A 4 layer PCB stackup is a common configuration used in printed circuit board design, offering a good balance between complexity, cost, and performance. It typically consists of two signal layers and two plane layers (power and ground). Here’s a standard 4-layer PCB stackup:

Typical 4-Layer PCB Stackup

1. Top Layer (Layer 1): Signal layer (outer layer)
   • Used for component placement and routing high-speed or critical signals.
   • Often includes components, traces, and pads.
2. Inner Layer 1 (Layer 2): Ground plane (GND)
   • Provides a low-impedance return path for signals.
   • Helps reduce electromagnetic interference (EMI) and improves signal integrity.
3. Inner Layer 2 (Layer 3): Power plane (VCC)
   • Distributes power to components.
   • Can be split into multiple voltage regions if needed.
4. Bottom Layer (Layer 4): Signal layer (outer layer)
   • Used for additional routing and component placement.
   • Often used for less critical signals or general-purpose routing.

Advantages of a 4-Layer Stackup

• Improved signal integrity due to dedicated ground and power planes.
• Better EMI shielding and noise reduction.
• Simplified routing with two signal layers.
• Suitable for moderate-speed digital and analog designs.

Stackup Thickness

The total thickness of a 4-layer PCB depends on the dielectric material (e.g., FR-4) and the desired impedance. A common total thickness is around 1.6 mm, but this can vary based on design requirements.

Impedance Control

For high-speed designs, controlled impedance traces are often required. The stackup can be designed to achieve specific impedance values (e.g., 50 Ω for single-ended signals or 100 Ω for differential pairs) by adjusting the dielectric thickness and trace width.

Example Stackup Configuration

Design Considerations

• Place high-speed signals on the top or bottom layers adjacent to the ground plane for better EMI performance.
• Use vias to connect signals between layers, but minimize via stubs for high-speed designs.
• Ensure proper decoupling capacitors are placed near power pins to stabilize the power distribution network.

4 Layer Stackup reference for designers:

Common 4-Layer Stackups

In 4-layer PCB designs, the arrangement of plane and signal layers is crucial, as it involves balancing power distribution, grounding, and signal routing. While 4-layer boards have limited space compared to higher-layer counts, they are often chosen when the routing complexity or grounding requirements exceed what a 2-layer PCB can handle. These stackups can accommodate diverse functionalities, such as combining power and RF circuits or integrating digital and RF sections. Several layer configurations are possible, each enabling a variety of design applications.

wo Internal Ground Planes

One common 4-layer stackup features two internal ground planes beneath the outer signal layers. This configuration is particularly useful for digital routing, including high-speed digital signals. It is often employed when routing is required on both outer layers and when controlled impedance is necessary for high-speed signal integrity. This is why many computer motherboards and other high-speed digital devices are built using this type of 4-layer stackup.

New designers might question where the power is placed in this arrangement. Interestingly, a dedicated power plane is not always necessary. Power can be routed on the surface layers using traces or large copper pours, providing flexibility in design.

Applications:

• Double-sided high-speed PCBs
• Mixed-signal PCBs (combining analog and digital circuits)

Two External Ground Planes

This stackup is essentially the inverse of the previous configuration, with the signal layers moved to the interior and the ground planes placed on the outer layers. While this arrangement may not be ideal for high-speed routing due to potential crosstalk between signals on the internal layers, it offers unique advantages for certain applications. The external ground planes provide excellent shielding, making this stackup suitable for specialized low-noise systems, such as sensitive analog circuits that require minimal interference.

Applications:

• Low-noise PCBs
• Specialty mixed-signal PCBs (combining analog and digital circuits with noise sensitivity)

This configuration is particularly useful in scenarios where noise suppression and shielding are critical, even if it sacrifices some of the benefits of high-speed signal routing.

Signal-Ground-Power-Signal

This 4-layer stackup integrates both signal and power domains into a single design by incorporating a dedicated power layer. The power layer can function as a full power plane or accommodate multiple power rails at different voltages, depending on the design requirements. This configuration is particularly useful when a PCB needs to support a high number of signals alongside significant power delivery, necessitating the inclusion of a dedicated power layer.

However, the presence of the power layer can limit the bottom signal layer's ability to handle high-speed signals effectively unless it is maintained as a plane layer. Despite this, the bottom layer can still be utilized for routing lower-speed signals or miscellaneous connections without encountering significant signal integrity (SI) or electromagnetic interference (EMI) issues.

Applications:

• Single-sided high-speed PCBs
• Power electronics PCBs

This stackup is ideal for designs that require a balance between signal routing and power distribution, making it a practical choice for applications involving power electronics or systems with mixed signal and power requirements.

Signal-Ground-Ground-Power

This 4-layer PCB stackup is commonly used when a design requires a robust ground plane to support signal integrity while also needing significant power routing capabilities. The additional ground plane on the third layer is often redundant for grounding purposes, so it can alternatively be utilized for routing some signals if necessary.

This configuration is particularly suited for designs with a lower signal count, where all signals can be accommodated on a single layer, but the power delivery demands justify a dedicated power layer. The power layer can be configured to handle multiple voltage rails or a single large plane for high-current applications, making it a practical choice for power-intensive designs.

Applications:

• Power electronics with a digital control section

This stackup is ideal for applications that combine power delivery requirements with digital signal processing, offering a balance between grounding, signal routing, and power distribution.

This stackup is widely used in consumer electronics, industrial controls, and communication devices. For more complex designs, higher layer counts (e.g., 6-layer or 8-layer PCBs) may be required.