High-salt-fog-resistant electroless nickel-gold plating solution for printed circuit board

Abstract

The application discloses a PCB high-salt-fog-resistant electroless nickel-gold plating solution and belongs to the technical field of PCB surface treatment. In view of the defects of high plating layer porosity, easy black disc generation, poor salt-fog resistance, high cost of additional hole sealing and poor weldability of a traditional ENIG (electroless nickel immersion gold) process, a nickel plating solution and a gold plating solution double-matching system is adopted. The nickel plating solution forms a dense low-porosity nickel-phosphorus alloy bottom layer by compounding nickel salt, primary and secondary reducing agents, a complexing agent, a buffer, a composite activation agent and an organic phosphine precipitation inhibitor; the gold plating solution forms a dense protective gold layer on the surface of the nickel layer by matching gold salt, a composite reducing agent, a complexing agent, a composite buffer, a plating layer stabilizer, a refiner and an antioxidant stabilizer, and the corrosion channel is blocked in two directions. The auxiliary reducing agent is strictly controlled to be added in a low amount, and the organic phosphine precipitation inhibitor is used to avoid the nickelous hypophosphite precipitation pitting problem, so that a bright nickel-gold plating layer can be prepared without hole sealing, the soldering rate is greater than or equal to 95%, there is no black nickel defect in 48h salt-fog resistance, the process is simplified, the cost is lower, and the system is suitable for PCB mass production.

Description

Technical Field

[0001] This invention relates to the field of printed circuit board surface treatment technology, and in particular to a high salt spray resistant electroless nickel-gold plating solution for printed circuit boards. Background Technology

[0002] During the immersion gold stage of the ENIG (Enhanced Electroless Gold) process, the gold solution can excessively corrode along the grain boundaries of the nickel layer, forming a loose and brittle nickel oxide layer. This oxide is dark gray or black, hence the name "black pad" or "black mat." Black pad defects can severely impair the solderability of the pads, leading to problems such as solder rejection, insufficient solder joint strength, or even cracking during soldering.

[0003] The fundamental reason for the insufficient corrosion resistance of ENIG coatings lies in their extremely thin gold layer, typically only 0.02-0.05 micrometers, making it difficult to achieve complete density and inevitably resulting in micropores on the surface. This is particularly problematic under high concentrations of chloride ions (Cl). - In a salt spray environment: (1) Cl - (1) It can attack the underlying nickel layer through the micropores of the gold layer; (2) The gold and nickel form a corrosion cell in the pores, which accelerates the corrosion process of nickel; (3) As the nickel layer is continuously eroded, the copper substrate is eventually exposed, causing the entire coating to lose its protective function.

[0004] To address the aforementioned issues, existing technologies often require an additional "sealing" process. This involves immersing or coating a sealant after electroplating or electroless plating to fill the micropores on the plating surface, forming a dense protective film to block corrosive media and improve resistance to corrosion such as salt spray. However, this process not only increases the number of steps and material costs but may also result in sealant residue affecting the solderability of the pads. Summary of the Invention

[0005] To address the problems mentioned above, the present invention provides a highly salt-resistant electroless nickel-gold plating solution for printed circuit boards, which has good salt resistance and corrosion isolation capabilities.

[0006] The solution adopted by the present invention to solve its technical problem is: a high salt spray resistant electroless nickel-gold plating solution for printed circuit boards, wherein the electroless nickel-gold plating solution comprises a nickel plating solution and a gold plating solution; The nickel plating solution comprises the following components by mass concentration: nickel salt 22-32 g / L, main reducing agent 28-38 g / L, auxiliary reducing agent 7-13 g / L, complexing agent 35-45 g / L, buffer 15-22 g / L, composite plating starter 5-10 mg / L, precipitation inhibitor 0.1-0.5 g / L, with the balance being deionized water; The nickel salt is selected from any one of nickel sulfate, nickel sulfamate, and nickel chloride; the main reducing agent is selected from any one of sodium hypophosphite and potassium hypophosphite; the auxiliary reducing agent is selected from any one of sodium phosphite and potassium phosphite, with the addition amount controlled in the low-dose range of 7-13 g / L, synergistically inhibiting the formation of nickel phosphite precipitation in conjunction with the precipitation inhibitor; the complexing agent used in the nickel plating solution is selected from any one of gluconic acid, aminoacetic acid, hydroxyethyl ethylenediaminetriacetic acid, glycine, and disodium ethylenediaminetetraacetate; the buffer is selected from any one of sodium dihydrogen phosphate, disodium hydrogen phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, sodium acetate, and potassium citrate; the composite plating initiator is composed of a thiol active component and a macromolecular weak adsorption inhibitor at a mass ratio of 1:0.3-1:0.6. It is a compound, wherein the thiol active component is selected from any one of sodium 3-mercapto-1-propanesulfonate, mercaptoacetic acid, mercaptopropionic acid, and mercaptopropanesulfonic acid; the macromolecular weak adsorption inhibitor is selected from at least one of polyethyleneimine, soluble chitosan, and polyallylamine hydrochloride; and the precipitation inhibitor is selected from any one of hydroxyethylidene diphosphonic acid and aminotrimethylphosphonic acid.

[0007] Furthermore, the gold plating solution comprises the following components at the following mass concentrations: gold salt 1-3 g / L, composite reducing agent 5-15 g / L, complexing agent 20-40 g / L, composite buffer 10-20 g / L, plating stabilizer 8-15 mg / L, refining agent 3-8 mg / L, antioxidant stabilizer 10-30 mg / L, with the balance being deionized water; The gold salt is selected from sodium gold sulfite and potassium gold sulfite; the composite reducing agent is a composite system of ascorbic acid and glucose in a mass ratio of 7:3-9:1, with ascorbic acid accounting for 70%-90% of the total mass of the reducing agent and glucose accounting for 10%-30%; the antioxidant stabilizer is 2-mercaptobenzimidazole; the complexing agent used in the gold plating solution is selected from disodium ethylenediaminetetraacetate, glycine, and gluconic acid; the composite buffer is composed of sodium acetate and phosphate in a mass ratio of 1:0.8-1:1.2, with the phosphate selected from at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, and potassium dihydrogen phosphate; the plating stabilizer is mercaptopropionic acid; and the refining agent is o-phenylenediamine.

[0008] In summary, the beneficial effects of the present invention are as follows: The electroless nickel plating solution of this application can form a dense nickel-phosphorus alloy layer on the surface of the PCB substrate, while the electroless gold plating solution uniformly deposits a dense gold layer on the surface of the nickel layer, which strengthens the protection and achieves high salt spray resistance of the PCB plating without the need for sealing.

[0009] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0010] Figure 1 This is a microscope image of the coating surface in Example 1; Figure 2 This is a scanning electron microscope image of the coating surface after 48 hours of salt spray resistance testing in Example 1; Figure 3 This is a microscope image of the coating surface in Comparative Example 1; Figure 4 This is a scanning electron microscope image of the coating surface after 24 hours of salt spray resistance testing, which is a comparative example 2. Detailed Implementation

[0011] To facilitate a clearer understanding of the present invention, the invention will be further described below with reference to specific embodiments and accompanying drawings. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0012] This application includes two systems, a nickel plating solution and a gold plating solution, each with its own functional additives.

[0013] The main reducing agent in the nickel plating solution is a phosphorus-containing reducing agent, which can release electrons to reduce nickel ions and simultaneously generate a low-porosity, dense nickel-phosphorus alloy. The auxiliary reducing agent slows down the nickel deposition rate and avoids coarse and loose grains. The precipitation inhibitor complexes phosphite ions to prevent nickel phosphite precipitation. The composite plating starter is formulated with macromolecular inhibitors and thiol active components in a fixed ratio to buffer the thiol adsorption intensity, refine the nickel layer grains, significantly reduce the porosity of the plating layer, and improve the corrosion resistance of the underlying layer.

[0014] The gold plating solution contains a specific ratio of composite reducing agent and antioxidant to maintain long-term reducing power. The plating stabilizer maintains the stability of the gold sulfite system, inhibiting gold ion disproportionation, gold layer oxidation, and crystallization defects. The grain refiner compresses the gold grain gaps, reduces grain boundary corrosion channels, and forms a continuous, dense, low-porosity gold protective layer on the nickel layer surface, preventing chloride ions from penetrating and corroding the underlying nickel layer. The synergistic effect of various additives in both the nickel and gold plating solutions creates a double-layered protective layer, significantly improving the overall salt spray corrosion resistance of PCB pads.

[0015] The preparation method of the nickel plating solution of this application is as follows: add a complexing agent to deionized water to dissolve it, add a buffer to adjust the pH to 4-5, add a precipitation inhibitor, a main reducing agent, an auxiliary reducing agent, and a composite plating starter in sequence, and finally add nickel salt and stir evenly to obtain a nickel plating solution.

[0016] The preparation method of the gold plating solution of this application is as follows: add a complexing agent to deionized water to dissolve it, add a buffer to adjust the pH to the range of 7.0-7.5, add an antioxidant stabilizer, a composite reducing agent, a coating stabilizer, and a refining agent in sequence, and finally add gold salt and stir evenly to obtain the gold plating solution.

[0017] Example 1

[0018] Electroless nickel plating solution: nickel sulfate 27g / L, sodium hypophosphite 33g / L, sodium phosphite 10g / L, gluconic acid 40g / L, sodium acetate 18g / L, composite plating starter 7mg / L (sodium 3-mercapto-1-propanesulfonate: polyethyleneimine = 1:0.4), hydroxyethylidene diphosphonic acid 0.2g / L, balance deionized water.

[0019] Chemical gold plating solution: Sodium gold sulfite 2g / L, composite reducing agent 10g / L (ascorbic acid: glucose = 9:1), disodium ethylenediaminetetraacetate 30g / L, composite buffer 15g / L (sodium acetate: sodium dihydrogen phosphate = 1:0.9), mercaptopropionic acid 11mg / L, o-phenylenediamine 5mg / L, 2-mercaptobenzimidazole 20mg / L, balance deionized water.

[0020] Example 2

[0021] Electroless nickel plating solution: nickel aminosulfonate 22g / L, potassium hypophosphite 28g / L, potassium phosphite 7g / L, glycine 35g / L, potassium dihydrogen phosphate 15g / L, composite plating starter 5mg / L (thioglycolic acid: soluble chitosan = 1:0.5), aminotrimethylphosphonic acid 0.1g / L, balance deionized water.

[0022] Chemical gold plating solution: sodium gold sulfite 1g / L, composite reducing agent 5g / L (ascorbic acid: glucose = 9:1), glycine 20g / L, composite buffer 10g / L (sodium acetate: potassium dihydrogen phosphate = 1:1), mercaptoacetic acid 8mg / L, o-phenylenediamine 3mg / L, 2-mercaptobenzimidazole 10mg / L, balance deionized water.

[0023] Example 3

[0024] Electroless nickel plating solution: nickel chloride 32g / L, sodium hypophosphite 38g / L, sodium phosphite 13g / L, disodium ethylenediaminetetraacetate 45g / L, potassium citrate 22g / L, composite plating starter 10mg / L (mercaptopropionic acid: polyallylamine hydrochloride = 1:0.6), hydroxyethylidene diphosphonic acid 0.5g / L, balance deionized water.

[0025] Chemical gold plating solution: Potassium gold sulfite 3g / L, composite reducing agent 15g / L (ascorbic acid: glucose = 7.5:2.5), gluconic acid 40g / L, composite buffer 20g / L (sodium acetate: disodium hydrogen phosphate = 1:1.1), mercaptopropionic acid 15mg / L, o-phenylenediamine 8mg / L, 2-mercaptobenzimidazole 30mg / L, balance deionized water.

[0026] Example 4

[0027] Electroless nickel plating solution: nickel sulfate 25g / L, sodium hypophosphite 30g / L, potassium phosphite 9g / L, hydroxyethyl ethylenediamine triacetic acid 38g / L, disodium hydrogen phosphate 16g / L, composite plating starter 6mg / L (mercaptopropanesulfonic acid: polyethyleneimine = 1:0.35), aminotrimethylphosphonic acid 0.3g / L, balance deionized water.

[0028] Chemical gold plating solution: Potassium gold sulfite 1.5g / L, composite reducing agent 8g / L (ascorbic acid: glucose = 8:2), disodium ethylenediaminetetraacetate 25g / L, composite buffer 12g / L (sodium acetate: potassium dihydrogen phosphate = 1:0.95), mercaptoacetic acid 10mg / L, o-phenylenediamine 4mg / L, 2-mercaptobenzimidazole 15mg / L, balance deionized water.

[0029] Example 5

[0030] Electroless nickel plating solution: Nickel sulfamate 30g / L, potassium hypophosphite 35g / L, sodium phosphite 11g / L, glycine 42g / L, sodium dihydrogen phosphate 20g / L, composite plating starter 9mg / L (sodium 3-mercapto-1-propanesulfonate: soluble chitosan = 1:0.45), hydroxyethylidene diphosphonic acid 0.4g / L, balance deionized water.

[0031] Chemical gold plating solution: Sodium gold sulfite 2.5g / L, composite reducing agent 12g / L (ascorbic acid: glucose = 8:2), glycine 35g / L, composite buffer 18g / L (sodium acetate: potassium dihydrogen phosphate = 1:1.05), mercaptopropionic acid 13mg / L, o-phenylenediamine 7mg / L, 2-mercaptobenzimidazole 25mg / L, balance deionized water.

[0032] The above-mentioned high salt spray resistant electroless nickel-gold plating solution is applied to the surface treatment of printed circuit boards, specifically including the following steps: Pretreatment: Select PCB boards with a size of 620mm×520mm and completed solder mask curing. Then, they are subjected to sandblasting, degreasing, water washing, micro etching, water washing, pre-immersion, activation, and water washing in sequence to clean and roughen the copper surface and deposit palladium metal, build a clean and compatible surface and catalytic sites, and prepare for the subsequent electroless nickel plating process.

[0033] Electroless nickel plating: The pre-treated PCB board is directly immersed in a nickel plating bath containing an electroless nickel plating solution for plating, depositing a dense nickel-phosphorus alloy layer. The operating temperature is 80℃, and the treatment time is 20 minutes; after electroless nickel plating, the board is rinsed with water.

[0034] Chemical gold plating: The PCB board after chemical nickel plating is directly immersed in a gold plating bath containing chemical gold plating solution to form a dense gold layer, protecting the nickel layer from oxidation and improving solderability. The operating temperature is 85℃, and the processing time is 4 minutes. After chemical gold plating, the board is rinsed with water and dried.

[0035] Comparative Example 1 The chemical nickel plating solution and chemical gold plating solution of Comparative Example 1 have the same composition as those of Example 1, but the polyethyleneimine macromolecular inhibitor and the hydroxyethylidene diphosphonic acid precipitation inhibitor in the composite plating initiator are removed; the antioxidant stabilizer 2-mercaptobenzimidazole is removed from the gold plating solution, and only sodium acetate without phosphate is added; the preparation method and application process are the same as those of the examples.

[0036] Comparative Example 2 The chemical nickel plating solution and chemical gold plating solution of Comparative Example 2 have the same composition as those of Comparative Example 1. The difference is that the application process adds a sealing process on the basis of the conventional steps of printed circuit board surface treatment. The sealing agent is 3g / L of γ-aminopropyltriethoxysilane, the operating temperature is 60℃, the treatment time is 8 minutes, and the sealing is completed by washing with water and drying.

[0037] The differences in the composition and ratio of the electroless nickel-gold plating solutions in Examples 1-5 are shown in Table 1.

[0038] Table 1

[0039] "-" indicates the deionized water component.

[0040] Experimental Test Coating appearance test: The coating of the PCB sample is observed under a microscope. The test results are judged according to the standard that "bright, relatively uniform, and golden yellow is good, and dull, uneven, and dark is poor".

[0041] Salt spray resistance test: A 5% sodium chloride solution was used as the salt spray test medium. The temperature of the salt water tank and salt spray chamber was 35℃, the temperature of the pressure tank was 47℃, and the spray pressure was 1.0 kgf / cm². 2 The PCB samples were placed in a salt spray chamber, and the surface morphology of the PCB sample plating was tested using a scanning electron microscope every 12 hours to record whether black nickel appeared.

[0042] Solderability test: Lead-free solder was applied using reflow soldering with a peak temperature of 260℃ and a holding time of 25 seconds to complete the soldering operation on the PCB samples. After soldering, the soldering effect was observed using a 300x magnifying glass, and the solder ratio was calculated. Solderability was judged according to the standard that "solder joints without pinholes and with uniform and sufficient solder coverage indicate good solderability, while the opposite indicates poor solderability."

[0043] The test results for the examples and comparative examples are shown in Table 2: Table 2

[0044] As can be seen from Table 2, Examples 1-5, using the chemical nickel-gold plating solution and application process provided by the present invention, all had good plating appearance without sealing treatment. After 48 hours of salt spray resistance, there was no black nickel phenomenon, good solderability, and the solder rate was consistently higher than 95%.

[0045] like Figure 1 As shown, the coating surface of Example 1 is bright and uniform, golden yellow in color, without obvious defects, and the coating has a good appearance.

[0046] like Figure 2 As shown, after 48 hours of salt spray resistance testing in Example 1, the coating surface showed no corrosion marks, the structure remained dense and intact, and no black nickel defects appeared.

[0047] like Figure 3 As shown, the coating of Comparative Example 1 is not bright, unevenly distributed, and dark in color, with a "poor" appearance.

[0048] like Figure 4 As shown, after adding the sealing process in Comparative Example 2, black nickel still appeared after 24 hours of salt spray resistance, indicating poor solderability and a solder rate of 70.4%.

[0049] The above results indicate that this invention introduces a composite sulfur-based plating initiator and an organophosphorus precipitation inhibitor into the nickel plating solution; the gold plating solution uses a fixed ratio of composite reducing agent, antioxidant, composite buffer, low-adsorption mercaptopropionic acid plating stabilizer and grain refiner. The multiple composite components synergistically optimize the nickel-gold plating structure, and without the need for additional sealing processes, it can simultaneously improve the PCB plating's salt spray corrosion resistance, suppress black pad defects, and ensure excellent pad solderability, meeting the high-reliability mass production requirements of high-end PCBs.

[0050] The embodiments described above are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and modifications made by those skilled in the art based on the invention shall fall within the scope of protection of the present invention.

Claims

1. A high salt spray resistant electroless nickel-gold plating solution for printed circuit boards, characterized in that, The electroless nickel-gold plating solution is divided into a nickel plating solution and a gold plating solution; The nickel plating solution comprises the following components in the indicated mass concentrations: nickel salt 22-32 g / L, main reducing agent 28-38 g / L, auxiliary reducing agent 7-13 g / L, complexing agent 35-45 g / L, buffer 15-22 g / L, composite plating starter 5-10 mg / L, precipitation inhibitor 0.1-0.5 g / L, with the balance being deionized water. The auxiliary reducing agent is selected from sodium phosphite and potassium phosphite, and is added in a low dose range of 7-13 g / L, in conjunction with the precipitation inhibitor to synergistically inhibit the formation of nickel phosphite precipitation; the composite plating initiator is composed of a thiol active component and a macromolecular weak adsorption inhibitor in a mass ratio of 1:0.3-1:0.6, wherein the thiol active component is selected from sodium 3-mercapto-1-propanesulfonate, mercaptoacetic acid, mercaptopropionic acid, and mercaptopropanesulfonic acid, and the macromolecular weak adsorption inhibitor is selected from at least one of polyethyleneimine, soluble chitosan, and polyallylamine hydrochloride; and the precipitation inhibitor is selected from any one of hydroxyethylidene diphosphonic acid and aminotrimethylphosphonic acid.

2. The high salt spray resistant electroless nickel-gold plating solution for printed circuit boards according to claim 1, characterized in that, The gold plating solution comprises the following components by mass concentration: gold salt 1-3 g / L, composite reducing agent 5-15 g / L, complexing agent 20-40 g / L, composite buffer 10-20 g / L, plating stabilizer 8-15 mg / L, refining agent 3-8 mg / L, antioxidant stabilizer 10-30 mg / L, with the balance being deionized water; The composite reducing agent is a composite system of ascorbic acid and glucose in a mass ratio of 7:3-9:1, combined with the antioxidant stabilizer 2-mercaptobenzimidazole to solve the problem of easy degradation of pure ascorbic acid; the composite buffer is composed of acetate and phosphate in a mass ratio of 1:0.8-1:1.2, wherein the acetate is selected from sodium acetate, and the phosphate is selected from at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, and potassium dihydrogen phosphate.

3. A high salt spray resistant electroless nickel-gold plating solution for printed circuit boards according to claim 1 or 2, characterized in that, The surface treatment process for printed circuit boards includes the following steps: S1 Pretreatment: The PCB substrate that has completed solder resist curing is sequentially subjected to sandblasting, degreasing, water washing, micro-etching, water washing, pre-immersion, activation, and water washing to construct catalytic palladium sites on the copper surface; S2 Electroless nickel plating: The substrate is immersed in the nickel plating solution and plated at 80°C for 20 min. After water washing, a dense, low-porosity nickel-phosphorus alloy underlayer is obtained; S3 Electroless gold plating: After nickel plating, the substrate is immersed in the gold plating solution and plated at 85°C for 4 min. After water washing and drying.

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