Multi-layer ceramic capacitors (MLCCs) have stacked layers of conductors and dielectric materials.  The dielectric materials have desirable and undesirable physical characteristics that change with voltage, temperature, impedance, etc. Like all engineering, you have to balance the competing priorities to select the best capacitor for your design.

Permittivity, ε, describes the ability of a material to store energy in an electric field.  You’ve probably already guessed that a high permittivity is a desirable characteristic in a capacitor dielectric.  Dielectrics with high permittivity have a strong interaction with the electric field.  One way materials interact is through atomic and molecular polarization.  In polarization, nuclei shift in one direction, and electron clouds shift in the opposite direction.  Think of it like pushing (but not releasing) a small child up a slide.  When you release the child, sometimes they’ll slide, and sometimes they’ll remain in place – it depends if the slide is polished steel or scratched plastic.

Linear polarization means that the moment the electric field disappears from the dielectric, the electrons and nuclei return to their original configuration.  In the child example, the child would move down the slide.  Ferroelectric (a misnomer having nothing to do with iron) materials tend to stay polarized after removing the electric field.  That means there is less energy to recover from the dielectric and that a threshold field voltage is required to return the atoms to their original configuration.  In our child example, the child would stick in place or perhaps only slide a little distance down the slide.  It takes work to get them back to their original position.

EIA RIS-198 divides dielectrics into three classes based on their temperature coefficients.  Class 1 and 2 dielectrics are in most MLCCs.  Class 3 dielectrics are in most disc capacitors, and I will not discuss Class 3 dielectrics here.

Class 1 materials have non-ferroelectric dielectrics that do not exhibit a polarization hysteresis during charge and discharge cycles.  That means that class 1 dielectrics return to their original polarization state as soon as the electric field disappears. Class 2 materials have a polarization hysteresis.  That means that the dielectric materials retain part of their polarization after removing the potential difference from the capacitor plates.  After a long period, many atoms stick in their polarization state and no longer help store energy.  Heating the material beyond the curie temperature (approximately 1 hour @ 150°C) can release the “stuck” atoms and return them to active use.

Above is an image of the electric field vs. polarization curves for a linear dielectric (on the left) and ferroelectric dielectric material (on the right)

The dielectric materials in this category have a Letter-Number-Letter designator.

EIA Class 1

Dielectric materials in this category are non-ferroelectric and have linear temperature coefficients (a directly proportional relationship).   They are thermally stable over their designed temperature range.

The class 1 capacitor with the best performance has a C0G dielectric.  These capacitors often have a C0G/NP0 designation, where NP0 is a military specification denoting a flat temperature coefficient.  C0G/NPO capacitors have a constant capacitance over their entire operating range -55°C to 125°C.  No other capacitors behave this well.

EIA Class 2

Class II dielectric materials are ferroelectric. The polarization varies with the applied electric field. The performance of these materials varies with temperature.    Engineers should determine the expected operating environments of their devices when deciding whether an X7R, a Y5V, or other dielectric since the capacitors will have similar characteristics in the controlled climate that surrounds most test benches.