The Principles of Ultrasonic Cleaning

Ultrasonic cleaning has found its most successful application in removal of insoluble particulate contamination from hard substrate surfaces. Contamination that is soluble or emulsifiable can usually be removed with facility by means of conventional methods in conjunction with suitable solvents or detergent solutions.

Such techniques, however, cannot adequately remove particulate matter in the micron and submicron size range to the extent that is necessary, for example, for the critical cleaning required in the microelectronics and optical industries or for the preparation of surface prior to the application of thin films or coatings.

A number of methods have been used for the purpose of removing micro particulates from hard surface. These includes pressure spraying or manual and mechanical scrubbing with solvents or detergent solution, vapor degreasing, ion bombardment, plasma, chemical, or ultrasonic cleaning, and ultraviolet / ozone cleaning. The intent of this discussion, however, is not to evaluate the relative merits of these methods but rather to describe ultrasonic technology……

Principles of Ultrasonic Cleaning

In general, ultrasonic cleaning consist of immersing a part in a suitable liquid medium, agitating or sonicating that medium with high-frequency (18 to 120 kHz) sound for a brief interval of time ( usually a few minutes), rinsing with clean solvent or water, and drying. The mechanism underlying this process is one in which microscopic bubbles in the liquid medium implode or collapse under the pressure of agitation to produce shock waves, which impinge on the surface of the part and, through scrubbing action, displace or loosen particulate matter from that surface. The process by which these bubbles collapse or implode is known as cavitation. High intensity ultrasonic fields are known to exert powerful forces that are capable of eroding even the hardest surfaces. Quartz. Silicon, and alumina, for example, can be etched by prolonged exposure to ultrasonic cavitation, and "cavitation burn" has been encountered following repeated cleaning of glass surfaces. The severity of this erosive effect has, in fact, been known to preclude the use of ultrasonic in cleaning of some sensitive, delicate components.

Ultrasonic cleaning has, however, been used to great advantage for extremely tenacious deposits, such as corrosion deposits on metals. In any case, cavitation forces can be controlled, thus given proper selection of critical parameters, ultrasonic can be used successfully in virtually any cleaning application that requires removal of small particulates.

Although the ultrasonic cleaning process has been used for over half a century, no reliable means of quantifying its cavitation activity has ever been developed. Indirect methods of measurements, such as erosion tests on metal surfaces, soil removal from weighted samples, acceleration of chemical reactions, thermodynamics studies, and white noise measurement, have been employed to a limited extent, but none of these methods has proved to be effective.

Thus, operators who seek to assess the performance of an ultrasonic cleaning system must rely almost exclusively on the evaluation of actual cleanliness levels achieved. Surface patterns produced on cavitating liquids can also be observed, as can the overall degree of agitation of the cleaning medium. Operators have also observed erosion patterns produced on aluminum foil erosion test," as it is called, has come to be recognized as a fairly dependable, albeit subjective, means of demonstrating the existence of cavitation in ultrasonically agitated media. The measure has been used not only to provide an indication of the distribution of the sonic field throughout the bath, but also to locate the sites of the nodes and antinodes of the standing sonic waves. It can also generate fairly reliable side – by- side comparisons of different ultrasonic cleaning systems. In no way, however, can it be used to obtain quantitative measurements of cavitation activity.