PCB Blog - PCB board etching process and process control

The process of PCB board from light board to showing circuit pattern is a relatively complicated process of physical and chemical reaction. This article analyzes its one-step etching. At present, the typical process of PCB (PCB board) processing adopts the "graphic electroplating method". That is, pre-plated a lead-tin anti-corrosion layer on the part of the copper foil that needs to be retained on the outer layer of the board, that is, the pattern part of the circuit, and then chemically corrodes the remaining copper foil, which is called etching.

1. Types of etching
It should be noted that there are two layers of copper on the board during etching. In the outer layer etching process, only one layer of copper must be completely etched away, and the rest will form the final required circuit. The characteristic of this type of pattern plating is that the copper plating layer only exists under the lead-tin resist layer. Another process method is to plate copper on the entire 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 disadvantage of copper plating on the whole board is that copper must be plated twice on all parts of the board and they must be corroded during etching. Therefore, when the wire width is very fine, a series of problems will occur. At the same time, side corrosion will seriously affect the uniformity of the line. In the processing technology of the outer circuit of the printed board, there is another method, which is to use a photosensitive film instead of the metal coating as a 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. At present, tin or lead-tin is a commonly used anti-corrosion layer, which is used in the etching process of ammonia-based etchant. Ammonia-based etchant is a commonly 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 actual production, but it is expected to be used in chlorine-free etching. Someone 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 cannot be used for etching of lead-
tin resist, and this process is not PCB The main method in the production of the outer layer of the board, so most people rarely care about it.

2. Etching quality and previous problems
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 defined, the etching quality must include the consistency of the wire line width and the degree of side etching. 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 PCB board 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 satisfactory. The structure of the etching equipment and the etching solutions of different compositions 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 PCB 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 actually existed in the process of removing the film or even before. As for the etching process of outer layer graphics, because the "inverted stream" phenomenon it embodies is more prominent than most printed board processes, many problems are reflected in it. At the same time, this is also due to the fact that etching is a part of a long series of processes that starts 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 PCB production process. Theoretically speaking, after the PCB board enters the etching stage, in the process of processing the PCB board by the pattern electroplating method, the ideal state should be: the total thickness of the electroplated copper and tin or copper and lead tin should not exceed the electroplating resistant photosensitive film The thickness of the electroplating pattern is completely blocked by the "walls" on both sides of the film and embedded in it. However, in actual production, after electroplating of PCB boards all over the world, the plating pattern is much thicker than the photosensitive pattern. In the process of electroplating copper and lead-tin, since the plating height 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 form "copper roots" on both sides after etching. The copper roots narrow the line spacing, causing the printed board to fail to meet Party A's requirements and may even be rejected. The rejection will greatly increase the production cost of the PCB board. In addition, in many cases, due to the formation of dissolution due to the reaction, in the PCB board industry, the residual film and copper may also form and accumulate 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 for processing and cleaning. And it affects work efficiency.

3. Equipment adjustment and interaction with corrosive solution
In PCB board 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 great extent on the good working condition of the equipment. At present, no matter what kind of etching solution is used, high-pressure spray must be used, and in order to obtain a neater line side and high-quality etching effect, the nozzle structure and spray method must be strictly selected. In order to obtain good side effects, many different theories have appeared, forming different design methods and equipment structures. These theories are often very different. But all theories about etching recognize the basic principle of keeping the metal surface in constant contact with fresh etching solution as quickly as possible. The chemical mechanism analysis of the etching process also confirmed the above point of view. 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 just produced; the other is to continuously provide ammonia (NH3) needed for the reaction. In the traditional knowledge of the PCB board industry, especially the suppliers of PCB board raw materials, it is recognized that the lower the monovalent copper ion content in the ammonia etching solution, the faster the reaction speed. This has been confirmed by experience. 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. Reducing monovalent copper from 5000ppm to 50ppm will increase the etching rate by more than one time. Since a large amount of monovalent copper ions are generated during the etching reaction, and because the monovalent copper ions are always tightly combined with the complexing group of ammonia, it is very difficult to keep its content close to zero. The monovalent copper can
be removed by converting monovalent copper into divalent copper through the action of oxygen in the atmosphere. The above purpose can be achieved by spraying. This is a functional reason for passing air into the etching box. However, if the air is too much, it will accelerate the loss of ammonia in the solution and make the PH value drop, 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 field of chemical etching (called photochemical etching or PCH) related to this, research work has begun and has reached the stage of etching machine structure design. In this method, the solution used is divalent copper, not ammonia-copper etching. It may be used in the PCB board 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 board 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 colloidal compaction produced by the etchant on the upper surface of the PCB board. The accumulation of colloidal slabs on the copper surface affects the jetting force on the one hand, and on the other hand prevents the replenishment of fresh etching solution, resulting in a reduction 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 key factor in the maintenance of etching equipment is to ensure that the nozzle is clean and free of obstructions to make the jet unobstructed. Clogging 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 board. 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. In many cases, slagging will accumulate. Excessive slagging will 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 both edge film and over-plating in the PCB board.