Capacitors (like all other human contrivances) eventually fail, either parametrically or catastrophically. Parametric failures are those in which a device continues to function, but has slowly degraded to a point where it no longer meets performance specifications. Catastrophic failures on the other hand, are characterized by abrupt, dramatic shifts in device characteristics that result in out-of-spec behavior, which might include self-disassembly, combustion, incandescence, etc.
Failure by dielectric breakdown is an electrical condition where the insulting properties of a dielectric material are unable to maintain leakage current below a specified level. Frequently resulting from application of voltage in excess of a device’s rated limit or operation outside of specified thermal limits, failures by dielectric breakdown tend to be self-aggravating, low-impedance (short circuit) failures. Consequently, they’re often spectacular though some capacitor types have the ability to endure minor dielectric breakdown events gracefully. Since dielectric breakdown and thermal failures can be either’s cause or effect, categorizing a failure event as due to one or the other can occasionally be difficult.
Thermal failures are those that occur due to excess device temperature. In circumstances where excess temperature causes a dielectric breakdown event, they’re usually short circuit failures. Thermal failures may also be considered a long-term phenomenon, whereby prolonged operation at elevated temperature results in device parameter shifts beyond allowable limits.
Mechanical failures are those in which physical damage to a device is the proximate cause of failure, and may manifest as out-of-spec parameters, short, or open circuits. Frequently encountered with ceramic capacitors, mechanical failures are usually caused during manufacturing and assembly processes, but may also occur in the field due to abuse or poor mechanical design.