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.




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

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