There are several reasons why high-precision multilayer PCBs (Printed Circuit Boards) typically have an even number of layers:

1. Symmetry and Mechanical Stability

Balanced Structure: Having an even number of layers helps achieve a balanced structure, which is essential for maintaining mechanical stability during manufacturing and operation. Any imbalance in layer distribution can lead to warping or mechanical stress.

Thermal Stability: Symmetry in layer arrangement helps to manage thermal expansion more effectively during soldering and operation, minimizing the risk of delamination.

2. Signal Integrity

Controlled Impedance: In multilayer PCBs, especially those designed for high-frequency applications, maintaining controlled impedance is crucial. With an even number of layers, the dielectric between the signal layers can be more easily controlled, helping maintain signal integrity and reducing electromagnetic interference (EMI).

Reference Planes: The presence of ground and power planes in multilayer designs creates a more effective return path for signals, which is easier to achieve with an even number of total layers.

3. Layer Pairing

Power/Ground Pairing: An even number of layers allows for proper pairing of power and ground layers. This allows for better power distribution across the board and improved grounding, essential for reducing noise and ensuring reliable performance.

Signal Layers: Signal layers can be arranged in pairs, with dedicated ground or power planes in between, enhancing performance in terms of impedance control and shielding.

4. Manufacturing and Design Considerations

Via and Layer Routing: An even number of layers facilitates better routing of vias. It allows for more efficient interconnections and helps minimize the size and spacing of vias, which can greatly affect the PCB’s performance and reliability.

Masking and Coating: When applying solder mask and other coatings, an even number of layers can help ensure uniform application and improve overall board aesthetics.

 5. Simplified Design Rules

Many PCB design tools and standards often have predefined specifications that favor even layers, making it easier for designers to conform to best practices and manufacturing capabilities.

Summary

The preference for even-numbered layers in high-precision multilayer PCBs is largely driven by considerations of mechanical stability, signal integrity, power distribution, manufacturing efficiency, and design simplicity. These factors collectively contribute to more reliable and high-performance electronic circuits.