Gary:  Electrical spacing is a design driven issue and is defined by par. 6.3
and Table 6-1 on page 39 of IPC-2221 (also see conductive material
requirements, par. 6.2 on page 35 of 2221).  Your best bet, from an assembly
acceptability standpoint, is to identify the minimum spacing required in the
field environment (who knows, it may be close to the "design" limit ;-) and
find some visual aid that you can use to relate that value to your inspectors
(diameter of a paper clip = 0.7mm [0.028 inch] and the thickness of a business
card = 0.3mm [0.012 inch]).  Note that column A5 of Table 6-1 depicts a
conductor spacing of 0.13mm [0.005 inch] for external conductors  when
conformal coated and Column A6 depicts the same value for external
leads/terminations which are uncoated.
Hope this helps a little.  Regards, Jim Moffitt, Moffitt Enterprises
Consulting

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