[SI-LIST] Re: Ground vias around signal via
- From: <Wolfgang.Maichen@xxxxxxxxxxxx>
- To: <aaditya.kandibanda@xxxxxxxxx>
- Date: Thu, 16 Oct 2014 06:29:30 +0000
Hello Aaditya,
the important term is "proper return path": In many cases (Scott and others
have already given examples) those vias are integral components to make the
return path "proper". Impedance control on a PCB requires that the return
current can flow close to the signal current - i.e. in the ground layer above
and/or below the signal trace, very close to it horizontally, or in a via close
to the signal via at a layer transition. If the return current needs to make a
detour, for example because the signal transitions across several layers and
there is no via close by to connect the respective reference planes, the you
can end up with a significant parasitic inductance, causing reflections,
bandwidth reduction, emissions etc.
If your reference planes are already closely stitched together then you already
have the return via - the question whether to assign a via to the particular
signal transition via or not is then rather philosophical.
Differential signals are much less susceptible as others have pointed out (as
long as true and complement signal are routed sufficiently close together and
the frequency is not excessively high, meaning a few GHz). Second, if the
return current does not need to change reference plane then obviously no return
via is necessary (that is your "Signal-Ground-Signal" example), but IMHO this
is a rather theoretical case - on a stripline transition between adjacent
signal layers (separated by one ground layer) still half the return current
needs to change planes, and even on 4-layer boards there are typically two
planes to jump across (assuming signals are routed as surface microstrips,
while the two inner layers are ground and power, as is customarily done).
Finally, capacitive coupling between planes helps in the high-frequency region
in cases where there is no close return via, but I would not rely on that
without a lot of experience and/or reliable simulation.
My 2 cents
Wolfgang
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Aaditya K
Sent: Thursday, October 16, 2014 5:09 AM
To: bowden.ivor@xxxxxxxxx
Cc: si-list
Subject: [SI-LIST] Re: Ground vias around signal via
By "proper" my intention now is, even if there is a reference plane change in
signal path and both the reference planes are stitched.
On Oct 15, 2014 8:04 PM, "Ivor Bowden" <bowden.ivor@xxxxxxxxx> wrote:
> Right, for example if stackup is sig - gnd - sig then why bother?
> Need to know how Aaditya defines "proper" before further comment.
> -Ivor
>
> On 10/15/2014 12:32 PM, Lee wrote:
> > You are correct. Can't think of any case where they add value
> commensurate
> > with their cost.
> >
> > Lee
> >
> > -----Original Message-----
> > From: Aaditya K
> > Sent: Wednesday, October 15, 2014 10:41 AM
> > To: si-list
> > Subject: [SI-LIST] Ground vias around signal via
> >
> > Hello Experts,
> > I have a question on ground vias placement around signal via.
> >
> > My opinion is, if we have a proper return path, they are not necessary.
> Am
> > I correct?
> >
> > Any situations they are useful other than shielding?
> >
> > How will they help? When do we need them?
> >
> > Please help.
> >
> > Thanks
> > Aaditya
> >
> >
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