Etching Process of Outer Circuit of Printed Circuit Board

The typical process of PCB 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 retained on the outer layer of the board, that is, the graphic part of the circuit, and then the rest of the copper foil is corroded by chemical means, which is called etching. It should be noted that there are two layers of copper on the board at this time. In the outer etching process, only one layer of copper must be completely etched away, and the rest will form 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 to coat the entire plate 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 must be plated twice and they must be corroded away during etching. Therefore, when the wire width is very fine, a series of problems will arise. At the same time, side corrosion will seriously affect the uniformity of the lines.

In the processing process of the outer circuit of the printed board, there is another method, which is 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, you can see etching in the inner layer making process. At present, 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 commonly used chemical liquid, which does not react 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.

Sulfate based etching solution, after use, the copper in it can be separated by electrolysis, so it can be reused. Due to its low corrosion rate, it is generally rare in actual production, but it is expected to be used in chlorine-free etching. Some people experimented with the use of sulfuric acid-hydrogen peroxide as an etching agent to corrode the outer pattern. For a number of reasons, including economic and liquid waste treatment aspects, this process has not been widely adopted 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 main method in the production of PCB outer layer, so most people rarely ask.

Etching quality and previous problems

The basic requirement for etching quality is to be able to completely remove all copper layers except under the resist layer, and that's it. In a strict sense, if it is to be precisely defined, then the etching quality must include the consistency of the wire width and the degree of side erosion. Due to the inherent characteristics of the current corrosion fluid, not only downward but also in all directions to produce etching, so 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, it varies widely, from 1:1 to 1:5. Obviously, a small side etching degree or a low etching factor is the 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, or in optimistic terms, it can be controlled. Some additives can reduce the degree of side corrosion. The chemical composition of these additives is generally a trade secret, and the respective developers are not disclosed to the outside world. As for the structure of the etching equipment, the following chapters will be devoted to the discussion.

In many ways, the quality of etching has existed long before the printed board entered the etching machine. Because there is a very close internal relationship between the various processes or processes of printed circuit 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 actually exist in the process of film removal and even before. For the etching process of the outer layer graphics, because the "pour stream" image reflected in it is more prominent than most printed board processes, many problems are finally reflected in it. At the same time, this is also because the etching is the last link in a long series of processes that start with the self-coating film, after which the outer pattern is transferred successfully. The more links there are, the greater the chance of problems. This can be regarded as a very special aspect of the printed circuit production process.

In theory, the printed circuit enters the etching stage. In the process of processing printed circuits by graphic electroplating, the ideal state should be that the sum of the thickness of 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" on both sides of the film and embedded in it. However, in real production, the world's printed 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, there is a tendency to lateral accumulation, which results in problems. A tin or lead-tin resist layer covering the top of the line extends to both sides to form a "edge", which covers a small amount of sensitive film under the "edge".

The "edge" formed by 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 cause incomplete etching. The lines form "copper roots" on both sides after etching, and the copper roots narrow the line spacing, resulting in the printed board does not meet the requirements of Party A and may even be rejected. The rejection will greatly increase the production cost of PCB. In addition, in many cases, due to the reaction and the formation of dissolution, in the printed circuit industry, residual film and copper may also form accumulation in the corrosive fluid and blocked in the nozzle of the corrosion machine and the acid pump, have to shut down for treatment and cleaning, and affect the work efficiency.

Equipment adjustment and interaction with corrosion solutions

In printed circuit processing, ammonia etching is a relatively fine and complex chemical reaction process. On the other hand, it is an easy task. Once the process is up-regulated, continuous production can be carried out. The key is to maintain a continuous working state once the machine is started, and it is not appropriate to dry and stop. The etching process depends greatly on the good working condition of the equipment. At present, no matter what kind of etching fluid is used, high pressure spray must be used, and in order to obtain a cleaner line side and high-quality etching effect, the structure and spray method of the nozzle must be strictly selected.

In order to get a good side effect, there are many different theories, forming different design methods and equipment structures. These theories are often vastly different. But all theories about etching recognize the basic principle of constantly exposing the metal surface to fresh etching fluid as quickly as possible. The chemical mechanism analysis of the etching process also confirmed the above viewpoint. In ammonia etching, assuming that all other parameters are constant, the etching rate is mainly determined by the ammonia (NH3) in the etching solution. Therefore, the purpose of the fresh solution and the etched surface is mainly two: one is to wash away the copper ions just produced; The second is the continuous supply of ammonia (NH3) required for the reaction.

In the traditional knowledge of the printed circuit industry, especially the suppliers of printed circuit raw materials, it is recognized that the lower the content of monovalent copper ions in ammonia etching solution, the faster the reaction speed. This has been borne out by experience. In fact, many ammonia etching liquid products contain special coordination groups (some complex solvents) of monovalent copper ions, whose role is to reduce monovalent copper ions (these are the technical secrets of their products with high reactivity), it can be seen that the influence of monovalent copper ions is not small. Reducing copper from 5000ppm to 50ppm will more than double the etching rate.

Due to the formation of a large number of monovalent copper ions during the etching reaction, 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. Copper can be removed by converting monovalent copper to bivalent copper through the action of atmospheric oxygen. The above purpose can be achieved by spraying.

This is a functional reason to pass air into the etching chamber. However, if there is too much air, it will accelerate the loss of ammonia in the solution and reduce the PH value, which will still reduce the etching rate. Ammonia in solution is also a variable that needs to be controlled. Some users use the practice of passing pure ammonia into the etched liquid storage tank. To do so, a PH meter control system must be added. When the PH value is lower than the given value, the solution is automatically added.

In the related field of chemical etching (also known as photochemical etching or PCH), research work has begun and reached the stage of structural design of the etching machine. In this method, the solution used is copper divalent, not ammonia-copper etching. It will likely 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 demanding than those in the PCB industry.

There is a study from the PCM industrial system that has not yet been officially published, but the results will be refreshing. Due to the strong project funding support, the researchers were able to make long-term changes in the design of etching devices and study the effects of these changes. For example, the best nozzle design is fan-shaped compared to conical nozzles, and the spray collector cavity (that is, the section of pipe into which the nozzle is screwed) also has an Angle to spray the workpiece entering the etching chamber at 30 degrees. If this change is not made, the way the nozzles on the collector cavity are installed will cause the injection Angle of each adjacent nozzles to not be exactly the same. The spray surface of the second group of nozzles is slightly different from that of the first group. This makes the ejected solution shape into a superimposed or crossed state. In theory, if the solution shapes cross each other, then the spray force of that part will be reduced, and it will not be able to effectively wash away the old solution on the etched surface while keeping the new solution in contact with it. This is especially true at the edges of the spray surface. The jet force is much smaller than in the vertical direction.

The study found that the latest design parameter is 65 pounds per square inch (that is, 4+Bar). There is a question of optimal injection pressure for each etching process and for each practical solution, and as of now, it is rare for the etching chamber to reach a injection pressure of more than 30 pounds per square inch (2Bar). There is a principle that the higher the density of an etching solution (i.e., the specific gravity or beaume degree), the higher the optimal injection pressure should be. Of course this is not a single parameter. Another important parameter is the relative mobility (or mobility) that controls its reaction rate in a solution.

About the etching state of the upper and lower panels, the leading edge and the back edge are different

A large number of problems related to etching quality focus on the etched part of the upper plate surface. It is important to understand this. These problems come from the effect of colloidal bonding produced by etching agents on the top surface of printed circuit boards. The accumulation of colloidal plate on the copper surface affects the injection force on the one hand, and blocks the replenishment of fresh etching fluid on the other hand, resulting in the reduction of etching speed. It is because of the formation and accumulation of colloidal plates that the etching degree of the upper and lower shapes of the plates is different. This also makes the first part of the board in the etching machine easy to etch completely or easily cause excessive corrosion, because the accumulation has not yet formed at that time, the etching speed is faster. Conversely, the part of the board that enters after the board enters has accumulated and slows down its etching rate.

Maintenance of etching equipment

The most critical factor in the maintenance of etching equipment is to ensure that the nozzle is clean and the spray is smooth without obstruction. Blockages or slagging can impact the surface under jet pressure. If the nozzle is not clean, it will cause uneven etching and scrap the entire PCB. Obviously, the maintenance of the equipment is to replace the broken parts and wear parts, including the replacement of the nozzle, the nozzle also has the problem of wear. In addition, the more critical issue is to keep the etching machine free of slagging, which will occur in many cases. Excessive accumulation of slagging will even affect the chemical balance of the etching solution. Similarly, if there is an excessive chemical imbalance in the etching fluid, the slagging will become more serious. The problem of slagging accumulation cannot be overemphasized. Once the etching fluid suddenly appears a large amount of slagging, it is usually a signal that the balance of the solution has a problem. This should be properly cleaned with a stronger hydrochloric acid or the solution should be supplemented.

Residual film can also produce slagging, a very small amount of residual film dissolved in the etching solution, and then formed copper salt precipitate. The slagging formed by the residual film indicates that the previous film removal process is not thorough. Poor film removal is often the result of the combination of edge film and electroplating.