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 [1] 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 [2]. 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