Quick answer: Most metallized film capacitors fail because their capacitance drops below the required tolerance, usually with a rising dissipation factor. The dominant root causes are electrode corrosion from moisture, corona demetallization in gas gaps between films, loss of insulation resistance (with a risk of thermal runaway), overheating from excessive current, and poor self-healing management — which in the worst case can produce melted “chimneys” through the winding, fire, or even explosion.
Understanding these failure modes is essential whether you design power electronics, qualify components, or troubleshoot field returns. This article maps each failure mode to its causes, its measurable effects, and its practical prevention.
How Failures Are Classified
Every capacitor failure traces back to one of three origins:
- Design: dielectric film too thin, insulation distances too small, metallization layer too thick or too thin, conductor wrongly sized.
- Production process: poor mechanical tension control during winding, inadequate drying (excess residual moisture), bad sealing.
- Application conditions: overvoltage, EMI, lightning, excessive temperature, high ambient humidity.
The complication: different causes can produce the same failure mode, which is why proper failure analysis always works backwards from the physical evidence.
The Role of Self-Healing — and Its Limits
Metallized film capacitors operate at much higher electric-field stress than film-foil types thanks to self-healing: when a breakdown punctures the polymer, the resulting current density evaporates the ~100 nm metal layer around the defect, electrically isolating it. If the design is right, the cleared area grows just enough to insulate the defect without damaging the film.
The electrode sheet resistance (Ω/square) is the key design parameter:
- A thicker metallization → lower resistance and lower Joule losses, but higher energy released per self-healing event, causing more damage.
- A thinner metallization → gentler self-healing, but higher series resistance.
A wrong choice of metallization resistance — or poor metallization control during film production — leads to violent self-healing that mechanically damages the film and dumps heat into adjacent layers. Local dielectric strength collapses, breakdown recurs, and chimneys of melted polypropylene can burn through the winding. These conductive channels drop the insulation resistance, leak current generates more heat, internal pressure rises — and in the worst case the result is fire ignition or explosion.
Failure Mode 1: Gradual Capacitance Loss
The most common failure mode of all: capacitance slowly drifts below tolerance, accompanied by increasing tan δ. This is the normal wear-out mechanism, typically occurring after the manufacturer-stated lifetime, and it is the basis of Weibull lifetime modelling (see our companion article on capacitor lifetime calculation). Every self-healing event removes a microscopic area of electrode; cumulative losses, corrosion, and corona accelerate the decline.
Detection: periodic capacitance and tan δ measurement against the specified tolerance.
Failure Mode 2: Electrode Corrosion (Moisture-Driven)
Plastic housings are never perfectly moisture-tight — polymers always show residual permeability. Once moisture reaches the metallized electrode, corrosion slowly thins the metal layer:
- Series resistance rises → dissipation factor increases → more heat is dissipated.
- Higher temperature reduces the dielectric strength → self-healing activity and capacitance loss accelerate.
- The process ends in loss of function.
Causes: insufficient drying during manufacturing, high-permeability housing materials, or a humid installation environment.
Prevention: proper vacuum drying and impregnation (vegetable oil or gas such as SF₆/N₂), hermetic or low-permeability packaging, and humidity control in the application.
Failure Mode 3: Corona Demetallization
When moisture — or poor winding control — lowers the dielectric strength of the gas trapped in gaps between film layers, partial discharges (“corona”) ignite in those gaps. The discharges erode the metallization, especially at electrode edges where the point effect concentrates the electric field, causing rapid capacitance drop. With segmented metallization, corona can also propagate along the non-metallized separation lines.
The critical parameter is the space factor — the ratio of dielectric thickness to total electrode spacing. The larger the gas gap, the more severe the problem. Space factor is notoriously hard to control in the curved regions of flat windings, which is why quality manufacturers:
- use high-performance winding machines with precise mechanical tension control,
- prefer round windings or stacked elements, sometimes winding on a large-diameter wheel and cutting the layers into stacks.
Prevention: tight space-factor control in production; dry, gas- or oil-impregnated elements; conservative field stress at electrode edges.
Failure Mode 4: Insulation Resistance Loss and Thermal Runaway
The third moisture-related failure mode attacks the capacitor’s parallel (insulation) resistance R_p. As R_p falls, leakage current between the electrodes rises. This mode dominates at low frequency, where:
tan δ = 1 / (R_p · ωC)
The danger is its runaway behaviour: lower insulation resistance → more leakage → more heat → higher temperature → further insulation decrease. Left unchecked, the loop ends with melted dielectric and chimney formation.
Prevention: moisture control at every stage, adequate insulation distances, and thermal design margins.
Failure Mode 5: Overheating from Excessive Current
High RMS current heats the capacitor, reducing the breakdown voltage of the dielectric; in severe cases the element melts outright. Current capability is specified through the equivalent series resistance (ESR, R_s) and the dissipation factor at each frequency, linked in the high-frequency domain by:
tan δ = R_s · ωC
Geometry matters: for high-power applications, short capacitor elements reduce the current path length and increase the number of parallel layers, cutting self-heating dramatically. This is why DC-Link capacitors for traction and inverter duty are built wide and short rather than tall and narrow.
Failure Mode Map: Cause → Effect → Consequence
| Failure mode | Typical causes | Measurable effect | End consequence |
|---|---|---|---|
| Capacitance loss | Cumulative self-healing, aging | C drop, tan δ rise | Loss of function |
| Electrode corrosion | Moisture ingress, poor drying | ESR rise, tan δ rise, heating | Accelerated C loss, failure |
| Corona demetallization | Bad space factor, humid gas gap | Fast C drop | Loss of function |
| Insulation resistance loss | Moisture in film, contamination | Leakage current, low-freq tan δ rise | Thermal runaway, melting |
| Current overheating | Ripple over spec, long current path | Hot spot, reduced breakdown voltage | Melting, chimney, fire/explosion |
| Bad self-healing | Wrong metallization resistance | Localized film damage, heat | Chimneys, insulation collapse |
How to Prevent Capacitor Failures: A Checklist
- Specify from the mission profile — voltage, temperature, and humidity peaks, not averages.
- Choose qualified manufacturers with controlled winding tension, vacuum drying, and impregnation processes.
- Verify metallization design appropriate to the field stress (self-healing behaviour vs. ESR).
- Derate voltage and temperature to cut the acceleration factors.
- Protect against transients — lightning, EMI, and surge events are application-side killers.
- Monitor in service: capacitance and tan δ trending catches wear-out before downtime does.
- Run accelerated qualification tests (e.g., per IEC 61071) for critical applications — 16 units × 2,000 h at 70 °C/1.4 × U_n with zero failures demonstrates 80% confidence that 90% of the population keeps >90% capacitance.
Frequently Asked Questions
What is the most common failure mode of metallized film capacitors? Gradual capacitance loss below the specified tolerance, accompanied by an increasing dissipation factor — the normal wear-out mechanism that defines end of life.
Can a film capacitor catch fire or explode? Yes, in the worst case. Poor self-healing management or insulation-resistance thermal runaway can melt conductive chimneys through the winding; the resulting leakage current and internal pressure can lead to fire or explosion. Correct design and derating make this extremely rare.
What is corona demetallization? Partial discharges in gas gaps between film layers that erode the electrode metallization, especially at electrode edges, causing fast capacitance loss. It is driven by moisture and poor space-factor control during winding.
Why does humidity damage film capacitors? Moisture permeates plastic housings and attacks three ways: it corrodes electrodes (raising ESR and tan δ), lowers the gas-gap dielectric strength (triggering corona), and reduces insulation resistance (causing leakage and thermal runaway).
How are early (“infant”) failures handled? They stem from design/process weaknesses and are screened out by manufacturers’ routine tests; they are covered by product warranty and are not part of the Weibull wear-out model.
Failure-Resistant Capacitors from JGS Technology
Failure modes are engineered out at the factory, not in the field. JGS Technology & Manufacturing controls every critical step — film quality, metallization, winding tension, drying, impregnation, and sealing — across our film, DC-Link, ceramic, and electrolytic capacitor ranges. With ISO 9001-compliant production, VDE, UL, and CQC certifications, and 25+ years of manufacturing experience, we supply capacitors built to survive real mission profiles.
Facing field failures or qualifying a new design? Talk to our engineers — we support failure analysis, selection, and OEM customization.