[hsdd] High-Speed Digital Design Newsletter - Think Small
- From: "Dr. Howard Johnson" <howie03@xxxxxxxxxx>
- To: <hsdd@xxxxxxxxxxxxx>
- Date: Wed, 1 Jun 2005 14:12:26 -0700
THINK SMALL
HIGH-SPEED DIGITAL DESIGN ? online newsletter ?
Vol. 8 Issue 04
If you read my last missive, you know I have been
working recently to understand crosstalk in BGA
packages (www.sigcon.com/Pubs/news/v8_03). This work
led me to some new experiments involving a ? guess
what ? gigantic scale model of a BGA package. The
model incorporates 100 balls, spaced on a pitch of 2-
1/4 inches (scale factor 57:1). Crosstalk
measurements taken from this model correspond
perfectly to the crosstalk actually accrued in the
spaces between the BGA balls underneath a realistic
BGA package.
The most excellent part of this model is that you
can easily change the assignment of ground balls in
the model package. This lets you instantly try out
various ground pin assignments, to study the
relationship of crosstalk to BGA ground ball
assignment.
I will publish the results of my measurements in the
form of tech online webcast, open to all viewers.
Here is the registration link:
http://seminar2.techonline.com/s/xilinx_jun0705
Greenwich Mean Time Tue, Jun 07, 2005 18:00
Eastern Daylight Time Tue, Jun 07, 2005 02:00 PM
Pacific Daylight Time Tue, Jun 07, 2005 11:00 AM
This broadcast is part of a series of talks
sponsored by Xilinx. Here is a link to other
broadcasts I have conducted for this series:
http://www.xilinx.com/products/virtex4/advantage/sipi.htm
This month's article, Think Small, addresses in a
general way the topic of physically scaling a BGA
package.
______________________________________________________
THINK SMALL
If you want to go fast, think small.
The three-dimensional rule for physical scaling of
electrical connections immutably controls the
performance of connectors, packages, component
bodies, vias, and many other common structures on a
printed circuit board. Technically, it applies to
any lossless physical structure, meaning any
structure that does not lose power due to resistive
losses, dielectric losses, or radiation losses in
any appreciable way.
For example, a BGA ball, while it may create a
signal reflection or create some crosstalk, does not
lose any power. The power you put into the structure
propagates through the circuit (S21), reflects back
to the source (S11), or goes into an adjacent
circuit (crosstalk). There may be a miniscule
resistive loss, but if the ball is used as a signal
via that loss is small enough to simply ignore. Such
a structure qualifies as perfectly lossless for the
purpose of applying the 3-D rule of scaling.
Quoting from my book, High-Speed Signal Propagation,
"When you change the physical dimensions of a
distributed circuit, modifying all geometrical
dimensions x, y, and z by a common factor k, without
changing either the electric permittivity or the
magnetic permeability, you find that all inductances
change by a factor of k and all capacitances change
by the same factor.
"If the circuit is passive and lossless (that is,
composed only of inductive and capacitive effects,
with no resistances) it will have been scaled in
time, whereby the new circuit should behave the same
as the old circuit, only its step response stretched
(or compressed) in time by the factor k. A network-
analyzer plot of the new system will show the same
frequency response as the old, only shifted in
frequency by a factor of 1/k. A resonance at
frequency f in the original circuit appears at
frequency f/k in the new circuit. As a consequence,
physically enlarging a system of physical conductors
lowers its resonant frequencies, while shrinking the
system physically raises them.
"The rule of physical scaling applies well to low-
loss conducting structures like metal plates,
conducting wires, connector pins, and semiconductor
packages. This rule is valid over any range of
physical scales for which useful conducting objects
may be constructed. It breaks down for certain
structures near the atomic level, for which the
conducting surfaces cannot be scaled due to the
inherent quantization of atomic matter. As far
physicists know, this law applies to structures of
galactic dimensions, although such structures have
not been tested to verify conformance with the
rule."
In the simplest terms, large objects have low
resonant frequencies, and small objects have higher
ones. This simple rule of scaling explains why a
flip-chip package is better than a surface mounted
package, why a surface-mount package is better than
a DIP, and (going even further back in time) a DIP
package is better than a miniature 9-pin tube
socket. Tiny dimensions push the parasitic
resonances up to frequencies above the bandwidth of
your signal, resulting in a perfectly flat frequency
response across the band that matters. When you want
to go fast, small is good.
OK, now let's apply this rule to something like a
BGA package. A package with 0.8 mm ball pitch has a
smaller form factor than a 1-mm pitch, and so should
work better at higher frequencies, right? Well, at
this point I must remind you that all three physical
dimensions, x, y, and z are involved in the scaling
process. Shrinking x and y is good, but not if it
comes at the expense of increasing the overall
height z. Yet, that is what often happens with
reduced form-factor BGA arrays. If you can, for
example, accomplish double-track routing with the
larger form-factor ball layout but only single-track
routing with the smaller form factor, it may
actually take more layers to completely route your
design, thus adding to z. Since most crosstalk and
reflection problems scale in proportion to z (total
sum of BGA height plus board thickness), things that
make the board thicker are generally bad ideas.
Best Regards,
Dr. Howard Johnson
______________________________________________________
Dr. Johnson will be presenting both his seminars at
Oxford University in the U.K. the week of June 21st,
2005, as part of a comprehensive week-long training
series:
http://www.conted.ox.ac.uk/cpd/electronics/courses/digital_design_week.asp
Questions & Comments: all students who attend our
High-Speed Digital Design seminars have the
opportunity to talk directly with Dr. Johnson about
signal integrity issues.
______________________________________________
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