In general, multilayer PCB design must adhere to two fundamental principles:

1. Each trace layer must have an adjacent reference layer (either a power or ground layer);

2. The adjacent main power layer and ground layer should be positioned as close as possible to maximize coupling capacitance. Below is an example illustrating the stack of an eight-layer board:

1. Stacking of single-sided and double-sided PCB boards

For two-layer boards, the limited number of layers eliminates concerns related to lamination. The primary focus for controlling EMI radiation is on routing and layout. The electromagnetic compatibility issues associated with single-layer and double-layer boards are becoming increasingly significant. This phenomenon largely stems from an excessively large signal loop area, which not only generates substantial electromagnetic radiation but also renders the circuit more susceptible to external interference. To enhance the electromagnetic compatibility of the circuit, the simplest approach is to minimize the loop area of critical signals.

Key signals: From an electromagnetic compatibility perspective, key signals refer primarily to those that produce significant radiation and those that are particularly sensitive to external disturbances. Signals capable of generating strong radiation are typically periodic, such as low-order clock signals or address signals. Conversely, signals that are sensitive to interference are often analog signals with lower levels.


Single and double-layer boards are typically utilized in low-frequency analog designs below 10 kHz:

1) Power traces on the same layer should be routed in a radial pattern to minimize their total length. 2) When routing power and ground wires, they should be kept close together. Place a ground wire adjacent to the key signal line, ensuring it is as close as possible to the signal line. This arrangement forms a smaller loop area, reducing the sensitivity of differential mode radiation to external interference. By positioning a ground wire next to the signal wire, a loop with minimal area is created, and the signal current will prefer this path over alternative ground routes.

3) In the case of a double-layer circuit board, a ground wire can be routed along the signal wire on the opposite side of the board, directly beneath the signal wire, with the ground trace made as wide as feasible. The loop area created in this manner equals the thickness of the circuit board multiplied by the length of the signal line.

### Two and Four-Layer Laminates

1. SIG-GND (PWR)-PWR (GND)-SIG;

2. GND-SIG (PWR)-SIG (PWR)-GND.

For these two stacking designs, a potential issue arises with the traditional 1.6 mm (62 mil) board thickness. The increased layer spacing becomes problematic, adversely affecting impedance control, interlayer coupling, and shielding. Notably, the greater spacing between power and ground planes diminishes board capacitance, hindering noise filtering.

The first scheme is generally applied in scenarios where there are numerous chips on the board. This arrangement can yield better signal integrity (SI) performance, though it may compromise electromagnetic interference (EMI) performance, which is primarily controlled through wiring and other details. Key considerations include placing the ground layer on the interconnecting layer with the highest signal density, facilitating the absorption and suppression of radiation; and increasing the board area to adhere to the 20H rule.

The second scheme is typically used when the chip density is low enough and there is ample space around the chips (to accommodate the necessary power copper layer). In this configuration, the outer layer of the PCB serves as the ground layer, while the two middle layers function as signal/power layers. Routing the power supply on the signal layer with wide traces can reduce the path impedance for power supply currents, as well as lower the impedance of the signal microstrip path. Additionally, signal radiation from the inner layer can be shielded by the outer layer. From an EMI control perspective, this represents the most effective four-layer PCB structure available.

**Note:** The two middle layers, which combine signal and power, should be separated, with wiring directions oriented vertically to prevent crosstalk. The board area must be managed appropriately to reflect the 20H rule. If controlling wiring impedance is a priority, care must be taken in arranging the traces, paving copper beneath the power and ground layers. Furthermore, the copper on the power supply or ground layer should be interconnected as much as possible to ensure DC and low-frequency connectivity.
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