Kenny:  The rationale for the part stand-off requirement is two-fold.  First
to assure sufficient standoff to allow cleaning systems to remove process
residuals, second to assure that a component with a conductive body is not
mounted directly on conductive traces.
- Paragraph 4.17 of Mil-Std-2000A states: "Devices mounted over circuitry.
 Parts mounted over protected surfaces, insulated parts over circuitry, or
surfaces without exposed circuitry, may be mounted flush.  Parts mounted over
exposed circuitry shall have their leads formed to allow a minimum of 0.25 mm
(0.010 inch) between the bottom of the component body and the exposed
circuitry. The clearance between the bottom of the component body and the
printed wiring surface shall not exceed 1.0 mm (0.040 inch)."
- By way of additional explanation.
The allowance for mounting parts flush to the pwb surface is predicated on
the assumption that both the pwb surface and the lower surface of the
component do not contain contaminates.  With a component such as you
described, there should (ideally) be minimal residue trapped beneath the
component during soldering and the cleaning system used should be capable of
extracting/removing process residuals from the lead/pwb interface area.
The allowance for mounting insulated parts flush over in circuitry is worded
in a manner to require that the parts be separately insulated from the
surface circuitry.  A separate insulator is required because, as far as DOD
has always been concerned, solder mask does not constitute a dielectric
barrier since dielectric withstanding is neither a design attribute or a
measured characteristic of ALL solder maskant materials.
The 0.040 maximum height limitation is imposed to assure nominal resistance
to X & Y axis vibration and shock by minimizing Z axis arm.
During the DOD/EPA/IPC tests conducted to identify alternative cleaning
technologies (eliminate CFC's) it was found that most all cleaning systems
were capable of extracting process residuals if a component standoff of 0.2
mm (0.008 inch) existed, hence the present requirement of ANSI/J-STD-001.
- Bottom line.  You should be able to maintain 0.010 inch standoff with
little or no problem.  The proof of the pudding however will be the results
of your cleanliness testing, not the dimensional measurement of standoff.
Hope this has helped some.  If additional information is needed e-mail me
direct.  Regards, Jim Moffitt

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