PCB Tech - Why are circuit boards always even-numbered layers

If the wiring does not require an extra layer, why use it? Wouldn't reducing the layers make the circuit board thinner? If there is one less circuit board, wouldn't it be cheaper? However, in some cases, adding a layer will reduce the cost.

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

The odd-numbered PCB needs to add a non-standard laminated core layer bonding process on the basis of the core structure process. Compared with the nuclear structure, the production efficiency of the factory that adds foil to the nuclear structure is reduced. Before lamination and bonding, the outer core requires additional processing, which increases the risk of scratches and etching errors on the outer layer.

The best reason not to design a PCB with an odd number of layers is that an odd number of layer circuit boards are easy to bend. When the PCB is cooled after the multilayer circuit bonding process, the different lamination tensions of the core structure and the foil-clad structure will cause the PCB to bend. As the thickness of the circuit board increases, the risk of bending of the composite PCB with two different structures of the PCBA OEM foundry is greater. The key to eliminating circuit board bending is to use a balanced stack. Although a PCB with a certain degree of bending meets the specification requirements, the subsequent processing efficiency will be reduced, resulting in increased costs. Because special equipment and craftsmanship are required during assembly, the accuracy of component placement is reduced, which will damage the quality.

When an odd-numbered PCB appears in the design, the following methods can be used to achieve balanced stacking, reduce PCB manufacturing costs, and avoid PCB bending. The following methods are arranged in order of preference.

1. A signal layer and use it. This method can be used if the power layer of the design PCB is even 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 power layer of the design PCB is odd and the signal layer is even. A simple method is to add a layer in the middle of the stack without changing other settings. First, follow the odd-numbered PCB layout, and then copy the ground layer in the middle to mark the remaining layers. This is the same as the electrical characteristics of a thickened ground layer.

3. Add a blank signal layer near the center of the PCB stack. This method minimizes stacking imbalance and improves the quality of the PCB. Wire the odd-numbered layers first, then add a blank signal layer, and mark the remaining layers. Used in microwave circuits and mixed media (dielectric constants of different media) circuits.