[SI-LIST] Re: fire extinguisher is close
- From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
- To: <ludvik@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sat, 2 Oct 2004 19:04:25 -0400
Ludvik,
Yes, this approach has been in use for quite a while.
A couple of things to keep in mind:
- as we push for narrower traces, you may not have
enough width to neck it further down reliably, but
this depends on the design
- more importantly, you want to compensate only for
the excess capacitance, which are the one or two stubs
on either vertical side of the layer transition, plus any
extra capacitance coming from an impedance mismatch
between the trace impedance and via-barrel impedance.
In your question you assume a top-to-bottom transition,
which eliminates the stubs. Still, you dont want to compensate
for the entire capacitance of the via., just what may be
above what is needed to match the trace impedance.
Note that the compensation goes both ways. If you have for
instance an 'inductive' via or any other electrically short
inductive interconnect, you can add small islands or bigger
pads to compensate for it. This may be the case if for
instance you have to neck down a trace (and it bocemes
higher impedance than required) in a dense area for routing reasons.
Regards,
Istvan Novak
SUN Microsystems
----- Original Message -----
From: <ludvik@xxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Saturday, October 02, 2004 4:57 PM
Subject: [SI-LIST] fire extinguisher is close
> Hello everybody,
>
>
> I have a question:
>
> Did some of you use the following via compensation?
>
>
>
> Assume top ustrip connected by via to bottom ustrip. This via could be
> modeled as C to ground, or in other words, as localized decrease in Zo of
> mentioned trace. Now if we would neck down top and bottom trace for some
> small distance, say 0.1", before and after via, we would introduce two
local
> increases of Zo. This really would be like a T (LCL) network with could
> perform better that via by itself. Of course there will be an increase of
> local R_skin, but maybe negligible and could be accounted for with Ls.
>
>
>
> Hopefully you will not flame me much for asking, and if yes no worry, my
> fire extinguisher is close.
>
> :o)
>
>
>
> Ludvik Kindl
>
> 650-868-2675
>
>
>
>
>
>
>
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