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Improve PCB etch process quality analysis
I. Introduction

The purpose of the etching: After the circuit plating is completed, the board removed from the electroplating apparatus is subjected to post-processing to complete the circuit board. Specifically there are the following steps:

        a. Stripping: The dry film of the anti-electroplating application is stripped with a syrup. After curing, the dry film partially dissolves under the strong liquid and partially peels off into flakes. In order to maintain the effect of the liquid medicine and the thorough washing, the performance of the filtration system is very important.

b. Line Etching: The non-conductive portion of the copper is dissolved away.

c. Stripping tin: Finally remove the etching resistant tin-lead plating. Regardless of pure tin or tin-lead layers with different composition ratios, the purpose of plating is to resist etching. After the etching is completed, it is to be stripped. Therefore, the step of stripping lead is only processing, and no additional value is generated. However, we must pay special attention to the following points. Otherwise, the increase in cost is the second reason. The outer routes that are difficult to complete have caused bad results here.

At present, the typical process of printed circuit board (PCB) processing uses "pattern plating." That is, a layer of lead-tin resist is pre-plated on the part of the copper foil to be reserved on the outer layer of the board, that is, the pattern part of the circuit, and then the remaining copper foil is chemically etched away.

It should be noted that there are two layers of copper on the board at this time. Only one layer of copper in the outer layer etching process must be etched away, and 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 layer. Another method is to plate copper on the entire board. The only part other than the photo-sensitive film is only the tin or lead-tin resist. This process is called "full-plate copper plating process." The biggest drawback of full-plate copper plating compared to graphic plating is that the surface must be plated with copper twice and must be etched away during etching. Therefore, when the wire width is very fine, a series of problems will arise. At the same time, side corrosion can seriously affect the uniformity of the lines.

In the processing technology of the outer circuit of the printed circuit board, there is another method that uses a photosensitive film instead of a metal plating layer as a resist layer. This method is very similar to the inner layer etching process. See the etching in the inner layer fabrication process.

Currently, tin or lead tin is the most commonly used resist layer used in the etching process of ammonia etching agents. Ammonia etchant is a commonly used chemical liquid that does not undergo any chemical reaction with tin or lead tin. Ammonia etchant mainly refers to ammonia/chlorinated ammonia etching solution. In addition, ammonia/sulfuric acid ammonia etching solutions can also be purchased on the market.

The sulphate-based etch liquid, after use, copper 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. Some people used sulfuric acid-hydrogen peroxide as an etchant to etch the outer layer. Due to many reasons including economics and waste liquid handling, this process has not been widely adopted in commercial sense. Furthermore, sulfuric acid-hydrogen peroxide cannot be used for the etching of lead-tin resists, and this process is not the main method in the production of PCB outer layers, so most people rarely care.

Second, the quality of etch and pre-existing problems

The basic requirement for the quality of the etching is to be able to completely remove all copper layers except under the resist layer. In a strict sense, if it is to be precisely defined, then the etching quality must include the uniformity of the wire width and the degree of undercut. Due to the intrinsic characteristics of the current etching solution, not only the downward direction but also the etching effect in the right and left directions, the side etching is almost unavoidable.

The problem of undercutting is one of the most frequently discussed etch parameters. It is defined as the ratio of the undercut width to the etch depth, called the etch factor. In the printed circuit industry, its scope of change is very broad, from 1:1 to 1:5. Obviously, a small degree of side etching or a low etching factor is the most satisfactory.

The structure of the etching equipment and the etching solutions of different compositions all have an effect on the etching factor or the side etching degree, or in an optimistic sense, can be controlled. The use of certain additives can reduce the degree of side etching. 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 etch has already existed before the printed board entered the etching machine. Because there are very close internal links between the various processes or processes of the printed circuit process, there is no process that is not affected by other processes and does not affect other processes. Many of the problems identified as etch quality have actually existed in the removal of films and even earlier processes. For the outer layer pattern etching process, many problems are finally reflected on it because the “riverside” phenomenon it embodies is more prominent than most of the printed circuit board processes. At the same time, this is also due to the fact that etching is the last step in a long series of processes starting with self-bonding and photosensitization, after which the outer layer pattern is transferred successfully. The more links, 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 etching stage, in the process of patterning the printed circuit by pattern plating, the ideal state should be: the sum of the thickness of copper and tin or copper and lead and tin after plating should not exceed the plating resistance The thickness of the photosensitive film allows the plating pattern to be completely blocked by the "walls" on both sides of the film and embedded therein. However, in actual production, the plating patterns of the printed circuit boards all over the world are greatly thicker than the photosensitive patterns after plating. In the electroplating of copper and lead-tin, the problem arises because the plating height exceeds that of the photosensitive film, which causes a tendency of lateral stacking. The tin or lead-tin-resist layer covered above the lines extends to both sides to form an "edge", and a small part of the photosensitive film is covered under the "edge".

The "edge" of tin or lead tin makes it impossible to completely remove the photosensitive film when removing the film, leaving a small part of the "residue" under the "edge." "Residue" or "residual film" remains underneath the "edge" of the resist and will cause incomplete etching. Lines form "copper roots" on both sides after etching, and the copper roots narrow the line spacing, resulting in printed boards that do not meet Party A requirements and may even be rejected. Due to the rejection, the production cost of the PCB is greatly increased.

In addition, in many cases, due to the reaction to form a dissolution, in the printed circuit industry, residual film and copper may also accumulate in the etching solution and plug in the corrosion machine nozzle and acid pump, had to stop processing and cleaning , which affects the efficiency of work.

Third, equipment adjustments and the interaction with corrosion solutions

In printed circuit processing, ammonia etching is a more elaborate and complicated chemical reaction process. On the contrary, it is an easy task. Once the process is turned on, production can continue. The key point is that once you start up, you need to maintain continuous working status and you should not stop work. The etching process depends to a great extent on the good working conditions of the equipment. At present, no matter what kind of etching liquid is used, high-pressure showering must be used, and in order to obtain a more neat line side and a high-quality etching effect, the nozzle structure and spray mode must be strictly selected.

In order to obtain a good side effect, many different theories emerged to form different design methods and equipment structures. These theories are often very different. However, all the theories concerning etching acknowledging that the most basic principle is to keep the metal surface in contact with the fresh etching solution as quickly as possible. The chemical mechanism analysis of the etching process also confirms the above viewpoint. In ammonia etching, assuming all other parameters are unchanged, the etching rate is mainly determined by ammonia (NH3) in the etching solution. Therefore, the use of fresh solution and etching the surface, its purpose is mainly two: First, flush away the copper ions just produced; Second, constantly provide the ammonia (NH3) required 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 the ammonia etching solution, the faster the reaction rate. This has been confirmed by experience. . In fact, many ammonia etch solutions contain special ligands (a few complex solvents) for monovalent copper ions. Their role is to reduce monovalent copper ions (these are the secrets of their products' high reactivity.) ), we can see that the impact of monovalent copper ions is not small. By reducing monovalent copper from 5000 ppm to 50 ppm, the etch rate will more than double.

Since a large amount of monovalent copper ions are generated during the etching reaction, and because the monovalent copper ions are always closely combined with the complexing groups of ammonia, it is very difficult to keep the content near zero. Conversion of monovalent copper to divalent copper by the action of oxygen in the atmosphere can remove monovalent copper. This can be achieved by spraying.

This is a functional reason for passing air into the etching tank. However, if the air is too much, it will accelerate the loss of ammonia in the solution and cause the PH value to drop. As a result, the etching rate will still decrease. Ammonia is also the amount of change that needs to be controlled in the solution. Some users use the practice of passing pure ammonia into the etching tank. To do so must add a PH meter control system. When the automatically measured pH result is lower than the given value, the solution is automatically added.

In the field of chemical etching (also referred to as photochemical etching or PCH) related to this, research work has begun and reached the stage of structural design of the etching machine. In this method, the solution used is divalent copper, not an ammonia-copper etch. It will likely be used in the printed circuit industry. In the PCH industry, etched copper foils typically have a thickness of 5 to 10 mils, and in some cases are quite thick. Its etch parameters are often more demanding than in the PCB industry.

There is a research result from the PCM industrial system that has not yet been officially published, but the results will be refreshing. Because of the support of a relatively strong project fund, researchers have the ability to change the design concept of etching equipment in the long-term sense, and at the same time study the effects of these changes. For example, compared with the conical nozzle, the optimal nozzle design uses a fan shape, and the spray manifold (that is, the tube into which the nozzle is screwed) also has a mounting angle that can spray 30 degrees into the workpiece in the etching chamber. If this change is not made, the nozzles on the manifold are installed so that the spray angle of each adjacent nozzle is not exactly the same. The second set of nozzles each had a slightly different spray surface corresponding to the first set (it indicates the spray operation). In this way, the sprayed solutions are in a superimposed or intersecting state. Theoretically, if the solution shapes cross each other, the jetting force at that portion will decrease, and the old solution on the etched surface will not be effectively washed away and the new solution will remain in contact therewith. This is particularly prominent at the edges of the shower surface. Its ejection force is much smaller than in the vertical direction.

This study found that the latest design parameters are 65 psi (4+Bar). Each etch process and each practical solution has a problem with the optimum injection pressure, and for the time being, the injection tank pressure in the etch tank reaches a level of 30 psi or more (2 Bar) or less. There is a principle that the higher the density of an etching solution (ie, specific gravity or glassy degree), the higher the optimum injection pressure. Of course this is not a single parameter. Another important parameter is to control the relative mobility (or mobility) of the reaction rate in the solution.

Fourth, the etching equipment maintenance

The most critical factor in the maintenance of the etching equipment is to ensure that the nozzles are clean and free of obstructions to make the injection smooth. Blocking or scumming will impact the layout under the pressure of the jet. 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 parts and wear parts, including the replacement of nozzles, and 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, slagging builds up. Excessive slag accumulation can even affect the chemical balance of the etching solution. Similarly, scumming can become more severe if excess chemical imbalance occurs in the etching solution. The problem of slag accumulation cannot be overemphasized. Once the etching solution suddenly shows a large amount of slagging, it is usually a signal that there is a problem with the balance of the solution. This should use strong hydrochloric acid for proper cleaning or additional solution.

Residues can also produce scum, and a very small amount of residue film is dissolved in the etching solution, and copper salt precipitates. The scum formed by the residue film shows that the preceding film removal process is not thorough. Bad film removal is often the result of edge film and over-plating.

Five, on the upper and lower plate surface, the introduction of the edge and the back side of the etching state is different

A large number of problems related to the quality of etching are concentrated on the etched portion of the upper plate surface. It is very important to understand this. These problems arise from the influence of gel-like board deposits produced by the upper board surface etchant of the printed circuit board. Colloidal plates accumulate on the copper surface, affecting the jetting force on the one hand and blocking the addition of fresh etching solution on the other hand, resulting in a decrease in the etching rate. It is due to the formation and accumulation of gelatinous plate members that the etching patterns of the upper and lower patterns of the board are different. This also makes it easier for the part where the board first enters in the etching machine to be etched thoroughly or easily to cause over-etching because the stacking is not yet formed and the etching speed is faster. Conversely, when the part that enters the board enters, the build-up has already formed and slows down the etching speed.