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KEMET Electronics Corp
PIEZOELECTRIC EFFECTS CERAMIC CHIP CAPACITORS

Singing Capacitors

Most dielectrics of ceramic capacitors exhibit a characteristic
identified as piezoelectric effects than can cause
unexpected signals in certain circuits. In some cases, the
piezoelectric effect may result in the appearance of
electrical noise, while in other cases; an acoustic sound may be
heard, emanating from the capacitor itself.

The basic element in most MLCCs is barium-titanate, or some close
derivative of this. Piezoelectric properties
are common to the barium-titanate structure. If you are old enough to
recall before CDs, acoustic recordings
were mostly sold on vinyl records and the signals were picked up using
a needle that was contained in a
spiraling groove cut in the record=92s surface. Within these grooves,
the needle would ride on roughened
surfaces that created mechanical vibrations of the needle. The needle
was connected to a crystal structure (in
less expensive turntables), and this crystal would generate an
electrical signal that correlated with the
vibration=92s frequency and magnitude, which was then amplified to
generate the speaker signals. What was the
crystal structure of these ceramic cartridges? They were based on
barium-titanate!
Piezoelectric effects can result in noise for ferroelectric ceramic
chips, such as those of the middle to high
dielectric constants like X5R, X7R, X8R, Y5V, Y5U, Z5U, etc.
Piezoelectricity occurs in all ferroelectric
dielectrics, regardless of manufacturer, and the means to reduce these
effects usually requires the dielectric
constant be lowered (the capacitance capability is also lowered with
the lower dielectric constant) while moving
to higher dielectrics (Y5V, Z5V are usually cheaper) creates a higher
susceptibility to this effect. Note that
there are no measurable piezoelectric effects in Class 1 capacitors,
such as C0G or NP0 - neither of which is
considered ferroelectric.

.... SNIP ...

Solutions to these noise problems may involve alternative types of
capacitors (e.g., tantalum, aluminum, tapolymer,
al-polymer, film, as this effect is unique to ceramic), leaded or
standoff capacitors (using leadframes
to eliminate the mechanical tie to the PCB eliminates a unified or
cumulative response of multiple ceramic
capacitors), using higher voltage ratings (lowers the stress), and
others. It should be pointed out that this effect
is lost at frequencies well above 30 kHz because the body cannot
respond fast enough to the changing stress
levels. The peak response region and the noise attributes dictates
that these capacitors should be used very
carefully in audio circuits, as well as high-gain circuits; and, be
careful that an audio application may not easily
appear to be an audio circuit (sleep mode conditions of microprocessors).
John D. Prymak =96 KEMET Applications Manager

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