[SI-LIST] Re: Current Return Vias
- From: "Mike Greim" <mike.greim@xxxxxxxxxxx>
- To: <cpad@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 25 Jul 2006 11:07:01 -0500
Hi=20Chris,
As=20always,=20the=20short=20answer=20is=20it=20depends.
If=20you=20are=20talking=20about=20a=20through=20hole=20component
then=20this=20is=20a=20real=20concern.=20=20If=20you=20set=20the=20wave
profile=20for=20the=20signal=20pin=20attach,=20odds=20are=20
you=20are=20going=20to=20have=20a=20cold=20solder=20joint=20on=20plane=20c=
onnects=20=20
If=20in=20fact=20you=20have=20a=20large=20multilayer=20backplane
with,=20for=20the=20sake=20of=20argument,=2010=20=20-=201oz=20gnd=20
planes=20with=20dead=20shorts,=20you=20might=20have=20to=20put
so=20much=20heat=20into=20the=20board=20to=20get=20solder=20flow
that=20you=20risk=20delamination.=20=20One=20needs=20to=20take=20
a=20look=20at=20the=20design=20to=20come=20up=20with=20the=20best=20
solution.=20=20Perhaps=20two=20spoke=20thermals=20on=20every
layer=20is=20preferable=20to=20leaving=20certain=20layers=20
out.=20=20It=20is=20design=20dependent.
One=20way=20to=20avoid=20this=20is=20in=20this=20situation=20is=20to
make=20use=20of=20press=20fit=20connectors.=20=20In=20many=20cases
this=20may=20be=20a=20preferable=20avenue=20for=20your=20manufacturing=20
folks.=20=20You=20get=20a=20solid=20via/plane=20attach=20and=20they
get=20reliable=20component=20to=20board=20attach.
If=20you=20are=20talking=20a=20surface=20mount=20component=20than
the=20argument=20doesn't=20hold=20a=20whole=20lot=20of=20water=20and
there=20are=20numerous=20studies=20to=20support=20this.=20The=20fanout
should=20provide=20enough=20thermal=20relief=20for=20a=20reliable
solder=20joint,=20even=20with=20dead=20shorts=20on=20all=20planes.=20
Best=20Regards,
Michael=20Greim
Platform=20Engineering
Newisys,=20Inc.
512-634-5553=20=20=20=20(V)
512-740-9097=20=20=20=20(C)
We=20will=20either=20find=20a=20way=20or=20make=20one=20=20=20=20=20-Hanni=
bal
The=20time=20is=20gone,=20the=20email's=20over
Thought=20I'd=20something=20more=20to=20say.......
-----Original=20Message-----
From:=20si-list-bounce@xxxxxxxxxxxxx=20[mailto:si-list-bounce@xxxxxxxxxxxx=
g]
On=20Behalf=20Of=20Chris=20Padilla=20(cpad)
Sent:=20Tuesday,=20July=2025,=202006=2010:44=20AM
To:=20si-list@xxxxxxxxxxxxx
Subject:=20[SI-LIST]=20Re:=20Current=20Return=20Vias
I=20get=20push=20back=20all=20the=20time=20in=20trying=20to=20connect=20al=
l=20"gnd"=20planes=20in
multi-layer=20stackups=20(say=2018=20layers=20+...and=20usually=20at=20our=
=20limit=20of=2026
layers).=20=20The=20reason=20is=20provide=20by=20the=20manufacturing=20fol=
ks=20in=20that=20they
cannot=20solder=20or=20get=20good=20solder=20flow=20because=20the=20mulitp=
le=20gnd=20planes
act=20like=20heat=20sinks.=20=20Therefore,=20I=20am=20forced=20to=20used=20=
thermal=20reliefs=20and
even=20in=20a=20lot=20of=20cases,=20I=20cannot=20even=20connect=20some=20g=
nd=20planes=20to=20the=20gnd
vias!
Any=20thoughts=20on=20this=20subject?
Thanks,
Chris=20Padilla
Cisco=20Systems
San=20Jose,=20CA=3D20
-----Original=20Message-----
From:=20si-list-bounce@xxxxxxxxxxxxx=20[mailto:si-list-bounce@xxxxxxxxxxxx=
g]
On=20Behalf=20Of=20Lee=20Ritchey
Sent:=20Monday,=20July=2024,=202006=203:44=20PM
To:=20Joel=20Brown
Cc:=20Kenny=20Frohlich;=20si-list@xxxxxxxxxxxxx
Subject:=20[SI-LIST]=20Re:=20Current=20Return=20Vias
Joel,
First,=20you=20should=20have=20stitched=20your=20ground=20planes=20to=20ea=
ch=20other=20every
where=20there=20is=20a=20ground=20pin=20on=20a=20device,=20so=20these=20gr=
ound=20planes=20should
be=20well=20tied=20to=20each=20other=20every=20place=20you=20have=20a=20co=
mponent=20whether=20it
be=20a=20bypass=20capacitor=20or=20an=20IC.=20=20(This=20argues=20for=20sp=
reading=20your=20bypass
capacitors=20=20evenly=20around=20the=20planes=20as=20has=20been=20advocat=
ed=20by=20a=20number
of=20members=20of=20this=20forum=20in=20the=20past.)=20=20Only=20where=20y=
ou=20might=20change
layers=20in=20an=20empty=20part=20of=20the=20PCB=20is=20this=20not=20so.=20=
Second,=20when=20you
engineer=20your=20PDS=20you=20need=20to=20insure=20that=20the=20impedance=20=
is=20a=20few
milliohms=20across=20all=20of=20the=20frequencies=20in=20your=20signals,=20=
so=20the=20Vdd
planes=20are=20"shorted"=20to=20these=20ground=20planes=20by=20this=20meth=
od.=20=20(=20To
achieve=20this=20low=20impedance=20across=20the=20broad=20range=20of=20fre=
quencies=20in=20your
signals,=20you=20will=20be=20using=20plane=20capacitance=20as=20well=20as=20=
a=20variety=20of
different=20values=20of=20capacitors.=20=20When=20done=20right,=20all=20of=
=20the=20planes=20are
"shorted"=20to=20each=20other=20and=20return=20currents=20have=20all=20the=
=20paths=20they
need.=20=20If=20the=20PDS=20isn't=20properly=20designed=20you=20can=20wind=
=20up=20with=20EMI
problems=20and=20unstable=20logic=20operation.
>=20[Original=20Message]
>=20From:=20Joel=20Brown=20<joel@xxxxxxxxxx>
>=20To:=20<leeritchey@xxxxxxxxxxxxx>
>=20Cc:=20Kenny=20Frohlich=20<kenny_frohlich@xxxxxxxxx>;=20<si-list@freeli=
sts.org>
>=20Date:=207/23/2006=2012:25:04=20PM
>=20Subject:=20[SI-LIST]=20Re:=20Current=20Return=20Vias
>
>=20=20I=20am=20currently=20working=20on=20board=20that=20has=2012=20layer=
s.
>=20Ground=20planes=20are=20layers=202,=205,=208,=20and=2011.=3D20=20If=20=
a=20high=20speed=20signal=20
>=20changes=20from=20layer=203=20to=20layer=209=20then=20its=3D20=20refere=
nce=20planes=20change=20
>=20from=20layers=202=20&5=20to=20layers=208=20&11.
>=20Since=20the=20return=20current=20flows=20under=20the=20signal=20trace=20=
on=20the=20reference
plane
>=20,=20this=20return=20current=20has=20jump=20from=20from=20one=20set=20o=
f=20planes=20to=20the=3D20=20
>=20other
set
>=20at=20the=20point=20signal=20changes=20layers=20through=20a=20via.=20Th=
e=20return=20
>=20current=3D20=20could=20jump=20through=20interplane=20capacitance=20or=20=
through=20a=20
>=20ground=20via=3D20=20located=20next
to
>=20the=20signal=20via.=20So=20in=20this=20situation=20I=20don't=20see=20w=
hy=20having=20a=20
>=20ground=3D20=20via=20nextto=20a=20signal=20would=20not=20be=20beneficia=
l?=20Does=20the=20
>=20interplane=3D20=20capacitance=20provide=20a=20lower=20impedance=20path=
=20than=20the=20
>=20ground=20via=20for=20the
>=20return=20current
method=20to
>=20jump=20planes?=20Please=20enlighten=20me.
>=20Joel
>
>
>=20Lee=20Ritchey=20wrote:=20Kenny,=20It=20is=20not=20true=20that=20you=20=
need=20a=20"return
current"
>=20vianext=20to=20each=20layer=20changing=20signal=20via.=20I=20continue=20=
to=20be=20amazed=3D20=20
>=20that=20engineers=20who=20are=20looked=20upon=20as=20SI=20experts=20say=
=20such=20things.=3D20=20
>=20Imagine=20you=20havea=204=20layer=20PCB,=20such=20as=20the=20mother=20=
board=20in=20a=20PC,=20where
>=20there=20are=20only=20twoplanes,=20one=20Vdd=20and=20one=20ground,=20wh=
ere=20would=20such=3D20=20
>=20vias=20connect?=20There
have
>=20been=20billions=20of=20these=20made=20to=20date=20that=20work=20just=20=
fine=20and=20have=20
>=20very=3D20=20fast=20signals=20on=20them.=20The=20return=20currents=20yo=
u=20are=20concerned=20
>=20about=20find
>=20their
way
>=20from=20plane=20to=20plane=20through=20the=20collection=20of=20decoupli=
ng=20capacitors=3D20
>=20and=20interplane=20capacitance=20that=20you=20had=20to=20engineer=20in=
to=20the=20power=3D20=20
>=20delivery=20system=20in=20order=20to=20make=20it=20stable.=20Focus=20on=
=20this=20and=20the=3D20=20
>=20return=20currents=20takecare=20of=20themselves.=20EMI=20is=20minimized=
=20he=20same=20way..
>=20[Original=20Message]
>=20From:=20Kenny=20Frohlich=20<kenny_frohlich@xxxxxxxxx>[1]=20To:
>=20<si-list@xxxxxxxxxxxxx>[2]Date:=207/22/2006=206:45:56=20AM=20Subject:=3D=
20=20
>=20[SI-LIST]=20Current=20Return=20Vias=20Dear=20Experts,=20I=20understand=
=20that=20I=20
>=20need=3D20=20to=20provide
ground
>=20vias=20next=20to=20via=20explictly=20for=20the=20purpose=20of=20lettin=
g=20return=20
>=20currents=3D20=20jump=20between=20layers.=20I=20know=20it's=20a=20requi=
rement=20for=20high=20
>=20speed=20signals,
especially
>=20differrential=20signals.=20Is=20this=20also=20required=20for=20low=20s=
peed=3D20=20
>=20single-ended=20signals=20(133Mhz=20or=20slower)?=20If=20this=20is=20a=20=
requirement,=3D20=20
>=20what=20would=20be=20a=20good=20signal=20via=20to=20ground=20via=20rati=
o?=20For=20example,=3D20=20
>=20there=20are=20five=20signal=20vias=20within=20a=201=20inch=20area,=20h=
ow=20many=20ground=20
>=20vias=3D20=20do=20I=20need?=20Thank=20you=20Kenny=3D20=20
>=20__________________________________________________=20Do=20You=20Yahoo!=
?=3D20=20
>=20Tired=20of=20spam?=20Yahoo!=20Mail=20has=20the=20best=20spam=20protect=
ion=20around
>
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>=20---=20Links=20---
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