The application of chemical deposition coatings can be divided into two categories. For the decorative metallization of plastics, a thin (0.3 to 1 μm) metal layer is deposited on the dielectric surface, and its thickness is then increased by electroplating techniques. In this case, the nature of the electroless deposited coating and the nature of the metal are not very important, it is important to ensure that this coating is dense and sufficiently conductive for subsequent electroplating and to provide the required adhesion of the metal layer . The metals used for the electroless deposition of the bottom layer are chosen for process convenience and cost. For this, nickel and copper coatings are used. Because the electroless nickel plating solution is more stable than the electroless copper plating solution, the composition is simpler, and nickel is more convenient.
Adhesion of coatings to non-conductive surfaces is determined by the state of the surface, while the nature of the metal (at least for nickel and copper) usually has only a slight effect on adhesion. Copper plating may be preferred due to its higher conductivity. In the production of printed circuit boards, a copper base layer is almost always used.
On the other hand, chemically deposited coatings are thicker and their use depends on their mechanical, electrical and magnetic properties. Very popular are nickel (nickel phosphorous and nickel boron) deposits on metal articles. The thickness of the copper plating is 20-30 μm, deposited on the plastic, has good conductivity and ductility, so it can be used in the production of printed circuit boards with additives. The entire circuit pattern was obtained chemically.
Cobalt and its alloy coatings can be used to take advantage of their specific magnetic properties; silver and gold coatings are used due to their good electrical conductivity, optical properties and stability.
Electroless plating can be performed by using the plating solution once (until most of the components in the solution are consumed and the reaction rate drops sharply), or by replenishing the substances consumed during the plating process. The long-term developed solution reduces the amount of plating waste and ensures higher labor productivity, but at the same time, it places stricter requirements on the plating solutions: they need to be stable and their parameters should not change significantly over time. In addition, special equipment is required to monitor and control the composition of this solution. For this reason, the long-term mining of solutions is only suitable for large-scale production processes.
Single-use solutions are more versatile, but they are less economical and less efficient. However, a single-use method can be used when the solution has a simple composition and the basic components (metal ions first) are completely consumed during the plating process, while the remaining components (such as ligands) are cheap and non-polluting. In this case, a single-use process is almost acceptable even in mass production.
An extreme case of single use of plating solutions is aerosol spraying, where 5 solutions are sprayed by a special Spray Gun where they impinge on or near the surface to be plated. One solution usually contains metal ions and the other solution contains a reducing agent. In this case, the reduction of metal ions should be fast enough to allow most of the metal to precipitate on the surface before the solution film flows out. This method is feasible for the deposition of such easily reducible metals as silver and gold, although this aerosol solution is also known for the deposition of copper and nickel. The aerosol spray method is great for depositing thin coatings on large, flat surfaces, similar to spray paint.
Since the components of the electroless plating solution, firstly the metal ions, may be toxic and pollute the environment, techniques for recovering metals from the spent plating solution and rinsing water have been developed. Other valuable solution components, such as ligands (EDTA, tartaric acid), can also be recovered.
Electroless plating usually does not require sophisticated equipment. Holding the bath tank needs to exhibit sufficient chemical inertness, it should not catalyze the deposition of the metal lining. Such containers are usually made of chemically stable plastic, but metal cans are also available, and they can be made of stainless steel or titanium. To prevent the deposition of metals on the walls, a sufficient positive potential is applied using a special current source (anodic protection). Electroplated parts can be mounted on racks, and small parts can be placed in barrels in the plating solution. Heating and filtering of the solution is done in the same way as the electroplating process. Special automatic devices have been developed to monitor and control the composition of the plating solution.
