Explain in detail the etching process of the outer circuit of the circuit board and the problems existing in the prior

2019-08-18 10:14:31

Currently, the typical process for printed circuit board (PCB) processing uses "graphic plating". That is, firstly, a layer of lead-tin resist is pre-plated on the portion of the copper foil to be retained on the outer layer of the board, that is, the pattern portion of the circuit, and then the remaining copper foil is chemically etched away, which is called etching.

It should be noted that there are two layers of copper on the board at this time. Only one layer of copper in the outer etching process must be completely etched away, and the rest will form the final required circuit. This type of pattern plating is characterized in that the copper plating layer is only present under the lead-tin resist layer. Another method is to plate copper on the entire board, and the portion other than the photosensitive film is only a tin or lead-tin resist. This process is called "full-plate copper plating process." The biggest disadvantage of full-plate copper plating compared to pattern plating is that the copper is plated twice throughout 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 line.

In the processing of the outer circuit of the printed circuit board, there is another method of replacing the metal plating layer with a photosensitive film as a resist layer. This method is very similar to the inner layer etching process, and can be referred to the etching in the inner layer fabrication process.

Currently, tin or lead tin is the most commonly used resist layer used in the etching process of ammonia etchants. Ammonia etchants are commonly used chemical solutions that do not undergo any chemical reaction with tin or lead tin. The ammonia etchant mainly refers to an ammonia/ammonia chloride etching solution. In addition, ammonia/ammonium sulfate etching solutions are also available on the market.

Sulfate-based etching solution, after use, copper 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 have experimented with sulphuric acid-hydrogen peroxide as an etchant to etch the outer layer. This process has not been widely adopted in the commercial sense due to many reasons including economy and waste disposal. 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 PCB outer layer, so most people are rarely interested.

Second, the etching quality and problems in the early stage

The basic requirement for the quality of the etch is to be able to completely remove all copper layers except the underside of the resist layer. Strictly speaking, if it is to be precisely defined, the etch quality must include the uniformity of the wire width and the degree of undercut. Due to the inherent characteristics of the current etching solution, etching is not only downward but also in the left and right directions, so side etching is almost inevitable.

The side etch problem is one of the often discussed etch parameters and is defined as the ratio of the etched width to the etch depth, called the etch factor. In the printed circuit industry, it varies widely from 1:1 to 1:5. Obviously, a small degree of undercut or a low etch factor is most desirable.

The structure of the etching equipment and the etchant of different compositions can affect the etch factor or the degree of etch, or, in optimistic terms, it can be controlled. Side additives can be reduced with certain additives. The chemical composition of these additives is generally a trade secret, and the respective developers are not disclosed to the outside world. As for the structural problems of the etching equipment, the following sections will be specifically discussed.

In many ways, the quality of the etch has existed long before the printed board entered the etch machine. Because there is a very close internal relationship 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 have actually existed in film removal and even in previous processes. For the etching process of the outer layer, many of the problems are finally reflected on it because the “downside” image is more prominent than most printed board processes. At the same time, this is also because the etching is the last ring in a long series of processes since the film is applied, and then the outer layer is transferred successfully. The more links, the more likely you are to have problems. This can be seen as a very special aspect of the printed circuit production process.

Theoretically, after the printed circuit enters the etching stage, in the process of pattern plating processing of printed circuit, the ideal state should be: the total thickness of copper and tin or copper and lead-tin after plating should not exceed the plating resistance. The thickness of the photosensitive film is such that the plating pattern is completely blocked by the "wall" on both sides of the film and embedded therein. However, in actual production, after printing on the printed circuit boards of the world, the plating pattern is much thicker than the photosensitive pattern. In the process of electroplating copper and lead-tin, since the plating height exceeds the photosensitive film, a tendency of lateral accumulation occurs, and the problem arises. The tin or lead-tin resist layer overlying the lines extends to the sides to form an "edge", and a small portion of the photosensitive film is placed 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 portion of "residual glue" under the "edge". The "residual" or "residual film" remains below the "edge" of the resist, causing incomplete etching. The lines form a "copper root" on both sides after etching. The copper root narrows the line spacing, causing the printed board to fail to meet the requirements of Party A and may even be rejected. Rejection will increase the production cost of the PCB.

In addition, in many cases, dissolution occurs due to the reaction. In the printed circuit industry, residual film and copper may form deposits in the etching solution and block in the nozzle of the corrosion machine and the acid-resistant pump, and have to be shut down and cleaned. And affect the efficiency of work.

Third, equipment adjustment and interaction with corrosion solutions

Ammonia etching is a relatively delicate and complex chemical reaction process in printed circuit processing. Conversely, it is another easy job. Once the process is transferred, production can be continued. The key is to maintain continuous operation once the machine is turned on. It is not advisable to stop and stop. The etching process relies to a great extent on the good working conditions of the device. For the time being, no matter which etching solution is used, high-pressure spraying must be used, and in order to obtain a neat line side and a high-quality etching effect, the nozzle structure and the spraying method must be strictly selected.

In order to get a good side effect, many different theories emerged, forming different design methods and equipment structures. These theories are often quite different. But all theories about etching acknowledge the basic principle of keeping the metal surface in constant contact with fresh etchant as quickly as possible. The above-mentioned viewpoint is also confirmed by the chemical mechanism analysis performed on the etching process. In ammonia etching, assuming all other parameters are constant, the etching rate is mainly determined by ammonia (NH3) in the etching solution. Therefore, there are two main purposes for using fresh solution and etching surface: 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 materials, it is recognized that the lower the monovalent copper ion content in the ammonia etching solution, the faster the reaction speed, which has been confirmed by experience. . In fact, many ammonia etchant products contain special ligands for monovalent copper ions (some complex solvents) that act to lower monovalent copper ions (these are technical tips for their products with high reactivity). ), it can be seen that the impact of monovalent copper ions is not small. By reducing the amount of monovalent copper from 5000 ppm to 50 ppm, the etching rate is more than doubled.

Since a large amount of monovalent copper ions are generated during the etching reaction, and since the monovalent copper ions are always tightly combined with the complexing group of ammonia, it is very difficult to keep the content close to zero. The conversion of monovalent copper to divalent copper by the action of oxygen in the atmosphere removes monovalent copper. The above purpose can be achieved by spraying.