When should I choose 2 layer PCB or 4 layer PCB?

2-layer PCB is enough to route any circuitry, at least in theory. One layer for vertical tracks, other for horizontal tracks, connect them with via’s. Unfortunately this is only an option if there is no need to route high-speed signals on the PCB. That is, no RF circuitry, no switched-mode power supplies, no long digital buses etc… The problem is that the return path for the signal currents is too long, the loop impedance is too high. As a consequence signal integrity cannot be ensured, the PCB tracks start acting as antennas, inductors and various other parts, destroying normal operation. The datasheet of most modern, high-performance semiconductors explicitly requires at least 4 layer PCB.

Even if the circuit seems to work, the radiated EMI might be too high to pass CE/FCC or the circuit might be sensitive to external interference. Except for the simplest circuits, at least 4 layers are needed to ensure that the device won’t emit too much radiation and would be robust against interference.

As shown above, high-frequency return current prefers a route that is close to the signal path. If there is no conductor there, we run into a lot of trouble!

4-layer PCB won’t improve the routing situation much, and won’t allow significant density increase. The two inner layers are used for ground, providing short return path for the signals, and power to provide clean and low resistance power supply to all subsystems. It’s also possible to reverse the layer stack. That can improve the radiated EMI but would also slow down signaling. A popular compromise is using a ground plane, but no power plane. It’s not as good as a standard 4-layer stack, but it’s orders of magnitude better than 2-layer.

4-layer PCB enables switched-mode PSU, most RF circuitry, and quite complicated digital logic. Some power devices rely on PCB cooling, rejecting heat to the ground plane through thermal via’s. They also require at least 4 layers.

Due to the aspect ratio limitations, blind and buried via’s are usually not available.

Price of a 4 layer PCB is roughly 3x-4x higher than that of a 2-layer one. However most of the time 2 layer is not a viable option.

6 or more layers unleash the full performance of PCB technology. Everything is possible, but comes at a price. More layers are only needed for routing and increasing the density. There are some devices, which due to package constraints, require at least 6 layers for correct operation and might work at reduced performance / increased EMI on 4 layers. Always read the datasheet / application notes.

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