Class III: Z5U and Y5V
There exists a third class of MLCC dielectrics. This type is known for two things, its 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 temperature.
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.
Click here to view this K-SIM 3.0 Project.
Ceramic Capacitor Physics
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.
It is all about the dipoles
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 similarity 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 freshy 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.
Designing and Engineering with Class II Ceramic Capacitors
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.
Capacitance 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.
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.
Ceramic Capacitor Aging
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.