1 Introduction: Definition and Importance of Electrolytic Capacitor Capacitance Failure

Electrolytic capacitors are crucial passive components in electronic circuits, widely used in applications such as power supply filtering, energy storage, and signal coupling. The stability of their capacitance directly impacts the performance and reliability of the entire circuit system. Capacitance failure in electrolytic capacitors typically refers to the capacitance value dropping below 80% of its rated initial value, which is an important technical indicator for determining the end of an electrolytic capacitor's life .

Capacitance failure not only leads to increased power supply ripple and reduced system stability but may also cause equipment restarts, performance degradation, and even serious accidents such as capacitor explosion and fire. Therefore, an in-depth analysis of the mechanisms, causes, and prevention strategies of electrolytic capacitor capacitance failure is of great significance for improving the reliability and service life of electronic equipment. This report will systematically analyze the internal mechanisms and external causes of electrolytic capacitor capacitance failure and propose corresponding diagnostic and prevention solutions.

2 Analysis of Internal Mechanisms of Capacitance Failure

2.1 Electrolyte Drying and Volatilization Mechanism

Electrolyte drying is the primary cause of electrolytic capacitor capacitance failure. Aluminum electrolytic capacitors contain a paper medium impregnated with electrolyte, which serves as the electrical contact between the cathode aluminum foil and the anode aluminum foil oxide layer. Over time, the electrolyte gradually volatilizes and decreases, especially when the internal temperature of the capacitor is high, where the volatilization rate significantly accelerates .

The volatilization rate of the electrolyte has an exponential relationship with temperature. Temperature increase leads to increased viscosity of the electrolyte and reduced ion mobility. When the electrolyte volatilizes to a certain extent, the thickened electrolyte finds it difficult to fully contact the oxide film layer on the rough surface of the corroded aluminum foil, reducing the effective plate area of the aluminum electrolytic capacitor and causing a sharp drop in capacitance . The evaporation of the electrolyte through the seal determines the operating time of the electrolytic capacitor. When the capacitor's electrolyte evaporates to a certain level, the capacitor will ultimately fail, a result accelerated by internal temperature rise .

Sealing quality is a critical factor affecting the volatilization rate of the electrolyte. Defects in the capacitor's sealing structure (such as rubber plug material, aluminum shell waist tightening process, etc.) can accelerate the volatilization of the electrolyte. Studies have shown that capacitors with poor sealing performance may see their capacity drop to below 50% of the initial value after working for several months in a high-temperature environment .

2.2 Chemical Degeneration and Performance Degradation of Electrolyte

The electrolyte undergoes chemical degeneration during long-term operation, leading to a decline in its electrical performance. Components within the electrolyte decompose, polymerize, and undergo other reactions under the influence of electric fields and temperature, increasing its resistivity and reducing its ionization capability. This chemical degeneration directly leads to an increase in the capacitor's Equivalent Series Resistance (ESR) and an increase in the loss tangent value .

Electrochemical processes also consume the electrolyte. Leakage current-induced electrochemical effects consume the electrolyte; the life of an aluminum electrolytic capacitor decreases as the leakage current increases. Leakage current increases with rising temperature: the leakage current at 25°C is less than one-tenth of that at 85°C. Leakage current also increases with applied voltage: the leakage current of a 400V withstand voltage aluminum electrolytic capacitor at rated voltage is about five times that at 90% of the rated voltage .

Furthermore, impurity ion contamination can also accelerate electrolyte degeneration. For example, impurities such as chloride and sulfate ions can corrode the aluminum foil, leading to oxide film damage. In environments containing corrosive gases, such as chemical plants, capacitors are more prone to performance degradation due to gas adsorption .

2.3 Oxide Film Damage and Aluminum Foil Corrosion Mechanism

The anode oxide film of an aluminum electrolytic capacitor, as the dielectric, is crucial for maintaining capacitance. Oxide film damage mainly occurs in the following forms:

Voltage Stress Damage: Overvoltage or reverse voltage can cause local breakdown of the oxide film. Especially after the capacitor has been stored for a long time, the dielectric oxide film may partially degrade, and suddenly applying the rated voltage may damage the oxide film .

Chloride Ion Erosion: Chloride ions cause significant damage to the alumina dielectric film. Over time without being powered on, the leakage current of an electrolytic capacitor increases. When chloride ions invade the capacitor's interior, they locally destroy the oxide film, leading to increased leakage current, further exacerbating oxide film damage .

Mechanical Stress Damage: Mechanical stresses such as vibration and impact may cause damage to the internal structure of the electrolytic capacitor, such as lead breakage or aluminum foil displacement, thereby affecting its performance and lifespan .

Table: Main Capacitance Failure Modes of Electrolytic Capacitors and Their Characteristics

3 External Causes and Accelerating Factors of Capacitance Failure

3.1 Impact of Temperature Stress on Capacitance

Temperature is the most critical external factor affecting the lifespan of electrolytic capacitors. According to the Arrhenius law, for every 10°C increase in temperature, the chemical reaction rate of the electrolytic capacitor approximately doubles, and its lifespan is halved. The internal temperature of an electrolytic capacitor is determined by both the ambient temperature and internal heating. Internal heating mainly comes from the power loss of the ripple current on the Equivalent Series Resistance (ESR), calculated as: P_loss = I_ripple² × ESR .

High-temperature environments accelerate electrolyte volatilization and oxide film aging. When the capacitor core temperature exceeds the rated value, the electrolyte viscosity surges, and the ESR can increase nearly tenfold. The increased equivalent series resistance generates more heat, causing greater volatilization of the electrolyte and creating a thermal runaway phenomenon. For example, for an electrolytic capacitor rated at 105°C, when its maximum core temperature exceeds 125°C, the electrolyte viscosity increases sharply, and the ESR of the electrolyte increases nearly tenfold .

Low-temperature environments also adversely affect capacitor performance. When the temperature decreases, the electrolyte viscosity increases, and ion mobility and conductivity decrease. When the electrolyte freezes, ion mobility becomes very low, resulting in very high resistance. When working in extremely cold regions (generally below -25°C), heating is necessary to ensure the electrolytic capacitor operates at a normal temperature .

3.2 Capacitance Degradation Induced by Electrical Stress

Voltage stress is another important factor leading to capacitance degradation. Overvoltage operation subjects the oxide film to excessive electric field strength, accelerating its aging process. Tests show that a commonly used imported 450V/470μF 105°C general-purpose 2000-hour electrolytic capacitor, when operated at 1.34 times the rated voltage, may leak and emit gas after 2 hours, with the top bursting open .

Ripple current is the main cause of internal heating in electrolytic capacitors. The magnitude of the ripple current directly affects the hot spot temperature inside the electrolytic capacitor. The allowable range of ripple current can be found in the electrolytic capacitor's datasheet. If it exceeds the range, a parallel connection can be used as a solution. In applications like rectification filtering (including high-frequency rectification filtering of switching power supplies) and power supply bypass for electric furnaces, the ripple current causes losses in the ESR, which turn into heat causing the capacitor to heat up .

Reverse voltage and frequent charge-discharge cycles can also accelerate capacitor failure. Even a small reverse voltage can destroy the oxide film insulation layer within minutes, leading to short-circuit failure of the capacitor. In actual circuit design, it should be ensured that the reverse voltage borne by the capacitor is less than 10% of its rated value .

3.3 Frequency Effects and Harmonic Influences

The impedance-frequency characteristic of electrolytic capacitors causes them to exhibit different capacitance characteristics at different frequencies. The resonant frequency is a key parameter determining their filtering performance. For solder lug and screw terminal aluminum electrolytic capacitors, the resonant frequency is between 1.5kHz and 150kHz. If the capacitor is used above the resonant frequency, its external characteristic becomes inductive .

In high-frequency switching power supplies, capacitors withstand not only the fundamental current but also higher harmonic currents. These harmonic currents cause additional power losses. If the current consists of the fundamental frequency and multiple harmonics, the power loss value generated by each harmonic must be calculated, and the results summed to obtain the total loss value. High-frequency harmonic currents, due to the skin effect, increase the effective ESR of the capacitor, leading to more severe heating problems .

Table: Degree of Influence of Different Factors on Electrolytic Capacitor Lifespan

4 Diagnostic Methods and Techniques for Capacitance Failure

4.1 Electrical Performance Testing Techniques

Diagnosing capacitance failure first relies on the accurate measurement of electrical performance parameters. Key parameters include capacitance value, ESR, loss tangent (tanδ), and leakage current. Using an LCR meter to measure the capacitance value at a specified frequency (usually 100Hz/120Hz) can determine the degree of capacity decay. ESR measurement is best performed at high frequency (e.g., 100kHz), as an increase in ESR is the most common and critical sign of failure .

Leakage current testing can reflect the health of the oxide film. Applying the rated working voltage and measuring whether the stabilized leakage current exceeds the standard (usually at the microamp level) can determine the insulation performance of the capacitor. Excessive leakage current often indicates defects or damage in the oxide film .

Online testing technology allows for assessing the capacitor's condition without disassembly. By monitoring the ripple voltage amplitude of the power supply, one can indirectly judge whether the ESR of the filter capacitor has increased. When the ripple voltage increases significantly, it often means the capacitor's ESR has increased notably or the capacitance has decreased .

4.2 Visual Inspection and Physical Analysis

Visual inspection is a simple and effective method to identify early capacitor failure. Common abnormal phenomena include: top bulging/vent opening (excessive internal pressure), bottom swelling, electrolyte leakage (with odor or residue), surface discoloration (brown-yellow, blackened), etc. .

X-ray透视技术 can non-destructively inspect the internal structure of the capacitor. X-ray imaging can observe whether the core winding is neat, whether the lead pins/connection strips are broken, and whether internal voids are too large (suggesting electrolyte drying), etc. This method is effective for batch testing and fault analysis .

解剖分析 is the most direct method for studying capacitor failure mechanisms. After carefully opening the shell, one can check if the electrolyte has dried up (the core appears white, not moist), the color/state of the electrolyte (should be clear; if darkened, cloudy, or with precipitate, it may have deteriorated), whether the electrolyte paper has become brittle, yellowed, or damaged, and whether the aluminum foil has corrosion perforations, etc. .

4.3 Thermal Analysis and Life Prediction

Thermal imaging technology can visually display the temperature distribution of the capacitor during operation, helping to identify overheating capacitors. The hot spot temperature on the capacitor surface correlates with the internal hottest spot temperature and is an important indicator for assessing the capacitor's operating state .

Accelerated life testing is a common method for predicting capacitor life. By increasing the operating temperature or applying over-stress, the capacitor aging process can be accelerated, thus obtaining life data in a relatively short time. According to the life formula Lop = A × 2^((B-Th)/C), where A is the life at the reference temperature, B is the reference temperature value, C is the temperature rise required to halve the capacitor's life, and Th is the hot spot temperature .

Table: Comparison of Electrolytic Capacitor Capacitance Failure Diagnostic Techniques

5 Prevention Strategies and Life Extension Solutions

5.1 Design Selection and Parameter Derating

Correct design selection is the primary link to prevent premature capacitor failure. When selecting an electrolytic capacitor, parameters such as its operating environment temperature, ripple current, and working voltage should be considered, leaving sufficient safety margin. The voltage selection for electrolytic capacitors generally involves a two-level derating, and using them at 80% of the rated value is reasonable .

Life calculation should be performed at the design stage. Based on the life formula, comprehensively consider the actual application's ambient temperature, ripple current, and heat dissipation conditions to calculate the expected life of the capacitor, ensuring it meets the equipment's design life requirements. For example, in a certain bus capacitor design, the capacitor life is required to reach 7 years under 25°C conditions .

Parallel application can reduce the ripple current borne by each capacitor and extend the overall life. At the same time, parallel application can also reduce the equivalent ESR and improve the filtering effect. In high-voltage or high-current applications, multiple capacitors in series or parallel should be considered to improve system reliability .

5.2 Operating Condition Control and Thermal Management

Thermal management is a key measure to extend capacitor life. Capacitors should be kept away from heating components, and forced air cooling should be used if necessary. Research shows that using a bolt mounting method, installing the capacitor on a heat sink with a thermal resistance of 2°C/W and a forced air cooling rate of 2m/s, the resulting capacitor thermal resistance Rth = 2.1°C/W, which is significantly improved compared to natural cooling conditions .

Installation method also affects the heat dissipation effect of the capacitor. Capacitors must be installed correctly to achieve their designed operating life. For example, the RIFA PEH169 series and PEH200 series should be installed vertically upright or horizontally, ensuring the safety vent faces upwards, so that hot electrolyte and vapor can be smoothly discharged from the safety vent in case of capacitor failure .

When capacitors are arranged very compactly, there should be at least a 5mm gap between adjacent capacitors to ensure adequate air flow. When using bolt installation, the control of the nut torque is very important: if it is too loose, the capacitor and the heat sink cannot be in close contact; if it is too tight, the thread may be damaged .

5.3 Regular Testing and Preventive Maintenance

Regular testing can detect potential faults in time to avoid sudden equipment failure. The testing cycle should be determined according to the capacitor's working conditions and importance. For capacitors in critical equipment, it is recommended to perform a comprehensive electrical parameter test at least once a year .

Pre-charging treatment is essential for capacitors that have been stored for a long time. Generally, for frequency conversion devices that have been stored for more than one year, a pre-charging operation is required for the DC bus capacitor before putting them into operation to restore the activity of the capacitor electrolyte. During charging, the voltage should be increased step by step, gradually raising the DC bus voltage to the rated value to prevent excessive current from damaging the capacitor .

The replacement standard should be clearly defined. When the capacitor's capacitance drops below 80% of the nominal value, or the ESR increases to more than twice the initial value, or obvious bulging or leakage appears, the capacitor should be replaced in time. When replacing, attention should be paid to replacing the entire group as much as possible to ensure consistent capacitor characteristics .

6 Conclusion and Outlook

Electrolytic capacitor capacitance failure is a complex issue involving multiple disciplines such as materials science, electrochemistry, and thermodynamics. This report systematically analyzed the internal mechanisms and external causes of capacitance failure and proposed corresponding diagnostic and prevention strategies. Research shows that electrolyte drying is the main cause of capacitance failure, and temperature is the key factor affecting the failure rate .

To improve the reliability of electrolytic capacitors, future research should focus on the following directions: development of new materials, such as high-stability electrolytes, solid electrolytes, etc.; structural optimization, such as improved sealing design, electrode structures with lower ESR; intelligent monitoring, integrating sensors to monitor capacitor status in real-time and achieve predictive maintenance. Through multidisciplinary collaborative innovation, it is expected to significantly improve the life and reliability of electrolytic capacitors to meet the increasing reliability requirements of electronic equipment .

As the "life short board" in electronic equipment, the reliability of electrolytic capacitors is related to the stable operation of the entire system. Through scientific selection, reasonable design, strict control of working conditions, and regular maintenance, it is entirely possible to control the failure rate of capacitors within an acceptable range, ensuring the reliable operation of electronic equipment throughout its full life cycle .