Failure cause analysis and application precautions of ceramic capacitor

Date：2022-01-17 14:46:50 Views：1853

Ceramic capacitors are made of high dielectric constant capacitor ceramics extruded into circular tubes, discs or disks as the medium, and silver is plated on the ceramics as the electrode by sintering infiltration method. The capacitor with small positive capacitance temperature coefficient is used in high stable oscillation circuit as loop capacitor and padding capacitor. Ceramic capacitors are easy to fail when used in inappropriate circuits.For silver electrode ceramic capacitors, the following failure forms may occur.

1. Influence of humidity on deterioration of electrical parameters

When the humidity in the air is too high, the water film condenses on the surface of the capacitor shell, which can reduce the surface insulation resistance of the capacitor. In addition, for semi sealed capacitors, moisture can also penetrate into the capacitor medium, reducing the insulation resistance and insulation capacity of the capacitor medium. Therefore, the influence of high temperature and high humidity environment on the deterioration of capacitor parameters is very significant. After drying and dehumidification, the electrical performance of the capacitor can be improved, but the consequences of water molecular electrolysis can not be eradicated. For example, when the capacitor works at high temperature, water molecules are electrolyzed into hydrogen ions (H +) and hydroxyl ions (OH -) under the action of electric field, and electrochemical corrosion occurs at the root of the lead. Even if it is dried and dehumidified, it is impossible to restore the lead.

2. Consequences of silver ion migration

Most inorganic dielectric capacitors use silver electrodes. When the semi sealed capacitor works at high temperature, the water molecules infiltrating into the capacitor produce electrolysis. The oxidation reaction occurs at the anode, and silver ions are combined with hydroxyl ions to produce silver hydroxide; The reduction reaction occurs at the cathode, and silver hydroxide reacts with hydrogen ions to produce silver and water. Due to the electrode reaction, the silver ions of the anode are continuously reduced to discontinuous metal silver particles to the cathode, which are connected into a tree by the water film and extend to the anode. Silver ion migration not only occurs on the surface of inorganic medium, but also diffuses into the interior of inorganic medium, resulting in increased leakage current. In severe cases, a complete short circuit between two silver electrodes can be used, resulting in capacitor breakdown.

Ion migration can seriously damage the silver layer on the surface of the positive electrode. Silver oxide with semiconductor properties is separated between the lead solder joint and the silver layer on the surface of the electrode, which increases the equivalent series resistance of the dielectric free capacitor, increases the loss of the metal part, and significantly increases the loss tangent of the capacitor.

As the effective area of the positive electrode decreases, the capacitance of the capacitor will decrease. The surface insulation resistance decreases due to the presence of silver oxide semiconductor on the dielectric surface between the two electrodes of inorganic dielectric capacitor. When the migration of silver ions is serious, a dendritic silver bridge is built between the two electrodes, which greatly reduces the insulation resistance of the capacitor.

3. Breakdown mechanism of ceramic capacitor at high temperature

Breakdown Failure of semi sealed ceramic capacitors is a common and serious problem when they work in high humidity environment. The breakdown phenomena can be divided into dielectric breakdown and surface inter electrode flashover breakdown. Dielectric breakdown can be divided into early breakdown and aging breakdown according to the time of occurrence. Early breakdown exposes the defects existing in capacitor dielectric materials and production processes. These defects lead to a significant reduction in the dielectric strength of ceramic dielectric, so that under the action of electric field in high humidity environment, electric breakdown occurs during withstand voltage test or at the initial stage of operation. Aging breakdown mostly belongs to the category of electrochemical breakdown. Due to the migration of silver in ceramic capacitors, the electrolytic aging breakdown of ceramic capacitors has become a very common problem. The conductive dendrites formed by silver migration locally increase the leakage current, which can cause thermal breakdown and break or burn the capacitor. Thermal breakdown often occurs in small tubular or disc-shaped ceramic dielectric capacitors, because local heating is serious during breakdown, and the thinner tube wall or smaller ceramic body is easy to burn or break.

The migration of silver ions seriously distorts the electric field at the edge of the capacitor, and the corona discharge voltage at the edge of the capacitor decreases significantly due to the condensation of water film on the surface of ceramic dielectric in high humidity environment, resulting in surface inter electrode arcing under working conditions. If it is serious, it will lead to flashover breakdown between electrodes on the surface of the capacitor. Surface breakdown is related to capacitor structure, distance between electrodes, load voltage, hydrophobicity and moisture permeability of protective layer. The main reason for the inter electrode flashover breakdown of the edge surface is that the dielectric edge is small. The silver ion migration and the formation of surface water film when working in a humid environment make the edge surface insulation of the capacitor. Due to the generation and development of silver ion migration, it takes a period of time, so at the beginning of the withstand voltage test, the failure mode is mainly dielectric breakdown until 500h after the test, As long as the failure mode is excessive, it is inter electrode flashover breakdown on the edge surface.

5. Fracture of laminated ceramic capacitor

The common failure of laminated ceramic capacitor is fracture, which is determined by the brittleness of its own medium. Because the laminated ceramic capacitor is directly welded on the circuit board, it directly bears various mechanical stresses from the circuit board, while the lead-in ceramic capacitor can absorb the mechanical stresses from the circuit board through the pins. Therefore, for laminated ceramic capacitors, the mechanical stress caused by different thermal expansion coefficients or circuit board bending will be the main factor for the fracture of laminated ceramic capacitors.

6. Fracture analysis of laminated ceramic capacitor

After the mechanical fracture of laminated ceramic capacitor, the electrode insulation spacing at the fracture will be lower than the breakdown voltage, which will lead to arc discharge between two or more electrodes and completely damage the laminated ceramic capacitor.

The prevention methods of mechanical fracture of laminated ceramic capacitor mainly include: reducing the bending of circuit board as much as possible, reducing the stress of ceramic chip capacitor on circuit board, and reducing the mechanical stress caused by the difference of thermal expansion coefficient between laminated ceramic capacitor and circuit board.To reduce the mechanical stress caused by the difference of thermal expansion coefficient between laminated ceramic capacitor and circuit board, the capacitor with small package size can be selected. For example, aluminum based circuit board should be packaged with less than 1810 as far as possible. If the capacitance is not enough, it can be solved by multiple parallel connection or lamination, It can also be solved by ceramic capacitors in the form of package with pins.

7. Electrode end of laminated ceramic capacitor is melted

When wave soldering is used to weld laminated ceramic capacitors, the electrode ends may be melted off by solder. The main reason is that the time of wave soldering laminated ceramic capacitor contacting high-temperature solder is too long. At present, the laminated ceramic capacitors in the market are divided into those suitable for reflow soldering process and those suitable for wave soldering process. If the laminated ceramic capacitors suitable for reflow soldering process are used for wave soldering, the melting of the electrode ends of laminated ceramic capacitors is likely to occur.

The elimination method is very simple, that is, when using wave soldering process, use laminated ceramic capacitors conforming to wave soldering process as much as possible; Or wave soldering process shall not be used as far as possible.

When using ceramic capacitors, pay attention to the following problems:

① High temperature (the temperature exceeds the rated service temperature);

② Overcurrent (the current exceeds the rated ripple current). When the ripple current exceeds the rated value, the ceramic capacitor will overheat, reduce the capacity and shorten the service life;

③ Over voltage (the voltage exceeds the rated voltage), when the voltage applied on the capacitor is higher than the rated working voltage, the leakage current of the capacitor will rise, and its electro-oxygen physical properties will deteriorate in a short time until damaged;

④ If it is used in the circuit with repeated rapid charge and discharge, such as rapid charge, its service life may be reduced due to the decrease of capacity and the sharp rise of temperature;

⑤ When the reverse voltage or AC voltage is applied, when the DC capacitor is connected to the circuit according to the reverse polarity, the capacitor will cause short circuit of the electronic circuit, and the resulting current will cause damage to the capacitor. If it is possible to apply positive voltage to the negative lead in the circuit, a non-polar capacitor shall be selected.

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