The ion transport caused by electrostatic force in electroplating solution is called ion migration. The migration rate is expressed as follows: u=zeoE/6 π r η Yes. Where u is the ion migration rate, z is the charge number of the ion, and eo is the charge amount of one electron (i.e. 1.61019CE is the potential, r is the radius of the hydrated ion η Is the viscosity of the electroplating solution. According to the calculation of the equation, it can be seen that under the action of the electric field. The greater the decrease in potential E, the smaller the viscosity of the electroplating solution, and the faster the rate of ion migration.

　　When electroplating, according to the theory of electrodeposition. The PCB located on the cathode is a non ideal polarized electrode. Copper ions adsorbed on the surface of the cathode obtain electrons and are reduced to copper atoms, resulting in a decrease in the concentration of copper ions near the cathode. Therefore, a copper ion concentration gradient will form near the cathode. The diffusion layer of the plating solution is the layer where the concentration of copper ions is lower than that of the main plating solution. The copper ion concentration in the main plating solution is relatively high, and it will diffuse towards the lower copper ion concentration near the cathode, occasionally supplementing the cathode area. A PCB is similar to a planar cathode, and the relationship between its current and the thickness of the diffusion layer is the COTTRELL equation:

　　The current is called the limiting Diffusion current ii, where I is the current, z is the charge number of copper ion, F is the Faraday constant, A is the cathode surface area, and D is the diffusion coefficient of copper ion (D=KT/6 π r η) Cb is the concentration of copper ions in the main plating solution, Co is the concentration of copper ions on the cathode surface, D is the thickness of the diffusion layer, K is the Boltzmann constant (K=R/N), T is the temperature, and r is the radius of copper hydrated ions η Is the viscosity of the electroplating solution. When the concentration of copper ions on the cathode surface is zero. The limiting Diffusion current depends on the concentration of copper ions in the main bath, the diffusion coefficient of copper ions and the thickness of the diffusion layer. When the concentration of copper ions in the main plating solution is high, the diffusion coefficient of copper ions is large, and the thickness of the diffusion layer is thin, it can be seen from the above equation. The greater the limiting Diffusion current.