Richard,
You've raised some good points here and I'll certainly stand
corrected, but it's been my experience with NRTLs that pcb
fabs are *strictly* creepage issues. That is to say, whenever
I've handed the artwork and bare board (the fab) to an
engineer at an NRTL for analysis, the only thing that has
been addressed to me is creepage.
Now, I can give you two scenerios where a bare fab clearance
would NOT be and would be an issue.
Assume two inner layer traces side by side.
Situation A : Board is continuous between trace A
and trace B. That is to say, there
is FR4 material between the traces.
----------------------
------ ------
| A | | B |
------ ------
----------------------
Situation B : Board is NOT continuous between trace A
and trace B. That is to say, there is an
air gap cut through the board between
the traces leaving the traces exposed.
--------+ +-----------
------| |------
| A | | B |
------| |------
--------+ +-----------
Clearly, even on an unpopluated board fab,
Situation B is strictly a clearance issue.
I bow to any correction to this, since my experience
has been - for board fabs creepage is the issue unless
something such as Situation B is present on the board.
Especially two traces sid-by-side on the outer layer
of the board.
I assume you're talking 1 or 2 oz copper for your traces?
Regards, Doug
