MLCC classes

MLCCs are probably the most widely used capacitors in the world. They come in many shapes, performance classes, voltage/current ratings, and application-specific types but, generally, all of them can be divided into two classes: I and II. Class III existed but is not supported by IEC anymore because of their fast degradation and high capacitance change with temperature and ageing; moreover, their maximum operating temperature was +55°C. Class IV capacitors were never considered by the IEC since this was a very old capacitor class which is not sold anymore.

When approaching a new design, it is important to select the right type of capacitor for each subfunction. However, not only the electrical properties but also mechanical and reliability-related ones are important since MLCCs are intensively used and it was discovered they are prone to failures - it's not rare to see a class II capacitor failing in short circuit.

As an example, two TDK, general purpose, 10nF, 0603, 50V, standard termination capacitors - C1608C0G1H103J080AA (C0G) and C1608X7R1H103K080AA (X7R) - are shown below for a quick and intuitive comparison.

Class I capacitors are very stable along the whole temperature range, with a very small variation; they exhibit an optimal performance also when polarized with high voltage since the used dielectric material is not ferroelectric. Class I capacitors have a very low capacitance change rate during their life, under temperature and voltage change. They don't have piezoelectric nor pyroelectric behaviour since the dielectric material is a paraelectric substance, which is typically calcium zirconate (CaZrO₃), calcium titanate (CaTiO₃): they don't 'move' (they don't shrink nor stretch) when an AC voltage is applied so they are very silent and less prone to mechanical damage - among class I capacitors it's quite rare to observe delamination, short circuits, cracks, heat-related failures. The drawback is the capacitance density: they have very low capacitance per unit volume if compared to class II MLCCs and other types of capacitors, so they are often used in signal conditioning circuits, as precise local bypass capacitors or when a high accuracy and low performance degradation in time is needed.

Class II MLCCs are made to have, typically, bigger capacitance values, but they are not temperature-stable and their capacitance strongly depends on the voltage applied. These capacitors should be used for decoupling only and their tolerance must be considered, especially during worst-case analysis. The dielectric materials used in this case have very high k (or relative electrical permittivity) but are typically pyroelectric and piezoelectric. This means that, when a voltage is applied, they change their structure, varying the actual capacitance and, in some cases, making a clicking or buzzing noise: this is because, if the voltage applied is not constant, such voltage makes the MLCC dielectric structure move back and forth, generating some noise that could be amplified by the PCB where it is mounted. Also, the temperature makes their capacitance change, due to gradual but appreciable phase change in the dielectric crystalline structure. One of the most used dielectric materials is barium titanate (BaTiO₃), which has a relative permittivity of around 6000.