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
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