Analysis of Improving the Quality of PCB Circuit Board Etching Process

Introduction

The purpose of etching: After the circuit is electroplated, the board removed from the electroplating equipment will be post-processed to complete the circuit board. Specifically, there are the following steps:

1. Peel off the Film: Peel off the dry film for anti-plating purposes with the potion. The hardened dry film is partially dissolved under the dope, and partially peeled off into flakes. In order to maintain the effect of the liquid and the completeness of the post-washing, the efficiency of the filtration system is very important.

2. Line Etching: Dissolve the copper in the non-conductor part.

3. Stripping tin and lead: Finally, remove the etch-resistant tin-lead coating. Regardless of pure tin or the tin-lead layer of each composition ratio, the purpose of plating is only for anti-etching, so after the etching is completed, it must be stripped off, so this step of stripping tin-lead is only for processing, and no added value is generated. But the following points still need special attention, otherwise the cost increase is the second, and the outer layer circuit that is finally completed will cause defects here.

At present, the typical process of printed circuit board (PCB) processing adopts “pattern plating method”. That is, pre-plating a lead-tin resist layer on the copper foil part that needs to be retained on the outer layer of the board, that is, the graphic part of the circuit, and then chemically corrode the rest of the copper foil.

It should be noted that there are two layers of copper on the board at this time. In the outer layer etching process, only one layer of copper must be completely etched away, and the rest will form the final required circuit. This type of pattern plating is characterized by the presence of a copper layer only under the lead-tin resist. Another process method is to coat the entire board with copper, and the part other than the photosensitive film is only a tin or lead-tin resist layer. This process is called “full board copper plating process”. Compared with pattern plating, the biggest disadvantage of full-board copper plating is that copper is plated twice everywhere on the board and must be etched away during etching. Therefore, a series of problems will arise when the wire width is very fine. At the same time, side corrosion can seriously affect the uniformity of the lines.

In the processing technology of the outer circuit of the printed board, there is another method, which is to use the photosensitive film instead of the metal coating as the resist layer. This method is very similar to the inner layer etching process, you can refer to the etching in the inner layer fabrication process.

At present, tin or lead tin is the most commonly used resist layer, which is used in the etching process of ammonia etchant. Ammonia etchant is a commonly used chemical liquid, which does not have any chemical reaction with tin or lead tin. Ammonia etchant mainly refers to ammonia water/ammonium chloride etching solution. In addition, ammonia water/ammonia sulfate etching solution is also available in the market.

Sulfate-based etching solution, after use, the copper in it can be separated by electrolysis, so it can be reused. Due to its low corrosion rate, it is generally rare in actual production, but it is expected to be used in chlorine-free etching. Some people tried to use sulfuric acid-hydrogen peroxide as an etchant to corrode the outer layer pattern. For many reasons including economics and waste disposal aspects, this process has not yet been adopted in a commercial sense. Furthermore, sulfuric acid-hydrogen peroxide cannot be used for the etching of lead-tin resist, and this process is not the main method in the production of the outer layer of PCB, so most people rarely care about it.

Etching Quality and Early Problems

The basic requirement for etching quality is to be able to completely remove all copper layers except under the resist layer, and that’s it. Strictly speaking, if it is to be accurately defined, the etching quality must include the consistency of the wire width and the degree of undercut. Due to the inherent characteristics of the current etchant, it not only etches downward but also in all directions, so side etching is almost inevitable.

The problem of side etching is one of the etching parameters that is often discussed. It is defined as the ratio of the width of the side etching to the depth of the etching, which is called the etching factor. In the printed circuit industry, it varies widely, from 1:1 to 1:5. Obviously, a small undercut degree or a low etch factor is the most satisfactory.

The structure of the etching equipment and the etching solution of different compositions will have an impact on the etching factor or side etching degree, or in optimistic terms, it can be controlled. The use of certain additives can reduce the degree of side etching. The chemical compositions of these additives are generally trade secrets, and their developers do not disclose them to the outside world.

In many ways, the quality of etching exists long before the printed board enters the etching machine. Because there is a very close internal connection 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 etch quality actually existed in the stripping process even earlier. For the etching process of the outer layer pattern, because the “reverse stream” phenomenon it reflects is more prominent than most printed board processes, many problems are finally reflected on it. At the same time, this is also because the etching is the last link in a long series of processes starting with self-adhesive film and photosensitive. After that, the outer layer pattern is successfully transferred. The more links there are, the greater the chance of problems. This can be seen as a very special aspect of the printed circuit production process.

Theoretically, after the printed circuit enters the etching stage, in the process of processing the printed circuit by the pattern plating method, the ideal state should be: the sum of the thicknesses of copper and tin or copper and lead-tin after electroplating should not exceed the thickness of the photosensitive film the electroplating resistance, so that the electroplating pattern is completely blocked by the “walls” on both sides of the film and embedded in it. However, in actual production, after electroplating of printed circuit boards all over the world, the coating pattern is much thicker than the photosensitive pattern. In the process of electroplating copper and lead-tin, since the height of the plating layer exceeds the photosensitive film, there is a tendency to accumulate laterally, and the problem arises. The tin or lead-tin resist covered above the lines extends to both sides to form “edges”, and a small part of the photosensitive film is covered under the “edges”.

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”. “Residual glue” or “residual film” is left under the resist “edge” and will cause incomplete etching. The lines form “copper roots” on both sides after etching, and the copper roots narrow the line spacing, resulting in the printed board not meeting the requirements of Party A, and may even be rejected. The production cost of the PCB is greatly increased due to rejection.

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 deposits in the etching solution and block the nozzle of the etching machine and the acid-resistant pump, and have to be shut down for processing and cleaning. , which affects work efficiency.