Detailed explanation of PCB etching process and key technology control in PCB production process

The process of printing circuit boards from copper-clad laminates to displaying circuit patterns is a complex physical and chemical reaction. This article will analyze the final step - etching. At present, the typical process for processing printed circuit boards (PCBs) adopts the "graphic plating method". First, a layer of lead tin corrosion-resistant coating is pre deposited on the copper foil that needs to be retained on the outer layer of the board, that is, on the graphic part of the circuit. Then, the remaining copper foil is chemically etched away, which is called etching.

1.Types of Etching

It should be noted that there are two layers of copper on the board during etching. In the outer 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 graphic electroplating is that the copper plating layer only exists below the lead tin corrosion-resistant layer.

Another process method is to copper coat the entire board, with only tin or lead tin corrosion-resistant layers on the parts outside the photosensitive film. This process is called "full board copper plating process". Compared with graphic electroplating, the biggest disadvantage of full board copper plating is that copper must be plated twice throughout the surface of the board and 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 lines.

In the processing technology of 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 corrosion-resistant layer. This method is very similar to the inner layer etching process, which can be referred to as etching in the inner layer manufacturing process.

At present, tin or lead tin is the most commonly used corrosion-resistant layer in the etching process of ammonia etchants Ammonia etchant is a commonly used chemical solution that does not undergo any chemical reaction with tin or lead tin. Ammonia based etchants mainly refer to ammonia/chlorinated ammonia etching solutions.

In addition, ammonia/ammonium sulfate etching solutions can also be purchased in the market. The etching solution based on sulfate can separate copper through electrolysis after use, making it reusable. Due to its low corrosion rate, it is generally not commonly seen in actual production, but it is expected to be used in chlorine free etching.

Someone is experimenting with using sulfuric acid hydrogen peroxide as an etchant to corrode the outer layer of the pattern. Due to various reasons including economic and waste liquid treatment, this process has not yet been widely adopted in commercial terms Furthermore, sulfuric acid hydrogen peroxide cannot be used for etching the lead tin corrosion-resistant layer, and this process is not the main method in PCB outer layer production, so the vast majority of people rarely pay attention to it.

2.Etching quality and pre-existing issues

The basic requirement for etching quality is to be able to completely remove all copper layers except for the corrosion-resistant layer, and that's all. Strictly speaking, in order to accurately define, etching quality must include the consistency of wire line width and the degree of lateral etching. Due to the inherent characteristics of the current corrosive solution, it not only etches downwards but also in all directions, so side corrosion is almost inevitable.

Side etching is a frequently discussed issue in etching parameters, defined as the ratio of side etching width to etching depth, known as the etching factor. In the printed circuit industry, its range of variation is very broad, from 1:1 to 1:5. Obviously, a small lateral etching degree or low etching factor is the most satisfactory.

The structure of etching equipment and the different compositions of etching solution will have an impact on the etching factor or side etching degree, or in optimistic terms, it can be controlled. The use of certain additives can reduce the degree of lateral corrosion. The chemical composition of these additives is generally a trade secret, and their respective developers do not disclose it to the outside world.

In many ways, the quality of etching already existed before the printed circuit board entered the etching machine. Because there are very close internal connections between various processes or techniques in printed circuit processing, there is no process that is not affected by other processes and does not affect other processes. Many issues identified as etching quality problems actually exist in the process of film removal or even earlier.

For the etching process of the outer layer graphics, due to its prominent "reverse stream" phenomenon compared to most printed circuit board processes, many problems are ultimately reflected on it. At the same time, this is also due to the fact that etching is the last step in a long series of processes starting from self coating and photosensitive, after which the outer layer pattern is successfully transferred. The more links there are, the greater the likelihood of problems occurring. This can be seen as a very special aspect of the production process of printed circuits.

In theory, after the printed circuit enters the etching stage, the ideal state in the process of graphic electroplating for printed circuit processing should be that the total thickness of copper and tin or copper and lead tin after electroplating should not exceed the thickness of the electroplating resistant photosensitive film, so that the electroplated graphic is completely blocked by the "walls" on both sides of the film and embedded inside. However, in actual production, printed circuit boards worldwide have a coating pattern that is much thicker than the photosensitive pattern after electroplating. In the process of electroplating copper and lead tin, due to the height of the coating exceeding the photosensitive film, there is a tendency for lateral accumulation, which leads to the problem. The tin or lead tin corrosion-resistant layer covering the upper part of the strip extends to both sides, forming a "edge" and covering a small part of the photosensitive film below the "edge".

The "edge" formed by tin or lead tin makes it impossible to completely remove the photosensitive film during film removal, leaving a small amount of "residual glue" below the "edge". Residual glue or film left under the "edge" of the resist will cause incomplete etching. After etching, the lines form "copper roots" on both sides, which narrow the line spacing and cause the printed board to not meet the requirements of Party A, and may even be rejected. Refusal will significantly increase the production cost of PCB.

In addition, in many cases, dissolution occurs due to reactions. In the printed circuit industry, residual films and copper may also accumulate in the corrosive solution and block the nozzle of the corrosion machine and acid resistant pump, requiring shutdown for processing and cleaning, which affects work efficiency.

3. Equipment adjustment and interaction with corrosive solution

In printed circuit processing, ammonia etching is a relatively fine and complex chemical reaction process. On the other hand, it is an easy job to do. Once the process is upgraded, continuous production can be carried out. The key is to maintain continuous operation once the machine is turned on, and it is not advisable to stop and start. The etching process heavily relies on the good working condition of the equipment. At present, regardless of the type of etching solution used, high-pressure spraying must be used, and in order to obtain a more neat line edge and high-quality etching effect, the nozzle structure and spraying method must be strictly selected.

To achieve good side effects, many different theories have emerged, forming different design methods and equipment structures. These theories are often vastly different. But all theories related to etching recognize the most fundamental principle, which is to continuously expose the metal surface to fresh etching solution as quickly as possible. The chemical mechanism analysis of the etching process also confirms the above viewpoint. In ammonia etching, assuming all other parameters remain constant, the etching rate is mainly determined by the ammonia (NH3) in the etching solution. Therefore, the purpose of using fresh solution to etch the surface is mainly twofold: first, to flush out the newly generated copper ions; The second is to continuously provide the ammonia (NH3) required for the reaction.

In the traditional knowledge of the printed circuit industry, especially among suppliers of printed circuit materials, it is widely recognized that the lower the content of monovalent copper ions in ammonia etching solution, the faster the reaction rate This has been confirmed by experience. In fact, many ammonia etching solution products contain special coordination groups of monovalent copper ions (some complex solvents), whose function is to reduce monovalent copper ions (these are the technical secrets of their products with high reactivity), indicating that the influence of monovalent copper ions is not small. Reducing monovalent copper from 5000ppm to 50ppm will increase the etching rate by more than double.

Due to the generation of a large amount of monovalent copper ions during the etching reaction, and the fact that monovalent copper ions are always tightly bound to the complex groups of ammonia, it is very difficult to maintain their content close to zero. By converting monovalent copper into divalent copper through the action of oxygen in the atmosphere, monovalent copper can be removed. The above purpose can be achieved by spraying.

This is a functional reason for introducing air into the etching chamber. However, if there is too much air, it will accelerate the loss of ammonia in the solution and cause a decrease in pH value, resulting in a decrease in etching rate. The amount of ammonia in solution also needs to be controlled. Some users adopt the method of introducing pure ammonia into the etching liquid storage tank. This requires the addition of a pH meter control system. When the automatically measured pH result is lower than the given value, the solution will be automatically added.

In the field of chemical etching (also known as photochemical etching or PCH) related to this, research work has begun and 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 can be quite large. Its requirements for etching parameters are often more stringent than those in the PCB industry.

4. Regarding the issue of different etching states between the upper and lower board surfaces, the imported edge and the rear imported edge

A large number of issues related to etching quality are concentrated on the etched parts on the upper surface of the board. Understanding this is very important. These issues arise from the influence of the adhesive formed by the etching agent on the upper surface of the printed circuit board. Colloidal plate deposits on the copper surface, which affects the spraying force and blocks the replenishment of fresh etching solution, resulting in a decrease in etching speed. It is precisely because of the formation and accumulation of gel like structures that the etching degree of the upper and lower graphics of the board is different. This also makes the part of the board that enters first in the etching machine easy to be completely etched or prone to over corrosion, because at that time, the accumulation has not yet formed and the etching speed is fast. On the contrary, when the part behind the board enters, it accumulates and slows down its etching rate.

5. Maintenance of etching equipment

The most critical factor in maintaining etching equipment is to ensure that the nozzle is clean, unobstructed, and the spray is smooth. Blockage or slagging will impact the surface under the action of jet pressure. If the nozzle is not clean, it will cause uneven etching and result in the entire PCB being scrapped.

Obviously, equipment maintenance involves replacing damaged and worn parts, including replacing nozzles, which also have wear issues. In addition, a more critical issue is to keep the etching machine free from slagging, which can occur in many cases Excessive slag accumulation can even affect the chemical equilibrium of the etching solution. Similarly, if there is an excessive chemical imbalance in the etching solution, slagging will become more severe. The issue of slag accumulation cannot be overemphasized. Once a large amount of slag suddenly forms in the etching solution, it is usually a signal that there is a problem with the equilibrium of the solution. This should be cleaned appropriately with strong hydrochloric acid or supplemented with the solution.

Residual film can also produce slag, and a very small amount of residual film dissolves in the etching solution, forming copper salt precipitation. The slagging formed by residual film indicates that the previous membrane removal process was not thorough. Poor film removal is often the result of both edge film and electroplating.