Aluminum electrolytic capacitors show various failure modes in different applications. (See Table 2.)
Table 2: Failure Modes and Causes
Failure Mode Table in Adobe PDF format – To view this file, either configure your web browser with a helper application for automatic launching of Abode Acrobat Reader or save to disk and manually open this file with Adobe Acrobat Reader.
Short circuits are caused by burred or rough foil edges as well as thin regions in the separator paper and deficiencies in the oxide film; both of which lead to dielectric breakdown. Since all products are subjected to aging before shipping, short circuited capacitors are rarely found in the field. However, inappropriate application factors such as excessive operating voltage, excessive ripple current and reverse voltage may result in short
Open circuit or poor contact of terminal connections can be caused by excessive mechanical stress such as high vibration levels while in service. Open circuits caused by the evaporation of electrolyte are normally observed as an end of life phenomenon during the wear-out failure period. When subjected to excessive operating voltages or excessive ripple currents, capacitors will show internal heat rise, and the internal pressure increases will accelerate evaporation of the electrolyte, allowing open circuits to take place. If halide ions come in contact with the capacitor elements, they will corrode foils and ter-minals causing an open circuit condition. The speed of this phenomenon is dependent on temperature and applied voltage. In this failure mode, an open circuit is usually preceded by gradual capacitance drop and tan d increase.
Capacitance Drop/Tan d Rise
In service, the electrolyte gradually evaporates through the seal, resulting in capacitance drop and tan d rise. These changes are accelerated by high ambient temperatures and ripple current. These phenomena are prominent during the wear-out failure period.
Leakage Current Increase
Capacitors in service are free from leakage current in-creases because of the continuous reformation of the dielectric with the applied voltage. However, capacitors stored for long periods of time without voltage applied show small leakage current increases which are allowable for normal applications. For applications in elec-tronic devices such as audio equipment, time constant circuits, etc., the use of specially designed, low leakage capacitors are recommended.
Reverse voltage and AC current are the major causes of an open vent failure. The safety vent is designed to open when abnormal internal pressure increases or heat rises occur due to excessive operating voltage, ripple current or when any other abnormal operating conditions exist.
Capacitors in normal service are free from electrolyte leakage. Electrolyte leakage is caused by mechanical factors such as lead stress or chemical deterioration of the seal. Electrical factors like excessive operating voltage, excessive ripple current, reverse voltage and AC current which cause internal heat rise and pressure in-creases may also cause electrolyte leakage.