[SI-LIST] Re: Differential via optimization
- From: Hal Murray <hmurray@xxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 17 Oct 2012 01:25:38 -0700
> The plane perforation raises the power-ground plane pair impedance locally
> in the pin field, creating new plane resonances. And equally importantly it
> becomes a signal-integrity issue, when the large antipads leave very little
> webbing for the traces to serve as return current path. In larger pinfields
> with regular pin pitch, such as under large packages and connectors, the
> periodic nature of disturbance on the traces not only increases the average
> impedance of the traces, but also creates sharp resonance suckouts in the
> transfer functions. ...
Many years ago, when I was working at DEC, one of the senior engineers sent
out a memo titled roughly "Don't cut your ground planes!". That was the
result of tracking down a SI problem due to a signal crossing a 20 pin DIP
where the anti-pads on the DIP were big enough to connect to their neighbors.
So some poor signal was crossing a pair 1-inch slits in the planes. It
almost worked, but it screwed up often enough to delay a high-profile
project, thus deserving a loud-mouth memo.
The memo was widely forwarded, so some/many of us were lucky enough to avoid
that problem in the future and/or start paying attention to SI issues.
How good are modern tools in this area?
If I specify an anti-pad to be X microns (or mils), I really need to know
that it's between X1 and X2. I assume I can get those numbers from
manufacturers. Are modern vendors smart enough to provide those numbers?
As a first step, X1 has to be big enough so that the plane won't short to the
via. X2 has to be small enough so that the webbing won't break through to a
nearby via. I think tools could automate those checks. How many of them do
it? Cleanly?
As a second step, I have to check that the impedeances/reflections are within
acceptable. limits. I haven't thought about this area.
Are their other areas to think about?
--
These are my opinions. I hate spam.
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