The Development of Ultrasonics

June 4, 2010 by · Leave a Comment 

The Development of Ultrasonic Cleaning

By: Edward W. Lamm October 2003

The history of ultrasonic cleaning dates back almost 70 years to the early 1930s. One of the laboratories at the Radio Corporation of America (RCA), located in New Jersey, discovered the use of ultrasonics for cleaning quite by accident. While using Freon to cool the internal components of a radio, they noticed a wave action surrounding a crystal that was operating at 300 kHz. Although this phenomenon had interesting characteristics, it was not actively pursued as a cleaning mechanism for years.

The majority of ultrasonic cleaning systems, which were developed later in the 1950s, were operated at 18 to 40 kHz. 18 kHz is the lowest frequency, which is still used routinely in industry today; however, a 6 kHz system was developed in Russia but was short lived. Up until the late 1980s most of the commercially available systems operated at 25 to 40 kHz.

The use of ultrasonics in this frequency range provided cleaning for thousands of applications where no other means of agitation was effective. The energy imparted to a component by ultrasonics is fairly aggressive. This is extremely beneficial in the case of soil that effectively adheres to the substrate. However, it is detrimental to a substrate that is delicate and can be damaged by the robust activity of ultrasonics. In the past seven years, a number of advances in the field of ultrasonics have had a positive impact on the ability to utilize this mode of soil removal on sensitive components. It is during this period that new developments in higher mid-range ultrasonic frequencies become an area of focus; but let us first look at how and why ultrasonics is so effective.

How It Works

In producing a clean part, 95% of the time mechanical energy is required to separate the soil from the substrate in both the wash and rinse steps of the cleaning process. A number of options are shown in Table 1. This chart shows methods of imparting energy to a part in a fluid in an effort to remove soil. The left-hand column shows the different mechanical energy methods. Parameters are shown for comparing and rating each method. If you examine the methods, you can see that each has a relative ability to remove soil and comes with an associated cost. While they all have the capability of providing a clean part, none truly compares to ultrasonics in both ability and cost.

For ultrasonic cleaning, two common rules of thumb that are accepted in the industry are: *If ultrasonics is used in the wash, use it in the rinse.

* For delicate components, use the next higher frequency in the rinse.

The second rule is recommended because a wash solution usually has a surfactant, which reduces surface tension. This tempers the ultrasonics in the wash step. The surface tension of DI water in the rinse is significantly higher, resulting in stronger cavitations that could damage a delicate surface. Going to a high frequency will reduce the cavitational energy of the DI rinse. To get a better understanding of this phenomenon, let us look more closely at the ultrasonic ranges in use today.*<100 kHz-”Conventional” Ultrasonics Range *100-250 kHz-”High Frequency” Ultrasonic Range

*>250 kHz-”Megasonic” Range

These ranges are determined by the energy in their cavitations. The conventional 40 kHz and lower frequencies have been around a very long time. The megasonic range also has a long history, but was not used for a substantial time period until the semiconductor industry investigated its ability to remove sub-micron particles without damaging the substrate. It has only been within the last few years that interest in high frequency has generated a number of products that are now available.

Where the Energy Comes From

The primary mechanism in ultrasonic cleaning is the energy released from a cavitation. As sound propagates from the transducer, it produces pressure gradients that compress and expand the liquid. (See Figure 1). As the expansion occurs, cavitation bubbles form. As the bubbles grow, they grow to a size where they can no longer support the weight and pressure of the liquid and thus collapse. When they collapse, they generate a tremendous shock wave on a microscopic level. Theorists have calculated that the localized temperatures and pressures generated reach thousands of degrees centigrade and hundreds of atmospheres.

The shock wave of energy from each cavitation is instrumental in the removal of particles when it occurs close to the surface to which the particle is bound. The closer to the surface the better. Figure 2 shows how close a sound wave, the pressure gradient and cavitation can get to a surface and penetrate the boundary layer. As the frequency is increased the boundary layer is reduced permitting the energy closer to the surface to remove smaller particles.

The boundary layer, next to the substrate’s surface where the sound does not penetrate, is essentially motionless. At 40 kHz, it is fairly thick at 2.8μ where smaller particles can hide out. (See Figure 3). As you increase frequency, the boundary layer is reduced, permitting the higher frequency to be closer to the surface and therefore the soil. For example, at 400 kHz the boundary layer is reduced to 0.98μ.

While cavitation offers the ability to clean in tight clearances that no other agitation can effectively penetrate, it also comes with the detrimental effect of aggressive energy that can damage “soft” materials. The lower frequency range has very intensive cavitations. At 20 kHz you can knock sand off a casting. However, you can soften the blow of ultrasonics by lowering the surface tension of the fluid. This can be accomplished by increasing temperature or adding a surfactant. However, adjusting the surface tension has its limitations. Some processes are sensitive to temperature and will not permit a high level. In others, the addition of a surfactant may be unacceptable since it becomes a contaminant to the process. The other option is to increase the ultrasonic frequency.

Cavitation strength is also important because with more delicate parts you do not want to damage the substrate as you get closer to the surface. If you look at relative cavitation energy for various frequencies, you get a plot that shows the loss of cavitation strength as the frequency of an ultrasonic field is increased (See Figure 4). Using 40 kHz as the relative standard at 1, you can see how other frequencies compare: 18 kHz is extremely aggressive, while 80 kHz and higher drop off in energy quite radically becoming virtually non-existent above 180 kHz.

If you could look at a typical tank and actually see the cavitations, there would be a substantial difference in the number and size of the cavitation bubbles at each frequency. To show the range of power, a tank with 1000 watts of energy at 40 kHz would appear as 500 bubbles at an energy level of 2 watts each. At 80 kHz, the same 1000 watt tank would exhibit 5,000 bubbles, but each at only 0.2 watts.

The resultant effect of a tank driven at three different frequencies can be observed in Figure 5. Three samples of heavy gauge foil were subjected to 40, 80 & 120 kHz, all at identical conditions in the same tank with DI water at 70°F. The damage to the sample at 40 kHz within 2 minutes was so great that voids were created. A sample of the same foil was exposed to 80 kHz for the same time period and tank conditions. The actual distortion of the foil surface is visible, but there is no severe damage like the holes created at 40 kHz. At 120 kHz with identical time and tank conditions, the cavitations are so gentle it is difficult to observe in a photo.

Another factor that is important to remember when examining higher frequencies with respect to delicate parts is their presentation to the energy field, or fixturing. At frequencies below 250 kHz, the energy fills the tank in about the same fashion. Typically, it enters the tank from the bottom surface at a wide angle and reflects off the interfacial surfaces. In its journey, the compressions and rarefactions create cavitations that penetrate every square inch of the fluid. This is important even as blind holes not presented directly to the radiating surface will see cavitations as long as they are filled with liquid.

As previously stated, when looking at cavitations as a function of frequency, we observed that above 250 kHz, the energy was almost insignificant. If we examine the resonant radius of the cavitation bubble as a function of frequency, we find that above 250 kHz the actual size of the bubble becomes so small that it is almost non-existent. At these frequencies, the wave movement through the fluid has a more significant impact on cleaning, and the presentation or fixturing becomes extremely important. Let us take a closer look at how the higher frequencies enter the tank

At 800 kHz, the energy introduced to the tank enters as a narrow beam relative to the lower frequencies. It moves through the fluid in a focused column without much divergence until it contacts the interface with the air. Here it does not reflect, but instead distorts the surface and the energy dissipates. This energy, as it travels though the media, exhibits strong shear forces, which

are good for stripping particles off flat surfaces. These forces sweep the soils away with streaming not cavitation, which is important in cleaning delicate components or soft materials. This action is best suited for flat components such as semiconductors wafers or disk-drive media. As previously mentioned, when the high frequency energy reaches the air/liquid interface, it is not reflected back into the fluid as in lower frequencies. It actually has sufficient strength to force the fluid upwards. Photo 1 shows how 800 kHz can push strongly enough to raise the water over 2 inches.

Other Damage Concerns

In addition to cavitation, damage can be induced by constantly bombarding a delicate component with a standing wave of ultrasonic energy. Even some typically robust parts can show etching formed by a standing wave. At 40 kHz, this would show up as a pattern of surface distortion forming lines approximately 3/4” apart, perpendicular to the radiating surface. Early in ultrasonic development, this type of damage was reduced by moving the part in the tank to minimize the formation of a pattern. Most modern day ultrasonic systems eliminate this damage by continually changing the frequency generated by 1 to 2 kHz. This feature is called sweeping the frequency and is successfully employed to prevent the standing wave.

Sweeping has been in use for the last 12 years. However, recently it has been found to induce damage itself. For very delicate mechanical components, the sweep rate can set up a low frequency harmonic, which can be damaging. This can be best illustrated by the famous Tacoma Narrows Bridge, which was destroyed by a continuous burst of wind. The wind repeatedly hit the bridge at its harmonic resonance causing fatigue in crucial structural supports. The twisting motion of the

harmonic at its maximum elevated the sidewalk on one side of the roadway to 28 ft above the sidewalk on the other side of the roadway. The bridge finally failed after 30 minutes at which time the structural integrity was exceeded. This occurred less than 5 months after completion of the bridge’s construction. The way to prevent this type of damage in ultrasonics is change the sweep rate so that a component is never continually hit with the same harmonic sweep rate of the frequency.

Latest Advances

The latest nuance in ultrasonic cleaning is to induce a range of frequencies into a system. The reasoning for this is quite simple. Each ultrasonic frequency has the ability to remove an optimal range of specifically sized particles. Lower frequencies are best at removing larger particles. Smaller particles are more effectively removed at higher frequencies. If a broader range of frequencies can be introduced to the same tank during the process, then a large range of particle sizes can be optimally removed. Thus a range of particle removal will be realized while also reducing the repetitive bombardment from one frequency.

Most of these unique ultrasonic developments have occurred in the past 10 years and was driven by the desire to miniaturize most everything in our lives. As we move further into the new century, I’m certain there will be other new technical insights in introducing sound to a fluid. Some will have merit and others won’t. As we’ve learned from the past…only time will tell.