Ultrasonic Parameters I
Barbara Kanegsberg and Ed Kanegsberg, November 2005
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 involving extremely
high forces and local temperatures up to 20,000°K, four times that
of the surface of the sun [2]. Interactions among parameters are generally
neither
independent nor linearly additive. As with any force used in cleaning or
extraction, ultrasonics is a balancing act to remove the unwanted contaminants
without negatively impacting 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 [3]. 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,
gravimetric
determination 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 [4]. There is a large variation among
chemicals
both in the
optimal temperature 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 [5] indicate that even heavy
duty
foil was significantly
eroded after 10 minutes of ultrasonic exposure. Effects of ultrasonics
on aluminum are qualitative but indicate the power of ultrasonics
to alter surfaces.
Cavitation caused
erosion as function of the temperature for
aluminum foil in different liquids [4]. Note- Low flashpoint
solvents must be used with appropriately designed equipment
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.
References:
1. B. Kanegsberg, E. Kanegsberg. Controlled Environments,
(August 2005).
2. K. Suslick, D. Flannigan. “Plasma Formation and Temperature Measurement
During Single-Bubble Cavitation”, Nature Vol. 434, (3 March 2005)
pp.52-55.
3. B. Kanegsberg, E. Kanegsberg. “Measuring Sonics”, Parts
1 to 3, A2C2 Magazine, (December, 2001, January, 2002, and February, 2002).
4. L. D. Rosenberg. "On the Physics of Ultrasonic Cleaning," Ultrasonic
News, (Winter 1960) p. 16.
5. 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 cleaning, 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.
