PCB etching process and process control2020-03-19
The process of printed circuit boards from light boards to displaying circuit patterns is a relatively complex physical and chemical reaction process. This article analyzes its final step, etching.
PCB etching is the process in which copper is removed from a circuit board layer. It is done through a chemical solution. This etching process is broken in two categoties: acidic etching and alkaline.
At present, the typical process of printed circuit board ( PCB ) processing is “graphic plating”. That is, a layer of lead-tin resist is pre-plated on the portion of the copper foil. It needs to be retained on the outer layer of the board, that is, the pattern portion of the circuit. Then the remaining copper foil is chemically etched away, which is called etching.
First, the type of pcb etching
Note that there are two layers of copper on the board during pcb etching. Only one layer of copper must be etched away in the outer layer etching process. The rest will form the final required circuit. This type of pattern plating is characterized in that the copper plating layer exists only under the lead-tin resist.
Another method of pcb etching is copper plating on the whole board, and the part other than the photosensitive film is only tin or lead-tin resist. This process is called “full-plate copper plating process”. Compared with pattern plating, the biggest disadvantage of full-plate copper plating is that copper must be plated twice on the entire surface of the board and they must be corroded during pcb 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.
During the process of printing circuit boards etching, there are two other methods that can be used. The first utilizes a photoresist film instead of a metal plating layer as the resist layer. This method is very similar to the inner layer etch in the inner layer manufacturing process, and is a more common choice for those with volume orders.
At present, tin or lead-tin is the most commonly used resist layer. It is used in the etching process of ammonia-based etchant. Ammonia-based etchant is a commonly used chemical solution. It does not react with tin or lead-tin. Ammonia etchant mainly refers to ammonia water / ammonia chloride etchant.
In addition, ammonia / ammonium sulfate pcb etching solutions are also commercially available. After using the sulfate-based etching solution, the copper in the pcb etching solution can be separated by electrolysis. So it can be reused. Due to its low corrosion rate, it is generally rare in practical production, but it is expected to be used in chlorine-free etching.
Some people have used sulfuric acid-hydrogen peroxide as an etchant to corrode the outer layer pattern. However, this method is not ideal due to the many downsides of this particular mix. First, it can be very dangerous, as mixing these chemicals is acidic and highly flammable. Second, the water that results from the reaction needs to be disposed of anyway, so this process creates more waste than necessary. Furthermore, sulfuric acid-hydrogen peroxide cannot be used for the etching of lead-tin resists. This process is not the main method in the production of PCB outer layers, so most people rarely ask for it.
Second, the pcb etching quality and problems
The basic requirement for the quality of pcb etching is to be able to completely remove all copper layers except the resist layer, and that’s it. In the strict sense, if it is to be accurately defined, the etching quality must include the consistency of the wire width and the degree of side etching. Due to the inherent characteristics of the current etching solution, not only downward but also left and right directions, side etching is almost inevitable.
The problem of undercut is often raised and discussed in the pcb etching parameters. It is defined as the ratio of the undercut width to the etch depth and is called the pcb etch factor.
In the printed circuit industry, it varies widely, from 1: 1 to 1: 5. Obviously, a small side etch or a low etch factor is most satisfactory.
The process of pcb etching is all about finding the right combination of settings to create a successful print. These settings include pcb etching solution strength, time, temperature and motor speed. The use of certain additives can reduce the side erosion. The chemical composition of these additives is generally a trade secret, and their respective developers are not disclosed to the outside world. In many ways, the quality of the etching has 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 pcb etching, there is no process that is not affected by other processes and does not affect other processes. Many of the issues identified as etching quality have actually existed in the process of removing the film and even before. As for the pcb etching process of the outer layer pattern, since the “down stream” image it embodies is more prominent than most printed board processes, many problems are finally reflected on it. At the same time, this is also because the pcb etching is a self-adhesive film, the last part of a long series of processes that began with photosensitivity.
After that, the outer layer pattern is successfully transferred. The more links there are, the greater the possibility of problems. This can be seen as a very special aspect of the printed circuit production process.
In theory, after the printed circuit enters the pcb etching stage, in the process of processing the printed circuit by the pattern plating method, the ideal state should be: the thickness of the copper and tin or copper and lead-tin after plating should not exceed the resistance to plating The thickness of the photosensitive film makes the plating pattern completely blocked by the “walls” on both sides of the film and embedded in it.
However, in actual production, after the electroplating of printed circuit boards all over the world, the plating pattern must be 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 of lateral accumulation, and the problem arises. The tin or lead-tin resist layer covered above the lines extends to both sides, forming a “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 when removing the film, leaving a small portion of “residual glue” under the “edge”. “Residual glue” or “residual film” left behind the “edge” of the resist 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. The rejection will greatly increase the production cost of the PCB. As a result, in many cases, residual film and copper built up in the corrosive solutions used to dip circuit boards are dissolved due to an exothermic reaction. The solution can cause blockages in the printing machine’s nozzles and acid-resistant pumps, resulting in work stoppage.
Third, equipment adjustment and interaction with the corrosion solution.
In printed circuit processing, ammonia pcb etching is a relatively fine and complex chemical reaction process. On the other hand, it is an easy task. Once the process is tuned, production can continue. The key is to maintain a continuous working state once the machine is turned on. The pcb etching process relies heavily on the good working conditions of the equipment.
No matter what kind is of pcb etching solution used, it should be rinsed with high pressure. The nozzle structure and spraying method is 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 quite different. But all theories about pcb etching recognize the basic principle of keeping the metal surface in contact with fresh etching solution as quickly as possible. The analysis of the chemical mechanism of the etching process also confirms the above point.
In ammonia pcb etching, assuming all other parameters remain unchanged, the etching rate is mainly determined by ammonia (NH3) in the etchant. Therefore, the use of fresh solution to etch the surface has two main purposes: one is to flush out the copper ions that have just been generated; the other is to continuously provide the ammonia (NH3) required for the reaction.
Cleaning of printed circuit boards (PCBs) is often done using ammonium hydroxide solutions. It has been observed that the etching speed is directly proportional to the concentration of free copper ions in the etchant. This has been confirmed by experience. In fact, many ammonia-based pcb etching solution products contain special ligands for monovalent copper ions (some complex solvents). Which reduces the monovalent copper ions (these are the technical secrets of their products with high reactivity) ). It can be seen that the effect of monovalent copper ions is not small. When the monovalent copper is reduced from 5000 ppm to 50 ppm, the pcb etching rate will be more than doubled.
Since a large amount of monovalent copper ions are generated during the pcb etching reaction, and because the monovalent copper ions are always tightly bound to the complexing group of ammonia. It is very difficult to maintain its content close to zero. Conversion of monovalent copper to divalent copper by the action of oxygen in the atmosphere can remove monovalent copper.
The above purpose can be achieved by spraying. This is one of the functional reasons for passing air into the etching box. However, if there is too much air, the ammonia loss in the solution will be accelerated and the pH value will be reduced. As a result, the pcb etching rate will still be reduced.
Ammonia is also the amount of change in the solution that needs to be controlled. Some users use pure ammonia into the pcb etching reservoir. To do so, a pH meter control system must be added. When the automatically measured pH is below the given value, the solution is automatically added. Currently, machines capable of pcb etching are being developed. They rely on photochemical methods (photoetching) rather than the ammonia-based methods that have been used since the 19th century. 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 the thickness is quite large. Its requirements for etching parameters are often more stringent than in the PCB industry.
Four, Regarding the upper and lower plate surfaces, the pcb etching status of the leading edge and the trailing edge are different.
A large number of problems related to pcb etching quality are concentrated on the etched portion of the upper plate surface. It is important to understand this. These problems come from the influence of the gel-like board structure caused by the etchant on the upper surface of the printed circuit board.
Gelatinous plate deposits accumulate on the copper surface, which on the one hand affects the spraying force, and on the other hand blocks the replenishment of fresh etching solution, causing a reduction in the pcb etching speed. It is precisely because of the formation and accumulation of gel-like slabs that the etching of the upper and lower graphics of the board is different. This also makes the part that the board first enters in the pcb etching machine easy to etch completely or easily cause over-corrosion. Because the deposit has not yet formed at that time, and the etching speed is fast. On the contrary, the part that entered after the board has formed a deposit when entering, and slows down the etching speed.
Five, the maintenance of pcb etching equipment
The most critical factor for the maintenance of pcb etching equipment is to ensure that the nozzle is clean and free of obstructions to make the spray unobstructed. Obstructions or slagging will impact the layout under the spray 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 and worn parts, including the replacement of the nozzles. The nozzles also have the problem of wear. In addition, a more critical issue is to keep the pcb etcher free of slagging, which in many cases will cause slagging to accumulate. Excessive slag accumulation can even affect the chemical balance of the pcb etchant.
Similarly, if excessive chemical imbalance occurs in the etchant, slagging will become more serious. The problem of slag accumulation cannot be overemphasized.
Once the pcb etching solution suddenly appears a lot of slagging, it is usually a signal that the balance of the solution is a problem. This should be properly cleaned with strong hydrochloric acid or added to the solution. Residual film can also produce slagging. 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 removing process was incomplete. Poor film removal is often the result of both the edge film and overplating.