The second step in designing a transparent
via is to recognize that, in general,
vias tend to be low impedance, so do what you can that is free to raise the impedance of the
vias.
The optimal method is to have the board shop fill and plate over the
vias. Most fab shops have thermally conductive fill materials that transfer heat almost as well as the open
via.
The differential impedance of the
vias will be between the impedance given by the twin rods and the rods with coupling to the planes, depending on the size of the anti-pads.
FIGURE 2 is a closeup of a through
via with solder mask filling the
via and solder paste printed with a segmented pattern, covering approximately 40 to 50% of the center pad.
A paper presented at last year's EMC Symposium considered the signal imbalance that ground-return
via placement has on nearby differential pairs of
vias.
In these cases, the use of blind
vias and buried
vias completely eliminates the through-hole
via stub, improving the signal integrity of the PCB.
Smaller
via sizes, which created smaller voids than larger
vias.
Plated through-hole (PTH)
via structures in high layer count printed circuit boards and thick backplanes/midplanes can significantly distort high-speed digital signals that pass through them.
One such discontinuity that is common in multilayered printed circuit boards is the
via. The accurate characterization of a printed circuit board
via is an important issue in the successful design of high speed circuits implemented on multilayered printed circuit boards.
Two amps through the 20 mil trace results in a 37.5[degrees] to 38[degrees]C temperature in both the trace and
via. As trace width increases, the trace can carry more current.
Our recommended method is to plug the
via with copper or epoxy and have it plated over by the fabricator.
Conductive fill, especially for packaging applications where
vias are small (50 to 75 [micro]m in size), is often completed using an electrolytic copper plating technique.