42 The PCB Design Magazine • October 2017
for different test boards with different
laminates. A peek preview of one such
board is shown in Figure 8, courtesy of
DuPont. The 6" x 6" open-edge board
has a 1-mil HK04J25 laminate with one
ounce copper.
So, the next time you do memory eye
simulations with PDN effects included,
make sure your models are causal and your
tools can properly handle them.
PCBDESIGN
References
1. "Be careful with Tlines in plane
models"
2. "Simulating Planes with SPICE"
3. "Simulating Complex Power-
Ground Plane Shapes with Variable-Size
Cell SPICE Grids," 11th Topical Meeting
on Electrical Performance of Electronic
Packaging, October 21–23, 2002, Mon-
terey, CA
4. Istvan Novak's home page
5. Cadence Design Systems
6. Ansys
7. Mentor
8. "Wideband frequency-domain charac-
terization of FR-4 and time-domain causality,"
IEEE Trans. Electromagn. Compat., vol. 43, no.
4, pp. 662–667, Nov. 2001.
9. "Frequency domain analysis and electrical
properties test method for PCB dielectric core
materials," DesignCon 2003 East, Boston, MA,
Jun. 2003.
10. Isola
11. DuPont
12. Istvan Novak's home page
Dr. Istvan Novak is a distinguished
engineer at Oracle, working on
signal and power integrity designs
of mid-range servers and new tech-
nology developments. With 25
patents to his name, Novak is
co-author of "Frequency-Domain
Characterization of Power Distribution
Networks." To contact Novak or read past
columns, click here.
AC resistance on the planes. If needed, correc-
tions for surface roughness can be added, too.
The resulting causal grid model can be simu-
lated in circuit simulators that can take frequency
dependent components. Unfortunately, the orig-
inal free Berkeley SPICE does not have this op-
tion,
but as long as we need only AC simulations,
there is a straightfor
ward, though tedious work-
around. We
can calculate the actual RLGC values
separately at each frequency point and then run
SPICE AC simulations at those single frequencies.
From the output files, we can put together the full
frequency dependent causal response.
To illustrate the potentials of causal and
frequency dependent plane models, Figure 6
shows how the lossy causal model captures the
gradual suppression of modal resonances as the
laminate gets thinner. We can see that with 10-
mil (0.25 mm) plane separation the impedance
swings are substantial at the resonance frequen-
cies, and as we go to 1-mil or thinner, we get
a significant reduction. As signal vias may go
through these power-ground cavities, capturing
the plane resonance correctly in frequency and
magnitude becomes important, especially when
we combine the plane models with signal inter-
connects for SI-PI co-simulation.
In a future column we will show the cor-
relation between simulated and measured data
CAUSAL POWER PLANE MODELS
Figure 7: Photo of a 6" x 6" laminate test board,
courtesy of DuPont.
