Etching process and process control of PCB processing

Etching process and process control of PCB processing

The process from the printed circuit board to the exposed circuit pattern is a relatively complex physical and chemical reaction process. This article will analyze the last step – etching. At present, the typical process of printed circuit board (PCB) processing adopts “graphic plating method”. That is, a layer of lead-tin resist is pre-plated on the copper foil part that needs to be retained on the outer layer of the board, that is, the graphic part of the circuit, and then the remaining copper foil is chemically etched away, which is called etching.

Types of etching

It should be noted that there are two layers of copper on the board when 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. The characteristic of this type of pattern plating is that 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 part other than the photosensitive film is just a 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 on all parts of the board and must be corroded during etching. Therefore, a series of problems will occur when the wire width is very fine. 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 board, there is another method, which is to use a photosensitive film instead of the metal plating layer as the resist layer. This method is very similar to the inner layer etching process. Please refer to Etching in the inner layer manufacturing process.

Currently, tin or lead-tin is the most commonly used resist layer, which is 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-based etchants mainly refer to ammonia/ammonium chloride etching liquid.

In addition, ammonia/ammonium sulfate etching solution is also available on the market. After use, the copper in the sulfate-based etching solution 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.

Someone tried using sulfuric acid-hydrogen peroxide as an etchant to corrode the outer layer pattern. Due to many reasons, including economic and waste disposal aspects, this process has not been widely adopted in a commercial sense. Furthermore, sulfuric acid-hydrogen peroxide cannot be used for etching lead-tin resist layers, and this process is not PCB It is the main method in outer layer production, so most people rarely care about it.

Etching quality and pre-existing problems

The basic requirement for etching quality is to be able to completely remove all copper layers except under the resist layer, and that’s it. Strictly speaking, if it is to be defined accurately, the etching quality must include the consistency of the wire width and the degree of undercutting. Due to the inherent characteristics of the current corrosive liquid, it not only etches downward but also in both left and right directions, so side etching is almost inevitable.

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

The structure of the etching equipment and the etching solutions with different components will have an impact on the etching factor or side etching degree, or in optimistic terms, it can be controlled. The degree of side corrosion can be reduced by using certain additives. 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 begins long before the printed board enters the etching machine. Because there are very close internal connections between various processes or processes in printed circuit processing, there is no process that is not affected by other processes and does not affect other processes. Many problems that are considered to be etching quality problems actually already exist in the film removal or even earlier processes.

For the etching process of the outer layer pattern, because the “streaming” phenomenon it embodies is more prominent than most printed board processes, many problems are ultimately reflected in it. At the same time, this is also because etching is the last step in a long series of processes that starts with self-adhesive film and photosensitization. After that, the outer layer pattern is successfully transferred. The more links there are, the greater the potential for 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 processing the printed circuit by pattern electroplating, the ideal state should be: the sum of the thickness of copper and tin or copper and lead-tin after electroplating should not exceed the electroplating resistance. The thickness of the photosensitive film causes the electroplated graphics to be completely blocked by the “walls” on both sides of the film and embedded inside. However, in actual production, after electroplating of printed circuit boards around the world, the plating pattern is much thicker than the photosensitive pattern. In the process of electroplating copper and lead-tin, since the height of the plating layer exceeds the photosensitive film, there is a tendency to accumulate laterally, and problems arise. The tin or lead-tin resist layer covering the line extends to both sides to form an “edge”, covering a small part of the photosensitive film 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 amount of “residual glue” under the “edge”. “Residual glue” or “residual film” remains under the “edge” of the resist, which will cause incomplete etching. The lines form “copper roots” on both sides after etching. The copper roots narrow the line spacing, causing the printed board to not meet Party A’s requirements and may even be rejected. Due to rejection, the production cost of PCB will be greatly increased.

In addition, in many cases, dissolution occurs due to reaction. In the printed circuit industry, residual film and copper may accumulate in the corrosive liquid and block the nozzle of the etching machine and the acid-resistant pump, and have to be shut down for processing and cleaning. , which affects work efficiency.

Equipment adjustment and interaction with corrosive solutions

In printed circuit processing, ammonia etching is a relatively delicate and complex chemical reaction process. On the other hand, it is an easy task. Once the process is adjusted up, production can continue. The key is that once the machine is turned on, it must maintain continuous working status and should not be stopped and started. The etching process relies heavily on the good working condition of the equipment. For now, no matter what kind of etching solution is used, high-pressure spray must be used, and in order to obtain neater line sides and high-quality etching effects, the structure of the nozzle and the spray method must be strictly selected.

In order to obtain good side effects, many different theories have emerged, forming different design methods and equipment structures. These theories are often very different. But all theories about etching admit the most basic principle, which is to keep the metal surface in contact with fresh etching liquid as quickly as possible. The chemical mechanism analysis of the etching process also confirmed the above point of view. In ammonia etching, assuming all other parameters remain unchanged, the etching rate is mainly determined by ammonia (NH3) in the etching solution. Therefore, there are two main purposes of using fresh solution to interact with the etched surface: one is to wash away the copper ions just produced; the other is to continuously provide the 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 generally accepted that the lower the content of monovalent copper ions 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), which function to reduce monovalent copper ions (these are the technical secrets of their products’ high reactivity. ), it can be seen that the influence of monovalent copper ions is not small. Reducing monovalent copper from 5000ppm to 50ppm will more than double the etching rate.

Since a large amount of monovalent copper ions are generated during the etching reaction, and because monovalent copper ions are always tightly combined with the complex group of ammonia, it is very difficult to keep its 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 to vent air into the etch 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 is also a variable quantity in solution that needs to be controlled. Some users adopt the practice of passing pure ammonia into the etching reservoir. To do this, a 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 structural design. In this method, the solution used is divalent copper, not ammonia-copper etching. It will potentially be used in the printed circuit industry. In the PCH industry, typical thicknesses of etched copper foil are 5 to 10 mils, and in some cases considerably thicker. Its requirements for etching parameters are often more stringent than those in the PCB industry.

Regarding the issue of different etching states between the upper and lower boards, the lead-in edge and the back-entry edge

A large number of problems related to etching quality are concentrated on the etched parts of the upper surface. It is very important to understand this. These problems come from the impact of gelatinous build-up produced by the etchant on the upper surface of the printed circuit board.

Colloidal agglomerates accumulate on the copper surface, which on the one hand affects the spray force, and on the other hand blocks the replenishment of fresh etching solution, resulting in a reduction in etching speed. It is precisely due to the formation and accumulation of gelatinous plaque that the etching degree of the upper and lower graphics on the board is different. This also makes it easier for the part of the board that enters the etching machine to be completely etched or cause excessive corrosion, because the accumulation has not yet formed at that time and the etching speed is faster. On the contrary, when the part of the board enters later, accumulation has formed and slows down its etching speed.

Maintenance of etching equipment

The most critical factor in the maintenance of etching equipment is to ensure that the nozzles are clean and free of obstructions to ensure smooth jetting. Blockage or slagging will impact the plate under the action of jet pressure. If the nozzle is unclean, uneven etching will occur and the entire PCB will be scrapped.

Obviously, equipment maintenance means replacing damaged and worn parts, including replacing nozzles, which also suffer from wear and tear. In addition, the more critical issue is to keep the etching machine free of slagging. In many cases, slagging accumulation will occur. Excessive slagging accumulation may even affect the chemical balance of the etching solution. Likewise, if there is an excessive chemical imbalance in the etching solution, slagging will become more severe. The problem of slag buildup cannot be overemphasized. Once a large amount of slagging suddenly occurs in the etching solution, it is usually a signal that there is a problem with the balance of the solution. This should be properly cleaned with strong hydrochloric acid or the solution should be replenished.

The residual film can also produce slagging. A very small amount of the residual film dissolves in the etching solution, and then forms a copper salt precipitation. The slagging formed by the residual film indicates that the previous film removal process was not thorough. Poor film removal is often the result of a combination of edge film and over-plating.Use privileges

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