Etching process of PCB outer layer circuit

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In printed circuit processing, ammonia etching is a relatively fine and complex chemical reaction process,
but it is also an easy job. As long as the process to achieve a pass, can carry out continuous production,
but the key is the boot after must maintain continuous working state, not suitable for intermittent production.
The etching process is very dependent on the equipment status, so the equipment must be kept in good condition at all times.

At present, no matter what the etching liquid, must use the high-pressure spray, and in order to get a neat side line and high quality etching effect, structure and methods on the selection of spray nozzle must be more stringent.

The way of making excellent side effect, research theory, design methods and equipment structure outside have different, and these theories are often different people.

However, one of the most basic principles has been recognized and confirmed by chemical mechanism analysis,
that is, as soon as possible to keep the surface of the metal constantly exposure to fresh etching solution.

In ammonia etching, assuming that all parameters remain unchanged, then the rate of etching will be determined mainly by the ammonia (NH3) in the etchant. Therefore, the primary purpose of using fresh solutions to interact with the etched surface is to remove two of the newly produced copper ions and to supply the required ammonia (NH3) for the reaction.

In the traditional knowledge of the printed circuit industry, especially the suppliers of printed circuit materials, they all agree that the lower the monovalent copper ion content in the ammonia etching solution, the faster the reaction rate.

In fact, many of the products of ammonia etching solution containing copper ion specific ligands (some complex solvent),
its role is to reduce the copper ion (secret technology products with high response ability), visible effects of cuprous ion is not small.

The price of copper is reduced from 5000ppm to 50ppm, and the etching rate is increased by more than twice.

Since a large amount of copper ion is produced during the etching reaction, and the valence copper ion is always tightly combined with the complexing group of ammonia, it is very difficult to keep the content near zero.

The use of spraying can be achieved by the role of oxygen in the atmosphere, the conversion of one valence copper to two valence copper, and the removal of one valence copper, which is the need to air into the etching box of a functional reasons. However, if the air is too large, it will accelerate the loss of ammonia in the solution and decrease the pH value and reduce the etching rate.

Ammonia changes in solution amount is also need to be controlled, some users will use pure ammonia into the etching liquid storage tank, but it must be with a pH meter control system, when the automatic monitoring of PH lower than the default value, will automatically add the solution.

In the field of chemical etching (also known as photochemical etching or PCH), research has already begun and reached the stage of structural design of the etching machine. The solution used in this method is two valence copper, not ammonia copper etching. It will probably be used in the printed circuit industry. In PCH industry, the typical thickness of etched copper foil is 5 to 10 mil (mils).

In some cases, the thickness is rather large. Its requirements for etching parameters are often more demanding than the PCB industry. There is a research that has yet to be formally published by the PCM industrial system, and it is believed that the results will be refreshing.

Because of the strong project fund support, the researchers have the ability to change the design ideas of the etching device from a long-term perspective and to study the effects of these changes.

For example, compared with the conical nozzle, the better design effect of fan nozzle, and spray manifold (i.e. the pipe nozzle screw into the) also has a mounting angle of the workpiece into the etching tank with a 30 degree spray, without such changes, manifold will install the nozzle the result of each phase lead nozzle injection angle is not consistent.

The respective spray surfaces of the second sets of nozzles are slightly different from the first set (which indicates the operation of the spray), so that the ejected solution forms a superposition or cross state.

In theory, if the shape of the solution cross, the jet force of the part will be lowered, and the old solution on the etching surface will not be effectively removed so that the new solution will come into contact with it.

This is especially true at the edge of the spray surface, where the jet force is much smaller than the vertical jet. The study found that the latest design parameter was 65 pounds per square inch (4+Bar).

Each etching process and each practical solution has an optimum jet pressure. For the time being, the injection pressure in the etching chamber is negligible at 30 psi / 2Bar.

But there is a principle, an etching solution density (i.e. the proportion or higher Baume), higher injection pressure should be higher. Of course, this is not a single parameter. Another important parameter is the relative mobility (or mobility) that controls the rate of reaction in the solution.

The problem of different etching States

Many of the quality problems involved in etching facets are concentrated in the etched portion of the upper plate, which arises from the influence of the colloidal cement produced by the etchant. Understanding this is important because colloidal bonding accumulates on the surface of copper.

On the one hand, the jet force is affected; on the other hand, the fresh etching solution is prevented from being added so as to reduce the speed of etching. Because of the formation and accumulation of colloidal hardening material, the etching degree substrate following graph is different,
first enter the substrate due to the accumulation has not yet formed, etching speed, it is easy to be completely etched or caused by corrosion,
and then into the substrate has been formed due to the accumulation, and slow down the etching speed.

Maintenance of etching equipment

The key factor in maintaining the etching equipment is to ensure the high cleanliness and no obstruction of the nozzle, so that the nozzle can be ejected smoothly. The obstruction or slagging will cause pressure on the jet and impact the surface of the plate. The nozzle is not clean, it will cause uneven etching and the integral circuit board scrap.

Obviously, the maintenance of the equipment is to replace the damaged parts and worn parts. Because of the wear problem of the nozzle, the nozzle should be included in the replacement. In addition, a more critical problem is to keep the etching machine free of slag,

because in many cases excessive accumulation of slag will affect the chemical balance of the etching solution. Similarly, if there is chemical imbalance in the etching solution, the situation of slagging will be more serious. When the etching liquid suddenly appears a lot of slagging, it is usually a signal that the balance of the solution has a problem. At this time, the stronger hydrochloric acid should be used for proper cleaning or the solution is added.

In addition, the residue film will also produce slag. A small amount of residual film is dissolved in the etching solution to form a copper salt deposit. This means that the front of the membrane to do the process is not complete, bad membrane is often the result of the edge of the membrane and electroplating together.

Problems needing attention in etching process

Reduce side corrosion and protrusion and increase etching coefficient

Side erosion will produce steep edges. Generally, the longer the printed board is in the etching solution, the more serious the side etching is.
Side etching will seriously affect the accuracy of printed wiring, and serious side etching will not be possible to fabricate fine wires. When the edges and edges are lowered, the etching coefficient increases, and the high etch coefficient indicates the ability to hold the fine wires so that the etched leads are close to the original size.

Whether tin lead alloy, tin, tin nickel alloy or nickel plating etchant, along the excessive lead will lead to lead short circuit. Because the edge is easy to tear down, electrical separation is formed between two points of the wire.


Etching process of PCB

There are many factors affecting the lateral corrosion, and what are some of the following?:

Etching method:

Soaking and bubbling etching will cause lateral erosion of large, small splash and spray type side etching etching, especially in the best spray etching.

Types of etching solutions:

Different etching liquid has different chemical composition, the etching rate is different, and the etching coefficient is different.

For example: the etching coefficient of acid cupric chloride etching solution is usually 3, while alkaline cupric chloride etching coefficient can reach 4.

Etching rate:

Slow etching will cause severe side corrosion. The quality of the etching is greatly related to the rate of etching. The faster the etching rate, the shorter the time the substrate stays in the etching process, the smaller the amount of etching will be, and the etched patterns will be clearer and more orderly.

PH of etching solution:

When the pH of the alkaline etching solution is higher, the side etching will increase. In order to reduce lateral corrosion, pH should generally be controlled below 8.5.

Density of etching solution:

The low density of alkaline etching solution will increase the side corrosion, and the choice of etching solution with high copper concentration is very beneficial to the reduction of side corrosion.

Copper foil thickness:

To achieve minimal etching of fine wire etching, it is best to use (ultra) thin copper foil. Moreover, the thinner the wire width is, the thinner the copper foil thickness is. Because the thinner the copper foil, the shorter it will be in the etching solution and the smaller the amount of corrosion.

Increasing the consistency of etching rate between substrate and substrate

In continuous plate etching, the higher the consistency of etching rate, the more uniform etching can be achieved. To achieve this requirement, the etching solution must always be maintained at the optimum etching state throughout the etching process. It is necessary to choose easy regeneration and compensation, and the etching rate and easy to control the etching solution, and selection can provide constant operating conditions and can process and equipment automatic control of solution parameters, by controlling the amount of dissolved copper, pH, concentration, temperature and solution flow uniformity of the spray system (or nozzle. Even the nozzle swing) consistency to achieve etching rate.

Increasing uniformity of etching rate on substrate surface

The uniformity of etching on the upper and lower sides of the substrate as well as the surface of the plate depends on the uniformity of the flow of the etchant.

In the process of etching, the etching rate of the upper and lower surfaces is often inconsistent. Generally speaking, the etching rate of the lower panel will be higher than the upper panel. Because of the accumulation of solution on the upper plate, the etching reaction is reduced, but the uneven etching of the upper and lower plates can be solved by adjusting the spray pressure of the upper and lower nozzles.

A common problem with etching is that it is difficult to etch all the boards at the same time.
Because the edges of the substrate are etched faster than the center, it is difficult to make all the etching clean at the same time.

The spray system is used to make the nozzle sway and spray, which is an effective solution. To further improve, the etching uniformity of the entire panel can be achieved by varying the spray pressure at the center and edge of the plate and by intermittent etching of the plate front and the rear end of the plate.

Improve safety handling and corrosion resistance of copper foil and laminates

In the etching plate (such as: when the inner plate of the multilayer board), substrate easy wound on roller and wheel transmission caused by the waste, so the equipment must be in etching plate can guarantee the smooth and reliable processing of thin laminates. At present, many equipment manufacturers in an etching machine or gear wheel to prevent additional winding, but it is a better method of additional swing tetrafluoroethylene coated line as the support sheet at the time of transmission.

For etching of thin copper foils (such as 1/2 or 1/4 ounces), copper surfaces must be secured from scratch or scratch. Sometimes severe vibrations can damage copper foil.

 

The pollution of copper to water is a common problem in the production of printed circuit, and the use of ammonia base etching solution is more important. Because of complex copper and ammonia, not easily by ion exchange method and alkali precipitation removal. Therefore, the use of copper free liquid to rinse the board (second spray operation method), can greatly reduce the amount of copper emissions. The air knife is then used to remove excess water from the plate before rinsing the water, thereby reducing the rinsing burden of water on the etching of copper.

In an automatic etching system, the concentration of copper is controlled by repetition. In the etching process of printed circuit board, with copper has been dissolved, when the dissolved ratio rising to more than a certain value, the system will automatically fill the water solution with ammonium chloride and ammonia, the proportion of adjusting to the appropriate scope. The general weight should be controlled at 18~240Be.

Effect of pH of solution

The pH value of the etching solution should be between 8.0~8.8. If the pH falls below 8, it will be detrimental to the metal resist.

On the other hand, copper etching solution cannot be completely complex synthesis of copper ions, the solution to form mud settling at the bottom of the groove, and the precipitates in the heater had encrusted, may damage the heater, will make the pump and the nozzle is blocked, causing difficulties for etching. If the pH of the solution is too high, the ammonia in the etchant is supersaturated, and free ammonia is released into the atmosphere, causing environmental pollution. Moreover, the pH of the solution increases as well as the extent of side etching, which in turn affects the accuracy of etching.

The influence of ammonium chloride content

The chemical reaction by etching regeneration can be seen: = Cu (NH3) 2, 1+ regeneration needs there is an excess of NH3 and NH4CL. If the solution of the lack of NH4CL, and makes a lot of, Cu (NH3) 2, 1+ not regeneration, the etching rate will be reduced, as well as the loss of etching ability. Therefore, the content of ammonium chloride has a great influence on the etching rate. As the etching proceeds, the ammonium chloride shall be continuously replenished. However, excessive CL content in the solution causes erosion of the resist layer. Generally, the NH4CL content in etching solution should be about 150g/L.

Influence of temperature

The etching rate and temperature have a great relationship, the etching rate can be accelerated with the increase of temperature, etching temperature lower than 40 DEG C, the etching rate and the etching rate will be very slow, too slow will increase the amount of side etching, the etching quality.

When the temperature is higher than 60 degrees, the etching rate will increase obviously, but the volatilization of NH3 will increase greatly, which will lead to environmental pollution and make the chemical composition of etching solution out of proportion. Therefore, generally should be controlled at 45 degrees ~55 degrees is appropriate.

Etching fluid adjustment

Automatic control adjustment

With the etching, the content of copper in the etching liquid increases continuously, and the specific gravity also increases gradually. When the copper concentration in the etching solution reaches a certain height, it should be adjusted in time. In automatic control equipment, the specific gravity controller is used to control the specific gravity of etching solution. When the specific gravity is increased, the solution is automatically discharged, and a new additive solution is added to adjust the proportion of the etching liquid to the allowable range. The filling solution should be prepared in advance and added into the barrel, so that the liquid level of the tank can be kept at a certain height.

Common problems in etching process

Etch rate reduction

The reduction of etching rate is related to many factors, so it is necessary to check the etching conditions (such as temperature, spray pressure, specific gravity of solution, pH value and the content of ammonium chloride, etc.) so as to reach the proper range.

Precipitation occurs in etching solution

It is caused by too low ammonia content (pH reduction) or water dilution solution (e.g. cooling system, water leakage, etc.). Too high a proportion of solution will cause precipitation.

Corrosion resistant coating is etched

It is caused by too low pH value of the etching solution or excessive CL content.

The surface of copper is black and etched.

Caused by an excessive amount of NH4CL in the etchant.

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