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Two common attributes of Class II and Class III dielectrics materials are electrostriction and piezoelectricity. Often mistaken as being related, these two contrasting properties can offer unfavorable performance in certain circuit applications. Electrostriction is a behavior of all dielectrics in which the material suffers mechanical deformation (changes in shape) under the application of an electric field. Class II and Class III dielectrics such as X7R, X5R, Z5U and Y5V are made using ferroelectric materials that display greater effects of electrostrictive movement. The mechanical deformation of a mounted surface mount MLCC constructed of a ferroelectric material can result in a sound emission such as an audible hum (i.e., “singing”), as the top left figure illustrates. Several capacitors mounted closely together on the board can amplify the sound to the point that it is noticeable.
Piezoelectricity, also referred to as the piezoelectric effect, is the ability of a material to generate a voltage and/or an electrical signal (noise) when subjected to an external mechanical stress or vibration. Synonymous with the term “microphonics”, MLCCs constructed of ferroelectric materials are piezoelectric in nature and can convert external stress, similarly to the way a microphone converts sound, into an electrical signal. The electrical signal is often unwanted (noise) and may be introduced into the circuit as illustrated here (top right). The extended terminations (lead wires) of leaded capacitors allows for mechanical isolation of the MLCC from the circuit board. This isolation inhibits the transfer of mechanical stress and vibration of the PCB transferring to the capacitor body thus reducing the potential for microphonic noise. The leads also provide an effective solution to “singing” capacitors by dampening the sound emission produced by electrostriction.
