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Ultrasonics, The Power of Sound Part 1 - Cleaning Forces in Perpective
By Barbara Kanegsberg, BFK Solutions LLC
“I always use ultrasonics.” or “I hate ultrasonics; we can’t use it.”
People have extreme opinions about ultrasonics. Some don’t trust ultrasonics and never use the technique; others attempt to ultrasonic clean for hours on end. If you avoid these extremes, you can improve your manufacturing productivity. Because ultrasonic cleaning is ubiquitous and because supply chains often constitute a long and winding road, ultrasonic cleaning probably happens somewhere in your process.
So, let’s begin by looking at ultrasonic cleaning as one of a number of cleaning forces.
Ultrasonic cleaning uses high-frequency sound, sound waves that are above the audible range. In ultrasonic cleaning, sound waves pass through a liquid producing rarefaction and compression. During rarefaction, vapor-filled “tears” in the liquid form the ultrasonic bubbles. During compression, the bubbles implode. The implosions result in localized, very transient high heat and force (pressure). This micro-cleaning action removes particles and thin films from the surface; if ultrasonics are not used prudently, there is also the potential to modify the surface, sometimes by erosion.
Ultrasonic cleaning uses high frequency sound waves in the range of approximately 20 KHz to approximately 500 KHz (the exact range depends on the source of the information; there is no standard definition). Ultrasonic cleaning is a very attractive technique for densely populated assemblies and blind holes because the technique is omni-directional rather than line-of-sight.
Most, but not all, cleaning processes involve both force and chemicals at the wash and rinse steps. Recall that most cleaning operations involve three steps, wash, rinse, and dry. The wash step uses a cleaning agent and often heat, force, and time to remove soil from the proximity of the surface and to keep the soil away from the surface. While the rinse step may continue the washing action, the primary purpose it to remove residual cleaning agent. Both steps have to be accomplished without damaging the part being cleaned. Let’s review some cleaning options. Some of them can be combined with ultrasonics.
Hand-wipe cleaning uses the force of elbow grease. The approach has also been termed ‘point of use’ cleaning. If you use a saturated wipe, cleaning and rinsing will occur primarily at the direct contact areas. Using spray bottles is a type of spray-in-air cleaning (discussed later in this article). If you submerge the part while wiping or brushing, there will be better efficiency than with simple immersion. However, effectiveness of washing and rinsing depends on the skill, strength, patience, and vigilance of the individual performing the task.
Immersion involves simply soaking the part to be cleaned in the cleaning chemistry or rinsing chemistry. Even if there is good solvency, the soil may not diffuse far from the surface within a practical amount of time. Soil may remain trapped at the wash step; cleaning agent may remain trapped at the rinse step. Immersion cleaning can readily be combined with ultrasonic ultrasonics. All other things being equal, the chemical medium markedly influences the power of ultrasonics.
Vapor phase cleaning involves suspending the part to be cleaned in solvent vapor; the vapor condenses on the part and drips off, carrying the soil with it. Because vapor phase cleaning involves freshly distilled solvent, it is the only time you are using perfectly clean solvent, with the exception of using brand-new cleaning agent. The extent of cleaning depends on solvency and temperature. You would use vapor phase cleaning with unblended organic solvents that are relatively low boiling or with azeotropes. Azeotropes are constant boiling mixtures. With azeotropes, the rate of evaporation is proportional to the percent composition, so the relative composition of the vapor does not vary. Often, vapor phase cleaning is used as a final step, after spray and/or ultrasonic cleaning. Vapor phase cleaning is often used as a final wash or a final rinse.
Moving the liquid below the surface encompasses several approaches. Some parts washers mechanically move parts up and down or side to side to achieve turbulation. For laboratory-scale operations, people use a stirring bar on a magnetic stirrer. Air jets and “Jacuzzi” type jets can be used to move the liquid. Other systems rotate the basket; and such rotation can often be productively used with ultrasonics to assure that the soil is moved away from the surface. Some systems use low-speed centrifugation to achieve mixing due to the coriolis force.
Spray-in-air is used in in-line (conveyor belt) systems as well as in cabinet washers. Spray-in-air has been used successfully in metal cleaning and in electronics defluxing. Spray-in-air is a line-of-site technique, sort of like using a water pistol. The part that needs to be cleaned has to be lined up with the spray of cleaning or rinsing chemistry. This means proper positioning of the part so that the wash and rinse chemistries can impact the surfaces to be cleaned. It seems simple; but you have no idea how often we inspect a cleaning system only to fine that the direction of the spray is nowhere near the location of the surface to be cleaned.
Vibratory and impingement cleaning use a solid media, often combined with a liquid, to physically impact the surface. Such cleaning may be thought of as surface finishing rather than cleaning. While it is true that such processes may impart an aesthetically desired surface polish, we regularly encounter impingement or vibratory processes that in effect serve as the functional cleaning process.
Anodic and cathodic processes involve electronic charge forces that may encompass cleaning or may simply involve surface finish.
Megasonic cleaning involves frequencies of 500+ KHz. The cleaning action is acoustic streaming, not cavitation, so cleaning is unidirectional. The technique is commonly used in wafer fabrication.
Some people are concerned about ultrasonic erosion of the surface; and that is a real possibility. It is also true that any force can eventually erode a surface – just think of the Grand Canyon. Using ultrasonic cleaning involves optimizing the process. As with any cleaning process, you have to balance the benefits of contamination removal with the risk for surface modification or product damage.
Next: Ultrasonic variables.
