PCB Blog - The etching process of the outer circuit of the PCB board

I. Overview

At present, the typical process of printed circuit boards processing adopts the "pattern plating method". That is, pre-plating a lead-tin resist layer on the copper foil part of the outer layer of the board that needs to be retained, that is, the graphic part of the circuit, and then chemically corrodes the rest of the copper foil, which is called etching. It should be noted that there are two layers of copper on the board at this time. 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. This type of pattern plating is characterized by the presence of a copper layer only under the lead-tin resist. Another processing method is that the whole board is copper-plated, and the part other than the photosensitive film is only a tin or lead-tin resist layer. This process is called the "full board copper plating process". Compared with pattern plating, the disadvantage of full-board copper plating is that copper is plated twice everywhere on the board and must be etched away during etching. Therefore, a series of problems will arise when the wire width is very fine. At the same time, side corrosion (see Figure 4) can seriously affect the uniformity of the lines.

In the processing technology of the outer circuit of the printed 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, you can refer to the etching in the inner layer fabrication process. At present, tin or lead-tin is a commonly used resist layer, which is used in the etching process of ammonia etchant. Ammonia 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 water/ammonium chloride etching solution. In addition, ammonia water/ammonia sulfate etching solution is also available 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 tried to use sulfuric acid-hydrogen peroxide as an etchant to corrode the outer layer pattern. Due to many reasons including economic and waste liquid treatment, this process has not been widely adopted in a commercial sense. Furthermore, sulfuric acid-hydrogen peroxide cannot be used for the etching of lead-tin resist, and this process is not PCB It is the main method in the production of the outer layer of the board, so most people rarely care about it.

2. Etching quality and early 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. Strictly speaking, if the ground is to be defined, the etching quality must include the uniformity of the wire width and the degree of side etching. Due to the inherent characteristics of the current etchant, it not only etches downward but also in all directions, so side etching is almost inevitable. The undercut problem is one of the etching parameters that is often discussed, which is defined as the ratio of the undercut width to the etching depth, called the etching factor. In the printed circuit industry, it varies widely, from 1:1 to 1:5. Obviously, a small undercut degree or a low etch factor is satisfactory. The structure of the etching equipment and the different compositions of the etching solution will have an impact on the etching factor or side etching degree, or in optimistic terms, it can be controlled. The use of certain additives can reduce the degree of side etching. The chemical compositions of these additives are generally trade secrets, and their developers do not disclose them 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 exists long before the printed board enters the etching machine. Because there is a very close internal connection 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 etch quality actually existed in the stripping process even earlier. For the etching process of the outer layer pattern, many problems are reflected in it because the "reverse stream" phenomenon it reflects is more prominent than most printed board processes. At the same time, this is also because the etching is a part of a long series of processes starting with self-adhesive film and photosensitive, after which the outer layer pattern is successfully transferred. The more links there are, the greater the chance 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, the cross-sectional state of the pattern should be as shown in Figure 2. In the process of processing printed circuits by pattern electroplating, the ideal state should be: the sum of the thicknesses of copper and tin or copper and lead and tin after electroplating should not exceed the thickness of the electroplating photosensitive film so that the electroplating pattern is completely covered with "on both sides of the film. wall" blocked and embedded in it. However, in actual production, after electroplating of printed circuit boards all over the world, the coating pattern is much thicker than the photosensitive pattern. In the process of electroplating copper and lead-tin, since the height of the plating layer exceeds the photosensitive film, there is a tendency to accumulate laterally, and the problem arises.

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". "Residual glue" or "residual film" is left under the resist "edge" and 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 not meeting the requirements of Party A, and may even be rejected. The production cost of the PCB board will be greatly increased due to rejection. 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 accumulate in the etching solution and block the nozzle of the etching 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 the corrosive solution

In PCB board processing, ammonia etching is a relatively fine and complex chemical reaction process. In turn, it is an easy job. Once the process has been turned up, production can continue. The key is that once it is turned on, it needs to maintain a continuous working state, and it is not advisable to stop 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 spraying must be used, and in order to obtain neat line sides and a high-quality etching effect, the structure and spraying method of the nozzle must be strictly selected. In order to obtain a good side effect, many different theories have emerged, resulting in different design methods and equipment structures. These theories are often very different. But all theories about etching acknowledge the basic principle of getting the metal surface in constant contact with fresh etchant as quickly as possible. The chemical mechanism analysis of the etching process also confirmed the above point. In ammonia etching, assuming all other parameters are constant, the etching rate is mainly determined by the ammonia (NH3) in the etching solution. Therefore, using the 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 the 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 generally recognized that the lower the content of monovalent copper ions in the ammonia-based etching solution, the faster the reaction speed. This has been confirmed by experience. . In fact, many ammonia-based etchant products contain special ligands (some complex solvents) for monovalent copper ions, which act to reduce monovalent copper ions (these are the technical secrets of their products' high reactivity ), it can be seen that the influence of monovalent copper ions is not small. Reducing the monovalent copper from 5000ppm to 50ppm will more than double the etch rate. Since a large amount of monovalent cupric ions are generated during the etching reaction, and because the monovalent cupric ions are always tightly combined with the complex group of ammonia, it is very difficult to keep the content close to zero. Monovalent copper can be removed by converting monovalent copper to divalent copper by the action of oxygen in the atmosphere. The above purpose can be achieved by spraying. This is one 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 etch rate. Ammonia is also a variable amount in a solution that needs to be controlled. Some users have adopted the practice of passing pure ammonia into the etching reservoir. To do so, a PH meter control system must be added. When the automatically measured pH is lower than a 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 has reached the stage of the structural design of the etcher. In this method, the solution used is copper divalent, not an ammonia-copper etch. It will likely be used in the printed circuit industry. In the PCH industry, etched copper foils are typically 5 to 10 mils thick, and in some cases considerably thicker. Its requirements for etching parameters are often more stringent than those in the PCB industry.

There is a study from PCM industrial systems that have not yet been officially published, but the results will be refreshing. Due to the relatively strong project funding support, researchers have the ability to change the design thinking of the etching device in the long run and study the effects of these changes. For example, compared with the conical nozzle, the nozzle design of the nozzle adopts a fan shape, and the spray manifold (that is, the tube into which the nozzle is screwed) also has an installation angle, which can spray the workpiece entering the etching chamber at a 30-degree angle. If not If such a change is made, the installation of the nozzles on the manifold will result in the spray angles of each adjacent nozzle are not exactly the same. The respective spray surfaces of the second group of nozzles are slightly different from those of the corresponding groups (see Figure 8, which shows the working conditions of the spray). In this way, the shapes of the sprayed solutions are superimposed or crossed. Theoretically, if the solution shapes cross each other, the jetting force of that part is reduced and cannot effectively wash the old solution off the etched surface while keeping the new solution in contact with it. This is especially true at the edges of the spray surface. Its jet force is much smaller than that in the vertical direction. This study found that the design parameter was 65 psi (ie 4+Bar). Every etching process and every practical solution has an injection pressure problem, and at present, it is very rare that the injection pressure in the etching chamber exceeds 30 psi (2Bar). There is a principle that the higher the density of an etching solution (ie specific gravity or degree of Baume), the higher the 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 solution.

4. Regarding the upper and lower boards, the lead-in edge and the rear-entry edge have different etching states

A large number of problems related to etching quality are concentrated on the etched part of the upper board surface. It is important to know this. These problems arise from the effects of colloidal build-up from etchants on the upper surface of the printed circuit board. The colloidal solids accumulate on the copper surface, which affects the ejection force on the one hand, and blocks the replenishment of fresh etching solution, on the other hand, resulting in a decrease in the etching rate. It is precise because of the formation and accumulation of colloidal solids that the etching degrees of the upper and lower patterns of the board are different. This also makes them part of the board that enters first in the etching machine easy to be completely etched or easy to cause over-corrosion, because the accumulation has not been formed at that time, and the etching speed is faster. Conversely, when the part entering the back of the board enters, the buildup is already formed and slows down its etch rate.

5. Maintenance of etching equipment

The key factor in the maintenance of etching equipment is to ensure that the nozzles are clean and free of obstructions to make the spraying smooth. Blockages or slagging can impact the layout under jet pressure. If the nozzle is not clean, the etching will be uneven and the entire PCB will be scrapped. Obviously, the maintenance of the equipment is to replace the damaged and worn parts, including the replacement of the nozzle, which also has the problem of wear and tear. In addition, the more critical issue is to keep the etching machine free of slagging, and slag accumulation will occur in many cases. Excessive accumulation of slag will even affect the chemical balance of the etching solution. Likewise, if the etchant exhibits an excessive chemical imbalance, slag formation will be exacerbated. The problem of slag buildup 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 properly cleaned with stronger hydrochloric acid or the solution should be replenished. The residual film can also produce slag, a very small amount of residual film is dissolved in the etching solution, and then the copper salt precipitate is formed. The slag formed by the residual film indicates that the previous film removal process is not complete. Poor film removal is often the result of a combination of edge film and overplaying on the PCB board.