One stack-up NOT to use on a six-layer board is the one
in Figure 5. The planes provide no shielding for the signal
and two of the signal layers (1 and 6) are not adjacent to a
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
signals, or better yet no signals at all (just mounting pads), are
on layers 1 and 6. If used, any unused area on layers 1 and 6
be provided with "ground fill" and tied into the primary ground plane,
with vias, at as many locations as possible.
________________Power Figure 5
This configuration satisfies only one (number 3) of our original objectives.
With six layers available the principle of providing two buried
for high-speed signals (as was done in Fig. 3) is easily implemented as
shown in Fig. 6. This configuration also provides two surface
for routing low speed signals.
________________Mounting Pads/Low Freq. Signals
________________High Freq. Signals
________________High Freq. Signals Figure 6
________________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.
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
________________Power Figure 8
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
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