In the field of PCB design, even-layer PCBs (2/4/6/8 layers) have always dominated the market, while odd-layer PCBs are quite rare. This is not about pursuing symmetrical aesthetics, but a comprehensive optimal solution balancing engineering, cost and reliability.
 
The core reason lies in the symmetrical lamination structure. The dielectric layers and copper foils of even-layer PCBs are distributed in a mirror-image pattern. After molding under high temperature and pressure, they can offset the internal stress caused by the difference in thermal expansion coefficients of materials, fundamentally avoiding PCB warpage and deformation. In contrast, the asymmetrical structure of odd-layer PCBs will lead to uneven shrinkage of the board surface during cooling. The warpage rate of odd-layer PCBs is 30%-50% higher than that of even-layer ones, which directly affects the yield rate of SMT soldering and long-term reliability.
 
From the manufacturing perspective, even-layer PCBs are compatible with standardized production lines, requiring no additional process adjustments and achieving higher material utilization rates. Odd-layer PCBs, however, need to adopt non-standard lamination schemes such as "2+1". This not only adds more production procedures, but also increases costs by 20%-35%.
 
In addition, even-layer design makes it easier to achieve the ideal stack-up of "signal-ground/power-signal", providing a continuous reference plane for high-speed signals, reducing electromagnetic interference and optimizing signal integrity.
 
Of course, odd-layer PCBs are not completely worthless—they still have applications in extremely cost-sensitive or special RF/sensing scenarios. But in most cases, designers will choose the "pseudo-odd-layer" strategy (e.g., using a 6-layer board to achieve the functions of a 5-layer board by adding a blank layer) to balance performance and economy.
 
In the end, the dominant position of even-layer PCBs is the best balance achieved by the electronics manufacturing industry among structural mechanics, electrical performance and cost.