Mechanism Analysis of Capacitor Failure(III)

Breakdown Mechanism of Ceramic Capacitor under High humidity

The breakdown failure of semi-sealed ceramic capacitors (such as patch capacitors) in high humidity environment is a common and serious problem. The breakdown can be divided into dielectric breakdown and surface arc breakdown. The dielectric breakdown can be divided into early breakdown and aging breakdown according to the time of occurrence。

Early breakdown exposed the defects in capacitor dielectric materials and production processes, which leads to a significant reduction in the electrical strength of ceramic dielectric, resulting in the electric field in high humidity environment. The capacitor produces an electric breakdown during the voltage test or at the beginning of the operation. Aging breakdown mostly belongs to the category of electrochemical breakdown.Early breakdown exposed the defects in capacitor dielectric materials and production processes, which leads to a significant reduction in the electrical strength of ceramic dielectric, resulting in the electric field in high humidity environment. The capacitor produces an electric breakdown during the voltage test or at the beginning of the operation. Aging breakdown mostly belongs to the category of electrochemical breakdown.

In addition, in the ceramic medium with titanium dioxide as the main component, the reduction reaction of titanium dioxide may also occur under the loading condition, which makes the titanium ion change from tetravalent to trivalent. The aging of ceramic dielectric reduces the dielectric strength of the capacitor and may lead to the breakdown of the capacitor. Therefore, the electrolytic breakdown of this kind of ceramic capacitor is more serious than that of ceramic dielectric capacitor without titanium dioxide。

The migration of silver ions makes the electric field at the edge of the capacitor seriously distorted, and the corona discharge voltage on the edge of the capacitor decreases significantly because of the water film condensation on the ceramic dielectric surface in high humidity environment. Under the working conditions, the phenomenon of interpolar arc flying on the surface is produced. When it is serious, it causes the breakdown of the fly-arc between the electrodes on the surface of the capacitor. Surface breakdown is related to capacitance structure, distance between poles, load voltage, hydrophobicity of protective layer and moisture permeability.The main reason is that the fly-arc breakdown of the edge surface is due to the small amount of the dielectric edge, the silver ion migration and the formation of the surface water film make the insulation resistance of the edge surface of the capacitor decrease significantly when the medium is working in the wet environment. it will cause corona discharge and eventually lead to breakdown. It is  severe especially in high humidity environments. Since the generation and development of silver ion migration takes a period of time, in the beginning of the stage of voltage test, the failure mode is mainly caused by medium breakdown until 500 h after the test. The main failure mode changes to the fly-arc breakdown of the edge surface.