Improve PCB etching process quality analysis

- Aug 18, 2018-

One, foreword

The purpose of the etching is to finish the circuit plating, remove the boards from the plating equipment, and finish the circuit board after processing.To be specific, there are several steps: a. Stripping: anti-electroplating of the dry film to remove the liquid medicine.The dry film after hardening is partially dissolved and partially peeled in this concentrated solution, and the filtration system is very important to maintain the efficacy of the solution and complete after washing.

B. Line etching: the dissolution of copper from non-conductive parts.

C. Tin lead stripping: finally remove the anti-etching tin lead coating.Regardless of pure tin or tin lead layer of each ingredients than its purpose is only resist etching plating, so after etching, will strip, so the strip tin lead steps for processing, only not generate additional value, but the following numbered still have to pay special attention to them, otherwise the cost increase is second, very not easy to do the outer circuit is bad here.

At present, the typical process of PCB processing is "pattern plating".The outer layer of the plate should be retained on the copper foil part, that is, the graphic part of the circuit is pre-coated with a layer of lead tin corrosion resistant layer, and then the rest of the copper foil is chemically corroded off.

Notice that there are two layers of copper on the board.In the outer etching process, only one layer of copper has to be completely etched off, and the rest will form the circuit needed eventually.This type of graphic plating is characterized by copper plating that exists only under the lead tin corrosion resistant layer.Another technique is to coat the entire plate with copper, and the outer part of the film is just a tin or lead corrosion resistant layer.This process is called "copper plate plating".In contrast to graphical electroplating, the biggest drawback of copper plating on a full plate is that the surface of the plate must be plated twice everywhere and must be corroded away when etching.So there's a whole set of problems when the wire width is very fine.At the same time, side corrosion can seriously affect the evenness of lines.

In the processing of the outer circuit of the printed board, another method is to use photosensitive film instead of metal coating for anti-corrosion layer.This method is very similar to the inner etching process, you can refer to the inner etching process.

At present, tin or lead tin is the most commonly used anti-corrosion layer, which is used in the process of ammonia etching agent. Ammonia etching agent is a commonly used chemical liquid, and no chemical reaction occurs with tin or lead tin.Ammonia etching agent mainly refers to ammonia water/ammonia chloride etching solution.In addition, ammonia/ammonium sulfate etching solution is available on the market.

Sulphate-based etching solution, in which the copper is separated by electrolysis, can be reused.Due to its low corrosion rate, it is rarely seen in actual production, but it is expected to be used in chlorine-free etching.Some experiments have been done with sulfuric acid - hydrogen peroxide as etching agent to corrode the outer figure.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 cannot be used for etching of lead-tin corrosion resistant layers, and this process is not the main method of making the outer layer of PCB, so most people rarely use it.

Ii. Quality of etching and problems existing in advance

The basic requirement for etching quality is to be able to completely remove all copper layers except those below the corrosion resistant layer.Strictly speaking, to be precisely defined, the etching quality must include consistency of wire width and degree of lateral erosion.Due to the inherent characteristics of the corrosion solution at present, the lateral erosion is almost inevitable.

Lateral etching is often discussed in the etching parameters, which is defined as the ratio of lateral etching width to etching depth, known as the etching factor.In the printed circuit industry, it varies widely, from 1:1 to 1:5.Obviously, small lateral erosion or low etching factor is the most satisfying.

The structure of the etching equipment and the etching solution of different components will affect the etching factor or lateral etching degree, or in optimistic words, it can be controlled.Some additives can reduce lateral erosion.The chemical composition of these additives is usually a trade secret, and their authors do not disclose it to the outside world.

In many ways, the quality of the etching was there long before the board got into the etcher.Because there is a very close internal connection between each process or process of printed circuit processing, there is no process which is not affected by other processes and does not affect other processes.Many of the problems identified as etching quality are actually present in the process of stripping or even prior art.In terms of etching technology of outer layer graphics, many problems are reflected in it because the inverted stream is more prominent than most printing plate processes.At the same time, this is also because the etching is self-stick film, the last step in a long series of processes that begin with photoreceptors, after which the outer pattern is transferred successfully.The more links, the greater the likelihood 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 printed circuit processing by pattern electroplating, the ideal state should be: the thickness of copper and tin or copper and lead tin after electroplating shall not exceed the thickness of electroplated photosensitive film, so that the electroplating figure is completely blocked by the "wall" on both sides of the film and embedded in it.However, in actual production, all the printed circuit boards in the world after electroplating, coating graphics are much thicker than sensitive graphics.In the process of electroplating copper and lead tin, the problem arises because the plating layer is higher than the photosensitive film.The tin or lead-tin corrosion resistant layer that covers the line runs sideways, forming a "edge" that covers a small portion of the photosensitive film below the edge.

The "edge" formed by tin or lead tin makes it impossible to remove the photosensitive film completely when removing the film, leaving a small amount of "glue" under the "edge".The "glue" or "film" left below the "edge" of the resist will cause incomplete etching.The "copper root" is formed on both sides of the line after etching. The copper root causes the line spacing to be narrowed, resulting in that the printed board does not meet party a's requirements and may even be rejected.As a result of rejection, PCB production costs will be greatly increased.

In addition, in many cases, dissolvable as a result of the reaction, residues of film and copper in the printed circuit industry may also accumulate in the corrosion solution and plug in the nozzle of the corrosion machine and the acid-resistant pump, which have to be shut down for treatment and cleaning, thus affecting the work efficiency.

Adjustment of equipment and interaction with corrosion solution

In printed circuit processing, ammonia etching is a rather fine and complex chemical reaction process.It is, in turn, an easy job.Once the process is adjusted upward, continuous production can be carried out.The key is that once started, it should be kept in continuous working state, not dry stop.The etching process depends to a great extent on the good working condition of the equipment.For now, no matter what etching solution is used, it is necessary to use high pressure spray, and in order to obtain the neat line side and high quality etching effect, the structure and spray mode of the nozzle must be strictly selected.

In order to get a good side effect, many different theories have appeared and different design methods and equipment structures have been formed.These theories are often quite different.But all theories of etching acknowledge the most basic principle of keeping metal surfaces in constant contact with fresh etching fluid as quickly as possible.The chemical mechanism analysis of the etching process also confirmed the above views.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, the purpose of using fresh solution and etching surface is mainly two.The second is to continuously supply 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 copper ions in ammonia etching solution, the faster the reaction speed.In fact, many ammoniacal etch products contain special coordination groups (complex solvents) of univalent copper ions, which are used to reduce univalent copper ions (these are the technical secret of their products' high reactivity). Thus, the effect of univalent copper ions is not small.The etching rate will be more than doubled when copper is cut from 5000ppm to 50ppm.

It is very difficult to keep the content close to zero due to the large amount of cupric ions generated during the etching reaction and the fact that cupric ions always bind closely to the complexyl group of ammonia.The conversion of univalent copper to bivalent copper by the action of oxygen in the atmosphere can remove univalent copper.The above purpose can be achieved by spraying.

This is one of the functional reasons to put air into the etching box.However, if there is too much air, it will accelerate the loss of ammonia in the solution and reduce the PH, which will still reduce the etching rate.Ammonia is also a variable that needs to be controlled in solution.Some users have adopted the practice of passing pure ammonia into the etching reservoir.To do this, a PH meter control system must be added.When the PH automatically measured is below the given value, the solution is added automatically.

In the related field of chemical etching (also known as photochemical etching or PCH), the research work has begun and reached the stage of structural design of etching machine.In this method, the solution used is copper divalent, not ammonia-copper etching.It will probably be used in the printed circuit industry.In the PCH industry, the typical thickness of etching copper foil is 5 to 10 mils, and in some cases it is quite thick.Its requirements for etching parameters are often more stringent than those of the PCB industry.

There is a study from the PCM industrial system that has not been officially published, but the results will be refreshing.The researchers have the ability to change the design thinking of the etching device in the long run, and to study the effects of these changes, thanks to the support of a stronger project fund.For example, compared with conical nozzle, the best nozzle design USES the fan, and spray collector chamber (i.e., the nozzle screw inside of the pipe) also has an installation Angle, to enter the etch tank artifacts in 30 degrees. Without such changes, the installation method of collecting chamber on the nozzle would lead to each adjacent nozzle spray Angle is not completely consistent.The spray surfaces of the second set of nozzles are slightly different from those of the first set (which represents the operation of the spray).This makes the ejected solution look like a superposition or a cross state.In theory, if the solution shapes cross each other, the ejection force on the part falls, unable to effectively flush out the old solution on the etched surface to keep the new one in contact.This condition is particularly pronounced at the edges of the spray surface.The injection force is much less than the vertical direction.

The latest design parameter, the study found, is 65 pounds per square inch (that is, 4+Bar).Each etching process and each practical solution has an optimal injection pressure problem, and for now, the injection pressure in the etching chamber above 30 pounds per square inch (2Bar) is minimal.There is a principle that the higher the density of an etching solution (i.e., the specific gravity or degree of bolivability), the higher the optimum injection pressure should be.This is not a single parameter, of course.Another important parameter is the relative mobility (or mobility) of the reaction rate in a solution.

Iv. Maintenance of etching equipment

The most critical factor for the maintenance of the etching equipment is to ensure that the nozzle is clean and free of obstructions to make the spray smooth.Clogging or slagging will hit the plate under jet pressure.If the nozzle is not clean, it will cause uneven etching and scrap the whole PCB.

Obviously, the maintenance of the equipment is to replace the damaged parts and wear parts, including replacing the nozzle, which also has wear problems.In addition, the more critical problem is to keep the etching machine free of slagging, which will happen in many cases. Excessive slagging may even affect the chemical balance of the etching solution.Similarly, if there is excessive chemical imbalance in the etching solution, the slagging will become more serious.The problem of slag accumulation cannot be overemphasized.When a large amount of slag suddenly appears in the etching solution, it is usually a signal that there is a problem with the balance of the solution.This should be cleaned properly with stronger hydrochloric acid or added to the solution.

Residues can also produce slag deposits, which dissolve in etching solution in very small amounts and form copper salt deposits.The slag formed by the residual film indicates that the former membrane removal process is incomplete.Poor film removal is often the result of both edge film and overplating.

About the problem of different etching state between the imported edge and the reentrant edge on upper and lower plate surface

A large number of problems involving the quality of the etching are concentrated in the etched parts of the upper surface.It is important to know this.These problems arise from the effects of the colloidal particles produced by the etching agent on the upper surface of the printed circuit board.The gelatinous slabs deposited on the copper surface affected the jet force on the one hand, and blocked the replenishment of fresh etching liquid on the other hand, resulting in the reduction of etching speed.It is precisely because of the formation and accumulation of colloidal slabs that the upper and lower patterns of the plate are etched to different degrees.This also makes the first part of the plate in the etcher more likely to be thoroughly etched or corroded, since the accumulation is not yet formed and the etching is faster.On the contrary, the part that enters after the board enters when the accumulation has formed and slows down its etching speed.