General Description
tends to de-age capacitors and is why re-reading of capac-
itance after 12 or 24 hours is allowed in military specifica-
tions after dielectric strength tests have been performed.
Effects of Mechanical Stress –
High “K” dielectric
ceramic capacitors exhibit some low level piezoelectric
reactions under mechanical stress. As a general statement,
the piezoelectric 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 capaci-
tors in extremely low level applications.
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:
L
o
=
L
t
V
t
V
o
X
Typical Curve of Aging Rate
X7R Dielectric
+1.5
0
Capacitance Change Percent
-1.5
T
t
T
o
Y
-3.0
-4.5
-6.0
-7.5
1
10
100
1000 10,000 100,000
Hours
where
L
o
= operating life
L
t
= test life
V
t
= test voltage
V
o
= operating voltage
T
t
= test temperature and
T
o
= operating temperature
in °C
X,Y
= see text
Historically for ceramic capacitors exponent X has been
considered as 3. The exponent Y for temperature effects
typically tends to run about 8.
A capacitor is a component which is capable of storing
electrical energy. It consists of two conductive plates (elec-
trodes) separated by insulating material which is called the
dielectric. A typical formula for determining capacitance is:
Characteristic
C0G (NP0)
X7R
Z5U
Y5V
Max. Aging Rate %/Decade
None
2
3
5
Figure 6
Effects of Frequency –
Frequency affects capacitance
and impedance characteristics of capacitors. This effect is
much more pronounced in high dielectric constant ceramic
formulation that is low K formulations. AVX’s SpiCap soft-
ware generates impedance, ESR, series inductance, series
resonant frequency and capacitance all as functions of fre-
quency, temperature and DC bias for standard chip sizes
and styles. It is available free from AVX.
C = .224 KA
t
C
= capacitance (picofarads)
K
= dielectric constant (Vacuum = 1)
A
= area in square inches
t
= separation between the plates in inches
(thickness of dielectric)
.224
= conversion constant
(.0884 for metric system in cm)
Capacitance –
The standard unit of capacitance is the
farad. A capacitor has a capacitance of 1 farad when 1
coulomb charges it to 1 volt. One farad is a very large unit
and most capacitors have values in the micro (10
-6
), nano
(10
-9
) or pico (10
-12
) farad level.
Dielectric Constant –
In the formula for capacitance given
above the dielectric constant of a vacuum is arbitrarily cho-
sen as the number 1. Dielectric constants of other materials
are then compared to the dielectric constant of a vacuum.
Dielectric Thickness –
Capacitance is indirectly propor-
tional to the separation between electrodes. Lower voltage
requirements mean thinner dielectrics and greater capaci-
tance per volume.
Area –
Capacitance is directly proportional to the area of
the electrodes. Since the other variables in the equation are
usually set by the performance desired, area is the easiest
parameter to modify to obtain a specific capacitance within
a material group.
38
ETC [ ETC ]
