Ultrasonics vs. Megasonics
07/01/2002
Critical Cleaning
Critical cleaning method can be a tough technology to grasp without careful approach
by Barbara Kanegsberg and Ed Kanegsberg
Ultrasonic and megasonic cleaning are essential tools in such surface preparation applications as mi cro electronics assembly, biomedical device manufacturing and wafer fabrication.
Equipment has evolved from simple tanks with on/off switches to sophisticated, complex and often incomprehensible cleaning systems.
While indispensable to achieving the stringent specifications for surface cleanliness and acceptable surface quality in critical applications, ultrasonic cleaning has become so routine that it may be used without a full appreciation of the potential and limitation of the techniques.
Both ultrasonic and megasonic cleaning use sound waves traveling through liquid, producing cycles of compression and rarefaction-a decrease in density and pressure.
The differences between ultrasonics and megasonics are in the frequencies of these sound waves and the effect these frequencies have on the liquid.
Waves produce both cavitation and acoustic streaming. During rarefaction, the liquid tears, or cavitates, to produce a vapor-filled bubble. Lower frequencies characterize ultrasonics, and with each cycle, the bubbles grow and finally implode at the speed of sound to produce local transient spots of heat and energy.
The forces of these small implosions physically remove particles. Because cavitation occurs throughout the cleaning solution, ultrasonics provides a cleaning force in small blind holes or areas unreachable by line-of-site cleaning (such as high-pressure spray).
Through a process known as acoustic streaming, the associated fluid motions also assist to solubilize thin-film soil by bringing fresh cleaning chemistry closer to the substrate, the surface to be cleaned, carrying away chemical-containing soil.
Two types of transducers, magnetostrictive and piezoelectric, convert electrical energy to vibratory mechanical energy and produce sound waves.
Magnetostrictive transducers use coiled wire to convert electrical energy to an alternating magnetic field, which induces mechanical vibrations at the desired frequency in nickel or some other magnetostrictive material. Piezo electric transducers use materials that change dimension in responses to an electrical charge, known as the piezoelectric effect. While both are reliable, piezoelectric transducers are more commonly used, particularly at higher frequencies, above 30 kHz.
The sonic boom of cleanliness
Ultrasonic cleaning is heavily influenced by the power amplitude or "loudness" and the frequency, or "pitch." At a given frequency, greater amplitude produces more force yielding stronger cleaning action, but with greater potential for product damage. Frequency also plays a role in cleaning action.
All other things being equal, as the frequency increases, one sees a decrease in bubble size as well as a decrease in the cavitation cycle time. This greater frequency of smaller bubbles is considered effective in removal of small particles with minimal invasiveness to the substrate. The exact frequency to use is pragmatic.
As a very rough rule-of-thumb, approximately 20 KHz is suitable for industrial, general metals cleaning, while 40 KHz is useful for many precision-cleaning applications. Frequencies of 60 to 80 KHz are often used for more delicate parts. Recently, 100- to 200-KHz systems have been introduced for very delicate parts cleaning. Some systems provide the option of multiple frequencies within a single tank.
Other variables related to equipment design come into play, including the wave shape (such as square waves resulting in more harmonic content), pulse cycle (on-off cycle of bursts of ultrasonic energy and quiescent periods), frequency sweep (a kind of vibrato around the main ultrasonic "note") and design of the transducer used to produce ultrasonics.
The physical characteristics of the cleaning agent or blend, including surface tension and viscosity and the chemical reactivity, combined with temperature effects provide additional variables.
Also, any dissolved gasses in the cleaning fluid can impact performance, usually negatively, by cushioning the forces of implosion. Many ultrasonic systems have the ability to de-gas the cleaning solution prior to using it for cleaning, usually by heating the liquid in combination with ultrasonic energizing.
Megasonic cleaning uses much higher frequencies, ranging from 360 kHz to 2,000 KHz. Megasonics are particularly useful for removal of sub-micron particles from flat surfaces. In contrast with ultrasonics, where the cavitation is transient or cyclic, at the high frequencies of megasonics the motions of the fluid result in stable cavitation, without implosions. There is relatively low likelihood of substrate damage. Megasonics is a line-of-sight process, although not quite the same as fluid directed at a surface processes, such as spray techniques.
In megasonics, cleaning occurs in a stream of fluid moving past the surface, like pebbles being dislodged in a flowing brook. Therefore, it is not suitable for complex, ornate structures or in blind holes.
The acoustic streaming involved in megasonics, as in ultrasonics, reduces the boundary layer, which normally shields a surface from fluid flowing past it. The cleaning solution is brought even closer to the substrate and therefore to the sub-micron particle, facilitating removal by the shear forces of fluid flow, and the high frequencies associated with megasonics make this boundary layer even thinner. Other factors such as time, temperature, power and chemistry also impact megasonic cleaning.
Sonic logic
The complete mechanisms of ultrasonics and megasonics, as well as the impact of variables on each cleaning application are not completely understood.
Assemble six experts in ultrasonic cleaning and you may receive nine clashing opinions. One issue is the competitive territoriality of each equipment supplier; or, more charitably, enthusiasm for a particular set of cleaning technologies.
Each equipment supplier is convinced that it has the logical approach to understanding ultrasonics; and the supplier may imply that the competitive claims range from promising, but misguided, to downright misleading.
One reason for the controversy is that there is no standardized, universally accepted method of comparing the performance of ultrasonic tanks. The classic test involves immersion of a piece of aluminum foil in the tank for a preset amount of time, and then observing the characteristic dimpled "orange peel" pattern.
Absence of such a pattern indicates lack of cavitation. However, beginning at approximately 70 to 100 KHz, the foil test is not effective. Ultrasonic meters have been introduced; and some users have found them to be helpful in comparing tank performance within a given facility. However, widespread utility remains to be determined. The only definitive method to determine efficacy of ultrasonics is through comparative cleaning tests.
Most ultrasonic systems are designed around cavitation of water. Manufac turers have not fully addressed the issue of cavitating solvents, particularly solvents with densities differing from water. It is relatively easy to cavitate solvents, which are heavier than water, particularly if the solvents are heated. It is more difficult to cavitate less-dense solvents such as alcohol and acetone.
Issues of flammability and exposure of employees to mists must also be addressed. Potential exposure problems include cleaning agents, additives to cleaning agents, soils, particles and traces of metals, so intelligent equipment design is a must. This involves working with your advisors as well as with equipment suppliers.
Testing is imperative. Choosing an ultrasonic or megasonic system based only on theory or on claims made by suppliers can lead to unpleasant surprises. Application-specific tests are needed to optimize soil removal and to minimize product damage.
Aqueous cleaning solutions, which are non-aggressive under conditions of heat and immersion, can become highly efficient with ultrasonics. Some can damage the substrate. For example, take a recent test of a magnesium component with a very adherent coating, capable of withstanding high temperatures and exposure to heated chlorinated solvent, in heated perchloroethylene with ultrasonics.
Both ultrasonics and megasonics can be effective tools and their range of applications overlap. Megasonic cleaning uses a very gentle gun to drag and lift particles from the surface, so it acts like a very high velocity jet, but one that can get much closer to the surface than an actual jet of liquid could.
Ultrasonic cleaning uses micro-demolition (actually multiple tiny implosions), producing many sites of energy to assist in soil removal and physically remove larger particles. Because it uses lower frequencies, it is capable of turning corners, so it is particularly useful in the surface preparation of complex components.
Acknowledgments: The authors would like to thank John Fuchs of Blackstone/Ney for his helpful comments and suggestions. Material from "Handbook for Critical Cleaning," Kanegsberg and Kanegsberg, ed, CRC Press, was used in preparation of this article.
Barbara Kanegsberg, president of BFK Solutions, LLC, an independent consulting company serving the cleaning and contamination control industry, has over 20 years of experience in process development. She is editor of the "Handbook for Critical Cleaning," CRC Press, 2001.
Ed Kanegsberg, vice president of BFK Solutions, LLC, is a chemical physicist and engineer with over 30 years involvement in all aspects of precision instrument development from instrument design to developmental testing and production.