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Date: | Wed, 18 Sep 96 10:34:50 PDT |
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Max,
We also are sensitive to EMI problems and are aware of the problems
caused by excessive anti-pad sizes, namely reducing the integrity
of the ground and power planes.
There is a IPC spec concerning
clearance area in planes in IPC-D-275 section 5.3.2.4.
According to this spec, there is a .010" minimum clearance between
the edge of the land and the plane. A fabrication allowance should
be added to this also. For us, we use a .035" via pad with a .018"
finished drill size. If we were to follow this spec, we should have a
.020" clearance using the Level C fabrication allowance. This would
translate to a anti-pad of .075", (pad size + .040" for .020" clearance).
Having said this, we don't follow the spec. We use a much smaller anti-pad, .055"
which gives us a .010" clearance. I have asked our board fabricators about
this reduced clearance and they told me this is not a problem.
Hope this helps.
Danny Gross
PCB Designer
E-Mu Systems, Inc.
Scotts Valley, CA
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