General Description
Effects of Time – Class 2 ceramic capacitors change
capacitance and dissipation factor with time as well as
temperature, voltage and frequency. This change with time is
known as aging. Aging is caused by a gradual re-alignment
of the crystalline structure of the ceramic and produces an
exponential loss in capacitance and decrease in dissipation
factor versus time. A typical curve of aging rate for semi-
stable ceramics is shown in Figure 4.
Effects of Frequency – Frequency affects capacitance
and impedance characteristics of capacitors. This effect is
much more pronounced in high dielectric constant ceramic
formulation than in low K formulations. AVX’s SpiCalci
software generates impedance, ESR, series inductance,
series resonant frequency and capacitance all as functions
of frequency, temperature and DC bias for standard chip
sizes and styles. It is available free from AVX and can be
downloaded for free from AVX website: www.avx.com.
If a Class 2 ceramic capacitor that has been sitting on the
shelf for a period of time, is heated above its curie point,
1
(125°C for 4 hours or 150°C for ⁄
2
hour will suffice) the part
will de-age and return to its initial capacitance and dissi-
pation factor readings. Because the capacitance changes
rapidly, immediately after de-aging, the basic capacitance
measurements are normally referred to a time period some-
time after the de-aging process. Various manufacturers use
different time bases but the most popular one is one day
or twenty-four hours after “last heat.” Change in the aging
curve can be caused by the application of voltage and
other stresses. The possible changes in capacitance due to
de-aging by heating the unit explain why capacitance changes
are allowed after test, such as temperature cycling, moisture
resistance, etc., in MIL specs. The application of high voltages
such as dielectric withstanding voltages also tends to de-age
capacitors and is why re-reading of capacitance after 12 or 24
hours is allowed in military specifications after dielectric
strength tests have been performed.
Typical Curve of Aging Rate
X7R
+1.5
Effects of Mechanical Stress – High “K” dielectric ceramic
capacitors exhibit some low level piezoelectric reactions
under mechanical stress. As a general statement, the piezo-
electric output is higher, the higher the dielectric constant of
the ceramic. It is desirable to investigate this effect before
using high “K” dielectrics as coupling capacitors in extreme-
ly low level applications.
0
-1.5
-3.0
-4.5
Reliability – Historically ceramic capacitors have been one
of the most reliable types of capacitors in use today.
The approximate formula for the reliability of a ceramic
capacitor is:
Lo
Lt
Vt
X
Tt
Y
-6.0
-7.5
=
ꢄ ꢄ
ꢄV ꢄ
T
o
o
where
1
10
100 1000 10,000 100,000
Hours
Lo = operating life
Lt = test life
Vt = test voltage
Tt = test temperature and
To = operating temperature
in °C
Characteristic Max. Aging Rate %/Decade
None
2
C0G (NP0)
X7R, X5R
Vo = operating voltage
X,Y = see text
Figure 4
Historically for ceramic capacitors exponent X has been
considered as 3. The exponent Y for temperature effects
typically tends to run about 8.
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