Design of Chip Ceramic Capacitor
Ceramic Capacitor
Basics
- A capacitor is an electrical device that stores energy in the electric field between a pair of closely spaced plates
- Capacitors are used as energy-storage devices, and can also be used to differentiate between high-frequency and low-frequency signals. This makes them useful in electronic filters
- Capacitance Value: Measure of how much charge a capacitor can store at a certain voltage
- MLCC: Multilayer Ceramic Chip Capacitor L-ayers of ceramic and metal are alternated to make a multilayer chip
Process of Making Capacitor:
The process of making ceramic capacitors involves many
steps.
Mixing: Ceramic
powder is mixed with binder and solvents to create the slurry, this makes it
easy to process the material.
Tape Casting: The
slurry is poured onto conveyor belt inside a drying oven, resulting in the dry
ceramic tape. This is then cut into square pieces called sheets. The thickness
of the sheet determines the voltage rating of the capacitor.
Screen Printing
and Stacking: The electrode ink is made from a metal powder that is
mixed with solvents and ceramic material to make the electrode ink. The
electrodes are now printed onto the ceramic sheets using a screen printing
process. This is similar to a t-shirt printing process. After that the sheets
are stacked to create a multilayer structure.
Lamination: Pressure
is applied to the stack to fuse all the separate layers, this created a
monolithic structure. This is called a bar.
Cutting: The bar
is cut into all the separate capacitors. The parts are now in what is called a
‘green’ state. The smaller the size, the more parts there are in a bar.
Firing: The
parts are fired in kilns with slow moving conveyor belts. The temperature
profile is very important to the characteristics of the capacitors.
Termination: The
termination provides the first layer of electrical and mechanical connection to
the capacitor. Metal powder is mixed with solvents and glass frit to create the
termination ink. Each terminal of the capacitor is then dipped in the ink and
the parts are fired in kilns.
Plating: Using an
electroplating process, the termination is plated with a layer of nickel and
then a layer of tin. The nickel is a barrier layer between the termination and
the tin plating. The tin is used to prevent the nickel from oxidizing.
Testing: The
parts are tested and sorted to their correct capacitance tolerances.
At this point the
capacitor manufacturing is complete. The parts could be packaged on tape and
reel after this process or shipped as bulk.
Types of Material Systems Used to make capacitors
There are
two material systems used today to make ceramic capacitors: Precious Metal
Electrode and Base Metal Electrode. The precious metal system is the older
technology and uses palladium silver electrodes, silver termination, then
nickel and tin plating. Today this material system is mostly used on high
voltage parts with a rating of 500V and higher. The base metal system is a
newer technology and uses nickel electrodes, nickel or copper termination, and
nickel and tin plating. This material system is typically used for parts with
voltage ratings lower than 500VDC.
Precious Metal Vs Base Metal System
MLCC Basics
The capacitance
value of a capacitor is determined by four factors. The number of layers in the
part, the dielectric constant and the active area are all directly related to
the capacitance value. The dielectric constant is determined by the ceramic
material (NP0, X7R, X5R, or Y5V). The active area is just the overlap between
two opposing electrodes.
The dielectric
thickness is inversely related to the capacitance value, so the thicker the
dielectric, the lower the capacitance value. This also determines the voltage
rating of the part, with the thicker dielectric having a higher voltage rating
that the thinner one. This is why the basic trade off in MLCCs is between
voltage and capacitance.
Critical
Specifications
Material
|
Dielectric
Constant
|
%
Capacitance Change
|
DF
|
NP0
|
15-100
|
<0.4% (-55 to 125C)
|
0.1%
|
X7R
|
2000-4000
|
+/-15% (-55 to 125C)
|
3.5%
|
Y5V
|
>16000
|
Up to 82% (-30 to 85C)
|
9.0%
|
- Dissipation factor: % of energy wasted as heat in the capacitor
- Dielectric Withstanding Voltage: Voltage above rating a capacitor can withstand for short periods of time
- Insulation resistance: Relates to leakage current of the part (aka DC resistance)
The critical specifications of a
capacitor are the dielectric constant, dissipation factor, dielectric
withstanding voltage, and insulation resistance. Dielectric constant: this
depends on the ceramic material used. The table shows different dielectrics and
some of their specifications. As you can see NP0 has the lowest dielectric constant,
followed by X7R which has a significantly higher constant, and Y5V which is higher
still. This is why the capacitance values for X7R capacitors are much higher
than NP0 capacitors, and Y5V has higher capacitance than X7R. The capacitance
change vs temperature is very small for NP0 parts from -55C to 125C, and gets
larger for X7R, then even larger for Y5V. So, the more capacitance a material
provides, the lower the stability of capacitance over temperature. Dissipation
Factor: this is the percentage of energy wasted as heat in the capacitor. As you
can see, NP0 material is very efficient, followed by X7R, then Y5V which is the
least efficient of the three materials. Dielectric withstanding voltage: this
refers to the momentary over voltage the capacitor is capable of withstanding
with no damage. Insulation resistance: this is the DC resistance of the capacitor;
it is closely related to the leakage current.
Characteristics
of Ceramic Capacitors
Low impedance, equivalent series resistance (ESR) and
equivalent Series Inductance (ESL). As frequencies increase, ceramic has bigger
advantage over electrolytic
The final part of this presentation will cover the
characteristics of ceramic capacitors. MLCCs have low impedance when compared
with tantalum and other electrolytic capacitors. This includes lower inductance
and equivalent series resistance (ESR). This allows ceramic capacitors to be
used at much higher frequencies than electrolytic capacitors.
Temperature
Coefficient: Describes change of capacitance vs. temperature. Ceramic materials
are defined by their temperature coefficient
Voltage
Coefficient: Describes change of capacitance vs voltage applied. Capacitance
loss can be as much as 80% at rated voltage. This is a property of ceramic
materials and applies to all manufacturers
Voltage Coefficient of Capacitance: describes change of
capacitance vs DC voltage applied. This is a property of ceramic materials and
applies to all manufacturers. The graph shows typical voltage coefficient
curves for 500VDC rated X7R and NP0 capacitors. Note that the capacitance of
the NP0 remains stable with applied voltage, while the X7R material can have a
capacitance loss of 80% at rated voltage.
Aging: X7R, X5R, and Y5V
experience a decrease in capacitance over time caused by the relaxation or
realignment of the electrical dipoles within the capacitor.
- For X7R and X5R the loss is 2.5% per decade hour and for Y5V it is 7% per decade hour, NP0 dielectric does not exhibit this phenomenon
- De-Aging: aging is reversible by heating the capacitors over the “Curie Point” (approx 125°C), the crystalline structure of the capacitor is returned to its original state and the capacitance value observed after manufacturing.
Aging: X7R, X5R, and Y5V
experience a decrease in capacitance over time caused by the relaxation or
realignment of the electrical dipoles within the capacitor. For X7R and X5R the
loss is 2.5% per decade hour and for Y5V it is 7% per decade hour, NP0
dielectric does not exhibit any aging. Aging is reversible by heating the
capacitors over the “Curie Point” (approx 125°C), the crystalline structure of
the capacitor is returned to its original state and the capacitance value
observed after manufacturing.
Johanson Part Number
Breakdown
This slide is for reference and shows the Johanson
Dielectrics part number breakdown.
Summary
- Manufacturing process and basic structure of ceramic capacitors
- Material systems and basic specifications of ceramic capacitors
o Precious Metal vs Base Metal
o Critical Specifications of MLCCs
- Characteristics of ceramic chip capacitors
o Low impedance, temperature coefficient, voltage coefficient, aging