Zynq-7000 PCB Design Guide
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UG933 (v1.8) November 7, 2014
Chapter 3:
Power Distribution System
plane, up through one via, through the connecting trace to the land, through the capacitor,
through the other land and connecting trace, down through the other via, and into the
other plane, as shown in
A connecting trace length has a large impact on the mounting’s parasitic inductance and if
used, should be as short and wide as possible. When possible, a connecting trace should
not be used and the via should butt up against the land. Placing vias to the side of the
capacitor lands or doubling the number of vias, further reduces the mounting’s parasitic
inductance.
Some PCB manufacturing processes allow via-in-pad geometries, an option for reducing
parasitic inductance. Using multiple vias per land is important with ultra-low inductance
capacitors, such as reverse aspect ratio capacitors that place wide terminals on the sides of
the capacitor body instead of the ends.
PCB layout engineers often try to squeeze more parts into a small area by sharing vias
among multiple capacitors. This technique should not be used under any circumstances.
PDS improvement is very small when a second capacitor is connected to an existing
capacitor’s vias. For a larger improvement, reduce the total number of capacitors and
maintain a one-to-one ratio of lands to vias.
The capacitor mounting (lands, traces, and vias) typically contributes about the same
amount or more inductance than the capacitor's own parasitic self-inductance.
X-Ref Target - Figure 3-7
Figure 3-7:
Example Cutaway View of PCB with Capacitor Mounting
Solderable End Terminal
0402 Capacitor Body
Surface Trace
Capacitor Solder Land
Via
PCB
GND
V
CC
Power and
Ground Planes
Mounted Capacitor Current Loop
UG933_c3_07_032811
