# [SI-LIST] Re: Current Return Vias

• From: Joel Brown <joel@xxxxxxxxxx>
• To: leeritchey@xxxxxxxxxxxxx
• Date: Sun, 23 Jul 2006 12:25:53 -0700
``` I am currently working on board that has 12 layers.
Ground planes are layers 2, 5, 8, and 11.
If a high speed signal changes from layer 3 to layer 9 then its reference
planes change from layers 2 &5 to layers 8 &11.
Since the return current flows under the signal trace on the reference plane
, this return current has jump from from one set of planes to the other set
at the point signal changes layers through a via. The return current could
jump through interplane capacitance or through a ground via located next to
the signal via. So in this situation I don't see why having a ground via
nextto a signal would not be beneficial? Does the interplane capacitance
provide a lower impedance path than the ground via for the return current to
Joel

Lee Ritchey wrote: Kenny, It is not true that you need a "return current"
vianext to each layer changing signal via. I continue to be amazed that
engineers who are looked upon as SI experts say such things. Imagine you
havea 4 layer PCB, such as the mother board in a PC, where there are only
twoplanes, one Vdd and one ground, where would such vias connect? There have
been billions of these made to date that work just fine and have very fast
signals on them. The return currents you are concerned about find their way
from plane to plane through the collection of decoupling capacitors and
interplane capacitance that you had to engineer into the power delivery
system in order to make it stable. Focus on this and the return currents
takecare of themselves. EMI is minimized he same way.. [Original Message]
From: Kenny Frohlich <kenny_frohlich@xxxxxxxxx>[1] To:
<si-list@xxxxxxxxxxxxx>[2]Date: 7/22/2006 6:45:56 AM Subject: [SI-LIST]
Current Return Vias Dear Experts, I understand that I need to provide ground
vias next to via explictly for the purpose of letting return currents jump
between layers. I know it's a requirement for high speed signals, especially
differrential signals. Is this also required for low speed single-ended
signals (133Mhz or slower)? If this is a requirement, what would be a good
signal via to ground via ratio? For example, there are five signal vias
within a 1 inch area, how many ground vias do I need? Thank you Kenny
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