Ultrasonics are a trusted cleaning technique. We use our own historical, anecdotal experiences to set acceptable parameters. With newer, more sophisticated systems, we also enlist the advice of ultrasonic equipment suppliers. While suggestions and recommendations abound, independent, comprehensive, and systematic studies of the impact of ultrasonic parameters (equipment, chemical, and physical parameters) on soil removal, surface quality, and structural integrity are limited.

Potential Substrate Impact
Any physical force has the potential for enhanced cleanliness and for undesirable substrate interaction. For most applications, appropriate ultrasonic cleaning processes are used without negative impact. Industrially, tanks and related fixturing are used daily over a period of years without unacceptable loss of functionality. Tanks and fixtures do not crumble into dust. Large stainless steel and titanium devices are relatively unlikely to show obvious ultrasonic damage.

For miniature, complex components with highly-specified surface characteristics ultrasonics parameters must be carefully defined. Cavitational erosion has been observed in highly polished parts as well as parts made of certain metals such as brass, beryllium, and aluminum. Edges, interfaces, and fine structure all have increased potential for ultrasonic damage. The care needed with optics is readily inferred from ultrasonic erosion of glass beakers.

The Boiling Point
When temperatures approach the boiling point of the liquid, increased vapor pressure inside cavitation bubbles cushions the impact of implosions thus lessening the force and cleaning action. In cleaning systems operated in a partial vacuum to reduce solvent emissions, the boiling point of the liquid is reduced from that at standard atmospheric pressure. In one such system, when ultrasonic cleaning in perchloroethylene was attempted within 30°C of the boiling point, the effect on aluminum foil decreased [1]. This does not necessarily mean that cavitation ceased, only that forces associated with it were diminished to the point that foil erosion essentially ceased. A somewhat lower temperature produced more effective cavitation and moreefficient cleaning.

The temperature of ultrasonic tanks increases during normal use, sometimes to the point that efficient cavitation ceases. Therefore, anecdotal reports of successful ultrasonics over excessive times (several hours) may be due more to the fact that soil removal occurs by heat and/or chemical action than fromeffective cavitation.

High Boiling Point Liquids
In one study, several bio-based methyl soyate solvents apparently do not cavitate well at ambient and elevated temperatures — and they are effectiveonly at elevated temperatures.

In some cases, little change in cleaning action was seen by employing ultrasonics [2]. We speculate that the relatively high solvent viscosity severely dampens cavitation. Efficacy of high boiling point cleaning chemistries will be enhanced by refinement of ultrasonic parameters.

Independent Studies Are a Must
Cost and time have been limiting factors in systematically evaluating ultrasonic performance and functionality. Therefore, both for extraction and for cleaning or decontamination, parameters are set based on experience, pragmatic observations,or vendor recommendations.

However, as devices become smaller and/or more complex (e.g. medical implants, nanotechnology devices), the potential for unintended surface modification increases. Currently, some manufacturers shy away from ultrasonics for devices with critical coatings or containing multiple metal and non-metal components. At the same time, ultrasonics cannot be avoided because it is the technique with the highest potential for excellent contamination control.

Independent, systematic studies of ultrasonic behavior, studies that are publicly available are needed. Studies must be designed to avoid issues of competition sensitivity on the part of device manufacturers, of ultrasonic equipment manufacturers, and of chemical companies and formulators. Such studies are essential for the progress of miniature, multi-material, and nano-scale devices.

References
*Adapted from a paper presented by B. Kanegsberg, “ASTM Symposium onCleanliness of Implants,” Reno, NV, May 2005.

References

1. R. Dowell, S. Norris, J. Unmack, B. Kanegsberg. “Choosing a Reliable, Environmentally-Preferred Process for Cleaning Prior to PVD,” presentation, Thirteenth Annual International Workshop on Solvent Substitution and The Elimination of Toxic Substances and Emissions, Scottsdale, AZ , (December, 2002).

2. B. Kanegsberg. “Comments to: Continuing JSSSWG”, presentation to Joint Services Solvent Substitution Working Group, Fairborn, OH, (November, 2004).

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; info@bfksolutions.com; www.bfksolutions.com.