Composition of electroless nickel plating solutions and basic principles for formulating plating baths

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Composition of electroless nickel plating solutions and basic principles in formulating plating baths – An excellent formulation of the plating bath is essential for obtaining the highest quality electroless nickel coating. A chemical nickel plating solution should include: nickel salts, reducing agents, chelating agents, buffers, accelerators, stabilizers, brighteners, wetting agents, etc. The main salt in chemical nickel plating solutions is the nickel salt, such as nickel sulfate, nickel chloride, nickel acetate, etc.; it is these salts that provide the nickel ions required for the chemical plating process. Nickel chloride was once used as the main salt in the past, but due to the fact that chloride ions not only reduce the corrosion resistance of the coating but also induce tensile stress, it is now rarely used. Compared to nickel sulfate, using nickel acetate as the main salt is beneficial for the properties of the coating. But it is not used because of its high price. In fact, the most ideal source of nickel ions should be nickel hypophosphite; using it prevents the accumulation of large amounts of sulfate ions in the plating bath, and it also avoids the introduction of large quantities of sodium ions as sodium hypophosphite is added during use. However, its cost makes it unsuitable for industrial application. The most commonly used one at present is nickel sulfate; there are two types of nickel sulfate with different numbers of crystal waters, due to slight differences in the manufacturing process. Since nickel sulfate is the main salt and is used in large quantities, it needs to be continuously added during plating. The impurity elements contained in it accumulate in the plating solution, resulting in a decrease in the plating rate and a shorter lifespan of the solution; these factors also affect the properties of the coating, especially its corrosion resistance. Therefore, when purchasing nickel sulfate, it is necessary to ensure that the supplier provides reliable composition test reports to maintain consistent quality across batches; special attention should be paid to controlling impurities that are harmful to the plating solution, especially heavy metal elements. Reductant: The most commonly used reductant is sodium hypophosphite, owing to its low cost, ease of controlling the plating solution, and the good properties of the alloy coating obtained. Sodium hypophosphite is readily soluble in water, and the pH of its aqueous solution is 6. It is a product obtained by dissolving white phosphorus in NaOH and heating it. Currently, the production level of sodium hypophosphite in China is very high, and it is exported in large quantities not only to meet domestic demand but also overseas. Chelating agents: In a chemical nickel plating solution, aside from the main salts and reducing agents, the most important component is the chelating agents. The differences in plating solution performance and their service life mainly depend on the choice of chelating agents and their combination. The first function of the complexing agent is to prevent precipitation in the plating solution, thereby increasing its stability and extending its service life. If no chelating agent is present in the plating solution, due to the low solubility of nickel hydroxide, a light green flocculent precipitate of hydrated nickel hydroxide forms in the acidic plating solution. When nickel sulfate is dissolved in water, it forms nickel hexaaqua ions, which have a tendency to hydrolyze; this hydrolysis results in an acidic environment, and at this point hydroxide precipitates are formed. If some chelating agents are present in the hexaaqua nickel ions, their resistance to hydrolysis can be significantly improved; it is even possible for them to exist in the form of nickel ions in an alkaline environment. However, as the pH value increases, the water molecules in nickel hexahydrate ions are replaced by OH groups, thereby accelerating hydrolysis. To completely suppress this hydrolytic reaction, all nickel ions must be chelated in order to achieve the highest stability that prevents hydrolysis. There are also relatively high levels of hypophosphite ions in the plating solution, but due to the high solubility of nickel hypophosphite, precipitation generally does not occur. As the plating solution is used over time, phosphite ions accumulate in the solution, increasing their concentration, which facilitates the precipitation of white NiHPO3·6H2O precipitates. The addition of a chelating agent significantly reduces the concentration of free nickel ions in the solution, thereby suppressing the precipitation of nickel phosphite at later stages in the plating process. The second function of the complexing agent is to increase the deposition rate; there are many studies showing an increase in the deposition rate upon the addition of this agent. The addition of a chelating agent significantly reduces the concentration of free nickel ions in the plating solution. According to the law of mass action, it is impossible for a decrease in the concentration of reactants to increase the reaction rate; therefore, this phenomenon can only be explained from a kinetic perspective. Simply put, after organic additives adsorb on the surface of the workpiece, they increase its activity, providing more activation energy for hypophosphite to release active atomic hydrogen, thereby accelerating the deposition reaction. The chelating agent also acts as an accelerator here. There are many chelating agents that can be used in electroless nickel plating, but those employed in such plating solutions are required to have high solubility, a certain degree of reactivity, and cost considerations also play an important role. Currently, the commonly used chelating agents are mainly some aliphatic carboxylic acids and their substituted derivatives, such as succinic acid, citric acid, lactic acid, malic acid, and glycine, or their salts. In alkaline baths, pyrophosphates, citrates, and ammonium salts are used. Unsaturated fatty acids are rarely used, as unsaturated hydrocarbons absorb hydrogen atoms when saturated, reducing the efficiency of the reducing agents. Common monocarboxylic acids such as formic acid and acetic acid are rarely used; acetic acid is commonly used as a buffer, while propionic acid is used as an accelerator. Stabilizers: The electroless nickel plating solution is a thermodynamically unstable system. For various reasons such as local overheating, an increase in pH value, or the influence of certain impurities, active particles—catalytic cores—inevitably form within the plating solution. This leads to intense homogeneous autocatalytic reactions, resulting in the generation of large amounts of Ni-P black powder. As a consequence, the plating solution decomposes rapidly, with numerous bubbles being released, causing irreparable economic losses. These black powders are highly efficient catalysts; they possess a very large specific surface area and high activity, which accelerates the spontaneous decomposition of the plating solution, rendering it unusable within a few minutes. The function of a stabilizer is to inhibit the spontaneous decomposition of the plating solution, allowing the plating process to proceed in an orderly manner under control. A stabilizer is a toxicant, that is, a toxic catalyst; just a trace amount of it is sufficient to prevent the spontaneous decomposition of the plating solution. Stabilizers should not be used in excess; excessive amounts can reduce the plating rate at best, or even prevent plating from occurring at worst. We roughly divide the stabilizers we used in the past into four categories: 1. Compounds of Group 16 elements S, Se, Te ； 2. Certain oxides ； 3. Heavy metal ions 4. Water-soluble organic compounds. What has been mentioned above uses hypophosphite as a reducing agent as an example, but the same basic principle applies to amine boride baths as well. However, in a strongly alkaline sodium borohydride bath at 90°C, some stabilizers tend to decompose, precipitate, and become ineffective. There are reports that thallium salts work well. Furthermore, thallium nitrate can also increase the deposition rate of the plating bath at lower temperatures. Thallium salts can deposit in Ni—B coatings, sometimes at concentrations as high as 6%. Accelerators: To increase the deposition rate of electroless plating, certain chemicals are added to the electroless nickel plating solution; these chemicals enhance the plating speed and are therefore known as accelerators. The mechanism of action of accelerators is believed to be that the oxygen atom in the reducing hypophosphite can be replaced by an external anion to form a coordination compound; in other words, the catalytic effect of the accelerator’s anion is due to the formation of a heteropolyacid. The reduced H-P bond strength due to steric hindrance facilitates the dehydrogenation of hypophosphite ions, or in other words, it increases the reactivity of hypophosphorous acid. Experiments show that the anions of short-chain saturated fatty acids and at least one inorganic anion have the effect of promoting hypophosphite dehydrogenation via oxygen substitution, thereby accelerating the deposition rate. Many complexing agents in electroless nickel plating also serve as accelerators. Buffer: During the chemical nickel plating process, hydrogen ions are generated, causing the pH value of the solution to decrease gradually as plating proceeds. To maintain a stable plating rate and ensure the quality of the coating, the chemical nickel plating system must have the ability to buffer the pH value; in other words, it needs to prevent large fluctuations in pH during the plating process, allowing the pH to remain within a certain normal range. A mixture of certain weak acids (or bases) and their salts can counteract the effects of small amounts of external acid or base, as well as dilution, on the pH value of a solution, keeping it within a narrow range; such substances are known as buffers. The buffering capacity of a buffer can be represented by a graph showing the relationship between pH value and acid concentration; a system in which the acid concentration fluctuates within a certain range while the pH value remains essentially constant has good buffering capacity. The monobasic or dibasic organic acids and their salts commonly used in electroless nickel plating solutions not only possess the ability to complex nickel ions but also have buffering properties. The HAC-NaAC system, which is commonly used in acidic plating baths, possesses good buffering capacity; however, acetate has very weak chelating ability, and it is generally not used as a chelating agent. Other components: Similar to electroplated nickel, a small amount of surfactant is added to the chemical nickel plating solution; it helps with the escape of gases and reduces the porosity of the coating. Furthermore, since the surfactant used also acts as a foaming agent, a layer of white foam forms on the surface of the plating solution during the plating process as a result of the large amount of gas being released and mixed around. This foam not only helps to retain heat, reduce evaporation losses from the plating solution, and lessen the acidic taste, but it also traps various kinds of dirt that may be floating in the solution, making it easier to remove them and thus keeping both the plated parts and the plating solution clean. Surfactants are substances that, when added in small amounts, can significantly reduce the surface tension and interfacial tension of solvents, thereby altering the state of the system. At the solid-liquid interface, the valence bond forces of atoms or molecules on the solid surface are unsaturated, resulting in a relatively higher energy compared to those of atoms or molecules inside the solid. Metal surfaces, in particular, belong to the category of high-energy surfaces; when they come into contact with a liquid, the surface energy always decreases. In other words, the solid-gas interface of a metal can easily be replaced by a solid-liquid interface (the definition of wetting is that the gas adsorbed on the solid surface is replaced by a liquid). Chemical nickel plating is a functional coating that is generally not used for decorative purposes, so brightness is not required. However, some people have achieved certain results by using brighteners for nickel plating, such as sodium phenyldisulfonate, in acidic electroless plating baths. Proteins, naphthalenesulfonic acid, fatty alcohol sulfonates, and saccharin are reported to also have a brightening effect in acetic acid-buffered plating baths. Stabilizers for certain metal ions also serve as brighteners, such as chromium ions, thallium ions, and copper ions; it is believed that this is the reason for their co-deposition with Ni-P. The addition of trace copper ions results in a mirror-like, shiny appearance by altering the structure of the coating. However, many manufacturers currently specify chromium-free coatings for electroless plating, so careful consideration must be given to the choice of brightener.