Designers may design printed circuit boards (PCBs) with an odd number of layers. If routing doesn't require an extra layer, why use it at all? Wouldn't reducing layers make the board thinner? If the circuit board had one less layer, wouldn't the cost be lower? However, in some cases, adding an extra layer can actually reduce costs.

Circuit board structure

Circuit boards have two different structures: core structure and foil structure.
 
In the core structure, all the conductive layers in the circuit board are coated on the core material; in the foil structure, only the inner conductive layer of the circuit board is coated on the core material, and the outer conductive layer is foil-clad dielectric board. All conductive layers are bonded together through a dielectric using a multi-layer lamination process.  
The nuclear material is the double-sided foil-coated board in the factory. Because each core has two sides, when fully utilized, the number of conductive layers of the PCB is an even number. Why not use foil on one side and core structure on the rest? The main reasons are: the cost of PCB and the curvature of PCB.  
       

Cost advantage of even-layer circuit boards

Because there is one less layer of dielectric and foil, the cost of raw materials for odd-numbered PCB boards is slightly lower than that of even-numbered PCBs. However, the processing cost of odd-numbered layer PCB is significantly higher than that of even-numbered layer PCB. The processing cost of the inner layer is the same; but the foil/core structure significantly increases the processing cost of the outer layer.  
Odd-layer PCB requires a non-standard laminated core layer bonding process based on the core structure process. Plants that add foil outside the core structure will experience reduced productivity compared to the core structure. The outer core requires additional processing before lamination can be bonded, which increases the risk of scratches and etching errors on the outer layer.  
 
       

Balanced structure avoids bending

The best reason not to design PCBs with odd layers is: odd-layer circuit boards tend to bend. When the PCB cools after the multilayer circuit bonding process, the different lamination tensions of the core and foil structures can cause the PCB to warp. As the board thickness increases, the risk of bending of a composite PCB with two different structures increases. The key to eliminating board flex is to use a balanced stackup.  
Although PCBs that are bent to a certain extent meet specification requirements, subsequent processing efficiency will be reduced, resulting in increased costs. Because assembly requires special equipment and processes, the accuracy of component placement is reduced, thus compromising quality.  
       

Use an even-numbered layer PCB

When odd-numbered PCB layers appear in the design, the following methods can be used to achieve balanced stacking, reduce PCB production costs, and avoid PCB bending. The following methods are arranged in order of preference.  
A signal layer and exploit it. This method can be used if the PCB is designed with an even number of power layers and an odd number of signal layers. The added layers do not increase costs, but can shorten delivery times and improve PCB quality.  
Add an additional power plane. This method can be used if the PCB is designed with an odd number of power layers and an even number of signal layers. An easy way to do this is to add a ground layer in the middle of the stack without changing other settings. First lay out the odd-numbered layers in the PCB, then copy the ground layers in the middle and mark the remaining layers. This is the same electrical characteristic as a thickened foil.  
Add a blank signal layer near the center of the PCB stackup. This approach minimizes stackup imbalance and improves PCB quality. Route the odd layers first, then add a blank signal layer and label the remaining layers. Used in microwave circuits and mixed media (mediums with different dielectric constants) circuits.  
       

Balanced Stacked PCB Advantages

Low cost, not easy to bend, shorten delivery time and ensure quality.