One stack-up NOT to use on a six-layer board is the one
shown
in Figure 5. The planes provide no shielding for the signal
layers,
and two of the signal layers (1 and 6) are not adjacent to a
plane.
The only time this arrangement works even moderately well is if all the
high frequency signals are routed on layers 2 and 5 and only very low
frequency
signals, or better yet no signals at all (just mounting pads), are
routed
on layers 1 and 6. If used, any unused area on layers 1 and 6
should
be provided with "ground fill" and tied into the primary ground plane,
with vias, at as many locations as possible.
________________Signal
________________Signal
________________Ground
________________Power Figure 5
________________Signal
________________Signal
This configuration satisfies only one (number 3) of our original
objectives.
With six layers available the principle of providing two buried
layers
for high-speed signals (as was done in Fig. 3) is easily implemented as
shown in Fig. 6. This configuration also provides two surface
layers
for routing low speed signals.
________________Mounting Pads/Low Freq. Signals
________________Ground
________________High Freq. Signals
________________High Freq. Signals Figure 6
________________Power
________________Low Freq. Signals
This is a probably the most common six-layer stack-up and can be
very effective in controlling emissions, if done correctly. This
configuration satisfies objectives 1, 2, & 4 but not objectives 3
&
5. Its main drawback is the separation of the power and ground
planes.
Due to this separation there is no significant interplane capacitance
between
power and ground Therefore, the decoupling must be designed very
carefully to account for this fact. For more information on
decoupling,
see our Tech Tip on Decoupling.
Not nearly as common, but a good performing stack-up for a six-layer
board is shown in Fig. 7.
________________Signal(H1)
________________Ground
________________Signal (V1)
Figure 7
________________Signal (H2)
________________Power
________________Signal (V2)
H1 indicates the horizontal routing layer for signal 1, and V1
indicates
the vertical routing layer for signal 1. H2 and V2 represent the
same for signal 2. This configuration has the advantage that
orthogonal
routed signals always reference the same plane. To understand why
this is important see section on Changing
Reference Planes in Part 6. The disadvantage is that the
signals
on layer one and six are not shielded. Therefore the signal
layers
should be placed very close to their adjacent planes, and the desired
board
thickness made up by the use of a thicker center core. Typical
spacing
for a 0.060" thick board might be
0.005"/0.005"/0.040"/0.005"/0.005".
This configuration satisfies objectives 1 and 2, but not 3, 4, or 5.
Another excellent performing six-layer board is shown in Fig. 8. It
provides two buried signal layers and adjacent power and ground planes
and satisfies all five objectives. The big disadvantage, however,
is that it only has two routing layers -- so it is not often used.
________________Ground/ Mounting Pads
________________Signal
________________Ground
________________Power Figure 8
________________Signal
________________Ground
It is easier to achieve good EMC performance with a six-layer board
than with a four-layer board. We also have the advantage of four
signal routing layers instead of being limited to just two. As
was
the case for four-layer boards, it is possible to satisfy four of our
five
objectives with a six-layer PCB. All five objectives can be
satisfied
if we limit ourselves to only two signal routing layers. The
configurations
of Figures 6, 7, and 8 all can all be made to perform very well from an
EMC point of view.
© 2001 Henry W. Ott Henry Ott Consultants, 48 Baker Road Livingston, NJ 07039 (973) 992-1793