Aluminium electrolytic capacitor — Failure Modes & Failure Rate

Polarised, large-value capacitor used for bulk filtering and energy storage. Failure rate dominated by electrolyte dryout — strongly Arrhenius-temperature-dependent (10 °C halves life). The single most replaced component in long-running power supplies.

λ typical

3.0×10-8 / h

Range

1.0×10-8 – 3.0×10-7

Source

SN 29500-3

Failure modes

Increased ESR (drift)

Root causes: Electrolyte loss through the rubber seal, accelerated by case temperature. Ripple-current heating compounds it.
Detection: ESR measurement in-circuit; ripple-voltage rise on the same DC rail; output-voltage instability under load.
Mitigation: Specify 105 °C (or 125 °C automotive-grade) parts; derate ripple current to ≤70% of rating; oversize the bulk capacitance so end-of-life ESR still meets the system spec.

Open-circuit

Root causes: End-of-life wearout: complete electrolyte dryout, vent activation, terminal corrosion.
Detection: Capacitance drop to near zero; dramatic ripple-voltage rise; visible domed top or vent opening.
Mitigation: Lifetime calculation per Arrhenius and ripple derating; preventive replacement before MTTF for safety-critical rails.

Short-circuit

Root causes: Dielectric breakdown from overvoltage or reverse polarity; mechanical damage to the foil.
Detection: Power-supply current limit trips; smoke; protective fuse opens.
Mitigation: Reverse-polarity protection on the input rail; surge-suppression on switching supplies; fuse the rail at a value below the foil's short-circuit safe-current.

Vent / explosion

Root causes: Severe overvoltage, reverse polarity, or internal short producing rapid gas generation.
Detection: Audible / olfactory event; loss of capacitor function.
Mitigation: Absolute-maximum input rating respected with 50% margin; mechanical separation from operator-accessible surfaces; conformal coating contains debris.

Typical applications

Switch-mode power supply input/output filtering, motor-drive DC bus, audio coupling, hold-up energy storage during input dropouts. Heavily used in any equipment with 50/60 Hz or 100/120 Hz ripple to suppress.

How to model in a fault tree

Treat the electrolytic as a single basic event with the open-circuit and increased-ESR modes combined into a total λ_DU. For redundant rail architectures, layer Beta-factor CCF on top — same batch and same temperature-stress profile is a strong common-cause driver, often invalidating "3 caps in parallel" redundancy claims. If the supply is the single source of power for an ASIL function, model the electrolytic explicitly as a basic event under the top OR.