While not always an important measure of system performance, the TDR response of a
via is a sensitive metric to compare how transparent two different
vias are.
It's good to keep paste off the
vias. Even inverted though, I'm not sure how the stencil would have worked.
The differential signal will not see the return
via; it will just see the two
vias that make up the differential pair.
Ideally, it is preferred to have solder mask imaged and developed with rings around the
vias, rather than filling the
vias.
In a paper delivered at the EMC SI 2015 Symposium, the relationship between a pair of traces carrying a differential signal and two
vias carrying a different differential signal was explored.
In many cases the power and ground planes under a BGA component have voids, as a result of through-hole
vias. These voids have negative effects on power integrity, such as:
Microvias, as indicated by their name, are very small
vias, usually less than 0.006" or 150 [micro]m in diameter.
As a general rule, increasing the number of
vias that need to be backdrilled and decreasing the maximum allowable residual stub length will significantly increase the manufacturing costs of the PCB/backplane.
This article presented an equivalent circuit for the characterization of a printed circuit board
via as well as the results of time domain reflectometry measurements taken on a test printed circuit board containing
vias whose geometries were swept.
FIGURE 2 provides a closeup of the
via, circled in Figure 1.
The images here show some pretty small
vias. I believe they're 0.3mm in diameter.
When stacked
vias or via-in-pad design are required there are a number of manufacturing options that can be used to fill the
vias.
