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 sub micron 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.
