The hottest PCB horizontal electroplating technolo

2022-08-11
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Horizontal electroplating technology of printed circuit board (Part 1)

I. overview

with the rapid development of microelectronics technology, the manufacturing of printed circuit board is developing rapidly in the direction of multilayer, lamination, functionalization and integration. Promote the design of printed circuits to adopt a large number of tiny holes, narrow spacing, and thin wires for the conception and design of circuit graphics, which makes the manufacturing technology of printed circuit boards more difficult, especially the aspect ratio of through holes in multilayer boards exceeds 5:1 and the deep blind holes widely used in laminated boards, so that the conventional vertical electroplating process can not meet the technical requirements of high-quality and high reliability interconnection holes. The main reason needs to be analyzed from the electroplating principle about the current distribution state. Through the 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 deposition on the surface and the edge of the hole. The Heilongjiang provincial government is determined that it is unable to ensure the standard thickness of the copper layer in the center of the hole, and sometimes the copper layer is extremely thin or free of copper layer, In serious cases, it will cause irreparable losses, resulting in a large number of multi-layer board scrapping. In order to solve the product quality problems in mass production, deep hole electroplating problems are currently solved from the aspects of current and additives. In the copper electroplating process of high aspect ratio printed circuit boards, most of them are carried out under relatively low current density with the assistance of high-quality additives, combined with appropriate air stirring and cathode movement. When the electrode reaction control area in the hole is enlarged, the effect of electroplating additives can be shown. 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 parts is increased, and the formation speed of crystal nuclei and the growth speed of grains are mutually compensated during the electrodeposition process of the coating, so as to obtain a high toughness copper layer

however, when the aspect ratio of through holes continues to increase or deep blind holes appear, these two process measures appear weak, resulting in horizontal electroplating technology. It is the continuation of the development of vertical electroplating technology, that is, a new electroplating technology developed on the basis of vertical electroplating technology. The key of this technology is to produce a suitable and mutually supporting horizontal electroplating system, which can make the plating solution with high dispersion ability show better function than the vertical electroplating method under the cooperation of improved power supply mode and other auxiliary devices

II. Brief analysis of the principle of horizontal electroplating

the method and principle of horizontal electroplating and vertical electroplating are the same. Both of them must have positive and negative poles. After being electrified, electrode reaction will occur to ionize the main components of the electrolyte and move the charged positive ions to the negative phase of the electrode reaction area; Charged negative ions move to the positive phase of the electrode reaction zone, resulting in metal deposition coating and gas emission. Because the process of metal deposition on the cathode is divided into three steps: that is, the hydrated ions of the metal diffuse to the cathode; The second step is that when the metal hydration ions pass through the electric double layer, they can be dehydrated and adsorbed on the surface of the cathode according to the customer's special requirements; The third step is that the metal ions adsorbed on the cathode surface accept electrons and enter the metal lattice. It is actually observed that the working tank is an unobservable heterogeneous electron transfer reaction between the solid-phase electrode and the interface of liquid-phase electroplating solution. Its structure can be explained by the electric double layer principle in electroplating theory. When the electrode is a cathode and in a polarized state, the cations surrounded by water molecules and positively charged are orderly arranged near the cathode due to electrostatic force. The phase setting surface formed by the cation center closest to the cathode is called the Helmholtz outer layer, which is about 1-10 nm away from the electrode. However, due to the total amount of positive charge carried by the cation in the outer layer of Helmholtz, the amount of positive charge is not enough to neutralize the negative charge on the cathode. The cation concentration of the solution layer is higher than that of the anion concentration of the solution far away from the cathode due to the influence of convection. This layer is smaller than the Helmholtz outer layer due to electrostatic force and is also affected by thermal motion. 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 plating solution. That is, the faster the flow rate of the plating solution is, the thinner the diffusion layer is, on the contrary, it is thicker. Generally, the thickness of the diffusion layer is about 5-50 microns. Farther away from the cathode, 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 layer of Helmholtz 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 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 caused 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. At the place closer to the surface of the solid electrode, due to the influence of its friction resistance, the flow of the electroplating solution becomes slower and slower, and the convection rate on the surface of the solid electrode is zero at this time. 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 ion transport in the diffusion layer is hardly affected by convection

under the action of electric field, the ions in the electroplating solution are subjected to electrostatic force and cause ion transport, which is called ion migration. The migration rate is expressed as follows:

u = ze0e/6p R h

where u is the ion migration rate, Z is the charge number of ions, E0 is the charge of one electron (i.e. 1.61019c), e is the potential, R is the radius of hydrated ions, and H is the viscosity of electroplating solution. According to the calculation of the equation, the greater the potential E drop, the smaller the viscosity h of the electroplating solution, and the faster the rate of ion migration

according to the electrodeposition theory, during electroplating, the printed circuit board located on the cathode is a non ideal polarization electrode. 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 close to the cathode. Therefore, a copper ion concentration gradient will be formed near the cathode. The plating solution with copper ion concentration lower than that of the main plating solution is the diffusion layer of the plating solution. While the copper ion concentration in the main plating solution is high, it will diffuse to the place with low copper ion concentration near the cathode to continuously supplement the cathode area. The printed circuit board is similar to a planar cathode, and the relationship between the current and the thickness of the diffusion layer is the Cottrell equation:

z fad[CB - co]

i = -------

d

where I is the current, Z is the charge number of copper ions, f is the Faraday constant, a is the cathode surface area, D is the diffusion coefficient of copper ions (d= kt/6 PR h), and CB is the concentration of copper ions in the main plating solution CO is the concentration of copper ions on the cathode surface, D is the thickness of the diffusion layer, K is the borzmann constant (k = R/N), t is the temperature, R is the radius of copper hydrated ions, and H 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:

zfadcb

ii = -

d

from the above formula, it can be seen that the limit diffusion current depends on the copper ion concentration of the main plating solution, the diffusion coefficient of copper ion and the thickness of the diffusion layer. When the concentration of copper ion in the main plating solution is high, the diffusion coefficient of copper ion is large, and the thickness of the diffusion layer is thin, the limit diffusion current is larger

according to the above formula, in order to reach a higher limit current value, appropriate process measures must be taken, that is, heating process method. Because increasing the temperature can increase the diffusion coefficient, and the acceleration can withstand the flow rate. A relatively high temperature rate can make it become 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 accelerating the convection rate can increase the limit diffusion current, so as to achieve the purpose of accelerating the plating rate. Horizontal electroplating forms eddy current based on 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 of PCB electroplating is how to ensure the uniformity of copper layer thickness on both sides of the substrate and the inner wall of the through hole. To obtain the uniformity of coating thickness, it is necessary to ensure that the flow rate of 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. 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, it can spray the plating solution quickly and vertically to the printed board, so as to accelerate the flow speed of the plating solution in the through hole, resulting in a rapid flow rate of the plating solution, forming eddy currents in the upper and lower parts of the substrate and the through hole, 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 inlet of the through hole will also have the phenomenon of reverse reflux. Coupled with the influence of the primary current distribution, it often causes the thickness of the copper layer at the inlet hole to be too thick due to the tip effect, and the inner wall of the through hole forms a dog bone shaped copper coating. According to the flow state of the plating solution in the through hole, that is, the size of eddy current and reflux, and the state analysis of the quality of the conductive plating through hole, the control parameters can only be determined through the process test method to achieve the uniformity of the plating thickness of the printed circuit board. Because the size of eddy current and reflux can not be known by theoretical calculation, only the measured process method is adopted. From the measured results, it is known that in order to control the uniformity of the thickness of the through-hole electroplating copper layer, we must adjust the controllable process parameters according to the aspect ratio of the through-hole of the printed circuit board, and even choose the electroplating copper solution with high dispersion ability, add appropriate additives and improve the power supply method, that is, electroplating with reverse pulse current to obtain the copper coating with high distribution ability

especially the increase in the number of micro blind holes in the laminate, not only the development of specialization and innovation should adopt the horizontal electroplating system for electroplating, but also the ultrasonic vibration should be used to promote the replacement and circulation of the plating solution in the micro blind holes, and then improve the power supply mode. Using the reverse pulse current and the actual test data to adjust the controllable parameters, we can obtain satisfactory results

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