Best Formats for 4-Layer PCB Stackups
4-layer PCBs are the workhorses of many low-density digital and RF PCBs that still require some controlled impedance routing. 4-layer boards are also very useful for diverse power connections, and they can be used in power electronics that require many varied components and power levels. As soon as a 2-layer board becomes too dense, ground planes get added to create a 4-layer board, making these stackups a new designers first jump into more advanced layout and routing.
If you find that you need a 4-layer PCB, or that your 2-layer PCB will no longer be adequate, how should you assign layers in the 4-layer stackup? There are three very common configurations used in 4-layer stackup designs, but you can still go further and create a more advanced stackup with these techniques.
Common 4-Layer Stackups
All stackups need to mix plane and signal layers while also assigning power and ground. 4-layer stackups have limited space, but typically they are used because the routing or grounding demands eliminate the use of a 2-layer PCB. 4-layer boards can sometimes mix diverse functions, such as power and RF or digital and RF. There are a few different combinations of layer arrangements that can enable many types of designs with diverse functions.
Two Internal GND Planes
Internal ground planes below surface layers are normally implemented to support digital routing, including high-speed digital routing. These stackups are most commonly used when routing is needed on both sides of the PCB, and possibly when controlled impedance is needed for high-speed signals. This is one reason why many computer motherboards are built on this type of 4-layer PCB stackup.
New designers might look at this stackup and wonder where power is being placed. Power does not always require a power plane, and power could be routed in the surface layers in these designs using traces. Power could also be placed in the surface layers using large copper pours.
Applications:
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Double-sided high-speed PCBs
4-layer PCB stackup with two internal planes.
Two External GND Planes
This is just an inversion of the previous stackup with the signals placed on the interior layers. This stackup might not be the best option for high-speed routing because there could be crosstalk between signals in the internal layers. But because of the ground shielding the exterior of the board, this stackup could be used for some specialized low-noise systems, such as specialized analog systems that require low noise.
Applications:
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Low-noise PCBs
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Specialty mixed-signal PCBs
4-layer PCB stackup with two external planes.
Signal-Ground-Power-Signal
This 4-layer stackup mixes signal and power domains into a single system through the use of a dedicated power layer. The power layer could be a power plane, or it could use many power rails at different voltages. The main reason to use this type of stackup is to have many signals and high power in the same PCB, which is why the stackup would require a power layer.
Because of the use of a power layer, the bottom signal layer may not be able to accommodate high-speed signals unless it is kept as a plane layer. However, you can route lower speed signals or miscellaneous low-speed signals in the bottom layer without experiencing SI/EMI problems.
Applications:
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Single-sided high-speed PCBs
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Power electronic PCBs
4-layer PCB stackup with dedicated power layer.
Signal-Ground-Ground-Power
This type of stackup for a 4-layer PCB is typically used when a ground plane is needed to support some signals, and the design needs large rails for power. The additional ground plane on the 3rd layer is usually redundant in these designs, so this layer could also be used for routing some signals.
These designs tend to have lower signal count such that all signals can fit onto a single layer, but they need the dedicated layer for power routing. This could involve routing of multiple voltages on different rails, or use of a single large plane to provide high current. Either case justifies the use of the power layer on the back side.
Applications:
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Power electronics with a digital section
4-layer PCB stackup with bottom-side power layer and internal grounds.
One question that is sometimes asked regarding these stackups is when to apply grounded copper fill in the signal layers. In these designs, we would prefer to apply fills only when balanced copper is needed in order to prevent warpage. The use of fill should be symmetric; only use pour in the mirrored layers on each side of the internal core, or in order to balance plane layers on one half of the board.
Whether you need to design a 4-layer PCB or a 40-layer PCB, use the best PCB design features OrCAD from Cadence. If you’re ready to take even more control over net logic and board layout, you can graduate to Allegro PCB Designer for a more advanced toolset and additional simulation options for systems analysis. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.
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