What are the reasons for capacitor failure?

Rules to be followed for capacitor selection
1、 Electrical parameter matching
1. Capacity
Filter circuit: It requires a large capacity (such as μ F level) to smooth voltage fluctuations. The formula is C ≥ Δ VIout × Δ t (Iout is the load current, Δ t is the ripple period, and Δ V is the allowable ripple voltage).
High frequency circuit: requires small capacity (such as pF level) to avoid slow response, typical application is the coupling capacitor of RF circuit (0.1pF~1nF).
Energy storage scenario: Select the capacity based on the discharge time constant to ensure sufficient energy is maintained after a power outage.
2. Voltage Rating
Safety margin: The rated voltage should be higher than the maximum operating voltage of the circuit, and it is recommended to leave a margin of 1.5 to 2 times. For example, choose a 25V capacitor for a 12V circuit.
Transient voltage: Consider the surge voltage of the switching power supply, and reserve a margin through oscilloscope measurement or reference to the data manual.
Polarized capacitors: Electrolytic capacitors and other polarized components should pay attention to the positive and negative poles, as reverse connection may cause explosions.
3. Equivalent series resistance (ESR)
Low ESR requirement: For high-frequency filtering (such as switching power input/output), ceramic capacitors or tantalum capacitors are preferred, with ESR as low as a few m Ω.
Power filtering: Aluminum electrolytic capacitors have a high ESR, but the total ESR can be reduced by parallel connection, or solid-state capacitors (ESR<100m Ω) can be selected.
Calculation formula: Power loss P=Irms2 × ESR, ensuring that the heating power is below the allowable value of the capacitor.
4. Temperature Coefficient
High precision circuits: such as oscillators and filters, choose capacitors with low temperature coefficients (such as NPC/C0G, ± 30ppm/℃).
General circuit: X7R (± 15%), X5R (± 15%) are suitable for most scenarios, Y5V (-80%~+30%) is only suitable for temperature stable environments.
2、 Environmental adaptability
1. Operating temperature range
Consumer electronics: Universal grade (-20 ℃~85 ℃), suitable for indoor scenes.
Industrial control: Industrial grade (-40 ℃~105 ℃), suitable for vehicle mounted and industrial control equipment.
Aerospace: High temperature level (-55 ℃~125 ℃), requiring vehicle certification such as AEC-Q200.
2. Moisture resistance and vibration resistance
High humidity environment: Use sulfur resistant electrode materials (such as silver palladium alloy) to avoid silver migration and short circuits.
Vibration scenario: Choose packaging with high mechanical strength (such as through core capacitors), or use spring washer anti loosening structure.
3. Special environment
High vacuum environment: Choose non-volatile materials (gas release rate<1 × 10 ⁻⁹ Pa · m ³/s).
In strong vibration scenarios, metal casing reinforcement (compressive strength>500N) or miniaturized packaging (such as 0402) can be used.
3、 Mechanical Structure and Packaging
1. Packaging form
SMD: Suitable for high-density PCB design, such as 0603 and 0805 packaging, saving space and high reliability.
Direct insertion (THT): Easy to maintain, suitable for high current or manual welding scenarios.
Through core capacitor: designed with a central through-hole, suitable for high-frequency filtering (such as input terminals of switching power supplies), with a rated current of up to 100A.
2. Size and polarity
Polarity identification: For electrolytic capacitors, attention should be paid to the positive and negative poles, pin length, or marking distinction (such as the long leg being positive).
Aperture matching: The aperture of the through-hole capacitor should be 0.3mm larger than the diameter of the wire to avoid mechanical stress.
4、 Cost and Supply Chain
1. Brand selection
Priority given to big brands: such as Murata, KEMET, and Yageo, with stable quality but high costs.
Cost performance balance: Aluminum electrolytic capacitors have low cost and are suitable for scenarios with low performance requirements, such as power filtering.
2. Supply chain management
Multi supplier strategy: Select at least 2 suppliers to avoid the risk of stockouts, while reducing costs through financial bargaining.
Spot reserve: Key capacitors are kept in stock for over 5000 pieces, supporting small-scale trial production (minimum order of 10pcs).
5、 Special application requirements
1. High frequency circuit
Material selection: NPO/C0G ceramic capacitor, temperature coefficient ± 30ppm/℃, self resonant frequency higher than operating frequency (e.g. 10MHz circuit selected from capacitors with resonant frequency of 50MHz).
Layout optimization: capacitor grounding wire length<5cm, reducing lead inductance.
2. Power filter
Combination scheme: Large capacity aluminum electrolytic capacitors (100 μ F~1000 μ F) are connected in parallel with low ESR solid-state capacitors to improve high-frequency filtering effect.
Ripple current: Calculate the effective value of ripple current Irms to ensure that it does not exceed the rated value of the capacitor.
3. Automotive Electronics
Certification requirements: Passed AEC-Q200 certification, temperature level -40 ℃~125 ℃, vibration tolerance meets ISO 16750-3.
Reliability design: Voltage derating to 70%, shell temperature derating to Tmax-20 ℃.
6、 Derating and reliability design
1. Voltage derating
General rule: The working voltage should not exceed 70%~80% of the rated value, for example, a 25V capacitor is used for an 18V circuit.
Transient voltage: Consider 1.5 times the safety margin to avoid long-term operation close to the rated voltage.
2. Temperature derating
Shell temperature control: The working temperature is 20 ℃ lower than the rated temperature, for example, a 105 ℃ capacitor is used in an 85 ℃ environment.
Thermal management: Adding heat dissipation vias or using thermal conductive adhesive in high-density PCBs to reduce capacitor temperature rise.
7、 Testing and maintenance
1. Incoming material inspection
Electrical testing: Use LCR meter to test capacity and ESR, withstand voltage test (2.5 times rated voltage, no breakdown for 60 seconds).
Appearance inspection: The encapsulation layer has no cracks, the pins are not oxidized, and the polarity markings are clear.
2. Regular maintenance
Appearance inspection: Check the appearance of the capacitor every month (such as cracking and leakage), and a crack length<2mm is considered normal.
Performance monitoring: Infrared thermometer detects temperature rise (Δ T<20 ℃), grounding impedance<0.1 Ω.
8、 Summary
The selection of capacitors requires comprehensive consideration of electrical parameters, environmental adaptability, mechanical structure, cost, and supply chain factors, and optimization based on specific application scenarios such as high frequency, power supply, and automotive electronics. The key rules include:
Capacity and voltage matching: leave sufficient safety margin to avoid overvoltage or insufficient capacity.
Temperature and ESR optimization: Select low temperature coefficient capacitors for high-precision circuits and low ESR models for high-frequency circuits.
Packaging and mechanical design: Surface mount packaging is suitable for high-density design, and through-hole capacitors are used for high-frequency filtering.
Brand and cost balance: Big brands ensure quality, cost-effective models reduce costs.
Derating and reliability: voltage and temperature derating design extends capacitor life.
By systematically matching parameters and adapting to the environment, it is possible to ensure the stable operation of capacitors in the circuit and improve the overall system reliability.