During the PCB etching process, the main chemical reaction that occurs is the redox reaction. Specifically, this process involves the reduction of copper ions to metallic copper, while the oxidant is reduced to a lower valence state or consumed. For example, in acidic copper chloride etching solution, Cu2+ in copper chloride has oxidizing properties and can oxidize the plate into Cu1+. The reaction is as follows: Etching reaction: Cu+CuCl2 = Cu2Cl2. The formed Cu2Cl2 is not easily soluble in water. In the presence of excess Cl-, it can form soluble complex ions. The reaction is as follows: Complexation reaction: Cu2Cl2 +4Cl- = 2[CuCl3]2-. As copper is etched, more and more Cl1+ is contained in the solution, and the etching ability will soon decrease until it finally loses its effectiveness. In order to maintain the etching ability, Cu1+ can be converted back into CU2+ through solution regeneration to ensure the etching ability.