When the current is passed through the electroplating bath, the anions and cations in the electroplating solution will migrate in opposite directions respectively, a phenomenon called electromigration. In fact, in addition to electromigration, there are two migration phenomena of diffusion and convection at the same time in the electroplating process. Now take the bright sulfuric acid copper plating solution with copper sulfate content of 200g/L and sulfuric acid content of 60g/L as an example to illustrate these three migration modes. By calculation (omitted), it can be known that the ion migration numbers of H+, SO42-, Cu2+ of the acid copper solution are 0.58, 0.30, and 0.12 respectively (the ion migration number refers to the proportion of the amount of electricity migrated by a certain ion in the plating solution that passes through the plating solution. ratio of total power). In addition, for convenience, the cathode and anode current efficiencies of the acid copper solution can be regarded as 100%.

1. Electromigration

According to Faraday's law, when the electricity passing through the electroplating bath is 1 Faraday, there will be 1 gram equivalent of substance reacting on the electrode. Let's first look at the anode of the above acid copper solution. When passing 1 Faradaic charge, 1 gram equivalent of metallic copper will be dissolved on the anode, resulting in 1 gram equivalent of Cu2+. And because in the acid copper solution, the ion migration number of Cu2+ is 0.12, it means that after electrification of 1 Faradaic charge, Cu2+ electromigrates out 0.12 g equivalent from the solution in the anode area. In this case, it is equivalent to an increase of 1-0.12=0.88 gram equivalents of Cu2+ in the solution in the anode region. It is also known that the ion migration numbers of H+ and SO42- are 0.58 and 0.30, respectively, which means that after electrifying 1 Faradaic charge, H+ electromigrates out of the anode area by 0.58 g equivalent (equivalent to leaving or adding 0.58 g equivalent in the anode area). SO42-), the electromigration of SO42- from the inside of the solution to the anode area is 0.30 grams equivalent, then the SO42- in the anode area increases by 0.58+0.30=0.88 grams equivalent. In this case, after energizing 1 Faradaic charge, the solution in the anode area is still electrically neutral because the amount of Cu2+ and SO42- also increases by 0.88 g equivalent (equivalent to an increase of 0.88 g equivalent of CuSO4 in the anode area) .

Let's take a look at the cathode of the acid copper solution. When passing 1 Faradaic charge, 1 gram equivalent of metallic copper will be deposited on the cathode, and 1 gram equivalent of Cu2+ will be reduced from the solution in the cathode region. However, 0.12 g equivalent of Cu2+ is electromigrated from the inside of the solution, and the Cu2+ in the cathode region is actually reduced by 1-0.12=0.88 g equivalent. It is also known that 0.58 grams of equivalent H+ (equivalent to 0.58 grams of equivalent SO42-) is electromigrated from the inside of the solution at this time (equivalent to 0.58 grams of equivalent SO42-), and 0.30 grams of equivalent SO42- is electromigrated from the cathode region, then the SO42- in the cathode region is reduced by 0.58+ 0.30=0.88 grams equivalent. In this way, after energizing 1 Faradaic charge, the solution in the cathode area is still electrically neutral because the amount of Cu2+ and SO42- is also reduced by 0.88 g equivalent (equivalent to a reduction of 0.88 g equivalent of CuSO4 in the cathode area) .

It can be seen from the above analysis that after the above-mentioned acid copper solution is electrified with 1 Faradaic power, the anode area increases by 0.88 g equivalent of CuSO4, while at the same time, the cathode area decreases by 0.88 g equivalent of CuSO4. At this point, the entire solution is still electrically neutral. Then, with the prolongation of electroplating time, will there be such a situation: CuSO4 in the anode area becomes more and more and CuSO4 in the cathode area becomes less and less, so that in the end CuSO4 in the cathode area is zero and no copper can be plated? Of course not , because in addition to electromigration, there are two other modes of ion migration in the electroplating process: diffusion and convection. It is precisely because of the two motions of diffusion and convection that the imbalance of substances near the electrode and the interior of the solution caused by electromigration can be rebalanced, thereby maintaining the normal progress of the electroplating process.

2. Diffusion

When there is a difference in concentration of a certain component in the plating solution, the movement of this component from the high-concentration area to the low-concentration area will occur even when the plating solution is completely stationary due to the thermal motion of the molecules. This movement is called diffusion. For example, drop a drop of ink into a glass of still water, and the ink will slowly spread to every corner of the water from where it was dropped, until the water turns into the color of ink. This is a diffusion phenomenon. Diffusion is a phenomenon of material migration due to the presence of a concentration field.

After the above-mentioned acid copper solution is energized, with the continuous increase of the concentration of CuSO4 in the anode area, the concentration difference between CuSO4 in the anode area and CuSO4 in other areas continues to increase. Diffusion to the cathode region, so that the material imbalance between the regions of the solution caused by electromigration can be alleviated. Since the anions and cations diffuse together in pairs, this movement does not cause current transfer.

3. Convection

The so-called convection refers to a phenomenon in which the particles of the solute migrate with the flow of the plating solution, and there is no relative motion between the solute and the plating solution (while there is relative motion between the diffused solute and the plating solution). For example, the air in a closed room contains unclean elements. When the front and rear windows of the room are opened, the circulating air will drive away the air in the original room and the unclean elements it contains. This is a convection phenomenon. Convection is a phenomenon of material migration due to the existence of a velocity field.

The convection in the plating solution is due to the reason of electrification or heating, which makes the concentration and temperature difference in the local area of ​​the plating solution, which causes the solution density to flow naturally, and drives the solute particles to move together. This type of convection is natural convection. When the electrode reacts, the solution turning caused by the precipitation of bubbles is also a kind of natural convection. There is also a kind of convection called forced convection, which refers to the convection generated by artificially flowing the plating solution, such as cathode movement and air stirring in rack plating,

The circulating jet of barrel plating into the drum belongs to forced convection.

During the convection process of the plating solution, some reactants or products of the electrode reaction can be brought over or brought over, thus playing an important role in conveying substances and rebalancing the unevenness of substances between regions of the solution. Because each part of the solution is electrically neutral when energized, this convection will not cause current transmission.

To sum up, when the current passes through the electroplating tank, electromigration causes the imbalance of substances near the electrode and the inside of the solution, and the diffusion and convection slow down this imbalance, thus ensuring the normal progress of the electroplating process.