PCB etching process and process control

Edit:China hardware manufacturing      View times      Time:2021-6-21    Print

Printed circuit board from the plate to show the line graphic process is a complex physical and chemical reaction process, which is the final step - etching parsing. At present, the printed circuit board (PCB) processing of typical process using "graphic electroplating method". Namely first on board outer copper foil part of the need to retain, also is the graphical portion of the circuit on the pre plating a layer of lead solder resist layer, then the rest of the copper foil with a chemically corrosion, referred to as the etching.

A. The kinds of etching

Note that the board with two layers of copper etching. There is only a layer of copper in the outer layer etching process is must be etched away, all the rest will be formed finally the circuit you need. This type of pattern plating, its characteristic is the copper plating layer is confined to lead solder resist layer below. Another technique is the entire board copper plating, coating outside of the part is only a tin or lead solder resist layer. This process is called "whole board copper plating process". Compared with the graphic electroplating, the biggest drawback is plate copper plating board face everywhere to two copper plating and etching must also drop their corrosion. So when very fine wire line width will produce a series of problems. At the same time, the serious influence line of lateral corrosion uniformity.

In the outer circuit PCB processing technology, there is another method, is made from photographic film to replace the metal coating corrosion layer. This method is very similar to the inner layer etching process, can refer to the inner process of etching. 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. Ammoniacal etching agent is mainly refers to the ammonia/ammonium chloride etching liquid. In addition, also can buy in the market the etching liquid ammonia/ammonium sulphate. Sulfate as the base of etching solution, after use, the copper can be separated with electrolytic method, thus can be reused. Because of its low corrosion rate, generally in the actual production, but is expected to be in no chloride etching. Someone test using sulfuric acid etching agent - hydrogen peroxide do to corrosion outer graphics. 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.

B. Etching quality and the problems in advance

The basic requirements of quality of etching is able to besides corrosion resistant layer below all of the copper layer completely remove clean, check this. In a strict sense, if you want to accurately define, then etching quality must include the consistency of the wire line width and the degree of lateral erosion. Due to the inherent characteristics of corrosive liquid, not only down but have etched on direction around each role, so the lateral erosion is almost inevitable.

Lateral erosion is etching parameters is often mentioned in the discussion of a, it is defined as the ratio of the width of lateral erosion and etching depth, referred to as the etching factor. In the printed circuit industry, its range is very broad, from 1:1 to 1:5. Obviously, the small lateral erosion degree or low etching factor is the most satisfying.

Etching equipment structure and different composition of etching fluid will have an effect on the etching factor or lateral erosion, or in positive terms, can be controlled. Using some additives can reduce the lateral erosion degree. The chemical composition of these additives generally belongs to the commercial secret, each developer is not revealed to the outside world. In many ways, etching, by the quality of existed long before the PCB into the etching machine. Because of printed circuit processing all processes or process between the internal is very closely linked, there is no one is not affected by other processes do not affect other craft process again. Many as etching quality problem, in fact, in the process before you go to film is even more. External layer etching process, due to its "stream" is like is outstanding than most of the PCB process, so many problems reflected in it in the end. At the same time, this is because the etching is the skins, photosensitive began a long series of the last part of the process, after the outer graphics that transfer was successful. The more links, the greater the likelihood of a problem. This can be seen as printed circuit a very special aspects in the process of 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. In real production, however, the world of printed circuit board after electroplating, plating graphics are much thicker than the photosensitive graphics. In the process of electroplating copper and lead, tin, due to the coating over the photosensitive film, lateral accumulation of trend, the resulting problems. Above the line covered with tin or lead solder resist layer to stretch, formed the "edge", cover a small number of photosensitive membrane under the "down".

Tin tin or lead form "along" makes when to membrane cannot remove coating thoroughly clean, leaving a small part of the "glue" under "along". The "glue" or "residual film" in the resist "along" below, will cause the etching of incomplete. Copper lines after etching on both sides of the form "root", narrow the distance between copper root, cause the PCB do not meet the requirements of party a, and even may be rejected. Due to rejection will be greatly increased the production cost of PCB.

C. Equipment adjustment and the interaction relationship of the corrosion solution

In printed circuit processing, ammoniacal etching is one of the more subtle and complex chemical reaction process. On the other hand it is a easy work. Once a process raised, can continuous production. The key is needs to keep continuous working condition, once turned on unfavorable dry and stopping. Etching process in great extent, dependent on equipment in good working condition. 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.

To obtain good side effect, appeared a lot of different theories, forming different design methods and equipment structure. These theories are often very different. But all the etching theory has admitted that a basic principle, namely fast as far as possible to the metal surface contact fresh etching liquid. The chemical mechanism of the etching process analysis also confirmed this point. In ammonia etching, the assumption is that all the other parameters constant, then the etching rate is mainly composed of etching liquid ammonia (NH3) to decide. Therefore with fresh solution and the etched surface effect, the main purpose has two: one is doing just produce copper ions; 2 it is needed to provide ongoing for reaction of ammonia (NH3).

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. A valence copper by 5000 PPM down to 50 PPM, etching rate will increase by more than double. 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. Through the role of oxygen in the atmosphere will be a price of copper into bivalent copper can remove monovalent copper. With spray can achieve the above purpose.

This is to get the air going into the etching of a functional reasons. But if the air is too much, and will accelerate solution of ammonia loss and decreased PH value, the result is still lower the etching rate. Ammonia in solution is also a need to control the amount of change. In some users access to pure ammonia etching, a reservoir. Do have to add a PH meter control system. When the results of the PH value of the measured automatically is lower than the given value, solution will be automatically added. In the related chemical etching (also called photochemical etching or PCH), in the field of research work has begun, and achieve the etching machine structure design of the stage. The solution in this method, use of bivalent copper, not ammonia - copper etching. It is likely to be used in printed circuit industry. In the PCH industry, typical thickness of copper foil etching for 5 to 10 mil (mils), in some cases the thickness is quite large. Its claim to the etching parameters often more demanding than in PCB industry.

D. About the top and bottom panel, and later into the import side etching condition of different problems

A lot of problems involving etching quality be etched parts was concentrated in the upper surface. Understand that this is very important. These problems from the PCB panel on colloidal harden material produced by etching agent. Colloidal harden up on copper surface, on the one hand affect the injection force, blocks the fresh etching liquid supplement, on the other hand caused the etching rate reduce. It is due to the formation and accumulation of the colloidal harden material board etching degree on the graphic below. It also makes the board in the etching machine easily etched into the part completely or likely to cause corrosion, because at that time accumulation has not yet formed, etching speed faster. On the other hand, the board after the part into the accumulation is already formed, etching and slow down its speed.

E. Etching equipment maintenance

Etching equipment maintenance is one of the most key factors to ensure the nozzle cleaning, no obstruction to jet flow. Obstruction or slagging under the effect of injection pressure impact. If the nozzle is not clean, so will cause uneven etching and makes the whole piece of PCB scrap.

Obviously, equipment maintenance is the replacement of damaged parts and wear parts, including the replacement of nozzle, the nozzle also has the problem of wear and tear. 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. Also, if the etching liquid chemical imbalance, excessive slagging will more and more serious. Slagging accumulation can't emphasize enough. Once the etching liquid suddenly appeared a lot of slagging, is usually a signal, the solution of the problems with balance. It should be with strong hydrochloric acid cleaning or solution for adding properly.

Residual material also can produce slagging, very little residual film dissolved in etching liquid, and then forming copper salt precipitation. The residual film formed by slagging way to membrane process not completely before that. Stripping bad is always caused by the edge membrane with a plating results.