PCB Tech - Methods and techniques to control PCB etching process

The process of the printed circuit board from the light board to the circuit pattern is a relatively complicated process of physical and chemical reactions. This article will analyze its final step-etching.

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.

Types of etching

It should be noted that there are two layers of copper on the board during etching. 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.

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 undercutting is one of the etching parameters that is often raised for discussion. It is defined as the ratio of the width of undercutting to the depth of 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.

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.

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.

Equipment adjustment and interaction with corrosive solution

In printed circuit processing, ammonia etching is a relatively delicate and complex chemical reaction process. On the other hand, it is an easy job. Once the process is up-regulated, production can be continued. The key is to maintain continuous working status once it is turned on, and it is not advisable to dry and stop. The etching process depends to a large extent on the good working condition of the equipment. At present, no matter what etching solution is used, high-pressure spray must be used, and in order to obtain a neater line side and high-quality etching effect, the nozzle structure and spray method must be strictly selected.

In order to obtain good side effects, many different theories have appeared, forming different design methods and equipment structures. These theories are often very different. But all theories about etching recognize the most basic principle, which is to keep the metal surface in contact with fresh etching solution as quickly as possible. The chemical mechanism analysis of the etching process also confirmed the above point of view. In ammonia etching, assuming that all other parameters remain unchanged, the etching rate is mainly determined by the ammonia (NH3) in the etching solution. Therefore, using fresh solution to etch the surface has two main purposes: one is to flush out the copper ions that have just been produced; the other is to continuously provide ammonia (NH3) needed for the reaction.

In the traditional knowledge of the printed circuit industry, especially the suppliers of printed circuit raw materials, it is recognized that the lower the monovalent copper ion content in the ammonia etching solution, the faster the reaction speed. This has been confirmed by experience. . In fact, many ammonia-based etching solution products contain special ligands for monovalent copper ions (some complex solvents), whose role is to reduce monovalent copper ions (these are the technical secrets of their products with high reactivity ), it can be seen that the influence of monovalent copper ions is not small. Reducing monovalent copper from 5000ppm to 50ppm will increase the etching rate more than doubled.

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

This is a functional reason for passing air into the etching box. However, if there is too much air, it will accelerate the loss of ammonia in the solution and decrease the pH value, resulting in a decrease in the etching rate. Ammonia in the solution is also the amount of change that needs to be controlled. Some users adopt the method of passing pure ammonia into the etching reservoir. To do so, a set of PH meter control system must be added. When the automatically measured PH result is lower than the given value, the solution will be added automatically.

In the related field of chemical etching (also known as photochemical etching or PCH), research work has begun and has reached the stage of etching machine structure design. In this method, the solution used is divalent copper, not ammonia-copper etching. It may be used in the printed circuit industry. In the PCH industry, the typical thickness of etched copper foil is 5 to 10 mils, and in some cases the thickness is quite large. Its requirements for etching parameters are often more stringent than those in the PCB industry.