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Analysis of Etching Process of Outer Circuit of Multilayer Circuit Board
1. Overview
Nowadays, the typical process of printed circuit board(PCB multi-layer circuit board) processing selects "graphic plating method". That is, pre-plated a layer of lead-tin anti-corrosion layer on the part of the copper foil that needs to be preserved on the outer layer of the board, that is, the pattern part of the circuit, and then chemically corrodes the other copper foil, which is called etching.
It should be noted that there are two layers of copper on the multilayer circuit board at this time. In the outer layer etching process, only one layer of copper must be completely etched away, and the others will form the required circuit after all. This type of pattern electroplating is characterized by the copper plating layer only exists under the lead-tin resist layer. Another process method is to plate copper on the entire multilayer circuit board, and the parts other than the photosensitive film are only tin or lead-tin resist. This process is called "full board copper plating process". Compared with pattern electroplating, the biggest disadvantage of copper plating on the entire board is that copper must be plated twice on the surface of the board and it is necessary to corrode them during etching. Therefore, a series of problems will occur when the wire width is very precise. At the same time, side corrosion will severely affect the uniformity of the line.
In the processing technology of the outer circuit of the PCB circuit board (multi-layer circuit board), there is another way, which is to use a photosensitive film instead of a metal coating as a corrosion-resistant layer. This method is very similar to the inner layer etching process, and you can refer to the etching in the inner layer manufacturing process.
Nowadays, tin or lead-tin is the most commonly used anti-corrosion layer, used in the etching process of ammonia-based etchant. Ammonia-based etchant is a widely used chemical liquid, which does not have any chemical reaction with tin or lead-tin. Ammonia etchant mainly refers to ammonia/ammonium chloride etching solution. In addition, ammonia/ammonium sulfate etching chemicals are also available on the market.
After using the sulfate-based etching solution, the copper in it can be separated by electrolysis, so it can be reused. Because of its low corrosion rate, it is generally rare in practice, but it is expected to be used in chlorine-free etching. Some people experimented with sulfuric acid-hydrogen peroxide as an etchant to corrode the outer layer pattern. Because of many reasons including economy and waste liquid treatment, this process has not been widely used in the commercial sense. Furthermore, sulfuric acid-hydrogen peroxide cannot be used for the etching of lead-tin resist, and this process is not the primary method in the production of the outer layer of the multilayer circuit board, so most people rarely care about it.
2. Regarding the upper and lower multilayer circuit board surfaces, the etching conditions of the leading edge and the trailing edge are different
Many problems related to the quality of etching will be combined with the etched part of the upper multilayer circuit board. It is very important to understand this. These problems come from the influence of the glue-like clumps produced by the etchant on the upper surface of the printed circuit board. The accumulation of colloidal slabs on the surface of the copper affects the eruption force on the one hand, and on the other hand blocks the compensation of the fresh etching solution, which constitutes a decrease in the etching speed. It is precisely because of the composition and accumulation of the colloidal slabs that the etching levels of the upper and lower patterns of the board are different. This also makes the part of the etching machine (multi-layer circuit board) that the board enters first is simply etched completely or simply constituted over corrosion, because at that time the accumulation has no structure and the etching speed is faster. On the contrary, the part of the multi-layer circuit board that enters after the board is accumulated when it enters and slows down its etching speed.
3. Equipment adjustment and interaction with corrosive solution
In the processing of printed circuits (multilayer circuit boards), ammonia etching is a more sophisticated and messy chemical reaction process. On the other hand, it is an easy job. Once the process is up-regulated, it can be produced continuously. The key is to insist on continuous operation once it is turned on, and it is not advisable to dry and stop. The etching process depends to a great extent on the outstanding operating conditions of the equipment. For now, no matter what kind of etching solution is used, it is necessary to use high-pressure spray, and in order to obtain a more regular line side and high-quality etching effect, it is necessary to strictly select the nozzle structure and spray method.
In order to obtain outstanding side effects, many different theories have been presented, which constitute different planning methods and equipment structures. These theories are often very different. But all the theories about etching recognize the most basic principle, that is, as quickly as possible to keep the surface of the metal touching the fresh etching solution. The chemical mechanism analysis of the etching process also proved the above point of view. In ammonia etching, assuming that all other parameters remain unchanged, the etching rate is primarily determined by the ammonia (NH3) in the etching solution. Therefore, using fresh solution and etching the appearance effect has two main purposes: one is to flush out the copper ions that have just occurred; the other is to continuously supply the ammonia (NH3) required for the response.
In the traditional common sense of the printed circuit industry, especially the suppliers of printed circuit materials, we recognize that the lower the monovalent copper ion content in the ammonia etching solution, the faster the response speed. This has been learned from experience Proof. In fact, many ammonia-based etching solution products contain special ligands for monovalent copper ions (some messy solvents), the effect of which is to reduce monovalent copper ions (these are the technical know-how of their products with high response ability ), it can be seen that the influence of monovalent copper ions is not small. If the monovalent copper is reduced from 5000ppm to 50ppm, the etching rate will more than double.
Because a lot of monovalent copper ions are generated during the etching reaction process, and because the monovalent copper ions are always tightly combined with the complexing group of ammonia, it is very difficult to maintain its content close to zero. Converting monovalent copper to divalent copper through the effect of atmospheric oxygen can remove monovalent copper. The above purpose can be achieved by spraying.
This is a functional reason for passing air into the etching box. However, assuming that there is too much air, it will speed up the loss of ammonia in the solution and decrease the pH value, and its effect will still reduce the etching rate. Ammonia in the solution is also the amount of modification that needs to be manipulated. Some users choose to pass pure ammonia into the etching reservoir. To do so, it is necessary to add a PH meter control system. When the actively measured pH effect is lower than a given value, the solution will actively increase.
In the related chemical etching (also known as photochemical etching or PCH) field, the research work has been preliminary and has reached the stage of etching machine structure planning. In this method, the solution used is divalent copper, not ammonia-copper etching. It may be used in the printed circuit industry. In the PCH industry, the typical thickness of etched copper foil is 5 to 10 mils (mils), and in some cases the thickness is quite large. Its requirements for etching parameters are often more stringent than those in the PCB industry.
There is a research effect from the PCM industrial system, which has not been officially announced, but its effect will be refreshing. Because of the relatively strong project fund support, the researchers are capable of making changes to the planning thinking of the etching equipment in a long-term sense, and discussing the effects of these changes together. For example, compared with a cone nozzle, the best nozzle plan is to use a fan shape, and the spray manifold (that is, the pipe into which the nozzle is screwed) also has a device viewpoint, which can erupt at 30 degrees to the workpiece entering the etching chamber. Assumptions Without such a modification, the installation method of the nozzles on the manifold will cause the ejection point of view of each adjacent nozzle to be not completely consistent. The spray surfaces of the second group of nozzles are slightly different from those of the first group (it indicates the spraying operation status). In this way, the shape of the sprayed solution becomes superimposed or crossed. Theoretically, assuming that the shapes of the solutions intersect each other, the ejection force of this part will decrease, and the old solution on the etching surface cannot be effectively washed away and the new solution can be touched. At the edge of the spray surface, this situation is particularly excellent. Its eruption force is much smaller than that in a straight direction.
This study found that the latest planning parameter is 65 pounds per square inch (ie 4+Bar). Every etching process and every useful solution has a problem of the best eruption pressure, and for now, the eruption pressure in the etching chamber reaches 30 psig (2Bar) or more is minimal. There is a principle that the higher the density of an etching solution (ie, specific gravity or Bomei degree), the higher the optimal eruption pressure should be. Of course this is not a single parameter. Another important parameter is the relative mobility (or mobility) that manipulates its response rate in the solution.
Fourth, etching quality and previous problems
The basic requirement for etching quality is to be able to completely remove and clean all the copper layers except underneath the resist layer, and that's it. Strictly speaking, assuming that it is precisely defined, the etching quality must include the consistency of the wire line width and the degree of undercutting. Because of the inherent characteristics of the current etching solution, not only downwards but also etching effects occur in all directions, so side etching is almost inevitable.
The problem of undercutting is often mentioned in the etching parameters, it is defined as the ratio of the width of undercutting to the depth of etching, which is called the etching factor. In the printed circuit industry, its modification plan is very extensive, from 1:1 to 1:5. Obviously, a small undercut degree or a low etch factor is the most satisfactory.
The structure of the etching equipment and the different components of the etching solution will affect the etching factor or the degree of side etching. Perhaps in Daguan terms, it can be manipulated. The use of certain increasing agents can reduce the degree of side erosion. The chemical components of these additives are generally trade secrets, and their respective developers will not leak them to the outside world. As for the structure of the etching equipment, the following chapters will specifically discuss it.
In many ways, the quality of etching has existed long before the printed circuit board (multilayer circuit board) entered the etching machine. Because the various processes or processes of printed circuit (multi-layer circuit board) processing have very close internal connections, there is no process that is not affected by other processes and does not affect other processes. Many problems identified as etching quality have actually existed in the process of removing the film and even more in the past. For the etching process of the outer layer graphics, because the "inverted stream" it exhibits is better than most printed board processes, many problems are ultimately reflected in it. At the same time, this is also because the etching is the final step in a long series of processes that are self-adhesive, and the initial photosensitive, after which the outer layer pattern is transferred successfully. The more links, the greater the possibility of presenting problems. This can be regarded as a very special aspect in the production process of printed circuits.
In theory, after the printed circuit enters the etching stage, in the process of processing the printed circuit by pattern electroplating, the ideal situation should be: the total thickness of copper and tin or copper and lead tin after electroplating should not exceed the resistance to electroplating The thickness of the photosensitive film makes the electroplated graphics completely blocked by the "walls" at both ends of the film and embedded in it. However, in actual production, printed circuit boards (multi-layer circuit boards) all over the world have much thicker plating patterns than photosensitive patterns after electroplating. In the process of copper and lead-tin electroplating, because the plating height exceeds the photosensitive film, the tendency of lateral accumulation occurs, and the problem occurs. The tin or lead-tin resist layer covering the top of the line extends to both ends, forming a "edge", covering a small part of the photosensitive film under the "edge".
The "edge" made of tin or lead tin makes it impossible to completely remove the photosensitive film when removing the film, leaving a small part of the "residual glue" under the "edge". The "residual glue" or "residual film" left under the "edge" of the resist will constitute an incomplete etching. After etching, the lines form "copper roots" at both ends. The copper roots narrow the line spacing, and the printed board does not meet the requirements of Party A, and may even be rejected. Because the rejection will greatly increase the production cost of the PCB multilayer circuit board.
In addition, in many cases, dissolution is caused by the reaction. In the printed multilayer circuit board industry, the residual film and copper may also form a buildup in the corrosive liquid and be blocked in the nozzle of the corrosion machine and the acid-resistant pump, and have to be shut down. Treatment and cleaning affect the efficiency of work.
5. Protection of etching equipment
The most critical factor for the protection of etching equipment is to ensure that the nozzles are clean and free from blockages to make the eruption unobstructed. Clogging or slagging will impact the layout under the effect of eruption pressure. Assuming that the nozzle is not clean, the etching will be uneven and the entire multilayer circuit board will be scrapped.
Obviously, the protection of equipment is the replacement of damaged and worn parts, including replacement nozzles, which also have the problem of wear. In addition, the more critical issue is to insist that there is no slagging in the etching machine. In many cases, there will be slag accumulation. Too much slag accumulation may even affect the chemical balance of the etching solution. Similarly, if the etching solution presents excessive chemical imbalance, the slagging will be more severe. The problem of slagging accumulation cannot be overemphasized. Once the etching solution suddenly shows a lot of slagging, it is usually a signal that the balance of the solution is problematic. This should be done with strong hydrochloric acid for proper cleaning or supplementation of the solution.
Residual film can also cause slagging, a very small amount of residual film dissolves in the etching solution, and then forms copper salt deposition. The slagging caused by the residual film indicates that the previous film removal process is incomplete. Poor film removal is often the result of edge film and over-plating.
