I. Introduction

　　The purpose of etching: After the circuit is electroplated, the PCB circuit board removed from the electroplating equipment is processed to complete the circuit board. Specifically, there are the following steps:

　　a. Peel off the film: peel off the dry film for anti-plating purposes with the potion. The hardened dry film is partially dissolved under the dope, and partially peeled off into flakes. In order to maintain the effect of the liquid and the completeness of the post-washing, the efficiency of the filtration system is very important.

　　b. Line etching: dissolve the copper in the non-conductor part.

　　c. Stripping tin and lead: Finally, remove the anti-etching tin-lead coating. Regardless of pure tin or the tin-lead layer of each composition ratio, the purpose of plating is only for anti-etching, so after the etching is completed, it must be stripped off, so this step of stripping tin-lead is only for processing, and no added value is generated. But the following points still need special attention, otherwise the cost increase is the second, and the outer layer circuit that is finally completed will cause defects here.

　　At present, the typical process of printed circuit board (PCB) processing adopts "pattern plating method". That is, pre-plating a lead-tin resist layer 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 chemically corrode the rest of the copper foil.

　　It should be noted that there are two layers of copper on the PCB 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 process method is to coat the entire board with copper, and the part other than the photosensitive film is only a tin or lead-tin resist layer. This process is called "full board copper plating process". Compared with pattern plating, the biggest 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 can seriously affect the uniformity of the lines.

　　In the processing technology of the outer circuit of the PCB 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, you can refer to the etching in the inner layer fabrication process.

　　At present, tin or lead tin is the most 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. For many reasons including economics and waste disposal aspects, this process has not yet been 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 the main method in the production of the outer layer of PCB, so most people rarely care about it.

　　2. Equipment adjustment and interaction with corrosive solution

　　In the processing of printed circuit boards, 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 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 a fresh solution to interact with the etching 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 board (PCB) 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 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 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.

Read recommendations:

Display board (six layers)

Computer card board (four layers)

Six-layer Immersion Gold Board (BGA)

DIP plugin

Automotive lighting PCB