PCB Tech - PCB inner layer micro short circuit phenomenon

PCB company’s products have been plagued by this "micro short circuit inside the circuit board" phenomenon recently. Because we have been unable to find evidence, we have finally made breakthroughs recently. The reason is that we finally found the phenomenon of micro short circuit in the circuit board. Existing defective boards, after analyzing the phenomenon with the PCB board factory, the cause of the defects currently points to [CAF (Conductive Anodic Filament, conductive anode filament, anode glass fiber filament leakage phenomenon)], also commonly known as [Glass Hour Leakage], At last there is a little eyebrow, otherwise the customer has already started to jump, why can't find the problem.

In fact, it is really not easy to find the bad phenomenon of this kind of CAF. First, you must find out where the circuit board is short-circuited, and then cut all the lines that can be cut, and gradually narrow the range of possible short-circuit phenomena, preferably to Is the via to the via, or which trace is to the line, or even to measure which layer of copper foil is a short circuit, so that the cross section will have a greater chance to be found Evidence of micro-short circuit.

▪ If you are not yet very clear about what CAF is, please refer to this article first

This time, I actually found two laboratories [X Special] and [XX Institute] to make slices. [X Special] said that there was no problem at all, while [XX Institute] believed that the gap in the glass fiber cloth might cause CAF. However, because the bad circuit boards that were sampled were different and the micro-short circuit problem was not always present, it is difficult to say which of the two laboratories' slice results was right or who was wrong.

Later, after we got the actual micro-short circuit board, we sent an engineer to the PCB board factory with the board and asked for a slice analysis on the spot. This time it was really confirmed that there was a CAF phenomenon. However, the board manufacturer believes that the reason for CAF is that the through hole (PTH via) and blind via (Blind Via) of our board are too close. Now the board manufacturer is beginning to overturn the originally stated recommended distance of 0.4mm and change it to at least More than 0.5mm. The hole edge to drill to drill is now as small as 0.1mm, but looking back and asking for a distance of 0.5mm, it is fatal and irresponsible.

However, we also confidently told the other party that when the board factory reviewed the PCB, it did not mention the hole-to-hole distance problem, and the project board factory should be more experienced than our system factory, even if the design is risky. However, the board factory still has to bear a relatively large responsibility, but we did not ask the board factory to compensate, but to ask the board factory to propose improvement measures and request preventive measures.

The following are the improvement measures proposed by the board factory for CAF:

1. Change design of PP filling material: PP filling material L is changed from S1000 to S1000H. The board factory said that S1000H has better resistance to CAF.

2. The design of PP stacking structure and ratio change: The core PP was changed from the original 5 sheets of 7628 sheets to 4 sheets of 7628, and also changed to high RC fillers. Because the thickness of Core's PP has become thinner, a layer of 3313 is added to the upper and lower PP layers of the Core to maintain the original thickness of the base material.

But this time we used EDX to hit Au instead of Cu. It seems that problems have already occurred in the process of gold processing. The board our company uses is ENIG.

▼The picture below is a report from the laboratory section. It can be found that the glass fiber cloth has cracks, and some conductive substances penetrate and grow along the gaps of the glass fiber bundles, but it is not yet at the stage of short-circuit.

The picture is a report from the laboratory section. It can be found that the glass fiber cloth has cracks, and some conductive substances penetrate and grow along the gaps of the glass fiber bundles, but the short circuit has not yet occurred.

▼When it is suspected that the PCB has CAF, you can first use electrical measurement and circuit cutting to gradually reduce the scope of the CAF, and you may have to remove the electronic parts on the board to remove possible interference factors first.

When it is suspected that the PCB has CAF, you can gradually reduce the scope of the CAF by measuring and cutting the circuit first, and you may have to remove the electronic parts on the board to remove possible interference factors first.

▼Slowly confirm the position of CAF, you can cooperate with Gerber to check whether the PCB structure has the problem of too close through holes or too close lines.

After slowly confirming the location of CAF, you can cooperate with Gerber to check whether the PCB structure has the problem of too close through holes or too close lines.

▼The picture below shows the appearance of the slicing board after confirming that the short circuit continues to occur. Before treatment with the chemical solution, a long strip of "copper" can be seen across the through hole and the blind hole, but this It is also possible that the copper on the wall of the through-hole is brought over during the slicing and grinding.

The picture is the slice of the board after confirming that the short circuit continues to occur. Before the treatment with the medicine, you can see a long strip of the same element across the through hole and the blind hole, but there are also It may just be that the copper on the wall of the through-hole is brought over during the slicing and grinding.

ACF (Conductive Anodic Filament, conductive pad filament, anode glass fiber filament leakage phenomenon). The picture has been treated with a syrup to clean the possible contamination during the grinding of the slices. It was punched out with EDX and found that the element of Au (gold) is between the through hole and the blind hole.

▼The Au (gold) element punched out with EDX is the first position between the through hole and the blind hole.

ACF (Conductive Anodic Filament, conductive pad filament, anode glass fiber filament leakage phenomenon). Using EDX, the Au (gold) element is located at the first position between the through hole and the blind hole.

▼The Au (gold) element punched out with EDX is located in the second position between the through hole and the blind hole.

ACF (Conductive Anodic Filament, conductive pad filament, anode glass fiber filament leakage phenomenon). Using EDX, the Au (gold) element is located at the second position between the through hole and the blind hole.