Don't know if you already have your answer
(I'm in "digest" mode, once a day)
and I can't tell you how that chart was derived,
but here are a few clues that might help you out:
1) the voltage breakdown you mention is THROUGH
the dielectric, in the Z-axis, in other words. This
is far different than the value across the surface
of the board, trace edge to edge.
2) IPC is a guideline for predicting what is needed
for the operation of the board, and the board will
work using those values. UL is a safety requirement,
and will have MUCH different values.
For example, your coffeepot might draw 6amps, and
you can calculate a value of copper that will operate
your coffeepot, and it WILL work. The building
SAFETY code, however, requires a wire gauge that
is much much much bigger. Don't confuse a minimum
design guideline with a customer's safety requirement.
good luck,
Jack
----- Message from - Bogert <[log in to unmask]> on Wed, 2 Feb 2005 18:55:57
-0500 -----
Subject: Technical basis for Table 6-1 of IPC-2221
February 2, 2005
Table 6-1 defines minimum electrical spacing requirements between traces on
a PWB. What is the technical basis for the Table? Is there a formula
associated with the values in the table? For example, if I have a PWB
operating at 30 volts DC, the minimum spacing required between internal
traces is 0.05 mm. What if a PWB operates at 30 VDC but only has 0.04 mm
spacing, is there a technical concern? Based on what formula or technical
consideration?
The minimum spacing in the Table are much less than the electric strength
requirements for FR4. For example, my understanding is that FR4 is about
1250 volts/mil. Therefore, at 1 mil of spacing the PWB should be able to
withstand 1250 volts between conductors.
The values in the table are also different than those included in UL 840.
Why?
Any help you can offer on this point would be appreciated.
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