Hello Dave, I hope that electronic slap didn�t hurt too much.
Robf,
I am sorry that you did not agree with my initial response, but the answer is very detailed and most of the support material I have is only in paper format so I need to either snail mail or fax it to Michael F (or any other interested party). If I had e-mailed my response directly to the poster, Dave�s face would have been slapped even harder because of the lack of �free and open communication that is the corner stone of internet forums.� So Dave, do you want to be the rock or the hard spot on this one?
However, in self-defense, I will describe the general guidelines and reference my sources. If you have access to a good library, I guess you may be able to find them. The material is about 40 pages, so the time and effort that I was originally willing to put forth on the copy/fax machine will now be allocated to the following:
Here goes. Ultrasonics (u/s) originally got a bad rap back in the 1950s when Westinghouse developed the process for the U.S. Air Force. At that time, the technology used very low frequencies (20 kHz - the lower the u/s frequency, the more aggressive the cleaning action) and very high power or power density (200 watts/gallon). This early technology was tested for cleaning PCBs with devastating results. The PCBs of that era were very fragile with weak wire bonds and the u/s technology was very strong and uncontrollable, so the two technologies were not compatible. As a result, the U.S. Military Specified that Ultrasonics should not be used for cleaning PCBs.
Since then, u/s has developed into the most controllable precision cleaning process available. Virtually every parameter can be managed. The frequency, the amplitude of the sonic wave, the power input (density), the bath temperature, the chemistry, the exposure time, use of multiple �sweep� frequencies, positioning and movement of parts are all controlled with today�s technology.
Today�s surface mount PCBs are more durable than the through-hole technology of the 50�s, so research by the EMPF laboratory, GEC Marconi, Smart Sonic and others have revealed that the two technologies are now very compatible for many PCB cleaning applications.
For most, ultrasonic technology is a mysterious science. Because the cleaning activity (cavitation) is microscopic, it is difficult to discern one system from another. Simply put, however, ultrasonic cavitation is the formation of tiny vapor bubbles, caused by pressure changes in a liquid cleaning media that collapse to form vacuum pockets throughout the bath. These collapsing vacuum pockets serve to �pull� contamination away from solid surfaces. So, ultrasonics is basically a �pulling action�.
Ultrasonic frequencies vary from 20 � 400 kHz. Power input to the bath is measured in watts of energy and vary depending on the cleaning task. Low frequency/High watt systems are usually designed to clean durable parts with difficult contaminants like rebuilt carburetors or aircraft wheel housings. High frequency/Low watt systems are usually designed for cleaning delicate parts with mild contaminants like disk drives and optics. An analogy can be made that the frequency is similar to the bristles of a brush. The lower the frequency, the stronger and larger the bristles. The wattage is similar to the force applied to the brush. The more watts, the more force applied to the brush.
This is one reason care should be taken when evaluating an ultrasonic cleaning system for they can vary tremendously. The wrong frequency or power could be devastating to misprinted PCBs.
Most ultrasonic systems designed for cleaning solder paste and adhesive use 40 kHz �sweep� frequency generators. �Sweep� frequency meaning that the ultrasonic generator will alternate within a tight range of frequencies (say 38 � 42 kHz) to prevent cavitation from concentrating in any one area of the bath. Lower frequencies could damage stencils and PCBs. Higher frequencies will not be able to form large enough vacuum pockets to effectively pull away the contaminants. The input power (watts), however, may vary tremendously from one supplier to another. This is also a good way to determine if an ultrasonic supplier is expert in PCB applications or just supplying generic ultrasonic equipment.
If cleaning tooling alone, high wattage systems may have an advantage (strong brush with lots of force). Unfortunately, if misprinted PCBs are to be cleaned in the same machine, the excessive power wattage could be detrimental.
It is important to work with the chemistry and machine supplier simultaneously. The chemistry will play an important role in determining the wattage necessary to properly clean all desired applications. Generic ultrasonic stencil cleaners that propose their machine may be used with any chemistry must power their machines to help compensate for inferior chemistries (more power is not always better when cleaning misprinted PCBs). When a machine is designed for use with a specific chemistry, the power can be tailored to safely clean tooling and PCBs. Guidelines for proper frequency, power and cleaning cycles of an ultrasonic system designed for cleaning misprinted PCBs are: �the ultrasonic frequency should be 40 kHz using a �sweep� frequency technology �the power input (density) to the bath should be less than 11 watts per liter or 42 watts per gallon (total watts of ultrasonic generator / total liters (gallons) of cleaning solution in bath). In general, the lower the power required to clean effectively, the better. �the ultrasonic wash cycle should be less than 10 minutes
References:
1. Robins, M., �Solutions for PCB Cleaning,� Electronic Packaging and Production, August 2000, pp. 42 - 49 2. Clouthier, R., �Improving Screen Print Yields,� Electronic Packaging & Production, July 1996, pp. 32- 36 3. Lester, N.J., �Surface-Mount Stencil Cleaning,� Circuits Assembly, November 1998, pp. 56 � 60 4. Bivins, B., �Cleaning Solutions for Stencils and Misprints,� Proceedings of the 1999 NEPCON West Conference, pp. 94-97 5. Seto, P., and Loew, N., �Stencil Cleaning,� SMT, May 2001, pp.66 � 72 6. Richards, B., Burton, P., and Footner, P., �Does Ultrasonic Cleaning of PCBs Cause Component Problems: An Appraisal,� IPC Technical Review, June 1990, pp.15 � 27 7. Vuono, B., and Crawford, T., �Ultrasonic Cleaning of Military PWAs,� EMPF TS0040, April 1991 8. Kenyon, W.G., �Ultrasonic Cleaning Acceptance Accelerates,� SMT, May 1995, pp. 21 9. Hymes, L, �Defluxing with Ultrasonics,� Circuits Assembly, September 1999, pp. 80-81 10. Hutchins, C.L., Equipping for the Cleanup, SMT, November 1992, pp 11-12 11. Mil-Std-2000A (Superseding Mil-Std-2000) February 14, 1991 12. Schreiber, B., �SMD Adhesive Cleaning,� Proceedings of the 2002 APEX Conference, pp. S27.3.1 � S27.3.6 13. Schreiber, W., �Environmentally Certified Technologies: A Case Study,� SMT, July 1999
Anyone interested in copies of these articles may still contact me rather than researching the technical libraries.
Regards, Bill Schreiber Smart Sonic Corporation
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