Fellow Designers,

After all this talk and investigation concerning thermals, fabrication
allowances, formulas, etc., I believe the formula(s) and/or table 5-10
and/or figure 5-18 in IPC-D-275 may be wrong.  Specifically, notice that the
formula given for calculating Land Size uses, in the example, (2 X minimum
annular ring). + fabrication allowance.  This makes sense if table 5-10 is
indicating the TOTAL fabrication allowance, plus and minus.  In the example,
this would indicate a Level C PCB, up to 12" in size and one must 'assume'
(there's that word, again!) that this means +/- 0.005", since it needs to
apply to both sides of the pad, as does the annular ring.  OK, if that's
true, then notice what figure 5-16 says for Clearance Area in Planes - the
annular clearance ring (anti-pad) around a hole is the (total fabrication
allowance X 2) plus the (electrical clearance X 2), considering that the
dimension they are indicating needs to applied for both sides of the hole.
Question - Why would the fabrication allowance for an anti-pad need to be
double that for a pad?  The objective for the pad is to insure a minimum
copper annular ring around the hole and the objective for the anti-pad is to
insure the electrical clearance between the plane and the edge of the hole.
Can someone shed some light on this?  Are one of the figures, tables or
formulas wrong?  Or, am I missing something?  Has IPC published a correction
to this?

Thanks, in advance, for any help you can provide!

hammering away....
Ray Humphrey, DynaCad Design Services
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