Decoupling must be handled in other ways, and we should choose a tight coupling between the signal and the current return plane. The advantages of a tight coupling between the signal layer and the current return plane outweigh the disadvantages of a slight loss of inter-layer capacitance.
Therefore, the easiest way to improve the EMC performance of four-layer boards is to put the signal layer as close to the plane as possible (<10mil) and use a large dielectric core (>40mil) between the power supply and the ground plane, as shown in figure 2.
This has three advantages and few disadvantages. The signal loop area is small and thus produces less differential mode radiation. For wiring layer to plane layer 5mil interval, the loop radiation can be reduced by 10dB or more compared with the same spacing cascade structure.
Second, the tight coupling of the signal routing to the ground reduces the plane impedance (inductance), thereby reducing the common-mode radiation of the cables connected to the plate.
Third, the tight coupling between routing and plane will reduce crosstalk between routing. For a fixed routing interval, crosstalk is proportional to the square of the routing height. This is one of the simplest, cheapest, and most overlooked ways to reduce radiation from a four-layer PCB.
Through this cascade structure, we satisfy both goals (1) and (2).
On the other hand, a bad cascade can greatly increase the radiation of both mechanisms. Four factors are important when considering lamination:
1. Number of floors;
2. Number and type of layers used (power supply and/or ground);
3. The order or sequence of layers;
4. Interlayer spacing.
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 PCB designers. When determining the number of layers, the following points should be taken into account:
1. Number and cost of wiring signals;
2. Frequency;
3. Does the product have to meet the launch requirements of Class A or Class B?
4. PCB is in the shielded or non-shielded housing;
5. EMC engineering expertise of the design team.
Usually just the first term. In fact, all projects are vital and should be considered equally. This last item is particularly important and should not be ignored if the optimal design is to be implemented with the least time and cost.
Using grounded and/or power plane multilaminates provides a significant reduction in radiation emission compared to two-laminates. The general rule of thumb is that four plates produce 15dB less radiation than two, all other factors being equal. A plate with a flat surface is much better than one without one, for the following reasons:
1. They allow signals to be wired as microstrip (or strip) wires. These structures are controlled impedance transmission lines that emit much less radiation than the random wiring used on the two-layer plates;
2. The ground plane significantly reduces ground impedance (and therefore ground noise).
Although two layers have been successfully used in unshielded enclosures of 20 to 25MHz, these cases are the exception rather than the rule. In the range of about 10-15 MHZ and above, multilayer boards should usually be considered.
When using multilaminates, you should try to achieve five goals. They are:
1. The signal layer shall always be adjacent to the plane;
2. The signal layer shall be tightly coupled (close) to its adjacent plane;
3, the power plane and the ground plane should be closely combined;
4. High-speed signals should be buried in the line between two planes, which can play a shielding role and inhibit the radiation of high-speed printed lines;
Multiple grounding surfaces have many advantages, as they will reduce the grounding (reference plane) impedance of the circuit board and reduce common mode radiation.
Typically, we are faced with a choice between signal/plane proximity coupling (target 2) and power/ground proximity coupling (target 3). With conventional PCB construction techniques, the flat capacitance between the adjacent power supply and the ground plane is insufficient to provide sufficient decoupling below 500 MHz.
Therefore, decoupling must be solved by other means, we should usually choose the tight coupling between the signal and the current return plane. The advantages of tight coupling between the signal layer and the current return plane outweigh the disadvantages caused by the slight loss of inter-plane capacitance.
Eight layers is the minimum number of layers that can be used to achieve all five of the above goals. On the fourth and sixth layers, some of these goals will have to be compromised. Under these conditions, you must determine which goals are most important to the design at hand.
The above paragraphs should not be interpreted to mean that you cannot make a good EMC design on a four or six layer board, because you can. It simply means that not all objectives can be achieved at the same time and some compromise is required.
Since all desired EMC goals can be achieved through eight layers, there is no reason to use more than eight layers, except to accommodate additional signal routing layers.
Another ideal goal from a mechanical point of view is to make the PCB cross section 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 configurations presented here use a symmetrical or balanced structure. If asymmetric or unbalanced structures are allowed, it is possible to build other cascading configurations.
Four layer board
The most common four-layer board structure is shown in figure 1 (the power plane and the ground plane can be reversed). It consists of four evenly spaced layers, the internal power plane and the ground plane, the two external wiring layers usually have orthogonal wiring directions.
Although this structure is much better than double panels, it has some less desirable features.
With respect to the target list in part 1, this stacked structure only satisfies the 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-layer board, we cannot correct both defects at the same time, so we have to decide which one is most important to us.
As mentioned earlier, with conventional PCB manufacturing techniques, the inter-layer capacitance between the adjacent power supply and the ground plane is insufficient to provide sufficient decoupling.