When cleaning water washable flux from PCBs using U/S, the final rinse can be with spray DI water for aesthetic purposes because the wash cycle is really just a long U/S rinse (no added chemistry). Fresh DI water should be continuously added to freshen the wash solution and it is best to add this water when the U/S are off. This will not only keep the wash solution fresh, it will also overflow the wash water to "skim" any floating contaminants off the surface.
However, when cleaning non-water washable contaminants, the most important factor is ALWAYS THE CHEMISTRY. U/S is just a mechanical scrubbing action (cavitation). If you do not identify the correct chemistry U/S will do little good. It is the same as trying to wash greasy hands without the correct soap, you can scrub all day long (mechanical action) but the grease will not come off without the right chemistry.
When using a chemistry, it is also important to use an U/S DI rinse. U/S will force the wash solution into tight tollerance areas and under components. If you feel that it is important to rinse the wash solution out from these areas, the only way to do so is with an U/S DI rinse. Spray rinses will not reach into these areas.
As far as damage, the EMPF Laboratory (Mil Spec) is recommending U/S for many applications. Dr. B.P. Richards of GEC Marconi has done the most research in this area. In a nut shell, the recommended parameters are: Use U/S of 40kHz or higher (the lower the frequency-the more aggressive the cleaning action)., Use cleaning cycles of 10 minutes or less, and use power input of 14 watts per liter or less, and, like any new cleaning process, TEST IT!
I have been working with U/S now for over 15 years for many different applications. When someone asks me if U/S will damage an SMT assembly, my response is "try putting your hand under the sprays of a high pressure inline washer-you could draw blood! If you put your hand into a 40kHz U/S bath, you can hardly feel it."
U/S got a bad reputation as a result of the 1950s experience of the U.S. Air Force. PCBs then were very delicate and U/S was a very new and uncontrollable cleaning technology using very low frequencies (20-25 kHz). Now SMT assemblies are more durable and U/S is the most controllable precision cleaning technology available. SMT and U/S are very compatible.
However, just as you would not buy or use a spray system without testing it, you should also test clean in the U/S system before implementing it.
Additional information is referenced on our Web Site: www.smartsonic.com click on the "Recommended Reading" button. Go to the article by Richard Clouther. Several articles are referenced by Mr. Clouthier regarding cleaning PCBs with U/S.
Feel free to call me if you have any other questions.
Bill Schreiber Smart Sonic Corporation Tel: 1(818) 909-6400 Fax: 1(818) 909-6409
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