There are various reasons for capacitor failure, which can be mainly attributed to the following categories:
1、 Electrical stress factors
Overvoltage:
The applied voltage exceeding the rated voltage (WV) of the capacitor may result in:
Dielectric breakdown: The insulation layer is permanently damaged, resulting in a short circuit or a significant increase in leakage current.
Accelerated aging: Long term operation at near or slightly above rated voltage will significantly shorten the lifespan.
Voltage reversal:
Mainly affecting electrolytic capacitors (aluminum electrolysis, tantalum electrolysis). Applying reverse voltage (anode to cathode negative pressure) may result in:
Internal heating: The cathodic oxide layer cannot effectively block reverse current, generating a large amount of heat.
Gas generation: The decomposition of electrolyte produces gas, and an increase in pressure may cause the pressure relief valve to operate or the housing to rupture.
Permanent damage: In severe cases, it can be directly short circuited or open circuited.
Overcurrent/Surge Current:
Instantaneous high currents (such as startup of switching power supplies, sudden load changes, short circuits) may cause:
Internal connection melting: The electrodes, leads, or solder joints inside the capacitor melt due to overheating (open circuit failure).
Overheating damage: The equivalent series resistance generates Joule heating, causing internal material degradation, electrolyte drying, or shell deformation.
High ripple current:
Long term operation at ripple currents higher than the rated value (commonly seen in the output filtering capacitors of switching power supplies):
Continuous heating: The heat generated by ESR accumulates continuously.
Accelerated aging: As the core temperature increases, the evaporation of electrolyte accelerates, and the ESR further increases, forming a vicious cycle that ultimately leads to a decrease in capacity, a sharp increase in ESR, or an open circuit.
2、 Environmental stress factors
High temperature:
Accelerated chemical reaction: It is the main accelerating factor for capacitor failure. Resulting in:
Electrolyte evaporation/drying: The main reason for the decrease in capacity, increase in ESR, and even open circuit of aluminum electrolytic capacitors.
Dielectric aging: The dielectric materials of all types of capacitors age faster at high temperatures, leading to a decrease in insulation performance (increased leakage current) and an increased risk of breakdown.
Aging of sealing materials: Rubber plugs and sealants become ineffective due to aging, losing their sealing function and accelerating the deterioration of internal materials.
Internal pressure increases: chemical reactions accelerate gas production.
Low temperature:
Electrolyte viscosity increase/solidification: The ESR of aluminum electrolytic capacitors increases sharply, and the capacity may temporarily decrease (reversible after temperature recovery).
Material brittleness: Ceramic capacitors and plastic shell capacitors may crack due to mechanical stress (such as PCB bending).
Temperature cycle/shock:
Thermal stress: Different materials have different coefficients of thermal expansion, and repeated and drastic temperature changes can lead to:
Internal structural stress: Stress is generated between electrodes, dielectric layers, and terminal materials, which may lead to delamination and cracking.
Solder joint fatigue: The solder joints of surface mount capacitors crack (open circuit) due to repeated expansion and contraction.
Sealing failure: The interface between the shell and the sealing material cracks due to stress, resulting in the loss of sealing.
High humidity:
Electrochemical migration: Under the action of DC bias and moisture, metal ions (such as silver ions in silver electrodes) migrate and grow on the surface of the medium, forming dendrites, leading to short circuits or increased leakage current (especially affecting ceramic capacitors and some thin film capacitors).
Corrosion: Moisture intrusion can cause corrosion of internal metal electrodes, leads, or external terminals, increasing ESR or leading to open circuits.
Decreased insulation performance: Moisture reduces the surface resistance of the medium.
Mechanical stress:
Vibration/Shock:
Broken leads/internal connections: causing an open circuit.
MLCC Microcracks: Multilayer ceramic capacitors are highly sensitive to PCB bending stress and can easily generate invisible microcracks in the internal ceramic dielectric layer, leading to high resistance, leakage, or intermittent short/open circuits.
PCB bending/deformation: Applying force to the PCB after installation is a common cause of microcracks in MLCC.
Excessive installation stress: Tightening the screws to fix the capacitor may cause deformation of the housing or internal damage.
3、 Material aging and manufacturing defects
Electrolyte drying up:
The inherent failure mechanism of aluminum electrolytic capacitors. High temperature, poor sealing, and long-term storage can accelerate this process. Resulting in a decrease in capacity and an increase in ESR.
Media aging:
All dielectric materials of capacitors will slowly degrade under the action of electric field, temperature, and time, resulting in a decrease in insulation resistance and an increase in losses. For high-K ceramic capacitors, the aging phenomenon is more pronounced (the capacity slowly decreases over time).
Electrochemical corrosion:
Internal materials such as aluminum foil and tantalum anodes undergo corrosion reactions under the influence of impurities, moisture, and electric fields, leading to performance degradation.
Manufacturing defects:
Raw material defects: pinholes in dielectric film, defects in metal foil, impurities in electrolyte, etc.
Process issues: poor winding/stacking, poor welding/connection, inadequate sealing, incomplete cleaning leaving corrosive residues, etc.
Design flaw: Unreasonable internal structural design leading to local overheating or stress concentration.
Virtual welding/cold welding:
Poor soldering results in excessive or intermittent connection resistance between capacitor terminals and PCB pads, manifested as open circuits or poor contacts.
4、 Other factors
Long term storage:
Electrolytic capacitors stored without electricity for a long time may cause degradation of the oxide film, and there may be significant leakage current when they are re energized (requiring "re empowerment" or "aging"). In extreme cases, the seal may fail.
Chemical pollution/corrosive environment:
Exposure to corrosive environments such as acid, alkali, salt spray, etc. can corrode the casing, terminals, or internal materials (if the seal fails).
Radiation:
Under strong radiation conditions such as space and nuclear environments, material properties may undergo irreversible changes.
Summary and prevention key points
Aluminum electrolytic capacitors: the most prone to failure. The main killers are high temperature (causing electrolyte to dry up), high ripple current (causing heating and drying up), overvoltage/back pressure, and long-term storage.
Tantalum electrolytic capacitors are extremely sensitive to overvoltage/reverse voltage and surge current, and are prone to "explosion" failure (often requiring derating for use). I'm also afraid of high temperatures.
Multilayer ceramic capacitors: highly sensitive to mechanical stress (PCB bending, impact) (microcracks). The DC bias effect (capacity decreases with increasing voltage) and aging are also characteristics. Easy to undergo electrochemical migration (under humid and biased conditions).
Thin film capacitors: relatively reliable, but afraid of overvoltage, high temperature, and welding heat damage. Metallized thin films have self-healing properties, but multiple self-healing processes can lead to a decrease in capacity.
Prevention key: correct selection (voltage, capacitance, type, temperature level), reasonable derating use, optimized heat dissipation design, control ripple current, avoid mechanical stress, ensure good welding, and store and use in suitable environments.
Understanding these failure mechanisms is crucial for circuit design, component selection, manufacturing, reliability testing, and fault analysis.