Rick,

This is very well written and quite understandable.

Rocky Hilburn
480.458.5709 (Office)
480.205.1351 (Mobile)
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-----Original Message-----
From: EmbeddedNet [mailto:[log in to unmask]] On Behalf Of Richard Ulrich
Sent: Tuesday, March 07, 2006 7:10 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
******************************************************