In the electronics manufacturing industry, cleaning is a problem we are all faced with everyday. In the broadest sense, cleaning is the removal of undesirable material from a particular area. In the more technical realm, cleaning is usually performed to make a particular material or component acceptable to proceed to the next level of processing. This task is becoming increasingly more difficult as component geometries continue to decrease while production scales escalate. Cleaning can be accomplished by a variety of means. One of the more common methods is immersion in a liquid. When this is the chosen methodology, it is usually a combination of chemistry and mechanical activity. Independently, each will remove some soil; together, the effect is multiplied many times.

Some typical sources of mechanical activity include brushing, spraying and ultrasonic. Brushing is used where the parts to be cleaned are geometrically simple, typically having a large flat surface with no recessed areas. For effective use of mechanical energy, a spray must be aimed such that it impinges directly on the soil to be removed. This makes component orientation critical. Ultrasonic is often chosen because it is not dependent upon either geometry or orientation for effective soil removal. Ultrasound waves travel in all directions in the solution and will actually pass through the components to reach and clean areas which are inaccessible.

Because ultrasonic energy penetrates into these crevices and cavities, any type of part or assembly can be cleaned. There are a number of benefits realized from the application of ultrasonics to precision cleaning. These include:

1. Speed - Ultrasonic is faster than conventional cleaning methods. Entire assemblies can be cleaned without disassembly. Often the labor saving advantages make ultrasonic the most cost-effective choice.

2. Consistency - The ultrasonic activity is micro in nature and reaches all areas for uniform cleaning. This is true for large or small parts, simple or complex parts handled one at a time or in batches. In many cases, ultrasonic cleaning is the only way to meet specifications as in the cleaning of precision parts or assemblies. Thorough soil removal is not operator dependent.

3. Compliance with safety and environmental regulations through reduction of dangerous chemical concentrations or substitution of less aggressive cleaning media.

4. Reduction of hazardous substances. In addition to the mechanical advantages provided by ultrasonic cleaning, there are a number of chemical or process benefits that can also be gained. These include:

• Speeding up the rate at which the soil dissolves by mixing the cleaning agent with the contaminant.
• Careering fresh solution to the soiled surface through a micro streaming effect.
• Attacking the molecular “cement” by which the soil attaches itself to a workpiece surface.
• Preventing formation of a neutral film on the workpiece surface that may impede cleaning.
• Raising the temperature of the liquid, thereby increasing the rate of chemical activity.

How ultrasonic stencil cleaning works
When high frequency sounds (20kHz and up) are applied to water or other liquids, a process known as cavitation occurs. Sound travels in waves that cause the molecules it touches to move back and forth in the direction of the wave. The higher the pitch (or “frequency”), the faster it causes molecules to move. In the ultrasonic cleaning chamber, high frequency sound vibrates the water or cleaning chemistry with such intensity that the liquid molecules pull apart. This produces air bubbles of the type you see when you boil water. Ultrasonic cleaning can be thought of as “cold boiling.”

When the bubbles in the ultrasonic cleaning chamber collapse, they leave a void. The surrounding molecules rush to fill in the cavity. When they reach the center, they collide with great force and bounce back. This creates a shock wave that runs outward from the collapsed bubble.

The shock wave leads to further cavitation. All of this imploding and colliding creates a powerful scrubbing action that loosens and displaces particulate matter from the surface. This explanation seems rather violent. Cavitation can, after all, wear away the blades of outboard motors, so why is it advertised as “safe for even your finest pitch stencils?” The higher the frequency of sound applied to the water, the greater the number of cavities created. However, as the number of cavities is increased, the amount of energy released by each cavity decreases. Therefore, you end up with many gentle “scrubbing bubbles” that are able to reach into tiny crevices and remove small particles, but not powerful enough to wear away your stencil.

Ultrasonic cleaners consist of three main parts: a tank, a transducer, and a generator. The generator converts standard electric current into higher frequency electrical energy, which is converted into ultrasonic waves in the cleaning tank by the transducer.

Many companies are opting for ultrasonic systems to clean their stencils, screens, and misprints because they offer greater precision, speed, and consistency than other cleaning methods.

The EMPF uses the Aqueous Technologies, Ultrasonic Stencil Cleaning System, Model AQ-201 SC. The EMPF uses this cleaner on all PCB assembly projects, military or commercial, to achieve the cleanliness specifications required by the customer. This system is a fully automatic ultrasonic stencil cleaner that quickly removes all flux types from stencils, screens and misprints. This cleaning system is designed to remove all paste types including rosin (R, RMA, RA), water soluble (OA) and no-clean pastes as well as all SMT adhesives. If you would like a demonstration or information on the Aqueous Technologies Ultrasonic Stencil Cleaning System, please contact Jeff Stong at (610) 362-1200, extension 224 or jstong@aciusa.org.