Sound is caused by the propagation of pressure and density fluctuations (compression and rarefaction of molecules) in an elastic medium. Frequencies ranging from 20 Hz to 20 kHz, are audible for humans. Vibrations with frequencies exceeding 20 kHz are called ultrasound.
Cleaning frequencies typically range from 35 kHz to 45 kHz. Through ultrasonic transducer elements attached to or directly fixed in the cleaning tank, under-inflation and overpressure waves spread throughout the cleaning bath. Cavitation creates a pressure jet (microjet) that helps remove the dirt particles from the surface.
In the case of cleaning undercomponents and complex geometries, ultrasonic cleaning can be useful since the sound waves are capable of reaching every angle of the mounted circuit board. Many industry cleaning machines have an automatic frequency adjustment for the modulation of the ultrasound (sweep) working frequency, which prevents “hot spots” and allows material-friendly cleaning.
According to J-STD-001 and MIL-STD-2000 Rev. A, the use of ultrasound in cleaning electronics is permitted. The IPC TM 650 22.214.171.124. standard recommends a test procedure before using an ultrasonic application for electronic components.