PCB Stack-up.pdf

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Henry Ott Consultants http://www.hottconsultants.com/index.html PCB Stack-Up Part 1. Introduction PCB stack-up is an important factor in determining the EMC performance of a product. A good stack-up can be very effective in reducing radiation from the loops on the PCB (differential-mode emission), as well as the cables attached to the board (common-mode emission). On the other hand a poor stack-up can increase the radiation from both of these mechanisms considerably. Four factors are important with respect to board stack-up considerations: 1. The number of layers, 2. The number and types of planes (power & ground), 3. The ordering or sequence of the layers, and 4. The spacing between the layers. Usually not much consideration is given except as to the number of layers. In many cases the other three factors are of equal importance. Item number four is sometimes not even known by the PCB designer. In deciding on the number of layers, the following should be considered: 1. The number of signals to be routed and cost, 2. Frequency, 3. If must meet Class A or B emission requirements, 4. Will the PCB be in a shielded or unshielded enclosure, 5. The EMC engineering expertise of the design team. Often only the first item is considered. In reality all the items are of critical importance and should be considered equally. If an optimum design is to be achieved in the minimum amount of time and at the lowest cost, the last item can be especially important and should not be ignored. Multi-layer boards using ground and/or power planes provide significant reduction in radiated emission over two layer PCBs. A rule of thumb, that is often used, is that a four-layer board will produce 15 dB less radiation than a two-layer board, all other factors being equal. Boards

containing planes 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 (and therefore the ground noise) significantly. Although two-layer boards have been used successfully in unshielded enclosures at 20 to 25 MHz, these cases are the exception rather than the rule. Above about ten or fifteen MHz, multi-layer boards should normally be considered. When using multi-layer boards there are five objectives that you should try to achieve. They are: 1. A signal layer should always be adjacent to a plane. 2. Signal layers should be tightly coupled (close) to their adjacent planes. 3. Power and Ground planes should be closely coupled together. 4. 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 very advantageous, since they will lower the ground (reference plane) impedance of the board and reduce the common-mode radiation.. Often we are faced with the choice between close signal/plane coupling (objective #2) and close power plane/ground plane coupling (objective #3). With normal PCB construction techniques, there is not sufficient inter-plane capacitance between the adjacent power and ground planes to provide adequate decoupling below about 500 MHz. The decoupling, therefore, will have to be taken care of by other means and we should usually opt for tight coupling between the signal and the current return plane. The advantages of tight coupling between the signal (trace) layers and the current return planes will more than outweigh the disadvantage caused by the slight loss in interplane capacitance. An eight-layer board is the fewest number of layers that can be used to achieve all five of the above objectives. On four and six layer board some of the above objectives will have to be compromised. Under

those conditions you will have to determine which objectives are the most important to the design at hand. The above paragraph should not be construed to mean that you can't do a good 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 compromise will be necessary. Since all the desired EMC objectives can be met with an eight-layer board, there is no reason for using more than eight layers other than to accommodate additional signal routing layers. Another desirable objective, from a mechanical point of view, is to have the 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, then layer seven should also be a plane. Therefore, all the configurations presented here use symmetrical, or balanced, construction. If a non-symmetrical, or unbalanced, construction is allowed additional stack-up configurations are possible. Part 2. Four-Layer Boards The most common four-layer board configuration is shown in Fig. 1 (power and ground planes may be reversed). It consists of four uniformly spaced layers with internal power and ground planes. The two external trace layers usually have orthogonal trace routing directions. _____________ Sig. _____________ Ground Figure 1 _____________ Power _____________ Sig. Although this configuration is significantly better than a two-layer board, it has a few, less that ideal characteristics. With respect to the list of objectives in Part 1, this stack-up only satisfies objective (1). If the layers are equally spaced, there is a large separation between the signal layer and the current return plane. There is also a large separation between the power and ground planes. With a four-layer board we cannot correct both of these deficiencies at the same time; therefore, we must decide which is most important to us. As mentioned previously, with normal PCB construction techniques there is not sufficient inter-plane capacitance between the adjacent power and ground planes to provide adequate decoupling. The decoupling, therefore, will have to be taken care of by other means and we should opt for tight coupling between the signal and the current return plane.

The advantages of tight coupling between the signal (trace) layers and the current return planes will more than outweigh the disadvantage caused by the slight loss in interplane capacitance. Therefore, the simplest way to improve the EMC performance of a four- layer board is to space the signal layers as close to the planes as possible (<0.010"), and use a large core (>0.040") between the power and ground planes as shown in Fig. 2. This has three advantages and few disadvantages. The signal loop areas are smaller and therefore produce less differential mode radiation. For the case of 0.005" spacing (trace layer to plane layer), this can amount to 10 dB or more reduction in the trace loop radiation compared a stack-up with equally spaced layers. Secondly, the tight coupling between the signal trace and the ground plane reduces the plane impedance (inductance) hence reducing the common-mode radiation from the cables connected to the board. Thirdly, the close trace to plane coupling will decrease the crosstalk between traces. For a fixed trace to trace spacing the crosstalk is proportional to the square of the trace height. This is one of the simplest, least costly, and most overlooked method of reducing radiation on a four-layer PCB. With this configuration we have satisfied both objectives (1) and (2). _____________ Sig. _____________ Ground Figure 2 _____________ Power _____________ Sig. What other possibilities are there for a four-layer board stack-up? Well, we could become a little non-conventional and reverse the signal layers and the plane layers in Fig. 2, producing the stack-up shown in Fig 3a. _____________ Ground. _____________ Sig. Figure 3a _____________ Sig. _____________ Power The major advantage of this stack-up is that the planes on the outer layers provide shielding to the signal traces on the inner layers. The disadvantages are that the ground plane may be cut-up considerably with component mounting pads on a high density PCB. This can be alleviated somewhat, by reversing the planes and placing the power plane on the component side, and the ground plane on the other side of the board. Secondly, some people don't like to have an exposed power

plane and thirdly, the buried signal layers make board rework difficult if not impossible. This stack-up satisfies objectives (1), (2), and partially satisfies objective (4). Two of these three problems can be alleviated with the stack-up shown in Fig. 3b, where the two outer planes are ground planes and power is routed as a trace on the signal planes. The power should be routed as a grid, using wide traces, on the signal layers. Two added advantages of this configuration are that; (1) the two ground planes produce a much lower ground impedance and hence less common-mode cable radiation, and (2) the two ground planes can be stitched together around the periphery of the board to enclose all the signal traces in a faraday cage. From an EMC point of view this configuration, if properly done, is the best stack-up possible with a four-layer PCB. Now we have satisfied objectives, (1), (2), (4), and (5) while using only a four-layer board. _____________ Ground. _____________ Sig./Pwr. Figure 3b _____________ Sig./Pwr. _____________ Ground A fourth possibility, not commonly used, but one that can be made to perform very well, is shown in Fig. 4. This is similar to Fig 2, but with the power plane replaced with a ground plane, and power routed as a trace on the signal layers. _____________ Sig./Pwr. _____________ Ground Figure 4 _____________ Ground _____________ Sig./Pwr. This stack-up overcomes the rework problem mentioned before, and still provides for the low ground impedance as a result of two ground planes. The planes however do not provide any shielding. This configuration satisfies objectives (1), (2), and (5) but not objectives (3) or (4). So, as you can see there are more options available, than you might have originally thought, for four layer board stack-up. It is possible to satisfy four of our five objectives with a four layer PCB. The configurations of Figures 2, 3b, and 4 all can be made to perform well from an EMC point of view.

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. Part 4. Eight-Layer Boards An eight-layer board can be used to add two more routing layers or to improve EMC performance by adding two more planes. Although we see examples of both cases, I would say that the majority of eight layer board stack-ups are used to improve EMC performance rather than add additional routing layers. The percentage increase in cost of an eight- layer board over a six-layer board is less than the percentage increase in going from four to six layers, hence making it easier to justify the cost increase for improved EMC performance. Therefore, most eight- layer boards (and all the ones that we will concentrate on here) consist of four wiring layers and four planes. An eight-layer board provides us, for the first time, the opportunity to easily satisfy all of the five originally stated objectives. Although there are many stack-ups possible, we will only discuss a few of them that have proven themselves by providing excellent EMC performance. As stated above, eight layers is usually used to improve the EMC performance of the board, not to increase the number of routing layers. An eight-layer board with six routing layers is definitely not recommended, no matter how you decide to stack-up the layers. If you need six routing layers you should be using a ten-layer board. Therefore, an eight-layer board can be thought of as a six-layer board with optimum EMC performance. The basic stack-up of an eight-layer board with excellent EMC performance is shown in Fig 9. ________________Mounting Pads/Low Freq. Signals ________________Pwr. ________________Gnd. ________________High Freq. Signals ________________High Freq. Signals Figure 9 ________________Gnd. ________________Pwr. ________________Low Freq. Signals/Test Pads This configuration satisfies all the objectives listed in Part 1. All signal

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