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A PCB leakage problem was caused by a low-power product. The original current of the whole machine was at the uA level. After being used for a period of time at room temperature, it was found that its power consumption increased, and the power consumption of some prototypes even reached the mA level.

After carefully eliminating component problems, we finally found a 5V voltage point. When the product is in sleep mode, it should be 0V, but it actually has a voltage drop of about 1.8V!

After patiently cutting the PCB circuit, I was surprised to find that two vias on the PCB with no electrical connection could actually test a resistance of several hundred ohms between each other. Looking at the original design, the two-layer board has a via spacing > 6mil and a hole wall spacing > 18mil. This design is a common drilling process in the PCB industry.

After washing away the ink and eliminating the conductive problem of impurities in the ink or the surface of the hole, the resistance between the vias still exists! After a period of puzzlement, I found that it was the leakage problem caused by the "CAF effect"!

What is the CAF effect:

CAF, the full name of which is Conductive Anodic Filamentation, refers to the leakage behavior of copper and copper salts that occurs when copper ions inside the PCB migrate from the anode (high voltage) along the micro-crack channels between the glass fibers to the cathode (low voltage).

As shown in the figure below, two adjacent vias are ground longitudinally and placed under an electron microscope for 100 times magnification. The plate appears dull in color, while the bright gold part is copper. It can be seen that there are copper dots and copper wires between the two vias.

The mechanism of CAF generation:

1. Conventional FR4 PCB boards are made of glass fibers woven into glass cloth, which is then coated with epoxy resin and semi-cured. The adhesion between the resin and the glass fiber is insufficient or the adhesiveness is poor during impregnation, and gaps are likely to appear between the two. In addition, during mechanical processing such as drilling, the tangential tension and longitudinal impact force further damage the resin adhesion, which may cause the glass fiber bundles to be loosened or separated, resulting in gaps.

In a high temperature and high humidity environment, the adhesion between the epoxy resin and the glass fiber is further deteriorated, and the chemical hydrolysis of the silane coupling agent on the surface of the glass fiber is promoted, forming a pathway for electron migration along the glass fiber reinforcement material.

2. Based on the above conditions, if there is a potential difference between two vias that are close to each other, the copper on the anode with a higher potential will be oxidized into copper ions. Under the action of the electric field, the copper ions migrate to the cathode with a lower potential. During the migration process, they combine with the impurity ions or OH- of the board to generate water-insoluble conductive salts, which are then deposited. As a result, the electrical spacing between the two insulating holes drops sharply, and in severe cases, they may even be directly connected to form a short circuit.

anode:

Cu → Cu2++2e–

H2O → H++OH-

cathode:

2H++2e– → H2

Cu2++2OH– → Cu(OH)2

Cu(OH)2 → CuO+H2O

CuO+H2O → Cu(OH)2 → Cu2++2OH–

Cu2++2e– → Cu

Before I realized the defects caused by the CAF effect, I was amazed at the resistance between two vias that were insulated from each other. Later, after searching for information, I found that many of my colleagues were also troubled by this problem. In fact, the CAF effect has become one of the more popular reliability issues in the PCB industry!

How to prevent or reduce the occurrence of CAF?

1. Improve the board's ability to resist CAF. For the circuit board substrate process, it can be improved by improving the ion purity of the material, using low-hygroscopic resin, and making sure that the glass cloth is fully soaked in the resin and well bonded. For engineers at the application end, when selecting boards, they can consider using CAF-resistant boards. For example, the board supplier Shengyi has CAF-resistant boards for selection.

2. Mechanical drilling or laser burning of PCB will generate high temperature. When the temperature exceeds the Tg point of the board, it will melt and form residues. These residues adhere to the hole wall and cause poor contact during copper plating. Therefore, slag removal must be performed before copper plating. The immersion treatment during the slag removal operation will cause certain erosion to the through-hole and may cause copper infiltration problems, making subsequent copper migration easier.

3. When designing PCB, increase the through-hole spacing. In addition, since the CAF channels are almost generated along the same glass fiber bundle, cross-posting adjacent through-holes can help reduce the occurrence of CAF.

4. Clean the surface of the PCBA, such as using a high-pressure air gun to remove dust, to avoid residual impurities that may cause unnecessary electrolysis of impurities.

In addition, the PCBA surface is coated with conformal coating to prevent the intrusion of water vapor, especially in high temperature and high humidity environments.

Regarding the leakage problem caused by the CAF problem, from the initial confusion to the sudden enlightenment later, there are two points that gave me a deeper understanding:

1. When you find the existence of a bug incredible, please maintain an objective attitude, because the current phenomenon is far from what you already know, and it is likely that there is a blind spot in your knowledge system.

When I encountered the CAF phenomenon, I asked the PCB manufacturer the question of "why there is resistance between mutually insulated vias". The manufacturer also found it incredible, but based on the empirical thinking that "I have made boards for decades and no customers have reported this problem", they have never been able to objectively face the existence of this problem. Under this premise, even with the best cooperation, the so-called verification can only stay at the level of self-certification of their own materials and processes that are in line with industry standards. However, CAF is a problem that cannot be 100% avoided in the current PCB industry. This forced me to find a third-party manufacturer to conduct a section analysis. After obviously seeing the copper between the holes, the problem has an irrefutable conclusion.

2. Electronic engineers on the application side, in addition to being able to use and know how to use electronic devices, must also have some knowledge of their basic materials.

led2015

1. By measuring other related voltage points, verify whether there are other abnormal voltage drops.

2. Use tools such as thermal imaging cameras or thermocouples to check whether circuits or components are abnormally hot when in sleep mode.

3. Perform circuit tracing and analysis to check for unexpected current loop leakage paths.

4. Check the components and lines connected to the 5V voltage point, especially the power management circuit and control circuit related to the sleep state.

lkh747566933

Use tools such as thermal imagers to check whether circuits and components are abnormally hot during sleep.

lkh747566933

This is a good piece of knowledge. I will also learn from it and improve myself.

xutong

I've learned something, thanks to the moderator!