The process of printing a circuit board from a light plate to a display line pattern is a process of complex physical and chemical reactions. This paper analyzes its last step – etching. At present, the typical process of printed circuit board (PCB) processing is “graphic plating”. That is, the outer part of the outer layer to be retained in the copper part, that is, part of the circuit on the part of the pre-plating layer of tin corrosion layer, and then chemically the remaining copper foil corrosion, known as etching.
The type of etching
It should be noted that there are two layers of copper on the board when etched. In the outer layer of the etching process, only one layer of copper must be etched away, and the rest will form the final desired circuit. This type of graphic plating, which is characterized by the presence of copper plating layer only below the lead tin resist layer.
Another method is the entire board are copper, photosensitive film outside the part is only tin or lead tin resist layer. This process is called “full board copper plating”. Compared with the graphics plating, the biggest drawback of the whole board copper plating is plated twice the copper surface and the etching must also be corroded them. So when the wire width is very fine will produce a series of problems. At the same time, side corrosion will seriously affect the uniformity of the line.
In the printed circuit board outer circuit processing technology, there is another way, is to use the photosensitive film instead of metal coating to do corrosion layer. This method is very similar to the inner layer etching process, which can be seen in the inner layer of the etching process.
At present, tin or lead tin is the most commonly used corrosion layer, used in the etching process of ammonia etching agent. Amine etching agent is a commonly used chemical liquid, with tin or lead tin does not occur any chemical reaction. Ammonia etchant mainly refers to ammonia / ammonium chloride etching solution.
In addition, in the market can also buy ammonia / ammonium sulfate etching liquid. The sulfate-based etching solution, after use, can be separated by electrolysis and can be reused. Because of its low corrosion rate, generally in the actual production is rare, but is expected to be used in chlorine-free etching.
It was tested with sulfuric acid-hydrogen peroxide as an etchant to corrode the outer layer. This process has not been widely used in commercial sense due to many reasons, including economic and waste disposal, etc. Furthermore, sulfuric acid-hydrogen peroxide can not be used for the etching of lead-tin resist, which is not a PCB Outside the main method of production, so most people rarely ask.
Etching quality and pre-existing problems
The basic requirement for the quality of the etch is to be able to completely remove all copper layers except the resist layer. In the strict sense, if the precise definition is to be made, the etch quality must include the consistency of the wire width and the degree of lateral erosion. Due to the inherent characteristics of the current corrosion solution, not only down and around the direction of the etching effect, so the side erosion is almost inevitable.
The pitting problem is an item that is often discussed in the etch parameter, which is defined as the ratio of the lateral erosion width to the etch depth, called the etch factor. In the printed circuit industry, it varies widely from 1: 1 to 1: 5. Obviously, small side erosion or low etch factor is the most satisfactory.
The structure of the etching apparatus and the etching solution of the different components have an influence on the etching factor or the side erosion, or, in an optimistic manner, it can be controlled. The use of certain additives can reduce the side erosion. The chemical composition of these additives are generally trade secrets, their developers are not disclosed to the outside world.
In many ways, the quality of the etch is as long as it has existed before the printed circuit board enters the etch machine. Because there are very close internal links between the processes or processes of the printed circuit processing, there is no process that is not affected by other processes and does not affect other processes. Many of the problems that have been identified as the quality of the etch are actually present in the process of filming or even more.
For the outer layer of the etching process, because it embodies the “back stream” is like most of the printed circuit board processes are prominent, so many problems are reflected in the last above it. At the same time, this is also because the etching is self-adhesive film, the beginning of a long series of photosensitive process in the last ring, after the outer graphics that transfer success. The more the number of links, the greater the likelihood of problems. This can be seen as a very special aspect of the printed circuit production process.
In theory, the printed circuit into the etching stage, the graphics plating process in the processing of printed circuit, the ideal state should be: after plating copper and tin or copper and lead and tin the sum of the thickness should not exceed plating The thickness of the photosensitive film, so that the plating pattern on both sides of the film “wall” to block and embedded in the inside. However, the reality of production, the world’s printed circuit board after plating, the coating graphics should be much thicker than the