PCB Horizontal Electroplating Process for High-Precision Circuit Boards (2)

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. The rate of its migration is expressed as follows: u = zeoE/6πrη to. Where u is the ion migration rate, z is the charge number of the ion, eo is the charge of an electron (ie 1.61019C), E is the 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 drop of the potential E, the smaller the viscosity of the electroplating solution, and the faster the ion migration rate.

According to the electrodeposition theory, during electroplating, the printed circuit board on the cathode is a non-ideal polarized electrode, and the copper ions adsorbed on the surface of the cathode gain electrons and are reduced to copper atoms, so that the concentration of copper ions close to the cathode increases reduce. Therefore, a copper ion concentration gradient is formed near the cathode. The layer of the plating solution with the copper ion concentration lower than the concentration of the main plating solution is the diffusion layer of the plating solution. However, the copper ion concentration in the main plating solution is higher, and it will diffuse to the place near the cathode where the copper ion concentration is lower, and continuously replenish 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 number of copper ions, F is the Faraday constant, A is the surface area of ​​the cathode, D is the diffusion coefficient of copper ions (D=KT/6πrη), Cb is the concentration of copper ions in the main bath, and Co is the cathode The concentration of surface copper ions, D is the thickness of the diffusion layer, K is the Portman constant (K=R/N), T is the temperature, r is the radius of copper hydrate ions, 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:

It can be seen from the above formula that the magnitude of the limiting 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 larger. According to the above formula, in order to achieve a higher limit current value, it is necessary to take appropriate technological measures, that is, the use of heating technology. Because increasing the temperature can increase the diffusion coefficient, increasing the convection rate can make it eddy and obtain a thin and uniform diffusion layer. From the above theoretical analysis, increasing the copper ion concentration in the main plating solution, increasing the temperature of the plating solution, and increasing the convection rate can increase the limit diffusion current and achieve the purpose of accelerating the plating rate. The horizontal plating is based on the formation of eddy currents due to the accelerated convection speed of the plating solution, 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.

The key to PCB electroplating is how to ensure the uniformity of the thickness of the copper layer on both sides of the substrate and the inner wall of the via hole. 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 is fast and consistent to obtain a thin and uniform diffusion layer. In order to achieve a thin and uniform diffusion layer, according to the structure of the current horizontal electroplating system, although many nozzles are installed in the system, the plating solution can be quickly and vertically sprayed to the printed board to accelerate the flow of the plating solution in the through holes. The flow rate of the plating solution is very fast, and eddy currents are formed on the upper and lower sides of the substrate and in the through holes, so that the diffusion layer is reduced and more uniform. However, when the plating solution suddenly flows into the narrow through-hole, the plating solution at the entrance of the through-hole will also have a phenomenon of reverse reflow. Coupled with the influence of the primary current distribution, the phenomenon often causes the electroplating of the hole at the entrance, the copper layer is too thick due to the tip effect, and the inner wall of the through hole constitutes a dog-bone-shaped copper plating layer. According to the flow state of the plating solution in the through hole, that is, the size of the eddy current and reflow, and the analysis of the quality of the conductive plated 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. Because the size of the eddy current and the backflow still cannot be known by the method of theoretical calculation, only the measured process method is used. From the measured results, it is known that in order to control the uniformity of the thickness of the through-hole copper electroplating layer, it is necessary to adjust the controllable process parameters according to the aspect ratio of the printed circuit board through-holes, and even choose a high-dispersion copper electroplating solution. , and then adding appropriate additives and improving the power supply mode, that is, using reverse pulse current for electroplating, can obtain a copper coating with high distribution ability.

Especially with the increase in the number of micro-blind holes in laminates, 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 micro-blind holes. The data can be adjusted to correct the controllable parameters, and satisfactory results can be obtained.