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Point/Counterpoint
December 31, 1969 |Estimated reading time: 4 minutes
Industry Experts voice differing opinions on ultrasonic vs. spray cleaning
Cleaning at the Speed of Sound
As spray-in-air technology gained market share, circuit assemblies have become more complex. Component density increased while standoff heights decreased, forcing spray-in-air machines to use more spray pressure, fluid flow and higher temperatures.
Michael Konrad"Ultrasonic cleaning depends on cavitation."
Today, spray-in-air technology is suited to modern circuit assemblies. But the same can not be said for stencil cleaning applications. To adequately clean a modern fine-pitch stencil using spray-in-air technology, one must ensure an adequate degree of fluid flow, spray pressure and fluid temperature.
Most stencil cleaning applications require the use of a chemical to solublize the flux. Although heat is not required in solder paste removal, it is required to activate the chemical defoamers commonly used in most stencil cleaning chemicals. But stencils should never be placed into an environment where they are subject to elevated temperatures or to moderate to high spray pressures.
Stencils are not durable, so, spray-in-air stencil cleaners by design can not be powerful. Therein lays the dilemma.
Unlike spray-in-air technology, ultrasonic technology does not use a spray pump, spray nozzles or any other type of mechanical force-producing mechanism. With ultrasonic technology, a generator directs energy at a specific frequency range through multiple ultrasonic transducers. Ultrasonic cleaning depends on cavitation. Agitation by millions of small and intense imploding bubbles creates a highly effective scrubbing action on the stencil's surface and within its apertures. As the ultrasonic frequency increases, the number of these cavities also increases, but the energy released by each cavity decreases, making higher frequencies ideal for small particle removal without stencil damage.
When a stencil is subjected to ultrasonic energy, the solder paste or adhesive simply falls off the stencil. Although ultrasonic technology is highly effective on all stencils, it is most impressive when used to clean ultra-fine-pitch (microBGA) stencils when, after cleaning, even the smallest aperture is solder ball-free.
Because ultrasonic energy does not produce an agitated state, foam is not produced. With the stencil cleaning chemical's foam production capability eliminated, one may operate the solder paste removal process at ambient temperatures without the threat of foam. An ambient or low fluid temperature, the absence of pumps, and high-pressure nozzles ensure that the stencil will not be damaged.
Clean stencils without damage. Cleaning at the speed of sound. That's the ultrasonic advantage.
Michael Konrad, president, may be contacted at Aqueous Technologies Corp., 9785 Crescent Center Drive, #302, Rancho Cucamonga, CA 91730; (909) 944-7771; Web site: www.aqueoustech.com.
The Practicalities of Spray Cleaning
Cleaning can be separated into two basic and very broad categories: immersion cleaning (vapor or liquid) and spray cleaning. The debate between equipment manufacturers as to the "best" type of process for cleaning a product has raged since the inception of automated cleaning.
Steve Cacciola"Cleaning chemistry is the most critical element."
Cleaning generally is influenced by these variables: chemistry, mechanical action, temperature and time. If one of these variables is fixed, cleanliness usually can be accomplished by adjusting one or more of the others. In my opinion, cleaning chemistry is the most critical element. If the chemistry does not adequately act on the soil, all the other variables must be adjusted to such extremes as to make the system design impractical. Mechanical action is the second most critical, and generally is present in a given process in the form of turbulation, spray impingement or ultrasonic implosion.
Though very effective in some processes, in the case of immersion cleaning the limiting factor typically is the geometry of the product being cleaned. Sometimes it is just too cumbersome to hold enough liquid in a single chamber (or chambers) to facilitate the complete immersion of a product. Holding large quantities of chemistry in single or multi-stage immersion tanks can pose floor space problems, and consideration also must be given to the raising and lowering of the product into the cleaning agent. Hoisting mechanisms sometimes can prove prohibitive if the product is large enough, not to mention the costs associated with creating enough turbulence to facilitate cleaning or elevating the temperature of a large cleaning bath. The means of mechanical agitation in immersion systems is limited to turbulent spray of some type or ultrasonic.
In most cases, it is easier and more practical to move the cleaning fluid to the product rather than vice-versa. Since holding reservoirs can become smaller, they are easier to heat, and filtration of the cleaning medium is best done when the product is not present. Whether moving the product past a fixed spray arm, or moving the spray arm past the product, areas of high and low pressure act to enhance the mechanical scrubbing action, and aid in dislodging and flushing away the soil. With either fixed or rotary spray, and the proper design of equipment, impingement angles can be changed fairly simply and nozzles can be interchanged if higher flow rates or cleaning pressures are necessary. If a rotating spray arm is the method of application selected, the simple movement of the fluid through the sprayer is enough to disperse the solution over the entire product and throughout fairly complex geometries.
Steve Cacciola, product manager, may be contacted at EMC Global Technologies, 4059 Skyron Dr., Doylestown, PA 18901; (215) 340-0650; E-mail: stevec@emcgti.com.