PCB Horizontal Electroplating Process for High-Precision Circuit Boards (1)

With the rapid development of microelectronics technology, the manufacturing of printed circuit boards is developing in the direction of multi-layer, lamination, functionalization and integration, which makes the manufacturing technology of printed circuit boards more difficult, and the conventional vertical electroplating process cannot meet the requirements of high technical requirements for high-quality, high-reliability interconnect holes that resulted in horizontal electroplating technology. This paper analyzes and evaluates the horizontal electroplating technology from the principle of horizontal electroplating, the basic structure of the horizontal electroplating system, and the development advantages of the horizontal electroplating, and points out that the use of the horizontal electroplating system is a great development and progress for the printed circuit industry.

Overview

With the rapid development of microelectronics technology, the manufacturing of printed circuit boards is developing rapidly in the direction of multi-layer, stack-up, functionalization and integration. It promotes the 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. In particular, the aspect ratio of through holes in multilayer boards exceeds 5:1 and the deep blind holes widely used in buildup boards make the conventional vertical electroplating process unable to meet the technical requirements of high-quality and high-reliability interconnect 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 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 the 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 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.

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 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 surface formed by the cation center point closest to 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, 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.