From: "Saved by Internet Explorer 11" Subject: PCB Stack-Up - Introduction Date: Thu, 2 Jul 2015 15:05:49 -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-1.html X-MimeOLE: Produced By Microsoft MimeOLE V6.1.7601.17609 =20 =20 =20 = PCB Stack-Up=20 - Introduction =20

Henry Ott=20 Consultants

Electromagnetic Compatibility Consulting = and=20 Training

PCB Stack-Up

Part 1. Introduction

PCB = stack-up is=20 an important factor in determining the EMC performance of a = product. A=20 good stack-up can be very effective in reducing radiation from the loops = on the=20 PCB (differential-mode emission), as well as the cables attached to the = board=20 (common-mode emission). On the other hand a poor stack-up can = increase the=20 radiation from both of these mechanisms considerably.=20

Four factors are important with respect to board stack-up = considerations:=20

1. The number of layers,
2. The number and types of = planes=20 (power and/or ground) used,
3. The ordering or sequence of the = layers,=20 and
4. The spacing between the layers.

Usually = not much=20 consideration is given except as to the number of layers. In many = cases=20 the other three factors are of equal importance. Item number four = is=20 sometimes not even known by the PCB designer. In deciding on the = number of=20 layers, the following should be considered:=20

1. The number of signals to be routed and cost,
2.=20 Frequency,
3. Will the product have to meet Class A or Class B = emission requirements,
4. Will the PCB be in a shielded or unshielded = enclosure,=20 and
5. The EMC engineering expertise of the design = team.

Often=20 only the first item is considered. In reality all the items are of = critical importance and should be considered equally. If an = optimum design=20 is to be achieved in the minimum amount of time and at the lowest cost, = the last=20 item can be especially important and should not be ignored.=20

Multi-layer boards using ground and/or power planes provide = significant reduction in radiated emission over two layer PCBs. A rule of = thumb, that=20 is often used, is that a four-layer board will produce 15 dB less = radiation than=20 a two-layer board, all other factors being equal. Boards = containing planes=20 are much better than those without planes for the following = reasons:

1. They allow signals to be routed in a microstrip (or = stripline) configuration. These configurations are controlled impedance transmission lines with much less radiation than the random traces = used on a two-layer board.
2. The ground plane decreases the ground = impedance=20 (and therefore the ground noise) significantly.

Although = two-layer=20 boards have been used successfully in unshielded enclosures at 20 to 25 = MHz,=20 these cases are the exception rather than the rule. Above about = ten or=20 fifteen MHz, multi-layer boards should normally be considered.=20

When using multi-layer boards there are five objectives = that=20 you should try to achieve. They are:

1. A signal layer should always be adjacent to a plane. =
2.=20 Signal layers should be tightly coupled (close) to their adjacent = planes. =20
3. Power and Ground planes should be closely coupled together. =
4.=20 High-speed signals should be routed on buried layers located between planes. In this way the planes can act as shields and contain = the radiation from the high-speed traces.
5. Multiple ground planes = are=20 very advantageous, since they will lower the ground (reference plane)=20 impedance of the board and reduce the common-mode = radiation..

Often=20 we are faced with the choice between close signal/plane coupling = (objective #2)=20 and close power plane/ground plane coupling (objective #3). With = normal=20 PCB construction techniques, there is not sufficient inter-plane = capacitance=20 between the adjacent power and ground planes to provide adequate = decoupling=20 below about 500 MHz. The decoupling, therefore, will have to be = taken care=20 of by other means and we should usually opt for tight coupling between = the=20 signal and the current return plane. The advantages of tight = coupling=20 between the signal (trace) layers and the current return planes will = more than=20 outweigh the disadvantage caused by the slight loss in interplane = capacitance.=20

An eight-layer board is the fewest number of layers that can be = used to=20 achieve all five of the above objectives. On four and six layer = board some=20 of the above objectives will have to be compromised. Under those=20 conditions you will have to determine which objectives are the most = important to=20 the design at hand.

The above paragraph should not be construed to mean that you can't do = a good=20 EMC design on a four- or six-layer board, because you can. It only = indicates that all the objectives cannot be met simultaneously and some=20 compromise will be necessary. Since all the desired EMC objectives = can be=20 met with an eight-layer board, there is no reason for using more than = eight=20 layers other than to accommodate additional signal routing layers.

Another desirable objective, from a mechanical point of view, is to = have the=20 cross section of the board symmetrical (or balanced) in order to prevent = warping. For example, on an eight-layer board if layer two is a = plane,=20 then layer seven should also be a plane. Therefore, all the = configurations=20 presented here use symmetrical, or balanced, construction. If a=20 non-symmetrical, or unbalanced, construction is allowed additional = stack-up=20 configurations are possible.

=A9 2000 Henry W. = Ott &nbs= p; = ; = 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

August=20 5, 2002