Introduction Capacitors, generally composed of two close, insulated conductors, are essential components of military electronic systems. Electrolytic capacitors constitute a significant proportion of military electronic products, and their reliability plays a crucial role in the overall system. According to statistics from relevant domestic departments, failures caused by capacitor selection and application account for approximately 55-85% of total capacitor failures, while failures due to capacitor quality issues account for approximately 15-45%. These data clearly show that improper selection and use are the primary causes of capacitor failures. Therefore, proper selection and application of electrolytic capacitors are of great importance to ensuring the quality and reliability of military products. 1. Selection of Electrolytic Capacitors In military electronic products, to ensure the reliability of the entire system, the following principles should be followed when selecting electrolytic capacitors: (1) Use components listed in the Qualified List of Military Electronic Components (QPL) as much as possible; (2) Use products from the Preferred List of Components (PPL) as much as possible; (3) Correctly select the quality grade of the components; (4) Use standard and general-purpose components as much as possible, and be cautious in selecting new and non-standard components; (5) When providing a component list, the meaning of the component markings must be clear to avoid purchasing components that do not meet the reliability requirements of the entire system. When selecting capacitors, designers should understand the main technical performance of electrolytic capacitors and compare their capacitance, working voltage, leakage current, loss tangent, temperature characteristics, impedance frequency characteristics, reverse withstand voltage, storage performance, and price. At the same time, attention should be paid to the material used in the capacitor. The most commonly used material is Z5U, which has stable performance, high dielectric constant, large capacitor capacity, and a self-resonant frequency of 1-20MHz; the maximum usable frequency is 50MHz. Another commonly used material is NPO, which has excellent high-frequency characteristics, but low dielectric constant and small capacitance, and is generally not used below 10 MHz. Electrolytic capacitors are divided into two main categories: aluminum electrolytic capacitors and tantalum electrolytic capacitors. Tantalum electrolytic capacitors are further divided into solid tantalum electrolytic capacitors and liquid tantalum capacitors. Each of the three types of capacitors has its own characteristics, and different considerations should be taken into account when selecting them. The advantages of aluminum electrolytic capacitors are that aluminum is widely distributed in my country, is inexpensive, has a large output, is lightweight, and has self-healing properties. Its disadvantages are high dielectric loss, poor insulation performance (high leakage current), large capacitance deviation, and a shelving effect. In addition, they are generally non-hermetically sealed components with aluminum casings, so their reliability is relatively poor. They have a narrow operating temperature range, especially poor negative temperature characteristics, generally -20℃ (some varieties can reach -40℃, and the Chinese military standard CDK series is -55℃). They are suitable for general civilian electronic products with low environmental requirements and some ground military electronic devices with low reliability requirements. Compared to aluminum electrolytic capacitors, tantalum electrolytic capacitors have advantages such as a wider ambient temperature range, typically -55℃ to +125℃; the insulation resistance of the tantalum oxide film is twice that of the aluminum oxide film, resulting in lower leakage current; within the operating voltage range of semiconductor devices (below 150V), tantalum capacitors of the same capacitance are twice the size of aluminum electrolytic capacitors, and also offer advantages such as lower losses, more stable performance, longer lifespan, and higher reliability. Tantalum electrolytic capacitors are suitable for military electronic equipment, but their high price limits their usage. Because the CA42 tantalum electrolytic capacitor uses epoxy resin encapsulation, the price of solid tantalum capacitors is significantly reduced, leading to its adoption in some civilian applications. However, since the CA42 epoxy resin solid tantalum capacitor is a non-sealed component, it is unsuitable for use in harsh environments like military equipment; therefore, fully sealed tantalum electrolytic capacitors should be used in military products. Electrolytic capacitors suffer from high dielectric loss, large capacitance error, and poor insulation performance, making them suitable only for filtering in 50-100 Hz electronic equipment power supplies, as well as for bypassing and coupling in low-frequency circuits, or in low-frequency circuits where capacitance error requirements are not high. Liquid tantalum capacitors have low leakage current and a particularly large product of capacitance per unit volume and operating voltage, making them especially suitable for large-capacity circuits under medium to high voltage conditions. However, these capacitors contain an acidic liquid; leakage can cause short circuits in the printed circuit board, leading to serious malfunctions. Therefore, liquid tantalum capacitors are prohibited in military electronic equipment. However, with technological advancements, some units have largely solved the sealing problem of liquid tantalum capacitors, and these capacitors now meet national military standards, achieving a reliability level of VI. Currently, they are being used in aerospace products, and other military products can refer to this. However, liquid tantalum capacitors must undergo leak testing and sealing before installation, and a significant derating must be incorporated into the design to ensure reliability. 2. Reliability Application of Electrolytic Capacitors 2.1 Appropriately Reducing the Operating Voltage Reducing the operating voltage of electrolytic capacitors is the most effective way to extend their lifespan and improve their reliability. This is because the failure rate of an electrolytic capacitor is directly proportional to the square of the ratio of the applied voltage to the capacitor's rated voltage. Electrolytic capacitors are most commonly used in power supply filtering circuits. Changes in input voltage or a sudden open circuit in the load will cause changes in the voltage across the capacitor. Without derating, the capacitor is likely to break down. Furthermore, the AC voltage entering from the input terminal is not a sinusoidal voltage; generally, the peak voltage of a non-sinusoidal voltage is higher than that of a sinusoidal voltage, which can significantly impact the capacitor's lifespan and reliability. Therefore, a significant derating of the electrolytic capacitor's operating voltage is necessary during design and use. The derating range depends on the overall reliability requirements of the device and the specific circuit in which the capacitor is used. Generally, it can be divided into three levels: Level 1 is 50% of the rated voltage; Level 2 is 60%; and Level 3 is 70%. High-voltage, high-capacity capacitors should have a larger derating range. The larger the capacitance of a capacitor, the larger the oxide film area, the greater the probability of dielectric defects, and the lower the reliability. 2.2 Fully Consider Ripple Voltage Electrolytic capacitors generally have positive and negative terminals. When used in pulsating circuits with both DC and AC voltage components, their operating characteristics require special attention. Electrolytic capacitors must meet the specified voltage polarity requirements across their terminals during use. When used in interstage coupling or pulse circuits, the DC voltage applied to the capacitor is superimposed with the amplitude of the AC voltage component. In some cases, the negative peak voltage of the AC component may exceed the positive DC voltage, causing the polarized capacitor to operate in reverse. This will cause a sharp increase in leakage current, thereby destroying the forward operating characteristics and causing failure. Therefore, when there is a pulsating AC component across the capacitor, the sum of the AC peak voltage and the applied DC voltage should not exceed the capacitor's rated operating voltage. This is because the AC component causes a much greater temperature rise in the capacitor than the DC component, so the ripple magnitude must be strictly controlled, generally not exceeding 20% ​​of the capacitor's rated operating voltage. Even for tantalum electrolytic capacitors, it should be controlled within 10%. Because ripple voltage can polarize the electrolyte and significantly affect the loss resistance RS, it is essential to effectively control the peak ripple voltage applied to the capacitor. General technical specifications stipulate that the permissible AC component refers to the allowable value under a power frequency of 50 Hz. If the operating frequency exceeds this condition, it can be calculated using the following formula: Where Ssh is the surface area of ​​the capacitor casing (cm²), t is the allowable temperature rise (°C) under a certain ambient temperature, f is the sinusoidal frequency of the pseudo-ripple voltage (Hz), C is the capacitance (μF), and tgδ is the loss tangent at the actual operating frequency. To ensure reliable capacitor operation, the ripple voltage applied to the capacitor should be less than the peak ripple voltage calculated using the above formula. Since polarized capacitors generally cannot withstand reverse voltage, polarized capacitors are not permitted in polarity-changing or purely AC circuits. Instead, non-polarized tantalum capacitors (such as the CA74 fully sealed solid tantalum capacitor) should be selected. A non-polarized tantalum capacitor is actually made by connecting two polarized tantalum capacitors back-to-back in series, ensuring that one tantalum capacitor is always in a positive polarity state in the AC circuit. 2.3 Operating Frequency of Electrolytic Capacitors Electrolytic capacitors are best suited for power frequency filtering or for bypassing or interstage coupling in low-frequency circuits. Lower circuit impedance generally leads to higher reliability. When an electrolytic capacitor is working, it functions like an electrolytic cell, with one electrode being the electrolyte. Because the resistance of the electrolyte is much higher than that of ordinary metal electrodes, the equivalent series resistance of an electrolytic capacitor is relatively large. Under DC or low-frequency conditions, the equivalent series resistance RS and equivalent inductance L are negligible compared to the dielectric insulation resistance Rp of the actual capacitor. However, as the frequency increases, both the equivalent series resistance RS and equivalent inductance L increase. The increase in equivalent series resistance RS is due to the "skin effect." The equivalent inductance L is caused by a magnetic field that is proportional to the frequency. Generally, as the frequency increases, the capacitive reactance XC decreases, while the inductive reactance XL increases. This indicates that (XC-XL)² will decrease with increasing frequency. When the frequency increases to a certain point (XC-XL)² = 0, the impedance Z = RS. At this point, the capacitor will resonate. This is the resonant point when the capacitor presents pure resistance to the circuit. When the frequency exceeds the capacitor's resonant point, the capacitor effectively becomes an inductor and ceases to function as a capacitor. Therefore, the operating frequency of a typical electrolytic capacitor should not exceed 20 kHz. Since the dielectric constant of most materials decreases significantly with increasing frequency, the allowable AC component becomes very small when the operating frequency exceeds 20 kHz, resulting in severe capacitance loss. In this case, a high-frequency ceramic or mica capacitor can be connected in parallel as the high-frequency path. Electrolytic capacitors, due to their large capacitance, can serve as the low-frequency path, and their capacitance should be more than 100 times greater than that of the high-frequency capacitor. Because most capacitor characteristics are affected by frequency to some extent, the operating frequency should be at least half the resonant frequency. Excessive operating frequency not only wastes a large amount of energy in the circuit but also causes the capacitor core to overheat, affecting its reliability. Generally, electrolytic capacitors should operate below 10 kHz. If the operating frequency exceeds 10 kHz, the effective capacitance will rapidly decrease until the capacitor impedance becomes purely resistive. Generally, within the range of 100 Hz to 100 kHz, the capacitance of solid tantalum capacitors exhibits better frequency characteristics than that of liquid tantalum capacitors. Liquid tantalum capacitors, however, show a 6% decrease in capacitance with frequency within the range of 1 kHz to 3 kHz. When the frequency increases by more than 10 kHz, the decrease can reach approximately 65%. This is primarily determined by the properties of the dielectric material. If electrolytic capacitors cannot meet the usage requirements, four-wire electrolytic capacitors or capacitors with other dielectrics can be used. 2.4 Appropriately reducing the operating ambient temperature: Under the same voltage conditions, the leakage current and losses of electrolytic capacitors increase with increasing temperature. When the temperature rises from room temperature (25℃) to 85℃, the leakage current typically increases threefold. Leakage current and losses are the main causes of capacitor heating. Therefore, reducing the operating ambient temperature of capacitors is a beneficial measure to extend their service life and improve reliability. Generally, the design should be based on this temperature derating requirement. Aluminum electrolytic capacitors should have their rated temperature reduced by 20–40℃; solid tantalum capacitors by 15–25℃; and liquid tantalum capacitors by 15–30℃. Aluminum electrolytic capacitors experience a sharp increase in losses at sub-zero temperatures, so their rated temperature is generally -20℃. 2.5 Prevention of instantaneous high current surges and circuit impedance: During use, as the ambient temperature increases, the leakage current of the capacitor also increases. When a large surge current passes through, the leakage current may experience an "avalanche" phenomenon, damaging the capacitor. To prevent this, the circuit impedance should be increased to ensure the circuit impedance is not less than 3 Ω/V. Otherwise, reliability will be correspondingly reduced. 2.6 Reliability of capacitor installation and soldering: If the capacitor is a recently manufactured product and its solderability meets requirements, tinning pretreatment is generally not required. However, if the storage time is long, tinning treatment is necessary before use. Tinning treatment should be controlled to exceed the 3.2 mm sealing depth specified in the technical specifications, and excessive time or temperature should be avoided to prevent melting of the seal or detachment of the leads from the electrodes. For chip capacitors, highly active and strongly acidic fluxes should be avoided to prevent incomplete cleaning, penetration, corrosion, and diffusion, which could affect product reliability. Simultaneously, the soldering temperature and time for chip capacitors should be controlled (generally 260℃/10 seconds). During installation, capacitors should be kept away from heat-generating components. For larger capacitors, the capacitor leads should not be used for mounting; to prevent breakage of the leads or damage to the seals during vibration or impact, clamping devices should be designed for fixation if necessary. During installation, the marked side of the capacitor should be exposed as much as possible for observation. 3. Conclusion Electrolytic capacitors are one of the most fundamental components of military electronic systems and are irreplaceable electronic components. The selection of electrolytic capacitors must follow the principles of electronic component selection, comprehensively weighing the environmental conditions of the system, electrical performance requirements, size and weight, reliability requirements, and cost. Only by correctly and rationally selecting and applying electrolytic capacitors can the quality and reliability of military electronic products be guaranteed.