From: "Saved by Internet Explorer 11" Subject: PCB Stack-Up - Part 3 Date: Thu, 2 Jul 2015 15:06:20 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.hottconsultants.com/techtips/pcb-stack-up-3.html X-MimeOLE: Produced By Microsoft MimeOLE V6.1.7601.17609 =20 =20 =20 = PCB Stack-Up=20 - Part 3 =20

Henry Ott=20 Consultants

Electromagnetic Compatibility Consulting = and=20 Training

PCB Stack-Up

Part 3. Six-Layer = Boards

Most=20 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.=20

One stack-up NOT to use on a six-layer board is the one shown = in=20 Figure 5. The planes provide no shielding for the signal layers, = and two=20 of the signal layers (1 and 6) are not adjacent to a plane. The = only time=20 this arrangement works even moderately well is if all the high frequency = signals=20 are routed on layers 2 and 5 and only very low frequency signals, or = better yet=20 no signals at all (just mounting pads), are routed on layers 1 and = 6. If=20 used, any unused area on layers 1 and 6 should be provided with "ground = fill"=20 and tied into the primary ground plane, with vias, at as many locations = as=20 possible.

________________Signal
= ________________Signal =20
________________Ground
= ________________Power &nbs= p; = ; =20 Figure 5
________________Signal
=20 ________________Signal

This configuration satisfies only one (number 3) of our original objectives.

With six layers available the principle of providing two buried = layers for=20 high-speed signals (as was done in Fig. 3) is easily implemented as = shown in=20 Fig. 6. This configuration also provides two surface layers for = routing=20 low speed signals.

________________Mounting Pads/Low Freq. Signals =
________________Ground
________________High Freq. = Signals =20
________________High Freq. Signals = Figure 6
________________Power
=20 ________________Low Freq. Signals

This is a probably the most common six-layer stack-up and can be = very=20 effective in controlling emissions, if done correctly. This = configuration=20 satisfies objectives 1, 2, & 4 but not objectives 3& 5. = Its main=20 drawback is the separation of the power and ground planes. Due to = this=20 separation there is no significant interplane capacitance between power = and=20 ground Therefore, the decoupling must be designed very carefully = to=20 account for this fact. For more information on decoupling, see our = Tech = Tip on=20 Decoupling.

Not nearly as common, but a good performing stack-up for a six-layer = board is=20 shown in Fig. 7.

________________Signal(H1)
________________Ground
________________Signal (V1) =
= &= nbsp; &n= bsp; &nb= sp; &nbs= p; = ; = =20 Figure 7
________________Signal (H2) =
________________Power=20
________________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=20 signal 2. This configuration has the advantage that orthogonal = routed=20 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=20 layer one and six are not shielded. Therefore the signal layers = should be=20 placed very close to their adjacent planes, and the desired board = thickness made=20 up by the use of a thicker center core. Typical spacing for a = 0.060" thick=20 board might be 0.005"/0.005"/0.040"/0.005"/0.005". This = configuration=20 satisfies objectives 1 and 2, but not 3, 4, or 5.

Another excellent performing six-layer board is shown in Fig. 8. It = provides=20 two buried signal layers and adjacent power and ground planes and = satisfies all=20 five objectives. The big disadvantage, however, is that it only = has two=20 routing layers -- so it is not often used.

________________Ground/ Mounting Pads
=20 ________________Signal
________________Ground =
= ________________Power &nbs= p; = ; = =20 Figure 8
________________Signal
=20 ________________Ground

It is easier to achieve good EMC performance with a six-layer = board than=20 with a four-layer board. We also have the advantage of four signal = routing=20 layers instead of being limited to just two. As was the case for=20 four-layer boards, it is possible to satisfy four of our five objectives = with a=20 six-layer PCB. All five objectives can be satisfied if we limit = ourselves=20 to only two signal routing layers. The configurations of Figures = 6, 7, and=20 8 all can all be made to perform very well from an EMC point of view. =

=A9 2001 Henry W. = Ott &nbs= p; = ; = Henry Ott Consultants, 48 Baker Road Livingston, = NJ 07039 (973) 992-1793

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Return to = HOC home=20 page.

Henry Ott Consultants

48 Baker Road Livingston, NJ 07039

Phone: 973-992-1793, FAX: = 973-533-1442

September 15, 2003