Failure Mechanism Analysis of Capacitor(II)

Breakdown Mechanism of Ceramic Capacitor under High humidity.

The breakdown failure of semi-sealed ceramic capacitors under high humidity 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 exposes defects in capacitor dielectric materials and production processes, which result in a significant reduction in the electrical strength of ceramic dielectric, resulting in capacitors during voltage tests or in the early stages of work under the action of an electric field in a high humidity environment. It produces an electric breakdown. Aging breakdown mostly belongs to the category of electrochemical breakdown. Due to the migration of silver in ceramic capacitors, electrolytic aging breakdown of ceramic capacitors has become a very common problem. The conductive dendrites formed by silver migration increase the leakage current locally, which can cause thermal breakdown and make the capacitor break or burn down. The phenomenon of thermal breakdown occurs mostly in small ceramic dielectric capacitors with tubular or circular shape, because local heat is severe during breakdown, and the thinner wall or smaller ceramic body is easy to burn out or break.

In addition, in the ceramic medium with titanium dioxide as the main medium, 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 leads to serious distortion of the electric field between the electrodes of capacitors, and the formation of water film on the ceramic dielectric surface in high humidity environment, which makes the corona discharge voltage on the edge surface of capacitors decrease significantly, resulting in the phenomenon of interpolar arc flying under the working conditions. 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 the breakdown of fly-arc between the poles of the edge surface. The reason is that the medium keeps the edge less, the silver ion migration and the formation of the surface water film make the insulation resistance of the edge of the capacitor decrease significantly, and cause the corona discharge. Eventually leading to a breakdown. It is especially severe in high humidity environments. Since the generation and development of silver ion migration takes a period of time, the failure mode is mainly dielectric breakdown at the initial stage of the voltage test, and the main failure mode does not transition to the fly-arc breakdown at the edge surface until 500 h after the test.