There are several potential problems with the 4-layer board design. First of all, for a traditional four-layer board with a thickness of 62 mil, even if the signal layer is on the outer layer and the power and ground layers are on the inner layer, the distance between the power layer and the ground layer is still too large.
If cost requirements are a priority, consider the following two alternatives to traditional 4-layer boards. Both solutions can improve EMI suppression performance, but only when the component density on the board is low enough and there is enough area around the components where the required power supply copper layer is placed.
The first one is the preferred solution. The outer layer of the PCB is the ground layer, and the two middle layers are the signal/power layer. The power supply on the signal layer is routed with wide traces, which makes the path impedance of the power supply current low, and the impedance of the signal microstrip path is also low. From an EMI control standpoint, this is the best 4-layer PCB structure available. In the second scheme, the outer layer takes the power and ground, and the middle two layers take the signal. Compared with the traditional 4-layer board, the improvement of this scheme is smaller, and the interlayer impedance is as poor as the traditional 4-layer board.
If you want to control the trace impedance, the above stacking scheme requires very careful routing of the traces under the power and ground copper islands. In addition, copper islands on power or ground planes should be interconnected as closely as possible to ensure DC and low frequency connectivity.
If the density of components on a 4-layer board is relatively large, it is best to use a 6-layer board. However, some stacking schemes in the 6-layer board design are not good enough to shield the electromagnetic field, and have little effect on reducing the transient signal of the power busbar. Two examples are discussed below.
In the first example, the power and ground are placed on the 2nd and 5th layers, respectively. Due to the high impedance of copper cladding in the power supply, it is very unfavorable for controlling common-mode EMI radiation. However, from the point of view of impedance control of the signal, this method is quite correct.
In the second example, the power supply and the ground are placed on the 3rd and 4th layers respectively. This design solves the problem of copper-clad impedance of the power supply. Due to the poor electromagnetic shielding performance of the first and sixth layers, the differential mode EMI increases. This design can solve the differential mode EMI problem if the number of signal traces on the two outer layers is minimal and the trace length is short (less than 1/20 of the wavelength of the highest harmonic of the signal). The suppression of differential mode EMI is particularly good by filling the non-component and trace-free areas on the outer layer with copper and grounding the copper area (every 1/20 wavelength interval). As mentioned earlier, the copper area should be connected to the internal ground plane at multiple points.
General high-performance 6-layer board design
Generally, the 1st and 6th layers are arranged as ground layers, and the 3rd and 4th layers are used for power and ground. EMI suppression is excellent due to two centered dual microstrip signal line layers between the power and ground planes. The disadvantage of this design is that there are only two trace layers. As mentioned earlier, the same stackup can be achieved with a traditional 6-layer board if the outer layer traces are short and copper is placed in the no-trace area.
Another 6-layer board layout is Signal, Ground, Signal, Power, Ground, Signal, which enables the environment required for advanced signal integrity designs. The signal layer is adjacent to the ground plane, and the power and ground planes are paired. Obviously, the downside is the unbalanced stacking of layers.
This usually brings trouble to processing and manufacturing. The solution to the problem is to fill all the blank areas of the third layer with copper. If the copper density of the third layer is close to the power layer or the ground layer after copper filling, this board can be loosely counted as a structurally balanced circuit board. The copper filling area must be connected to power or ground. The distance between the connecting vias is still 1/20 wavelength, not necessarily everywhere, but ideally should be connected.
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