• Use a different dielectric - C0G can replace X7R for low cap values, if the package size increase is acceptable.
• Use a different type of capacitor, such as tantalum (the 1206 0.1 uF 50 volt ceramic can sometimes be directly replaced by a tantalum equivalent, the 3216.
• Use a leaded part, rather than SMT - the leads tend to decouple the mechanical stress from the chip
• Use a smaller footprint SMT part, to minimize the span on the board, which can help to isolate the chip from flex and vibration effects
• Minimize vibration on the board by changing the board mount system, adding dampening materials near the chip, or by relocating the chip.
• Use a part with thicker dielectric, usually corresponding to a higher voltage rating. This reduces the voltage gradient, which reduces piezoelectric noise, if the package size increase is acceptable.
• Use a chip with greater overall thickness, which helps to prevent physical distortion and stress.
   

New capacitor from Murata cuts acoustic noise by 30dB    Info at EETimes 3/5/2009 http://www.eetimes.eu/products/passives/215800711

Murata’s GJ8 series of multi-layer ceramic chip capacitors (MLCCs) has been specifically developed to reduce acoustic noise in consumer and industrial electronics applications. Sound can be generated by the MLCCs at the input to the DC-DC converter in a notebook PC, or by capacitors in the control circuit of the LCD module in a mobile phone. This problem is caused by the expansion and contraction of the dielectric element in some MLCCs, which causes the PCB to vibrate at the amplitude of the voltage applied. When the frequency of the voltage applied approaches audio frequencies, a noise can be heard.

Murata’s GJ8 series has been specially designed to reduce this problem and is available with capacitance between 1 and 10µF. The acoustic noise generated by 4.7 to 10µF GJ8 series capacitors represents an improvement of at least 10dB over ordinary MLCCs; for 1µF models the improvement is up to 30dB. The series is available in low-profile models with thicknesses of just 1.25, 1.60 or 2.50 mm depending on capacitance. Rated voltage is up to 50V.

Aya Tonooka,
Murata Electronics (UK) Ltd.,
Oak House, Ancells Road, Ancells Business Park, Fleet, Hampshire GU51 2QW , United Kingdom
Tel: +44 (0) 1252 811666
Fax: +44 (0) 1252 811777
E-mail: ayatonooka@murata.com
Web: www.murata.eu



Reduce acoustic noise from capacitors.  Adding parts or cutting PCB slots can make a difference.  EDN Article, Damian Bonicatto, Landis+Gyr, Pequot Lakes, MN; Edited by Martin Rowe and Fran Granville -- EDN, February 17, 2011
http://www.edn.com/article/512775-Reduce_acoustic_noise_from_capacitors.php

Some surface-mount capacitors exhibit acoustic noise when operated at frequencies in the audio range. A recent design uses 10-μF, 35V X5R 1206 ceramic capacitors that produce noticeable acoustic noise. To quiet such a board, you can use acoustically quiet capacitors from manufacturers such as Murata and Kemet. Unfortunately, they tend to cost more than standard parts. Another option is to use capacitors with a higher voltage rating, which could reduce the noise. Those parts may also be more expensive than standard capacitors. A third path is to make a physical change to the PCB (printed-circuit board).

A ceramic capacitor expands when you apply a voltage and contracts when you reduce the voltage. The PCB flexes as the capacitor changes size because the ends of the capacitor mechanically couple to the PCB through solder (Reference 1).

Figure 1a shows a capacitor with no applied voltage, and Figure 1b shows an exaggerated condition of PCB flexing when you apply voltage to a capacitor. Applying the voltage makes the PCB operate as a speaker. Keeping that fact in mind, consider two methods for improving the situation. The first technique is relatively simple: If your circuit uses one capacitor, replace it with two in parallel, each with half the capacitance of the noisy capacitor. This approach lets you place a capacitor on top of the board and the other on the bottom of the board; the capacitors lie directly above each other, and their orientations are the same. As the upper capacitor tries to flex the board down, the lower capacitor tries to flex the board up. These two stresses tend to cancel each other, and the PCB generates little sound.
 

Adding a second capacitor increases cost but not as much as replacing the noisy capacitor with one that might not create noise. A ceramic capacitor from Digi-Key sells for approximately 27 cents (1000). A quieter KPS-series part from Kemet costs approximately $1.50. The second method involves making a slot in the PCB near each end of the capacitor (Figure 2). When the capacitor expands and contracts, it flexes only a small portion of the PCB, which should reduce the noise.
 

A test with five 10-μF, 25V ceramic capacitors connected in parallel showed that putting three capacitors on top of the PCB and two on the bottom reduces the noise by 14 dBA (acoustic decibels). Routing a slot on both sides of the five capacitors reduces the noise by 15 dBA. Both are substantial noise reductions. A Murata JG8-series capacitor reduces the noise by 9.5 dBA. Combining these techniques should further reduce the noise.