Samsung SCLSeries manual Reliability Prediction, Reliability Nomograph

●RELIABILITY PREDICTION

Solid tantalum capacitors exhibit no degration failure mode during shelf storage and show a constantly decreasing failure rate(i.e. , absence of wearout mechanism) during life tests. this failure rate is dependent upon three important application conditions:DCvoltage, temperature, and circuit impedance.

Estimates of these respective effects are provided by the reliability nomograph.(Figure 8.)

The nomograph relates failure rate to voltage and temperature while the table relates failure rate to impedance. These estimates apply to steady-state DC condition, and they assume usage within all other rated conditions.

Standard conditions, which produce a unity failure rate factor, are rated voltage, +85℃, and 0.1 ohm- per-volt impedance.

While voltage and temperature are straight-forward, there is sometimes difficulty in determining impedance. What is required is the circuit impedance seen by the capacitor. If several capacitors are connected in parallel, the impedance seen by each is lowered by the source of energy stored in the other capacitors. Energy is similarly stored in series inductors.

Voltage "de-rating" is a common and useful approach to improved reliability. It can be persued too far, however , when it leads to installation of higher voltage capacitors of much larger size.

It is possible to lose more via higher inherent failure rate than is gained by voltage derating. SAMSUNG typically recommends 50% derating, especially in low impedance circuits.

Failure rate is conventionally expressed in units of percent per thousand hours. As a sample calculation, suppose a particular batch of capacitors has a failure rate of 0.5% / Khr under standard conditions.

What would be the predicted failure rate at 0.7times rated voltage, 60℃ and 0.6Ω/V?

The nomgraph gives a factor of 7 × 10-2and the table gives a factor of 0.4.

The failure rate estimate is then :

0.5× 7 × 10-2 × 0.4

=1.4 × 10-2 or 0.014%/Khr

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	102
	101
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Connect the temperature
and applied voltage ratio	10-1
of interest with a straight
edge. The multiplier of
failure rate is given at the
inersection of this	10-2
line with the model scale.
Given T1&v1 Read Failure
Rate Multiplier F1	10-3
Given T, & F2
Read Reguired Voltage V2
Given F3 & V3
Read Allowable Temp T3	10-4
	10-5

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0.1

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SCL Series

Fig.8 Reliability Nomograph