I agree with Mike completely.  Too many people make the mistake of simply
wiping wtih IPA or some other solvent and calling it "clean".  Solder spheres
become trapped in between the solder mask and the pads as well as in vias.
These spheres are contaminated with the IPA and generally do not meld with
the new solder paste.  The result- solder balls (and sometimes worse).
When a misprint occurs, wipe the majority of the solder off the board with
IPA.  Then, either run the board through an aqueous cleaner with spray and
immersion or put it in an ultrasonic stencil cleaner.  My research a number
of years ago uncovered the same wisdom Mike pointed out.  Ultrasonics will
not harm the component per se.  One interesting phobia is the fear of
cracking ceramic capacitors.  I learned from my friend and colleague John
Maxwell (he had done numerous comprehensive studies) that if the capacitor
was manufactured improperly  - specifically if the dielectric had been
cofired incorrectly, there may be a tendency to crack in ultrasonics.
However, this is a component that will likely fail quickly in the field
anyway (assuming it makes it past ICT) with or without ultrasonics. The
ultrasonics act, in this case, as ESS !

Phil Zarrow
ITM, Inc.
Durham, NH USA
www.ITM-SMT.com

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