The Etching Process of The Outer Circuit of PCB (3)

3. Etching quality and previous problems

The basic requirement for etching quality is to be able to completely remove all copper layers except under the resist layer. Strictly speaking, if it is to be accurately defined, then the etching quality must include the consistency of the wire width and the degree of undercutting. Due to the inherent characteristics of the current etching solution, which not only produces an etching effect on the downward direction but also on the left and right directions, side etching is almost inevitable.

The problem of undercutting is one of the etching parameters that is often raised for discussion. It is defined as the ratio of the width of undercutting to the depth of etching, which is called the etching factor. In the printed circuit industry, it has a wide range of changes, from 1:1 to 1:5. Obviously, a small undercut degree or a low etching factor is the most satisfactory.

The structure of the etching equipment and the etching solutions of different compositions will affect the etching factor or the degree of side etching, which can be controlled. The use of certain additives can reduce the degree of side erosion. The chemical composition of these additives is generally a trade secret, and the respective developers do not disclose it to the outside world.

In many ways, the quality of etching has existed long before the printed board enters the etching machine. Because there are very close internal connections between the various processes or processes of printed circuit processing, there is no process that is not affected by other processes and does not affect other processes. Many of the problems identified as etching quality actually existed in the process of removing the film or even before. As for the etching process of outer layer graphics, because the “inverted stream” phenomenon it embodies is more prominent than most printed board processes, many problems are finally reflected in it. At the same time, this is also because the etching is the last step in a long series of processes starting with self-sticking and photosensitive, after which the outer layer pattern is successfully transferred. The more links, the greater the possibility of problems. This can be seen as a very special aspect of the printed circuit production process. In the process of processing printed circuits by pattern electroplating, the ideal state should be that the total thickness of the electroplated copper and tin or copper and lead tin should not exceed the thickness of the electroplating resistant photosensitive film, so that the electroplated pattern is completely covered on both sides of the film. The “wall” blocks and is embedded in it. However, in actual production, after electroplating printed circuit boards all over the world, the plating pattern is much thicker than the photosensitive pattern. In the process of electroplating copper and lead-tin, because the plating height exceeds the photosensitive film, a tendency of lateral accumulation occurs, and the problem arises from this. The tin or lead-tin resist layer covering the lines extends to both sides to form a “edge”, covering a small part of the photosensitive film under the “edge”.

The “edge” formed by tin or lead tin makes it impossible to completely remove the photosensitive film when removing the film, leaving a small part of “residual glue” under the “edge”. The “residual glue” or “residual film” left under the “edge” of the resist will cause incomplete etching. The lines formed “copper roots” on both sides after etching. The copper roots narrowed the line spacing, causing the printed board to not meet the requirements of Party A, and may even be rejected. Rejection will greatly increase the production cost of PCB. In addition, in many cases, due to the formation of dissolution due to the reaction, in the printed circuit industry, the residual film and copper may also form and accumulate in the corrosive liquid and be blocked in the nozzle of the corroding machine and the acid-resistant pump, and it has to be shut down for processing and cleaning, which affects work efficiency.

4. Regarding the upper and lower board surfaces, the etching state of the leading edge and the trailing edge are different

A large number of problems related to etching quality are concentrated on the etched part of the upper plate surface. It is very important to understand this. These problems come from the influence of the glue-like clumps produced by the etchant on the upper surface of the printed circuit board. The accumulation of colloidal slabs on the copper surface affects the jetting force on the one hand, and on the other hand prevents the replenishment of fresh etching solution, resulting in a reduction in the etching speed. It is precisely because of the formation and accumulation of colloidal slabs that the degree of etching of the upper and lower patterns of the board is different. This also makes the first part of the board in the etching machine easy to be etched completely or to cause over-corrosion, because the accumulation has not yet formed at that time, and the etching speed is faster. On the contrary, the part that enters behind the board has already formed when it enters, and slows down its etching speed.

Residual film can also produce slagging, a very small amount of residual film dissolves in the etching solution, and then forms copper salt precipitation. The slagging formed by the residual film indicates that the previous film removal process is not complete. Poor film removal is often the result of both edge film and over electroplating.

XPCB Limited is a manufacturer specializing in the production of high-precision double-sided, multi-layer and impedance, blind buried vias, and thick copper circuit boards. The products cover HDI, thick copper, backplanes, rigid-flex combined, buried capacitance and buried resistance, Golden Finger and other kinds of circuit boards, which can meet the needs of customers for all kinds of products.