I. Overview

　　Now, the typical process of printed circuit board (PCB multi-layer circuit board) processing adopts "pattern plating method". That is to say, a lead-tin resist layer is pre-plated on the copper foil part to be preserved on the outer layer of the board, that is, the graphic part of the circuit, and then other copper foils are chemically etched away, which is called etching.

　　It should be noted that there are two layers of copper on the multi-layer circuit board at this time. In the outer layer etching process, only one layer of copper must be completely etched away, and the others 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 that the entire multilayer circuit 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 "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 it is necessary to corrode them when etching. Therefore, a series of problems will occur when the wire width is very precise. At the same time, side corrosion will seriously affect the uniformity of the lines.

　　In the processing technology of the outer circuit of the PCB circuit board (multilayer 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 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 widely used chemical liquid, which does not have any chemical reaction with tin or lead-tin. Ammonia etchant mainly refers to ammonia water/ammonia chloride etching solution. In addition, ammonia water/ammonia sulfate etching solution is also available in the market.

　　After using the sulfate-based etching solution, the copper in the middle can be separated by electrolysis, so it can be reused. Because of its low corrosion rate, it is generally rare in practical production, but it is expected to be used in chlorine-free etching. Some people experimented with sulfuric acid-hydrogen peroxide as an etchant to corrode the outer layer pattern. For many reasons, including economics and waste treatment, this process has not been widely used 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 primary method in the fabrication of the outer layer of multilayer circuit boards, so most people rarely care about it.

　　2. Regarding the upper and lower multilayer circuit boards, the etching conditions of the leading edge and the rear edge are different.

　　Many problems involving etching quality will be combined with the etched part of the upper multilayer circuit board. It is important to know this. These problems arise from the effects of gel-like build-up that occurs with etchants on the upper surface of printed circuit boards. The jelly-like compacts accumulate on the copper surface, which affects the eruption force on the one hand, and blocks the compensation of the fresh etching solution on the other hand, which constitutes a decrease in the etching rate. It is precisely because of the composition and accumulation of the colloidal compacts that the etching degrees of the upper and lower patterns of the board are different. This also makes the part of the board in the etching machine (multilayer circuit board) that is simply etched to be completely or simply constituted over-etched, because the accumulation is not constituted at that time, and the etching speed is faster. On the contrary, when the part of the multi-layer circuit board enters the back part, the accumulation has been formed, and its etching speed is slowed down.

　　3. Equipment adjustment and interaction with corrosive solution

　　In the processing of printed circuits (multilayer circuit boards), ammonia etching is a relatively precise and complex chemical reaction process. In turn it is an easy job. Once the process is turned up, production can continue. The key is that once the machine is turned on, it needs to maintain continuous operation, and it is not advisable to stop and stop. The etching process depends to a great extent on the excellent working condition of the equipment. For now, no matter what kind of etching solution is used, it is necessary to use high-pressure spray, and in order to obtain a more regular line side and high-quality etching effect, it is necessary to strictly select the structure of the nozzle and the spray method.

　　In order to obtain outstanding side effects, many different theories are presented, which constitute different planning methods and equipment structures. These theories are often distinct. But all theories about etching recognize the most basic principle, which is to keep the metal surface in constant contact with fresh etching solution as quickly as possible. The chemical mechanism analysis of the etching process also proves the above point. In ammonia etching, assuming all other parameters remain unchanged, the etching rate is mainly determined by the ammonia (NH3) in the etching solution. Therefore, the use of fresh solution and etching surface effect mainly has two purposes: one is to flush out the copper ions that have just occurred; the other is to continuously supply the ammonia (NH3) required for the reaction.

　　In the traditional knowledge of the printed circuit industry, especially the suppliers of printed circuit materials, we recognize that the lower the content of monovalent copper ions in the ammonia-based etching solution, the faster the reaction speed. This has been proved by experience prove. In fact, many ammonia-based etchant products contain special ligands for cupric ions (some messy solvents), the effect of which is to reduce cupric ions (these are the technical tricks that make their products highly responsive ), 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.

　　Because a lot of monovalent cupric ions are generated in the process of etching reaction, and because monovalent cupric ions are always tightly combined with the complexing group of ammonia, it is very difficult to keep its content close to zero. Converting monovalent copper to divalent copper by the effect of atmospheric oxygen enables removal of monovalent copper. The above purpose can be achieved by spraying.

　　This is one functional reason to pass air into the etching chamber. But if there is too much air, it will accelerate the loss of ammonia in the solution and reduce the pH value, and its effect will still reduce the etching rate. Ammonia in solution is also an amount of change that needs to be controlled. Some users choose to pass pure ammonia into the etching reservoir. To do so, it is necessary to add a PH meter control system. When the actively measured pH effect falls below a given value, the solution will actively increase.

　　In the related field of chemical etching (also known as photochemical etching or PCH), the research work has already begun and has reached the stage of structural planning of the etching machine. In this method, the solution used is divalent copper, 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.

Read recommendations:

Six-layer Immersion Gold Board (BGA)

Six-layer Immersion Gold Board (BGA)

DIP plugin

Mobile phone board

What to pay attention to when choosing a high frequency circuit board.Automotive Electronics PCB