[SI-LIST] Re: Ground vias around signal via
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 15 Oct 2014 23:56:25 -0700
In a construction like this:
S1
thin
Vss
thin
S2
Big gap
S3
thin
Vdd
thin
S4
Striplines on S2 couple strongly to the Vss plane and only weakly to the
Vdd plane. The complement is true for S3. This greatly reduces the
amount of energy that rattles around in the Vss/Vdd cavity due to a
transition between S1 and S2, or S4 and S3 because the striplines on S2
or S3 themselves act to confine the injected energy.
Years ago Bruce Archambeault published his hierarchy of layer
transitions for EMI control:
1. None
2. Opposite sides of a continuous plane
3. Across a single voltage cavity (such as Vss) where the top and bottom
of the cavity have been stitched well (this may or may not happen
naturally as a result of IC Vss and bypass cap Vss vias)
4. Across a cavity with two voltages stitched together with stiff bypass
I will add that the plane in the case of 1 & 2 s/b the same voltage as
used for return path in the transmitting and receiving packages.
Steve
On 10/15/2014 11:29 PM, Wolfgang.Maichen@xxxxxxxxxxxx wrote:
> 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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