In PCB design, one common question arises: if an extra layer is not required for routing, why use it? Reducing the number of layers theoretically makes the board thinner and lowers costs. However, in some cases, adding an additional layer can actually reduce costs.

Core Structures of Multilayer PCBs

PCBs have two main structures: core structure and foil structure. In a core structure, all conductive layers are applied directly onto the core material. In a foil structure, only the inner conductive layer is applied to the core, while the outer layers are laminated with a foil and dielectric layer. Through the multilayer lamination process, all conductive layers are bonded together via dielectric material.

Why is an Even Number of Layers Preferred in PCB Design?

1. Cost Advantages
   A PCB with an odd number of layers requires a special lamination process, which increases manufacturing costs. The internal layer processing costs remain the same, but processing external layers is more complex, leading to higher costs and a greater risk of errors and reduced efficiency.
2. Minimizing Board Warpage
   An odd number of layers makes the PCB more prone to warping. During cooling after multilayer lamination, differences in material tension cause the board to bend. The thicker the board, the higher the risk of deformation. A balanced structure helps eliminate this issue. Even minor warping increases costs due to the need for specialized equipment and reduces the accuracy of component placement.

Methods for Achieving a Balanced Stack-Up

1. Add a Signal Layer
   If the power layer count is even, and signal layers are odd, adding a single signal layer will not increase the cost, but it will improve quality and reduce production time.
2. Add a Power Layer
   If the power layers are odd, an additional ground layer can be added without affecting electrical characteristics, creating a more balanced stack.
3. Add a Blank Signal Layer at the Center
   This approach is particularly useful for microwave and mixed dielectric boards. First, route the board with an odd number of layers, then add a blank signal layer to improve quality.

Advantages of a Balanced Stack-Up

Using an even number of layers can reduce costs, improve quality and stability, minimize warping, and shorten lead times for the final board.

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