A high-density heat source with good directionality, monochromaticity and coherence. Using these properties, the surface of the metal or part can be strengthened (including laser transformation hardening laser alloying and laser cladding, etc.) to change the microstructure of the metal or part surface and improve the wear resistance, corrosion resistance and fatigue resistance of the parts. Laser processing has high precision, concentrated energy, and low thermal impact on parts.
Electroplating and electroless plating have evolved over the long term as a traditional surface technology, and the process is very mature. Researchers have combined lasers with traditional surface technologies to do a lot of research in this cross-cutting field.
The laser can irradiate the surface of the substrate in the electroplating or electroless plating process, or before or after this, so it can be divided into pre-plating, in-plating and post-plating laser treatment depending on the stage of laser processing intervention. Among them, the laser treatment after plating can be divided into three types according to the combination of the plating layer and the substrate: In the first type, if the plating layer and the substrate are melted by the laser, and the plating layer is completely diluted by the substrate, the laser surface is alloyed. In the second type, if the plating layer and the substrate are partially melted, the plating layer is partially diluted by the substrate, and the plating layer and the substrate interface are metallurgically bonded, which is a preset laser cladding. The third type is laser heat treatment if the coating and the substrate are not melted without metallurgical bonding. The first and second types of coatings are preset layers for laser processing and are part of the laser processing process. The third type is to improve the performance of the coating. It is a part of the electroplating or electroless plating process. The factors affecting the bonding between the coating and the substrate are mainly the material of the substrate and the coating, the thickness of the coating and the specific energy of the laser Es=P/DV ( P is the laser power, D is the laser spot area, and V is the laser scanning speed.) As a laser surface alloying, laser cladding, and pre-coating before laser gradient cladding, the plating layer may be different by one layer or several layers. The material composition of the thickness.
Conventional electroplating is carried out between two electrodes immersed in an electrolyte, and the metal substrate to be plated is placed in a salt solution containing a certain metal, and the metal ions are driven and deposited on the surface of the substrate by electrolysis. Metal plating. The analysis of the rapid-electrolytic deposition process shows that the formation of electroplating on the cathode substrate mainly includes three processes of ion migration, charge transfer and lattice, and the deposition rate is mainly determined by the ion migration rate and charge exchange rate. Further studies have shown that the ion migration process mainly has three modes of diffusion, convection and electromigration. To increase the rate of these three modes, the following measures can be taken: (1) increasing the electrolyte temperature, and (2) stirring the electrolyte, (3) Increase the voltage between the poles or reduce the distance between the poles. (4) Increase the concentration of the plating solution.
The basic principle of laser plating, the laser beam output by the laser is focused by a lens and projected onto the surface of the cathode to form a very high optical power density in a small area near the cathode. When the irradiated cathode material absorbs the laser energy, the temperature in the local minute region near the electrolyte-cathode interface suddenly rises, creating a steep temperature gradient and causing strong convection in the electrolyte, thereby agitating the solution. Temperature rise and agitation cause an increase in ion mobility in the localized region, an increase in the cathodic reduction reaction and a positive shift in the equilibrium potential, which ultimately leads to an increase in the plating rate. Therefore, the mechanism of laser plating can be summarized as follows: the absorption of laser energy by the cathode causes a sudden rise in photoinduced temperature, resulting in a large increase in the electrochemical reaction in a localized range, resulting in a sharp acceleration of the deposition process in a locally exposed region of the cathode surface. This theoretical explanation for the laser-induced plating enhancement effect is called the thermal model of laser plating. There is also a light model used to explain the electroplating enhancement effect. This model considers that the enhancement of the electroplating reaction is due to the photolysis effect, that is, the laser promotes the increase of the electrochemical reaction rate caused by the self-decomposition of the electrolyte ions, resulting in the plating rate. Increase.
The laser beam can reach a very high power (energy) density after being focused by the optical system. For example, the output beam of a hydrogen laser with a power of 1 watt is focused by a lens into a spot having a diameter of several tens of micrometers, and the power density can reach 104-105. W/cm. Upon absorption of the laser energy by the illuminated cathode material, the temperature in the local micro-region near the cathode interface of the electrolyte suddenly rises to produce a steep temperature gradient, causing strong convection in the electrolyte, thereby agitating the solution. Temperature rise and agitation cause an increase in ion mobility in the local region, an increase in the cathodic reduction reaction and a positive shift in the equilibrium potential, which ultimately leads to an increase in the deposition rate of the localized portion of the cathode and an increase in the plating current. We define an enhancement ratio E equal to the ratio of the plating current density at the time of laser irradiation to the background plating current density at the time of no laser irradiation, which can be expressed by the following formula:
Laser-enhanced electroplating is a high-density laser beam irradiation liquid/solid interface, causing local temperature rise and micro-zone agitation, thereby inducing or enhancing the chemical reaction in the irradiation zone, causing the decomposition of liquid substances and depositing a reaction on the solid surface. Product. In 1978, IBM in the United States first studied laser plating, using a 1.5W hydrogen ion laser or a hydrogen ion laser, and irradiating the focused laser beam on the tungsten cathode. The deposition rate of nickel was 600-1000 times higher than that without the laser beam. .
In 1980, the Prccious me tal Reaserch Institute succeeded in laser electrodeposition of gold, palladium, copper, silver and nickel. However, the electrode substrate is required to be extremely thin, has a high thermal conductivity, and uses a strong beam to obtain a high energy density.
In 1981, Puippe and Von Gutfeld studied the principle of the action of the enhanced plating (LEP). It is believed that the change of the static potential and the charge transfer rate of the electrode is the main reason for the increase of the deposition rate.
In 1983, Gelchincki et al. reported that the “laser ablation technique†was gold-plated, and the workpiece was pre-coated with an organic coating. After laser irradiation, the coating in the irradiated area was removed to expose the substrate, thereby depositing gold. The quality of the coating obtained by this technique is good, but there are technical problems such as local scorching and contamination of the solution.
In 1984, Sandia Laboratories of the United States proposed the fabrication of integrated circuits using a process called "plasma laser deposition." The method is to let the silicon hydride gas enter the reaction chamber at a low pressure, generate a plasma under the action of a laser and deposit it on the substrate.
In order to improve the electrodeposition speed, Von Gutfeld et al. studied the new technology of laser jet tempering plating in 1985, so that the laser beam is an overview.
Completely adapted to the needs of high aspect ratio via plating. However, due to the complexity and particularity of the electroplating process, the horizontal plating technology is presented. There are still several technical issues in the design and development of horizontal plating systems. This needs to be improved in practice. Despite this, the use of horizontal plating systems is a significant development and advancement for the printed circuit industry. Because of the use of this type of equipment in the manufacture of high-density multi-layer boards, it shows great potential, saving manpower and working time and producing speed and efficiency higher than traditional vertical plating lines. Moreover, the energy consumption is reduced, the waste liquid waste gas to be treated is reduced, and the process environment and conditions are greatly improved, and the quality level of the plating layer is improved. The horizontal plating line is suitable for large-scale production of 24 hours of uninterrupted operation. The horizontal plating line is slightly more difficult than the vertical plating line during commissioning. Once the debugging is completed, it is very stable and the plating solution should be monitored at any time during the use. The liquid is adjusted to ensure stable operation for a long time.
PCB manufacturing is rapidly developing in the direction of multi-layer, layering, functionalization and integration. It has promoted the design and design of circuit patterns using tiny holes, narrow pitches and thin wires in a large number of printed circuit designs, with the rapid development of microelectronics technology. The PCB manufacturing technology is more difficult, especially the aspect ratio of the through-hole of the multi-layer board exceeds 5:1 and the deep blind hole which is widely used in the laminated board, so that the conventional vertical plating process cannot satisfy the high quality and high reliability. Technical requirements for interconnecting holes. The main reason is to analyze the current distribution state from the electroplating principle. The actual current distribution shows 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 deposit on the surface. The edge of the hole cannot ensure that the copper layer in the center of the hole should reach the specified thickness. Sometimes the copper layer is extremely thin or has no copper layer. In severe cases, irreparable damage will occur, resulting in a large number of multi-layer boards being scrapped. In order to solve the problem of product quality in mass production, the problem of deep hole plating is currently solved from the aspects of current and additives. In the high aspect ratio PCB electroplating copper process, most of them are assisted by high-quality additives, with appropriate air agitation and cathode movement, and the electrode reaction control area in the hole is increased under relatively low current density conditions. The role of the electroplating additive can be displayed, and the movement of the cathode is very beneficial to the improvement of the deep plating ability of the plating solution, the polarization of the plated part is increased, the formation speed of the crystal nucleus and the growth rate of the crystal grain during the electrocrystallization process of the plating layer. Mutual compensation to obtain a high toughness copper layer.
These two process measures appear to be weak, however, when the aspect ratio of the through hole continues to increase or a deep blind hole occurs. Then horizontal plating technology occurs. The continuation of the development of vertical plating technology, that is, the novel plating technology developed on the basis of the vertical plating process. The key to this technology is to produce a compatible horizontal plating system that can achieve high-dispersion plating, improved power supply and other auxiliary devices, showing better performance than vertical plating. effect.
Second, the principle of horizontal plating
Electrode reaction occurs after energization to cause ionization of the main component of the electrolyte. Horizontal plating and vertical plating methods and principles must have the same yin and yang poles. The charged positive ions are moved to the negative phase of the reaction zone of the electrode; the charged negative ions move to the positive phase of the reaction zone of the electrode, so that the metal deposit plating and the evolved gas occur. Because the process of metal deposition in the cathode is divided into three steps: the hydration ions of the metal diffuse toward the cathode; the second step is that the metal hydration ions are gradually dehydrated while passing through the electric double layer, and adsorbed on the outer surface of the cathode; The step is that the metal ions adsorbed on the surface of the cathode receive electrons and enter the metal lattice. From the actual observation, the case of the working tank is that an out-of-phase electron transport reaction cannot be observed between the interface of the solid phase electrode and the liquid phase plating solution. 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, the cations surrounded by water molecules and positively charged are arranged in order at the cathode due to electrostatic force. Nearby, the phase of the cation center point closest to the cathode is called Helmholtz (the outer layer of Helmholtz, the distance of the outer layer from the electrode is about 1-10 nm. But due to the outer layer of Helmholtz The total amount of positive charge of the cation, the amount of positive charge is not enough to neutralize the negative charge on the cathode. The bath farther from the cathode is affected by convection, and the concentration of the cation in the solution layer is higher than the concentration of the anion. The layer is smaller than the Helmholtz outer layer due to the electrostatic force, and is affected by the thermal motion. The cation arrangement is not as tight and tidy as the Helmholtz outer layer. This layer is called the diffusion layer. The thickness 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 the thickness is. The thickness of the diffusion layer is generally about 5-50 microns. Farther from the cathode, the convection is reached. Plating layer The main body plating solution. The convection effect of the solution affects the uniformity of the plating solution concentration. The copper ions in the diffusion layer are transported to the Helmholtz outer layer by diffusion and ion migration. The copper ions in the liquid are transported to the surface of the cathode by convection and ion migration. During the horizontal plating process, the copper ions in the plating solution are transported to the vicinity of the cathode to form an electric double layer in three ways.
And the flow of the plating solution caused by the temperature difference. The closer to the outer surface of the solid electrode, the convection of the plating solution occurs by externally internal mechanical stirring and pump agitation, and the electrode itself is oscillated or rotated. Due to the influence of the frictional resistance thereof, the flow of the plating solution becomes slower and slower, and the convection rate of the surface of the solid electrode at this time is zero. The rate gradient layer formed between the electrode surface and the convection bath is referred to as 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.
The ions in the plating solution are subjected to electrostatic forces to cause ion transport to be called ion migration. The rate of its migration is expressed as follows: u=zeoE/6πrη. Where u is the ion migration rate, z is the number of ions, and eo is the charge of one electron (ie, 1.61019CE 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 Under the action of electrophoresis, the larger the potential E falls, the smaller the viscosity of the plating solution and the faster the rate of ion migration.
When plating, according to the theory of electrodeposition. The PCB on the cathode is a non-ideal polarized electrode, and the copper ions adsorbed on the outer surface of the cathode obtain electrons and are reduced to copper atoms, and the concentration of copper ions near the cathode is lowered. Therefore, a copper ion concentration gradient is formed in the vicinity of the cathode. This layer of plating solution having a copper ion concentration lower than that of the main body plating solution is a diffusion layer of the plating solution. The concentration of copper ions in the main plating solution is high, and it diffuses to a place where the concentration of copper ions near the cathode is low, and the cathode region is supplemented from time to time. The PCB is similar to a planar cathode, and the relationship between the magnitude of the current and the thickness of the diffusion layer is the COTTRELL equation.
The current is called the ultimate diffusion current ii where I is the current, z is the number of charges of copper ions, F is the Faraday constant, A is the cathode surface area, and D is the copper ion diffusion coefficient (D=KT/6πrη) Cb is the main plating solution. Copper ion concentration, Co is the concentration of copper ions on the cathode surface, D is the thickness of the diffusion layer, K is the Bohmann constant (K=R/N)T is the temperature, r is the radius of the copper hydrated ion, and η is the plating solution. Viscosity. When the copper ion concentration on the cathode surface is zero.
The magnitude of the ultimate diffusion current is determined by the copper ion concentration of the bulk 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 body plating solution is high, the diffusion coefficient of copper ions is large, and the thickness of the diffusion layer is thin, it can be seen from the above formula. The ultimate diffusion current is greater.
To achieve a higher limit current value, it is known from the above formula. Appropriate process measures must be taken, that is, the heating process is adopted. Since increasing the temperature causes the diffusion coefficient to increase, increasing the convection rate makes it a vortex to 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 increase the ultimate diffusion current, and achieve the purpose of accelerating the plating rate. The convection speed of the plating solution is accelerated. The formation of eddy currents can effectively reduce the thickness of the diffusion layer to about 10 microns. Therefore, when using a horizontal plating system for electroplating, the current density can be as high as 8A/dm2.
It 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 get the uniformity of the thickness of the coating, the key to PCB plating. It is necessary to ensure that the flow rates of the plating solution on both sides of the printed board and in the through holes are fast and consistent to obtain a thin and uniform diffusion layer.
In order to achieve a thin uniform diffusion layer, in view of the structure of the current horizontal plating system, although many nozzles are installed in the system, the plating solution can be sprayed vertically and vertically to the printed board to accelerate the flow of the plating solution in the through hole. The velocity causes the flow rate of the plating solution to be fast, and eddy currents are formed in the upper and lower surfaces of the substrate and in the through holes, so that the diffusion layer is lowered 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 may have a reverse reflow phenomenon, and when the influence of the current distribution is often caused, the operation often causes the hole at the entrance to be plated. 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 layer in the shape of a dog bone. According to the state of the flow of the plating solution in the through hole, that is, the size of the eddy current and the reflow, the state analysis of the quality of the conductive plated through hole can only be determined by the process test method to determine the uniformity of the control parameter to the thickness of the PCB plating. Because the size of eddy currents and reflows cannot be known by theoretical calculations, only the measured methods are used. It is known from the measured results that to control the uniformity of the thickness of the via copper plating layer, it is necessary to adjust the controllable process parameters according to the aspect ratio of the PCB through hole, and even select the electroplating copper solution with high dispersibility, and then add Appropriate additives and improved power supply methods use reverse pulse current for electroplating to obtain a copper coating with high distribution capability.
Not only the horizontal plating system is used for electroplating, especially the number of micro-blind holes in the laminate is increased. Ultrasonic vibration should also be used to promote the replacement and circulation of the plating solution in the micro-blind hole, and then improve the power supply mode by using the reverse pulse current and the actual test data to adjust the controllable parameters, and satisfactory results can be obtained.
Third, the basic structure of the horizontal plating system
The way in which the PCB is placed is changed from vertical to parallel plating. At this time, the PCB is a cathode, according to the characteristics of horizontal plating. The current supply method includes a horizontal plating system using conductive clips and conductive rollers. From the convenience of the operating system, the use of roller conductive supply is more common. The conductive roller in the horizontal plating system has the function of transferring the PCB in addition to being a cathode. Each conductive roller is equipped with a spring device, which is designed to meet the needs of different thickness PCB (0.10-5.00mm) plating. However, in the case of electroplating, the portion in contact with the plating solution may be plated with a copper layer, and the system cannot be operated for a long time. Therefore, in the current horizontal plating systems, most of the cathodes are designed to be switchable to anodes, and a set of auxiliary cathodes can be used to electrolytically dissolve copper on the coated rollers. For repair or replacement purposes, the new plating design also allows for easy removal or replacement of parts that are prone to loss. The anode is an array of insoluble titanium baskets, which are placed on the upper and lower positions of the PCB. They are filled with copper with a diameter of 25 mm and a phosphorus content of 0.004-0.006%. The distance between the cathode and the anode is 40 mm.
The plating solution is alternately and rapidly flowed up and down in the closed plating tank, and the flow of the plating solution is a system composed of a pump and a nozzle. And can ensure the uniformity of the flow of the plating solution. The plating solution is sprayed vertically to the PCB, and the surface of the PCB forms a jet vortex. The ultimate goal is to achieve rapid flow of the plating solution on both sides of the PCB and through holes to form eddy currents. In addition, a filter system is installed in the tank, wherein the filter mesh is 1.2 micrometers for filtering the particulate impurities generated during the electroplating process to ensure clean and non-polluting of the plating solution.
Also consider the ease of operation and automatic control of process parameters. Because in the actual plating, when manufacturing a horizontal plating system. With the size of the PCB, the size of the through hole aperture and the required thickness of the copper, the transmission speed, the distance between the PCBs, the size of the pump horsepower, the direction of the nozzle and the current density, etc. It is necessary to carry out actual testing and adjustment and control to obtain the copper layer thickness 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 high-quality products, the through-hole processing (including plating holes) of the PCB in accordance with the process sequence constitutes a complete horizontal plating system, which is to meet the needs of new product development and market.
Fourth, the development advantages of horizontal plating
The quality of the products is more reliable. The development of horizontal plating 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. The advantage is that it is more advanced than the vertical plating process currently used. Can achieve large-scale production. Compared with the vertical plating process, it has the following advantages:
No need to carry out manual loading, 1) adapt to a wide range of sizes. Achieving full automation, and improving and ensuring no damage to the surface of the substrate during the operation process, is extremely advantageous for achieving large-scale production.
There is no need to leave a clamping position, 2) during the process review. Increase the practical area and greatly reduce the loss of raw materials.
The substrate is made under the same conditions, and 3) horizontal plating is controlled by full-time computer. Ensure the uniformity of the plating of the outer surface of each PCB and the hole.
The plating bath is cleaned, the plating solution is added and replaced, and 4) from a management perspective. Automated operation can be fully realized, and there is no management runaway problem caused by human error. Since horizontal plating uses multi-stage horizontal cleaning, 5) it can be measured from actual production. Significantly save the amount of washing water and reduce the pressure of sewage treatment.
Reduce the direct impact on the process environment by contamination of the work space and evaporation of heat, 6) because the system uses closed operation. Greatly improve the working environment. In particular, when the baking sheet is used, the loss of heat is reduced, the unnecessary consumption of energy is saved, and the production efficiency is greatly improved.
Electroplating and electroless plating have evolved over the long term as a traditional surface technology, and the process is very mature. Researchers have combined lasers with traditional surface technologies to do a lot of research in this cross-cutting field.
The laser can irradiate the surface of the substrate in the electroplating or electroless plating process, or before or after this, so it can be divided into pre-plating, in-plating and post-plating laser treatment depending on the stage of laser processing intervention. Among them, the laser treatment after plating can be divided into three types according to the combination of the plating layer and the substrate: In the first type, if the plating layer and the substrate are melted by the laser, and the plating layer is completely diluted by the substrate, the laser surface is alloyed. In the second type, if the plating layer and the substrate are partially melted, the plating layer is partially diluted by the substrate, and the plating layer and the substrate interface are metallurgically bonded, which is a preset laser cladding. The third type is laser heat treatment if the coating and the substrate are not melted without metallurgical bonding. The first and second types of coatings are preset layers for laser processing and are part of the laser processing process. The third type is to improve the performance of the coating. It is a part of the electroplating or electroless plating process. The factors affecting the bonding between the coating and the substrate are mainly the material of the substrate and the coating, the thickness of the coating and the specific energy of the laser Es=P/DV ( P is the laser power, D is the laser spot area, and V is the laser scanning speed.) As a laser surface alloying, laser cladding, and pre-coating before laser gradient cladding, the plating layer may be different by one layer or several layers. The material composition of the thickness.
Conventional electroplating is carried out between two electrodes immersed in an electrolyte, and the metal substrate to be plated is placed in a salt solution containing a certain metal, and the metal ions are driven and deposited on the surface of the substrate by electrolysis. Metal plating. The analysis of the rapid-electrolytic deposition process shows that the formation of electroplating on the cathode substrate mainly includes three processes of ion migration, charge transfer and lattice, and the deposition rate is mainly determined by the ion migration rate and charge exchange rate. Further studies have shown that the ion migration process mainly has three modes of diffusion, convection and electromigration. To increase the rate of these three modes, the following measures can be taken: (1) increasing the electrolyte temperature, and (2) stirring the electrolyte, (3) Increase the voltage between the poles or reduce the distance between the poles. (4) Increase the concentration of the plating solution.
The basic principle of laser plating, the laser beam output by the laser is focused by a lens and projected onto the surface of the cathode to form a very high optical power density in a small area near the cathode. When the irradiated cathode material absorbs the laser energy, the temperature in the local minute region near the electrolyte-cathode interface suddenly rises, creating a steep temperature gradient and causing strong convection in the electrolyte, thereby agitating the solution. Temperature rise and agitation cause an increase in ion mobility in the localized region, an increase in the cathodic reduction reaction and a positive shift in the equilibrium potential, which ultimately leads to an increase in the plating rate. Therefore, the mechanism of laser plating can be summarized as follows: the absorption of laser energy by the cathode causes a sudden rise in photoinduced temperature, resulting in a large increase in the electrochemical reaction in a localized range, resulting in a sharp acceleration of the deposition process in a locally exposed region of the cathode surface. This theoretical explanation for the laser-induced plating enhancement effect is called the thermal model of laser plating. There is also a light model used to explain the electroplating enhancement effect. This model considers that the enhancement of the electroplating reaction is due to the photolysis effect, that is, the laser promotes the increase of the electrochemical reaction rate caused by the self-decomposition of the electrolyte ions, resulting in the plating rate. Increase.
The laser beam can reach a very high power (energy) density after being focused by the optical system. For example, the output beam of a hydrogen laser with a power of 1 watt is focused by a lens into a spot having a diameter of several tens of micrometers, and the power density can reach 104-105. W/cm. Upon absorption of the laser energy by the illuminated cathode material, the temperature in the local micro-region near the cathode interface of the electrolyte suddenly rises to produce a steep temperature gradient, causing strong convection in the electrolyte, thereby agitating the solution. Temperature rise and agitation cause an increase in ion mobility in the local region, an increase in the cathodic reduction reaction and a positive shift in the equilibrium potential, which ultimately leads to an increase in the deposition rate of the localized portion of the cathode and an increase in the plating current. We define an enhancement ratio E equal to the ratio of the plating current density at the time of laser irradiation to the background plating current density at the time of no laser irradiation, which can be expressed by the following formula:
Laser-enhanced electroplating is a high-density laser beam irradiation liquid/solid interface, causing local temperature rise and micro-zone agitation, thereby inducing or enhancing the chemical reaction in the irradiation zone, causing the decomposition of liquid substances and depositing a reaction on the solid surface. Product. In 1978, IBM in the United States first studied laser plating, using a 1.5W hydrogen ion laser or a hydrogen ion laser, and irradiating the focused laser beam on the tungsten cathode. The deposition rate of nickel was 600-1000 times higher than that without the laser beam. .
In 1980, the Prccious me tal Reaserch Institute succeeded in laser electrodeposition of gold, palladium, copper, silver and nickel. However, the electrode substrate is required to be extremely thin, has a high thermal conductivity, and uses a strong beam to obtain a high energy density.
In 1981, Puippe and Von Gutfeld studied the principle of the action of the enhanced plating (LEP). It is believed that the change of the static potential and the charge transfer rate of the electrode is the main reason for the increase of the deposition rate.
In 1983, Gelchincki et al. reported that the “laser ablation technique†was gold-plated, and the workpiece was pre-coated with an organic coating. After laser irradiation, the coating in the irradiated area was removed to expose the substrate, thereby depositing gold. The quality of the coating obtained by this technique is good, but there are technical problems such as local scorching and contamination of the solution.
In 1984, Sandia Laboratories of the United States proposed the fabrication of integrated circuits using a process called "plasma laser deposition." The method is to let the silicon hydride gas enter the reaction chamber at a low pressure, generate a plasma under the action of a laser and deposit it on the substrate.
In order to improve the electrodeposition speed, Von Gutfeld et al. studied the new technology of laser jet tempering plating in 1985, so that the laser beam is an overview.
Completely adapted to the needs of high aspect ratio via plating. However, due to the complexity and particularity of the electroplating process, the horizontal plating technology is presented. There are still several technical issues in the design and development of horizontal plating systems. This needs to be improved in practice. Despite this, the use of horizontal plating systems is a significant development and advancement for the printed circuit industry. Because of the use of this type of equipment in the manufacture of high-density multi-layer boards, it shows great potential, saving manpower and working time and producing speed and efficiency higher than traditional vertical plating lines. Moreover, the energy consumption is reduced, the waste liquid waste gas to be treated is reduced, and the process environment and conditions are greatly improved, and the quality level of the plating layer is improved. The horizontal plating line is suitable for large-scale production of 24 hours of uninterrupted operation. The horizontal plating line is slightly more difficult than the vertical plating line during commissioning. Once the debugging is completed, it is very stable and the plating solution should be monitored at any time during the use. The liquid is adjusted to ensure stable operation for a long time.
PCB manufacturing is rapidly developing in the direction of multi-layer, layering, functionalization and integration. It has promoted the design and design of circuit patterns using tiny holes, narrow pitches and thin wires in a large number of printed circuit designs, with the rapid development of microelectronics technology. The PCB manufacturing technology is more difficult, especially the aspect ratio of the through-hole of the multi-layer board exceeds 5:1 and the deep blind hole which is widely used in the laminated board, so that the conventional vertical plating process cannot satisfy the high quality and high reliability. Technical requirements for interconnecting holes. The main reason is to analyze the current distribution state from the electroplating principle. The actual current distribution shows 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 deposit on the surface. The edge of the hole cannot ensure that the copper layer in the center of the hole should reach the specified thickness. Sometimes the copper layer is extremely thin or has no copper layer. In severe cases, irreparable damage will occur, resulting in a large number of multi-layer boards being scrapped. In order to solve the problem of product quality in mass production, the problem of deep hole plating is currently solved from the aspects of current and additives. In the high aspect ratio PCB electroplating copper process, most of them are assisted by high-quality additives, with appropriate air agitation and cathode movement, and the electrode reaction control area in the hole is increased under relatively low current density conditions. The role of the electroplating additive can be displayed, and the movement of the cathode is very beneficial to the improvement of the deep plating ability of the plating solution, the polarization of the plated part is increased, the formation speed of the crystal nucleus and the growth rate of the crystal grain during the electrocrystallization process of the plating layer. Mutual compensation to obtain a high toughness copper layer.
These two process measures appear to be weak, however, when the aspect ratio of the through hole continues to increase or a deep blind hole occurs. Then horizontal plating technology occurs. The continuation of the development of vertical plating technology, that is, the novel plating technology developed on the basis of the vertical plating process. The key to this technology is to produce a compatible horizontal plating system that can achieve high-dispersion plating, improved power supply and other auxiliary devices, showing better performance than vertical plating. effect.
Second, the principle of horizontal plating
Electrode reaction occurs after energization to cause ionization of the main component of the electrolyte. Horizontal plating and vertical plating methods and principles must have the same yin and yang poles. The charged positive ions are moved to the negative phase of the reaction zone of the electrode; the charged negative ions move to the positive phase of the reaction zone of the electrode, so that the metal deposit plating and the evolved gas occur. Because the process of metal deposition in the cathode is divided into three steps: the hydration ions of the metal diffuse toward the cathode; the second step is that the metal hydration ions are gradually dehydrated while passing through the electric double layer, and adsorbed on the outer surface of the cathode; The step is that the metal ions adsorbed on the surface of the cathode receive electrons and enter the metal lattice. From the actual observation, the case of the working tank is that an out-of-phase electron transport reaction cannot be observed between the interface of the solid phase electrode and the liquid phase plating solution. 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, the cations surrounded by water molecules and positively charged are arranged in order at the cathode due to electrostatic force. Nearby, the phase of the cation center point closest to the cathode is called Helmholtz (the outer layer of Helmholtz, the distance of the outer layer from the electrode is about 1-10 nm. But due to the outer layer of Helmholtz The total amount of positive charge of the cation, the amount of positive charge is not enough to neutralize the negative charge on the cathode. The bath farther from the cathode is affected by convection, and the concentration of the cation in the solution layer is higher than the concentration of the anion. The layer is smaller than the Helmholtz outer layer due to the electrostatic force, and is affected by the thermal motion. The cation arrangement is not as tight and tidy as the Helmholtz outer layer. This layer is called the diffusion layer. The thickness 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 the thickness is. The thickness of the diffusion layer is generally about 5-50 microns. Farther from the cathode, the convection is reached. Plating layer The main body plating solution. The convection effect of the solution affects the uniformity of the plating solution concentration. The copper ions in the diffusion layer are transported to the Helmholtz outer layer by diffusion and ion migration. The copper ions in the liquid are transported to the surface of the cathode by convection and ion migration. During the horizontal plating process, the copper ions in the plating solution are transported to the vicinity of the cathode to form an electric double layer in three ways.
And the flow of the plating solution caused by the temperature difference. The closer to the outer surface of the solid electrode, the convection of the plating solution occurs by externally internal mechanical stirring and pump agitation, and the electrode itself is oscillated or rotated. Due to the influence of the frictional resistance thereof, the flow of the plating solution becomes slower and slower, and the convection rate of the surface of the solid electrode at this time is zero. The rate gradient layer formed between the electrode surface and the convection bath is referred to as 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.
The ions in the plating solution are subjected to electrostatic forces to cause ion transport to be called ion migration. The rate of its migration is expressed as follows: u=zeoE/6πrη. Where u is the ion migration rate, z is the number of ions, and eo is the charge of one electron (ie, 1.61019CE 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 Under the action of electrophoresis, the larger the potential E falls, the smaller the viscosity of the plating solution and the faster the rate of ion migration.
When plating, according to the theory of electrodeposition. The PCB on the cathode is a non-ideal polarized electrode, and the copper ions adsorbed on the outer surface of the cathode obtain electrons and are reduced to copper atoms, and the concentration of copper ions near the cathode is lowered. Therefore, a copper ion concentration gradient is formed in the vicinity of the cathode. This layer of plating solution having a copper ion concentration lower than that of the main body plating solution is a diffusion layer of the plating solution. The concentration of copper ions in the main plating solution is high, and it diffuses to a place where the concentration of copper ions near the cathode is low, and the cathode region is supplemented from time to time. The PCB is similar to a planar cathode, and the relationship between the magnitude of the current and the thickness of the diffusion layer is the COTTRELL equation.
The current is called the ultimate diffusion current ii where I is the current, z is the number of charges of copper ions, F is the Faraday constant, A is the cathode surface area, and D is the copper ion diffusion coefficient (D=KT/6πrη) Cb is the main plating solution. Copper ion concentration, Co is the concentration of copper ions on the cathode surface, D is the thickness of the diffusion layer, K is the Bohmann constant (K=R/N)T is the temperature, r is the radius of the copper hydrated ion, and η is the plating solution. Viscosity. When the copper ion concentration on the cathode surface is zero.
The magnitude of the ultimate diffusion current is determined by the copper ion concentration of the bulk 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 body plating solution is high, the diffusion coefficient of copper ions is large, and the thickness of the diffusion layer is thin, it can be seen from the above formula. The ultimate diffusion current is greater.
To achieve a higher limit current value, it is known from the above formula. Appropriate process measures must be taken, that is, the heating process is adopted. Since increasing the temperature causes the diffusion coefficient to increase, increasing the convection rate makes it a vortex to 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 increase the ultimate diffusion current, and achieve the purpose of accelerating the plating rate. The convection speed of the plating solution is accelerated. The formation of eddy currents can effectively reduce the thickness of the diffusion layer to about 10 microns. Therefore, when using a horizontal plating system for electroplating, the current density can be as high as 8A/dm2.
It 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 get the uniformity of the thickness of the coating, the key to PCB plating. It is necessary to ensure that the flow rates of the plating solution on both sides of the printed board and in the through holes are fast and consistent to obtain a thin and uniform diffusion layer.
In order to achieve a thin uniform diffusion layer, in view of the structure of the current horizontal plating system, although many nozzles are installed in the system, the plating solution can be sprayed vertically and vertically to the printed board to accelerate the flow of the plating solution in the through hole. The velocity causes the flow rate of the plating solution to be fast, and eddy currents are formed in the upper and lower surfaces of the substrate and in the through holes, so that the diffusion layer is lowered 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 may have a reverse reflow phenomenon, and when the influence of the current distribution is often caused, the operation often causes the hole at the entrance to be plated. 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 layer in the shape of a dog bone. According to the state of the flow of the plating solution in the through hole, that is, the size of the eddy current and the reflow, the state analysis of the quality of the conductive plated through hole can only be determined by the process test method to determine the uniformity of the control parameter to the thickness of the PCB plating. Because the size of eddy currents and reflows cannot be known by theoretical calculations, only the measured methods are used. It is known from the measured results that to control the uniformity of the thickness of the via copper plating layer, it is necessary to adjust the controllable process parameters according to the aspect ratio of the PCB through hole, and even select the electroplating copper solution with high dispersibility, and then add Appropriate additives and improved power supply methods use reverse pulse current for electroplating to obtain a copper coating with high distribution capability.
Not only the horizontal plating system is used for electroplating, especially the number of micro-blind holes in the laminate is increased. Ultrasonic vibration should also be used to promote the replacement and circulation of the plating solution in the micro-blind hole, and then improve the power supply mode by using the reverse pulse current and the actual test data to adjust the controllable parameters, and satisfactory results can be obtained.
Third, the basic structure of the horizontal plating system
The way in which the PCB is placed is changed from vertical to parallel plating. At this time, the PCB is a cathode, according to the characteristics of horizontal plating. The current supply method includes a horizontal plating system using conductive clips and conductive rollers. From the convenience of the operating system, the use of roller conductive supply is more common. The conductive roller in the horizontal plating system has the function of transferring the PCB in addition to being a cathode. Each conductive roller is equipped with a spring device, which is designed to meet the needs of different thickness PCB (0.10-5.00mm) plating. However, in the case of electroplating, the portion in contact with the plating solution may be plated with a copper layer, and the system cannot be operated for a long time. Therefore, in the current horizontal plating systems, most of the cathodes are designed to be switchable to anodes, and a set of auxiliary cathodes can be used to electrolytically dissolve copper on the coated rollers. For repair or replacement purposes, the new plating design also allows for easy removal or replacement of parts that are prone to loss. The anode is an array of insoluble titanium baskets, which are placed on the upper and lower positions of the PCB. They are filled with copper with a diameter of 25 mm and a phosphorus content of 0.004-0.006%. The distance between the cathode and the anode is 40 mm.
The plating solution is alternately and rapidly flowed up and down in the closed plating tank, and the flow of the plating solution is a system composed of a pump and a nozzle. And can ensure the uniformity of the flow of the plating solution. The plating solution is sprayed vertically to the PCB, and the surface of the PCB forms a jet vortex. The ultimate goal is to achieve rapid flow of the plating solution on both sides of the PCB and through holes to form eddy currents. In addition, a filter system is installed in the tank, wherein the filter mesh is 1.2 micrometers for filtering the particulate impurities generated during the electroplating process to ensure clean and non-polluting of the plating solution.
Also consider the ease of operation and automatic control of process parameters. Because in the actual plating, when manufacturing a horizontal plating system. With the size of the PCB, the size of the through hole aperture and the required thickness of the copper, the transmission speed, the distance between the PCBs, the size of the pump horsepower, the direction of the nozzle and the current density, etc. It is necessary to carry out actual testing and adjustment and control to obtain the copper layer thickness 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 high-quality products, the through-hole processing (including plating holes) of the PCB in accordance with the process sequence constitutes a complete horizontal plating system, which is to meet the needs of new product development and market.
Fourth, the development advantages of horizontal plating
The quality of the products is more reliable. The development of horizontal plating 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. The advantage is that it is more advanced than the vertical plating process currently used. Can achieve large-scale production. Compared with the vertical plating process, it has the following advantages:
No need to carry out manual loading, 1) adapt to a wide range of sizes. Achieving full automation, and improving and ensuring no damage to the surface of the substrate during the operation process, is extremely advantageous for achieving large-scale production.
There is no need to leave a clamping position, 2) during the process review. Increase the practical area and greatly reduce the loss of raw materials.
The substrate is made under the same conditions, and 3) horizontal plating is controlled by full-time computer. Ensure the uniformity of the plating of the outer surface of each PCB and the hole.
The plating bath is cleaned, the plating solution is added and replaced, and 4) from a management perspective. Automated operation can be fully realized, and there is no management runaway problem caused by human error. Since horizontal plating uses multi-stage horizontal cleaning, 5) it can be measured from actual production. Significantly save the amount of washing water and reduce the pressure of sewage treatment.
Reduce the direct impact on the process environment by contamination of the work space and evaporation of heat, 6) because the system uses closed operation. Greatly improve the working environment. In particular, when the baking sheet is used, the loss of heat is reduced, the unnecessary consumption of energy is saved, and the production efficiency is greatly improved.
Jiangmen Hongli Energy Co.ltd , https://www.honglienergy.com