Quick answer: The lifetime of a metallized film capacitor is best predicted with a Weibull statistical model combined with three experimentally determined acceleration factors: an Arrhenius law for temperature, an inverse power or exponential law for voltage, and a power law for humidity. For example, a capacitor rated at 50 FIT at 40 °C and half its rated voltage has an expected lifetime of about 2.1 million hours — but at 70 °C and full rated voltage, that drops to roughly 60,000 hours (~7 years).
Capacitors often represent only a small fraction of a system’s cost, yet their failure can cause disproportionate physical and financial damage — from inverter downtime to scrapped appliances. That is why power electronics engineers need reliable lifetime prediction before a capacitor ever enters the field. This guide explains the physics, the math, and the practical steps.
What Does “Lifetime” Mean for a Capacitor?
A capacitor’s lifetime is its time to failure, where failure is defined as the inability to fulfil its specified function. For metallized film capacitors, the most common end-of-life criterion is the capacitance dropping below its tolerance band (e.g., −5% or −10%), usually accompanied by a rising dissipation factor (tan δ).
Failure behaviour follows the classic bathtub curve:
- Early failures (“infancy”) — a rapidly decreasing failure rate caused by design or process weaknesses. Reputable manufacturers screen these out with routine production tests, and they are covered by product warranty. They are not part of the Weibull wear-out model.
- Wear-out failures — after infancy, the failure rate rises with time and follows a predictable statistical law that depends on voltage, temperature, and humidity.
The Weibull Model: The Industry Standard for Lifetime Prediction
The Weibull failure rate is:
λ(t) = p · λ₀ · (λ₀t)^(p−1)
where:
- p = the Weibull shape (slope) parameter,
- λ₀ = the inverse of the time by which 63% of a sample population has failed (constant in time, but dependent on temperature, voltage, and humidity),
- λ(t) = the inverse of the mean time to failure (MTTF). In the wear-out region, λ(t) increases with time.
The probability that a capacitor is still working after time t — the survivor (reliability) function — is:
R(t) = e(−(λ₀t)p)
Multiplying R(t) by the number of capacitors in a batch gives the expected number still functional at time t.
Exponential model = special case. Some manufacturers (e.g., TDK/Epcos) use a simpler exponential model, which is just a Weibull model with p = 1 and a constant failure rate λ(t) = λ₀. To stay accurate, they limit it to a defined “service life,” after which the failure rate starts climbing.
FIT: How Failure Rates Are Quoted
Failure rates are often given in FIT (failures in time): the number of failures per 10⁹ operating hours of one component. 300 FIT equals 3×10⁻⁷ h⁻¹. A manufacturer might specify, for example, 150 FIT within a 100,000-hour lifetime expectancy.
How Reliability Level Changes Expected Lifetime
For the exponential model, the expected lifetime scales with the reliability level you require:
| Reliability (%) | Lifetime (1/λ₀, h) |
|---|---|
| 36.8 | 1.000 |
| 50 | 0.693 |
| 63.2 | 0.500 |
| 80 | 0.223 |
| 90 | 0.105 |
| 95 | 0.051 |
| 98 | 0.020 |
Demanding 95% reliability instead of 63.2% cuts the usable lifetime figure by roughly a factor of ten — a crucial point when interpreting datasheet numbers.
The Three Aging Acceleration Factors
Capacitance loss accelerates with temperature, voltage, and humidity. Each is modelled with its own law, and the parameters (Eₐ, n, m) must be determined experimentally — typically via a test plan at three different levels of each stress.
1. Temperature — Arrhenius Law
t(T) = t_Tn · exp[ (Eₐ/k_B) · (1/T − 1/T_n) ]
A good fit to published manufacturer data (Epcos/Vishay) is obtained with Eₐ/k_B = 7000 K. Between 40 °C and 70 °C, this yields an acceleration factor of about 7.1 — meaning every degree of unnecessary heat costs real service life.
2. Voltage — Inverse Power or Exponential Law
t_U = t_Un · (U/U_n)^(−n) or t_U = t_Un · exp[−α(U − U_n)/U_n]
Between 0.7 and 1.3 × U_n the two laws barely differ; typical values are n = α = 3.5 (Vishay data fits an exponential factor closer to 5). Operating at full rated voltage instead of half accelerates aging by a factor of roughly 5 to 10, depending on technology.
3. Humidity — Power Law
t(RH) = t_Hn · (RH_n/RH)^m
Humidity matters especially for plastic-cased capacitors, because moisture permeates polymers. Inside the component it lowers the dielectric strength of the gas gap (triggering corona demetallization), corrodes the electrodes, raises the loss factor, and reduces insulation resistance.
The Combined Lifetime Equation
All three factors multiply together:
t(T,U,RH) = t_ref · exp[(Eₐ/k_B)(1/T − 1/T_n)] · (U/U_n)^(−n) · (RH_n/RH)^m
Worked example: A capacitor specified at 50 FIT (40 °C, U_n/2, exponential model) has a lifetime expectancy of 2.1×10⁶ h. At 70 °C and U_n, the combined acceleration reduces this to approximately 60,000 hours — about 7 years. Same capacitor, radically different life: the operating conditions decide.
Real-World Lifetime: Use the Mission Profile
No application runs at a single constant stress. The correct engineering method is a two-step process:
- Determine the law parameters (Eₐ, n, m) experimentally.
- Convert each phase of the customer’s mission profile into reference-condition aging weight, then sum the contributions.
Example for a solar inverter:
| Duration (%) | Voltage (U_n) | Temperature (°C) | Converted aging weight @ reference (%) |
|---|---|---|---|
| 50 | 0 | 20 | 5 |
| 30 | 0.8 | 60 | 25 |
| 20 | 1.0 | 90 | 70 |
Notice the asymmetry: the 20% of time spent at maximum voltage and 90 °C consumes 70% of the total aging budget. Designing around average conditions instead of peak conditions is one of the most common lifetime-sizing mistakes.
Verifying Lifetime: Qualification Testing
Theory must be backed by test. The confidence level that a proportion p of capacitors survives is governed by a binomial law. The number of parts that must pass an accelerated test with zero failures is:
n = log(1 − CL) / log(p)
Example: testing 16 capacitors for 2,000 h at 70 °C and 1.4 × U_n with no failures gives 80% confidence (CL = 80%) that at least 90% of the population retains more than 90% of its capacitance — equivalent to 60,000 h at U_n and 40 °C once acceleration factors are applied.
A landmark real-world validation: during the 1990s, Montena Components (now Maxwell Technologies) and French railway operator SNCF qualified oil-impregnated metallized film capacitors for the TGV high-speed train (1800 V d.c., 8 mF input filter bank, 20-year life requirement, ±10% tolerance). Laboratory tests at 70 °C and 280 V/µm reached 10% capacitance loss after 20,000 h (Weibull p = 1.2, λ₀ = 1/120,000 h⁻¹); scaled through the acceleration factors, this supported an expected lifetime of about 2 million hours at 35 °C and 200 V/µm — consistent with the in-service fleet measurements.
7 Practical Ways to Extend Capacitor Lifetime
- Derate the voltage. Operating at 0.7–0.8 × U_n can multiply lifetime several times over.
- Control hot-spot temperature. Cooling and layout matter more than the ambient rating suggests.
- Design for the mission profile, not the average. Peak phases dominate the aging budget.
- Manage humidity. Sealed housings, controlled storage, and proper impregnation (oil or gas) prevent corrosion and corona.
- Respect ripple current limits. Excess current means excess self-heating; use short, wide capacitor elements with many parallel layers for high-power designs.
- Specify the right dielectric. Polypropylene for low losses; PET where volumetric efficiency and temperature headroom justify the higher loss factor.
- Demand test data. Weibull parameters and accelerated-test evidence separate serious suppliers from catalogue resellers.
Frequently Asked Questions
How long do metallized film capacitors last? Typically 100,000 hours or more at rated conditions; a capacitor specified at 50 FIT at 40 °C and half rated voltage corresponds to roughly 2.1 million hours of expected lifetime. Lifetime falls steeply with higher temperature, voltage, and humidity.
What is the end-of-life criterion for a film capacitor? Most commonly, capacitance dropping below its tolerance (e.g., −10%), usually together with an increase in the dissipation factor tan δ.
What is a FIT rating? FIT (“failures in time”) is the number of failures per 10⁹ component-hours. 300 FIT = 3×10⁻⁷ failures per hour.
How much does temperature shorten capacitor life? Per the Arrhenius law with Eₐ/k_B = 7000 K, raising the temperature from 40 °C to 70 °C accelerates aging by a factor of about 7.1.
Why is Weibull better than a simple exponential model? The exponential model assumes a constant failure rate (Weibull p = 1), which only holds within a limited “service life.” The Weibull model captures the rising failure rate of genuine wear-out, giving realistic predictions across the whole life.
Source Quality Capacitors from JGS Technology
Lifetime prediction only works when the capacitor itself is built to a controlled, qualified process. JGS Technology & Manufacturing supplies film capacitors — including DC-Link, snubber, and AC filter types — plus ceramic and electrolytic capacitors, with 25+ years of industry experience, ISO 9001-compliant production, and VDE, UL, and CQC certifications. Our factory-direct model and in-house R&D support help you select and validate the right capacitor for your mission profile.
Need help sizing a DC-Link capacitor for your inverter or power supply? Contact our engineering team for technical support and samples.
