What are the common failure causes of aluminum electrolytic capacitors?

The common failure causes of aluminum electrolytic capacitors primarily stem from the combined effects of their structural characteristics (liquid electrolyte, aluminum oxide dielectric film, and polarity) and external operating conditions. The following is a systematic classification of failure causes along with explanations of their underlying mechanisms:

1. Electrolyte drying up (the primary cause)

• Mechanism: Aluminum electrolytic capacitors rely on an internal liquid electrolyte to maintain cathode conductivity and the self-healing capability of the oxide film. As usage time extends, the electrolyte gradually evaporates through the sealing rubber—especially under high-temperature conditions (in accordance with the Arrhenius law: for every 10°C increase in temperature, the evaporation rate roughly doubles).

• Consequences:

  • The equivalent series resistance (ESR) has increased significantly;

  • Capacity decline (reduced effective electrode area);

  • The leakage current may first decrease and then increase abnormally.

  • This ultimately leads to a loss of filtering performance and causes the power supply ripple to exceed the specified limit.

II. High-Temperature Accelerated Aging

• Operating at long-term temperatures close to or exceeding the rated upper limit (e.g., 105℃) will:

  • Accelerate electrolyte evaporation;

  • Promotes the degradation of the alumina dielectric film;

  • This causes the sealing material (nitrile rubber) to harden and shrink, leading to leakage.

  • Shortened lifespan (according to the “10℃ rule,” at 125℃, the lifespan may be only 1/16 of the rated value).

3. Ripple Current Overload

• When the actual ripple current exceeds the specification limits:

  • Excessive Joule heat is generated inside the capacitor due to ESR ((P = I_{\text{rms}}^2 × ESR));

  • The temperature continues to rise, creating a vicious cycle of “heat accumulation → ESR increase → increased heating”;

  • This ultimately leads to bulging, bursting, or open circuits.

4. Apply a reverse voltage or an AC voltage.

• Aluminum electrolytic capacitors are polarized components, and their anodic oxide film remains stable only under forward voltage.

• If connected in reverse or subjected to a pure AC voltage:

  • The oxide film is reduced and damaged;

  • The leakage current increases sharply (reaching the mA level);

  • It rapidly heats up and produces gas, often swelling or even exploding within just a few minutes.

V. Overvoltage Operation

• Long-term operating voltage exceeds the rated value (e.g., using a 450V capacitor in a 500V circuit):

  • The oxide film experiences excessive electric field strength, leading to local breakdown.

  • Induce electrochemical decomposition of the electrolyte, producing hydrogen and oxygen;

  • The internal air pressure rises sharply, causing the safety valve to open or the shell to rupture.

6. Leakage caused by seal failure

• Sealing rubber aging (with a lifespan of approximately 5–8 years), mechanical stress, or improper installation (such as horizontal installation affected by gravity) can lead to seal failure.

• After the electrolyte (pH ≈ 3–5, acidic) leaks out:

  • Corrodes PCB copper foil and surrounding components;

  • Cause a secondary short-circuit fault;

  • The capacitor's performance deteriorates sharply due to fluid loss.

7. Manufacturing Process Defects

• Empowering (transforming) defects: Insufficient or uneven oxide film thickness leads to reduced dielectric strength.

• Excessive impurities in aluminum foil (e.g., Fe, Si > 0.01%): Increases leakage current;

• Uneven winding tension: Causes internal microshorts or poor contact;

• The sealing ring is not properly assembled: There is a potential leakage risk right from the factory.

8. Environmental and Mechanical Stress

• High-humidity environment: Moisture penetration accelerates corrosion and the hydration reaction of the medium.

• Vibration or shock: Lead breakage and loose internal connections can result in an open circuit.

• Welding thermal shock: Excessive reflow soldering temperature or prolonged soldering time can damage the internal structure.

9. Long-term disuse

• Even when not powered on, the electrolyte will still slowly evaporate.

• The oxide film lacks the electric field required for “self-repair” and may therefore degrade.

• When power is restored, an excessively high initial leakage current may occur, requiring a “boost recovery” procedure (gradual voltage increase).

Summary: Correspondence between Failure Modes and Root Causes

• Capacity degradation and increased ESR → electrolyte drying and high-temperature aging;

• Bulging, bursting → Overpressure, reversed polarity, ripple overload, gas generation;

• Leakage → Seal failure, safety valve activation, and shell corrosion;

• Short circuit → dielectric breakdown, impurity penetration, reverse connection;

• Open circuit → Lead breakage, complete drying of the electrolyte, and disconnection of internal connections.