Engineering Guide

Why Do Capacitors Fail? Top Causes and Prevention

Capacitors are the most failure-prone component on most circuit boards. Understanding why they fail helps you choose better parts and design more reliable circuits. This guide covers every major failure mechanism — from heat and ESR to ripple current and electrolyte drying.

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The 6 Ways Capacitors Fail

Thermal Stress

Excessive operating temperature

#1 Cause

ESR Rise

Aging increases internal resistance

Gradual

Ripple Current

I²R self-heating from AC current

Power supply

Voltage Stress

Spikes exceeding rated voltage

Sudden

Electrolyte Drying

Slow evaporation over years

Age-related

Manufacturing Defect

Bad electrolyte or oxide layer

Rare

Failure Mechanisms in Detail

We've analyzed thousands of failed units. Here is exactly what happens inside the can when things go wrong.

1. Thermal Stress (Heat)

Heat is the single biggest killer of capacitors. The relationship between temperature and life follows the Arrhenius equation. In our experience, engineers often underestimate ambient temperature rise inside an enclosure.

The 10°C Rule

Every 10°C rise above operating temp halves the life.

The Cold Benefit

Every 10°C below max rating doubles the life.

Operating Temp	Rated 5,000h @ 105°C	Rated 2,000h @ 85°C
105°C / 85°C (max)	5,000 hours	2,000 hours
95°C / 75°C	10,000 hours	4,000 hours
85°C / 65°C	20,000 hours	8,000 hours
75°C / 55°C	40,000 hours	16,000 hours
65°C / 45°C	80,000 hours	32,000 hours
45°C / 25°C	320,000 hours	128,000 hours

Senior Engineer's Note: Don't trust the room thermostat. Measure the temperature right next to the capacitor with a thermocouple. Stagnant air pockets in a chassis can be 15-20°C hotter than the ambient room air.

2. ESR Rise (Aging)

ESR (Equivalent Series Resistance) is the most critical health metric. As electrolyte dries, resistance spikes. This creates a "death spiral": Higher ESR = More Heat = Faster Drying = Higher ESR.

Healthy Capacitor

ESR: 0.05–0.5Ω

Efficient power delivery
Minimal self-heating
Filters ripple effectively

Failing Capacitor

ESR: 2.0–50Ω+

Overheats rapidly
Causes VCC sag / resets
Passes ripple to downstream chips

Pro Tip: Check datasheets for "Tan δ" (Dissipation Factor). While similar to ESR, Tan δ is better for signal loss comparison, while ESR is critical for power handling.

3. Ripple Current Overload

This is the silent killer in power supplies. AC ripple current flowing through the capacitor's internal resistance generates heat (P = I²rms × ESR).

Frequency Multiplier Trap

Datasheet ripple ratings are often at 120Hz. If you are designing a switcher at 100kHz, you must apply the frequency multiplier.

Example: 1000µF cap rated 1.5A @ 120Hz.
Multiplier for 100kHz = 1.5.
True capacity = 1.5A × 1.5 = 2.25A.

Corrective Action: If components are running hot, check your ripple current calculation. Parallel capacitors to share the current load (I/2 means P/4 per cap!).

4. Voltage Stress & Spikes

Voltage ratings are not suggestions. Exceeding them punch-throughs the dielectric.

Electrolytic Failure

Catastrophic. The oxide layer breaks down, gas forms rapidly, and the vent ruptures.

Film/Ceramic Failure

Often "self-healing" (film) or short-circuit (ceramic). Film caps can survive minor spikes by vaporizing the electrode around the fault.

Capacitor Type	Senior Engineer's Derating Rule
Aluminum Electrolytic	Max 80% of rated voltage (e.g., use 50V cap for 40V rail)
Tantalum (Solid)	Max 50% of rated voltage (CRITICAL - prone to fire)
Ceramic (MLCC)	De-rate 50% to avoid DC Bias capacitance loss
Film	Max 80% of rated voltage

5. Electrolyte Drying

Aluminum caps are chemically active devices. The electrolyte will evaporate through the rubber bung effectively putting an expiration date on the part.

Definition of "End of Life"

Users often think failure means "dead." Manufacturers define end-of-life as:
• Capacitance drops by 20%
• ESR increases by 200-300%

6. Counterfeits & Defects

The modern risk isn't just manufacturing defects—it's bad supply chain. Counterfeit capacitors are rampant. They often look identical but lack the high-purity foil and stable electrolyte of the real brand.

Stick to the "Big 5" for critical rails: Nichicon, Rubycon, Panasonic, Nippon Chemi-Con, and Vishay.

Capacitor Failure Prevention Checklist

Derate voltage: Use 50–80% of rated voltage depending on capacitor type

Choose 105°C rated capacitors over 85°C — the life extension is significant

Verify ripple current ratings at your operating frequency, not just 120Hz

Add adequate ventilation and spacing from heat-generating components

Use low-ESR capacitor series in high-ripple applications (SMPS, inverters)

Plan for replacement every 7–10 years in mission-critical equipment (UPS, medical, industrial)

Buy from authorized distributors to avoid counterfeit or defective stock

Test ESR periodically on critical capacitors — rising ESR is the early warning sign

Capacitor Failure FAQ

What causes capacitors to blow?

Capacitors typically blow when internal pressure rises from overvoltage, reverse polarity, severe ripple current heating, or electrolyte breakdown. The built-in safety vent may rupture to release pressure, which appears as a blown or burst capacitor.

What is the most common cause of capacitor failure?

Heat is the number one killer. Every 10°C increase above the rated temperature roughly halves the capacitor's expected life. In most real-world failures, the root cause is excessive operating temperature — from poor ventilation, proximity to hot components, or ripple current self-heating.

Can a capacitor fail without any visible signs?

Yes. Many failures are "parametric" — the capacitor looks fine but its electrical parameters have drifted out of spec. Capacitance may drop 20–40%, ESR may increase 5–10x, or leakage current may rise. These invisible failures cause circuits to malfunction without obvious visual clues.

How long should a capacitor last?

It depends on the type and operating conditions. Aluminum electrolytic capacitors are rated for 2,000 to 10,000 hours at maximum temperature. At lower temperatures, life extends dramatically — at 20°C below max, life roughly doubles. Film and ceramic capacitors can last 100,000+ hours because they have no liquid electrolyte to dry out.

Do ceramic capacitors fail?

Yes, but differently. Ceramic capacitors fail from cracking (mechanical stress, thermal shock, flex), which creates shorts. They don't have electrolyte to dry out, so age-related degradation is much slower. The main risks are board flex during assembly and thermal cycling.

How can I prevent capacitor failure?

The top strategies are: 1) Derate voltage — use 50-80% of rated voltage, 2) Keep temperatures low — improve ventilation, add thermal management, 3) Use appropriate ripple current ratings — don't undersize, 4) Choose 105°C rated caps over 85°C, 5) Select reputable manufacturers, 6) Plan for replacement every 7-10 years in critical systems.

Why do capacitors fail in power supplies?

Power supply capacitors face the harshest conditions: high ripple current, high ambient temperature, voltage transients, and continuous operation. The output filter capacitors handle the most ripple current and typically fail first. Input capacitors face voltage spikes from the mains. Both positions benefit from low-ESR, high-temperature rated capacitors.

What is ESR and why does it matter for failure?

ESR (Equivalent Series Resistance) is the real resistance inside a capacitor. When AC current flows through this resistance, it generates heat (P = I²R). As the capacitor ages, ESR increases, which generates more heat, which accelerates aging — creating a positive feedback loop that leads to failure. Monitoring ESR is the best way to predict upcoming failure.

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