Micro-Slicing And Drilling of Circuit Boards (2)

3.3 Grinding:

Use the cutting force of sandpaper to grind the sample to the center of the hole, so as to observe the cross-section of the hole wall. In order to save time and mass production, the rapid grinding method is often used with a fast turntable. It can be pasted on the surface of the disk with sandpaper with adhesive backing, or the gauze paper can be fixed by the edge turn-shaped fixer, or the paper has a center hole and is inserted into the rotating shaft. When wet and rotating at high speed, the sandpaper will be flat on the disk and can be ground. A small number of simple cut samples can be smoothed on ordinary sandpaper by hand, and even the turntable can be omitted. The sandpaper number used above is appropriate as follows.

3.3.1 No. 220 rough grinding until the two parallel lines of the hole wall fault will appear, pay attention to spray water or other liquids to reduce heat.

3.3.2 Use No. 400 and regrind until the “indication line” of the “center of the hole” appears.
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3.3.3 Use fine sandpaper above 600 to lightly grind it a few times to correct the non-parallel oblique grinding.

3.4 Polishing

To see the truth of the slice, careful polishing must be done to remove scratches from the sandpaper. When a large amount is used, the rotary felt is added with alumina suspension as an auxiliary agent to perform micro-contact polishing. Note that the direction of the cutting sample should be changed frequently during polishing to achieve a more uniform effect until the sand marks completely disappear. A small amount of cutting can be changed to a general cloth head and rubbed with copper ointment. It is also necessary to change the direction of throwing, front and rear, left and right and circular motion. When the craftsmanship is good, the effect is more clear than the high-speed turntable polishing, more able to preserve the truth, but more time-consuming. The polishing pressure is lighter and the number of reciprocations is more, the effect is better, and the truth of the copper surface obtained by oil-based polishing is better than that of water-based polishing.

3.5 Microetch:

After the polished surface is washed with water or dilute alcohol and dried, micro-etching can be performed to find out the respective layers of the metal and the crystallization condition. This kind of micro-etching seems simple, but it is necessary to see a clear and delicate real image. But it is not easy, and it is not always successful. When it doesn’t work, you can only toss it a few times and redo the micro-etch to find out the real image. The formula of the micro-etching solution is as follows:
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“10 cc ammonia + 10 cc pure water + 2~3 drops of hydrogen peroxide”
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After mixing evenly, you can use a cotton swab to dip the liquid and wipe it gently on the surface of the slice for about 2 seconds. Pay attention to the phenomenon of bubbles on the copper surface, immediately rinse off the etching liquid with water after 2 to 3 seconds, and immediately dry it with toilet paper. Do not continue to oxidize the copper surface, otherwise brown-black and rough copper surface will appear under 100x microscope, and good micro-etching will show bright red copper color, and the crystallization and demarcation are clear.

Note that the above micro-etching solution can only last for 1 hour at most, and the cotton swab should be replaced after 1-2 times of use, so as not to contaminate the crystals on the copper surface on the cut surface.
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In the early days, the method of adding a small amount of sulfuric acid and salt to chromic acid (Cr03) has become outdated, and it will make the tin-lead layer black, which is not suitable for reuse. The tin and lead surface of the ammonia water method is still white, and the common black spot distribution is the phenomenon of a large amount of lead.
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For the slices to be researched and judged, careful polishing and micro-etching work must be done, otherwise it will only be a waste of effort. Generally, a large number of slices are shipped, so they have to be collectively polished and micro-etched before inspection.

3.6 Photographs

If the original polishing sheet is 100 points, the inverted image seen under the microscope can only be seen at the level of 85% to 95% according to the performance of the microscope, and when it is photographed with a polaroid, it is best to only see 80% to 90%. When the instant photo is turned into a slideshow, of course, there is a discount of 75%to 85%. But for the sake of record and communication, photography is the best method. It is very expensive, and you must have a picture to go to the face, otherwise it is meaningless. The most difficult part of taking pictures is the alignment of the focal length, which is much more difficult.
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3.6.1 The visual focal length is not exactly the same as the photographic focal length, so it is not acceptable to take the visual as the standard.
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3.6.2 The amount of light required for exposure = light intensity x time. A good photo should try to extend the time and reduce the light intensity. After adding various filters, you can get photos with different effects.
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3.6.3 The surface of the image must be flat, otherwise when the multiple is large (above 100x), there will be local clear and local blurring.