TAJB335016 datasheet(39/51 Pages) AVX

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Figure 4. Correction factor to failure rate F for series resistance

R on basic failure rate FB for a typical component

(60% con. level).

Example calculation

Consider a 12 volt power line. The designer needs about

10µF of capacitance to act as a decoupling capacitor near a

video bandwidth amplifier. Thus the circuit impedance will be

limited only by the output impedance of the board’s power

unit and the track resistance. Let us assume it to be about

2 Ohms minimum, i.e. 0.167 Ohms/Volt. The operating

temperature range is -25°C to +85°C. If a 10µF 16 Volt

capacitor was designed in the operating failure rate would

be as follows.

FT = 1.0 @ 85°C

FR = 0.85 @ 0.167 Ohms/Volt

FU = 0.08 @ applied voltage/rated

voltage = 75%

Thus

FB = 1.0 x 0.85 x 0.08 x 1 = 0.068%/1000 Hours

If the capacitor was changed for a 20 volt capacitor, the

operating failure rate will change as shown.

FU = 0.018 @ applied voltage/rated voltage = 60%

FB = 1.0 x 0.85 x 0.018 x 1 = 0.0153%/1000 Hours

3.2 Dynamic.

As stated in Section 1.2.4, the solid Tantalum capacitor has

a limited ability to withstand voltage and current surges.

Such current surges can cause a capacitor to fail. The

expected failure rate cannot be calculated by a simple

formula as in the case of steady-state reliability. The two

parameters under the control of the circuit design engineer

known to reduce the incidence of failures are derating and

series resistance.

The table below summarizes the results of trials carried out

at AVX with a piece of equipment which has very low series

resistance with no voltage derating applied. That is the

capacitor was tested at its rated voltage.

Results of production scale derating experiment

As can clearly be seen from the results of this experiment,

the more derating applied by the user, the less likely the

probability of a surge failure occurring.

It must be remembered that these results were derived from

a highly accelerated surge test machine, and failure rates in

the low ppm are more likely with the end customer.

A commonly held misconception is that the leakage current

of a Tantalum capacitor can predict the number of failures

which will be seen on a surge screen. This can be disproved

by the results of an experiment carried out at AVX on 47µF

10V surface mount capacitors with different leakage

currents. The results are summarized in the table below.

Leakage current vs number of surge failures

Again, it must be remembered that these results were

derived from a highly accelerated surge test machine,

and failure rates in the low ppm are more likely with the end

customer.

AVX recommended derating table

For further details on surge in Tantalum capacitors refer

to J.A. Gill’s paper “Surge in solid Tantalum capacitors”,

available from AVX offices worldwide.

An added bonus of increasing the derating applied in a

circuit, to improve the ability of the capacitor to withstand

surge conditions, is that the steady-state reliability is

improved by up to an order. Consider the example of a 6.3

volt capacitor being used on a 5 volt rail.

The steady-state reliability of a Tantalum capacitor is affected

by three parameters; temperature, series resistance and

voltage derating. Assume 40°C operation and 0.1

Ohms/Volt series resistance.

Number tested

Number failed surge

Standard leakage range

10,000

0.1 µA to 1µA

Over Catalog limit

10,000

5µA to 50µA

Classified Short Circuit

10,000

50µA to 500µA

Capacitance

Number of

50% derating

No derating

and Voltage

units tested

applied

47µF 16V

1,547,587

0.03%

1.1%

100µF 10V

632,876

0.01%

0.5%

22µF 25V

2,256,258

0.05%

0.3%

Voltage Rail

Working Cap Voltage

3.3

6.3

510

≥24

Series Combinations (11)

Circuit resistance

ohms/volt

3.0

0.07

2.0

0.1

1.0

0.2

0.8

0.3

0.6

0.4

0.6

0.2

0.8

0.1

1.0

Technical Summary and

Application Guidelines

TAJB335016 Datasheet(PDF) 39 Page - AVX Corporation

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