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 hermetically sealed capacitors, moisture can also penetrate into the dielectric of the capacitor, reducing the insulation resistance and insulation capacity of the dielectric of the capacitor. Therefore, the influence of high temperature and humidity on the deterioration of capacitor parameters is very significant. After drying and dehumidification, the electrical performance of the capacitor can be improved, but the consequence of water molecular electrolysis cannot be eradicated. For example, when the capacitor works at high temperature, the water molecules are electrolyzed into H + and oh - ions under the action of electric field, and the roots of leads produce electrochemical corrosion. It is impossible to recover the lead wire even if it is dried and dehumidified.
2. Consequences of silver ion migration
Most of the inorganic dielectric capacitors use silver electrodes. When the semi sealed capacitors work at high temperature, the water molecules penetrating into the capacitors produce electrolysis. In anode, silver ion combines with hydrogen ion to produce silver hydroxide; in cathode, silver hydroxide reacts with hydrogen ion to produce silver and water. Due to the electrode reaction, the silver ions in the anode are continuously reduced to discontinuous metal silver particles, which are connected by water film to form dendrites and extend to the anode. Silver ion migration not only occurs on the surface of the inorganic medium, but also spreads to the interior of the inorganic medium, resulting in the increase of leakage current. In serious cases, a complete short circuit between two silver electrodes can be used, leading to the breakdown of the capacitor.
The silver layer on the positive electrode surface can be seriously damaged by ion migration. The lead solder joint and the silver layer on the electrode surface are separated by silver oxide with semiconductor properties, which makes the equivalent series resistance of the dielectric free capacitor increase, the loss of the metal part increases, and the loss tangent of the capacitor increases significantly.
As the effective area of the positive electrode decreases, the capacitance of the capacitor will decrease. The surface insulation resistance of the capacitor is reduced by the presence of silver oxide semiconductor on the surface of the dielectric between the two electrodes. When the migration of silver ions is serious, a dendriform silver bridge is built between the two electrodes, which makes the insulation resistance of the capacitor greatly reduced.
To sum up, silver ion migration will not only worsen the electrical performance of the non sealed inorganic dielectric capacitor, but also may cause the dielectric breakdown field strength to decline, and finally lead to the breakdown of the capacitor.
It is worth mentioning that the failure of silver electrode low-frequency ceramic monolith capacitor caused by silver ion migration is much more serious than other types of ceramic dielectric capacitor, because of its one-time firing process and multilayer lamination structure. In the first sintering process of silver electrode and ceramic medium, silver participates in the solid-state reaction on the surface of ceramic medium and infiltrates into the ceramic silver contact to form the interface layer. If the ceramic medium is not dense enough, the silver ion migration can not only occur on the surface of the ceramic medium, but also penetrate the ceramic medium layer. There are many gaps in the multilayer lamination structure, the electrode position is not easy to be accurate, and the amount of edge on the dielectric surface is small. When the two ends of the lamination layer are coated with the outer electrode, the silver slurry penetrates into the gap, reducing the insulation resistance of the dielectric surface, shortening the path between the electrodes, and the short path phenomenon is easy to occur when the silver ions migrate.
3. Breakdown mechanism of ceramic capacitors at high temperature
The 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 to surface flashover breakdown. According to the time of occurrence, dielectric breakdown can be divided into early breakdown and aging breakdown. Early breakdown exposes the defects in the dielectric materials and production process of capacitors. These defects lead to a significant reduction in the dielectric strength of ceramic media, so that under the action of electric field in high humidity environment, electric breakdown occurs in the process of withstand voltage test or in the early stage of operation of capacitors. Aging breakdown belongs to the category of electrochemical breakdown. Due to the silver migration of ceramic capacitors, the electrolytic aging breakdown of ceramic capacitors has become a very common problem. The conductive dendrite formed by silver migration can increase the leakage current locally, which can cause thermal breakdown and break or burn down the capacitor. The phenomenon of thermal breakdown often occurs in the small ceramic dielectric capacitors with tubular or wafer shape. Because of the serious local heating during breakdown, the thinner tube wall or smaller ceramic body is easy to burn or break.
In addition, the reduction reaction of titanium dioxide may occur in the ceramic medium mainly composed of titanium dioxide under the loading condition, which makes the titanium ion change from tetravalent to trivalent. The dielectric strength of the capacitor is significantly reduced by the aging of the ceramic medium, which may cause breakdown of the capacitor. Therefore, the electrolytic breakdown of the ceramic capacitor is more serious than that of the ceramic dielectric capacitor without titanium dioxide.
Silver ion migration causes serious distortion of the electric field at the edge between the electrodes of the capacitor, and the corona discharge voltage on the edge of the capacitor decreases significantly due to the water film on the surface of the ceramic medium in the high humidity environment, which results in the phenomenon of surface pole arc under the working conditions. When it is serious, it will cause flashover breakdown between electrodes on the surface of capacitor. Surface breakdown is related to capacitor structure, distance between electrodes, load voltage, hydrophobicity and moisture permeability of protective layer. The main reason of edge surface flashover breakdown is that the amount of dielectric edge is small. The migration of silver ions and the formation of water film on the surface make it take a while for the generation and development of the migration of silver ions on the edge surface of the capacitor. Therefore, at the beginning of the voltage withstand test, the failure mode is mainly dielectric breakdown. After 500h of the test, as long as the failure mode is excessive It is edge surface flashover breakdown.
4. Improvement of electrode materials
Silver electrodes have been used in ceramic capacitors. The migration of silver ions and the accelerated aging of ceramic containing titanium are the main reasons for the failure of ceramic capacitors. Some manufacturers use nickel electrode instead of silver electrode to produce ceramic capacitors, and use electroless nickel plating on ceramic substrate. Because the chemical stability of nickel is better than that of silver and the electric mobility is lower, the performance and reliability of ceramic capacitors are improved.
For example, the single stone low-frequency ceramic dielectric capacitor with silver electrode as its electrode can not obtain dense ceramic medium due to the under firing of the ceramic material when silver electrode and ceramic material are sintered at 900 ℃, so there is a large porosity; in addition, the commonly used cosolvent barium oxide of silver electrode will penetrate into the ceramic body, and under high temperature, it depends on the good infiltration and "mutual melting" ability between barium oxide and silver to make the electrode and media There is a phenomenon of thermal diffusion in the interior, that is, the phenomenon of "porcelain sucking silver" seen from the macroscopic view. With the introduction of barium oxide into the ceramic body, silver greatly reduces the effective thickness of the dielectric, resulting in the reduction of the insulation resistance and reliability of the product. In order to improve the reliability of the single stone capacitor, silver palladium electrode was used instead of the electrode which usually contains barium oxide, and 1% 5 × glass powder was added to the material formula. It eliminates the phenomenon of thermal diffusion of metal electrode to the ceramic medium layer during the first sintering at high temperature, which can promote the densification of the ceramic material sintering and greatly improve the performance and reliability of the product. Compared with the original process and medium material, the reliability of the capacitor has been improved by 1 ~ 2 orders of magnitude.
5. Rupture of laminated ceramic capacitor
The most common failure of laminated ceramic capacitor is fracture, which is determined by the brittleness of its own medium. As the laminated ceramic capacitor is directly welded on the circuit board, it can bear all kinds of mechanical stress from the circuit board directly, while the lead ceramic capacitor can absorb the mechanical stress from the circuit board through the pin. Therefore, for the laminated ceramic capacitor, the mechanical stress caused by the different coefficient of thermal expansion or the bending of the circuit board will be the most important factor for the fracture of the laminated ceramic capacitor.
6. Fracture analysis of laminated ceramic capacitor
After the mechanical fracture of the laminated ceramic capacitor, the electrode insulation distance at the fracture point will be lower than the breakdown voltage, which will cause the arc discharge between two or more electrodes and completely damage the laminated ceramic capacitor.
The main methods to prevent the mechanical fracture of the laminated ceramic capacitor are: to reduce the bending of the circuit board as much as possible, to reduce the stress of the ceramic chip capacitor on the circuit board, to reduce the mechanical stress caused by the difference of the thermal expansion coefficient between the laminated ceramic capacitor and the circuit board.
How to reduce the stress of laminated ceramic capacitor on the circuit board will be described in the following, and will not be described here. To reduce the mechanical stress caused by the difference of thermal expansion coefficient between the laminated ceramic capacitor and the circuit board, we can choose the capacitor with small package size to slow down. For example, the aluminum based circuit board should use the package below 1810 as much as possible. If the capacitance is not enough, we can use the method of multiple parallel connection or the method of lamination, or we can use the ceramic capacitor with pin package form Device to solve.
7. Electrode end of laminated ceramic capacitor is melted
In the wave soldering of laminated ceramic capacitors, the electrode end may be melted off by solder. The main reason is that the time of wave soldering is too long. There are two kinds of laminated ceramic capacitors in the market: one is suitable for reflow soldering and the other is suitable for wave soldering. If the laminated ceramic capacitor suitable for reflow soldering is used for wave soldering, it is likely that the electrode end of the laminated ceramic capacitor will be melted. As for the time characteristics of high-temperature soldering tin that the electrode end of the laminated ceramic capacitor can bear under different welding processes, it is described in detail in the applicable precautions of the laminated ceramic capacitor later, which is not covered here.
The elimination method is to use laminated ceramic capacitors that conform to the wave soldering process as much as possible when using the wave soldering process, or not to use the wave soldering process as much as possible.
