Why Most PCBs Are Even number Layers?

PCB designers may design odd-numbered printed circuit boards. If layout does not require an extra layer, why use it? Does reducing the layer won't make the board thinner? If the circuit board one layer less, isn't the cost less?
 
However, in some cases, adding a layer will reduce costs.
 
Because less one layer prepreg and foil, the cost of raw materials for odd-numbered PCBs is slightly lower than for even-numbered PCBs. However, the processing cost of odd-numbered PCBs is significantly higher than that of even-numbered PCBs. The processing cost of the inner layer is the same, but the foil and core structure significantly increases the processing costs of the outer layer.
 
The odd-numbered PCB needs to add a non-standard stack-up bonding process based on the core structure process. Compared with the core, the production efficiency of plant that add foils outside the core will decline. The outer core requires additional processing before stack-up, which increases the risk of the outer layer being scratched and erroneously etched.
 

The best reason to design a PCB without an odd-numbered layer is that the odd-numbered circuit boards are easy to bow and twist.
 
When the PCB is cooled after the multilayer circuit bonding process, the different lamination tensions at the cooling of the core structure and the foil structure cause the PCB to bow and twist. As circuit board thickness increases, the risk of PCBA OEM bow and twist with two different structures composite PCBs increases. The key to eliminating board bow and twist is to use a balanced stack. Although a certain degree of bow and twist of the PCB meets the specification requirements, subsequent processing efficiency will be reduced, resulting in increased costs. Because of the need for special equipment and processes during assembly, the placement accuracy of the components is reduced and the quality will be impaired.
 
When odd-numbered PCBs appear in the design, the following methods can be used to achieve balanced stacking, lower PCB fabrication costs, and avoid PCB bow and twist. The following methods are ranked according to the preferred level.
 
1. Add a signal layer and use it. This method can be used if the power supply layer of the PCB is designed to be even number and the signal layer is odd. The added layer does not increase the cost, but it can shorten the delivery time and improve the PCB quality.
 
2. Add an additional power layer. This method can be used if the PCB power supply layer is designed to be odd and the signal layer is even. A simple method is to add a ground layer in the middle of the stack without changing other settings. First, layout the odd-numbered PCBs, and then duplicate the ground layer in the middle and mark the remaining layers. This is the same as the electrical properties of the foil of the thickened ground layer.

3. Add a blank signal layer near the center of the PCB stack. This approach minimizes stack imbalance and improves PCB quality. First, layout the odd-numbered PCBs, and then added a blank signal layer and mark the remaining layers. Used in microwave circuits and mixed media (mediums with different dielectric constants).