PCB Blog - Introduction of PCB board Horizontal Electroplating Technology

I. Overview

With the rapid development of microelectronics technology, the manufacturing of PCB board is developing rapidly in the direction of multi-layer, stack-up, functionalization, and integration. It promotes the design and design of circuit patterns using tiny holes, narrow spacing, and thin wires in printed circuit design, which makes the manufacturing technology of printed circuit boards more difficult, especially since the aspect ratio of through holes in multilayer boards exceeds 5:1 and the product The deep blind holes widely used in the laminate make the conventional vertical electroplating process unable to meet the technical requirements of high-quality and high-reliability interconnection holes. The main reason for this is to analyze the current distribution state from the electroplating principle. During actual electroplating, it is found that the current distribution in the hole presents a waist drum shape, and the current distribution in the hole gradually decreases from the edge of the hole to the center of the hole, resulting in a large amount of copper deposited on the surface and the hole. At the edge of the hole, it is impossible to ensure the standard thickness of the copper layer in the center of the hole where copper is required. Sometimes the copper layer is very thin or there is no copper layer. In severe cases, it will cause irreparable losses, resulting in a large number of multi-layer boards being scrapped. In order to solve the product quality problem in mass production, the problem of deep hole electroplating is currently solved from the aspects of current and additives. Most of the copper electroplating processes for high aspect ratio printed circuit boards are carried out at relatively low current densities with the aid of high-quality additives, moderate air agitation, and cathode movement. The effect of electroplating additives can only be displayed by increasing the electrode reaction control area in the hole. In addition, the movement of the cathode is very beneficial to the improvement of the deep plating ability of the plating solution, and the polarization degree of the plated part increases. The formation speed of the crystal nucleus and the growth speed of the crystal grains compensate for each other, so as to obtain a high-toughness copper layer. However, when the aspect ratio of the through-hole continues to increase or deep blind holes appear, these two process measures become ineffective, thus resulting in horizontal electroplating technology. It is the continuation of the development of vertical electroplating technology, that is, a novel electroplating technology developed on the basis of the vertical electroplating process. The key to this technology is to create a horizontal electroplating system that is compatible with each other so that the plating solution with high dispersibility can be better than the vertical electroplating method with the improvement of the power supply mode and the cooperation of other auxiliary devices.

2. Introduction to the principle of horizontal electroplating

The methods and principles of horizontal electroplating and vertical electroplating are the same, and both must have cathode and anode electrodes. After electrification, an electrode reaction occurs to ionize the main components of the electrolyte, so that the charged positive ions move to the negative phase of the electrode reaction zone; the charged negative ions move toward the electrode. The positive phase shift of the reaction zone then produces a metal deposition coating and outgassing. Because the process of metal deposition at the cathode is divided into three steps: that is, the hydrated ions of the metal diffuse to the cathode; the second step is that the metal hydrated ions are gradually dehydrated and adsorbed on the surface of the cathode when passing through the electric double layer; The first step is that the metal ions adsorbed on the surface of the cathode accept electrons and enter the metal lattice. The actual observation of the working tank is an unobservable out-of-phase electron transfer reaction between the solid-phase electrode and the interface of the liquid-phase plating solution. Its structure can be explained by the principle of the electric double layer in electroplating theory. When the electrode is a cathode and is in a polarized state, cations with positive charges surrounded by water molecules are arranged in an orderly manner at the cathode due to electrostatic force. Nearby, the phase plane formed by the cation center point near the cathode is called the Helmholtz outer layer, and the distance between the outer layer and the electrode is about 1-10 nanometers. But due to the total amount of positive charge carried by the cations in the Helmholtz outer layer, the positive charge is insufficient to neutralize the negative charge on the cathode. The plating solution farther from the cathode is affected by convection, and the concentration of cations in the solution layer is higher than that of anions. This layer is smaller than the Helmholtz outer layer due to the electrostatic force and is also affected by thermal motion. The cation arrangement is not as compact 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 bath. That is, the faster the flow rate of the plating solution, the thinner the diffusion layer, and vice versa. Generally, the thickness of the diffusion layer is about 5-50 microns. It is farther from the cathode, and the plating solution layer reached by convection is called the main plating solution. Because the convection produced by the solution will affect the uniformity of the concentration of the plating solution. The copper ions in the diffusion layer are transported to the outer Helmholtz layer by means of diffusion and ion migration in the plating solution. The copper ions in the main bath are transported to the cathode surface by 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.

The convection of the plating solution is generated by the external and internal mechanical stirring and pump stirring, the swing or rotation of the electrode itself, and the flow of the plating solution caused by the temperature difference. The closer to the surface of the solid electrode, the flow of the electroplating solution becomes slower and slower due to the influence of its frictional resistance, and the convection rate on the surface of the solid electrode at this time is zero. The rate gradient layer formed from the electrode surface to the convective plating solution is called the flow interface layer. The thickness of the flow interface layer is about ten times that of the diffusion layer, so the transport of ions in the diffusion layer is hardly affected by convection. 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 = zeon/6πrη to. Where u is the ion migration rate, z is the charge number of the ion, so 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 a 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 current, z is the charge number of copper ions, F is the Faraday constant, A is the cathode surface area, and D is the diffusion coefficient of copper ions (D=KT/6πrη), Cb is the concentration of copper ions in the main bath, Co is the concentration of copper ions on the cathode surface, D is the thickness of the diffusion layer, and K is the Portman 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:

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, the thickness of the diffusion layer is thin, and 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 onto 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 current distribution, the phenomenon often causes the electroplating of the hole at the entrance. The copper layer is too thick due to the effect, and the inner wall of the through-hole forms a copper plating layer in the shape of a dog bone. 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.

3. Basic structure of horizontal electroplating system

According to the characteristics of horizontal electroplating, it is an electroplating method in which the way of placing the printed circuit board is changed from vertical to parallel plating liquid level. At this time, the printed circuit board is the cathode, and some horizontal electroplating systems use conductive clips and conductive rollers for the supply of current. From the point of view of the convenience of the operating system, it is more common to use the supply method of roller conduction. In addition to being the cathode, the conductive rollers in the horizontal electroplating system also have the function of conveying the printed circuit board. Each conductive roller is equipped with a spring device, and its purpose is to adapt to the needs of electroplating of printed circuit boards (0.10-5.00mm) of different thicknesses. However, during electroplating, the parts in contact with the plating solution may be plated with a copper layer, and the system cannot operate for a long time. Therefore, most of the current horizontal electroplating systems are designed to switch the cathode to an anode and then use a set of auxiliary cathodes to electrolytically dissolve the copper on the plated rollers. For maintenance or replacement purposes, the new plating design also allows for easy removal or replacement of wear-prone areas. The anode is made of an array of insoluble titanium baskets that can be adjusted in size, which are placed on the upper and lower positions of the printed circuit board, and are filled with 25mm diameter spherical, soluble copper with a phosphorus content of 0.004-0.006%, and the distance between the cathode and the anode. is 40mm. The flow of the plating solution is a system composed of a pump and a nozzle, which makes the plating solution flow rapidly in the closed plating tank, alternately back and forth, and up and down, and can ensure the uniformity of the plating solution flow. The plating solution is sprayed vertically on the printed circuit board, forming a wall jet eddy current on the surface of the printed circuit board. The ultimate goal is to achieve the rapid flow of the plating solution on both sides of the printed circuit board and through holes to form eddy currents. In addition, a filter system is installed in the tank, and the filter mesh used is 1.2 microns to filter out the particulate impurities generated during the electroplating process to ensure that the plating solution is clean and pollution-free.

In the manufacture of horizontal electroplating systems, the ease of operation and automatic control of process parameters should also be considered. Because in the actual electroplating, with the size of the printed circuit board, the size of the through-hole diameter and the required copper thickness, the transmission speed, the distance between the printed circuit boards, the size of the pump horsepower, the nozzle The setting of process parameters such as the direction of the copper and the current density, etc., all need to be tested, adjusted and controlled in order to obtain the thickness of the copper layer that meets the technical requirements. It must be controlled by a computer. In order to improve the production efficiency and the consistency and reliability of the quality of the sub-products, the pre-and post-processing of the through-holes (including the plated-through holes) of the printed circuit board are based on the process procedures to form a complete horizontal electroplating system, which is suitable for the development and launch of new products. need.

4. The development advantages of horizontal electroplating

The development of horizontal electroplating technology is not accidental, but an inevitable result of the need for special functions of high-density, high-precision, multi-functional, high-aspect-ratio multilayer printed circuit board products. Its advantage is that it is more advanced than the currently used vertical rack plating process, the product quality is more reliable, and it can achieve large-scale production. It has the following advantages compared to the vertical plating process method:

1) It can adapt to a wide range of sizes, without manual installation and hanging, and realize all automatic operations, which is extremely beneficial to improve and ensure that the operation process does not damage the surface of the substrate, and is extremely beneficial to large-scale production.

2) In the process review, there is no need to leave a clamping position, increase the practical area, and greatly save the loss of raw materials.

3) The whole process of horizontal electroplating is controlled by a computer, so that the substrate is under the same conditions to ensure the uniformity of the plating on the surface and holes of each printed circuit board.

4) From the perspective of management, the cleaning of the plating tank, and the addition, and replacement of the plating solution, can be fully automated, and the management will not be out of control due to human errors.

5) It can be known from the actual product that the horizontal electroplating adopts multi-stage horizontal cleaning, which greatly saves the amount of cleaning water and reduces the pressure of sewage treatment.

6) Because the system adopts closed operation, it reduces the pollution of the working space and the direct influence of the evaporation of heat on the process environment and greatly improves the working environment. Especially when baking the plate, because the heat loss is reduced, the unnecessary consumption of energy is saved and the production efficiency is greatly improved.

5. Summary

The emergence of horizontal plating technology is entire to meet the needs of high aspect ratio through-hole plating. However, due to the complexity and particularity of the electroplating process, there are still some technical problems in the design and development of the electroplating system. This needs to be improved in practice. Nonetheless, the use of horizontal electroplating systems is a great development and advancement for the printed circuit industry. Because the application of this type of equipment in the manufacture of high-density multilayer boards shows great potential, it can not only save manpower and operating time, but also produce faster and more efficient than traditional vertical electroplating lines. Moreover, the energy consumption is reduced, the waste liquid, wastewater, and waste gas to be treated are reduced, the process environment and conditions are greatly improved, and the quality level of the electroplating layer is improved. The horizontal electroplating line is suitable for 24-hour uninterrupted operation of large-scale production. The horizontal electroplating line is slightly more difficult to debug than the vertical electroplating line. Once the debugging is completed, it is very stable. The plating solution is adjusted to ensure stable operation for a long time on PCB board.