Samsung SCLSeries - Manual (Page 20)

SCL Series
● 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 -2 and
the table gives a factor of 0.4.
The failure rate estimate is then :
0.5 Ч 7 Ч 10 -2 Ч 0.4
= 1.4 Ч 10-2 or 0.014%/Khr
T F V
Connect the temperature
and applied voltage ratio
of interest with a straight
edge. The multiplier of
failure rate is given at the
inersection of this
line with the model scale.
Given T1&v1 Read Failure
Rate Multiplier F1
Given T, & F2
Read Reguired Voltage V2
Given F3 & V3
Read Allowable Temp T3
120
110
100
90
80
70
60
50
40
30
20
10 2
10 1
10 0
10 -1
10 -2
10 -3
10 -4
10 -5
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Fig.8 Reliability Nomograph