Mechanism of action of acidic copper plating additives

There are many types of copper acid additives, but the most studied are brighteners, levelers, grain refiners and Cl-. In recent years, researchers have done a lot of research on the mechanism of action of these additives.

1. Brightener

There is no unified opinion on the mechanism of action of brightener so far. There are many theories that explain it to a certain extent, such as fine crystal theory, crystal plane orientation theory, colloidal film theory and electron free flow theory. The brightener first has a certain effect of enlarging and polarizing. The brightness is caused by the grain size being smaller than the visible light wavelength (that is, less than 0.4 μm) and having a certain orientation arrangement of structures. This structural surface should be parallel to the surface. The gloss of the coating depends not only on whether the coating surface or substrate surface is smooth, but also on the fineness of the coating grains. When the brightener is properly adsorbed, the local growth of the plating is inhibited, the microscopicity of crystallization is promoted, and the visible light does not reflect randomly. At this time, the surface of the plating is smooth and bright.

2. Leveling agent

There are many mechanisms of action of leveling agents, but the theory that has been accepted by most people so far is the "diffusion control resistance theory" proposed by O. Kardos in 1974. This theory holds that the leveling agent first adsorbs the electrode and acts as a barrier to electrodeposition. At the same time, the leveling agent molecules adsorbed on the electrode surface are constantly consumed during the electrode deposition process, and their consumption rate is greater than the diffusion rate of the leveling agent from the solution body to the electrode surface, that is, its leveling effect is controlled by diffusion.

3. Grain refining agents.

Factors that can lead to dislocation and increase nucleation can play a role in refining grains. The grain refining agent increases polarization and generates high overpotentials to ensure high saturation of adsorbed atoms, or increases disorder in which adsorbed atoms enter the lattice by itself, thereby hindering the diffusion of adsorbed atoms to the growth center. Grain refinement has a certain effect on obtaining bright and smooth coatings, but is not a necessary condition. Grain refinement can significantly improve the coating performance.

4. Cl-

In the early 1960s, Cl- was not added to the copper acid plating solution, and Cl- was not added to the plating solution until the late 1960s.

Regarding the mechanism of action of Cl-, it is generally believed that it can form a water-insoluble CuCl complex with Cu2+. The latter is strongly adsorbed on the metal surface of the plating part to form a CuCl thin film, hindering the electrodeposition of copper, achieving the purpose of refining the plating layer crystallization and improving the crystal morphology and orientation.

However, there is a critical value for the addition of Cl-. When it is less than 30 mg/L, a complete film layer cannot be formed, and the application effect cannot be achieved; when it is greater than 100 mg/L, water-soluble CuCl2-, CuC132-, CuCl43-, etc. will be formed, which will increase the cathode current and the coating will be rough and dull.

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