PCB stacking is an important factor to determine EMC performance of products. Good layering can be very effective in reducing radiation from the PCB loop (differential mode emission), as well as from cables connected to the board (common mode emission).
On the other hand, a bad cascade can greatly increase the radiation of both mechanisms. Four factors are important for consideration of plate stacking:
1. Number of layers;
2. The number and type of layers used (power and/or ground);
3. The order or sequence of layers;
4. The interval between layers.
Usually only the number of layers is considered. In many cases, the other three factors are equally important, and the fourth is sometimes not even known to the PCB designer. When determining the number of layers, consider the following:
1. Signal quantity and cost of wiring;
3. Does the product have to meet the launch requirements of Class A or Class B?
4. PCB is in shielded or unshielded housing;
5. EMC engineering expertise of the design team.
Usually only the first term is considered. Indeed, all items were vital and should be considered equally. This last item is particularly important and should not be overlooked if optimal design is to be achieved in the least amount of time and cost.
A multilayer plate using a ground and/or power plane provides a significant reduction in radiation emission compared to a two-layer plate. A general rule of thumb used is that a four-ply plate produces 15dB less radiation than a two-ply plate, all other factors being equal. A board with a flat surface is much better than a board without a flat surface for the following reasons:
1. They allow signals to be routed as microstrip lines (or ribbon lines). These structures are controlled impedance transmission lines with much less radiation than the random wiring used on two-layer boards;
2. The ground plane significantly reduces ground impedance (and therefore ground noise).
Although two plates have been successfully used in unshielded enclosures of 20-25mhz, these cases are the exception rather than the rule. Above about 10-15mhz, multilayer panels should usually be considered.
There are five goals you should try to achieve when using a multilayer board. They are:
1. The signal layer should always be adjacent to the plane;
2. The signal layer should be tightly coupled (close to) to its adjacent plane;
3, the power plane and the ground plane should be closely combined;
4, high-speed signal should be buried in the line between two planes, plane can play a shielding role, and can suppress the radiation of high-speed printed line;
5. Multiple grounding planes have many advantages because they will reduce the grounding (reference plane) impedance of the board and reduce common-mode radiation.
In general, we are faced with a choice between signal/plane proximity coupling (Objective 2) and power/ground plane proximity coupling (objective 3). With conventional PCB construction techniques, the flat plate capacitance between the adjacent power supply and the ground plane is insufficient to provide sufficient decoupling below 500 MHz.
Therefore, decoupling must be addressed by other means, and we should generally choose a tight coupling between the signal and the current return plane. The advantages of tight coupling between the signal layer and the current return plane will outweigh the disadvantages caused by a slight loss of capacitance between the planes.
Eight layers is the minimum number of layers that can be used to achieve all five of these goals. Some of these goals will have to be compromised on four – and six-ply boards. Under these conditions, you must determine which goals are most important to the design at hand.
The above paragraph should not be interpreted to mean that you cannot do a good EMC design on a four – or six-tier board, as you can. It just shows that not all objectives can be achieved at once and that some kind of compromise is required.
Since all desired EMC goals can be achieved with eight layers, there is no reason to use more than eight layers except to accommodate additional signal routing layers.
From a mechanical point of view, another ideal goal is to make the cross-section of the PCB board symmetrical (or balanced) to prevent warping.
For example, on an eight-layer board, if the second layer is a plane, then the seventh layer should also be a plane.
Therefore, all of the configurations presented here use symmetrical or balanced structures. If asymmetrical or unbalanced structures are allowed, it is possible to build other cascading configurations.
Four layer board
The most common four-layer plate structure is shown in Figure 1 (the power plane and ground plane are interchangeable). It consists of four evenly spaced layers with an internal power plane and a ground plane. These two external wiring layers usually have orthogonal wiring directions.
Although this construction is much better than double panels, it has some less desirable features.
For the list of targets in Part 1, this stack only satisfies target (1). If the layers are equally spaced, there is a large gap between the signal layer and the current return plane. There is also a large gap between the power plane and the ground plane.
For a four-ply board, we cannot correct both defects at the same time, so we must decide which is most important to us.
As mentioned earlier, the interlayer capacitance between the adjacent power supply and the ground plane is insufficient to provide adequate decoupling using conventional PCB manufacturing techniques.
Decoupling must be handled by other means, and we should choose a tight coupling between the signal and the current return plane. The advantages of tight coupling between the signal layer and the current return plane will outweigh the disadvantages of a slight loss of interlayer capacitance.
Therefore, the simplest way to improve the EMC performance of the four-layer plate is to bring the signal layer as close to the plane as possible. 10mil), and uses a large dielectric core between the power source and the ground plane (> 40mil), as shown in Figure 2.
This has three advantages and few disadvantages. The signal loop area is smaller, so less differential mode radiation is generated. For the case of a 5mil interval between the wiring layer and the plane layer, a loop radiation reduction of 10dB or more can be achieved relative to an equally spaced stacked structure.
Second, the tight coupling of signal wiring to the ground reduces the planar impedance (inductance), thus reducing the common-mode radiation of the cable connected to the board.
Third, the tight coupling of the wiring to the plane will reduce crosstalk between the wiring. For fixed cable spacing, crosstalk is proportional to the square of cable height. This is one of the easiest, cheapest, and most overlooked ways to reduce radiation from a four-layer PCB.
By this cascade structure, we satisfy both objectives (1) and (2).
What other possibilities are there for the four-layer laminated structure? Well, we can use a bit of an unconventional structure, namely switching the signal layer and plane layer in Figure 2 to produce the cascade shown in Figure 3A.
The main advantage of this lamination is that the outer plane provides shielding for signal routing on the inner layer. The disadvantage is that the ground plane may be heavily cut by the high-density component pads on the PCB. This can be alleviated to some extent by reversing the plane, placing the power plane on the side of the element, and placing the ground plane on the other side of the board.
Second, some people don’t like having an exposed power plane, and third, buried signal layers make it difficult to rework the board. The cascade satisfies objective (1), (2), and partially satisfies objective (4).
Two of these three problems can be mitigated by a cascade as shown in Figure 3B, where the two outer planes are ground planes and the power supply is routed on the signal plane as wiring.The power supply shall be raster routed using wide traces in the signal layer.
Two additional advantages of this cascade are:
(1) The two ground planes provide much lower ground impedance, thus reducing common-mode cable radiation;
(2) The two ground planes can be sewn together at the periphery of the plate to seal all signal traces in a Faraday cage.
From an EMC point of view, this layering, if done well, may be the best layering of a four-layer PCB. Now we have met goals (1), (2), (4) and (5) with only one four-layer board.
Figure 4 shows a fourth possibility, not the usual one, but one that can perform well. This is similar to Figure 2, but the ground plane is used instead of the power plane, and the power supply acts as a trace on the signal layer for wiring.
This cascade overcomes the aforementioned rework problem and also provides low ground impedance due to the two ground planes. However, these planes do not provide any shielding. This configuration satisfies goals (1), (2), and (5), but does not satisfy goals (3) or (4).
So, as you can see there are more options for four-layer layering than you might initially think, and it’s possible to meet four of our five goals with four-layer PCBS. From an EMC point of view, the layering of Figures 2, 3b, and 4 all work well.
6 layer board
Most six-layer boards consist of four signal wiring layers and two plane layers, and six-layer boards are generally superior to four-layer boards from an EMC perspective.
Figure 5 shows a cascading structure that cannot be used on a six-layer board.
These planes do not provide shielding for the signal layer, and two of the signal layers (1 and 6) are not adjacent to a plane. This arrangement only works if all the high frequency signals are routed at layers 2 and 5, and only very low frequency signals, or better yet, no signal wires at all (just solder pads) are routed at layers 1 and 6.
If used, any unused areas on floors 1 and 6 should be paved and viAS attached to the main floor in as many locations as possible.
This configuration satisfies only one of our original goals (Goal 3).
With six layers available, the principle of providing two buried layers for high-speed signals (as shown in Figure 3) is easily implemented, as shown in Figure 6. This configuration also provides two surface layers for low-speed signals.
This is probably the most common six-layered structure and can be very effective in controlling electromagnetic emission if done well. This configuration satisfies goal 1,2,4, but not goal 3,5. Its main disadvantage is the separation of power plane and ground plane.
Because of this separation, there is not much interplane capacitance between the power plane and the ground plane, so careful decoupling design must be undertaken to cope with this situation. For more information on decoupling, see our Decoupling technique tips.
An almost identical, well-behaved six-layer laminated structure is shown in Figure 7.
H1 represents the horizontal routing layer of signal 1, V1 represents the vertical routing layer of signal 1, H2 and V2 represent the same meaning for signal 2, and the advantage of this structure is that orthogonal routing signals always refer to the same plane.
To understand why this is important, see the section on signal-to-reference planes in Part 6. The disadvantage is that layer 1 and layer 6 signals are not shielded.
Therefore, the signal layer should be very close to its adjacent plane and a thicker middle core layer should be used to make up the required plate thickness. The typical 0.060 inches thick plate spacing is likely to be 0.005 “/ 0.005” / 0.040 “/ 0.005” / 0.005 “/ 0.005”. This structure satisfies Goals 1 and 2, but not goals 3, 4 or 5.
Another six-layer plate with excellent performance is shown in Figure 8. It provides two signal buried layers and adjacent power and ground planes to meet all five objectives. However, the biggest drawback is that it only has two wiring layers, so it is not used very often.
Six – layer plate is easier to obtain good electromagnetic compatibility than four – layer plate. We also have the advantage of four signal routing layers instead of being limited to two.
As was the case with the four-layer circuit board, the six-layer PCB met four of our five goals. All five goals can be met if we limit ourselves to two signal routing layers. The structures in Figure 6, Figure 7, and Figure 8 all work well from an EMC perspective.