Industrial Electronics

Factors affecting capacitor reliability

13 February 2022
Source: Vishay Interconnectivity/CC BY 2.0

The subject of reliability and practice of reliability engineering is an underappreciated thing! A comprehensive treatise on the formulae and derivation of failure rate estimates for all common types of capacitors is beyond the scope of what can be covered here. However, this will cover the necessary basics and provide sources of detailed information for those who need to dig in further, for the sake of their product’s success.

In an electronic circuit, capacitors, especially those of the electrolytic type, are one of the highest failure rate components, so particular emphasis should be put on their operating conditions. Unacceptably high failure rates will prevail if they are used near, at or over their operating temperature or voltage ratings. “Operating temperature” is the ambient temperature of the capacitor’s environment plus the rise caused by AC ripple current flowing through the equivalent series resistance (ESR) in the capacitor. Calculating the rise caused by ripple current, if necessary, involves the thermal resistance from the core of the capacitor to the ambient air, the ESR and the ripple current. The power dissipated in the capacitor is equal to the ripple current squared times the ESR. To ensure a reliable design, the actual operating voltage and the rated maximum voltage allowed must be compared and analyzed in order to apply optimum derating for reliability, availability and cost considerations.

A quick look at the manufacturer’s specifications for an aluminum electrolytic capacitor may list an unnerving “lifetime” or “life validation test” such as 2,000 or 4,000 hours. This is the time until the capacitance, or ESR, or leakage current will drift a specified amount when the capacitor is used at its maximum rated temperature and voltage. It is not necessarily the time to complete failure.

What is the magnitude of reliability improvement when the voltage and temperature are reduced relative to the maximum ratings? The magnitude can be surprising. Some guidelines and formulae follow. Some manufacturers have reliability calculators on their websites, so it is not usually necessary to use these, but they are given so the reader can see the significant degree of reliability improvement that will be realized by derating.

Aluminum electrolytic

The major failure modes and parameter drift are caused by chemical reactions, so the Arrhenius law applies. In other words, reducing the operating temperature by 10° C will result in a doubling of lifetime. Deviations from this law may occur at lower temperatures and manufacturers’ specifications or application notes will provide this data.

Regarding operating voltage, MIL-HDBK-217 on reliability predictions states that failures decrease by half for each 20% of voltage derating, for example an operating voltage of 40 V when using a capacitor rated 50 V maximum. The effects of temperature and the effects of voltage are interrelated to some extent, and a formula for lifetime prediction is:

LT = LR x (ER/EO) x 2(deltaT/10) Liquid electrolyte

LT = LR x 10(deltaT/2) Polymer electrolyte

Where:
LT = operating life under stated temperature and voltage (capacitor core temperature, not just ambient)
LR = the life at rated limits
ER = rated voltage limit
EO = operating voltage
deltaT = difference between rated operating temperature and capacitor core temperature in C

Solid tantalum electrolytic

Solid tantalum capacitor lifetime is affected by operating voltage to a greater degree than the aluminum electrolytic. Great care should be taken to avoid overvoltage, even short pulses. In most cases tantalums will have a longer lifetime than aluminum electrolytics, given equal capacitance and voltage ratings. They will also be smaller, and considerably more expensive. Aluminum electrolytics are available in much higher voltage ratings than tantalums.

LT = LR x (ER/EO)3 x 2(deltaT/10)


Where:
LT = failure rate under stated temperature and voltage
LR = the failure rate at rated limits
ER = rated voltage limit
EO = operating voltage
deltaT = difference between rated operating temperature and capacitor core temperature in C

Ceramic

Ceramic capacitors normally have a longer lifetime than either of the two technologies above. The effects of voltage and temperature are given by:

L = LR x (ER/EO)3 x (8 x TR/TO)

Where:
L = operating life under stated temperature and voltage
LR = life at rated temperature and voltage limits
ER = rated voltage limit
EO = operating voltage
TR = rated temperature limit
TO = operating temperature in ° C (usually assumed to be same as ambient)

Film

In film capacitors, the effect of reducing temperature is like the aluminum electrolytic, but the effect of voltage derating is much greater.

L = LR x (ER/Eo)7 x 2(deltaT/10)

Where:
L = operating life under stated temperature and voltage
LR = life at rated temperature and voltage
ER = rated voltage limit
Eo = operating voltage
deltaT = difference between rated operating temperature and capacitor core temperature in ° C.

Other factors to consider that affect capacitor reliability include humidity, vibration, thermal shock, storage time and the number of large-swing charge-discharge cycles. Altitude or other air pressure change will change the thermal characteristics of the system. Consult the manufacturer’s specifications, FAQs and application notes. If subject to high humidity, use a conformal coating after studying the characteristics and cost tolerance of the various types (parylene, silicone, urethane, acrylic).

For vibration, normally we are looking at wire leads and SMT connection points breaking under the stress or brittle breaking under the stress of circuit board flexure at resonant nodes. Consider using adhesives to lock down the parts, and capacitor clamps for the larger ones. Secure the circuit board so that it cannot flex or use vibration isolation mounts. For circuits that may be exposed to large and rapid temperature swings, consult the manufacturer’s specifications and thermal shock test data, and consider measures that can limit the slew rate of temperature such as enclosures that have thermal mass and even thermostatically controlled heating elements.

Here is one example of a set of lifetime calculators, from CDE.

https://www.cde.com/tech-center/life-temperature-calculators

A tool for use with ceramic capacitors:

https://www.electronicproducts.com/tools/ceramic-capacitor-life#

For tantalums, from Vishay:

https://www.vishay.com/dt/capacitors/tantalum-reliability-calculator-list/

System failure rate calculation from TDK:

https://product.tdk.com/en/contact/faq/capacitors-0108.html

About the author:

Terry Conrad has performed 32 years of research, product design and technology management. He has been employed in quality and reliability engineering, design engineering, project management and as president in consumer electronics and military acoustic products design and production firms. Terry is currently an independent consultant, primarily in the fields of acoustics, batteries and component engineering. His clients have included General Dynamics, Molex, Lockheed Martin, Thomson-RCA and many small companies. He can be reached at tconrad@sealandinnovations.com

To contact the author of this article, email engineering360editors@globalspec.com


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