1、 Overview

At present, the typical process of printed circuit board (PCB) processing adopts “graphic electroplating method”. That is, a layer of lead tin corrosion resistant layer is pre plated on the copper foil to be retained on the outer layer of the board, that is, the graphic part of the circuit, and then the rest of the copper foil is chemically corroded, which is called etching.

  It should be noted that there are two layers of copper on the board at this time. In the outer etching process, only one layer of copper must be completely etched, and the rest will form the final required circuit. This type of pattern plating is characterized in that the copper plating layer only exists below the lead tin resist layer. Another process is that the whole board is plated with copper, and the part other than the photosensitive film is only tin or lead tin resist layer. This process is called “full board copper plating process”. Compared with pattern plating, the biggest disadvantage of full board copper plating is that copper must be plated twice everywhere on the board surface, and they 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.

Introduction to etching process of printed circuit board PCB

In the processing technology of outer circuit of printed board, another method is to use photosensitive film instead of metal coating as anti-corrosion layer. This method is very similar to the inner layer etching process. You can refer to the etching in the inner layer manufacturing process.

At present, tin or lead tin is the most commonly used resist layer, which is used in the etching process of ammonia etchant. Ammonia etchant is a commonly used chemical solution, which does not have any chemical reaction with tin or lead tin. Ammonia etchant mainly refers to ammonia / ammonia chloride etching solution. In addition, ammonia / ammonia sulfate etching solution can also be purchased on the market.  

The copper in the sulfate based etching solution can be separated by electrolysis after use, 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 etch the outer pattern with sulfuric acid hydrogen peroxide as etchant. Due to many reasons including economy and waste liquid treatment, this process has not been widely used in the commercial sense. Further, sulfuric acid hydrogen peroxide can not be used for the etching of lead tin resist layer, and this process is not the main method in the production of PCB outer layer, so most people pay little attention to it.

2、 Etching quality and existing problems

The basic requirement for etching quality is to completely remove all copper layers except under the resist layer, that’s all. Strictly speaking, if it is to be accurately defined, the etching quality must include the consistency of conductor linewidth and the degree of side corrosion. Due to the inherent characteristics of the current corrosive solution, it can etch not only downward, but also left and right directions, so side corrosion is almost inevitable.

Side etching problem is often discussed in etching parameters. It is defined as the ratio of side etching width to etching depth, which is called etching factor. In the printed circuit industry, it varies widely from 1:1 to 1:5. Obviously, a small side etching degree or low etching factor is the most satisfactory.

The structure of the etching equipment and the etching solution with different components will affect the etching factor or side etching degree, or in an optimistic word, it can be controlled. Some additives can reduce the degree of side corrosion. The chemical composition of these additives is generally a trade secret, and their developers do not disclose it to the outside world. As for the structure of etching equipment, the following chapters will be devoted to discussion.

In many ways, the quality of etching has existed long before the printed board entered the etching machine. Because there is a very close internal relationship between 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 problems identified as etching quality have actually existed in the previous process of film removal or even more. For the etching process of outer graphics, many problems are finally reflected in it because its “inverted stream” image is more prominent than most PCB processes. At the same time, this is also because etching is the last step in a long series of processes starting from film pasting and photosensitivity. After that, the outer pattern is transferred successfully. The more links, the greater the possibility of problems. This can be regarded as a very special aspect in the production process of printed circuit.

Theoretically, after the printed circuit enters the etching stage, in the process of processing printed circuit by graphic electroplating, the ideal state should be that the total thickness of copper and tin or copper and lead tin after electroplating should not exceed the thickness of electroplating resistant photosensitive film, so that the electroplating pattern is completely blocked by the “wall” on both sides of the film and embedded in it. However, in real production, after electroplating, the plating pattern of printed circuit boards all over the world is much thicker than the photosensitive pattern. In the process of electroplating copper and lead tin, because the coating height exceeds the photosensitive film, there is a trend of transverse accumulation, and the problem arises. The tin or lead tin resist layer covered above the strip extends to both sides to form a “edge”, and a small part of the photosensitive film is covered 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 the “residual glue” under the “edge”. “Residual glue” or “residual film” left under the “edge” of the resist will cause incomplete etching. The lines form “copper roots” on both sides after etching, which narrows the line spacing, resulting in the printed board not meeting the requirements of Party A and may even be rejected. Rejection will greatly increase the production cost of PCB.  

In addition, in many cases, dissolution is formed due to reaction. In the printed circuit industry, residual film and copper may also accumulate in the corrosive solution and block in the nozzle of the corrosive machine and the acid resistant pump, so they have to be shut down for treatment and cleaning, which affects the 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. Conversely, it is an easy job. Once the process is adjusted, continuous production can be carried out. The key is that once the machine is started, it needs to maintain a continuous working state and should not be stopped. The etching process depends on the good working condition of the equipment to a great extent. At present, no matter what kind of etching solution is used, high-pressure spraying must be used, and in order to obtain neat line sides and high-quality etching effect, the nozzle structure and spraying mode must be strictly selected.

In order to get good side effects, many different theories have emerged, forming different design methods and equipment structures. These theories are often quite different. However, all theories on etching recognize the most basic principle, that is, keep the metal surface in contact with fresh etching solution as soon as possible. The chemical mechanism analysis of the etching process also confirms the above view. In ammonia etching, assuming that all other parameters remain unchanged, the etching rate is mainly determined by ammonia (NH3) in the etching solution. Therefore, there are two main purposes for the interaction between fresh solution and etched surface: one is to flush out the newly generated copper ions; The second is to continuously provide ammonia (NH3) required for 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 content of monovalent copper ion in ammonia etching solution, the faster the reaction speed. This has been confirmed by experience. In fact, many ammonia etchant products contain special coordination groups of monovalent copper ions (some complex solvents), which are used to reduce monovalent copper ions (these are the technical secrets of their products with high reaction ability). It can be seen that the influence of monovalent copper ions is not small. If the monovalent copper is reduced from 5000 ppm to 50 ppm, the etching rate will be more than doubled.

Because a large number of monovalent copper ions are generated in the process of etching reaction, and because monovalent copper ions are always tightly combined with the complex group of ammonia, it is very difficult to keep their content close to zero. 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 reduce the pH value, which will still reduce the etching rate. Ammonia in solution also needs to be controlled. Some users use the method of passing pure ammonia into the etching storage tank. 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 reached the stage of etching machine structure design. In this method, the solution used is divalent copper, not ammonia copper etching. It will likely 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 it is quite large. Its requirements for etching parameters are often more stringent than those in PCB industry.

 

There is a research result from PCM industrial system, which has not been officially published, but the result will be refreshing. With strong project fund support, researchers have the ability to change the design idea of etching device in a long-term sense and study the effects of these changes. For example, compared with the conical nozzle, the best nozzle design adopts a sector, and the spray collecting cavity (i.e. the pipe into which the nozzle is screwed) also has an installation angle, which can spray the workpiece entering the etching chamber at 30 degrees. If such a change is not made, the installation mode of the nozzle on the collecting cavity will cause the injection angle of each adjacent nozzle to be not completely consistent. The spray surfaces of the second group of nozzles are slightly different from those of the first group (it indicates the working condition of the spray). In this way, the shape of the sprayed solution becomes a superimposed or crossed state. Theoretically, if the solution shapes cross each other, the ejection force of this part will be reduced, and the old solution on the etching surface can not be effectively washed away to keep the new solution in contact with it. This situation is particularly prominent at the edge of the spray surface. Its jet force is much smaller than that in the vertical direction.

The study found that the latest design parameter is 65 psi (i.e. 4 + bar). Every etching process and every practical solution has an optimal injection pressure. At present, the injection pressure in the etching chamber is more than 30 pounds per square inch (2bar). There is a principle that the higher the density (i.e. specific gravity or Baume) of an etching solution, the higher the optimal injection pressure. Of course, this is not a single parameter. Another important parameter is the relative mobility (or mobility) of the reaction rate in solution.

4、 On the upper and lower plate surfaces, the etching states of the leading edge and the trailing edge are different

A large number of problems related to etching quality focus on the etched part of the upper plate surface. It is important to understand this. These problems come from the influence of colloidal structures produced by etchant on the upper surface of printed circuit board. Colloidal deposits on the copper surface, on the one hand, affect the jet force, on the other hand, block the replenishment of fresh etching solution, resulting in the reduction of etching speed. It is due to the formation and accumulation of colloidal structures that the etching degree of the upper and lower graphics of the board is different. This also makes the first part of the board in the etching machine easy to be etched thoroughly or easy to cause over corrosion, because the accumulation has not been formed at that time and the etching speed is fast. On the contrary, when the part behind the board enters, the accumulation has been formed and its etching speed is slowed down.

5、 Maintenance of etching equipment

The key factor for the maintenance of etching equipment is to ensure that the nozzle is clean and unobstructed without obstruction. Blockage or slagging will impact the layout under the action of jet pressure. If the nozzle is dirty, it will cause uneven etching and scrap the whole PCB.

Obviously, the maintenance of the equipment is to replace the damaged and worn parts, including the nozzle. The nozzle also has the problem of wear. In addition, the more critical problem is to keep the etcher free of slagging, and slagging accumulation will occur in many cases. Excessive slagging accumulation will even affect the chemical balance of the etching solution. Similarly, if there is excessive chemical imbalance in the etching solution, the slagging will become more and more serious. The problem of slagging and accumulation cannot be overemphasized. Once a large amount of slagging occurs suddenly in the etching solution, it is usually a signal that the balance of the solution is wrong. This should be properly cleaned with strong hydrochloric acid or added to the solution.

The residual film can also produce slags. A very small amount of residual film is dissolved in the etching solution, and then copper salt precipitation is formed. The slagging formed by the residual film indicates that the previous film removal process is not complete. Poor film removal is often the result of edge film and over plating.

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