The etching process of pcb prototype outer circuit
At present, the typical process of printed circuit board (PCB) processing uses the "pattern plating method". That is, pre-plated a layer of 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. 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. 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 biggest disadvantage of full-board copper plating is that copper must be plated twice on all parts of the board and they must all be corroded during etching. Therefore, when the wire width is very fine, a series of problems will arise. At the same time, side corrosion will seriously affect the uniformity of the line.
In the process of processing the outer circuit of the printed circuit board, there is another method, which is to use a photosensitive film instead of a 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 the most commonly used anti-corrosion layer, used in the etching process of ammonia-based etchant. Ammonia-based etchant is a commonly used chemical liquid, and 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.
Sulfate-based etching solution, after use, 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 outer layer production, 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 accurately defined, then the etching quality must include the consistency of the wire line 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 a frequently discussed item in the etching parameters. It is defined as the ratio of the undercutting width to the etching depth, which is 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 etching factor is the most 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. As for the structure of the etching equipment, the following chapters will be specifically discussed.
From many aspects, the quality of etching has existed long before the printed board entered 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" 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 that, 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.