Formulation of brightener and stabilizer for electroless nickel plating

At present, the widely used chemical nickel plating solution formula can be roughly divided into acidic plating solution and alkaline plating solution. Although the composition of electroless nickel plating solution is adjusted according to different applications, it is generally composed of main salt, reducing agent, complexing agent, buffer, stabilizer, accelerator and surfactant. The function of each component is discussed below. (1) main salt The main salt of electroless nickel plating solution is soluble nickel salt, which provides metal nickel ions and is an oxidant in chemical reduction reaction. Useful nickel salts include nickel sulfate (NiS04? 7H20), nickel chloride (NiCl2? 6H20), nickel acetate [Ni(CH3COO)2], nickel sulfamate [Ni(NH2S03)2] and nickel hypophosphite [Ni(H2P02)2], etc. Nickel chloride was used as the main salt in the early stage, but it was no longer used because the existence of Cl- would reduce the corrosion resistance of the coating and produce tensile stress. Nickel acetate and nickel hypophosphite are expensive, and the main salt used at present is nickel sulfate. There are two kinds of crystal water nickel sulfate due to different preparation processes: NiS04? 6H20 and NiS04? 7: 20. Often used in NiS04? 7H20, relative molecular weight 280.88, green crystal. At 100℃, its solubility in 100g water is 478.5 g, and the prepared solution is dark green with a pH value of 4.5. Kinematically, with the increase of Ni2+ concentration in the bath, the deposition rate should increase. However, the experiment shows that the concentration of main salt has little effect on the deposition rate due to the action of complexing agent (except when the concentration of nickel salt is particularly low). In general, the concentration of nickel salt in electroless nickel plating solution is kept at 20 ~ 40g/L, or contains 4 ~ 8g/L. When the concentration of nickel salt is so high that a part of free Ni2+ exists in the plating solution, the stability of the plating solution decreases, and the obtained coating is usually dark and uneven. In the usual range of main salt concentration, the ratio of nickel salt to complexing agent and nickel salt to reducing agent has an influence on the deposition rate of nickel, and they all have a reasonable range. The molar ratio of Ni2+ to h2po 2- should be between 0.3 and 0.45 to ensure the maximum deposition rate and good stability of electroless nickel plating solution. (2) The reductants used in electroless nickel plating are sodium hypophosphite, sodium borohydride and hydrazine, whose structural similarity contains two or more active hydrogens, and the reduction of Ni2+ is carried out by catalytic dehydrogenation of the reductants. Ni-P alloy coating was obtained by sodium hypophosphite, Ni-B alloy coating was obtained by sodium borohydride, and pure nickel coating was obtained by hydrazine. In electroless nickel plating, sodium hypophosphite is mostly used as reducing agent because of its low price, easy control of plating solution and excellent performance of nickel-phosphorus alloy coating. Sodium hypophosphite is easily soluble in water, and the pH value of the aqueous solution is 6. The redox potential of hypophosphite ion is-1.065 V (pH = 7) and -0.882 V (pH-4.5), and it is-1.57 V in alkaline medium, so hypophosphite is a strong reducing agent. The results show that the change of hypophosphite concentration will affect the deposition rate only if the complexing agent ratio is appropriate. With the increase of hypophosphite concentration, the deposition rate of nickel increases. However, the concentration of hypophosphite is also limited, and its molar ratio to nickel salt concentration should not be greater than 4. Otherwise, it is easy to cause the coating roughness and even induce the instantaneous decomposition of the plating solution. Generally, the content of sodium hypophosphite is 20 ~ 40g/L. At the same time, the research shows that the highest pH value is beneficial to improve the deposition rate of nickel and the utilization rate of sodium hypophosphite, but at the same time, the phosphorus content in the coating decreases under the condition of ensuring the sufficient stability of electroless nickel plating solution. (3) Complexing agent The complexing agent in electroless nickel plating solution can not only control the concentration of free Ni2+ available for reaction, but also inhibit the precipitation of nickel phosphite, improve the stability of the plating solution and prolong the service life of the plating solution. Some complexing agents also act as buffers and accelerators to improve the deposition rate of the plating solution and affect the comprehensive properties of the coating. Complexing agents for electroless nickel plating generally contain hydroxyl groups, carboxyl groups and amino groups. Commonly used complexing agents include lactic acid, glycolic acid (glycolic acid), malic acid, glycine and citric acid. Complexing agents in alkaline electroless nickel plating solution include citrate, pyrophosphate and ammonia water. Usually, each plating solution has a main complexing agent, which is used in conjunction with other auxiliary complexing agents. Different kinds of complexing agents and different amounts of complexing agents have great influence on the deposition rate of electroless nickel plating. Reasonable selection of complexing agent and its dosage can not only obtain higher deposition rate of coating under the same conditions, but also stabilize the plating solution and prolong its service life. Fundamentally speaking, whether the electroless nickel plating solution is stable in work depends not only on whether some stabilizer is added to the plating solution, but also on the selection, collocation and dosage of complexing agent. Therefore, the choice of complexing agent should not only make the coating deposition speed fast, but also make the plating solution stable, with long service life and good coating quality. The concentration of complexing agent should be at least able to complex all nickel ions. Therefore, the concentrations of lactic acid, glycolic acid and aminoacetic acid should be at least twice that of Ni2+, while the concentrations of tartaric acid and citric acid should be at least equal to that of Ni2+. If the concentration of complexing agent is not enough to complex all Ni2+, so that the concentration of free Ni2+ in the solution is too high, the stability of the plating solution will decrease and the quality of the coating will deteriorate. (4) In the process of electroless nickel plating, in addition to the precipitation of nickel and phosphorus, the buffer will also produce hydrogen ions, which will lead to the continuous decrease of the pH value of the solution, which will not only slow down the precipitation speed, but also affect the quality of the coating. Therefore, a buffer must be added to the electroless nickel plating solution to make the solution have buffering ability, that is, the pH value of the plating solution will not change too much during the plating process and can be kept within a certain range. (5) Stabilizer electroless nickel plating solution is a thermodynamically unstable system. In the electroplating process, if local overheating is caused by improper heating method, or local pH value is too high due to improper adjustment and replenishment of plating solution, and impurities are introduced or formed due to pollution of plating solution or lack of sufficient continuous filtration, etc. A fierce autocatalytic reaction will occur in the plating solution, producing a large amount of nickel-phosphorus black powder, which will decompose the plating solution in a short time. Therefore, stabilizers should be added to the plating solution. The function of stabilizer is to inhibit the spontaneous decomposition of plating solution and make the electroplating process orderly and controllable. Stabilizers can be preferentially adsorbed on the particle surface to inhibit the catalytic reaction, thus masking the catalytic active center and preventing the nucleation reaction on the particle surface, but it does not affect the normal electroless plating process on the workpiece surface. However, it must be noted that the stabilizer is a toxic agent in electroless nickel plating, that is, a counter-catalyst, and the spontaneous decomposition of the plating solution can be inhibited only by adding a small amount. Stabilizers should not be used excessively. If it is excessive, it will slow down the plating speed. If it is heavy, it will not start plating again. Be sure to use it carefully. Stabilizers commonly used in electroless nickel plating are as follows. ① Heavy metal ions, such as Pb2+, Sn2+, Cd2+, Zn2+, Bi3+, etc. ② Compounds of sulfur, selenium and tellurium of group VI A elements, such as thiourea, thiosulfate and thiocyanate. ③ Some oxygenated compounds, such as AsO2-, M0042-, NO2-, IO3-, etc. ④ Some unsaturated organic acids, such as maleic acid. (6) Accelerator The component that can improve the nickel deposition rate in electroless nickel plating solution is called an accelerator. Its mechanism of action is thought to be to activate hypophosphite ions and promote them to release atomic hydrogen. Many complexing agents in electroless nickel plating also act as accelerators. F- 1 in inorganic ions is a common accelerator, but its concentration must be strictly controlled. High dose will not only slow down the deposition rate, but also affect the stability of the plating solution. The research shows that many substances as stabilizers in electroless nickel plating solution can play the role of accelerators when they exist in a small amount in nickel plating solution. For example, when the dosage of thiourea is 5 mg/L, it acts as a stabilizer, and when the dosage is reduced to 1 mg/L, it acts as an accelerator. (7) In the electroless nickel plating solution with other components, in addition to the above-mentioned main components, surfactants are sometimes added to suppress the pinhole of the coating, and brighteners are added to improve the brightness of the coating. However, sodium dodecyl sulfate, a commonly used surfactant in nickel plating solution, is not suitable for electroless nickel plating solution because it often causes incomplete color spots on the coating.