The reaction principle can be expressed by the equation of reaction of FeCl3 solution with metallic Cu: 2FeCl3 + Cu == 2FeCl2 + CuCl2;

Although iron is more active than copper, it can replace metallic copper from copper sulfate solution, and it produces ferrous sulfate itself. However, ferric iron has stronger oxidation than divalent copper, so metallic copper can react with ferric iron. The printed circuit board is provided with a thin layer of copper plate on one side of the composite material board. When the circuit is printed, the circuit part is printed on the side of the copper plate with anti-corrosion paint, and is placed in the ferric chloride solution without copper printed on the coating. It is eroded and only the circuit part is left.