Multilayer ceramic capacitors come in a wide variety of sizes and rated voltages. They are also available in multiple dielectric types, each of which describes how the rated capacitance changes over temperature. Many times, successful engineering is the careful balancing of tradeoffs between device characteristics with the needs of the application. Selecting the right MLCCs for the application is no different and having a clear understanding of the differences between dielectrics is necessary to make that decision.
Very shortly after an engineer’s first exposure to MLCCs they are quickly confronted with an alphabet soup of letters and numbers: C0G, X7R, X5R, U2J, etc. What do they all mean and what do they refer to? The meaning of those letters was created the Electronic Industries Alliance standard number 198 which defines the so-called “Temperature Coefficient of Capacitance” or TCC.
Class I capacitors are those that are considered “ultra-stable” across a variety of conditions. Class I capacitors are primarily made of calcium zirconate, a dielectric material that is very stable across temperature but has much lower relative permittivity than class II, and therefore has much lower overall capacitance. The tolerance of capacitance across a -55C to 125C temperature range is measured in PPM. For example, using the decoding table above, C0G has a capacitance tolerance of -30ppm across that temperature range and U2J has a tolerance of -750ppm across that same range. U2J has more capacitance than C0G, like 2 to 4 times but still much less than class II. Generally, having more capacitance comes at a cost of temperature stability.
While it appears similar, the temperature coefficient designation for class II is different primarily because of the drastically different material set. These types of capacitors are made using barium titanate (more of that later). This material has much higher dielectric constant than class I materials, like 1,000 to 10,000 times as much. That amount of capacitance comes at a price, it is not as stable over temperature. The way to decode the alphabet soup of is slightly easier than their class I counterparts. In this case the first letter is the lower temperature extreme, the second letter is the upper temperature extreme, and the final letter is the capacitance tolerance over that range. So, using that decoder, X7R is +-15% capacitance tolerance from -55C to 125C.
There exists a third class of MLCC dielectrics. This type is known for two things, it's very high capacitance and its temperature instability. Although still made with barium titanate, just like X7R and X5R, they are much less stable than class II. For example, a Z5U can vary as much as -56% in the relatively narrow range of 10C to 85C. But how can they be so different if they are made with the same materials? Well, that is where the different manufacturers apply their expertise in materials science. Certain dopants are added to the barium titanate material to flatten out the curve of relative permittivity vs temperature such that it becomes more stable across temperatures.
With our simulation tool, K-SIM, you can explore how temperature affects capacitors. In the following example we compare U2J, X7R, and Z5U of similar capacitance values.
Temperature coefficients and tolerance over a temperature range is all fine and dandy, but explaining the next effects fully requires a bit of a dive into the physics and even chemistry of the dielectric material itself. Strap in, this is going to get interesting.
A lot of the magic of a capacitor comes from the dielectric material itself. Some people would describe a dielectric as an insulator that prevents the two electrodes from shorting. That is true, but there is a bit more to dielectrics than just that. In a word, dipoles. A quick Wikipedia search will show that a dielectric is that it is “an electrical insulator that can be polarized” with the application of an external electric field. A piece of rubber is a great insulator, but it is a terrible dielectric. You can’t polarize rubber (very effectively). It is the presence of those dipoles in the dielectric material that makes for an effective capacitor. KEMET uses two main types of materials for ceramic dielectrics. Are you ready for some phrases that are going to take you back to chemistry class? First off is barium titanate (BaTiO3), which is used for our Class II/III dielectrics. Those are our X5Rs and X7Rs, among others. Next is our calcium zirconate, that’s what we use in our class I dielectrics. That would be C0G and U2J. This is where things get really interesting, calcium zirconate is paraelectric and barium titanate is ferroelectric. Those properties have some similarities to the concepts of paramagnetism and ferromagnetism, which is introduced in early physics classes.
In ferroelectric materials, the dipoles are permanently present and will align themselves with an electric field. In paraelectric materials, the dipoles appear spontaneously aligned with the application of an external electric field. The dipoles created by class II dielectrics are a result of the materials and structure of the barium titanate itself.
When freshly fired and sintered, the microcrystalline structure of barium titanate is a face-center-cubic (FCC) structure with the titanium atom in the middle of the lattice. As the material shrinks in size the titanium atom is dislodged from its position in the center of the cube and creates a charge density difference across the structure. This is the origin of the dipole in class II MLCCs. The entire ceramic material doesn’t polarize in the same direction uniformly, as the ceramic material aligns itself, grain boundaries are formed due to imperfections and differences in particle sizes. This forms domains that have a general polarized direction. It is these domains that generally align themselves with an electric field and contribute to capacitance. This is all because of that dislodged titanium atom found in class II dielectrics.
The effects brought upon by the ferroelectric nature of class II dielectrics has impacts to engineering and circuits that rely on class II capacitors. The so-called DC-bias effect, microphonics, and aging are all due to the dipoles created by the displacement of the titanium atom in barium titanate.
The terms “DC Bias” and “Voltage Coefficient” refer to the loss 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 their 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.
The above K-SIM plot shows a comparison of the DC-bias effect between class II and class I capacitors.
Click here to see the K-SIM 3.0 Project.
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 this usually occurs during reflow. Typically, you can find the aging rate in the catalog for a particular part type. Below is an example of aging rates:
K-SIM 3.0 also includes a ceramic capacitor aging calculator.
For example, a freshly fired 22uF X5R capacitor will have 16.8uF of capacitance after 5,000 hours or about half a year.
Click here to view K-SIM 3.0 Project.
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. That mechanical distortion can resonate with the PCB itself causing sound in the audible range.
While simple at first glance much is going on in the physics and science behind ceramic capacitors. Tools like K-SIM 3.0 are aimed at facilitating the selection of these components by allowing the simulation of these effects under particular circuit conditions.