Ultrasonic & Megasonic Cleaning

High power ultrasound has been used in cleaning applications for decades. Switching from cleaning with ozone-depleting solvents to aqueous processes and green chemicals in the 1990s has created a significant need for more advanced ultrasonic technology. A second compelling reason was the need to achieve higher standards of cleanliness in constantly advancing technologies for medical, automotive, semiconductor and disk drive industries. New systems with higher frequencies were developed. Some included new single frequency, multiple variable or fixed frequencies. Simultaneously, new chemistries were developed, such as ozone-safe solvents, new aqueous cleaning solutions, and advanced inhibitors that help to protect various steels during aqueous cleaning, prior to PVD coatings, and during storage or shipping abroad. Most of these new developments have evolved during the past two decades. With the use of far greater volumes of water in cleaning, a big effort has been made to develop systems to take care of recycling the water and saving the environment.

High Power Ultrasonics and Megasonics

Ultrasonic cleaning has been and is commonly used in applications such as microelectronics, hard-disk drives, medical and biomedical devices and instruments, components of automotive, aerospace, optics, glass, tools, jewelry, filters, heat exchangers, various molds, and weapons. An understanding of the basis of the technology and how it works can help in identifying the best frequency and power level for an application. However, what is good for one application might not be as good for another, regardless of how similar.

Typically, other parameters such as material composition, component design, surface finish, parts handling and cleaning chemicals must be taken into consideration when developing a process and selecting the proper ultrasonics and cleaning chemistry for an application.

Currently the available frequency range is from 20 kHz to 1MHz, with a good number of sweep high frequencies available from 20-500 kHz. Also, the current technology provides a uniform ultrasonic activity throughout the cleaning vessel, the lack of which was a major disadvantage in earlier stages of the technology.

The difference between ultrasonics and megasonics is mainly in how the sound waves interact with a fluid at a given frequency. At low frequencies (roughly below 800 kHz), the high-power sound waves interact with a fluid to create micro vacuum bubbles that grow to critical volumes and then implode with a great force. This phenomenon is known as cavitation. The products of such implosions are wave shocks, along with small, high-velocity microstreaming. However, at frequencies higher than 850 kHz, the ultrasound waves interact with fluids to give mainly high-energy acoustic streaming with very mild to zero cavitation. Recent testing has shown that no cavitation is detected above 5 MHz.

The mode of action applied on immersed surfaces is different. That is why ultrasonics is used to clean complex geometries while megasonics seems to be limited mainly to cleaning of flat surfaces.

Mechanism of Cavitation Formation

Cavitation is generated through nucleation, growth, and violent collapse or implosion. Transient cavities (bubbles) range in size from 50-150 μm in diameter at 25 kHz. They are produced during the half cycles—the rarefaction phase of the sound wave. The liquid molecules are extended outward against and beyond the liquid natural physical elasticity/bonding/attraction forces, generating vacuum nuclei, which continue to grow to a critical volume. A violent collapse occurs during the next compression phase, releasing the stored energy in different forms.

Cavitations are generated in microseconds. At the 20-kHz frequency, it is estimated that the pressure is about 50–70 MPa (500 atm), and the transient localized temperatures are about 9,000°F (5,000°C). The implosion changes the bubble into a micro jet one-tenth the bubble size, which travels at speeds up to 250 mph (400 km/hr) toward the hard surface.

The bubble size becomes smaller at higher frequencies as a function of the wavelength. For example, at 100 – 150 kHz, it is estimated to be about half the size of cavitations generated at 40 – 70 kHz. For water at ambient temperature, the minimum amounts of energy needed to be above the threshold are about 0.3 and 0.5 W/cm2 (per transducer radiating surface) for 20 and 40 kHz, respectively.

Ultrasonic Frequency Options

Different ultrasonic frequencies should be selected for different applications. The majority of applications use frequencies in the range of

20–200 kHz. Megasonics for nano particle removal encompasses frequencies above

850 kHz.

20–40 kHz – Used for heavy-duty cleaning of items such as engine blocks and heavy metal parts and for removal of heavy greasy soils.

40–80 kHz – Used for general cleaning of machine parts, optics and other components. This frequency range is very good at removing small to medium particles.

80–190 kHz – Used for gentle cleaning of optics, disk drive components and other sensitive parts.

190–500 kHz – Used for ultra-fine cleaning of semiconductor wafers, ultrathin ceramics, optics and highly polished metallic mirrors or reflectors.

The ultrasonic power delivered to a cleaning tank must be adequate to cavitate the entire volume of liquid with the workload in place. Watts per gallon is the unit often used to measure the level of ultrasonic power in a cleaning tank. As tank volume is increased, the number of watts per gallon required to achieve the required performance is reduced.

Cleaning parts that are very massive or that have a high ratio of surface to mass may require additional ultrasonic power. Excessive power may cause cavitation erosion or “burning” on soft metal parts. If a wide variety of parts is to be cleaned in a single cleaning system, an ultrasonic power control is recommended to allow the power to be adjusted as required for various cleaning needs. Part exposure to both the cleaning chemical and ultrasonic energy is important for effective cleaning. Care must be taken to ensure that all areas of the parts being cleaned are flooded with the cleaning liquid. Parts baskets and fixtures must be designed to allow penetration of ultrasonic energy and to position the parts to assure they are exposed to the ultrasonic energy. It is often necessary to individually rack parts in a specific orientation or rotate them during the cleaning process to thoroughly clean internal passages and blind holes.

Cleaning of Surfaces: Criteria and Needs

A safe ultrasonic surface cleaning, by general definition, is the freeing of a surface from contaminants that are adhered chemically or physically to that surface, without inflicting any damage to the surface. Contaminants may be grouped into three categories:

Organic contaminants include lubricating oils, cutting and machining fluids and oils, fingerprints, carbon, organic media in buffing, polishing and lapping compounds, waxes, silicone oils, mold release compounds, coolants, polymers, adhesives, photoresist residues, lacquers, paints, inks, antifoam additives, and residual biocides.

Inorganic contaminants include various metal oxides in buffing and lapping compounds, polishing compounds, inorganic salts, dust, metal fumes, slivers, and other metal oxides.

Particulate contaminants include environmental debris, skin flakes, cosmetics and hair, in addition to others from the previous examples.

An ultrasonic cleaning process involves much more than just the scrubbing action of cavitation forces. In a 24/7 production setting, special attention must be given to a whole set of parameters to achieve a consistent high yield of components that meet their preset cleanliness specs.

The process/equipment design must take into consideration the use of compatible and effective cleaning chemistry, parts handling, process parameter controls, the sound frequency, the sound power amplitude, the machine configuration and construction, equipment maintenance and proper packaging. Many high-end industry standards for particle removal sizes are in the nanometer range.

Why Ultrasonics?

Though ultrasonics is widely used, some professionals still have little experience with it and don’t understand the advantages it can bring. Ultrasonic cleaning is powerful enough to remove strongly adhered contaminants. It provides excellent penetration and cleaning in the smallest crevices and between tightly spaced parts.

Many industries have recognized the efficacy of ultrasonic cleaning and replaced  conventional cleaning methods with ultrasonic technology. They have discovered numerous advantages, including a greatly reduced per-component cost of cleaning. Although capital costs, chemical consumption and service charges are typically higher, those increases are more than countered by lower labor costs and decreased time needed to clean a component to spec.

During testing, ultrasonic cleaning has been shown to be more efficient than conventional methods in removal of contaminants (“dirt”). In one previous comparison, A.H. Crawford explains in “Large Scale Ultrasonic Cleaning, Ultrasonics 6” (1968, p. 211) that the method was 99 percent effective, compared with brush cleaning at 92 percent, steam degreasing at 35 percent, forcible motion in fluid at 30 percent, and spraying at 14 percent.

Components cleaned ultrasonically may be more reliable and have a longer lifespan than those cleaned by other methods. The improved surface finish can help to reduce wear and damages from friction. And because fewer parts have to be rejected through inefficient or damaging cleaning, cost of materials is reduced as well. These advantages combine to help improve productivity.

A typical ultrasonic cleaning system (Figure 2) consists of one or two ultrasonic wash stations, two reverse-cascade rinses, a recirculating hot air dryer and an optional vacuum dryer for special applications. The ultrasonics can be incorporated in all tanks. In a simple cleaning application, at least two ultrasonic tanks are required, one in the wash and one in the first rinse tank. One frequency can be used in all stations. However, the new trend is to use low frequency in the wash station(s) and higher frequencies in the rinses. Consultation and testing are essential to determine which frequencies best fit a process without causing any erosion damage to the components.

Ultrasonic cleaning has been used to overcome extremely tenacious deposits. Cavitation forces can be controlled given proper selection of critical parameters. Power ultrasound can be used successfully in virtually any cleaning application that requires removal of small particulates.

However, prolonged exposure to high-intensity ultrasonic fields is known to exert powerful forces that are capable of eroding even the hardest of surfaces, including quartz, silicon and alumina. ‘‘Cavitation burn’’ has been encountered following repeated cleaning of glass surfaces. The solid material itself does not affect the existence of cavitation.

Principle Mechanisms

Cavitation and acoustic streaming are the two principal mechanisms that work together in all forms of ultrasonic cleaning. But the relative contribution of each is a function of frequency. At low ultrasonic frequencies, cavitation is very strong and dominates the cleaning process. At high ultrasonic frequencies, cavitation bubbles are very small, but acoustic streaming velocities can be very high. Thus, at ultra high frequencies, acoustic streaming dominates the cleaning process and less of the cleaning action is occurring from cavitation effects. It should be noted that the cavitation strength increases rapidly with decreasing frequency. Also, increasing the frequency increases cavitation abundance (bubble density).

Implosions near the part surface will release energy that collides with and fragments (or disintegrates) the contaminants, allowing the cleaning chemicals to dissolve or displace the contaminants at a very fast rate. The implosion also produces dynamic pressure waves, which carry the contaminant fragments away from the part surface. The implosion is also accompanied by high-speed microstreaming currents of the liquid molecules. The cumulative effect of millions of continuous tiny implosions in the liquid medium is what provides the necessary mechanical energy to break physically bonded contaminants, speed up the hydrolysis of chemically bonded contaminants, and enhance the solubilization of ionic contaminants.

Cleaning Chemistry

The chemical cleaning medium is a crucial factor in enhancing the removal rate of various contaminants. Even with the best ultrasonics, a poor cleaning chemical choice can still cause a process to fail.

After assessing the chemical and physical nature of the contaminants, cleaning fluids are selected on the basis of compatibility with the substrate material(s), environmental considerations, and cleanliness specifications. Aqueous and solvent cleaning each have advantages and disadvantages. With the proper chemistry, aqueous cleaning is universal and achieves better cleaning results.

In fact, cleaning is more complex than just extracting the contaminants from the component and moving them away from the surface. Soil loading and the encapsulation/dispersion of contaminants directly affect the lifetime of the cleaning medium and, therefore, operating cost. Chemical formulations are different, not all cleaning chemicals will work the same, and no single chemistry is universal. For example, solvents are appropriate for removing organic contaminants, but not for removing inorganic salts. Also, the solvent must cavitate well with ultrasonics and be compatible with components to be cleaned. Collective properties for an aqueous cleaner, such as wettability, stability, soil loading, oil separation, effectiveness, dispersion or encapsulation of solid residues, ability to rinse readily, and disposal considerations – all must be addressed in the selection process. With so many factors to consider, it may be best to involve the advice of an expert to aid in the decision.

Saving the Environment

In a manufacturing plant, wastewater is diverse in nature. It can come from manufacturing processes, floor scrubbers, mop buckets, rinse tanks, parts washers and many other sources. Reducing the amount of waste water sent to the ground contributes tremendously to the quality of local drinking water.

Different technologies and methods have been suggested or developed for recycling water used in cleaning, but there is a distinct difference between dealing with water used in cleaning bath chemistry and water collected from the rinses. Used bath water requires extensive treatment, and recovery takes a long time, whereas rinse water can be recycled, treated and restored to pure deionized water in a short time. The reason is obvious—the high chemical concentration in the wash solution.

Different methods have been developed to treat the used cleaning chemistry.  Evaporative disposal (boiling or high speed aeration) of wastewater is safe and naturally extracts the water from the waste stream, leaving behind only the solid waste (usually about 5 – 10 percent of the total volume). The drawback is that the process is slow, has a high energy cost, and is not suitable for large volumes of rinse water.

Rinse Water Recycling

Deionized water is recommended in most ultrasonic cleaning applications. To help save water and cost, water purification technologies such as ultra filtration, reverse osmosis and carbon/ion exchange systems were developed and tested. The last of these has proven to be the choice of many process engineers.

The system is designed to purify the rinse water and send it back to the rinse station. It can also be used to deionize the incoming source water to initially fill the cleaning system. The purity of the water at circulating rates of up to 12 gpm is more than 10 MOhm, based on the system configuration.

The system uses granular activated carbon with cation and anion tanks, but mainly mixed cation and anion resins in one tank. First, the system filters all insoluble residues. The organic residues are adsorbed on the carbon, and the ion exchange occurs on the resin surfaces.

A typical system (Figure 3) also has a pre-filter at 1-5 µm and a post-filter at 0.5 to 1 µm. More savings are realized this way, as the circulating rinse water temperature can be maintained at temperatures below 140ºF using a minimum amount of heat energy when compared to heating room temperature water to about 120ºF at high flow rate.

Periodically, the carbon is replaced and specialized companies will collect the ion exchange media to be regenerated.

Convincing Solution

Properly designed, ultrasonic energy can contribute significantly to the speed and effectiveness of many immersion cleaning and rinsing processes. It is especially beneficial in increasing the effectiveness of today’s preferred aqueous cleaning chemistries and, in fact, is necessary in many applications to achieve the desired level of cleanliness. Ultrasonics and aqueous chemistries combined can often give results surpassing those previously achieved using solvents. Ultrasonics is not a technology of the future; it is very much a technology of today. PC

Sami B. Awad, Ph.D. is the head of Ultrasonic Apps, LLC, an advisory service group specializing in ultrasonic applications. The group helps current users in solving production issues and new users in all aspects of a new project, from system/process design to production optimization, including selecting frequencies, equipment, chemistry and waste minimization. Dr. Awad has decades of experience in ultrasonic applications and innovative chemistries for precision cleaning and surface treatments. He has a Ph.D. in organic chemistry, served as principle scientist with Henkel Surface Technology, and was the V.P. of technology and the director of the lab for 19 years at Crest Ultrasonics Corp. For more information, call 610-930-5070 or visit ultrasonicapps.com.