Capacitors are important components used in many electronic circuits to store electric charge. However, like any component, capacitors can fail over time. Understanding the common causes of capacitor failure can help engineers design more robust systems and perform predictive maintenance.
How do capacitors work?
A capacitor consists of two conductive plates separated by an insulating material called a dielectric. When voltage is applied across the plates, electric charge accumulates on the plates to store energy in the electric field between the plates. The amount of charge a capacitor can store depends on the capacitance, which is determined by the plate area, plate separation distance, and properties of the dielectric material. Capacitors resist changes in voltage based on their capacitance and the current flowing into or out of them.
Leaking dielectric
One of the most common causes of capacitor failure is a compromised dielectric that allows current to leak between the plates. The dielectric insulates the two plates to prevent internal short circuits. However, damage or wear over time can cause the dielectric to break down and conduct current. This allows charge to leak off the plates, reducing capacitance and energy storage. Eventually, the capacitor will fail as an effective charge storage device. Some common ways the dielectric can become damaged include:
- Overvoltage – Applying too much voltage across the capacitor can exceed the dielectric strength, causing it to break down and conduct.
- Electrical treeing – Tiny fissures in the dielectric develop into branch-like trees that degrade insulation.
- Thermal stresses – Heat causes the dielectric to degrade, become brittle, melt, or react chemically.
- Physical cracks and punctures – Mechanical damage like cracks or holes in the dielectric allow charge leakage.
- Contamination – Impurities in the dielectric create sites for conduction and lowered resistance.
Dried out electrolyte
Electrolytic capacitors use a metallic conducting liquid electrolyte as one of the plates. However, the electrolyte can dry out over time, causing increased equivalent series resistance (ESR) and power loss. Higher ESR decreases the ripple current handling capability. Eventually, the dried out electrolyte will cause the capacitor to fail as charge can no longer move in and out of the plates. This drying out process accelerates at higher temperatures which speed up evaporation. Electrolytic capacitors are polarized, so reversed voltage bias can also dry out the electrolyte prematurely.
Dielectric absorption
Dielectric absorption occurs when charge is absorbed and stored in the dielectric material itself. After a capacitor has been charged and then discharged, a small amount of residual charge can remain trapped in polarization of the dielectric. This effectively reduces capacitance and introduces delays in timing circuits since it takes time for the trapped charge to dissipate. Dielectric absorption also causes leakage current to flow as the trapped charge discharges after the capacitor is disconnected from a voltage source. Ceramic and solid tantalum capacitors are most prone to dielectric absorption compared to film or electrolytic types.
Temperature effects
Temperature changes can accelerate many failure mechanisms in capacitors. High temperatures increase chemical and electrochemical reactions including electrolyte evaporation, dielectric breakdown, and dielectric absorption. Thermal cycling and mechanical stresses induced by thermal expansion and contraction can also damage the dielectric over time leading to leakage current or short circuits. Every 10°C increase in temperature typically halves the expected lifetime of a capacitor. Capacitors should be operated well below their maximum temperature rating to avoid premature failure.
Poor manufacturing
Defects introduced in manufacturing can lead to early capacitor failure. Common defects include:
- Contamination – Impurities or moisture left in the dielectric lower insulation resistance.
- Poor sealing – Allows electrolyte to dry out or oxygen and moisture to enter.
- Damaged plates or leads – Can cause short circuit failures.
- Weak dielectric – Insufficient thickness or low dielectric strength leads to early breakdown.
- Unbalanced windings – Uneven plate area stresses localized regions leading to hot spots.
Ripple current and voltage
Capacitors have ripple current and voltage ratings that if exceeded, can overheat and stress the components. High ripple currents heat up the equivalent series resistance (ESR) while ripple voltage heats up the dielectric. For electrolytic capacitors, too much ripple current accelerates electrolyte evaporation. Both ripple current and voltage accelerate dielectric absorption and thermal stresses. Keeping ripple current and voltage below component ratings extends capacitor lifetime.
Voltage transients
Voltage spikes above the maximum capacitor voltage rating can puncture and short the dielectric. Lightning strikes, switching loads, or inductive circuit phenomena like reverse EMF can produce damaging voltage transients. Use surge absorbers, voltage clamps, snubbers, or transient voltage suppressors to protect capacitors from transients exceeding their ratings.
Reverse biasing
Reverse biasing occurs when the polarity of the applied voltage is opposite the polar orientation of an electrolytic capacitor. This causes the dielectric oxide layer in aluminum electrolytic capacitors to break down leading to shorts. Tantalum electrolytic capacitors can also be damaged by reverse biasing. Always observe correct polarity orientation for polarized capacitors to prevent reverse biasing.
Mechanical damage
Crushing, bending, or shocking a capacitor can damage the encapsulation, plates, or dielectric leading to failure.Apply appropriate mounting and handling methods to avoid cracking ceramic chip capacitors. Use adequate protective packing materials for larger capacitors that may be more prone to mechanical damage during transport. Avoid dropping capacitors or exposing them to excessive shock and vibration which can compromise solder joints or internal structures.
Soldering damage
Excessive heat during soldering can melt internal separator films or degrade the dielectric. Capacitors may also be damaged by flux impurities contaminating the dielectric. Use proper soldering methods at the correct temperatures and times to join capacitors without damaging them. Allow adequate cooling between soldering steps when heat-sensitive components are involved.
Oxidation and corrosion
Oxidation and corrosion degrades the conductive plates and terminals over time leading to increased ESR and power loss. Aluminum electrolytic capacitors are especially susceptible to corrosion induced failures. Keeping capacitors in a cool, dry environment helps minimize oxidation and corrosion. Hermetically sealed encapsulation also protects against environmental contaminants.
Electrolysis
Electrolysis can corrode and eat away the conductive plates of a capacitor, especially if bias voltages or transient currents cause reversed polarity conditions. Minimize electrolysis failure mechanism by preventing sustained reverse voltages across capacitors.
DC bias and voltage drift
Applying DC bias voltage or experiencing voltage drift over time can accelerate capacitor aging. Polarized film and electrolytic capacitors are most affected, exhibiting shorter lifetimes under DC bias conditions. AC application extends service life for film and aluminum electrolytic capacitors. Keeping voltage variations within specification also improves capacitor reliability.
ESR degradation
As capacitors age, the equivalent series resistance (ESR) gradually increases due to degradation of electrodes and electrolyte. This increases power loss and heat generation leading to thermal runaway failure. ESR degradation over time limits capacitor lifetime. Monitoring ESR allows predicting end of life before total failure.
Dissipation factor changes
The dissipation factor of capacitors indicates dielectric losses and tends to increase over time as capacitors age. Like ESR, monitoring dissipation factor trends allows detecting imperfections and changes before hard failure occurs. Increased dissipation factor signifies deteriorating insulation resistance.
Charge redistribution
After long periods without voltage bias, charge can redistribute across the plates of some capacitor types. This can lead to localized charge buildups that puncture the dielectric when voltage is reapplied. Conditioning reforms the dielectric by applying proper voltage before use to avoid damage from charge redistribution issues.
Passivation layer breakdown
Aluminum electrolytic capacitors contain a thin aluminum oxide passivation layer that can break down over time, especially under reverse bias conditions. This leads to increased leakage current. Tantalum capacitors can also suffer dielectric layer breakdown, accelerated by thermal stresses and voltage transients.
Dendrite formation
Electrolytic capacitors slowly accumulate metallic debris and growth called dendrites through electrochemical reactions between the electrolyte and electrodes. Over many years, this conductive buildup eventually shorts the dielectric. Higher voltages and temperatures accelerate dendrite formation.
Dielectric breakdown
The insulating dielectric material has a limited dielectric strength above which conduction rapidly increases leading to hard shorts and failure. Dielectric breakdown is accelerated by voltage transients, voltage reversal, overvoltage, contamination, and thermal stresses. Insufficient dielectric thickness also lowers breakdown strength.
Conclusion
Capacitor failure typically results from aged dielectric allowing current leakage between plates. This can stem from various factors degrading the dielectric over time including overvoltage, high ripple currents, thermal stresses, mechanical damage, and contamination. Poor manufacturing can also introduce defects leading to early failures. Monitoring parameters like ESR and dissipation gives early warning of degradation before total failure. With careful design, component selection, and protective measures, capacitors can achieve long service life in electronic systems.
