PCB etching process and process control

The process of the printed circuit board from the light board to the circuit pattern is a relatively complex process of physical and chemical reactions. This article analyzes the last step-etching.

1. Types of etching

       In the processing technology of the outer circuit of the printed circuit board, there is another method, which is to use the photosensitive film instead of the metal coating as the resist 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.

       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. Some people tried to use sulfuric acid-hydrogen peroxide as an etchant to corrode the outer layer pattern. Due to many reasons including economy and waste liquid treatment, this process has not been widely used in a commercial sense. Furthermore, sulfuric acid-hydrogen peroxide can not be used for the etching of lead-tin resist, and this process is not the main method in the production of PCB outer layer, so most people rarely care about it.

2. Etching quality and problems in the early stage

       The basic requirement for etching quality is to be able to completely remove all the copper layers except under the resist layer, and that's it. Strictly speaking, if it is to be accurately defined, then the etching quality must include the consistency of the wire width and the degree of undercutting. Due to the inherent characteristics of the current etching solution, which not only produces an etching effect on the downward direction but also on the left and right directions, side etching is almost inevitable. The problem of undercutting is one of the etching parameters that is often raised for discussion. 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, it has a wide range of changes, from 1:1 to 1:5. Obviously, a small undercut degree or a low etching 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, or in optimistic terms, it can be controlled. The use of certain additives can reduce the degree of side erosion. The chemical composition of these additives is generally a trade secret, and the respective developers do not disclose it to the outside world. In many ways, the quality of etching has existed long before the printed board enters the etching machine. Because there are very close internal connections 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 existed in the process of removing the film or even before. For the etching process of the outer layer graphics, because the "inverted stream" phenomenon it embodies 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 step in a long series of processes starting with self-sticking and photosensitive, after which the outer layer pattern is successfully transferred. The more links, the greater the possibility of problems. This can be seen as a very special aspect of the printed circuit production process.

Theoretically speaking, after the printed circuit enters the etching stage, in the process of processing the printed circuit by the pattern plating method, the ideal state should be: the total thickness of the electroplated copper and tin or copper and lead tin should not exceed the resistance to electroplating The thickness of the photosensitive film makes the electroplating pattern completely blocked by the "walls" on both sides of the film and embedded in it. However, in actual production, after electroplating printed circuit boards all over the world, the plating pattern is much thicker than the photosensitive pattern. In the process of copper and lead tin electroplating, since the height of the plating layer exceeds the photosensitive film, a tendency of lateral accumulation occurs, and the problem arises from this. The tin or lead-tin resist layer covering the lines extends to both sides to form a "edge", covering a small part of the photosensitive film under the "edge". The "edge" formed by tin or lead tin makes it impossible to completely remove the photosensitive film when removing the film, leaving a small part of "residual glue" under the "edge". The "residual glue" or "residual film" left under the "edge" of the resist will cause incomplete etching. The lines formed "copper roots" on both sides after etching. The copper roots narrowed the line spacing, causing the printed board to not meet the requirements of Party A, and may even be rejected. Rejection will greatly increase the production cost of PCB. In addition, in many cases, due to the formation of dissolution due to the reaction, in the printed circuit industry, the residual film and copper may also form and accumulate in the corrosive liquid and be blocked in the nozzle of the corrosive machine and the acid-resistant pump, and have to be shut down for processing and cleaning , Which affects work efficiency.

3. Equipment adjustment and interaction with corrosive solution

       In printed circuit processing, ammonia etching is a relatively delicate and complex chemical reaction process. On the other hand, it is an easy job. Once the process is up-regulated, production can be continued. The key is to maintain continuous working status once it is turned on, and it is not advisable to dry and stop. The etching process depends to a large extent on the good working condition of the equipment. In order to obtain good side effects, many different theories have emerged, forming different design methods and equipment structures. These theories are often very different. In ammonia etching, assuming that all other parameters remain unchanged, the etching rate is mainly determined by the ammonia (NH3) in the etching solution. Therefore, using fresh solution to etch the surface has two main purposes: one is to flush out the copper ions that have just been produced; the other is to continuously provide ammonia (NH3) needed for the reaction. In fact, many ammonia-based etching solution products contain special ligands for monovalent copper ions (some complex solvents), 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. If the monovalent copper is reduced from 5000ppm to 50ppm, the etching rate will be more than doubled. This is a functional reason for passing air into the etching box. However, if there is too much air, it will accelerate the loss of ammonia in the solution and decrease the pH value, resulting in a decrease in the etching rate. Ammonia in the solution is also the amount of change that needs to be controlled. Some users adopt the method of passing pure ammonia into the etching reservoir. To do so, a set of PH meter control system must be added. When the automatically measured PH result is lower than the given value, the solution will be added automatically. In the related field of chemical etching (also known as photochemical etching or PCH), research work has begun and has reached the stage of structural design of the etching machine. 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, and in some cases the thickness is quite large. Its requirements for etching parameters are often more stringent than those in the PCB industry.

4. Regarding the upper and lower board surfaces, the etching state of the leading edge and the trailing edge are different

       A large number of problems related to etching quality are concentrated on the etched part of the upper plate surface. 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 slabstock on the copper surface affects the spraying force on the one hand, and on the other hand prevents the replenishment of fresh etching solution, resulting in a decrease in the etching speed. It is precisely because of the formation and accumulation of colloidal slabs that the degree of etching of the upper and lower patterns of the board is different. This also makes the first part of the board in the etching machine easy to be etched completely or to cause over-corrosion, because the accumulation has not yet formed at that time, and the etching speed is faster. On the contrary, the part that enters behind the board has already formed when it enters, and slows down its etching speed.

5. Maintenance of etching equipment

  The most critical factor in the maintenance of etching equipment is to ensure that the nozzle is clean and free of obstructions to make the jet unobstructed. Blockages or slagging will impact the layout under the action of jet pressure. If the nozzle is not clean, it will cause uneven etching and scrap the entire PCB. Obviously, equipment maintenance is the replacement of damaged and worn parts, including replacement of nozzles. The nozzles also have 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 slag may even affect the chemical balance of the etching solution.

       Similarly, if there is excessive chemical imbalance in the etching solution, slagging will become more serious. The problem of slag accumulation cannot be overemphasized. Once a large amount of slagging occurs suddenly in the etching solution, it is usually a signal that there is a problem with the balance of the solution. This should be done with strong hydrochloric acid for proper cleaning or supplementation of the solution. Residual film can also produce slagging, a very small amount of residual film dissolves in the etching solution, and then forms copper salt precipitation. The slagging formed by the residual film indicates that the previous film removal process is not complete. Poor film removal is often the result of edge film and over-plating.