Capacitors Fail for Understandable Reasons

When an aluminum electrolytic capacitor fails in the field, the cause is usually not random but one of a few systematic problems: capacitance loss from a hot life, an ESR rise from aging or a poor grade, leakage from a fault, or a bulge and vent from over-voltage or reverse polarity. This article presents a method for diagnosing the common capacitor problems in CapXon parts, so an engineer can move from symptom to root cause quickly.

Capacitance Loss

A capacitor that measures low capacitance has usually aged faster than expected, because the electrolyte evaporates over time and the rate accelerates with temperature. Check the case temperature the capacitor experiences against the design assumption, and check the ripple current against the rating at that temperature. A capacitor that runs hot, whether from its own ripple heating or from a nearby heat source, loses capacitance far faster than the datasheet endurance suggests. If the loss is much faster than the life estimate, the thermal design or the ripple margin needs to change.

ESR Rise

The ESR rises as the electrolyte dries, so a high ESR is often the first sign of aging and the cause of extra ripple heating. Measure the ESR at the frequency of interest, because the ESR at 100 kHz matters more than at 120 Hz for a switching rail, and compare it with the datasheet. A part with a high ESR heats more for the same ripple, which accelerates the aging further, so replace it with a lower-ESR or larger part.

Overheating

If the capacitor runs hotter than expected, the first check is the ripple current against the rating at the case temperature, and the second is the airflow and the clearing around the can. A blocked airflow, a dense layout or a nearby hot component raises the ambient, and a too-small can runs hotter for the same ripple. Improve the spacing and the airflow, move the capacitor away from the heat source, and consider a bigger can or a lower-ESR part. Keep the ripple within the rating for the temperature.

Leakage and Insulation Problems

An unexpected leakage current usually indicates a fault rather than a normal condition. Check for over-voltage, a transient that exceeded the rating, contamination on the board or a poor solder joint, and confirm the storage and the shelf time, because a long-stored capacitor may need a reforming charge. Measure the leakage after the specified voltage application time, and replace any unit that does not recover.

Bulge and Vent

A bulged can or a vent that has opened is a serious failure and almost always means over-voltage, reverse polarity or a severe over-temperature. Check the applied voltage against the rating including the switching overshoot, verify the polarity of the assembly, and review the inrush and the ripple. Any bulged or vented unit must be removed from service and the cause fixed before the design is released.

A Systematic Method

Work from the simple to the complex: confirm the voltage and the polarity, then the temperature and the ripple, then the ESR and the capacitance, then the layout and the environment. Keep a known-good reference capacitor, measure the case temperature and the ripple of each new design on the bench, and record them so a later change is immediately visible. A design that runs cool and within its ripple rating will meet its endurance, and a design that does not will fail early regardless of the brand.

Getting Help

BeiLuo supplies genuine CapXon capacitors with an import declaration, a certificate of origin and a RoHS compliance file, and our FAE team can help you diagnose a failure and choose a design change that resolves the root cause rather than masking it. Send the symptoms, the measured temperature and ripple and the board photo, and we will help you interpret them and select a part with a longer life for the same envelope.