Re: [SI-LIST] : Surface Mount Cap Lead Inductance

Dennis Tomlinson (det@tellabs.com)
Tue, 11 Aug 1998 13:27:43 -0500

Ray Anderson wrote:
>=20
> You will probably find that the ESL of the capacitor
> is only a small part of the "mounted inductance" of the part
> when it is placed on a PCB. A larger percentage of the total
> loop inductance that your circuit sees associated with a mounted
> capacitor comes from the following factors:
>=20
> 1 Pad Geometry
> connector traces
> via location
> Number of vias
>=20
> 2 Stackup
> distance from mounting pads to power planes
>=20
> All those factors that influence the loop area seen by the curr=
ent
> flowing through the capacitor influence the mounted inductance. The amo=
unt
> contributed by the capacitor is a small part and is usually overwhelmed=
by
> the other constituents unless you do a VERY good job in minimizing the =
other
> contributions (which can be done). The ESL of the capacitor by itself i=
s a
> very strong function of the height dimension of the part.
>=20
> It is the mounted inductance of a bypass capacitor that mostly =
determines
> the effectivness of the bypassing that the part provides.
>=20
> Ray Anderson
>=20
> Sun Microsystems Inc.

A note on mounted inductance,

A component geometry called "via-in-pad" for SMT bypass capacitors will m=
inimize
the PCB contribution to mounted inductance. Any high quality PCB fabricat=
or
will have this capability, and manufacturing techniques exist which preve=
nt or limit
the solder thieving that manufacturing types fear so much. The same or si=
milar techniques
are needed to work with BGA packages, so ...

Via-in-pad (VIP) techniques get the loop area (and therefore inductance) =
down to it's
absolute minimum (for a given capacitor). With large body tantalum caps, =
it's possible
to make their ESL a major contributor to mounted inductance. With smaller=
body ceramic
surface mount capacitors, VIP becomes essential as the PCB becomes the gr=
eater contributor.

BTW, I have some vendor data from Kemet showing ESL of approximately 1.3 =
nH for=20
C1206 X7R and COG ceramics in the 0.001u to 0.01u range. I also have some=
SPICE
models from Kemet showing 2.5 nH for a T491D476M010 tantalum chip cap. Fi=
nally,
I have SPICE models for some 0603 NPO caps with ESL's ranging from 0.68 n=
H for
1.0 pF, to 1.82 nH for 220 pF.
These inductances are attributable to the capacitor body and are highly d=
ependent on
construction technique.

Fellow VIP crusader,

Dennis

>=20
> > From: Vigliarolo Roberto <Vigliarolo@asamrt.interbusiness.it>
> > To: si-list@silab.eng.sun.com
> > Subject: Re: [SI-LIST] : Surface Mount Cap Lead Inductance
> > Date: Tue, 11 Aug 1998 17:55:15 +0200
> > X-Priority: 3
> > MIME-Version: 1.0
> > Content-Transfer-Encoding: quoted-printable
> >
> > You can easily calculate the ESL of your capacitor by looking at the
> > frequency of the dip in the impedance vs frequency chart usually
> > provided by capacitors manufacturer. Fresonance=3D1/(2*pi*sqrt(ESL*C)=
)
> >
> >
> >
> > > ----------
> > > Da: Mike Mayer[SMTP:Mike.Mayer@heurikon.com]
> > > Inviato: luned=EC 20 luglio 1998 22.40
> > > A: si-list@silab.Eng.Sun.COM
> > > Oggetto: [SI-LIST] : Surface Mount Cap Lead Inductance
> > >
> > > I was looking at a note from howard Johnson about decoupling ("Bypa=
ss
> > > Multi-Valued Arrays"):
> > >
> > > http://www.sigcon.com/news/1_17.htm
> > >
> > > I started looking into it but can't seem to find any information on
> > > lead inductance for surface mount packages, especially the types us=
ed
> > > by tantalum caps (7343, 6032, etc.). Has anyone seen a source for t=
his
> > > information?
> > >
> > > --
> > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> > > =3D=3D=3D=3D=3D=3D=3D
> > > Mike Mayer Artesyn Communication
> > > Products, Inc
> > > Madison, WI
> > > http://www.artesyn.com/cp
> > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> > > =3D=3D=3D=3D=3D=3D=3D
> > >