Subject: PCB Stack-Up - Part 3
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Date: Thu, 2 Jul 2015 15:06:20 -0700

Henry Ott
Consultants
Electromagnetic Compatibility Consulting and
Training

PCB Stack-Up

Part 3. Six-Layer Boards
Most
six-layer boards consist of four signal routing layers and two planes.
From an EMC perspective a six-layer board is usually preferred over a
four-layer board.

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

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Henry Ott Consultants
48 Baker Road Livingston, NJ 07039
Phone: 973-992-1793,   FAX: 973-533-1442

September 15, 2003