Deep Analysis Of PCB Etching Process

- Aug 04, 2018-

I. types of 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 completely, and the rest will form the necessary circuit.This type of graphic plating is characterized by the copper plating layer only exists below the lead - tin corrosion - resistant layer.The other process is to coat the whole plate with copper, and the rest of the film is only tin or lead tin corrosion resistant layer.This process is called "full plate copper plating process".Compared with graphic plating, the biggest drawback of full-plate copper plating is that it has to be plated twice on all parts of the plate and corroded when etched.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 the line.In the process of printing plate outer circuit, another method is to use photosensitive film instead of metal coating as corrosion resistant layer.This method is very similar to the inner etching process. You can refer to the etching in the inner etching process.At present, tin or lead tin is the most commonly used anti-corrosion layer and is used in the etching process of ammonia etching agents. Ammonia etching agents are commonly used chemical liquids and do not react with tin or lead tin.Ammonia etching agent mainly refers to ammonia water/chlorinated ammonia etching solution.In addition, you can also buy ammonia/sulfuric acid ammonia etching liquid on the market.Sulphate-based etch solution, in which copper can be separated by electrolysis, can be reused.Due to its low corrosion rate, it is seldom seen in actual production, but it is expected to be used in chlorine-free etching.Some people have tried to use sulfuric acid - hydrogen peroxide as etching agent to corrode the outer graphics.This process has not been widely used in the commercial sense for many reasons, including economy and waste liquid treatment. Furthermore, sulfuric acid - hydrogen peroxide water cannot be used for the etching of lead - tin corrosion resistant layer, which is not the main method in the outer production of PCB. 

Ii. Problems of etching quality and preexistence

The basic requirement for etch quality is the ability to remove all copper layers except under the anti-etch layer completely.In a strict sense, if the etching quality is to be defined accurately, then the etching quality must include the consistency of wire width and the extent of side etching.Due to the inherent characteristics of the corrosion solution, the side corrosion is almost inevitable, because it is not only downward but also has etching effect on the left and right directions.The problem of side erosion is often discussed in the etching parameters. It is defined as the ratio of the width of side erosion 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, small side erosion or low etching factor is the most satisfactory.The structure of the etching equipment and the etching solution of different components can influence the etching factor or side etching degree, or, in optimistic terms, can be controlled.Some additives can reduce side erosion.The chemical composition of these additives is generally a trade secret, and their respective researchers are not disclosed to the outside world.In many ways, the quality of etching has existed long before the plate entered the etching machine.Because there is a very close internal connection between each process or process 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 that have been identified as problems with etching quality have actually existed in membrane removal and even earlier processes.As for the etching process of exterior layer graphics, many problems are finally reflected in it because the "pour stream" represented by it is more prominent than most printing plate technologies.Also, this is because etching is the last link in a long series of processes that start with a self-coating, photoreceptor.The more links there are, the more likely there are problems.This can be regarded as a very special aspect of the production of printed circuit.In theory, when the printed circuit enters the etching stage, the ideal state for the graphic plating process of the printed circuit should be: the sum of the thickness of copper and tin or copper and lead tin after plating should not exceed the thickness of the electroplated photosensitive film, so that the electroplating pattern is completely blocked and embedded by the "wall" on both sides of the film.However, in real production, the world's printed circuit boards after plating, coating graphics are much thicker than photosensitive graphics.In the process of plating copper and lead tin, the problem arises because the coating height exceeds that of the photosensitive film, which leads to the tendency of horizontal accumulation.The tin or lead tin resist coating above the line extends to both sides, forming a "edge" that covers a small portion of the photosensitive film below the edge.The "edge" of tin or lead tin prevents the photoreceptor film from being completely removed when the film is removed, leaving a small amount of residual glue under the edge.The "residual glue" or "residual film" left under the anti-corrosion agent "along" will cause incomplete etching.After etching, the lines form "copper roots" on both sides. The copper roots narrow the line spacing, causing the printing plate to not meet the requirements of party a and may even be rejected.Rejection will greatly increase the production cost of PCB.In addition, in many cases, as a result of reaction, dissolution is formed. In the printed circuit industry, the residual film and copper may also accumulate in the corrosion solution and be stuck in the nozzles of the corrosion machines and acid proof pumps, and have to be stopped for treatment and cleaning, which affects the working efficiency. 

3. Equipment adjustment and interaction with corrosion solution

Ammonia etching is a relatively fine and complex chemical reaction process in printing circuit processing.On the other hand, it's an easy job to do.Once the process is up-regulated, continuous production can be carried out.The key is to keep working continuously once the machine is started up.The etching process depends greatly on the good working condition of the equipment.At present, no matter what kind of etch fluid is used, high pressure spray must be used, and in order to obtain a neat line side and high quality etch effect, the structure and spray way of the nozzle must be strictly selected.In order to get a good side effect, there are many different theories, forming different design methods and equipment structures.These theories are often quite different.But all the etching theories acknowledge the basic principle of keeping metal surfaces in constant contact with fresh etching fluids as quickly as possible.Chemical mechanism analysis of the etching process also confirmed the above viewpoints.In ammonia etching, the etching rate is mainly determined by ammonia (NH3) in the etching solution, assuming all other parameters remain unchanged.Therefore, the purpose of using fresh solution and etching surface is twofold.The other is to keep providing the ammonia (NH3) needed for the reaction.In the traditional knowledge of the printed circuit industry, especially the suppliers of printed circuit materials, it is generally accepted that the lower the content of monovalent copper ions in ammonia etching solution, the faster the reaction speed. This has been proved by experience.In fact, many ammonia etching products contain special ligands of monovalent copper ions (some complex solvents), whose role is to reduce monovalent copper ions (these are the technical secrets of their products with high reactivity ability), so the influence of monovalent copper ions is not small.Cutting the price of copper from 5000ppm to 50ppm more than doubles the etching rate.It is difficult to keep the content near zero because a large amount of monovalent copper ions are generated during the etching process, and because the monovalent copper ions are always tightly bound to the complexation base of ammonia.The removal of monovalent copper can be achieved by converting univalent copper into divalent copper by the action of oxygen in the atmosphere.This can be achieved by spraying.This is one of the functional reasons to pass 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 that needs to be controlled in solution.Some users use the practice of injecting pure ammonia into the etch tank.This must be done with a PH meter control system.The solution is added automatically when the automatically measured PH value is lower than the given value.In the related field of chemical etching (also called photochemical etching or PCH), the research work has begun and reached the stage of structural design of etching machines.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 typical thickness of etched copper foil is 5 to 10 mils, in some cases quite large.Its requirements for etching parameters are often more stringent than those in the PCB industry. 

Iv. About the upper and lower panel, the etching status of the leading edge and the trailing edge is different

A lot of problems related to the quality of etching focus on the etched part of the upper plate.It is important to understand this.These problems are caused by the influence of the adhesive of the etching agent on the upper board of the printed circuit board.On the one hand, the injection force is affected, and on the other hand, the addition of fresh etching solution is prevented, resulting in the reduction of etching speed.It is because of the formation and accumulation of colloidal board that the etching degree of the upper and lower graphics of the board is different.This also makes the first part of the plate in the etching machine easy to etch thoroughly or to cause excessive corrosion, because at that time the accumulation has not been formed, etching speed is fast.On the contrary, the part that enters behind the plate is formed when it enters, and the etching speed is slowed down. 

V. maintenance of etching equipment

The most important factor for the maintenance of etching equipment is to ensure that the nozzle is clean and free of obstruction and the spray is free.Blockage or slag may impact the plate under jet pressure.If the nozzle is not clean, it will cause uneven etching and scrap the whole PCB.The maintenance of the equipment is to replace the damaged and worn parts, including the replacement of the nozzle, the nozzle also has wear problems.In addition, the more critical problem is to keep the etching machine free of slag deposition, which will occur in many cases. Excessive slag accumulation may even affect the chemical balance of the etching solution.Also, if the etch fluid is chemically unbalanced, the slag will become more serious.The problem of slag accumulation cannot be overemphasized.Once the etching solution suddenly appears a large amount of slag formation, it is usually a signal that the solution balance problems.This should be used strong hydrochloric acid for proper cleaning or to supplement the solution.The residual film can also produce slag. A very small amount of the residual film is dissolved in the etching solution, and then copper salt deposits are formed.The slag formed by residual film indicates that the process of defilm is not complete.Bad defilm is often the result of edge film and overplating.