PCB etching process and process control

The process of printing a printed circuit board from the light board to the display of the line pattern is a relatively complicated process of physical and chemical reactions. This article analyzes the final step, etching. Currently, typical processes for printed circuit board (PCB) processing use pattern plating. That is, 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.

1.Types of etching

It should be noted that there are two layers of copper on the board during etching. Only one layer of copper in the outer etch 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.

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 liquid, and does 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. The sulphate-based etching solution can be repeatedly used after being used, and the copper can be separated by electrolysis. 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. Due to many reasons including economy and waste disposal, this process has not been widely used in the commercial sense. Further, sulfuric acid-hydrogen peroxide cannot be used for etching of lead-tin resist, and this process is not PCB. The main method in outer production, so most people rarely care.

2.Etching quality and problems in advance

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 frequently 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.

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 “downstream” 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 it is that problems will arise. 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. A 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 during film removal, leaving a small portion of “residual glue” under the “edge”. “Residual glue” or “residual film” remains under the “edge” of the resist, which will result in 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 not meet the requirements of Party A and may even be rejected. The rejection of the PCB will greatly 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.

3.Equipment adjustment and interaction with corrosion solution

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 tuned, production can continue. The key is to maintain continuous working status once it 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 recognize the most 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 ammoniacal etching, assuming all other parameters are constant, the etch rate is primarily determined by the ammonia (NH3) in the etchant. Therefore, the effect of using fresh solution and etching surface is mainly two: one is to wash away 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. This 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 effect 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. Monovalent copper can be removed by converting monovalent copper to divalent copper by the action of oxygen in the atmosphere. The above purpose can be achieved by spraying.

This is a functional reason to pass air into the etching box. However, if there is too much air, the ammonia loss in the solution is accelerated to lower the pH, and as a result, the etching rate is lowered. Ammonia is also a variable in the solution that needs to be controlled. Some users use the practice of passing pure ammonia into the etched reservoir. In doing so, a set of pH meter control systems must be added. When the automatically measured PH result is lower than the given value, the solution is 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 structural design of the etching machine. In this method, the solution used is divalent copper, not an ammonia-copper etch. It will likely be used in the printed circuit industry. In the PCH industry, etched copper foils typically have a thickness of 5 to 10 mils, and in some cases are quite thick. Its requirements for etch parameters are often more demanding than in the PCB industry.

4.Regarding the upper and lower plates, the etching state of the lead-in and the back-in edge is different.

A large number of problems related to etching quality are concentrated on the portion to be etched on the upper surface. It is very important to understand this. These problems arise from the effects of gel-like slabs produced by the etchant on the upper surface of the printed circuit board. Glue-like plate deposits accumulate on the copper surface, which on the one hand affects the ejection force, and on the other hand blocks the replenishment of the fresh etching solution, resulting in a decrease in the etching rate. It is precisely because of the formation and accumulation of the gelatinous plates that the upper and lower patterns of the board are etched to different degrees. This also makes it possible for the portion where the board first enters in the etching machine to be easily etched or to cause excessive corrosion because the deposition is not formed at that time, and the etching speed is faster. Conversely, when the portion that enters the board enters, the deposit is formed and the etching speed is slowed down.

5.Maintenance of etching equipment

The most critical factor in the maintenance of the etching equipment is to ensure that the nozzle is clean and that there is no obstruction and the ejection is smooth. Blockage or slagging can impact the screen under jet pressure. If the nozzle is not clean, it will cause uneven etching and scrap the entire PCB.

Obviously, the maintenance of the equipment is to replace the damaged parts and the wear parts, including the replacement of the nozzles, and the nozzles also have the problem of wear. In addition, the more critical problem is to keep the etchant free of slagging. In many cases, slag accumulation will occur. Too much slag accumulation will even affect the chemical balance of the etchant. Similarly, if there is an excessive chemical imbalance in the etchant, the slagging becomes more severe. The problem of slagging accumulation cannot be overemphasized. Once the etchant suddenly exhibits a large amount of slagging, it is usually a signal that there is a problem with the balance of the solution. This should be done with a strong hydrochloric acid for proper cleaning or supplementation of the solution.

The residual film can also produce slag, and a very small amount of residual film is dissolved in the etching solution, and then a copper salt precipitate 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 a combination of edge film and overplating.

Grace Zheng

Email: sales06@andwinpcb.com

Skype: andwinpcb