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on the fabrication panels which are
rectangles and thus increases overall
board cost.
Neither of these methods are
satisfactory for most products which
must compete in a price-sensitive
market.
The second method employs a
glass weave style that has the glass
uniformly spread across the surface.
Figure 3 is a photo of 1067 glass
weave cloth with a 4 mil (102 micron)
trace running across it. As can be
seen, there are no resin filled voids
in the example. Because of this,
the difference in travel time of the
two sides of the differential pair is
minimized. In fact, we have been able
to build quite large systems where the
skew over 30 inches (76 cm) is less
than 2 ps.
This sounds like the perfect answer to
the skew problem. It would be if the
methods by which the glass is spread
were uniform between glass weavers.
Unfortunately, this has proven not
to be true. The driver for spreading
the glass weave is the cell phone
manufacturers. Their reason for
spreading the glass is to make laser
drilling of blind vias more uniform, not
to preserve signal integrity. At present,
there is no sure way to guarantee
uniform spreading of the glass, so
controlling skew by manipulating PCB
laminates is problematic.
Dealing with signal-path
loss
As mentioned at the start of this
article, advances in semiconductor
technology have resulted in
transceivers that can tolerate as much
as 38 dB of loss in the signal path at
32 Gb/s. This has made it possible to
design systems with large backplanes
with plug in modules. When the move
to 56 Gb/s is made the materials that
are available as laminates no longer
have loss values that allow the design
of the very large routers required in
server farms and large IT centers.
Figure 4 is the loss vs. frequency for
a variety of materials that might be
used to create next generation high
performance Internet products.
Notice that the two curves labeled
“cable” have far lower loss than any
of the laminate systems used to
manufacture current products. This
loss is representative of what twinax
cable can achieve. This solves the
problem of how to achieve 56 Gb/s
in large systems without the need to
resort to optical interconnects.
More reliable &
economical than PCB
laminates
Since the signal integrity problem
at high data rates in large systems
Figure 5. Backplane using twinax cabling in place of PCB traces
Figure 6. Differential cable insertion loss, 2.6-3m
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