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

Mark Randol ([email protected])
Tue, 11 Aug 1998 11:36:17 -0700

Ray Anderson wrote:
> 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:
> 1 Pad Geometry
> connector traces
> via location
> Number of vias
> 2 Stackup
> distance from mounting pads to power planes

This isn't lead inductance per say, but since we've drifted a
bit to what can contribute to resonances:

3 - The orientation of the cap as mounted to the PCB can make
a difference too. ATC (American Technical Ceramics - uses a mulitlayer construction, and suggests
the plates be oriented perpendicular to the PCB surface. This
reduces component to board capacitance <?>. I've made resonance
problems disappear and VCO's resonate in band by just rotating
the caps from "flat" to "vertical". (ref ATC Bulletin #201,
Recommended Attachment Techniques for Multilayer Capacitors)

ATC is the only manufacturer I know of that has orientation
markings, so maybe others construct theirs differently?

Mark Randol, RF Measurements Engineer   | Motorola SPS, Inc.
(602)413-8052 Voice                     | M/S EL379
(602)413-4150 FAX                       | 2100 E. Elliot Road
[email protected]                | Tempe, AZ 85284