[SI-LIST] Re: Bypass Capacitor Selection

Hal,

You can get some illustrative measured numbers from the EPEP2003
paper ""Frequency-Dependent Characterization of Bulk and Ceramic
Bypass Capacitors", posted at
http://home.att.net/~istvan.novak/papers.html

The presentation material is a longer version of the manuscript, with
more data. The measured data include: 0402 cap, 0508 with
multiple vias, IDC cap and capacitor arrays with via in pad.

To give some specific answers to your questions:
- you can see from the published data that -as always- there may
not be a single yes/no answer to which capacitor or which
connection is better.  In the C-R-L series equivalent circuit, all
three elements may be frequency dependent.  The L(f) function
for a particular capacitor furthermore depends on heavily on
the connection geometry.  Figures 24-28 compare three
different eight-terminal capacitors: two interdigitated capacitors
from different manufacturers and one capacitor array from a
third manufacturer.  All parts were measured in the same test fixture.
The L(f) curves reveal that interdigitated capacitors are always
better (lower inductance) compared to capacitor arrays (as
long as the internal geometries of the parts are identical) because
the capacitor array has separate strips of capacitors in the package
to bridge the two sides, whereas the interdigitated capacitor has a
single and much wider capacitor body.
- before we may want to optimize the external connection, is it worth
comparing the internal structures of different capacitors.  Most
importantly,
the thickness of cover-layer significantly impacts the inductance.  You
can see in Figure 27 of the presentation material that the same nominal
interdigitated capacitor from two different manufacturers had significantly
different L(f) curves.,  In fact, the worse of the two was even slightly
worse
(higher L) than the capacitor array.
- the most important factor in the L(f) function is the distance of the
capacitor body from the underlying plane (this spacing also includes the
thicklness of capacitor cover layer).  Second on the list is the spacing
and number of vias.  This all assumes that the vias are connected with the
shortest possible escape trace.  Via in pad can reduce the spacing between
vias, but manufacturing dictates a minimum spacing between vias anyway.
Therefore it is unlikely that interdigitated caps or capacitor arrays can be
hooked up with multiple vias in a single pad, even if we consider
laser-drilled
microvias.  Reverse-geometry caps, however, can be made with multiple
laser vias in the same pad.
- the above considerations are mostly electrical.  Cost always depends on
volume and the particular manufacturer.  In general, blind vias in pads will
add to the cost.  Assuming the same package style, reverse geometry
parts tend to have 1.5-3 times higher cost, interdigitated capacitors are
several times higher.

Regards and HAPPY NEW YEAR to all,

Istvan Novak
SUN Microsystems


"
----- Original Message -----
From: "Hal Murray" <hmurray@xxxxxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Cc: <hmurray@xxxxxxxxxxxxxxx>
Sent: Wednesday, December 31, 2003 2:45 AM
Subject: [SI-LIST] Re: Bypass Capacitor Selection


>
> How do broadside caps compare with arrays and/or the AVX IDC caps?  How to
> they compare with "normal" caps with multiple vias?
>
> I assume an array gets one via per pad.  (Does it make sense to use more?)
>
> Broadside caps leave more flexibility to the designer.  Let's assume I use
> the same number of vias for the same package size array.  Is that the
right
> number?  How do the parts cost compare?  How well does each approach work?
>
> Another topic tangled in this discussion is where the vias are located.
> How do vias next to a pad compare with a via in the pad?  How much do
> vias-in-pads cost relative to vias next to pads?  Does the answer to this
> depend upon what type of cap we are talking about?
>
>
>
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