The process of printed circuit board from light board to display circuit graph is a more complex physical and chemical reaction process, this paper analyzes its last step – etching. At present, the typical process of printed circuit board (PCB) processing is “graphic electroplating method”. That is, on the outer layer of the board to retain the copper foil part, that is, the graphic part of the circuit is pre-plated with a layer of lead tin corrosion resistant layer, and then chemically corroded the rest of the copper foil, known as PCB etching.
Type of PCB etching
Note that there are two layers of copper on the etched plate. In the outer etching process only one layer of copper must be etched away completely, and the rest will form the final circuit required. This type of graphic plating is characterized by the presence of a copper-plated layer only beneath a lead-tin resist.
Another process is to coat the entire plate with copper, leaving only a tin or lead-tin resist outside the film. This process is called “whole plate copper plating process”. The biggest disadvantage of full plate copper plating compared to graphic plating is that it has to be copper plated twice everywhere and must be etched away. Therefore, when the wire width is very fine, there will be a series of problems. At the same time, side corrosion will seriously affect the uniformity of lines.
In the process of outer circuit of printed board, there is another method, which is to use photosensitive film instead of metal coating as corrosion resistance layer. This method is very similar to the inner etching process, see etching in the inner fabrication process.
At present, tin or lead-tin is the most commonly used corrosion resistant layer, used in the etching process of ammonia etching agent. Ammonia etching agent is a widely used chemical solution, and tin or lead tin does not occur any chemical reaction. Ammonia etching agent mainly refers to ammonia water/ammonia chloride etching solution.
In addition, ammonia/ammonia sulfate etching solution is also available on the market. Sulphate based etching solution in which the copper can be separated by electrolysis after use and thus reused. Due to its low corrosion rate, it is rarely seen in actual production, but it is expected to be used in chlorine-free etching.
Some people try to use sulfuric acid – hydrogen peroxide as etching agent to corrode the outer pattern. This process has not been widely adopted in a commercial sense for a number of reasons, including economics and waste treatment. Furthermore, sulfuric acid and hydrogen peroxide can not be used to etch the lead-tin corrosion resistant layer, and this process is not the main method in the manufacture of PCB outer layer, so most people rarely pay attention to it.
Etching quality and existing problems
The basic requirement for etching quality is to be able to completely remove all copper layers except those below the resist layer, and that’s it. Strictly speaking, etching quality must include consistency of wire width and degree of lateral erosion if it is to be defined accurately. Due to the inherent characteristics of the current corrosion solution, etching not only down but also in all directions, so side erosion is almost inevitable.
Lateral erosion is often discussed among etching parameters, which is defined as the ratio of lateral erosion width to etching depth, known as the etching factor. In the printed circuit industry, it varies widely from 1:1 to 1:5. Obviously, small lateral etch or low etch factor is most satisfactory.
The structure of the etching equipment and the etching fluid of different components can influence, or optimistically control, the etching factor or lateral erosion. Lateral erosion can be reduced by using certain additives. The chemical composition of these additives is generally a trade secret and is not disclosed to the outside world by their developers.
In many ways, the quality of etching existed long before the printed board entered the etching 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 etching quality actually exist in the process of film removal even before.
For the etching process of outer layer graphics, because it reflects the “inverted stream” image is more prominent than most printed board processes, so many problems are finally reflected in it. At the same time, this is also because etching is the last step in a long series of processes starting with the film, photographic, after which the outer image is successfully transferred. The more links there are, the more likely things are to go wrong. This can be regarded as a very special aspect of the printed circuit production process.
Theoretically speaking, printed circuit into the etching stage, in the graphic electroplating process printed circuit, the ideal state should be: after electroplating copper and tin or copper and lead tin thickness of the sum should not exceed the thickness of electroplating sensitive film, so that electroplating graphics are completely blocked by the “wall” on both sides of the film and embedded in the inside. However, in real production, the printed circuit board in the world after electroplating, coating graphics are much thicker than photosensitive graphics. In the process of electroplating copper and lead tin, because the height of the coating exceeds the photosensitive film, it tends to accumulate horizontally, and the problem arises. A tin or lead-tin corrosion resistant layer covering the top of the line extends to the sides, forming an “edge” under which a small portion of the photosensitive film is covered.
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” below the “edge”. The “residual glue” or “residual film” remains below the “edge” of the resist, resulting in incomplete etching. “Copper root” is formed on both sides of the line after etching, and the copper root makes the line spacing narrow, resulting in the printed board does not meet the requirements of Party A, and may even be rejected. Rejection will greatly increase the cost of PCB production.
In addition, in many cases, due to the formation of reaction and dissolution, in the printed circuit industry, residual film and copper may also be accumulated in the corrosion liquid and blocked in the nozzle of the corrosion machine and acid pump, have to stop processing and cleaning, and affect the work efficiency.
Equipment adjustment and interaction with corrosive solution
Ammonia etching is a delicate and complex chemical reaction process in printed circuit processing. On the other hand, it is an easy task. Once the process is upgraded, continuous production can be carried out. The key is that once the boot needs to maintain continuous working state, not dry stop. The etching process depends greatly on the good working condition of the equipment. At present, no matter what kind of etching solution is used, high pressure spray must be used, and in order to obtain a neat line side and high quality etching effect, the structure and spray method of the nozzle must be strictly selected.
In order to get good side effect, there are many different theories, forming different design methods and equipment structure. These theories are often very different. But all the theories of etching recognize the most basic principle, which is to keep the metal surface exposed to fresh etching solution as quickly as possible. Chemical analysis of the etching process also supports this view. In ammoniacal etching, assuming all other parameters are constant, the etching rate is mainly determined by ammonia (NH3) in the etching solution. Therefore, with fresh solution and etching surface action, its purpose is mainly two: one is to wash away just generated copper ions; The other is to keep supplying 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 recognized that the lower the content of copper ion in the ammonia etching solution, the faster the reaction rate. This is borne out by experience. In fact, many ammonia etching solution products contain a special coordination base of copper ion (some complex solvents), its role is to reduce the copper ion (these are their products with high reaction ability of the technical secret), it can be seen that the influence of copper ion is not small. Dropping copper from 5,000 PPM to 50ppm more than doubles the etching rate.
Due to the formation of a large number of univalent copper ions in the etching reaction process, and because the univalent copper ions are always closely combined with the complex group of ammonia, so it is very difficult to keep its content close to zero. Conversion of univalent copper to divalent copper by the action of atmospheric oxygen can remove univalent copper. This can be achieved by spraying.
This is a functional reason to pass air into the etching chamber. However, if there is too much air, the loss of ammonia in the solution will be accelerated and the PH value will drop, which will still reduce the etching rate. Ammonia is also a variable that needs to be controlled in solution. Some users use pure ammonia channeling into etched reservoirs. To do so, a PH meter control system must be added. When the PH value measured automatically falls below a given value, the solution is automatically added.
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 will probably be used in the printed circuit industry. In the PCH industry, the thickness of etched copper foil is typically 5 to 10 mils, and in some cases quite large. It often requires more stringent etching parameters than in the PCB industry.
On the upper and lower board surface, the import side and the back into the side of the etching state is different
A large number of problems related to etching quality are focused on the etched part of the upper plate. This is very important to understand. These problems come from the effect of gelatinize produced by etching agent on the surface of printed circuit board. The accumulation of colloidal compaction on the copper surface affects the injection force on the one hand, and prevents the replenishment of fresh etching solution on the other hand, resulting in the decrease of etching speed. It is because of the formation and accumulation of colloidal material that the etching degree of the upper and lower graphics of the board is different. This also makes it easy to etch thoroughly or easily cause corrosion in the first part of the board in the etching machine, because the accumulation has not formed at that time, the etching speed is faster. On the contrary, when the part of the plate enters after the accumulation has formed, and slows down its etching speed.
Maintenance of pcb etching equipment
The most critical factor in the maintenance of etching equipment is to ensure that nozzles are clean and free of obstructions to allow smooth injection. Obstruction or slagging can impinge on the plate under injection pressure. If the nozzle is not clean, it will cause uneven etching and make the whole PCB scrap.
Obviously, maintenance of equipment involves replacing damaged and worn parts, including replacing nozzles, which are also subject to wear. In addition, the more critical problem is to keep the etching machine does not exist slagging, in many cases there will be slagging accumulation. Too much slagging accumulation will even affect the chemical balance of etching solution. Similarly, if there is excessive chemical imbalance in the etching fluid, slagging will become more serious. The problem of slagging accumulation cannot be overemphasized. Once a large amount of slagging occurs suddenly 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 supplemented with the solution.
Residual film can also produce slagging material, a very small amount of residual film dissolved in the PCB etching solution, and then formed copper salt precipitate. The slagging formed by residual film indicates that the previous film removal process is not thorough. Poor film removal is often the result of edge film and electroplating.