Mechanism Analysis of Capacitor Failure (II)

    The consequences of silver ion migration

    Most inorganic dielectric capacitors use silver electrodes. When semi-sealed capacitors work at high temperature, water molecules infiltrating into the capacitors produce electrolysis. The oxidation reaction occurs at the anode and the silver ion binds with the hydroxide ion to form silver hydroxide. A reduction reaction occurs at the cathode and silver hydroxide reacts with hydrogen ions to produce silver and water. As a result of the electrode reaction, the silver ions of the anode are continuously reduced to discontinuous metal silver particles to the cathode, and connected to the water film to form a tree extending to the anode

Silver ion migration not only occurs on the surface of inorganic medium, but also diffuses into the inorganic medium, resulting in the increase of leakage current and the complete short circuit between the two silver electrodes, which leads to the breakdown of the capacitor.

    Silver ion migration can seriously destroy the silver layer on the surface of the positive electrode. The silver oxide with semiconductor properties is spaced between the lead solder joint and the silver layer on the electrode surface, which increases the equivalent series resistance of inorganic dielectric capacitor. The angle tangent of the loss of capacitor increases significantly with the increase of metal loss

As the effective area of the positive electrode decreases, the capacitance of the capacitor will decrease as a result. The surface insulation resistance is reduced by the existence of silver oxide semiconductors on the dielectric surface between the two electrodes of inorganic dielectric capacitors. When the silver ion migrates seriously, a dendritic silver bridge is built between the two electrodes, which greatly reduces the insulation resistance of the capacitor.

    In conclusion, silver ion migration will not only worsen the electrical properties of unsealed inorganic dielectric capacitors, but also may lead to a decrease in dielectric breakdown field strength, and finally lead to capacitor breakdown.