The gravure electroplating uses sulfate copper plating, and the basic components of the plating solution are copper sulfate and sulfuric acid. Copper sulfate provides copper ions in the plating solution, while sulfuric acid helps prevent the hydrolysis of copper salts, improves the conductivity of the plating solution, and reduces cathodic polarization. Due to the high current efficiency of the plating solution (close to 99%), a thicker coating can be achieved. Of course, the purity and stability of various chemicals must be ensured. The main components of the copper plating solution are as follows:

(1)    Copper sulfate (CuSO₄·5H₂O)

Blue crystals, with particle sizes like rice grains, should be as free of yellow as possible, and industrial grade is acceptable. Depending on production conditions and different requirements, some companies specify the copper sulfate content as 200–250g/L, some as 210–230g/L, and others as 180–220g/L. A low copper sulfate content results in a low allowable working current density and low cathodic current efficiency. The increase in copper sulfate content is limited by its solubility, and in electroplating, as the sulfuric acid content increases, the solubility of copper sulfate correspondingly decreases. Therefore, the copper sulfate content must be below its solubility to prevent precipitation.

(2)    Sulfuric acid (H₂SO₄)

can significantly reduce the resistance of the plating solution and prevent the hydrolysis precipitation of copper sulfate. Its chemical reaction formula is as follows: CuSO₄+2H₂O==Cu(OH)₂+H₂SO₄ If a smell like ammonia is emitted, there is a problem. If the concentration of the bath solution is not appropriate, it will produce monovalent copper ions (Cu⁺), causing the copper surface to have pitting and pinholes. When the sulfuric acid content is low, the coating is rough, and the anode is passivated. Increasing the sulfuric acid content can improve the uniform plating ability, but some brighteners are prone to decomposition. Some companies specify the content as 40–60g/L, some as 55–65g/L, and others as 60–70g/L. Some companies control the copper sulfate content at 180–270g/L and the sulfuric acid content at 55–65g/L, achieving very good results, making the copper layer reach optimal hardness and extending the storage period. At a temperature of 15°C, the corresponding relationship between the copper sulfate + sulfuric acid content and the Baume degree is shown in the table.

(3)    Chloride ions (Cl^-)

are obtained by adding a small amount of hydrochloric acid. Chloride ions are an essential non-polar anion in acidic bright copper plating. Without chloride ions, it is impossible to obtain an ideal bright copper layer, but excessive chloride ion content can cause pitting in the copper layer and affect brightness and smoothness, and should be used according to the requirements of the additives.