Electrolytic capacitors are common components in electronic circuits, and their reliability directly affects the lifespan and stability of the entire electronic equipment. Among various failure modes, leakage is one of the most common and destructive. This article systematically analyzes the causes, mechanisms, hazards, and corresponding preventive measures of electrolytic capacitor leakage.

1. Phenomenon and Hazards of Electrolytic Capacitor Leakage

Electrolytic capacitor leakage refers to the phenomenon where the internal electrolyte seeps or leaks from the casing seal. The leaked electrolyte typically appears light yellow or brown, is somewhat viscous, and has a slightly acidic odor.

The electrolyte is slightly acidic, with a pH value of approximately 3-5. Once leaked, it can severely contaminate and corrode other components and the printed circuit board around the capacitor. This not only causes a decrease in the insulation resistance of the circuit board, leading to signal distortion or circuit short circuits, but may also cause intermittent equipment failures, increasing repair difficulty and costs.

Simultaneously, as the electrolyte leaks and the capacitor interior gradually dries out, it loses the ability to repair the anode oxide film dielectric, leading to capacitor breakdown or failure due to deterioration of electrical parameters. Such secondary failures triggered by leakage often have more severe consequences than the failure of the capacitor itself.

2. Analysis of Main Causes of Leakage

2.1 Sealing Structure Defects

The sealing structure of an electrolytic capacitor is the first line of defense against electrolyte leakage, but its design flaws or process issues often lead to leakage:

• Aging of Sealing Materials: For capacitors using rubber plug seals, the rubber gradually ages and cracks over time (typically 5-8 years), losing its sealing properties. High-temperature environments (>85°C) significantly accelerate this aging process.

• Mechanical Sealing Process Issues: Sealing structures where the aluminum negative foil is clamped between the shell edge and the sealing plate are prone to leakage points at the shell edge. Uneven sealing pressure or mechanical damage during production can also create micro-leakage channels.

• Insufficient Structural Design: Traditional sealing structures may not cope well with rapid internal pressure changes, especially under conditions of high ripple current or overvoltage.

2.2 Improper Installation and Use

Incorrect installation and usage methods significantly increase the risk of leakage:

• Incorrect Installation Method: Manufacturers often specify vertical installation, but some applications use horizontal installation, leading to uneven internal pressure distribution and accelerated seal failure.

• Electrical Stress Overload: When the working voltage has excessive AC components, or if overvoltage or reverse voltage exists, the leakage current increases sharply. The electrolysis process accelerates gas generation, increasing the internal pressure.

• Mechanical Stress Impact: Continuous mechanical vibration in environments like vehicles or industrial equipment can cause fatigue cracking of the sealing structure.

2.3 Environmental Factors

Environmental conditions significantly impact the sealing performance of electrolytic capacitors:

• Temperature Fluctuations: Rapid temperature changes cause different degrees of thermal expansion and contraction of internal components, damaging the integrity of the sealing interface. Low temperatures reduce the elasticity of sealing materials, while high temperatures accelerate chemical aging.

• Humidity and Corrosive Environments: High humidity can accelerate the electrochemical corrosion of metal parts of the seal. Corrosive gases in industrial environments can erode the capacitor casing and sealing interfaces.

3. In-depth Analysis of Leakage Mechanisms

3.1 Electrochemical Processes and Gas Generation

Complex electrochemical phenomena occur during the operation of electrolytic capacitors. When the oxide film dielectric has many defects, or harmful anions such as chloride or sulfate ions are present, the leakage current increases significantly. These harmful ions hinder the repair process of the oxide film, forming local micropores (diameter 10-100nm).

Under the action of the electric field, the electrolyte decomposes: 2H₂O→2H₂↑+O₂↑, generating gas that accumulates inside the shell. As the operating time extends, the internal pressure continues to rise (can reach 5-8 atmospheres). When the pressure exceeds the bearing capacity of the sealing structure, leakage occurs.

3.2 Material Degradation and Interface Failure

The electrolyte itself undergoes chemical changes over time. The volatilization rate of DMF (Dimethylformamide) based electrolyte through sealing gaps follows the Arrhenius equation; the volatilization rate doubles for every 10°C increase in temperature. Simultaneously, the solvent in the electrolyte gradually depletes and volatilizes, increasing the solution's acid value and corroding the oxide film layer.

The mismatch in the coefficient of thermal expansion (CTE) between different materials at the sealing interface (such as aluminum shell, rubber plug, electrolyte) can also cause problems. The significant difference in CTE between aluminum foil (CTE≈23×10⁻⁶/°C) and electrolytic paper (CTE≈5×10⁻⁶/°C) creates mechanical stress at the interface during temperature fluctuations.

4. Leakage Characteristics in Different Application Scenarios

4.1 Consumer Electronics

In consumer electronics like phone chargers and set-top boxes, capacitor leakage often manifests as slow seepage, which is not easy to detect initially, but the corrosive damage to high-density circuit boards is often irreversible. Leakage in these applications is mainly related to high-temperature environments and capacitor quality.

4.2 Industrial and Automotive Electronics

Capacitor leakage in equipment like industrial power supplies and automotive TV/DVD/radio is often closely related to factors such as vibration and temperature cycling. Leakage in these environments can be more sudden and often accompanied by other failure modes like explosion or open circuit.

5. Leakage Detection and Preventive Measures

5.1 Leakage Detection Methods

• Visual Inspection: Use a 10x magnifying glass to observe the state of the pressure relief valve, the degree of shell deformation, and look for traces of electrolyte residue.

• Electrical Parameter Testing: A capacitance drop exceeding 20% (standard requirement is ≤±20%) or an ESR increase exceeding 200% of the initial value can be precursors to leakage.

• Advanced Detection Techniques: X-ray can inspect the internal winding structure and lead connection status; SEM can observe the characteristics of oxide film defects.

5.2 Prevention and Improvement Measures

• Design Level: Adopt an explosion-proof shell structure by adding a fold seam on the upper part of the metal shell. When internal pressure rises, the seam opens, increasing the internal volume and reducing pressure. Optimize the sealing structure design and use composite sealing materials to enhance aging resistance.

• Manufacturing Process: Improve aluminum foil purity to reduce harmful impurities like chloride ions; improve riveting工艺 to avoid burrs damaging the oxide film; strictly control sealing process parameters.

• Application Recommendations: Ensure installation according to specifications, preferably vertical; avoid overvoltage and excessive ripple current; consider additional fixation measures in high-vibration environments; control the operating temperature within the rated range.

6. Typical Case Analysis

A model of automotive DVD equipment reported frequent failures. Analysis found that the filter electrolytic capacitor in the power module was leaking. The root cause was that the capacitor was installed near heating components, and long-term high-temperature operation accelerated the aging of the rubber sealing ring. Simultaneously, the vehicle's vibration environment created micro-cracks at the sealing interface. The solutions included selecting a 105°C high-temperature series capacitor, improving the capacitor installation position to reduce operating temperature, and adding shock absorption fixtures. After implementation, the failure rate dropped significantly.

Conclusion

Electrolytic capacitor leakage is a complex issue involving materials, structure, process, and application environment. By understanding the causes and mechanisms of leakage and adopting targeted prevention and detection measures, the reliability and service life of electronic equipment can be significantly improved. With the development of new materials and processes, the sealing performance of electrolytic capacitors is continuously improving. However, careful design for different application environments and correct usage remain key to preventing leakage.