PCB Design and Manufacturing Meet Horizontal Plating

With the rapid development of microelectronics technology, high-speed PCB design has also developed rapidly and steadily, and PCB manufacturing is also developing in the direction of multilayer, layered, functional and integrated, making PCB manufacturing technology more difficult, especially The aspect ratio of the through hole of the multilayer board exceeds 5:1 and the deep blind hole used in a large number of laminated boards makes the conventional vertical electroplating process unable to meet the technical requirements of high quality and high reliability interconnection holes, thus resulting in horizontal Electroplating technology.

Horizontal electroplating technology is a continuation of the development of vertical electroplating technology, that is, a novel electroplating technology developed on the basis of vertical electroplating technology. Its key is to manufacture a suitable horizontal electroplating system, which can make the plating solution with high dispersibility, and with the cooperation of improving the power supply mode and other auxiliary devices, show more excellent functional effects than the vertical electroplating method.

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 PCB products is an inevitable result. Its advantage is that it is more advanced than the current vertical rack plating process, the product quality is more reliable, and it can realize large-scale production. It has the following advantages over the vertical plating process method:

1. It is suitable for a wide range of sizes, no manual installation is required, and all automatic operations are realized, which is extremely beneficial to improving and ensuring that the operation process does not damage the substrate surface, and is extremely beneficial to the realization of large-scale production.

2. In the process review, there is no need to reserve a clamping position, which increases the practical area and greatly saves the loss of raw materials.

3. The horizontal electroplating adopts the whole process of computer control, so that the substrate is under the same conditions to ensure the uniformity of the coating on the surface and holes of each PCB.

4. From a management point of view, the cleaning of the electroplating tank, the addition and replacement of the electroplating solution, can be fully automated, and the management will not be out of control due to human errors.

5. It is known from actual production that since horizontal electroplating adopts multi-stage horizontal cleaning, it can greatly save the amount of cleaning water and reduce the pressure of sewage treatment.

6. Since the system adopts closed operation, it reduces the pollution of the working space and the direct impact of heat evaporation on the process environment, greatly improving the working environment. Especially when the plate is baked, due to the reduction of heat loss, the unnecessary consumption of energy is saved and the production efficiency is greatly improved.

The method and principle of horizontal electroplating and vertical electroplating are the same, and both must have negative and positive poles. After electrification, an electrode reaction occurs to ionize the main components of the electrolyte, so that charged positive ions move to the negative phase of the electrode reaction area; charged negative ions move to the electrode. The positive phase of the reaction zone moves, thus producing metal deposition coatings and gas evolution.

Because the process of metal deposition on the cathode is divided into three steps: that is, the hydration ions of the metal diffuse to the cathode; the second step is that the metal hydration ions are gradually dehydrated and adsorbed on the surface of the cathode when they pass 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. From the actual observation of the working tank, it is an unobservable heterogeneous electron transfer reaction between the solid-phase electrode and the liquid-phase plating solution interface.

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, positively charged cations surrounded by water molecules are arranged in an orderly manner on the cathode due to electrostatic force. Nearby, the phase surface formed by the cation center point closest to the cathode is called the outer layer of Helmholtz, and the distance from the outer layer to 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 amount of positive charge is not enough 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 movement. The cation arrangement 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 bath. That is to say, the faster the flow rate of the plating solution, the thinner the diffusion layer, and vice versa, the thickness of the general diffusion layer is about 5-50 microns. It is farther away from the cathode, and the plating solution layer reached by convection is called the main plating solution.

Because the convection generated 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 layer of Helmholtz by the diffusion of the plating solution and the migration of ions. The copper ions in the main bath are transported to the surface of the cathode 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 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, due to the influence of its frictional resistance, the flow of the electroplating solution becomes slower and slower, and the convection velocity on the surface of the solid electrode at this time is zero. The velocity gradient layer formed between the electrode surface and the convective bath 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 an 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 by the formula as follows: u=zeoE/6πrη. Among them, u is the ion migration rate, z is the charge number of the ion, eo is the charge amount 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 plating solution. According to the calculation of the equation, it can be seen that the greater the potential E drop, the smaller the viscosity of the electroplating solution and the faster the ion migration rate.

According to the electrodeposition theory, during electroplating, the PCB located on the cathode is a non-ideal polarized electrode, and the copper ions adsorbed on the surface of the cathode obtain 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 layer of plating solution whose copper ion concentration is lower than that of the main plating solution is the diffusion layer of the plating solution. However, the concentration of copper ions in the main plating solution is high, and it will diffuse to the place near the cathode where the concentration of copper ions is low, and continuously replenish the cathode area. The PCB 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 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 Bosman 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 limit diffusion current depends on the concentration of copper ions in 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, it is known that in order to achieve a higher limiting current value, appropriate technological measures must be taken, that is, the heating method is 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 velocity of the plating solution to form an eddy current, 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. In order to obtain the uniformity of the thickness of the coating, it is necessary to ensure that the flow rate of the plating solution on both sides of the printed board and in the through hole is fast and consistent, so as to obtain a thin and uniform diffusion layer. To achieve a thin and uniform diffusion layer, as far as the structure of the current horizontal electroplating system is concerned, 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 hole. 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 plating solution at the entrance of the through hole will also have a phenomenon of reverse flow, coupled with the influence of the primary current distribution, it often causes the hole at the entrance to be electroplated. , due to the tip effect, the thickness of the copper layer is too thick, 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 eddy current and reflow, and the state 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 PCB plating thickness.

Because the size of the eddy current and backflow is still not known through theoretical calculation methods, so only the actual measurement process method is used. From the measured results, it is known that to control the uniformity of the thickness of the through-hole copper plating layer, it is necessary to adjust the controllable process parameters according to the aspect ratio of the PCB through-hole, and even choose a high-dispersion copper plating solution, and then add Appropriate additives and improved power supply methods, that is, the use of reverse pulse current for electroplating, can obtain copper coatings with high distribution capabilities.

Especially the increase in the number of micro-blind holes in laminated boards requires not only the use of a horizontal electroplating system for electroplating, but also the use of 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 counter-pulse current and actual test results. The data can be adjusted to adjust the controllable parameters, and satisfactory results can be obtained.

According to the characteristics of horizontal electroplating, it is an electroplating method that changes the way of placing the PCB from vertical to parallel to the surface of the plating solution. At this time, the PCB is the cathode, and some horizontal electroplating systems use conductive clips and conductive rollers as the current supply method. From the convenience of the operating system, it is more common to use the supply method of roller conduction. In addition to being a cathode, the conductive roller in the horizontal electroplating system also has the function of conveying PCB. Each conductive roller is equipped with a spring device, which is designed to meet the needs of electroplating PCBs (0.40-5.00mm) of different thicknesses.

However, during electroplating, the parts in contact with the plating solution may be plated with a copper layer, and the system will not work for a long time. Therefore, most of the current horizontal electroplating systems are designed such that the cathode can be switched to an anode, and then a group of auxiliary cathodes can be used to electrolytically dissolve the copper on the plated rollers. For maintenance or replacement, the new electroplating design also takes into account the easy-to-wear parts for easy removal or replacement. The anode adopts an array of insoluble titanium baskets that can be adjusted in size, and is placed on the upper and lower positions of the PCB. It is filled with spherical copper with a diameter of 25mm and a phosphorus content of 0.004-0.006%. The distance between the cathode and the anode is 40mm.

The flow of the plating solution is a system composed of pumps and nozzles, so that the plating solution flows back and forth, up and down alternately and rapidly in the closed plating tank, and can ensure the uniformity of the plating solution flow. The plating solution is sprayed vertically to the PCB, forming a jet vortex on the surface of the PCB. The ultimate goal is to achieve rapid flow of plating solution on both sides of the PCB and through holes to form an eddy current. In addition, the tank is equipped with a filter system, which uses a filter with a mesh size of 1.2 microns to filter out the granular impurities produced during the electroplating process to ensure that the plating solution is clean and pollution-free.

When manufacturing a horizontal electroplating system, it is also necessary to consider the convenience of operation and the automatic control of process parameters. Because in actual electroplating, with the size of the PCB, the size of the through-hole aperture and the required copper thickness, the transmission speed, the distance between the PCBs, the size of the pump horsepower, the direction of the nozzle and the current density The setting of process parameters such as high and low requires actual testing, adjustment and control to obtain the copper layer thickness that meets the technical requirements. It must be controlled by computer. In order to improve the production efficiency and the consistency and reliability of high-end product quality, the pre- and post-processing of PCB through-holes (including plated-through holes) according to the process procedures to form a complete horizontal electroplating system is to meet the needs of new product development and launch.

The above is the knowledge about horizontal electroplating. When high-speed PCB design and manufacturing meet horizontal electroplating, more needs will be met.