PCB Stackup Symmetrical vs Asymmetrical

Printed Circuit Boards (PCBs) are important components in electronics. They provide a platform for interconnecting electronic components and enabling the flow of electrical currents. PCBs come in different shapes and sizes, and can have various stackup configurations. Two common stackup configurations are symmetrical and asymmetrical stackups. In this blog post, we will compare the two stackup configurations and highlight their advantages and disadvantages.

Symmetrical Stackup

In a symmetrical stackup configuration, both sides of the PCB have the same layer sequence and thickness. This means that the layers are mirrored on both sides, and the thickness of each layer is the same. Symmetrical stackups are commonly used in 4-layer, 6-layer, 8-layer, and 10-layer PCBs.

One advantage of a symmetrical stackup is that it offers better control of the impedance of the PCB traces. Because the layers are mirrored, the impedance remains more balanced, which helps to reduce signal delays and reflections.

Another advantage of a symmetrical stackup is that it offers better heat dissipation. The symmetry of the stackup configuration means that the heat spreading is more uniform, which helps to reduce thermal hotspots.

However, one downside of a symmetrical stackup is that it can be more expensive to manufacture. This is because both sides of the PCB need to have the same layer sequence, which can require more intricate manufacturing steps.

Asymmetrical Stackup

In an asymmetrical stackup configuration, the layers on each side of the PCB are different in terms of sequence and thickness. This means that the layers on one side are not mirrored on the other side, and the thickness of the layers can vary. Asymmetrical stackups are commonly used in high-layer count PCBs, such as 12-layer and above.

One advantage of an asymmetrical stackup is that it offers more design flexibility. Because the layer sequence and thickness can vary, designers have more options to optimize the PCB for their specific requirements, such as reducing crosstalk and improving signal integrity.

Another advantage of an asymmetrical stackup is that it can be more cost-effective to manufacture. Because the layer sequence and thickness can vary, manufacturers can optimize the manufacturing steps to reduce costs.

However, one downside of an asymmetrical stackup is that it can be more challenging to control the impedance of the PCB traces. Because the layers are not mirrored, the impedance can be less balanced, which can result in signal delays and reflections.

Conclusion

In conclusion, whether to use a symmetrical or asymmetrical stackup configuration depends on the specific requirements of the PCB. Symmetrical stackups are better suited for low-layer count PCBs and applications that require better impedance control and heat dissipation. Asymmetrical stackups are better suited for high-layer count PCBs and applications that require more design flexibility and cost-effectiveness.

Both stackup configurations have their advantages and disadvantages. It is important to carefully consider the specific requirements of the PCB before deciding on the stackup configuration.

References

• Lee Ritchey, "PCB Stackup Design Considerations for Signal Integrity in High-Speed Designs," Signal Integrity Journal, 2019.
• Mike Santarini, "The basics of PCB stackups," EE Times, 2020.
• "What is PCB Stack-Up? Advantages and Disadvantages!" Technotronix, 2021.