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"Jeff Seeger" <simon.ipc.org!bort.mv.net!rapidcad!jseeger> |
From [log in to unmask] Wed Jun 26 15: |
55:12 1996 |
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Gary Willard wrote:
> My main question is, are there any major concerns or advantages in
> having the vias connecting directly into the plane without relief, I
> vaguely recollect concern expressed over reliability of these
> connections if extreme thermal cycling is undertaken.
There are definite electrical advantages to direct plane contact,
we've done this for years. The inductance of the supply/return
loop drops enough to notice (important for high-speed switching
along with proper placement of vias and decoupling), as does re-
sistance (important for high loading).
Plane performance on dense boards improves as the planes are
noticably less "broken up". Also plane splits are a little easier
to accomplish without leaving bad etch geometries behind.
I agree that if you're doing your power/gnd routing well, there
is a price to pay in rework. I don't think the thermal relief
image makes any difference to via reliabilty over thermal cycling,
since the usual thermal geometry looks like a normal pad at the
junction to the PTH barrel.
Our SOP is to call them "direct hits" with a small flash (so every
one knows that we know). For some fabricators, who require therm-
al relief, we point to the appropriate thermal geometry for sub-
stitution.
I wish I knew why they do that. I was told once it was due to
"pink ring" but gather that's old news. Fabricators?
Regards,
Jeff Seeger Applied CAD Knowledge Inc
Chief Technical Officer Tyngsboro, MA 01879
[log in to unmask] 508 649 9800
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