Recently, I encountered a problem of PCB leakage. The cause was a low-power product. The current of the whole machine was 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 individual prototypes even reached the mA level. After carefully eliminating the component problem, I finally found a 5V voltage point, which should be 0V when the product is in sleep state, but it has a voltage drop of about 1.8V! After patiently cutting the PCB circuit, I was surprised to find that two vias without electrical connection on the PCB can test the resistance value of several hundred ohms between each other. Check out the original design, two-layer board, via hole spacing >6mil pad spacing, hole wall spacing >18mil, such a design is a common drilling process in the PCB industry. Wash off the ink and eliminate the conductive problem of impurities on the surface of the ink or the hole, and the measured resistance between the holes still exists! After thinking for a while, I found out that it was the leakage problem caused by the "CAF effect"!
What is the CAF effect:
CAF, the full name of Conductive Anodic FilamentaTIon (CAF: Conductive Anodic FilamentaTIon), refers to the migration of copper ions inside the PCB from the anode (high voltage) to the cathode (low voltage) along the micro-crack channel between the glass fiber filaments. The leakage behavior of copper and copper salts.
Longitudinal grinding of two adjacent vias, placed under an electron microscope to magnify 100 times, the plate is dim, and the bright golden part is copper. It can be seen that between the two vias, there are copper dots and copper dots. silk exists.
The mechanism of CAF generation:
1. The conventional FR4 PCB board is made of glass fiber woven into glass cloth, and then coated with epoxy resin and semi-cured. The adhesion between the resin and the glass fiber is insufficient or the glue is poor during impregnation, and there is a gap between the two. In addition, in the machining process such as drilling, the adhesive force of the resin is affected by the action of tangential pulling force and longitudinal impact force. The further damage may cause the glass fiber bundle to be pulled or separated to form a gap. In the environment of high temperature and high humidity, the adhesion between epoxy resin and glass fiber is further deteriorated, and the chemical hydrolysis of silane coupling agent on the surface of glass fiber is promoted, forming a path for electron migration along the glass fiber reinforcement material;
2. Based on the above conditions, if there is a potential difference between the two vias that are closer at this time, the copper on the anode with the higher potential will be oxidized to copper ions, and the copper ions will move to the cathode with a lower potential under the action of the electric field. Migration, in the process of migration, combined with impurity ions or OH- of the plate, a water-insoluble conductive salt is formed and deposited, so the electrical distance between the two insulating holes drops sharply, and in severe cases, it can even lead to direct conduction. 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 bad effect caused by the CAF effect, I felt incredible about the phenomenon of resistance between two vias that were insulated from each other. It is one of the more popular reliability issues in the PCB industry!
How to prevent or reduce the occurrence of CAF?
1. Improve the ability of the board to resist CAF. For the circuit board substrate process, it can be improved from the aspects of improving the ion purity in the material, using low hygroscopic resin, and the glass cloth being fully soaked in the resin and bonding well. For application-side engineers, CAF-resistant sheets can be considered when selecting sheets. In the sheet selection of the following sheet material supplier Shengyi, there are CAF-resistant sheets for selection.
2. The mechanical drilling or laser hole burning of the PCB will generate high temperature. When the Tg point of the board is exceeded, it will melt and form residues. These residues adhere to the hole wall and cause poor contact during copper plating. Therefore, it must be removed before copper plating. Slag operation, the soaking treatment in the slag removal operation will cause certain erosion of the through holes and may bring about the problem of copper infiltration, which makes the subsequent copper migration easier;
3. When designing the PCB, increase the spacing of the through holes. In addition, since the CAF channels are almost generated along the same glass fiber bundle, the cross release of adjacent through holes helps to reduce the occurrence of CAF;
What is the cause of PCB leakage
4. Clean the surface of the PCBA, such as using a high-pressure air gun for dust cleaning, to avoid the electrolysis of unnecessary impurities caused by residual impurities. In addition, coating the surface of PCBA with conformal paint to avoid the intrusion of water vapor, especially in the geographical environment of high temperature and high humidity.
What is the cause of PCB leakage? For the leakage problem caused by this CAF problem, from the trouble at the beginning to the sudden enlightenment later, there are two points that gave me a deeper understanding:
1. When you feel incredible about the existence of a bug phenomenon, please keep an objective attitude, because the current phenomenon is far from what you already know, it is probably just a blind spot in your knowledge system. . When I encountered the CAF phenomenon, I threw a question to the PCB manufacturer about "why is there resistance between mutually insulated vias?" The manufacturer also thought it was incredible, but the other party based on "the board that he has built for decades does not have it." The customer has responded to this problem" empirical thinking, and has never been able to objectively face the existence of this problem. Under this premise, no matter how good the cooperation is, the so-called verification can only stay in self-certification materials. , The process belongs to the level of industry norms, but CAF is a problem that cannot be 100% avoided for the current PCB industry. As a result, I was forced to find a third-party manufacturer to conduct sectional analysis. After I clearly saw the copper between the holes, there was an irrefutable conclusion on this issue;
2. Electronic engineers on the application side should have a knowledge of electronic devices, in addition to their ability to use and use them, but also to have a knowledge of their basic materials. It is like dealing with capacitors, resistors, inductors and other devices every day, but many people are unaware of the manufacturing process and basic material composition of these devices, and this is the root of the electrical characteristics of these devices. For example, under different operating frequency characteristics, why does a capacitor have resistive, inductive and capacitive components; or why an aluminum electrolytic capacitor will explode when the reverse connection voltage is unknown, and it will definitely explode when the reverse voltage is unknown. ?