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

I. Overview

At present, the typical process of printed circuit board (PCB) processing adopts 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. This type of pattern electroplating is characterized by 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 copper plating on the whole board is that copper must be plated twice on all parts of the board and all of them 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 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 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.

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 the etching of lead-tin resist, and this process is not PCB The main method in outer 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 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 side etching is one of the etching parameters that is often raised for discussion. It is defined as the ratio of the width of the side etching to the depth of the etching, which is called the etching factor. In the printed circuit 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 the most satisfactory.

The structure of the etching equipment and the different components of the etching solution 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.

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 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" phenomenon 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 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 printed circuit production process.

Theoretically speaking, after the printed circuit enters the etching stage, in the process of processing the printed circuit 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 resistance to electroplating The thickness of the photosensitive film makes the electroplated graphics completely blocked by the "walls" on both sides of the film and embedded in it. However, in actual production, after electroplating printed circuit boards all over the world, the plating pattern is much thicker than the photosensitive pattern. In the process of electroplating copper and lead-tin, because 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 formed "copper roots" on both sides after etching. The copper roots narrowed the line spacing, causing the printed board to not meet the requirements of Party A, and may even be rejected. Rejection will greatly increase the production cost of PCB.

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 form and accumulate in the corrosive liquid and be blocked in the nozzle of the corroding machine and the acid-resistant pump, and it has to be shut down for processing and cleaning., Which affects work efficiency.