Metallized film capacitors are widely used in power electronics, reactive power compensation, renewable energy systems, and industrial automation due to their excellent self-healing capability, low loss, and high reliability. However, under harsh operating conditions such as high temperature, humidity, overvoltage, and mechanical stress, their performance gradually deteriorates, eventually leading to failure.

 

The common failure mechanisms of metallized film capacitors can generally be classified into four categories: electrochemical corrosion, dielectric breakdown, capacitance degradation, and structural failure. In practical applications, these failures are often driven by multi-physics coupling effects involving electric field, temperature, humidity, and mechanical stress.

 

I,Common Failure Modes and Typical Manifestations

Failures of metallized film capacitors usually involve both electrical parameter abnormalities and physical structural damage.

 

Failure Mode	Typical Manifestation	Impact on Equipment
Capacitance Degradation	Gradual capacitance reduction while remaining within the rated range until sudden failure occurs	Reduced compensation performance, timing errors, oscillation instability
Insulation Failure	Increased leakage current and reduced insulation resistance	Higher thermal loss, increased risk of thermal runaway
Dielectric Breakdown	Dielectric film melting and puncture, forming conductive paths	Short-circuit burnout and complete equipment failure
Structural Failure	Internal fractures, solder joint detachment, package cracking	Open-circuit failure and interruption of current flow

 

II,Core Failure Mechanisms of Metallized Film Capacitors

1. Electrochemical Corrosion and Moisture Ingress

Electrochemical corrosion is one of the primary aging mechanisms in AC filtering and power compensation applications.

 

When the sealing performance of a metallized film capacitor is inadequate, moisture can penetrate into the internal structure, reducing the air breakdown voltage and accelerating ionization between film layers. The ozone generated during this ionization process oxidizes the metallized electrodes (Zn/Al), forming non-conductive oxides such as ZnO and Al₂O₃. As oxidation progresses, the effective electrode area gradually decreases, resulting in continuous capacitance degradation.

 

In environments where relative humidity exceeds 85%, electrochemical migration may also occur inside the metallized layer, forming conductive dendrites that can eventually trigger inter-electrode short circuits.

 

In sulfur-containing or acidic gas environments, the corrosion rate may increase by 3–5 times. Corrosion of terminal tin plating significantly increases contact resistance, leading to overheating and connection failure.

 

Key Effects

• Capacitance degradation
• Reduced insulation resistance
• Terminal overheating
• Short-circuit risk

 

2. Electrical Stress and Repeated Self-Healing Losses

One of the key characteristics of metallized film capacitors is their self-healing capability. When a localized dielectric breakdown occurs, the metallized layer around the fault rapidly vaporizes, isolating the damaged area and allowing the capacitor to continue operating normally.

However, repeated self-healing events gradually consume the effective metallized electrode area, leading to cumulative capacitance reduction and weakened voltage withstand capability.

 

Experimental studies show that:

• Frequent self-healing discharge significantly accelerates capacitance degradation
• Dielectric withstand voltage decreases together with capacitance reduction
• Lower remaining capacitance results in poorer insulation performance

 

3.Overvoltage Effects

Overvoltage is a direct trigger for catastrophic dielectric breakdown.

 

Because capacitor power loss increases approximately with the square of operating voltage, long-term overvoltage operation accelerates dielectric aging and internal heating. Meanwhile, transient surge voltages caused by switching operations or grid disturbances may reach several times the rated voltage, directly puncturing the dielectric layer.

 

According to IEEE research:

When electric field strength reaches 10⁶ V/cm, internal discharge probability rises exponentially with temperature

For every 10 ℃ increase in temperature, the probability of partial discharge approximately doubles

 

Key Effects

• Accelerated self-healing consumption
• Increased internal temperature rise
• Dielectric puncture
• Thermal runaway
• Sudden catastrophic failure

 

4.Multiphysics Coupling Accelerated Aging Mechanisms

Under extreme operating conditions, metallized film capacitor failures are typically caused by coupled interactions between electric field, temperature, humidity, and mechanical stress.

 

4.1. Electric Field–Temperature Coupling

High temperature reduces the dielectric strength and dielectric constant of polypropylene (PP) film, resulting in localized electric field enhancement. The increased electric field further raises internal power dissipation and temperature, creating a positive feedback loop.

This phenomenon produces localized "hot spots," where temperatures may rise to several hundred degrees Celsius, eventually melting the dielectric film and causing catastrophic breakdown.

 

Consequences

• Local thermal concentration
• Partial discharge intensification
• Film melting
• Thermal breakdown failure

 

4.2. Temperature–Mechanical Stress Coupling

The thermal expansion coefficients of aluminum metallization and polypropylene dielectric film differ significantly. During temperature cycling, substantial interfacial shear stress is generated.

 

The stress level may reach up to 50 MPa under repeated thermal cycling conditions. Once the material fatigue limit is exceeded, microcracks form in the metallized layer.

 

At the same time, elevated temperature accelerates:

• Metal diffusion
• Oxidation reactions
• Aluminum oxide layer growth
• The oxidation growth rate roughly triples for every 10 ℃ increase in temperature.

 

Consequences

• Metallization cracking
• Increased ESR
• Reduced electrical conductivity
• Accelerated aging

 

4.3. Mechanical Stress Coupling

Mechanical stress during PCB assembly, transportation, vibration, and installation can also significantly affect capacitor reliability.

PCB bending stress exceeding 2000 microstrain, along with long-term vibration or impact loading, may cause:

• Internal film cracking
• Solder joint fatigue
• Terminal detachment
• Package deformation

 

These mechanical microcracks also become pathways for moisture ingress and corrosion propagation, further accelerating electrochemical aging.

 

Consequences

• Open-circuit failure
• Intermittent electrical contact
• Moisture penetration
• Reduced operational lifetime

 

5.Manufacturing and Process Defects

Manufacturing defects are another important source of early failure in metallized film capacitors.

 

Common process-related defects include:

• Impurities in raw materials
• Uneven metallized layer thickness
• Pinhole defects in dielectric film
• Incomplete vacuum drying and dehumidification
• Poor encapsulation quality

 

These defects create localized electric field concentration points, making partial discharge and dielectric breakdown more likely during operation.

Residual internal moisture introduced during packaging further accelerates corrosion and insulation degradation from the initial stage of service life.

 

Consequences

Early-life failure

Localized dielectric breakdown

Reduced reliability

Shortened service life

 

III,Conclusion

The reliability of metallized film capacitors is strongly influenced by electrical stress, environmental conditions, thermal management, mechanical loading, and manufacturing quality. Among all failure mechanisms, electrochemical corrosion, repeated self-healing consumption, dielectric breakdown, and multiphysics coupling effects are the dominant factors affecting long-term performance and service life.

 

To improve capacitor reliability and operational lifetime, the following measures are critical:

• Enhanced sealing and moisture protection
• Proper thermal management and ventilation
• Overvoltage and harmonic suppression
• Reduced mechanical stress during installation
• High-quality dielectric film manufacturing and encapsulation processes

 

With optimized design, material selection, and environmental protection, metallized film capacitors can achieve significantly improved stability, safety, and operational durability in modern power electronic systems.