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PCB etching process and process control.

Jan 08, 2018

The process of printed circuit board from light board to show line graph is a complex process of physical and chemical reaction, and this paper will analyze the last step of it - etching.

At present, the typical process of PCB processing is "graphic electroplating".

That is, the copper foil that needs to be preserved in the outer layer of the board is precoated with a layer of lead tin corrosion resistant layer in the graphic part of the circuit, and then the remaining copper foil is corroded by chemical means, which is known as etching.


I. type of etching.

It is important to note that there are two layers of copper on the board.

There is only one layer of copper that must be etched in the outer etching process, and the rest will form the circuit that will eventually be needed.

This type of graphic electroplating is characterized by copper plating only under the corrosion layer of lead tin.


Another technique is that the entire board is coated with copper, and the other part of the film is only tin or lead tin resist.

This process is called "all-plate copper plating process".

The biggest drawback to the plating of the whole plate is that it has to be plated twice in every place and must be corroded when it is etched.


Therefore, a series of problems will arise when the wire width is very fine.

At the same time, lateral corrosion can seriously affect the uniformity of the line.


In the process of printing plate outer circuit, there is another method, which is to use the photosensitive film instead of metal coating to resist the corrosion layer.

This method is very similar to the inner etching process, and can be seen in the internal production process.


At present, tin tin or lead is the most commonly used, corrosion resistant layer used in ammonia etching process of etching agent. Ammonia etching agent is used widely in chemical liquid, lead and tin or tin without any chemical reaction.

Ammonia etching agent mainly refers to ammonia water/ammonium chloride etching solution.


In addition, it can be bought in the market ammonia/ammonium sulphate etching solution.

With sulfate - based etching solution, the copper can be separated by electrolysis and can be reused.

Due to its low corrosion rate, it is seldom seen in actual production, but it is expected to be used in non-chlorine etching.


Some experiments have been done to etch the outer figure with sulphuric acid - hydrogen peroxide.

Due to many reasons such as including economic and waste liquid treatment, this process has not been extensively used in commercial sense. Furthermore, sulfuric acid, hydrogen peroxide, cannot be used to lead solder resist layer etching, and this process is not PCB outer production, the main method, so the vast majority of people very few takers.


Ii. Problems of etching quality and early stage.

The basic requirement for etching quality is to remove all the copper layers except the corrosion resistant layer.

Strictly speaking, the etch quality must include the consistency of wire linewidth and the degree of lateral erosion if it is to be precisely defined.

Due to the inherent characteristics of the current corrosion fluid, it is not only the downward but also has the etching effect on the left and right directions, so the lateral erosion is almost inevitable.


Lateral erosion is a term that is often discussed in etching parameters, which is defined as the ratio of lateral etch width to etch depth, known as etching factor.

In the printing circuit industry, its range is wide ranging from 1:1 to 1:5.

Obviously, small lateral erosion or low etching factor is the most satisfactory.


The structure of the etching equipment and the etching of different components will have an effect on etching factors or lateral erosion, or, in optimistic terms, to control them.

Some additives can reduce the lateral erosion.

The chemical components of these additives are generally commercial secrets and are not disclosed to the outside world.


In many ways, the quality of the etching has existed long before the plate has 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 without affecting other processes.

Many of the problems identified as etching quality have actually existed in the process of removing membranes and even more.


For the etching process of the outer layer graphics, because the inverted stream is more prominent than most printing plates, many problems are finally reflected on it.

At the same time, this is also because etching is the last ring in a long series of processes that begin with a self-adhesive film, and then the outer layer is transferred successfully.

The more links, the greater the likelihood of problems.

This can be seen as a very special aspect of printed circuit production.


Theoretically, entered the stage of etching printed circuit, the graphic method in the machining process of printed circuit boards, electroplating ideal should be: after the plating of copper and tin or copper and lead the thickness of the tin sum resistance should not be more than the thickness of the plating coating, electroplating graphics completely is blocked by the "wall" on both sides of the membrane and embedded in it.

However, in real production, the printed circuit boards of the world are much thicker than photosensitive graphs after electroplating.

In the process of electroplating copper and lead tin, the problem arises because the coating height exceeds the photosensitive film, resulting in the trend of transverse accumulation.

The tin or tin corrosion resistant layer that is covered by the line extends to both sides, forming a "edge" and covering a small portion of the film under the "edge".


The "edge" of tin or tin makes it impossible to remove the photosensitive film completely and leave a small amount of "glue" under the "edge".

"Residual glue" or "residual film" remains under the "edge" of the corrosion inhibitor, resulting in incomplete etching.

The lines formed "copper root" on both sides after etching, and the copper root narrowed the line spacing, causing the printing plate not to meet the requirements of party a and may even be rejected.

Due to the rejection, the production cost of PCB will be greatly increased.


In addition, in many cases, because of the reaction and form dissolved, in the printed circuit industry, the residual film and copper may also be formed in corrosive fluid accumulation and plugging in the nozzle and acid proof pump corrosion machine, had to stop treatment and cleaning, and affect the working efficiency.


3. Equipment adjustment and interaction with corrosion solution.

Ammonia etching is a more elaborate and complex chemical reaction process in printed circuit processing.

In turn, it is an easy job.

Once the process is raised, it can be continuously produced.

The key is to maintain a continuous working condition once the machine is turned on.

The etching process relies heavily on the good working state of the equipment.

For now, no matter use what kind of etching liquid, must use high pressure spray, and in order to obtain a neat line side and the high quality of etching effect, must be strictly choose the way to the structure of the nozzle and spray.


In order to get a good side effect, many different theories have emerged, forming different design patterns and equipment structure.

These theories are often very different.

But all the theories about etching acknowledge the basic principle of keeping the metal surface constantly exposed to fresh etching.

The chemical mechanism analysis of the etching process also confirmed the above viewpoint.

In ammonia etching, assuming all other parameters are unchanged, the etching rate is determined mainly by ammonia (NH3) in the etching solution.

Therefore, the purpose of using fresh solution and etching surface is mainly two: one is to wash off the newly produced copper ion;

The second is the constant supply of ammonia (NH3) needed to react.


In the traditional knowledge of printed circuit industry, especially printed circuit materials suppliers, generally accepted, etching liquid ammonia sex of the lower price of copper ion content, the faster the reaction speed. This has been confirmed by experience.

In fact, many of ammonia etching liquid products contain special dentate valence copper ions (complex solvent), its function is to reduce the price of copper ions (which is their products have high ability to respond to technical tips, influence of monovalent copper ion is not small.

Reducing the price of copper from 5000ppm to 50ppm will more than double the etching rate.


Because of a price in etching reaction generated in the process of a large number of copper ions, and due to the price of copper ion with ammonia complexation always tightly together, so it is very hard to keep its content is close to zero.

The use of oxygen in the atmosphere to convert a copper into a divalent copper can remove a copper price.

This can be accomplished by sprinkling.


This is the functional reason to pass air into the etch box.

But if there is too much air, it speeds up the ammonia loss in the solution and lowers the PH, which still reduces the etching rate.

Ammonia also needs to be controlled in solution.

Some users adopt the method of etching pure ammonia into the etching reservoir.

This must be done with a PH control system.

The solution is automatically added when the automatically measured PH is lower than the given value.


In the field of chemical etching (also known as photochemical etching or PCH), the work has begun and has reached the stage of structural design of the etching machine.

In this method, the solution used is divalent copper, not ammonia - copper etching.

It will probably be used in the printed circuit industry.

In PCH industry, the typical thickness of etching 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.


4. On the upper and lower boards, the import side and the back edge etching state are different problems.

A large number of problems involving etching quality are focused on the part of the upper plate that is etched.

It is important to understand this.

These problems are derived from the effect of the adhesive on the board of the printed circuit board.

The colloidal substrate is deposited on the copper surface, which affects the spray force on the one hand and prevents the addition of fresh etching solution on the other hand, resulting in the reduction of etching speed.

It is precisely because of the formation and accumulation of colloidal boards that the etching of the top and bottom of the board is different.

This also makes it easy to etch the parts of the board in the etching machine, which are easy to be etched and corroded, since the accumulation is not yet formed and the etching rate is faster.

On the contrary, the part of the board that enters after entering into the accumulation has formed, and slowed down the etching rate.


5. 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 obstruction.

Blocking or slagging will impact the layout under the action of jet pressure.

If the nozzle is not clean, then can cause etch inhomogeneity and make whole block PCB scrap.


Obviously, the maintenance of the equipment is to replace the damaged parts and wear parts, including the replacement of the nozzle, and the nozzle also has the problem of wear.

In addition, more key problem is to keep the etching machine does not exist and slagging, in many cases will appear slagging accumulation. Slagging accumulation is overmuch, may even affect the etching liquid chemical equilibrium.

In the same way, if the etching solution produces excessive chemical imbalance, the slag will become more serious.

The problem of slag accumulation cannot be overemphasized.

Once the etching solution suddenly appears a large amount of slagging, it is usually a signal that the equilibrium of the solution is in question.

This should be properly cleaned or supplemented with a strong hydrochloric acid.


The residue can also produce slag, and a very small amount of film is dissolved in the etching solution, and then copper salt precipitates.

The scum formed by the residual film indicates that the membrane process is not complete.

Dysplasia is often the result of the combination of marginal film and electroplating.

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