PCB News - Circuit board factory: how to improve the quality of etching process analysis

Circuit board factory: how to improve the quality of etching process analysis

1. Preface

The purpose of etching: After the circuit is electroplated, the PCB circuit board removed from the electroplating equipment will be processed to complete the circuit board. Specifically, there are the following steps:

a. Stripping film: peel off the dry film for electroplating purposes with a potion. The hardened dry film is partially dissolved in the concentrated liquid and partially peeled into flakes. In order to maintain the effect of the liquid medicine and thoroughly wash it with water, the efficiency of the filtration system is very important.

b. Circuit etching: dissolve the copper in the non-conductor part.

c. Strip tin-lead: finally remove the anti-etching tin-lead plating. Regardless of the pure tin or the tin-lead layer of various composition ratios, the purpose of plating is only to resist etching, so after the etching is completed, it must be stripped, so this step of stripping tin and lead is only processing and does not produce added value. However, special attention must be paid to the following points, otherwise the cost increase will be the second, and the hard-to-finish outer circuit will cause defects here.

At present, the typical process of printed circuit board (PCB) processing adopts the "pattern plating method". That is, pre-plated a layer of lead-tin anti-corrosion layer on the part of the copper foil that needs to be retained on the outer layer of the board, that is, the pattern part of the circuit, and then chemically corrodes the remaining copper foil.

It should be noted that there are two layers of copper on the PCB circuit board at this time. 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. This type of pattern plating is characterized by 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 parts other than the photosensitive film are only tin or lead-tin resist. This process is called "full board copper plating process". Compared with pattern electroplating, the biggest disadvantage of full-board copper plating is that copper must be plated twice on all parts of the board and they must be etched away during 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 line.

In the processing technology of the outer circuit of the PCB printed circuit board, there is another method, which is to use the photosensitive film instead of the metal coating as the anti-corrosion layer. This method is very similar to the inner layer etching process, and you can refer to the etching in the inner layer manufacturing process.

At present, tin or lead-tin is the most commonly used anti-corrosion layer, used in the etching process of ammonia-based etchant. Ammonia-based etchant is a commonly used chemical liquid, which does not have any chemical reaction with tin or lead-tin. Ammonia etchant mainly refers to ammonia/ammonium chloride etching solution. In addition, ammonia/ammonium sulfate etching chemicals are also available on the market.

After using the sulfate-based etching solution, the copper in it can be separated by electrolysis, so it can be reused. Because of its low corrosion rate, it is generally rare in actual production, but it is expected to be used in chlorine-free etching. Someone tried to use sulfuric acid-hydrogen peroxide as an etchant to corrode the outer layer pattern. Due to many reasons including economy and waste liquid treatment, this process has not been widely adopted in a commercial sense. Furthermore, sulfuric acid-hydrogen peroxide can not be used for the etching of lead-tin resist, and this process is not the main method in the production of PCB outer layer, so most people rarely care about it.

2. Equipment adjustment and interaction with corrosive solution

In the processing of printed circuit boards, ammonia etching is a relatively delicate and complex chemical reaction process. On the other hand, it is an easy job. Once the process is up-regulated, production can be continued. The key is to maintain continuous working status once it is turned on, and it is not advisable to dry and stop. The etching process depends to a large extent on the good working condition of the equipment. At present, no matter what etching solution is used, high-pressure spray must be used, and in order to obtain a neater line side and high-quality etching effect, the nozzle structure and 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 recognize the most basic principle, which is to keep the metal surface in contact with fresh etching solution as quickly as possible. The chemical mechanism analysis of the etching process also confirmed the above point of view. In ammonia etching, assuming that all other parameters remain unchanged, the etching rate is mainly determined by the ammonia (NH3) in the etching solution. Therefore, using fresh solution to etch the surface has two main purposes: one is to flush out the copper ions that have just been produced; the other is to continuously provide ammonia (NH3) needed for the reaction.

In the traditional knowledge of the printed circuit board (PCB circuit board) industry, especially the suppliers of printed circuit raw materials, it is recognized that the lower the monovalent copper ion content 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), whose role is to reduce monovalent copper ions (these are the technical secrets of their products with high reactivity ), it can be seen that the influence of monovalent copper ions is not small. If the monovalent copper is reduced from 5000ppm to 50ppm, the etching rate will be more than doubled.

Because a large amount of monovalent copper ions are generated during the etching reaction, and because the monovalent copper ions are always tightly combined with the complexing group of ammonia, it is very difficult to keep its content close to zero. The 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 for passing air into the etching box. However, if there is too much air, it will accelerate the loss of ammonia in the solution and decrease the pH value, resulting in a decrease in the etching rate. Ammonia in the solution is also the amount of change that needs to be controlled. Some users adopt the method of passing pure ammonia into the etching reservoir. To do so, a set of 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 chemical etching (also known as photochemical etching or PCH) field, 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 may 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 research result from the PCM industrial system, which has not yet been officially published, but the result will be refreshing. Because of the relatively strong project fund support, researchers have the ability to change the design of the etching device in the long-term sense, and at the same time study the effects of these changes. For example, compared with the conical nozzle, the best nozzle design uses a fan shape, and the spray manifold (that is, the pipe into which the nozzle is screwed) also has an installation angle, which can spray 30 degrees of the workpiece into the etching chamber. If such a change is not made, the installation method of the nozzles on the manifold will cause the spray angle of each adjacent nozzle to be not completely the same. The spray surfaces of the second group of nozzles are slightly different from those of the first group (it shows the working conditions of the spray). In this way, the shapes of the sprayed solutions become superimposed or intersected. Theoretically, if the shapes of the solutions cross each other, the jetting force of this part will be reduced, and the old solution on the etched surface cannot be effectively washed away while keeping the new solution in contact with it. This situation is especially prominent at the edge of the spray surface. Its ejection force is much smaller than the vertical direction.

This study found that the latest design parameter is 65 pounds per square inch (ie 4+Bar). Every etching process and every practical solution has a problem of the best spray pressure, and at present, the spray pressure in the etching chamber reaches 30 pounds per square inch (2Bar) or more. There is a principle that the higher the density of an etching solution (ie, specific gravity or glass degree), the higher the optimal injection pressure should be. Of course this is not a single parameter. Another important parameter is the relative mobility (or mobility) that controls the reaction rate in the solution.

3. Etching quality and previous problems

The basic requirement for etching quality is to be able to completely remove all the copper layers except under the resist layer, and that's it. Strictly speaking, if it is to be accurately defined, then the etching quality must include the consistency of the wire width and the degree of undercutting. Due to the inherent characteristics of the current etching solution, which not only produces an etching effect on the downward direction but also on the left and right directions, side etching is almost inevitable.

The problem of side etching is one of the etching parameters that is often raised for discussion. It is defined as the ratio of the width of the side etching to the depth of the etching, which is called the etching factor. In the printed circuit industry, it has a wide range of changes, from 1:1 to 1:5. Obviously, a small undercut degree or a low etching factor is the most satisfactory.

The structure of the etching equipment and the different components of the etching solution will affect the etching factor or the degree of side etching, or in optimistic terms, it can be controlled. The use of certain additives can reduce the degree of side erosion. 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 has existed long before the printed board enters the etching machine. Because there are very close internal connections 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 existed in the process of removing the film or even before. For the etching process of the outer layer graphics, because the "inverted stream" phenomenon it embodies is more prominent than most printed board processes, many problems are finally reflected in it. At the same time, this is also because the etching is the last step in a long series of processes starting with self-sticking and photosensitive, after which the outer layer pattern is successfully transferred. The more links, the greater the possibility of problems. This can be seen as a very special aspect of the printed circuit production process.

Theoretically speaking, after the printed circuit enters the etching stage, in the process of processing the printed circuit by the pattern plating method, the ideal state should be: the total thickness of the electroplated copper and tin or copper and lead tin should not exceed the resistance to electroplating The thickness of the photosensitive film makes the electroplating pattern completely blocked by the "walls" on both sides of the film and embedded in it. However, in actual production, after electroplating printed circuit boards all over the world, the plating pattern is much thicker than the photosensitive pattern. In the process of electroplating copper and lead-tin, because the plating height exceeds the photosensitive film, a tendency of lateral accumulation occurs, and the problem arises from this. The tin or lead-tin resist layer covering the lines extends to both sides to form 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 part of "residual glue" under the "edge". The "residual glue" or "residual film" left under 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 fail to meet the requirements of Party A and may even be rejected. The rejection will greatly increase the production cost of the PCB circuit board.

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

Fourth, the maintenance of etching equipment

The most critical factor in the maintenance of etching equipment is to ensure that the nozzle is clean and free of obstructions to make the jet unobstructed. Blockages or slagging will impact the layout under the action of jet pressure. If the nozzle is not clean, the etching will be uneven and the entire PCB circuit board will be scrapped.

Obviously, equipment maintenance is the replacement of damaged and worn parts, including replacement of nozzles. The nozzles also have the problem of wear. In addition, the more critical issue is to keep the etching machine free of slagging. In many cases, there will be slagging accumulation. Too much slagging accumulation may even affect the chemical balance of the etching solution. Similarly, if there is excessive chemical imbalance in the etching solution, slagging will become more serious. The problem of slag 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 done with strong hydrochloric acid for proper cleaning or supplementation of the solution.

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

5. Regarding the upper and lower PCB circuit board surface, the etching state of the leading edge and the trailing edge are different

A large number of problems related to etching quality are concentrated on the etched part of the upper plate surface. It is very important to understand this. These problems come from the influence of the glue-like clumps produced by the etchant on the upper surface of the printed circuit board. The accumulation of colloidal slabstock on the copper surface affects the spraying force on the one hand, and on the other hand prevents the replenishment of fresh etching solution, resulting in a decrease in the etching speed. It is precisely because of the formation and accumulation of colloidal slabs that the degree of etching of the upper and lower patterns of the board is different. This also makes the first part of the board in the etching machine easy to be etched completely or to cause over-corrosion, because the accumulation has not yet formed at that time, and the etching speed is faster. On the contrary, the part that enters behind the board has already formed when it enters, and slows down its etching speed.