[SI-LIST] : bypass cap question (long, simple)

Lawrence Butcher ([email protected])
Sat, 31 Jan 1998 00:58:28 -0800

Imagine that I build a 4 layer board. Imagine that there were two chips on
it, labeled U1 and U2. Imagine that I route the board strictly manhatten
style. All horizontal wires are on top above the ground plane, and all
vertical wires are on the bottom below the power plane.
| |
| U1 ------* |
| | |
| | |
| | |
| U2 |

Normally, I would put bypass caps under U1 and bypass caps under U2.
I would cosy them up so that there was minimum distance between the
caps and the power supply pins on the chip.

Consider the image currents running on the power and ground planes.
An image current will sit directly under each wire. But that current
will have a hard time following the wire through the via, because it
would have to hop from the ground plane to the power plane.

It seems clear that a capacitor might be needed at that via site to
give the current in one plane a chance to hop to the other. Even
though there are no components nearby.

Intuition rarely substitutes for calculation. Question: Is this true?
How much capacitance? How does that vary if there are 40 wires instead
of 1? How does the number change with frequency?

The above illustrates a real problem. I am building an 8-layer board,
with a tentative stackup of:

1 Horizontal
2 GND Plane
3 V33 Plane
4 Vertical
5 Horizontal
6 V5 Plane
7 GND Plane
8 Vertical

My component placement places all of the 3 volt components above the
midline of the board, and all of the 5 volt components below the midline.

Therefore, there are NO bypass caps from the 5V plane to ground in the
top half of the board, and NO bypass caps from the 3.3V plane to ground
in the bottom half of the board.

A trace running horizontally on layer 5 in the top half of the board
will have an image current running on the V5 plane, and that current
has no way to get to the ground plane at a via site. Same for traces
running on layer 4 in the botton half of the board.

I want to add about 1 cap per square inch (about 50 more bypass caps).
Half will be between V5 and ground in the top half of the board, and half
will between V33 and ground in the bottom half of the board.

These components are there only to deal with my paranoia about image
currents. They would be placed even though there are already tons of
bypass caps in the same area, but exclusively to only one power plane.

My coworkers have doubts. They especially don't like 50 more caps when
there are no nearby chips connected to the power planes I am concerned with.

Most of my signals are changing at 100 MHz, but there is a bunch of 33 MHz
activity running around. (Fast, for me).

Is this a non-existent problem, or a real one? Comments?