Micro-Slicing And Drilling of Circuit Boards (3)

4. Interpretation:

The clear and cute picture of the slice is just a method of processing. To be able to interpret various phenomena in the image and use it as a basis for decision-making, it requires more knowledge of PCB boards, especially the causes and methods of improvement. Only the cooperation of experience is necessary, and it cannot be achieved in a short period of time. The following will explain the common shortcomings with the slides, so that the reader can do a more in-depth understanding.

4.1 The through hole of the empty board is cut straight and sliced (including the board that has been fried with oil or sprayed with tin), and various phenomena can be seen, such as:

Sheet structure, hole copper thickness, hole copper integrity, broken copper (VOID), flow tin situation, drilling alignment and layer to layer regisatration, Annular ring, etching situation, scum (Smear) situation, pressure plate and drilling situation, (gouging, nail head, Nailheading), copper infiltration broom (Wicking), hole copper floating (Pullaway), negative etchback, etc., are described separately below:

4.1.1 Hole Copper Thickness

At least 1 mil or more, when the micro-etching is good, you can clearly see the layers of primary copper and secondary copper or even thickened copper. It should be noted that some processes may have a large difference in the thickness of the hole copper. The thickness of the left and right copper walls on the slice can be clearly seen.

4.1.2 Hole Copper Complete Situation

Whether there is a step plating of the Nodule, Inclusion, orifice, and the crystallization of the copper layer.

4.1.3 Broken Copper

Only three holes are allowed to appear in one slice. Holes are the biggest cause of blow holes during soldering. If the holes appear at the same height of the straight slice, it can be interpreted or judged as a full hole ring break.

4.1.4 Flow Tin

The results of the half-moon flow tin of capillary phenomenon can be seen, and the highest level can be seen as a 50 micro-inch special interlayer of copper-tin alloy (IMC), a thin white strip between copper and tin.

4.1.5 Alignment Situation

The interlayer alignment and the alignment between drilling and printing can be seen from the length of the inner layers on both sides of the hole wall.

4.1.6 Etching

The undercut can be seen and the etch factor calculated, as can the sidewalls of the printed or dry film.

4.1.7 Slack

You can see the situation of de-smear or etch back. Excessive de-smear will cause the glass protruding hole wall to be rough and the hole copper to be uneven. It may also cause hole blowing. When there is insufficient smear removal, there is a black line or separation between the inner layer and the hole copper.

4.1.8 Drilling and Pressing Plate

Whether the hole wall is rough and digging, whether the nail head exceeds 50%, and whether the dielectric layer after pressing the plate is too thin (Dielectic thickness).

4.1.9 Seepage Hole

The reason is that after the hexavalent chromium desmear liquid eats the silicon oxide layer on the glass surface, the copper layer is deposited and infiltrated, but it should not exceed 1 mil.

4.1.10 Hole Wall Floats Away

It is because the stress of the copper layer is too high and the adhesion of the copper is poor, causing the large piece to float after being heated.

4.1.11 Etch Back

It may be caused by excessive micro-etching during the PTH process. At this time, the inner layer will retreat slightly. It is very likely that the copper layer that has been plated during one copper plating will float again, and then continue to thicken after electroplating, causing the floating part. The parts and the substrate are all plated with copper at the same time, and it takes a very clever technique to see the truth.

* Pay attention to the above shortcomings. If you use a simple hand-glued section, you can still be more careful to do horizontal sectioning and do in-depth re-proof, but if it is a cylindrical formal section, you can’t do anything.

4.2 Through-hole slices filled with tin, generally with the thermal stress test at 288°C, 10 seconds, the following situations can be seen:

4.2.1 Corner Cracking

When soldering at high temperature, the board will expand in the Z direction. If the ductility of the copper plating layer is not good, (at least 10% ductility. 062 board will not break the corner), it will be pulled at the corner. At this time, the activated carbon treatment of the copper plating tank can solve the problem, and the hole copper fracture may also appear in other positions.

4.2.2 Resin Recession

The wall of the hole is intact before soldering. After filling the tin, the resin is partially hardened, or volatilizes and escapes, causing local subsidence to shrink from the back of the hole copper.

4.2.3 Lamination Void

The situation is similar to that of the resin subsidence in the A area (hot area) of the board, but this kind of void appears in the B area of the board (press plate area, or non-through hole area) when serious, and even delamination occurs.

4.2.4 Lifted Land

Due to the severe Z-direction expansion and then the shrinkage, the float-off at the periphery of the ring after the thermal stress test shall not exceed 1 mil.

4.2.5 Blow hole

The hole formed by the hole in the hole in the copper layer of the hole wall is vaporized and blown out (Outgassing) at high temperature to drive away the liquid tin. This type of through hole is called a blow hole.

4.2.6 Micro-Crack of Inner Copper Foil

It is caused by the expansion of the Z-square and can only be seen with good craftsmanship.

4.2.7 Solder abity of through holes.

4.3 Oblique section (45°, 30°)

It can be seen that the relationship between the conductors between the layers, the situation that the blackened dust between the upper conductors of this layer moves with the flow of glue, and the situation that there are many joint surfaces between the hole wall and the inner layer, use a 40x solid microscope to observe, but the grinding craftsmanship of the film is more difficult, and it is not easy to take pictures.

4.4 Horizontal Slicing

The simple method is to lay the cut sample flat, pour glue, and then attach the upright grip with strong instant glue to facilitate grinding and cutting. This horizontal method can further prove the simple vertical cut, but the craft is more difficult. Be careful and slow grinding to avoid mistakes, especially when the copper foil is at 1/2 OZ or 1/4 OZ, you must be very careful. A little unevenness can make mistakes. Horizontal slicing can also see slag removal, hole copper thickness, rough drilling, etc. In general, vertical slicing can be seen and a special picture of flat cutting can be seen. E.g:

4.4.1 Pink Ring, Red Ring, Red Halo

It is because the drilling action is too violent or the acid liquid penetrates into the blackened layer before the PTH copper melting, and eats the copper oxide to expose the primary color of the copper metal surface. The size of this pink circle is also an indicator of the quality of the process.

4.4.2 Alignment Between Printing and Drilling

It is easiest to see the whole picture on the flat section. The minimum width of the flat ring and whether there is a break out are more clear and more true than the straight section.

4.4.3 The distribution of hole copper thickness is also more accurate than straight cut.

4.5 Cut Holes:

It is necessary to observe the panorama of the half-circular wall with a 40x solid microscope, which can be seen more completely and more closely to the real situation. The following are the characteristics of cutting holes.

4.5.1 The Real Situation of Blowing Holes

On the walls of the holes where the tin is sprayed and melted, it can be clearly seen that there is a still picture of the gas blowing out. Anyone can understand it at a glance and be impressed. It is more effective and powerful than any text and language explanation.

4.5.2 The condition of the original borehole wall before plating, such as the case where the longitudinal glass bundle is hollowed out and the case where the whole furrow appears.

4.5.3 After passing through the electroless copper (chemical copper), you can grind the back as much as possible, and do the backlight method to check whether the copper wall is well covered or has holes.

The simple way is:
Take a 500 ml beaker and cover the outside of the side wall and the bottom of the cup with tape, make the bottom of the cup face up, put the light source of a small flashlight in the cup, and cut a small slit on the tape at the bottom of the cup to let the light out, and then press the hole surface of the cut-hole sample upwards on the optical slit. Under the 20 or 40 times solid microscope, you can clearly see whether the hole wall glass has been covered with copper layer, and there are any light spots or hazy light. Leakage, that is, there is a problem with the coverage of the surface copper layer. The copper layer is opaque and must be completely black to indicate that the copper layer has been completely covered.

5 Conclusion:

Slicing to the circuit board is like X-ray to the doctor, it can find out the truth of the problem, find out the troubles of the production line, and find out the solution. Good slicing often has unexpected benefits, making the person who does it have a great sense of achievement, so it is constantly pursued by the industry and researchers.