Multilayer Ceramic Leaded Capacitors

The Capacitor
GENERAL INFORMATION
A capacitor is a component which is capable
of storing electrical energy. It consists of two conductive
plates (electrodes) separated by insulating material which is
called the dielectric. A typical formula for determining
capacitance is:
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.
Potential Change – A capacitor is a reactive
component which reacts against a change in potential
across it. This is shown by the equation for the linear
charge of a capacitor:
Iideal = C dV
dt
where
I = Current
C = Capacitance
dV/dt = Slope of voltage transition across capacitor
Thus an infinite current would be required to instantly
change the potential across a capacitor. The amount of
current a capacitor can “sink” is determined by the
above equation.
Equivalent Circuit – A capacitor, as a practical device,
exhibits not only capacitance but also resistance and
inductance. A simplified schematic for the equivalent
circuit is:
RP
Dielectric Constant – In the formula for capacitance
given above the dielectric constant of a vacuum is
arbitrarily chosen as the number 1. Dielectric constants
of other materials are then compared to the dielectric
constant of a vacuum.
Dielectric Thickness – Capacitance is indirectly proportional to the separation between electrodes. Lower voltage requirements mean thinner dielectrics and greater
capacitance 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.
Energy Stored – The energy which can be stored in a
capacitor is given by the formula:
E = 1⁄2CV2
L
RS
C
C = Capacitance
Rs = Series Resistance
L = Inductance
Rp = Parallel Resistance
Reactance – Since the insulation resistance (Rp)
is normally very high, the total impedance of a capacitor
is:
Z=
where
冑 R + (X - X )
2
S
2
C
L
Z = Total Impedance
Rs = Series Resistance
XC = Capacitive Reactance =
1
2 π fC
XL = Inductive Reactance = 2 π fL
The variation of a capacitor’s impedance with frequency
determines its effectiveness in many applications.
E = energy in joules (watts-sec)
V = applied voltage
C = capacitance in farads
2
Phase Angle – Power Factor and Dissipation Factor are
often confused since they are both measures of the loss
in a capacitor under AC application and are often almost
identical in value. In a “perfect” capacitor the current in
the capacitor will lead the voltage by 90°.
The Capacitor
Insulation Resistance – Insulation Resistance is the
resistance measured across the terminals of a capacitor
and consists principally of the parallel resistance RP shown
in the equivalent circuit. As capacitance values and hence
the area of dielectric increases, the I.R. decreases and
hence the product (C x IR or RC) is often specified in ohm
farads or more commonly megohm microfarads. Leakage
current is determined by dividing the rated voltage by IR
(Ohm’s Law).
I (Ideal)
I (Actual)
Loss
Angle
Phase
Angle
f
V
IR s
In practice the current leads the voltage by some other
phase angle due to the series resistance RS. The complement of this angle is called the loss angle and:
Power Factor (P.F.) = Cos f or Sine Dissipation Factor (D.F.) = tan for small values of the tan and sine are essentially equal
which has led to the common interchangeability of the two
terms in the industry.
Equivalent Series Resistance – The term E.S.R. or
Equivalent Series Resistance combines all losses both
series and parallel in a capacitor at a given frequency so
that the equivalent circuit is reduced to a simple R-C series
connection.
Dielectric Strength – Dielectric Strength is an expression
of the ability of a material to withstand an electrical stress.
Although dielectric strength is ordinarily expressed in volts,
it is actually dependent on the thickness of the dielectric
and thus is also more generically a function of volts/mil.
Dielectric Absorption – A capacitor does not discharge
instantaneously upon application of a short circuit, but
drains gradually after the capacitance proper has been discharged. It is common practice to measure the dielectric
absorption by determining the “reappearing voltage” which
appears across a capacitor at some point in time after it
has been fully discharged under short circuit conditions.
Corona – Corona is the ionization of air or other vapors
which causes them to conduct current. It is especially
prevalent in high voltage units but can occur with low
voltages as well where high voltage gradients occur. The
energy discharged degrades the performance of the
capacitor and can in time cause catastrophic failures.
CERAMIC CAPACITORS
E.S.R.
C
Dissipation Factor
The DF/PF of a capacitor tells what percent of the
apparent power input will turn to heat in the capacitor.
Dissipation Factor = E.S.R. = (2 π fC) (E.S.R.)
XC
The watts loss are:
Watts loss = (2 π fCV2) (D.F.)
Very low values of dissipation factor are expressed as their
reciprocal for convenience. These are called the “Q” or
Quality factor of capacitors.
Multilayer ceramic capacitors are manufactured by mixing
the ceramic powder in an organic binder (slurry) and casting it by one technique or another into thin layers typically
ranging from about 3 mils in thickness down to 1 mil or
thinner.
Metal electrodes are deposited onto the green ceramic
layers which are then stacked to form a laminated
structure. The metal electrodes are arranged so that their
terminations alternate from one edge of the capacitor to
another. Upon sintering at high temperature the part
becomes a monolithic block which can provide extremely
high capacitance values in small mechanical volumes.
Figure 1 shows a pictorial view of a multilayer ceramic
capacitor.
Multilayer ceramic capacitors are available in a wide range of
characteristics, Electronic Industries Association (EIA) and
the military have established categories to help divide the
3
The Capacitor
CERAMIC
LAYER
ELECTRODE
TERMINATE
EDGE
TERMINATE
EDGE
END
TERMINATIONS
ELECTRODES
MARGIN
Figure 1
EIA Temperature Compensating Ceramic temperature
characteristics in accordance with EIA-198.
4
basic characteristics into more easily specified classes. The
basic industry specification for ceramic capacitors is EIA
specification RS-198 and as noted in the general section
it specifies temperature compensating capacitors as Class
1 capacitors. These are specified by the military under
specification MIL-PRF-20. General purpose capacitors with
non-linear temperature coefficients are called Class 2
capacitors by EIA and are specified by the military under
MIL-C-11015 and MIL-PRF-39014. The new high reliability
military specification, MIL-PRF-123 covers both Class 1
and Class 2 dielectrics.
Class 1 – Class 1 capacitors or temperature compensating
capacitors are usually made from mixtures of titanates
where barium titanate is normally not a major part of the
mix. They have predictable temperature coefficients and
in general, do not have an aging characteristic. Thus they
are the most stable capacitor available. Normally the
T.C.s of Class 1 temperature compensating capacitors are
C0G (NP0) (negative-positive 0 ppm/°C). Class 1 extended
temperature compensating capacitors are also manufactured in T.C.s from P100 through N2200.
Class 2 – General purpose ceramic capacitors are called
Class 2 capacitors and have become extremely popular
because of the high capacitance values available in very
small size. Class 2 capacitors are “ferro electric” and vary in
capacitance value under the influence of the environmental
and electrical operating conditions. Class 2 capacitors
are affected by temperature, voltage (both AC and DC),
frequency and time. Temperature effects for Class 2
ceramic capacitors are exhibited as non-linear capacitance
changes with temperature.
The Capacitor
Table 2: MIL and EIA Temperature Stable and General Application Codes
EIA CODE
Percent Capacity Change Over Temperature Range
MIL CODE
Symbol
Temperature Range
A
-55°C to +85°C
B
-55°C to +125°C
C
-55°C to +150°C
Cap. Change
Zero Volts
Cap. Change
Rated Volts
R
+15%, -15%
+15%, -40%
W
+22%, -56%
+22%, -66%
X
+15%, -15%
+15%, -25%
Y
+30%, -70%
+30%, -80%
Z
+20%, -20%
+20%, -30%
Symbol
Temperature characteristic is specified by combining range and change
symbols, for example BR or AW. Specification slash sheets indicate the
characteristic applicable to a given style of capacitor.
In specifying capacitance change with temperature for
Class 2 materials, EIA expresses the capacitance change
over an operating temperature range by a 3 symbol code.
The first symbol represents the cold temperature end of the
temperature range, the second represents the upper limit
of the operating temperature range and the third symbol
represents the capacitance change allowed over the
operating temperature range. Table 2 provides a detailed
explanation of the EIA system.
Effects of Voltage – Variations in voltage affects only the
capacitance and dissipation factor. The application of DC
voltage reduces both the capacitance and dissipation
RS198
Temperature Range
X7
X5
Y5
Z5
-55°C to +125°C
-55°C to +85°C
-30°C to +85°C
+10°C to +85°C
Code
Percent Capacity Change
D
E
F
P
R
S
T
U
V
±3.3%
±4.7%
±7.5%
±10%
±15%
±22%
+22%, -33%
+22%, - 56%
+22%, -82%
EXAMPLE – A capacitor is desired with the capacitance value at 25°C
to increase no more than 7.5% or decrease no more than 7.5% from
-30°C to +85°C. EIA Code will be Y5F.
factor while the application of an AC voltage within a
reasonable range tends to increase both capacitance and
dissipation factor readings. If a high enough AC voltage is
applied, eventually it will reduce capacitance just as a DC
voltage will. Figure 2 shows the effects of AC voltage.
Capacitor specifications specify the AC voltage at which to
measure (normally 0.5 or 1 VAC) and application of the
wrong voltage can cause spurious readings. Figure 3 gives
the voltage coefficient of dissipation factor for various AC
voltages at 1 kilohertz. Applications of different frequencies
will affect the percentage changes versus voltages.
D.F. vs. A.C. Measurement Volts
AVX X7R T.C.
10.0
50
Dissipation Factor Percent
Capacitance Change Percent
Cap. Change vs. A.C. Volts
AVX X7R T.C.
40
30
20
10
0
12.5
Figure 2
25
37.5
Volts AC at 1.0 KHz
Curve 1 - 100 VDC Rated Capacitor
8.0 Curve 2 - 50 VDC Rated Capacitor
Curve 3 - 25 VDC Rated Capacitor
6.0
Curve 3
Curve 2
4.0
2.0
Curve 1
0
.5
1.0
1.5
2.0
2.5
AC Measurement Volts at 1.0 KHz
50
Figure 3
5
The Capacitor
Capacitance Change Percent
Cap. Change vs. D.C. Volts
AVX X7R T.C.
2.5
0
Capacitance Change Percent
1600
1200
800
AVX X7R T.C.
0
-5
-7.5
10
KHz
100
KHz
1
MHz
10
MHz
100
MHz
1
GHz
Frequency
25%
50%
75%
Percent Rated Volts
100%
Effects of Frequency – Frequency affects capacitance and
dissipation factor as shown in Figures 6 and 7.
Variation of impedance with frequency is an important
consideration for decoupling capacitor applications. Lead
length, lead configuration and body size all affect the impedance level over more than ceramic formulation variations.
(Figure 8)
+20
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 semistable ceramics is shown in Figure 9 and a table is given
showing the aging rates of various dielectrics.
+10
0VDC
0
RVDC
-10
-20
-30
-55 -35
-15
+5
+25 +45 +65 +85 +105 +125
Temperature Degrees Centigrade
Figure 5
Cap. Change vs. Frequency
AVX C0G (NP0) T.C.
0
AVX X7R T.C.
-10
-20
1
KHz
10
KHz
100
KHz
1
MHz
Frequency
Figure 6
1
KHz
Figure 7
-10
-30
AVX
C0G (NP0)
T.C.
400
Typical Cap. Change vs. Temperature
AVX X7R T.C.
Capacitance Change Percent
2000
-2.5
Figure 4
6
“Q” vs. Frequency
"Q" Factor
The effect of the application of DC voltage is shown in
Figure 4. The voltage coefficient is more pronounced for
higher K dielectrics. These figures are shown for room temperature conditions. The combination characteristic known
as voltage temperature limits which shows the effects of
rated voltage over the operating temperature range is
shown in Figure 5 for the military BX characteristic.
10
MHz
100
MHz
1
GHz
If a ceramic capacitor that has been sitting on the shelf for a
period of time, is heated above its curie point, (125°C for 4
hours or 150°C for 1⁄2 hour will suffice) the part will
de-age and return to its initial capacitance and dissipation
factor readings. Because the capacitance changes rapidly,
immediately after de-aging, the basic capacitance
measurements are normally referred to a time period sometime 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.
The Capacitor
Typical Curve of Aging Rate
X7R Dielectric
Impedance vs. Frequency
Effect of Capacitance – AVX SpinGuards
+1.5
0
1.00
.001mF
.01mF
0.10
.1mF
.33mF
0.01
1
10
100
Log Frequency, MHz
1000
Capacitance Change Percent
Log Impedance, Ohms
10.00
-1.5
-3.0
-4.5
-6.0
-7.5
Impedance vs. Frequency
Effect of Dielectric – AVX DIPGuards
1
1000 10,000 100,000
Hours
Max. Aging Rate %/Decade
None
2
3
5
Figure 9
.0
R
F
X7
47
.0
F
R
22
X7
Z5U
F
0.1
.22
Log Impedance, Ohms
)
F
1
.0
0
(NP
G F
C0 01 .0
R
X7
R
X7
F
0.1
1
10
100
200
Log Frequency, MHz
Impedance vs. Frequency
Effect of Lead Length – Military CKR05 .01mF
100.0
Log Impedance, Ohms
100
Characteristic
C0G (NP0)
X7R
Z5U
Y5V
10.0
1.0
10
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 capacitors 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:
.500"
.250"
.062"
0"
10.0
Lo
=
Lt
共共
Vt
Vo
X
共共
Tt
To
Y
where
Lo = operating life
T t = test temperature and
To = operating temperature in °C
L t = test life
V t = test voltage
Vo = operating voltage X,Y = see text
1.0
0.1
1
10
100
Log Frequency, MHz
Figure 8
1000
Historically for ceramic capacitors exponent X has been
considered as 3. The exponent Y for temperature effects
typically tends to run about 8.
7
The Capacitor
General Electrical and Environmental Specifications
Many AVX ceramic capacitors are purchased in accordance
with Military Specifications, MIL-PRF-39014, MIL-C-11015,
MIL-PRF-20, MIL-PRF-55681, and MIL-PRF-123 or according
to individual customer specification. When ordered to these
specifications, the parts will meet the requirements set
forth in these documents. The General Electrical and
Environmental Specifications listed below detail test
conditions which are common to the foregoing and to most
ceramic capacitor specifications. If additional information is
needed, AVX Application Engineers are ready to assist you.
Capacitance – Capacitance shall be tested in accordance
with Method 305 of MIL-STD-202.
Class 1 dielectric to 1000 pF measured at 1 MHz, ± 100
KHz, > 1000 pF measured at 1 KHz ± 100 Hz both at 1.0
± 0.2 VAC.
Class 2 dielectrics (except High K) to 100 pF shall be
measured at 1 MHz ± 100 KHz, > 100 pF measured at 1
KHz ± 100 Hz both at 1.0 ± 0.2 VAC.
High K dielectrics measured at 1 KHz ± 100 Hz with less
than 0.5 VAC or less applied.
Dissipation Factor – D.F. shall be measured at the same
frequency and voltage as specified for capacitance.
Dielectric Strength – The dielectric strength shall be
measured in accordance with Method 301 of MIL-STD-202
with a suitable resistor in series with the power supply to
limit the charging current to 50 ma. max.
Insulation Resistance – Insulation Resistance shall be
measured in accordance with Method 302 of MIL-STD-202
with rated voltage or 200 VDC whichever is less applied.
The current shall be limited to 50 ma. max. and the charging
time shall be 2.0 minutes maximum.
Burn-In – (Where specified.) 100% of the parts shall be
subjected to 5 cycles of Thermal Shock per Method 107
Test Condition A of MIL-STD-202 followed by voltage
conditioning at twice rated voltage and maximum rated
temperature for 100 hours or as specified. After Burn-In,
parts shall meet all initial requirements.
Barometric Pressure – Capacitors shall be tested in
accordance with Method 105 of MIL-STD-202 Test
Condition D (100,000 ft.) with 100% rated voltage applied
for 5 seconds with current limited to 50 ma. No evidence of
flashover or damage is permitted.
Solderability – Capacitors shall be tested in accordance
with Method 208 of MIL-STD-202 with 95% coverage of
new solder.
8
Vibration – Capacitors shall be tested in accordance with
Method 208 Test Condition D of MIL-STD-202 with the
bodies rigidly clamped. The specimens shall be tested in 3
mutually perpendicular planes for a total of 8 hours with
125% rated DC voltage applied. No evidence of opens,
intermittents or shorts is permitted.
Shock – Capacitors shall be tested in accordance with
Method 213 Condition 1 (100 Gs) of MIL-STD-202 with the
bodies rigidly clamped. No evidence of opens, intermittents
or shorts is permitted.
Thermal Shock and Immersion – Capacitors shall be
tested in accordance with Method 107 Condition A of
MIL-STD-202 with high test temperature (maximum rated
operating temperature) followed by Method 104 of
MIL-STD-202 Test Condition B.
Moisture Resistance – Capacitors shall be tested in
accordance with Method 106 of MIL-STD-202 with rated
voltage or 100 VDC whichever is less applied for the first 10
cycles.
Resistance to Solder Heat – Capacitors shall be tested in
accordance with Method 210 of MIL-STD-202 with
immersion to .050 of body. AVX Ceralam capacitors are
manufactured with solder which melts at a temperature
greater than 450°F.
General Considerations – The application of voltage or
temperature usually causes temporary changes in the
capacitance of Class 2 ceramic capacitors. These changes
are normally in the positive direction and may cause out-oftolerance capacitance readings. If a capacitance reading is
made immediately after a dielectric strength or insulation
resistance test and parts are high capacitance, they should
be re-read after a minimum wait of 12 hours.
The Capacitor
BASIC CAPACITOR FORMULAS
I. Capacitance (farads)
English: C = .224 K A
TD
.0884
KA
Metric: C =
TD
XI. Equivalent Series Resistance (ohms)
E.S.R. = (D.F.) (Xc) = (D.F.) / (2 π fC)
XII. Power Loss (watts)
Power Loss = (2 π fCV2) (D.F.)
XIII. KVA (Kilowatts)
KVA = 2 π fCV2 x 10 -3
II. Energy stored in capacitors (Joules, watt - sec)
E = 1⁄2 CV2
XIV. Temperature Characteristic (ppm/°C)
T.C. = Ct – C25 x 106
C25 (Tt – 25)
III. Linear charge of a capacitor (Amperes)
dV
I=C
dt
XV. Cap Drift (%)
C1 – C2
C.D. =
C1
IV. Total Impedance of a capacitor (ohms)
Z=
冑R
2
+ (XC - XL )2
V. Capacitive Reactance (ohms)
1
xc =
2 π fC
S
XVI. Reliability of Ceramic Capacitors
Vt
L0
X
Tt
Y
=
Lt
Vo
To
( ) ( )
VI. Inductive Reactance (ohms)
xL = 2 π fL
XVII. Capacitors in Series (current the same)
Any Number:
1 = 1 + 1 --- 1
CT
C1
C2
CN
C1 C2
Two: CT =
C1 + C2
VII. Phase Angles:
Ideal Capacitors: Current leads voltage 90°
Ideal Inductors: Current lags voltage 90°
Ideal Resistors: Current in phase with voltage
XVIII. Capacitors in Parallel (voltage the same)
CT = C1 + C2 --- + CN
VIII. Dissipation Factor (%)
D.F.= tan (loss angle) = E.S.R. = (2 πfC) (E.S.R.)
Xc
IX. Power Factor (%)
P.F. = Sine (loss angle) = Cos (phase angle)
f
P.F. = (when less than 10%) = DF
XIX. Aging Rate
A.R. = %
XX. Decibels
Pico
Nano
Micro
Milli
Deci
Deca
Kilo
Mega
Giga
Tera
X 10-12
X 10-9
X 10-6
X 10-3
X 10-1
X 10+1
X 10+3
X 10+6
X 10+9
X 10+12
D C/decade of time
db = 20 log V1
V2
X. Quality Factor (dimensionless)
Q = Cotan (loss angle) = 1
D.F.
METRIC PREFIXES
x 100
SYMBOLS
K
= Dielectric Constant
f
= frequency
Lt
= Test life
A
= Area
L
= Inductance
Vt
= Test voltage
TD
= Dielectric thickness
= Loss angle
Vo
= Operating voltage
V
= Voltage
f
= Phase angle
Tt
= Test temperature
t
= time
X & Y = exponent effect of voltage and temp.
To
= Operating temperature
Rs
= Series Resistance
Lo
= Operating life
9