Why do Electrolytic Capacitors fail and even Explode?

Several factors are responsible for electrolytic capacitor failures over time during service. Some causes are inherent by nature of capacitor, while others are governed by service conditions. Electrolytic capacitors may aluminium electrolytic, tantalum or niobium types, and present discussion will be limited to aluminium types, as these are the most common.

Aluminium electrolytic capacitors can heat up and ultimately explode if treated badly. Several factors can lead to this end. Aluminium electrolytic capacitors are provided with pressure vents, or a gas release safety mechanism in case of excessive pressure build up inside the container. It is worthwhile to first examine causes of failures of aluminium electrolytic capacitors.

Degradation of oxide layer dielectric

Positive electrode of aluminium electrolytic capacitors is made by formation of an extremely thin oxide layer by electrochemical reaction of electrolyte on aluminium foil by passing current through electrolyte and anode in one direction.

Reversal of current passage will revert the chemical reaction and erode the oxide dielectric layer, exposing the base metal and creating a short circuit (since electrolyte itself is part of cathode). Therefore, connections of electrolytic capacitors must be done by following the polarities mentioned.

Exceeding the terminal voltage limit

The oxide dielectric layer is formed through electrochemical reaction by increasing the formation current up to a desired limit necessary for the rated service voltage. The capacitor cannot withstand any voltage above this limit. An overvoltage will cause damage to this oxide layer, and offer a short-circuit path to current, thereby heating the capacitor and even cause blowing it up.

Reverse Polarity

Reverse polarity voltages can cause poor performance and damage to capacitor. Electrolytic capacitors have a thin nanometer scale oxide layer formed on its anode through chemical reaction of an electrolyte by passing current in one direction. Reversing this direction will actually cause reverse reaction and damage the oxide layer, making the capacitor unserviceable. Hence electrolytic capacitors can only work when a specified polarity is observed during connections and service. In case a reverse voltage is applied / generated across the terminals, it can lead to catastrophic capacitor failure.

Ripples and harmonics

Ripples or harmonics are another major adverse factor. High ripple currents or presence of harmonic currents means high AC currents in capacitor (overlapped on DC base). Loss factor being much higher than that of electrostatic capacitors, this leads to capacitor heating and eventual failure. So it is important to keep ripples under control. Manufacturer specifications often mention permissible ripple content.

Similarly, an increase in frequency of voltage across terminals also causes current increase, leading to catastrophic failure. An overvoltage on capacitor terminals cannot be accepted for long period, being detrimental to capacitor life. A substandard capacitor will not perform in same way as a genuine one, and will fail very early in life due to various reasons. Storage conditions of capacitor as recommended must be observed, otherwise extreme conditions of heat, humidity or atmospheric salt / chemicals can damage the capacitor.

Storage and weather conditions

Properties of electrolytic capacitor materials, their electrodes, dielectrics and construction (including hermetic sealing and insulation sleeves) contribute to their atmospheric and electrical behaviour. Their storage conditions also affect the dielectric layer, and a capacitor lying idle for long periods of a year or more has to be reconditioned before being put to service.

One of the main causes of capacitor failures over life is the slow evaporation of electrolyte over time, made worse by any increased temperature. The evaporation increases ESR of capacitor, and reduces its value. This leads to localized heating inside capacitor, accelerating the degradation. This factor had prompted a few companies in the past to suggest a ‘preventive maintenance’ to replace capacitor after specifies period of service, particularly in critical equipment.

Protection mechanism and precautions

Vent / gas release mechanism of aluminium capacitor bursts in case of pressure buildup inside the can and gas is released through the same, keeping the environment and personnel safe in case of catastrophic failure of capacitor. It is important to keep the vent free of any obstruction in its mounted condition. If the vent does not operate due to any reason, pressure buildup inside will lead to dangerous explosion, contents of capacitor will be thrown out violently, can cause damage to nearby things / personnel and could even cause fires. Capacitors mounted on PCB must have their vent facing upwards (not on PCB side).

The capacitor should not be mounted in circuit or on PCB near a heat-producing component, else it can fail by overheating. It may be noted that the PVC Insulation sleeve on capacitor is meant to isolate the container electrically from live container, and is used to print capacitor specifications / ratings. The sleeve can soften / melt under excessive heat, and can also get damaged by scratching of surface by sharp objects. Electrical safety from shock is then stands compromised.