced by the high and low hydrogen evolution overpotential, the capability of the plating layer for catalyzing the oxidation of the hypophosphite can be indirectly represented by testing the hydrogen evolution catalytic performance of the prepared nickel plating layer. The higher the hydrogen evolution catalytic activity of the nickel-plated layer, the higher its ability to catalyze the oxidation of hypophosphite. The cathodic hydrogen evolution curve of the prepared nickel-plated layer in a 1.0mol/L NaOH (25 ℃) solution is shown in FIG. 4. As can be seen from FIG. 4, the nickel plating layer obtained by the dual reducing agents begins to obviously increase in the hydrogen evolution curve of-1.4V to-1.2V, while the nickel plating layer obtained by the single reducing agent begins to obviously increase in the hydrogen evolution curve of-1.6V to-1.4V, which indicates that the nickel plating layer obtained by the dual reducing agents has higher hydrogen evolution catalytic activity and stronger oxidation capability of catalyzing hypophosphite ions.

MTO test: continuously using and continuously adding, testingAnd (4) stability of the plating solution. Assuming that the concentration of palladium ions in the initial plating solution is 1 g.L^-1Adding 1 g.L by continuous supplement in the continuous use process^-1The reduction of palladium ions to the substrate is referred to as 1MTO, and the industry generally requires that the bath have a service life of no less than 3 MTO. Therefore, if the plating solution can be made to be 5MTO, the plating solution has high stability. In the experiment, the experiment is designed by taking the principle that the concentration of palladium ions is calculated as a standard and the palladium ions are replenished every time the concentration of the palladium ions is reduced by 10 percent, and the stability of the plating solution is represented by taking the deposition rate of chemical plating palladium in each MTO as an index.

In the chemical palladium plating process, all effective components are continuously consumed and carried out, and the deposition rate and the stability of the plating solution are gradually reduced. Effective components need to be supplemented in time to realize continuous and stable use of the plating solution, so that the stability of the performance of the plating solution is maintained.

Statistics shows that when 1m2 copper sheets are plated, 200ml of plating solution is brought out approximately, and the copper sheets with the thickness of 2.5cm x 4cm are used for double-sided plating in the experiment, so that a basis is provided for a supplementing process through theoretical calculation. The specific theoretical calculation is as follows: plating the two sides of each red copper sheet to be plated, wherein the area is as follows: 2.5cm 4cm 2 to 20cm2, and the addition was started when the palladium ion consumption decreased by 10%, calculated from the palladium ion concentration in the initial bath. The reaction consumption of the tetraamminepalladium sulfate and the sodium hypophosphite is calculated according to the consumption of palladium ions, the thickness of palladium plated on each red copper sheet in the experiment is about 0.8 mu m, and each component in each piece of plating solution carried out is also required to be added by calculation. In conclusion, when 5 pieces of plating are performed, 0.1 g.L < -1 > of palladium ions are consumed, the replenishment is started, and when 10 times of replenishment is performed, one MTO is completed. The amounts of each substance added per MTO are shown in Table 7. After the effective components are supplemented each time, the pH value of the plating solution is required to be adjusted to 7 and the volume is determined.

Table 7.

The electroless palladium plating process was designed for 6 MTOs and the electroless palladium deposition rate was measured for each MTO with the results shown in table 8 below. From the table, it can be found that the electroless palladium plating rate can be kept in a relatively stable state in 6 MTOs, and can meet the requirement of at least 3 MTOs in industry, which indicates that the plating solution has good continuous use stability.

Table 8.

It is to be understood that the present invention has been described with reference to certain embodiments, and that various changes in the features and embodiments, or equivalent substitutions may be made therein by those skilled in the art without departing from the spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims (4)

1. The chemical plating solution applied to the chemical nickel-palladium-gold plating layer of the flexible circuit board is characterized by comprising a chemical nickel plating solution and a chemical palladium plating solution, wherein the chemical nickel plating solution comprises the following components:

sodium citrate: 20-30 g/L;

double reducing agents: 10-15g/L of sodium hypophosphite and dimethylamino borane: 1-3g/L of a mixture;

amine chloride: 5-15 g/L;

nickel sulfate: 25-35 g/L;

the precoating obtained by the alkaline nickel preplating formula by using a dimethylamino borane-sodium hypophosphite double reduction system non-palladium activation technology is subjected to chemical nickel plating by using sodium hypophosphite as a reducing agent, so that a nickel plating layer with more excellent performance by using a single sodium hypophosphite reduction system can be obtained;

the electroless palladium plating solution comprises the following components:

palladium salt: 0.5-2 g.L of tetraamminepalladium sulfate^-1；

Main complexing agent: ethylenediamine 4-10 g.L^-1And B23-9 g.L of disodium amine tetraacetate^-1A mixture of (a);

auxiliary complexing agent: sodium citrate 5-20 g.L^-1；

Reducing agent: sodium hypophosphite 5-21 g.L^-1；

A stabilizer: 0.5-1.5ppm of 4-dimethylamino pyridine;

pH buffer: sodium dihydrogen phosphate 0.2-1.2 g.L^-1And disodium hydrogen phosphate 0.1-0.5 g.L^-1A mixture of (a).

2. The electroless nickel-palladium-gold plating solution for the electroless nickel-palladium-gold plating of the flexible circuit board as claimed in claim 1, wherein the electroless nickel plating solution comprises the following components:

sodium citrate: 25 g/L;

sodium hypophosphite: 12 g/L;

amine chloride: 10 g/L;

nickel sulfate: 30 g/L;

dimethylamino borane: 2 g/L.

3. The electroless plating solution for electroless nickel, palladium and gold plating of flexible circuit boards as claimed in claim 1, wherein the electroless palladium plating solution comprises the following components:

palladium salt: tetraammine palladium sulfate 1 g.L^-1；

Main complexing agent: ethylenediamine 4 g.L^-1Disodium ethylene diamine tetraacetate 4 g.L^-1；

Auxiliary complexing agent: sodium citrate 10 g.L^-1；

Reducing agent: sodium hypophosphite 8 g.L^-1；

A stabilizer: 1ppm of 4-dimethylaminopyridine;

pH buffer: sodium dihydrogen phosphate 0.6 g.L^-1Disodium hydrogen phosphate 0.2 g. L^-1。

4. The chemical plating process applied to the chemical nickel, palladium and gold plating layer of the flexible circuit board is characterized by comprising the following steps which are sequentially carried out:

carrying out chemical nickel plating on the surface of the matrix to obtain a nickel plating layer;

the electroless nickel plating solution for electroless nickel plating is the electroless nickel plating solution according to any one of claims 1 to 3;

the precoating obtained by the alkaline nickel preplating formula by using a dimethylamino borane-sodium hypophosphite double reduction system non-palladium activation technology is subjected to chemical nickel plating by using sodium hypophosphite as a reducing agent, so that a nickel plating layer with more excellent performance by using a single sodium hypophosphite reduction system can be obtained;

performing chemical palladium plating on the surface of the substrate to obtain a palladium plating layer;

the electroless palladium plating solution for electroless palladium plating is the electroless palladium plating solution of any one of claims 1 to 3;

the temperature of the chemical palladium plating is 51-57 ℃, and the pH value is 7.0-7.6.

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