Why is the Number of PCB Layer Mostly Even?

Most of the PCB layers are even layers, such as common 4 layer boards and 6 layer boards. Why is it so? In fact, even PCBs do have more than odd layer PCBs, and they have more advantages.

1. Lower cost

     Because of the lack of a layer of dielectric and foil, the raw material cost of odd layer PCBs is slightly lower than that of even layer PCBs. However, the processing cost of odd layer PCBs is significantly higher than that of even layer PCBs. The processing cost of the inner layer is the same, but the foil/core structure obviously increases the processing cost of the outer layer.

2. Balance structure to avoid bending

 Odd layer PCB is easy to bent. When the PCB is cooled after the multilayer circuit bonding process, different lamination tensions can cause the PCB to bend. As the thickness of the PCB increases, the risk of bending of a composite PCB with two different structures increases. The key to eliminating PCB bending is to use balanced stack. Although a PCB with a certain degree of bending can meet the specification requirements, the subsequent PCBA processing efficiency will be reduced, resulting in increased costs. Because special equipment and processes are required during assembly, if the accuracy of component placement is reduced, the quality will be damaged.

How to balance stacking and reduce costs for odd layer PCBs? When an odd layer PCB appears in the design, the following methods can be used to balance the stacking, reduce the cost of PCB production, and avoid PCB bending

1. When the power layer of the design PCB is even and the signal layer is an odd number, a signal layer can be added. This method can make the added layer not increase the cost, can also shorten the delivery time and improve the quality of the PCB.

2. When the power layer of the design PCB is odd and the signal layer is even number, an additional power layer can be added. It is also possible to add a ground layer in the middle of the stack without changing other settings, first follow the odd layer PCB wiring, then copy the ground layer in the middle, and then mark the remaining layers. This method is the same as the electrical characteristics of the thickened ground layer.

3. Add a blank signal layer near the center of the PCB stack. This method can minimize the imbalance of the stack and improve the quality of the PCB.

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