You can find Multilayer Ceramic Capacitors (MLCC) available in a broad range of packages, sizes, and dielectric materials. Depending on their characteristics, these capacitors are separated by dielectric classification as Class I, II, or III. There are several types of dielectrics, each with different characteristics. This post introduces the differences between MLCC classification, DC bias, aging, and piezoelectric noise characteristics that are inherent in many ceramic capacitors.
The Electronic Industries Alliance (EIA) Standard 198 defines the temperature coefficient of capacitance (TCC) of ceramic capacitors. Using these definitions, you will see MLCC Dielectrics designations like Y5V, X7R, and C0G. Each letter has a meaning. You can use the charts in Figure 1 ([and Figure 2) to decode these designations.
[Figure 1 Chart]
Class I: C0G
Sometimes referred to as NP0, C0G are considered ultra-stable. Using the chart to decode the “name”, we can see that the TCC for C0G is ± 30 ppm/°C over the rated temperature range. In other words, the capacitance of C0G will only vary slightly due to change in temperature.
KEMET’s Commercial Grade-C0G capacitors are made using a unique formulation of Calcium Zirconate. This material is paraelectric, which provides its stability with applied DC voltage.
Since the classification does not define the material set used, other manufacturers may use different formulations or different material sets entirely.
Class II and III
Class II and III dielectrics use a slightly different naming system from Class I.
- The first letter represents the lower temperature.
- The second digit represents the upper temperature.
- The third letter indicates the maximum capacitance change that will occur throughout the lower and upper-temperature range.
For example, let’s look at X7R in the Class II/III chart. The X is -55°C, the 7 is +125°C while the R means capacitance variation of ±15% within the stated temperature range.
The difference between Class II and Class III is simply how much the capacitance will change over a certain temperature. Typically, Barium Titanate is the material used for Class II and III dielectrics. This material is ferroelectric, which is the source of the capacitance instability.
As you move further up in class, some of the negative characteristics of the dielectric are amplified.
Changes with applied voltage
The terms “DC Bias” and “Voltage Coefficient” refer to the lost in capacitance with applied voltage. This effect occurs in ferroelectric materials, like the Barium Titanate used in most X5R and X7R capacitors. Depending on the dielectric formulation, these capacitors can lose more than 70% of its rated capacitance with applied voltage!
One way to achieve smaller chip sizes while maintaining the same level of capacitance is to reduce the dielectric thickness. This design difference results in higher voltage stress, resulting in more capacitance loss.
KEMET’s K-SIM lets you simulate a ceramic capacitor’s voltage, with applied DC Voltage. It can also plot the expected capacitance change with applied voltage. It is available at ksim.kemet.com.
Class-I dielectrics do not exhibit DC bias, especially those formulated with Calcium Zirconate.
Aging is another characteristic exhibited by ferroelectric, or Class II and III dielectrics. While manufacturing the ceramic capacitor, the dielectric is exposed to temperatures more than 1000°C. For Barium Titanate devices, the Curie temperature can be in the range of 130°C to 150°C, depending on the particular formulation.
When exposed to the Curie temperature, the crystalline structure aligns to a tetragonal pattern. Once cooled, the ceramic’s crystalline structure changes to a cubic change. As this structure changes, so does the material’s dielectric constant.
Over time, the capacitance will continue to decline. It is possible to reset this aging cycle by “resetting” the material, by exposing it to its Curie temperature.
Typically you can find the aging rate in the catalog for a particular part type. Below is an example of aging rates:
Finally, the crystal structure of Barium Titanate gives the ceramic its piezoelectric, or microphonic, characteristic. When external stresses are applied to the dielectric material, the Titanium molecule oscillates back and forth. Electric signals can mechanically distort the dielectric. This distortion, or movement, creates a characteristic “buzzing” noise that some customers experience when using ceramic capacitors in their design.
The capacitance on the label, may not be the capacitance you get. The characteristics discussed in this post may change how much capacitance the part will have during the operation or life of your system.
Of course, this post is not complete description of the differences between the ceramic dielectrics. There are other subtle differences that you need to be taken into account when using ceramic capacitors. This information should provide a good starting point when choosing a suitable MLCC or starting a discussion with a KEMET Field Application Engineer.