The Principle Of Horizontal Electroplating Is Fully Explained in One Article!

With the advancement of microelectronics technology, printed circuit board manufacturing is developing rapidly in the direction of multilayer, layered, functional and integrated. The traditional vertical electroplating process can no longer meet the requirements of high-quality and high-reliability interconnection holes. Technical requirements. Therefore, horizontal electroplating technology came into being. It is a continuation of the development of vertical electroplating technology, that is, a new electroplating technology developed on the basis of vertical electroplating technology. Today, we will introduce the principle of horizontal electroplating!

Principle of Horizontal Plating

The method and principle of horizontal electroplating and vertical electroplating are the same. They must have yin and yang poles. Electrode reaction occurs after electrification, which ionizes the main components of the electrolyte, causing the charged positive ions to move to the negative phase of the electrode reaction zone; the charged negative ions move to the positive phase of the electrode reaction zone, resulting in metal deposition coating and gas emission. Because the metal deposition process on the cathode is divided into three steps: the metal hydrated ions diffuse to the cathode; the second step is when the metal hydrated ions pass through the electric double layer, they are gradually dehydrated and adsorbed on the surface of the cathode; the third step is to adsorb on the surface of the cathode. The metal ions on the surface of the cathode accept electrons and enter the metal lattice. Due to static electricity, this layer is smaller than the Helmholtz outer layer and is affected by thermal movement. The arrangement of cations is not as tight and neat as the Helmholtz outer layer. This layer is called the diffusion layer. The thickness of the diffusion layer is inversely proportional to the flow rate of the plating solution. That is, the faster the flow rate of the plating solution, the thinner and thicker the diffusion layer. Generally, the thickness of the diffusion layer is about 5-50 microns. In the place far away from the cathode, the plating solution reached by convection is called the main plating solution. Because the convection of the solution will affect the uniformity of the plating solution concentration. The copper ions in the diffusion layer are transported to the Helmholtz outer layer through diffusion and ion migration. The copper ions in the main plating solution are transported to the cathode surface through convection and ion migration. In the horizontal electroplating process, the copper ions in the plating solution are transported to the vicinity of the cathode in three ways to form an electric double layer.

Under the action of the electric field, the ions in the electroplating solution are subjected to electrostatic force to cause ion transport, which is called ion migration. Its migration rate is expressed by the formula as follows: u=zeoE/6πrη required. Where u is the ion migration rate, z is the charge number of the ion, eo is the charge of one electron (ie 1.61019C), E is the electric potential, r is the radius of the hydrated ion, and η is the viscosity of the electroplating solution. According to the calculation of the equation, it can be seen that the greater the potential E drop, the lower the viscosity of the electroplating solution, and the faster the ion migration rate.

The convection of the plating solution is caused by external and internal mechanical stirring and pump stirring, the oscillation or rotation of the electrode itself, and the flow of the plating solution caused by the temperature difference. At a position close to the surface of the solid electrode, due to its frictional resistance, the flow of the electroplating solution becomes slower and slower, and the convection velocity on the surface of the solid electrode is zero. The velocity gradient layer formed from the electrode surface to the convection groove is called the flow interface layer. The thickness of the flow interface layer is about 10 times that of the diffusion layer, so the ion transport in the diffusion layer is hardly affected by convection.

According to the theory of electrodeposition, during the electroplating process, the printed circuit board on the cathode is a non-ideal polarized electrode. The copper ions adsorbed on the surface of the cathode gain electrons and are reduced to copper atoms, which reduces the concentration of copper ions near the cathode. Therefore, a copper ion concentration gradient is formed near the cathode. The plating solution whose copper ion concentration is lower than that of the main plating solution is the diffusion layer of the plating solution. The high copper ion concentration in the main plating solution will diffuse to the low copper ion concentration near the cathode, constantly replenishing the cathode area. The printed circuit board is similar to a flat cathode, and the relationship between the magnitude of the current and the thickness of the diffusion layer is the COTTRELL equation:

Where I is the current, z is the charge of copper ions, F is the Faraday constant, A is the cathode surface area, D is the copper ion diffusion coefficient (D=KT/6πrη), Cb is the copper ion concentration in the main plating solution, and Co is the cathode The concentration of copper ions on the surface, D is the thickness of the diffusion layer, K is the Bowman constant (K=R/N), T is the temperature, r is the radius of the copper hydrate ion, and η is the viscosity of the electroplating solution. When the copper ion concentration on the cathode surface is zero, its current is called the limiting diffusion current ii:

Principle of Horizontal Plating

The key to PCB electroplating is how to ensure the uniformity of the copper layer thickness on both sides of the substrate and the inner wall of the through hole. In order to obtain the uniformity of the coating thickness, it is necessary to ensure that the flow rate of the plating solution on both sides of the printed board and in the through holes should be fast and consistent to obtain a thin and uniform diffusion layer. In order to obtain a thin and uniform diffusion layer, according to the current horizontal electroplating system structure, although many nozzles are installed in the system, it can quickly and vertically spray the plating solution onto the printed board, thereby speeding up the plating solution in the through hole Therefore, the flow rate of the plating solution is very fast, and a vortex is formed on the upper and lower parts of the substrate and the through hole, so that the diffusion layer is reduced and more uniform. However, under normal circumstances, when the plating solution suddenly flows into a narrow through hole, the plating solution at the entrance of the through hole will also reverse backflow. In addition, due to the influence of the primary current distribution and the tip effect, the thickness of the copper layer at the entrance hole is too thick, and the inner wall of the through hole forms a dog-bone copper coating. According to the flow state of the plating solution in the through hole, that is, the size of eddy current and reflow, and the state analysis of the quality of the conductive plating through hole, the control parameters can only be determined by the process test method to achieve the uniformity of the plating thickness of the printed circuit board. Since the magnitude of eddy current and backflow cannot be calculated theoretically, only the method of measurement process can be used. It can be seen from the measurement results that in order to control the uniformity of the copper plating thickness of the through-holes, it is necessary to adjust the controllable process parameters according to the aspect ratio of the through-holes of the printed circuit board. The power supply method is reverse pulse current electroplating to obtain copper plating with strong distribution ability.

It can be seen from the above formula that the limit diffusion current is determined by the copper ion concentration of the main plating solution, 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, the limiting diffusion current is greater. According to the above formula, it is known that in order to achieve a higher limit current value, appropriate process measures must be taken, that is, the heating process must be adopted. Because increasing the temperature can increase the diffusion coefficient, increasing the convection rate can make it a vortex and obtain a thin and uniform diffusion layer. From the above theoretical analysis, increasing the concentration of copper ions in the main plating solution, increasing the temperature of the plating solution, and increasing the convection rate can all increase the limiting diffusion current and achieve the purpose of accelerating the plating rate. Horizontal electroplating is based on the acceleration of the convection speed of the plating solution to form eddy currents, which can effectively reduce the thickness of the diffusion layer to about 10 microns. Therefore, when the horizontal electroplating system is used for electroplating, the current density can be as high as 8A/dm2.

Especially with the increase in the number of blind holes in the laminate, not only the horizontal electroplating system should be used for electroplating, but also ultrasonic vibration should be used to promote the replacement and circulation of the plating solution in the blind holes, and then the power supply method should be improved and reverse pulse current should be used. Adjust controllable parameters with actual test data.

Horizontal electroplating is an electroplating method developed based on vertical electroplating. From a certain point of view, it is the perfection and extension of vertical electroplating. Therefore, it is very important to understand the principle of horizontal electroplating. I hope this article can provide you with some help!