- the ceramic capacitor is available in many versions including leaded disc ceramic capacitors, surface mount mulilayer ceramic capacitors,MLCCs etc.
Ceramic capacitors are one of the most widely used forms of capacitor used in electronics equipment these days.
Ceramic capacitors have also been used for many years, being found in valve or tube circuits dating from the 1930s.
Today ceramic capacitors area available in a variety of formats ranging from leaded components to surface mount technology, SMT varieties. As leaded versions disc ceramic capacitors are widely available, and as SMT devices, multilayer ceramic capacitors or MLCCs are available in all the common formats. As such these ceramic capacitors are used in virtually every type of electronics equipment.
The actual performance of the ceramic capacitors is highly dependent upon the dielectric used. Using modern dielectrics, very high values are available, but it is also necessary to check parameters such as the temperature coefficient and tolerance. Different levels of performance are often governed by the dielectric used, and therefore it is necessary to choose the type of dielectric in the ceramic capacitor.
Ceramic capacitors range in value from figures as low as a few picofarads to around 0.1 microfarads. In view of the wide range and suitability for RF applications they are used for coupling and decoupling applications in particular. Here they are by far the most commonly used type being cheap and reliable and the loss factor is particularly low although this is dependent on the exact dielectric in use.
Ceramic capacitor basics
Ceramic capacitors are the workhorses of the capacitor world these days. Ceramic capacitors are used in millions as a result of a combination of their cost and performance. There is a wide variety of dielectrics that can be used as described below, but as the name of the ceramic capacitor suggests, they are all ceramic in nature.
In order to ensure that sufficient levels of capacitance can be obtained within a single capacitor package, ceramic capacitors, like types of capacitor have multiple layers. This increases the level of capacitance to enable the required values of capacitance to be achieved.
Ceramic capacitors are available now in three main types although other styles are available:
- Leaded disc ceramic capacitors for through hole mounting which are resin coated
- Surface mount multilayer ceramic capacitors
- Specialist microwave bare leadless disc ceramic capacitors that are designed to sit in a slot in the PCB and are soldered in place
Although it is possible to obtain other types of ceramic capacitor, these are the main types that can be found today. Of these the surface mount variety is used in the greatest quantities by far because of the manufacturing methods used these days for electronic equipment.
Ceramic capacitors have a variety of different ceramic dielectrics as the basis of the capacitor. Ceramic dielectrics are made from a variety of forms of ceramic dielectric. The exact formulas of the different ceramics used in ceramic capacitors vary from one manufacturer to another but common compounds include titanium dioxide, strontium titanate, and barium titanate.
In view of the wide variation of ceramics used in capacitors the EIA (Electronic Industries Alliance) classifies ceramics into groups. In general the lower the group or class the better the overall characteristics, but this is usually at the expense of size. Types within each class define the working temperature range, temperature drift, tolerance, etc.
- Class 1: Class 1 ceramic capacitors are the most stable forms of ceramic capacitor with respect to temperature. They have an almost linear characteristic and their properties are almost independent of frequency within normal bounds.
The common compounds used as the dielectrics are magnesium titanate for a positive temperature coefficient, or calcium titanate for capacitors with a negative temperature coefficient. Using combinations of these and other compounds it is possible to obtain a dielectric constant of between 5 and 150. Also temperature coefficients of between +40 and -5000 ppm/C may be obtained.
Class 1 capacitors also offer the best performance with respect to dissipation factor. This can be important in many applications. A typical figure may be 0.15%. It is also possible to obtain very high accuracy (~1%) class 1 capacitors rather than the more usual 5% or 10% tolerance versions. The highest accuracy class 1 capacitors are designated C0G or NP0.
- Class 2: Class 2 capacitors offer better performance with respect to volumetric efficiency, but this is at the cost of lower accuracy and stability. As a result they are normally used for decoupling, coupling and bypass applications where accuracy is not of prime importance. A typical class 2 capacitor may change capacitance by 15% or so over a -50C to +85C temperature range and it may have a dissipation factor of 2.5%. It will have average to poor accuracy (from 10% down to +20/-80%). Howeer for many applications these figures would not present a problem.
- Class 3: Class 3 ceramic capacitors offer a still high volumetric efficiency, but again this is at the expense of poor accuracy and stability and a low dissipation factor. They are also not normally able to withstand high voltages. The dielectric used is often barium titanate that has a dielectric constant of up to about 1250.
A typical class 3 capacitor will change its capacitance by -22% to +50% over a temperature range of +10C to +55C. It may also have a dissipation factor of around 3 to 5%. It will have a fairly poor accuracy (commonly, 20%, or -20/+80%). As a result, class 3 ceramic capacitors are typically used as decoupling or in other power supply applications where accuracy is not an issue. However they must not be used in applications where spikes are present as these may damage the capacitor if they exceed the rated voltage.
EIA temperature coefficient codes
In order that the performance of ceramic capacitors can be standardised and easily defined, a set of codes has been defined by the EIA (Electrical Industries Association). These codes enable ceramic capacitor performance to be defined in an easily managed way. The codes are different, though for class 1 and class 2 ceramic capacitors.
Class 1 capacitor codes:
Less common is the EIA code for temperature compensated capacitors. This comprises a three character code:
- The first character is a letter which gives the significant figure of the change in capacitance over temperature in ppm/C
- The second character is numeric and gives the multiplier
- The third character is a letter and gives the maximum error in ppm/C
The table below details what each of the EAI codes means.
As an example, one common type of class 1 capacitor is a C0G and this will have 0 drift, with an error of ±30PPM/C.
Class 2 capacitor codes
In order to define the class of temperature coefficient of a particular capacitor, a three letter code designated by the EIA is used. For non-temperature-compensating capacitors this EIA code comprises of three characters:
- The first character is a letter. This gives the low-end operating temperature.
- The second is numeric and this provides the high-end operating temperature.
- The third character is a letter which gives capacitance change over that temperature range.
The table below details what each of the EAI codes means.
|X||-55C (-67F)||2||+45C (+113F)||D||+/-3.3%|
|Y||-30C (-22F)||4||+65 (+149F)||E||+/-4.7%|
|Z||+10C (+50F)||5||+85 (+185F)||F||+/-7.5%|
|T||+22% / -33%|
|U||+22% / -56%|
|V||+22% / -82%|
Two very common examples of class 2 ceramic capacitors are the X7R capacitor which will operate from -55°C to +125°C with a capacitance change of up to ±15%, and the Z5U capacitor which will operate from +10°C to +85°C with a capacitance change of up to +22% to -56%.
Comparison of ceramic capacitor performance levels
The different types of ceramic capacitor have very different levels of performance. As a result they are used in different areas. Capacitors using dielectric like NP0 and C0G are very stable, but those like X7T and Y5V vary more widely, but have advantages in terms of the overall capacitance density and hence capacitor size.
The graph below shows some of the variation in performance of the different ceramic capacitor dielectrics.
Variation of capacitance of different ceramic capacitor dielectrics
SMD / SMT ceramic capacitors
The vast majority of ceramic capacitors that are used today are in the form of surface mount technology devices. Millions of these ceramic capacitors are used every day in every form of mass produced electronics equipment.
SMD / SMT ceramic capacitors are shaped in the form of a rectangular block or cuboid. The capacitor itself consists of the ceramic dielectric in which a number of interleaved precious metal electrodes are contained. This structure gives rise to a high capacitance per unit volume. The inner electrodes are connected to the two terminations, either by silver palladium (AgPd) alloy in the ratio 65 : 35, or silver dipped with a barrier layer of plated nickel and finally covered with a layer of plated tin (NiSn).
Care must be taken when soldering these capacitors. If heat is applied for too long, then the terminations can be damaged. Fortunately modern versions are far more robust than much older capacitors which used to suffer from metalisation if heat was applied for too long. Despite this care should be taken, especially if these components are being soldered manually. Normally production methods using infra-red reflow with carefully controlled heat profiles is to be recommended.
SMT / SMC ceramic capacitors are normally contained within standard package sizes. These have various designations as described in the table below:
|Ceramic Capacitor Package Designations|
|1812||4.6 x 3.0||0.18 x 0.12|
|1206||3.0 x 1.5||0.12 x 0.06|
|0805||2.0 x 1.3||0.08 x 0.05)|
|0603||1.5 x 0.8||0.06 x 0.03|
|0402||1.0 x 0.5||0.04 x 0.02|
|0201||0.6 x 0.3||0.02 x 0.01|
It can be noted that the package designation is derived from the package size in 0.01 inch increments.
Wired ceramic capacitors
While the majority of ceramic capacitors that are used are in the form of SMT or SMD components, large quantities of wired components are still used. A large proportion of the wired ceramic capacitors are in the form of disc ceramic capacitors. As the name suggests these electronic components are shaped in the form of a disc.
By Ian Poole
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