The Ultrasonic Cleaning Process

by Maurice O'Donoghue


Ultrasonic cleaning has found its most successful application in the  removal of insoluble particulate contamination from hard substrate surfaces.  Contamination that is soluble or emulsifiable can usually be removed with facility by  means of conventional methods in conjunction with suitable solvents or  detergent solutions.

Such techniques, however, cannot adequately remove  particulate matter in the micron and sub-micron size range to the extent that is  necessary, for example, for the critical cleaning required in the microelectronics  and optical industries or for the preparation of surfaces prior to the  application of thin films or coatings.

A number of methods have been used for the purpose of removing  microparticulates from hard surfaces. These include pressure spraying or manual and  mechanical scrubbing with solvents or detergent solutions; vapor degreasing; ion  bombardment; plasma, chemical, or ultrasonic cleaning; and ultraviolet/ozone  cleaning. The intent of this discussion, however, is not to evaluate the relative  merits of these methods but rather to describe ultrasonic technology......

Principles Of Ultrasonic Cleaning

In general, ultrasonic cleaning consists of immersing a part in a  suitable liquid medium, agitating or sonicating that medium with high-frequency  (18 to 120 kHz) sound for a brief interval of time (usually a few minutes),  rinsing with clean solvent or water, and drying. The mechanism underlying this  process is one in which microscopic bubbles in the liquid medium implode or  collapse under the pressure of agitation to produce shock waves, which impinge on the surface of the part and, through a scrubbing action, displace or  loosen particulate matter from that surface. The process by which these bubbles collapse or implode is known as cavitation.

High intensity ultrasonic fields are known to exert powerful forces  that are capable of eroding even the hardest surfaces. Quartz, silicon, and  alumina, for example, can be etched by prolonged exposure to ultrasonic  cavitation, and "cavitation burn" has been encountered following repeated cleaning  of glass surfaces. The severity of this erosive effect has, in fact, been  known to preclude the use of ultrasonics in the cleaning of some sensitive,  delicate components.

Ultrasonic cleaning has, however, been used to great  advantage for extremely tenacious deposits, such as corrosion deposits on metals.  In any case, cavitation forces can be controlled; thus, given proper  selection of critical parameters, ultrasonics can be used successfully in  virtually any cleaning application that requires removal of small particulates.

Although the ultrasonic cleaning process has been used for over half a century, no reliable means of quantifying its cavitation activity has  ever been developed. Indirect methods of measurement, such as erosion tests  on metal surfaces, soil removal from weighted samples, acceleration of  chemical reactions, thermodynamic studies, and white noise measurement, have been employed to a limited extent, but none of these methods has proved to be effective.

Thus, operators who seek to assess the performance of  an ultrasonic cleaning system must rely almost exclusively on the  evaluation of actual cleanliness levels achieved. Surface patterns produced on  cavitating liquids can also be observed, as can the overall degree of agitation of  the cleaning medium. Operators have also observed erosion patterns produced  on aluminum foil following exposure to ultrasonic cavitation. This  "aluminum foil erosion test," as it is called, has come to be recognized as a  fairly dependable, albeit subjective, means of demonstrating the existence of  cavitation in ultrasonically agitated media. The measure has been used not only to  provide an indication of the distribution of the sonic field throughout the  bath, but also to locate the sites of the nodes and antinodes of the standing  sonic waves. It can also generate fairly reliable side-by-side comparisons of  different ultrasonic cleaning systems. In no way, however, can it be used to  obtain quantitative measurements of cavitation activity.

The cavitation intensity in a sonic field is largely determined by  three factors:

  1. The frequency and amplitude of the radiating wave
  2. The colligative properties of the medium, including vapor pressure,  surface tension, density, and viscosity
  3. The rheological properties of the liquid, including static  condition, turbulent flow, and laminar flow.

Let us now examine each of these three factors in greater detail.

Frequency and Amplitude.


The radiating-wave  frequencies most commonly used in ultrasonic cleaning, 18-120 kHz, lie just above  the audible frequency range. In any cleaning system, however, the harmonics  of the fundamental frequency, together with vibrations originating at the  tank walls and liquid surface, produce audible sound. Thus, an operating  system that is fundamentally ultrasonic will nonetheless be audible, and low  frequency (20-kHz) systems will generally be noisier than higher-frequency  (40-kHz) systems.

Moreover, ultrasonic intensity is an integral function  of the frequency and amplitude of a radiating wave; therefore, a 20-kHz  radiating wave will be approximately twice the intensity of a 40-kHz wave for any  given average power output, and consequently the cavitation intensity  resulting from a 20-kHz wave will be proportionately greater than that resulting  from a 40-kHz wave.

The cavitation phenomenon will, of course, occur  less frequently at 20 kHz, but this is not thought to have a significant  bearing on cleaning effectiveness. However, the longer wavelengths of  low-frequency ultrasonic systems result in substantially different standing-wave  patterns throughout the liquid medium.

The standing or stationary waves produced by ultrasonics in liquid  media result from the simultaneous transmission of the surface-reflected wave  motion and the wave motion originating at the transducer radiating surface. The fixed points of minimum amplitude are called nodes, and the points of  maximum amplitude are called antinodes.

Obviously, the distance between  the nodes and antinodes of the 20-kHz standing wave (2 in.) will be  approximately twice that of the 40-kHz wave. Because cavitation takes place primarily  at the antinodes, the distance between cavitation sites will thus be larger with 20-kHz than with 40-kHz radiation, and the 20-kHz waves will also  have larger dead zones (i.e., zones with little or no cavitation activity)

It is for this reason that cleaning resulting from 20-kHz radiation is likely to be less homogeneous and less consistent, even though this frequency  produces more intense cavitation. Much of the inhomogeneity in ultrasonic fields  can, however, be reduced or wholly eliminated through the use of sweep  frequencies, or radiating waves with a multitude of different frequencies. By this  means, several overlapping standing waves can be generated at the same time,  thereby eliminating much of the dead zone.

The amplitude of the radiating wave is directly proportional to the  electrical energy that is applied to the transducer. In order for cavitation to be  produced in a liquid medium, the amplitude of the radiating wave must have a  certain minimum value, which is usually rated in terms of electrical input power to the transducer. No cavitation can occur below this threshold value,  and the use of electrical power over and above the minimum level results not in more intense cavitation activity but rather in an increase in the  overall quantity of cavitation bubbles. The minimum power requirement for the  production of cavitation varies greatly with the colligative properties and  temperature of the liquid and with the nature and concentration of dissolved  substances.

The Colligative Properties of the Liquid.


The  intensity with which cavitation takes place in a liquid medium varies greatly with the colligative properties of that medium, which include vapor pressure, surface tension, viscosity, and density, as well as any other property  that is related to the number of atoms, ions, or molecules in the medium. In  ultrasonic cleaning applications, the surface tension and the vapor pressure  characteristics of the cleaning fluid play the most significant roles in determining  cavitation intensity and, hence, cleaning effectiveness.

The energy required to form a cavitation bubble in a liquid is proportional to both surface  tension and vapor pressure. Thus, the higher the surface tension of a liquid,  the greater will be the energy that is required to produce a cavitation  bubble, and, consequently, the greater will be the shock-wave energy that is  produced when the bubble collapses. In pure water, for example, whose surface  tension is about 72 dyne/cm, cavitation is produced only with great difficulty  at ambient temperatures.

It is, however, produced with facility when a surface-active agent is added to the liquid, thus reducing the surface tension to about 30 dyne/cm. In the same manner, when the vapor pressure of a liquid is low, as is the case with cold water, cavitation is  difficult to produce but becomes less and less so as temperature is increased.  Every liquid, in fact, has a characteristic/temperature relationship in which  cavitation exhibits maximum activity within a fairly narrow temperature range.

The Rheological Properties of the Liquid.


The  flow characteristics, or rheological properties, of the cleaning fluid play a highly significant role in ultrasonic cleaning applications.

Static fluid conditions, for example, are highly conducive to the formation of the standing wave patterns that characterize intense ultrasonic fields, and  hence it would seem likely that cavitation intensity would be maximized under  such conditions.

In fact, however, optimum performance is seldom  achieved in static fields, since continuous purification of the cleaning fluid  either by overflow or by recycle filtration‹a process that necessitates fluid  change of up to 50% of the total bath volume per minute‹is often a prerequisite to effective cleaning. And, contrary to what one might anticipate under  such conditions, little or no cavitation activity is lost to this fluid flow  when it is properly introduced into the bath. In fact, improvement in overall surface impingement and homogeneity of cleaning can be realized with  this method.

In designing a cleaning process, one must give primary consideration  to the size, configuration, and capacity of the ultrasonic tank so that  this structure will be able to accommodate the parts to be cleaned in  sufficient quantity to fulfill production requirements.

Of course, every  individual cleaning application has its own set of variables, such as the number of parts per load, the orientation and spacing of these parts, and the  fixturing arrangements. There are, however, certain basic rules that can be used  as guidelines in making design-related determinations. These are as  follows:

Tank Loading

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The total surface area of the  substrates, measured in square inches, should not be much greater than the tank  volume, measured in cubic inches. In other words, the total surface area should  not be greater than 230 sq. in. per gallon of tank capacity.

Work Baskets and Fixtures

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Work baskets or  fixtures should have as little mass as possible, should be made of metal  (preferably stainless steel or some other hard, sound reflecting material), and  should be of open construction so that there will be minimal interference with  the free passage of both sound waves and cleaning fluids.

Work Orientation


The parts in the cleaning  system should be arranged in such a way that they are evenly spaced throughout  the tank volume, and they should be positioned so that their narrowest  dimensions are oriented toward the transducer radiating surfaces. Large, flat  surfaces that face transducers tend to screen sonic radiation from the bulk of  the cleaning fluid.

Location of Transducers.


Whenever possible,  sonic transducers should be placed on the largest sides or on the bottom of  the tank to allow for maximum distribution of the sonic energy throughout  the cleaning solution.

Power Requirements.


The ultrasonic power  requirements of almost all cleaning applications, expressed in terms of  electrical-input wattage to the transducers, lie in the range of 50-100 W per gallon of  cleaning fluid, or 2.8-3.6 W per square inch of transducer radiating surface.  These values apply only to piezoelectric transducers, which are the most  commonly used transducers in ultrasonic cleaning systems today.

Cleaning Fluid.


It is essential that cleaning fluids be selected on the basis of

  1. The chemical and physical nature of the contaminants to be removed;  and
  2. The identity of the substrate material.

Insoluble particulate contaminants can, for example, be divided into  two groups:

  1. Water-wettable or hydrophilic particles, including metal particles,  metal oxides, minerals, and inorganic dusts
  2. Non-water-wettable or hydrophobic substances, including plastic  particles, smoke and carbon particles, graphite dust, and organic chemical dusts.

Substrate surfaces, too, can be divided into hydrophilic and  hydrophobic groups. Rarely are hydrophobic contaminants found on hydrophilic  substrates or vice versa, but when this is the case, cleaning is best accomplished  simply through rinsing with a suitable solvent. Hydrophilic particles on  hydrophilic substrates, on the other hand, are best removed with aqueous detergent  solutions, while hydrophobic particles on hydrophobic substrates are most  effectively removed by the use of organic solvents.

Conclusion

In this article, efforts have been made to describe the unique  cleaning capabilities of the ultrasonic process. When they are properly employed, ultrasonic cleaninds can provide a highly effective means of  removing insoluble particulates from hard substrate s
 

 

About the Author:

Maurice O'Donoghue is one of the pioneers in the ultrasonic cleaning industry. He had a BS and MS in chemistry and worked in the chemical industry in Cyanamid / Union Carbide and Purex companies prior to joining the ultrasonic cleaning industry in the early 1970s. He was a Lab Manager at Crest Ultrasonic Corporation then worked as an independent  Consultant in the same industry. He was well respected by all his colleagues, staff and his customers for his wide experience, knowledge, dedication and thoroughness in his work.
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