Yuan Li:
- The ability to clean underneath components depends on the cleaning system in
use.
- In the distant past (when CFC's were being withdrawn from use due to the
Montreal protocols) there was a group formed (IPC/DOD/EPA) to study the
ability of alternative (aqueous and semi-aqueous) cleaning systems to remove
process residuals.  Test results showed that most (but not all) altenative
cleaning systems tested could remove residuals from components which were
0.006 inch above the pwb surface.  Consequently, the requirement for SMT
components on an IPC Class 3 Product (which you presumably want to be clean)
is that components have to be mounted 0.2mm (0.008 inch) above the pwb surface
unless you can demonstrate satisfactory cleaning with a lesser clearance.
- The 0.2mm vertical clearance is a good rule-of-thumb or guideline, but
satisfactory cleaning depends on too many variables to make any really precise
statements about vertical clearance.
Hope the above helps some.  Jim Moffitt

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