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

Larry Smith (ldsmith@lisboa.eng.sun.com)
Tue, 11 Aug 1998 10:07:45 -0700 (PDT)

Tanmoy Roy, John Prymack and myself will be presenting a paper
in October at the EPEP conference that goes into a little more
depth on the mounted inductance of decoupling capacitors. We
find that there are 3 major contributers capacitor inductance:

=09pad and via design=09=091.00 to 4.00 nH
=09plane spreading inductance=090.01 to 0.50 nH
=09capacitor height=09=090.20 to 0.40 nH
=09
The three components sum together. Inductance is essentially a
measurement of the flux or B field in the environment caused by
current going around a loop. The bigger the loop, the more the
flux and the more the inducance. =20

As Ray stated below, the pad and via design are the most important
contributers to capacitor inductance. The design of the power planes
(PCB stackup) is second in importance. Third in importance is the
capacitor height. If the capacitor manufacturer has compensated out
the first two contributors to inductance (given you the self
inductance of the capacitor) the third (and least important) part is
all you get. Inductance numbers quoted for surface mount devices
are often mis-interpreted or mis-understood.

Come to the EPEP in West Point NY on October 26-28 and get the
rest of the story!

regards,
Larry Smith
Sun Microsystems

> Date: Tue, 11 Aug 1998 09:17:13 -0700 (PDT)
> From: Ray Anderson <raymonda@radium.eng.sun.com>
> Subject: Re: [SI-LIST] : Surface Mount Cap Lead Inductance
> To: Vigliarolo@asamrt.interbusiness.it
> Cc: si-list@silab.eng.sun.com
> Mime-Version: 1.0
>=20
>=20
> =09You 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
> =091=09Pad Geometry
> =09=09=09connector traces
> =09=09=09via location
> =09=09=09Number of vias
> =09=09=09
> =092=09Stackup
> =09=09=09distance from mounting pads to power planes
> =09=09=09
> =09All those factors that influence the loop area seen by the current
> flowing through the capacitor influence the mounted inductance. The amoun=
t=20
> contributed by the capacitor is a small part and is usually overwhelmed b=
y=20
> the other constituents unless you do a VERY good job in minimizing the ot=
her=20
> contributions (which can be done). The ESL of the capacitor by itself is =
a=20
> very strong function of the height dimension of the part.
>=20
> =09It is the mounted inductance of a bypass capacitor that mostly=20
determines
> the effectivness of the bypassing that the part provides.
>=20
>=20
> Ray Anderson
>=20
> Sun Microsystems Inc.=09=09
>=20
>=20
>=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
> >=20
> > 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))
> >=20
> >=20
> >=20
> > > ----------
> > > Da:=09Mike Mayer[SMTP:Mike.Mayer@heurikon.com]
> > > Inviato:=09luned=EC 20 luglio 1998 22.40
> > > A:=09si-list@silab.Eng.Sun.COM
> > > Oggetto:=09[SI-LIST] : Surface Mount Cap Lead Inductance
> > >=20
> > > I was looking at a note from howard Johnson about decoupling ("Bypass
> > > Multi-Valued Arrays"):
> > >=20
> > > http://www.sigcon.com/news/1_17.htm
> > >=20
> > > I started looking into it but can't seem to find any information on
> > > lead inductance for surface mount packages, especially the types used
> > > by tantalum caps (7343, 6032, etc.). Has anyone seen a source for thi=
s
> > > information?
> > >=20
> > > --=20
> > > =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
> > >=20