Ultrasonic cleaning is widely accepted as an invaluable tool for minimizing contamination of critical devices, particularly where complex geometries such as blind holes are present. In order to use this powerful technique effectively, it is necessary to identify multiple parameters and to optimize conditions. A recent ASTM symposium concerned with residue on biomedical implants  featured ultrasonics both for cleaning and for extractive determination of contamination residues. Optimizing ultrasonic effectiveness involves removal of undesirable contaminants. Because ultrasonic cleaning generates significant force, effectiveness also involves minimizing the potential for substrate damage.
Ultrasonic cavitation quality is dependent on a number of factors including the frequency, amplitude, the specific aqueous or solvent chemistry time, and pressure. The potential for ultrasonic erosion is often assumed to be minimum, particularly where more sophisticated ultrasonic systems are employed. For example, higher frequencies and lower amplitude are preferred with delicate substrates. However, where the surface itself is more complex, more germane to performance, and contains what might be termed microstructure (or nanostructure), the potential for damage during ultrasonic cleaning must not be ignored.
Variations in any of the parameters can affect ultrasonic performance. The impact of ultrasonic variables is a matter of ongoing debate even by the experts in the field. Cavitation is a powerful mechanism involvingextremely high forces and local temperatures up to 20,000°K, four times thatof the surface of the sun . Interactions among parameters are generally neitherindependent nor linearly additive. As with any force used in cleaning or extraction,ultrasonics is a balancing act to remove the unwanted contaminants without negativelyimpacting the surface quality of the substrate.
We know of no universally-accepted test for ultrasonic performance. Ultrasonic cleaning processes tend to be based on experience or on the advice of equipment vendors. The current tests of ultrasonic performance are primarily empirical as part of the overall manufacturing process:
- contamination is reduced to the level that performance is not impacted
- there is no apparent adverse structural or surface impact
A number of cavitation meters or devices have been proposed . However, to date, there has not been universal commercialization or acceptance of a quantifiable measure of ultrasonics performance. For both aqueous and solvent media, erosion of standard weight aluminum foil after 30 to 45 seconds of ultrasonic action has been the only widespread indicator of ultrasonic functionality. Typically, an “orange peel” pattern is observed; there may also be sufficient erosion of the foil that holes appear. Less commonly, gravimetricdetermination is employed.
However, the aluminum foil cavitation test is at best a qualitative and subjective measure of performance. It is qualitative because the test depends on the thickness and handling of the foil, the solvent, the temperature, the frequency, power, wave form, etc. It is subjective because we generally do not quantify the degree of erosion. Further, there are differences in opinion as to what an appropriate orange peel pattern should look like. Some individuals look for erosion of the foil in the form of holes or perforations; others consider the appearance of excessive foil perforation to indicate an overly-aggressive cleaning or extraction environment.
Ultrasonic effectiveness is strongly dependent on the chemical and temperature, and there is an optimum temperature for each chemical. Figure 1 illustrates erosion of aluminum foil as a function of temperature for a number of commonly used liquids . There is a large variation among chemicals both in the optimaltemperature and peak height. The higher the peak, the more efficient the ultrasonic action and also the potential for surface damage. The effects are also dependent on substrate thickness and time. For short exposure times, only an orange peel pattern may be formed. For longer times, noticeable erosion occurs. Recent tests  indicate that even heavy duty foil was significantly eroded after 10 minutes of ultrasonic exposure. Effects of ultrasonics on aluminum are qualitativebut indicate the power of ultrasonics to alter surfaces.
The provisos are not meant to imply that we should abandon ultrasonics and instead massage each critical component with an extra-soft toothbrush Quite the contrary, ultrasonics is a powerful and, with appropriate understanding, highly controllable force for cleaning and extraction.
Next month we will continue our discussion of the impacts of various parameters on ultrasonic performance including specialized process options.
*Adapted from a paper presented by B. Kanegsberg, “ASTM Symposium on Cleanliness of Implants”, Reno, NV, May 2005.
- B. Kanegsberg, E. Kanegsberg. Controlled Environments, (August 2005).
- K. Suslick, D. Flannigan. “Plasma Formation and Temperature Measurement During Single-Bubble Cavitation”, Nature Vol. 434, (3 March 2005) pp.52-55.
- B. Kanegsberg, E. Kanegsberg. “Measuring Sonics”, Parts 1 to 3, A2C2 Magazine, (December, 2001, January, 2002, and February, 2002).
- L. D. Rosenberg. “On the Physics of Ultrasonic Cleaning,” Ultrasonic News, (Winter 1960) p. 16.
- B. Kanegsberg, E. Kanegsberg. “Parameters in Ultrasonic Cleaning for Implants and other Critical Devices,” ASTM Symposium on Cleanliness of Implants, Reno, NV, (May 2005).
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and precision cleaning, surface preparation, and contamination control. They are the editors of “Handbook for Critical Cleaning,” CRC Press.Contact them at BFK Solutions LLC., 310-459-3614; email@example.com; www.bfksolutions.com.