## 1. Piezoelectric Effect in MLCCs
Mechanism:
* Certain dielectric ceramics, especially ferroelectric BaTiO₃, become electrically polarized under an applied voltage.
* This polarization induces a small mechanical deformation in the ceramic lattice, producing a dimensional change in the capacitor body.
* The effect is linear at low fields and is strongest in class II (X7R, Y5V) ceramics, which are highly polarizable.
Manifestation in MLCCs:
* When AC voltage is applied, the MLCC may vibrate or emit audible sound, sometimes referred to as “singing capacitors.”
* This is a micro-mechanical movement of the ceramic layers coupled to the capacitor terminations.
Impact:
* In most applications, the effect is minor and harmless, but in high-precision analog circuits, vibrations can induce microphonic noise.
* Can also affect mechanical stress and solder joint fatigue in surface-mount assembly under repeated AC excitation.
## 2. Electrostrictive Effect in MLCCs
Mechanism:
* Electrostriction is a quadratic coupling between electric field and strain, meaning the mechanical deformation is proportional to the square of the applied electric field, regardless of polarity.
* All dielectric materials exhibit electrostriction to some extent, but ferroelectric ceramics like BaTiO₃ show the effect more strongly.
Manifestation in MLCCs:
* Even under DC bias, the MLCC body may experience minute dimensional changes.
* Under AC, the strain oscillates at twice the excitation frequency due to the quadratic relationship.
* This can contribute to acoustic noise similar to the piezoelectric effect, but with a different frequency signature.
## 3. Why It Happens in MLCCs
* Material Choice: High-permittivity, ferroelectric ceramics are required for high capacitance in small packages, which inherently have piezoelectric/electrostrictive properties.
* Layer Structure: Multilayer stacking amplifies the total mechanical response since hundreds of layers deform collectively.
* Electrode Clamping: Electrodes and solder joints convert microscopic strain into observable vibration or audible noise.
## 4. Design Implications
* Audible Noise: AC-driven MLCCs in audio or power circuits can produce hums or clicks.
* Microphonic Effects: Sensitive analog circuits (e.g., MEMS sensors or precision amplifiers) may pick up mechanical vibrations as electrical noise.
* Mitigation: Use class I (C0G/NP0) MLCCs for low-noise applications, as they have negligible ferroelectric polarization and almost no piezoelectric/electrostrictive response.
Summary:
Some MLCCs show piezoelectric or electrostrictive effects because BaTiO₃-based ferroelectric ceramics deform mechanically when polarized. This deformation can generate vibrations or acoustic noise under AC or DC voltage. The effect is intrinsic to high-permittivity, ferroelectric dielectrics used in class II MLCCs, and it is minimized in class I, stable dielectric types.
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