Yet another alternative approach to engineer the “transparent via” is to
adjust the more than a dozen specific design features of a via to
reduce its impact on the signal. These include features such as the non
functional pads, the length of the residual stubs, the clearance holes,
spacing between the vias, capture pads and even the shape of the traces
feeding the vias. The challenge is exploring this entire design space to
find a high volume production capable design.
The solution is an algorithm Sanmina-SCI created to efficiently explore
design space to find the optimize combination of features to
approximate a transparent via. The result is Opti-vias, which leverage
all the options in the Sanmina tool box.
Of course, the dielectric losses from laminate materials and conductor
losses from rough copper play a significant role in backplane
performance. But, it’s not just about the materials. There are many
design factors that also contribute to line loss.
“At the end of the day it’s all about what the signal sees. What
influences the eye pattern is the performance of that net on that board
and a large part of it is the total attenuation of that net, not just
the Df or the copper losses. The design choices, based on specific
assumed values of Dk and Df also influence the final performance.”
Figure 3. 6 Gbps eye comparing conventional long stub and terminated stub, courtesy of Sanmina-SCI