Excellent and interesting.

Thanx

Jim

James J. Hickman, PhD

Hickman Associates Inc    [log in to unmask]

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Raleigh, NC 27615

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-----Original Message-----
From: EmbeddedNet [mailto:[log in to unmask]] On Behalf Of Richard Ulrich
Sent: Tuesday, March 07, 2006 9:10 AM
To: [log in to unmask]
Subject: [EM] Circuitree Embedded Passive Column (DRAFT)

Here's a draft of my next column on embedded passives.  Any comments
would be appreciated.

- Rick

for "Embedded Passive Update" column, CircuiTree, May 2006


High-Capacitance Thick Film Dielectrics?

Rick Ulrich

In my last column, two months ago, I bemoaned the sad state of
affairs with screen-printable thick film dielectrics.  Even when
packed with 95%+ by weight with randomly-dispersed high-k barium
titanate and printed to only 10 microns thick, the overall k is a
mere 30 and delivers only about 3 nF/cm2.  One way of increasing the
capacitance of this approach would be to somehow stack the high-k
phase so that it is vertically continuous over as wide a lateral area
as possible.  Some kind of self-assembly or self-orientation would be
necessary, and I know of work going on in that direction.  But
there's another trick that's been receiving new attention lately that
may be able to significantly increase the capacitance density of
screen-printable thick films.  The concept is pretty simple: load the
polymer up with conductive metal particles, as opposed to the
insulating high-k particles that I described in the previous column.
Rao Tummala, C. P. Wong, and their team at Georgia Tech have been
exploring this lately and have managed to get effective dielectric
constants of over 100 by dispersing Ag and Al particles in epoxy.

To understand how this works, think about drilling a vertical hole
one molecule wide all the way through the dielectric.  The materials
and their configurations encountered along the way are what the field
sees.  In the case of randomly-dispersed 10 micron k = 1000 barium
titanate particles scattered in k = 3 epoxy, this hole would pass
through alternating regions of high and low k material, which the
field would see as capacitors in series.  The result is an overall k
in between the two, but much closer to the low end at about 30 since
adding caps in series lowers the overall value of the set.  But, if
the dispersed particles are instead conductive, then all of the
voltage drop is over the epoxy phase alone.  The hole might pass
through 10 microns of total applied film thickness, but only a small
fraction of that will be actual dielectric, depending the
concentration of conductive particles that were mixed in.  In
essence, this is a clever way to make a very thin polymer film that
is easily screen printed.  As you approach the percolation point, the
minimum concentration of metal particles that causes them to start
touching each other, the effective film thickness is very small and
high specific capacitance can be obtained.

Using 80 wt% of 3.0 micron Al in epoxy, they've managed to get an
effective k of  109 and a dissipation factor of only 2%.  I say
"effective k" because the material that is actually acting as the
dielectric has the same k it had in bulk form, about 3, but is spread
very thinly.  The value of 109 is based on the entire film thickness
so this is the k it acts like.  Printed to a total thickness of 10
microns gives a specific capacitance of almost 10 nF/cm2, a factor of
three better than high-k dispersed in epoxy and better than 30 times
what you'd get from unfilled epoxy.  This latter number also
represents the ratio of the printed film thickness to the effective
thickness.  The downside to this method is that you have to be very
close to the percolation point to get these high values and, in this
vicinity, the effective k and the dissipation factor change rapidly
with particle loading, making tolerance a major challenge.  Once the
particles reach percolation and actually begin touching, the
dissipation factor skyrockets since the film becomes leaky.  Much
past this, it would not function as a dielectric at all, so you need
to be just a fraction of a percent under percolation for best
results.  But Georgia Tech is working on getting around this issue by
using metal particles that have an oxide coating so that even when
they touch, there is no electrical continuity.

The primary application for this would be decoupling, since tolerance
is not a major issue as long as a certain minimum amount of
capacitance is achieved.  Also, dissipation factor is not nearly so
much a detriment in decoupling as it is in filters and A/C
converters.  In fact, some folks claim that a high dissipation factor
is beneficial to decoupling since it helps remove high-frequency
noise on the power/ground planes.

Their results will soon be published in IEEE Transactions of Advanced
Packaging along with a considerable amount of information on the
processability of this sort of material.  Much work remains to be
done, but it's an interesting and promising approach that could
eventually give tens of nF/cm2 with a screen-printable film.



******************************************************
Dr. Richard Ulrich
Professor
Dept. of Chemical Engineering
3202 Bell Center
Univ. of Arkansas
Fayetteville, AR   72701

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(479) 575-5645
******************************************************