PCB Tech - Application of Metallographic Slicing Technology in PCB

This article introduces the production process of metallographic slicing in detail, and discusses the application of metallographic slicing technology in PCB production through the use of a large number of pictures and examples, especially its application in solving quality problems in production.

Printed circuit boards are an indispensable part of electronic components and are widely used in the electronics industry. Their quality must be determined by certain testing techniques. The PCB manufacturing process is complicated, and if a quality problem occurs in one of the links, the printed circuit board will be scrapped. Then the inspection of printed circuit boards must be divided into in-process inspection and finished product inspection. Our commonly used inspection methods are visual inspection with magnifying glass and backlight inspection. As one of the inspection methods, metallographic slicing technology is adopted by printed circuit board manufacturers because of its small investment and wide application range. Metallographic slicing is a destructive test that can test multiple properties of printed circuit boards. For example: resin contamination, plating cracks, hole wall delamination, solder coating condition, interlayer thickness, plating thickness, plating thickness in the hole, side erosion, inner layer ring width, interlayer overlap, plating quality, rough hole wall Degree and so on. In short, as doctors use X-rays to treat patients, it can observe the defects and conditions of the fine structure of the surface and cross-section of the printed circuit board. I have a certain understanding of it in my work. It is briefly described as follows in several aspects:

1. Microsectioning production process

The metallographic slice production process is as follows:

Extract the production board to be inspected - take a sample - precision cut to fit the mold size - inlay - rough grinding - fine grinding - polishing - micro-etching - observation

1) Extract the production board that needs to be metallographic sliced on the production line.

2) Use a shear to cut the center and edge of the sample to be metallographically sectioned.

3) Use a precision cutting machine to cut the sample to fit the size of the mold, and pay attention to keeping the cutting surface parallel or perpendicular to the surface to be observed.

4) Take a special mold for metallographic sectioning, and place the sample upright in the mold with the part to be inspected facing upwards. Take a paper cup and mix the cold buried resin (solid) and curing agent (liquid) at a volume ratio of 2:1, stir evenly, and pour into the mold until the sample is completely submerged. Leave the mold for 10-20 minutes until the resin is completely cured .

2. The role of metallographic slicing technology in the production process of printed circuit boards

The quality of printed circuit boards, the occurrence and resolution of problems, and the improvement and evaluation of processes all require metallographic sections as the basis for objective inspection, research and judgment.

2.1 The role in the quality inspection and control of the production process

The PCB production process is complicated, and the various processes are interrelated. If the quality of the final product is to be reliable, the quality of the semi-finished board in each process of the intermediate link must be good. How to judge the quality of the production board in the process? Metallographic slicing technology will provide us with the basis.

2.1.1 Hole roughness inspection after drilling process

In order to ensure the quality of the hole metallization of the printed circuit board, the roughness of the hole wall after drilling must be tested. It can be used as a test board, drill holes with different sizes of drill bits, take a sample, make a metallographic section, and measure the roughness with a reading microscope. In order to make the measurement more accurate, the sample can be metallized and then sectioned.

2.1.2 Detection of resin contamination and etchback effect

The printed circuit board produces instantaneous high temperature when drilling, and the epoxy glass substrate is a poor thermal conductor. Heat is highly accumulated during drilling, and the surface temperature of the hole wall exceeds the glass transition temperature of the epoxy resin, resulting in a thin ring Oxygen resin contamination. After the multi-layer board is drilled, if the holes are metallized without etching, the signal line in the multi-layer board will not be connected, which will affect the quality of the board. By making metallographic sections, the removal effect of resin contamination after etchback can be detected, which is beneficial to control the quality of the multilayer board.

2.2 The role in solving quality problems in the production process

In the production process of printed circuit boards, various quality problems often occur. If the metallographic slicing technology can quickly find the cause of the problem, prescribe the right medicine in time, take effective measures to save production costs, ensure on-time delivery, and win customer satisfaction. First, I will introduce the solution of some problems:

2.2.1 Plating void problem (plating void)

1. Poor copper sinking. a. The characteristics of the slices are: symmetrical ring-shaped holes appear in the holes, and the patterned copper plating can be seen in the slices covering the whole plate copper plating and chemical copper. b. Reason analysis: no copper in the symmetrical hole: it is essentially ring-shaped without copper, because there are bubbles in the hole during the copper sinking process, so that the chemical solution and the hole wall cannot contact, so that the copper sinking reaction cannot occur. .

2. Dry film into the hole. a. Slicing feature: There is no copper in the air port position, and there is asymmetry. b. Reason analysis: After the dry film is pasted, the board stays for too long, and the board is placed in the vertical direction, causing the dry film to flow into the hole. During the pattern plating, the position cannot be plated with copper and tin. This will happen after the film is removed. The dry film at the location is removed, and the copper at the location is also etched away during etching, resulting in no copper in the hole.

2.2.2 Undercut

The outer layer pattern of the multilayer PCB is obtained through an etching process. When the board passes through the etching solution, the unnecessary copper layer is removed. For the thick copper plate with electroplating, the etching solution will also attack the unprotected copper surface on both sides of the circuit due to the existence of the effusion effect, causing etching defects like mushrooms. When a normal copper plate is etched, the etching solution not only erodes the copper layer of the line in the vertical direction, but also corrodes the copper layer in the horizontal direction, so that the cross section of the line after etching is similar to a trapezoid. Generally, the bottom of the line is wider than the top, which is called side etching. . The degree of side etching is expressed by the width of the side etching.

In PCB production, if the side erosion is too severe, it will affect the accuracy of the printed wires, making it even impossible to make fine wires. In addition, undercutting is prone to protruding edges. Excessive protruding edges will cause short circuits in the wires. Through the production of metallographic sections, the serious situation of side etching can be observed, so as to find out the reasons that affect the etching and improve it. For boards with a line width/line spacing of 6mil or more, the etching line width control is relatively simple, and the film line width compensation can be increased. For boards with a line width/line spacing of 4-5 mils, it is much more difficult to control the etching line width. Generally, 90% of the board is etched cleanly to control the production. In order to reduce the side corrosion, strict control of copper concentration, PH value, temperature and spraying methods are usually adopted. For example, the influence of the etching method is changed from splashing to spraying, the etching effect is good, and the side etching is also reduced; the adjustment of the etching rate, the slow etching rate will cause severe side erosion, and the etching rate will be accelerated; check the pH value of the etching solution, because If the PH value is higher, the side corrosion will increase, so try to reduce its PH value; the low density of the etching solution can easily cause side corrosion, so choose an etching solution with a high copper concentration. After targeted improvements, the problem of side erosion will be well resolved.

3. Conclusion

The production process of printed circuit boards is a complicated process and a process of mutual cooperation among multiple processes. The quality of the process quality control will directly affect the quality of the final product. In terms of quality control, metallographic slicing technology plays an important role. In addition, in the production of printed circuit boards, there will always be quality problems of one kind or another. To solve these problems well, we must make effective use of metallographic slicing technology to give full play to its fast and accurate advantages to accurately find out the cause of the problem. On this basis, through optimizing process parameters, improving human error, and carrying out strict production.