Thanks Bill !
Yeah, I'm high with the BaTiO3 k in particle form. I'll tone that
down to about a thousand, and the point of the column still works.
CP and I (and Len Schaper) have a little difference of opinion on how
that flake conductor mechanism works. For those not familiar with
it, the flakes are conductive, not dielectric, and you get this huge
effective k near the percolation point. As your slides show, you
also get a huge DF. CP thinks its some sort of double layer effect,
but Len and I think that it's really just a way to get an effectively
small polymer layer. If you drill that molecule-wide hole down
through this sort of dielectric, you pass alternately through polymer
and metal flake. The flakes don't support any field, but the polymer
in between them is very thin, especially near percolation, of
course. So, if you add up the total amount of polymer you pass
through, it's very thin and you get a high capacitance. I prefer to
call this an "effective k" instead of a "k", because you really
haven't changed the dielectric constant of anything, you just have a
thin dielectric in the form of a printable thick film. I expect that
breakdown voltage and leakage measurements would show this. I'm not
sure, though, why the DF is so high.
I know of at least one other company that messed around with this
some years before CP was looking at it, but they dropped it because
it was too hard to manufacture that close to the percolation point,
giving very poor tolerance.
Thanks for the comments and, yes, "exits" should be "exists".
- Rick
******************************************************
Dr. Richard Ulrich
Professor
Dept. of Chemical Engineering
3202 Bell Center
Univ. of Arkansas
Fayetteville, AR 72701
[log in to unmask]
(479) 575-5645
******************************************************
On Dec 21, 2005, at 6:31 AM, William J Borland wrote:
>
>
>
>
> Rick,
>
> Just a couple of points:
>
> 1. Barium titanate tends to have dielectric constants in powder
> form of
> around 2000 ( work done at Rolla). Thus is due to the crystal size
> being
> small and the Curie point being at 125C. So mixing a K of 2000 with
> a K of
> 4 will get you even lower composite K if you use the log mixing rule
>
> 2. Putting extremely high levels of powder into a screen printable
> composition creates difficulty in printing and when cured has reduced
> cohesive strength
>
> Some work by C.P Wong at Ga Tech was directed towards getting high K
> composite systems by mixing flake conductor particles into a resin
> and at
> a particular loading of particles forming millions of small thin
> capacitors
> ( in parallel) but we tried it and it never came to anything. Here
> is a
> presentation on it. There may be some papers published on it
> somewhere.
>
> (See attached file: PRC HiK material-0901.ppt)
>
> Merry Christmas and Happy New Year
>
> Regards
>
>
>
> Rick Ulrich
> <[log in to unmask]
>> To
> Sent by: [log in to unmask]
>
> EmbeddedNet cc
> <[log in to unmask]
> org>
> Subject
> [EM] Embedded Passives
> Column #14
>
> 12/20/2005 02:39
> PM
>
>
> Please respond to
> D-50 Embedded
> Devices Committee
> Forum
> <[log in to unmask]
> org>; Please
> respond to
> Rick Ulrich
> <[log in to unmask]
>>
>
>
>
>
>
>
> Here’s a draft of my upcoming column for CircuiTree magazine,
> probably to run in March. I like to show these around to my
> colleagues in the business to make sure they are as accurate as
> possible, so please look it over in the next few days and let me know
> if there is anything you think needs changing. I'll be in the EPUG
> conference call tomorrow, too.
>
> - Rick Ulrich
>
>
>
>
> for “Embedded Passive Update” column, CircuiTree, March 2006
>
>
> Ferro-Filled Polymer Dielectrics: Promises and Problems
>
>
> Rick Ulrich
>
>
> It would be the perfect board-level embeddable dielectric. A screen-
> printable paste, storable for months, processable by standard methods
> familiar to all board shops, curable in minutes at mild temperatures,
> low waste, no degrading effects on other layers, and high
> capacitance. And it exits for you to use today. Well, all except
> for that high capacitance part.
>
> The idea is simple: mix a high-k ferroelectric powder into a curable
> polymer binder, screen print, and cure in place. For example, BaTiO3
> can be produced in bulk as submicron particles with dielectric
> constants in the thousands. For the binder phase, I know of no
> polymer with a k higher than about 12, so the selection of the binder
> phase is driven by usability considerations such as printability,
> cure conditions, and stability. The natural choice is either an
> epoxy or a polyimide, both with k’s in the range of 3 to 4. So let’s
> mix barium titanate particles with k = 10,000 and an epoxy with k =
> 3. Naturally, you want as much high-k filler as possible, so how
> much can you put in? The densest possible spherical packing is
> hexagonal close-pack at 74% by volume which, as every teenager that
> works in the grocery store knows, is how you stack oranges so they
> won’t roll away. Since barium titanate has a density near 5.9 and
> epoxy is usually a little less than one, the mixture ends up about
> 95% by weight of the high-k phase.
>
> The advantage of this approach is that much of the processing, and
> all of the high temperature steps necessary to get high k from the
> ferroelectric phase, can be done in advance of application to the
> organic substrate. Processing is additive so there is no patterning
> and little waste. No vacuum equipment is required and cure
> temperatures are comfortably low for the rest of the board and
> components. Because the films are thicker than sputtered, sol-gel or
> CVD, the working voltages are higher, on the order of 100’s of volts,
> and leakage at common board voltages is almost too low to be
> measured. Pinholes can be eliminated through multiple printings.
>
> But, despite the overwhelming preponderance of k = 10,000 phase over
> k = 3 phase, the overall dielectric constant of the random mixture
> will end up being only about 10 - 40, much closer to that of the low-
> k phase. You can understand why the mixing rules are not kind to
> this approach if you imagine drilling a molecule-wide hole down
> through this compound dielectric from one plate to the other. You
> would alternately pass through regions of high-k and low-k materials,
> and that’s exactly what the electric field between the plates sees:
> dielectrics in series. As capacitors are placed in series, the
> overall value drops, and the same effect causes the overall k of a
> randomly-dispersed composite material to be close to the lower-k
> phase. It doesn’t matter which one is dispersed and which is
> continuous, each field line sees this as alternating stacked
> dielectrics. A printed 10 micron thick film would deliver about 1 to
> 4 nF/cm2, and that’s about the most you can get from this approach.
>
> Attempts have been made to increase the high-k loading by using a
> multi-disperse set of filler sizes and shapes, with the idea that the
> small particles will nestle in between the big ones, and this can
> give up to 85% by volume or about 98% by weight. But this still does
> not increase the overall capacitance density very much and also
> creates problems of printability, adhesion and mechanical stability
> at such high solids loadings. Another problem is that screen
> printing or stenciling is not amenable to tight tolerances and there
> is no technology currently available for continuous trimming of
> embedded caps. Trim tabs can always be used to decrease the value in
> a stepwise fashion by removing capacitor area, but this method
> requires extra area of its own, further decreasing the effective
> capacitance density.
>
> But it’s still a good idea. It does provide an order of magnitude
> more capacitance than unfilled polymers, but two orders of magnitude
> less than the more-expensive thin-film paraelectrics (up to about 300
> nF/cm2) and pure ferroelectric films (1000’s of nF/cm2). The trick
> to increasing capacitance is to somehow stack the high-k phase so
> that it is vertically continuous over as wide an area as possible.
> Some kind of self-assembly or self-orientation would be necessary,
> then we’ll have a technology that can significantly advance the cause
> of embedded passives.
>
>
>
>
> ******************************************************
> Dr. Richard Ulrich
> Professor
> Dept. of Chemical Engineering
> 3202 Bell Center
> Univ. of Arkansas
> Fayetteville, AR 72701
>
> [log in to unmask]
> (479) 575-5645
> ******************************************************
>
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