Analysis of Improving the Quality of Pircuit Board Etching Process (2)

3. Equipment adjustment and interaction with corrosive solution

In printed circuit processing, ammonia etching is a relatively fine and complex chemical reaction process. In turn it is an easy job. Once the process has been turned up, production can be continued. The key is that once it is turned on, it needs to maintain a continuous working state, and it is not advisable to stop. The etching process depends to a great extent on the good working condition of the equipment. 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 structure and spraying method of the nozzle must be strictly selected.

In order to obtain a good side effect, many different theories have emerged, resulting in different design methods and equipment structures. These theories are often very different. But all theories about etching acknowledge the basic principle of getting the metal surface in constant contact with fresh etchant as quickly as possible. The chemical mechanism analysis of the etching process also confirmed the above point. In ammonia etching, assuming all other parameters are constant, the etching rate is mainly determined by the ammonia (NH3) in the etching solution. Therefore, using a fresh solution to interact with the etching surface has two main purposes: one is to flush out 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 raw materials, it is generally recognized that the lower the content of monovalent copper ions in the ammonia-based etching solution, the faster the reaction speed. This has been confirmed by experience. In fact, many ammonia-based etchant products contain special ligands (some complex solvents) for monovalent copper ions, which act 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 the monovalent copper from 5000ppm to 50ppm will more than double the etch rate.

Since a large amount of monovalent cupric ions are generated during the etching reaction, and because monovalent cupric ions are always tightly combined with the complexing group of ammonia, it is very difficult to keep their 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 chamber. 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 etch rate. Ammonia is also a variable amount in solution that needs to be controlled. Some users have adopted the practice of passing pure ammonia into the etching reservoir. PH meter control system must be added. When the automatically measured pH is lower than 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 structural design of the etching machine. In this method, the solution used is copper divalent, not an ammonia-copper etch. 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 (mils), and in some cases the thickness is quite large. Its requirements for etching parameters are often more stringent than those in the PCB industry.

There is a study from PCM industrial systems that has not yet been officially published, but the results will be refreshing. Because of the relatively strong project funding support, the researchers have the ability to change the design thinking of the etching device in the long term and study the effects of these changes. For example, compared to conical nozzles, the optimal nozzle design is fan-shaped, and the spray manifold (that is, the tube into which the nozzle is screwed) also has an installation angle that can spray the workpiece entering the etching chamber at a 30 degree angle. Without such a change, the nozzles on the manifold are mounted in such a way that the spray angle of each adjacent nozzle is not exactly the same. The respective spray surfaces of the second group of nozzles are slightly different from those of the first group (it represents the operation of the spray). In this way, the shapes of the sprayed solutions are superimposed or crossed. Theoretically, if the solution shapes cross each other, the jetting force of that part is reduced and cannot effectively wash the old solution off the etched surface while keeping the new solution in contact with it. This is especially true at the edges of the spray surface. Its jet force is much smaller than that in the vertical direction.

This study found that the latest design parameter is 65 psi (ie 4+Bar). Every etching process and every practical solution has an optimum injection pressure, and at present, it is very rare that the injection pressure in the etching chamber exceeds 30 psi (2Bar). There is a principle that the higher the density of an etching solution (ie specific gravity or degree of Bomer), the higher the optimum injection pressure should be. This is not a single parameter. Another important parameter is the relative mobility that controls its reaction rate in solution.