Re: [SI-LIST] : Capacitor Inductance (was: Comment on Johnson's article)

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From: Istvan Novak - Board Design Technology (inovak@pompom.east.sun.com)
Date: Mon Nov 08 1999 - 11:41:55 PST


Larry,

I beleive at the dimensions that we are now using around bypass capacitors we cannot use
the 'incremental' counting of total inductance. As you stated accurately, it is the
mounted inductance that matters. The mounted inductance (except the plane inductance) is
really an inductance of a loop, where the inductance of a stand alone capacitor is its
partial inductance. If we move the capacitors and planes closer, it will increase the
mutual inductance, and it eventually will reduce the loop inductance as well. So for a
500pH mounted inductance of a 805 capacitor, the self inductance of the capacitor may be
higher than 500pH to start with, and it is reduced by the mutual inductance.

Istvan Novak
SUN Microsystems

        Date: Mon, 8 Nov 1999 10:14:19 -0800 (PST)
        From: Larry Smith <Larry.Smith@eng.sun.com>
        Subject: [SI-LIST] : Capacitor Inductance (was: Comment on Johnson's article)
        To: si-list@silab.eng.sun.com
        MIME-Version: 1.0
        Content-MD5: FFDb67h6eZ/PaEWBkrB//A==
        
        I wanted to address several comments that have come back on the
        subject of inductance of decoupling capacitors and vias. Rather
        than further increasing the traffic on SI-list, I'll do it all
        in one email. I have changed the thread title to better reflect
        the subject at hand.
        
> From: "Rick Brooks" <ribrooks@nortelnetworks.com>
>
> Has anybody ever tried 0508 caps (not 0805) with multiple trace/vias to
> the planes?
> It seems like a better idea for low inductance, but I don't know about
> the cost or availability.
>
> - Rick Brooks
        
        The 0508 capacitors have a little less inductance than 0805 capacitors
        because the current path is wider and shorter. But this only effects
        the 'partial inductance' due to the capacitor, which is usually small
        compared to the total inductance. To understand the total inductance,
        we must examine the entire loop associated with the capacitor
        current. That includes not only the capacitor but the mounting pads,
        traces (if any) vias and power planes underneath the capacitor.
        
        For this discussion, let's define inductance in terms of energy stored
        in the B field caused by the current flowing in this loop. Energy =
        (L/2)*I^2, or one-half L I-squared. The energy is proportional to
        inductance and the square of the current.
        
        To reduce the inductance, we have to reduce the energy stored in the B
        field. The best way to do that is to reduce the 'loop area' traversed
        by the current because the B field is associated with this loop. The
        loop may be marginally reduced by using shorter, wider capacitors, but
        there are other things that are much more important. For example, the
        distance between the vias to the power planes and the height of the
        vias between the power planes and the solder pads. These are the
        factors that greatly effect the inductance. As I mentioned in a
        previous note, the partial inductance due to an 0805 capacitor is 0.2
        to 0.4 nH, depending on the height. The partial inductance contributed
        by the height and separation of vias is 0.5 to 2nH. The mounting
        inductance must be carefully controlled before it makes sense to use
        'low inductance' capacitors.
        
> From: Keith Howard <howardk@modman.eng.sun.com>
>
> Larry,
>
> The low inductance capacitors from AVX have 8 or more leads, so require
> multiple vias into the planes.
> Unless you're talking about multiple vias in parallel with the required
> vias.
>
> Keith
        
        Some configurations of the AVX capacitor have multiple leads in
        a vdd/gnd checkerboard pattern, which nearly forces you to make a
        checkerboard pattern of vias (possibly 8 of them) under the capacitor.
        This is VERY good for low inductance. We have now broken up the loop
        into at least 8 parallel loops, greatly reducing the inductance.
        
        But in many cases, you could have just put the multiple vias in parallel
        (not in a checkerboard pattern) and mounted a dumb old 0805 capacitor
        on them. There would be almost as much improvement in the inductance
        and you can skip the expensive part. That is because the inductance
        is in the design of the vias, mounting pads and distance to the power
        planes, with only a minor amount attributed to the capacitor.
        
> Date: Fri, 05 Nov 1999 17:53:09 -0800
> From: Ron Miller <rmiller@Brocade.COM>
>
> Also note that ATC recommends that their microwave capacitors be
> mounted on their side so that the resonances from the top layers and
> bottom layers of the cap are all the same length from the input and output
> trace, which shifts the SRF up somewhat.
>
> For those caps that have very thin height it probably doesn't matter, but
> for the cube shaped ones it does.
>
> Ron Miller
        
        After significantly reducing the pad and via inductance, the biggest
        inductance contribution is from the height of the capacitor. For large
        'cubic' shaped capacitors, current must go a long distance up the post
        of the capacitor before it can cross over through the plates of the
        capacitor and go back down the other post. This essentially increases
        the loop area of the capacitor. Note that power planes in the PCB may
        be 10 mils or less below the surface of the PCB, but capacitors may be
        40 mils high. With carefully chosen PCB stackups (power planes near
        the surface) and carefully designed vias and solder pads, it is
        possible for the dominant inductance to be in the height of the
        capacitor.
        
        As Ron has mentioned above, you can see the effect of the capacitor
        height inductance on an impedance analyzer just by turning the cubic
        capacitor 90 degrees on it's side. It is difficult to know which
        direction the capacitor posts go just by looking at it. But if you
        mount it on an impedance analyzer with the posts vertical, two
        impedance dips can sometimes be seen. The low frequency resonance is
        from the mounted loop inductance. The higher frequency minimum is due
        to inductance internal to the capacitance. Current is sloshing around
        between the top and bottom plates. There is inductance associated with
        the current-loop path inside of the capacitor. Turn the capacitor on
        it's side so that the capacitor posts are perpendicular to the mounted
        inductance, and the 'anti-resonant' peak will go away. This
        anit-resonance is due to a parallel resonant circuit and behaves just
        like the parallel resonant circuit formed by inductive capacitors
        mounted on PCB power planes, but all inside of a single component.
        
        regards,
        Larry Smith
        Sun Microsystems
        
        
        
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Istvan Novak Sun Microsystems, Inc.
Istvan.Novak@Sun.Com Workgroup Servers, BDT Group,
                        One Network Drive, Burlington, MA 01803
                        Phone: (781) 442 0340

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