The most important reliability feature of film capacitors is their self-healing capability, i.e. their ability to clear faults (such as pores or impurities in the film) under the influence of a voltage.
The metal coatings, vacuum-deposited directly onto the plastic film, are only 20 ... 50 nm thick. If the dielectric breakdown field strength is exceeded locally at a weak point, a dielectric breakdown occurs. In the breakdown channel, the high temperatures reached (up to 6000 K) transform the dielectric into a highly compressed plasma that forces its way out. The thin metal coating in the vicinity of the channel is totally evaporated by interaction with the plasma, retreating from the breakdown channel. The rapid expansion of the plasma causes it to cool after a few microseconds, thus quenching the discharge before a greater loss of voltage takes place. The insulated region thus resulting around the former faulty area will cause the capacitor to regain its full operation ability.
Figure 1. Schematic of the self-healing area during electrical breakdown
Note: At low voltages, anodic oxidation of the metal coatings leads to an electrochemical self-healing process.
The air inside the microgaps that are normally present in and around the capacitor (e.g., inside and between the films, near the corners) may get ionized, leading to a destructive process called corona effect.
This occurs when the intensity of the electric field in the capacitor exceeds the dielectric rigidity of the air. In these conditions, small corona discharges take place, producing two undesirable effects:
- Removal of the film metallization edges, and thus drop of capacitance.
- Occasionally, structural damages caused by the bombardment of the film with ions and electrons might occur as well. This produces a reduction of the breakdown voltage level of the dielectric, and eventually might give rise to a short-circuit or fire.
The voltage at which this phenomenon starts is called corona starting voltage VCD. Its value is determined, above all, by the internal construction of the capacitors (which determines the field strength at the edges); it also depends, to a lesser extent, on the thickness of the dielectric. This voltage limit can be raised, in particular, by using internal series connection designs. However, if a capacitor is used below VCD, no corona effect will be observed and the corresponding degradation can be neglected.
If a capacitor that has remained charged for a long period of time is submitted to a brief discharge in short-circuit, within seconds or minutes the voltage on the capacitor will tend to recover to a fixed percentage of its original value (typically between 0.01% and 2%). This effect is known as dielectric absorption. The recharging comes from polarization processes in the dielectric material, and is largely independent of the capacitance of a capacitor and the thickness of its dielectric.
The phenomenon has a particularly unfavorable effect in sample and hold applications, in which charges are to be stored for comparison or measuring purposes. Measurements of dielectric absorption are performed to IEC 60384-1.
Under AC regime, buzzing noise is produced in film capacitors by the mechanical vibration of the films, due to the Coulomb force existing between electrodes of opposite polarity. Buzz noise becomes louder if the applied voltage waveform presents distortion and/or high frequency harmonics.
Buzz noise does not affect the capacitor structure, nor its electrical characteristics or reliability.
Cracks (SilverCaps only)
During the fabrication process of stacked-film capacitors, the single layers of metallized film are faced with mechanical strains during the winding, schooping, tempering and sawing of the capacitor. Furthermore, during the soldering process, the local heating-up of a capacitor might force a quick evacuation of the humidity stored between the film layers, producing a similar strain. All these mechanical strains may be released naturally by cracks, which mainly occur at the bottom side of the capacitors, between the leads.
Intense endurance tests of such components have proven that cracks have no influence on the electrical properties or the reliability of the uncoated capacitors (SilverCap), so the occurrence of such cracks is not dangerous for the application.
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