You are in charge of improving the cleaning of your company’s product. You think ultrasonic cleaning is needed but need to know which frequency is best. In this two-part article, we will explore aspects of ultrasonic cleaning to make it easier to choose; in some cases, a system with more than a single frequency may be optimum.
High-frequency sound is a powerful tool in critical cleaning. The sound creates the agitation of the molecules of the liquid that dislodges soils and results in cleaning. The nature of the agitation is strongly dependent on the frequency of the sound. Cleaning can be accomplished using frequencies ranging from a “low bass” of ~20 kHz to a “high soprano” of over 1MHz. The higher frequencies, generally >400 kHz, are called megasonics. There is no sharp definition for where ultrasonics becomes megasonics.
Sound waves consist of repeating cycles of rarefaction and compression. At lower frequencies, the cohesive tensile forces that hold liquid molecules together can be overcome during rarefaction by sufficiently strong or loud sound and create cavitation, voids or vapor bubbles in the liquid. During the subsequent compression, these voids implode, generating strong shock waves that impact contaminated surfaces and provide cleaning action.
Understanding and controlling the variables is a key to avoid fear of ultrasonics, “ultrasoniphobia.” Any cleaning agent and any cleaning process has the potential to damage the part being cleaned. Cavitation creates microscopic areas of extremely high heat and high pressure. Particularly at low frequencies and high power density, delicate or very thin parts may be eroded or crazed. This is how thin aluminum foil works to demonstrate the presence of ultrasonic cavitation. The foil becomes dimpled and eventually destroyed by strong ultrasonic action. Harder metals, like stainless steel, heavy gauge foils and parts that do not have sharp edges, are far less prone to damage.
When the ultrasonic frequency is higher, there is less rarefaction time during which ultrasonic voids grow, so the bubbles are smaller and the implosion is less violent. This means less of a potential for surface damage. The aluminum foil test for cavitation is much harder to observe at 130kHz than at 40 kHz. In addition, the higher the frequency, the higher the efficiency for dislodging small particles. Therefore, for cleaning delicate parts or when particles to be removed are small, frequencies exceeding 100kHz may prove best.
At high frequencies, energy from cavitation lessens as a cleaning mechanism. Acoustic streaming becomes the primary action. Think of acoustic streaming as a back and forth scrubbing motion. This motion is unidirectional, in the direction of the sound wave propagation. Therefore, megasonics, where acoustic streaming is the dominant mechanism, is most effective for cleaning flat accessible surfaces such as semiconductor wafers. For parts with holes or intricate shapes, lower frequencies, where omnidirectional cavitation energy is present, should be considered.
As a product cleaning professional, you have probably encountered the acronym TACT. The letters represent four key facets for effective cleaning, Temperature, Action, Chemistry, and Time. Action is the aspect that I have discussed here, but ultrasonic cleaning involves all four facets. In a subsequent article, I’ll explain how the others play not only a strong but an interactive role as well and need to be part of the consideration of the appropriate frequency or frequencies.