1. Overview

With the rapid development of microelectronics technology, printed circuit board manufacturing has developed rapidly in the direction of multi-layer, multi-layer, functionalization and integration. Promote printed circuit design using a large number of tiny holes, narrow pitches, and thin wires for the conception and design of circuit patterns, making printed circuit board manufacturing technology more difficult, especially the aspect ratio of multi-layer board through holes exceeding 5: 1 and the product The large number of deep blind holes used in the laminate makes the conventional vertical plating process unable to meet the technical requirements of high-quality, high-reliability interconnect holes. The main reason is to analyze the current distribution state from the principle of electroplating. Through actual plating, it is found that the current distribution in the hole is waist-shaped, and the current distribution in the hole gradually decreases from the edge of the hole to the center of the hole, causing a large amount of copper to be deposited on the surface and At the edge of the hole, the standard thickness of the copper layer at the center of the hole where copper is required cannot be guaranteed. Sometimes the copper layer is very thin or no copper layer. In severe cases, it will cause irreparable losses and cause a large number of multilayer boards to be scrapped. In order to solve the problem of product quality in mass production, the current and additives are used to solve the problem of deep hole plating. In high-aspect ratio printed circuit board copper electroplating processes, most of them are carried out under the condition of relatively low current density with the aid of high-quality additives, with appropriate air stirring and cathode movement. The electrode reaction control area in the hole can be enlarged so that the effect of the plating additives can be displayed. In addition, the cathode movement is very conducive to the improvement of the deep plating ability of the plating solution, the polarization of the plated part is increased, and the electrocrystallization process of the coating layer is increased. The formation rate of the crystal nuclei and the growth rate of the grains are mutually compensated, thereby obtaining a high-toughness copper layer.
However, when the aspect ratio of the through-holes continues to increase or deep blind holes appear, these two process measures become weak, so horizontal electroplating technology is produced. It is a continuation of the development of vertical plating technology, that is, a new plating technology developed on the basis of vertical plating technology. The key to this technology is to produce a suitable and compatible horizontal electroplating system, which can make the plating solution with high dispersing ability, with the improvement of the power supply method and other auxiliary devices, it shows better than the vertical electroplating method. Functional role.

2. Introduction to the principle of horizontal plating

The method and principle of horizontal electroplating and vertical electroplating are the same, both of which must have yin and yang poles. After the current is applied, the electrode reaction will cause the main components of the electrolyte to ionize, so that the charged positive ions will move to the negative phase of the electrode reaction zone; The normal phase of the reaction zone moves, which results in metal deposition and outgassing. Because the process of metal deposition on the cathode is divided into three steps: the metal hydrated ions diffuse to the cathode; the second step is that the metal hydrated ions are gradually dehydrated and adsorbed on the surface of the cathode as they pass through the electric double layer; The step is that metal ions adsorbed on the cathode surface accept electrons and enter the metal lattice. From the actual observation of the working tank, an unobservable heterogeneous electron transfer reaction between the interface of the solid-phase electrode and the liquid-phase plating solution is not observed. The structure can be explained by the principle of electric double layer in electroplating theory. When the electrode is a cathode and is in a polarized state, it is surrounded by water molecules and has positively charged cations, which are arranged in an orderly manner on the cathode due to electrostatic forces. Nearby, the phase surface composed of the cation center point closest to the cathode is called the outer layer of Helmholtz, and the distance between the outer layer and the electrode is about 1-10 nm. However, due to the total positive charge of the cations in Helmholtz’s outer layer, its positive charge is not enough to neutralize the negative charge on the cathode. The plating solution far from the cathode is affected by convection, and the cation concentration of the solution layer is higher than the anion concentration. This layer is smaller than the outer layer of Helmholtz due to electrostatic force and is affected by thermal movement. The cation arrangement is not as tight and neat as the outer layer of Helmholtz. This layer is called a 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 the diffusion layer is, and the thicker it is. 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 effect of 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 layer of Helmholtz by the plating solution by diffusion and ion migration. However, the copper ions in the main plating solution are transported to the cathode surface by convection and ion migration. In the process of horizontal electroplating, 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 mechanical stirring and pump stirring from the outside and inside, 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 more the flow of the plating solution becomes slower and slower due to the influence of its frictional resistance. The rate gradient layer formed from the electrode surface to the convection bath is called the flow interface layer. The thickness of the flow interface layer is about ten times the thickness of the diffusion layer, so the transport of ions in the diffusion layer is hardly affected by convection.Under the action of galvanic ion, ion transport in electroplating solution is called ion migration due to electrostatic force. The migration rate is expressed by the formula as follows: u = zeoE / 6πrη. Where u is the ion migration rate, Y is the charge number of the ions, eo is the charge amount of one electron (ie, 1.61019C), E is the potential, r is the radius of the hydrated ion, and η is the viscosity of the plating solution. According to the calculation of the equation, it can be seen that the larger the potential E drops, the smaller the viscosity of the plating 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. The copper ions adsorbed on the surface of the cathode get electrons and are reduced to copper atoms, so that the concentration of copper ions near the cathode. reduce. Therefore, a copper ion concentration gradient is formed near the cathode. The plating solution having a copper ion concentration lower than that of the main plating solution is a diffusion layer of the plating solution. However, the copper ion concentration in the main plating solution is high, and it will diffuse to the place near the cathode where the copper ion concentration is low, constantly replenishing the cathode area. The printed circuit board is similar to a planar cathode, and the relationship between the magnitude of the current and the thickness of the diffusion layer is the COTTELEL equation:

Where I is the current, Y is the charge number 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 Botman constant (K = R / N), T is the temperature, r is the radius of the copper hydrate ion, and η is the viscosity of the plating solution. When the copper ion concentration on the cathode surface is zero, its current is called the limit diffusion current ii:As can be seen from the above formula, the magnitude of the limiting diffusion current depends on the copper ion concentration of the main plating solution, the diffusion coefficient of the 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 the copper ions is large, and the thickness of the diffusion layer is thin, the limit diffusion current becomes larger. According to the above formula, it is known that in order to reach a higher limit current value, appropriate process measures must be taken, that is, a heating process method. 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, thereby achieving the purpose of accelerating the plating rate. Horizontal plating 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 plating system is used for electroplating, its current density can be as high as 8A / dm2.

The key to electroplating of printed circuit boards 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. To obtain the uniformity of the coating thickness, it is necessary to ensure that the flow rate of the plating solution in both sides of the printed board and 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, in view of the current structure of the horizontal plating system, although many nozzles are installed in the system, the plating solution can be sprayed quickly and vertically onto the printed board to accelerate the flow of the plating solution in the through holes. The speed causes the flow rate of the plating solution to be very fast, and eddy currents are formed on the upper and lower surfaces of the substrate and in the through holes, so that the diffusion layer is reduced and more uniform. However, usually when the plating solution suddenly flows into the narrow through hole, the reverse of the plating solution at the entrance of the through hole will also occur. In addition, the effect of the current distribution once will often cause the plating of the hole at the entrance. Due to the tip effect, the thickness of the copper layer is too thick, and the inner wall of the through hole constitutes a copper plating in the shape of a dog bone. According to the state of the plating solution flowing in the through-holes, that is, the size of the eddy current and reflow, and the state analysis of the quality of the conductive plating through-holes, the process parameters can only be used to determine the control parameters to achieve the uniformity of the plating thickness of the printed circuit board. Because the size of the eddy current and recirculation is still not known through theoretical calculation methods, only the measured process method is used. It is known from the measured results that to control the uniformity of the thickness of the plated through-hole copper plating, we must adjust the controllable process parameters according to the aspect ratio of the printed circuit board through-holes, and even choose a high-dispersion copper plating solution. Then, add appropriate additives and improve the power supply mode, that is, use the reverse pulse current for electroplating to obtain a copper coating with high distribution ability.

In particular, the number of micro-blind holes in the multilayer board increases. Not only should the horizontal plating system be used for electroplating, but also ultrasonic vibration to promote the replacement and circulation of the plating solution in the micro-blind holes, and then improve the power supply method by using reverse pulse current and actual testing. Data to adjust the controllable parameters, you can get satisfactory results.

3. The basic structure of the horizontal plating system

According to the characteristics of horizontal plating, it is a method of placing printed circuit boards from a vertical type to a parallel plating liquid surface. At this time, the printed circuit board is the cathode, and the horizontal plating system of the current supply method uses two kinds of conductive clips and conductive rollers. From the convenience of the operating system, the use of roller conductive supply is more common. In addition to being a cathode, the conductive roller in the horizontal plating system also has the function of conveying printed circuit boards. Each conductive roller is equipped with a spring device, the purpose of which can meet the needs of electroplating of printed circuit boards (0.1 to 5,000 mm) of different thicknesses. However, the parts that come into contact with the plating solution during electroplating may be coated with copper, and the system will not work for a long time. Therefore, most of the horizontal electroplating systems currently manufactured have cathodes designed to be switchable to anodes, and then a set of auxiliary cathodes can be used to dissolve the copper on the plating rollers. For maintenance or replacement, the new plating design also takes into account the vulnerable parts for easy removal or replacement. The anode uses an array of insoluble titanium baskets that can be adjusted in size. It is placed above and below the printed circuit board. It contains 25mm-diameter spheres with a phosphorus content of 0.004-0.06% soluble copper, cathode and anode. The distance between them is 40mm.The flow of the plating solution is a system consisting of a pump and a nozzle, which enables the plating solution to flow rapidly in the closed plating tank back and forth, up and down, and to ensure the uniformity of the plating solution flow. The plating solution is sprayed vertically onto the printed circuit board, and a punched wall jet vortex is formed on the printed circuit board surface. The ultimate purpose is to achieve rapid flow of the plating solution on both sides of the printed circuit board and through holes to form eddy currents. In addition, a filtering system is installed in the tank, and the filter used therein has a mesh size of 1.2 microns to filter out particulate impurities generated during the plating process to ensure that the plating solution is clean and free of pollution.

When manufacturing a horizontal electroplating system, also consider the convenience of operation and automatic control of process parameters. Because in actual electroplating, with the size of the printed circuit board, the size of the hole diameter of the through hole and the required copper thickness, the transfer 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 and current density requires actual testing, adjustment, and control in order to obtain a copper layer thickness that meets technical requirements. It must be controlled by a computer. In order to improve production efficiency and consistency and reliability of high-end product quality, through-hole pre- and post-processing (including plated holes) of printed circuit boards are constructed according to process procedures to form a complete horizontal electroplating system to meet new product development and market launch.

4. The development advantages of horizontal plating

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

(1) A wide range of applicable sizes, without manual mounting and mounting, and realizing all automated operations. It is extremely beneficial to improve and ensure that the operation process does not damage the surface of the substrate and achieve large-scale 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) Horizontal electroplating adopts full computer control to make the substrate under the same conditions to ensure the uniformity of the surface of each printed circuit board and the plating layer of the hole.

(4) From a management point of view, the plating tank can be completely automated from cleaning, addition and replacement of the plating solution, and it will not cause management runaway problems due to human error.

(5) It can be measured from actual production that the horizontal electroplating adopts multiple stages of 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 direct impact of pollution on the working space and heat evaporation on the process environment, and greatly improves the working environment. Especially when baking, the loss of heat is reduced, the unnecessary consumption of energy is saved and the production efficiency is greatly improved.

5. Summary

The emergence of horizontal electroplating technology is completely to meet the needs of high aspect ratio through-hole electroplating. However, due to the complexity and particularity of the electroplating process, there are still several technical problems in the design and development of horizontal electroplating systems. This needs to be improved in practice. Nevertheless, the use of horizontal electroplating systems is a great development and progress for the printed circuit industry. Because this type of equipment shows great potential in the manufacture of high-density multilayer boards, it can not only save manpower and operating time, but also produce faster and more efficiently than traditional vertical plating lines. In addition, it reduces energy consumption, waste liquid and waste gas to be treated, and greatly improves the process environment and conditions, and improves the quality level of the plating layer. The horizontal plating line is suitable for large-scale production for 24 hours of uninterrupted operation. The horizontal plating line is slightly more difficult than the vertical plating line during commissioning. Once the commissioning is completed, it is very stable. At the same time, it is necessary to monitor the plating solution at any time during use The bath is adjusted to ensure long-term stable operation.