Now the typical process of printed circuit board (PCB multi-layer circuit board) processing uses the "graphic plating method". That is, a layer of lead-tin corrosion resistance layer is pre-plated on the copper foil part that needs to be preserved on the outer layer of the board, that is, the graphic part of the circuit, and then the other copper foil is corroded by chemical methods, which is called etching.
It should be noted that there are two layers of copper above the multilayer circuit board at this time. In the outer etching process, only one layer of copper is necessary to be etched away, and the rest will constitute the final required circuit. This type of graphic plating is characterized by a copper-coated layer that exists only under the lead tin resist layer. Another process method is that the entire multilayer circuit board is plated with copper, and the part outside the sensitive film is only tin or lead-tin resist layer. This process is called "full plate copper plating process". Compared with graphic plating, the biggest disadvantage of full-plate copper plating is that the surface of the plate is twice plated with copper and it is necessary to corrode them when etching. Therefore, when the wire width is very precise, a series of problems will occur. At the same time, side corrosion will severely affect the uniformity of the lines.
In the processing process of the outer circuit of the PCB board (multi-layer circuit board), there is another way to use the sensitive film instead of the metal coating as the corrosion resistance layer. This method is very similar to the inner layer etching process, which can be seen in the inner layer production process.
Nowadays, tin or lead tin is the most commonly used corrosion resistance layer used in the etching process of ammonia etching agents. Ammonia etching agent is a widely used chemical liquid, which does not have any chemical reaction with tin or lead tin. Ammonia etching agent mainly refers to ammonia water/ammonia chloride etching solution. In addition, ammonia/ammonia sulfate etching solution can also be bought in the market.
The sulfate based etching solution can be used, and the copper in it can be separated by electrolysis, so it can be used repeatedly. Because of its low corrosion rate, it is generally rare in practical production, but it is expected to be used in chlorine-free etching. Some people experiment with sulfuric acid-hydrogen peroxide as an etching agent to corrode the outer pattern. For many reasons, including economic and liquid waste treatment, this process is not widely used in a commercial sense. Further said, sulfuric acid-hydrogen peroxide, can not be used for lead tin corrosion resistance layer etching, and this process is not the first way in the production of multi-layer circuit board outer layer, so most people rarely ask.
Many of the problems that touch the etching quality are connected to the etched part of the upper multilayer circuit board surface. It is important to understand this. These problems come from the effect of colloidal bonding on the etching agent of the top surface of the printed circuit board. The accumulation of colloidal plate on the copper surface affects the eruption force on the one hand, and blocks the compensation of fresh etching liquid on the other hand, which constitutes the decrease of etching speed. It is precisely because of the composition and accumulation of colloidal slab that the etching degree of the upper and lower patterns of the board is different. This also makes the complete or simple formation of the simple etching of the first part of the board in the etching machine (multilayer circuit board) over corrosion, because the accumulation is not formed at that time, and the etching speed is faster. On the contrary, the part that enters after the multilayer circuit board is deposited when it enters and slows down its etching speed.
In the processing of printed circuit (multilayer circuit board), ammonia etching is a relatively sophisticated and messy chemical reaction process. On the other hand, it is an easy task to perform. Once the process is adjusted, continuous production can be carried out. The key is to insist on continuous operation once started, and it is not appropriate to stop and stop. The etching process depends to a great extent on the excellent working condition of the equipment. For now, no matter what kind of etching fluid is used, it is necessary to use high pressure spray, and in order to achieve more regular line sides and high-quality etching effects, it is necessary to strictly choose the structure of the nozzle and spray method.
In order to obtain excellent side effects, many different theories are presented, which constitute different planning methods and equipment structures. These theories are often very different. But all theories about etching recognize the basic principle of constantly touching the metal surface with fresh etching liquid as quickly as possible. The chemical mechanism analysis of the etching process also proves the above viewpoint. In ammonia etching, assuming that all other parameters are unchanged, the etching rate is mainly determined by the ammonia (NH3) in the etching solution. Therefore, the purpose of using fresh solution and etching appearance effect is mainly two: one is to wash away the copper ion that has just occurred; The second is the continuous supply of ammonia (NH3) required for the reaction.
In the traditional common sense of the printed circuit industry, especially the suppliers of printed circuit materials, we recognize that the lower the content of monovalent copper ions in ammonia etching solution, the faster the response speed. This has been proved by experience. In fact, many ammonia etching solution products contain special coordination groups (some messy solvents) of monovalent copper ions, and the effect is to reduce monovalent copper ions (these are the skills of their products with high response ability), which shows that the influence of monovalent copper ions is not small. When copper is reduced from 5000ppm to 50ppm, the etching rate will more than double.
Because of the formation of many monovalent copper ions in the etching reaction process, and because the monovalent copper ions are always tightly bound to the complex group of ammonia, it is very difficult to maintain its content close to zero. The conversion of monovalent copper to bivalent copper by the effect of atmospheric oxygen can remove monovalent copper. The above purpose can be achieved by spraying.
This is a functional reason to pass air into the etching chamber. However, if too much air is assumed, it will accelerate the loss of ammonia in the solution and reduce the PH value, and the effect is still to reduce the etching rate. Ammonia in solution is also a variable that needs to be manipulated. Some users choose to pass pure ammonia into the etched liquid storage tank. To do so, it is necessary to add a PH meter control system. When the PH effect of the active measurement is lower than the given value, the solution will actively increase.
In the related field of chemical etching (also known as photochemical etching or PCH), the research work is now preliminary and has reached the stage of structural planning of the etching machine. In this method, the solution used is copper divalent, not ammonia-copper etching. It will probably be used in the printed circuit industry. In the PCH industry, the typical thickness of etched copper foil is 5 to 10 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 PCM industrial systems, which is not officially announced, but its effects will be refreshing. Because of the relatively strong project fund support, researchers have the ability to change the planning thinking of etching equipment from a long-term sense, and discuss the effects of these changes. For example, compared to conical nozzles, the optimal nozzle planning uses a fan, and the spray collector cavity (that is, the section of pipe into which the nozzle is screwed) also has a device viewpoint that emits a 30-degree eruption of the workpiece entering the etching chamber. Assuming that no such change is made, the arrangement of the nozzles on the collector cavity will result in the view point of the eruption of each adjacent nozzles being inconsistent. The spray surface of the second group of nozzles is slightly different from that of the first group (it indicates the operation of the spray). This results in a superimposed or crossed shape of the ejected solution. Theoretically, if the solution shapes cross each other, then the eruption force of this part will be reduced, and the old solution on the etched surface cannot be effectively washed away while the new solution is kept in touch with it. This is particularly good at the edges of the spray surface. The eruption force is much less than in a straight direction.
The study found that the latest planning parameter is 65 pounds per square inch (4+Bar). There is a question of the optimal eruption pressure for each etching process and for each useful solution, and as of now, it is very rare for the eruption pressure in the etching chamber to reach more than 30 pounds per square inch (2Bar). There is a principle that the higher the density of an etching solution (i.e. specific gravity or beaume 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 controls the response rate in the solution.
The basic requirement for etching quality is to be able to completely remove and clean all copper layers except under the resist layer, and that's it. In a strict sense, assuming that to be precisely defined, the etching quality must include the consistency of the wire width and the degree of side erosion. Because of the inherent characteristics of the corrosion fluid nowadays, the etching effect occurs not only down but in all directions, so the side erosion is almost inevitable.
The problem of lateral erosion is one of the etch parameters that is often discussed. It is defined as the ratio of lateral erosion width to etching depth, which is called the etching factor. In the printed circuit industry, its change planning is very wide, from 1:1 to 1:5. Significantly, small side etch degrees or low etching factors are most satisfactory.
The structure of the etching equipment and the different composition of the etching fluid will have an impact on the etching factor or the degree of side erosion, and perhaps, in noble terms, it can be manipulated. Some additives can reduce the degree of lateral corrosion. The chemical composition of these additives is generally a trade secret, and the respective researchers are not leaked to the outside world. As for the structure of the etching equipment, the following chapters will be devoted to it.
In many ways, the quality of etching has already existed before the printed circuit board (multi-layer circuit board) entered the etching machine. Because there is a very close internal connection between the various processes or processes of printed circuit (multi-layer circuit board) processing, there is no process that is not affected by other processes and does not affect other processes. Many of the problems identified as etching quality have already existed in practice in film removal and even earlier processes. For the etching process of the outer layer graphics, because the "pouring stream" that it shows is superior to most printed board processes, many problems are ultimately reflected in it. Also, this is because etching is the final link in a long series of processes of self-adhesive film, initial photosensitivity, after which the outer pattern is transferred successfully. The more links, the greater the possibility of presenting a problem. This can be seen as a very special other aspect of the printed circuit production process.
In theory, after the printed circuit enters the etching stage, in the process of processing the printed circuit by the graphic electroplating method, the ambition should be: the total thickness of the copper and tin or copper and lead tin after electroplating should not exceed the thickness of the electroplating sensitive film, so that the electroplating pattern is completely blocked by the "wall" at both ends of the film and embedded in it. However, in actual production, the world's printed circuit boards (multi-layer circuit boards) after electroplating, the coating pattern is much thicker than the sensitive pattern. In the process of electroplating copper and lead tin, because the coating height exceeds the photosensitive film, the trend of lateral accumulation occurs, and the problem occurs. The tin or lead tin resist layer covered above the line extends to both ends to form the "edge", and a small amount of sensitive film is covered under the "edge".
The "edge" composed of tin or lead tin makes it impossible to completely remove the sensitive film when the film is removed, leaving a small part of the "residual glue" below the "edge". The "residual glue" or "residual film" left behind the "edge" of the resist will constitute incomplete etching. The two ends of the line form a "copper root" after etching, and the copper root makes the line interval narrow, 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 PCB multilayer circuit boards.
In addition, in many cases, because of the reaction and dissolution, in the printed multi-layer circuit board industry, the residual film and copper may also constitute accumulation in the corrosive liquid and blocked in the nozzle of the corrosion machine and the acid pump, and have to shut down for processing and cleaning, which affects the efficiency of the operation.
The most critical factor in the protection of etching equipment is to ensure that the nozzle is clean and free of blockages to make the eruption smooth. Blockages or slagging can impact the surface under the effect of eruption pressure. Assuming that the nozzle is not clean, it will constitute uneven etching and scrap the entire multi-layer circuit board.
Obviously, the protection of the equipment is to replace the broken parts and worn parts, including the replacement of the nozzle, the nozzle has the same wear problem. In addition, the more critical issue is to insist that there is no slagging in the etching machine, which will present slagging accumulation in many cases. Too much slagging will even affect the chemical balance of the etching solution. Similarly, if the etching fluid presents an excessive chemical imbalance, the slagging will become more severe. The problem of slagging accumulation cannot be overemphasized. Once the etching fluid suddenly presents a lot of slag formation, it is usually a signal that the balance of the solution is problematic. This should be properly cleaned with a stronger hydrochloric acid or the solution should be supplemented.
Residual film can also be slag formation, a very small amount of residual film dissolved in the etching solution, and then formed copper salt deposition. The slagging formed by the residual film indicates that the previous film removal process is incomplete. Poor film removal is often the effect of edge film and electroplating together.
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