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......
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:
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:
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
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.
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.
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.
It is essential that cleaning fluids be selected on the basis of
Insoluble particulate contaminants can, for example, be divided into two groups:
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.
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
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