From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Thu Nov 09 2000 - 04:50:41 PST
Another potential problem area, is when a trace runs too close to a plane
split. Having only half a ground plane under your signal trace, increases
its characteristic impedance in the area where it approaches the split.
It's the same problem if you route too closely to the edges of your board.
In your figure, the trace under the lower pad of C2, and the one further to
the left (T4), are examples, although their segment lengths appear short.
Even the serpentined trace T3 that crosses the vertical plane split, might
be problematic where it runs parallel to the split. Stitching caps don't
help this problem.
It is easy to find traces that cross plane splits (much more reliably than
doing it visually), by putting "keep-out" areas completely surrounding each
plane split on all adjacent routing layers. Then all potential problems
show up as DRC errors and can be fixed or waved as appropriate. Making the
keep-out area wider than the split width, also avoids the problem I
mentioned of routing too close to the split.
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