If wiring does not require extra layers, why use it? Does the reduced layer not make the board thinner? If the circuit board less, is not the cost of lower it? However, in some cases, adding a layer will reduce the cost.

PCB board has two different structures: core structure and foil structure.

In the core structure, all the conductive layer in the PCB board is deposited on the core material. In the foil structure, only the inner conductive layer of the PCB is deposited on the core material, and the foil layer is used as the outer conductive layer. All of the conductive layers are bonded together by a medium using a multi-layer laminating process.

Nuclear material is the double-sided foil in the factory. Because each core has two faces, fully utilized, the number of conductive layers on the PCB is even. Why not use foil and the rest of the nuclear structure? The main reason is: the cost of PCB board and PCB board bending.

The cost advantage of even-numbered PCB boards

Because less than a layer of media and foil, odd PCB board raw material costs slightly lower than the even layer PCB. But the processing cost of the odd-layer PCB board is significantly higher than the even-layer PCB board. The inner processing costs are the same; however, the foil / core structure significantly increases the handling cost of the outer layer.

The odd-numbered PCBs need to be added on the basis of the core structure process to a non-standard laminated core layer bonding process. Compared with the nuclear structure, the production efficiency of the plant added to the nuclear structure will be reduced. Prior to laminating bonding, the outer core requires an additional process, which increases the risk of scratches and etching errors in the outer layer.

Balance the structure to avoid bending
The odd reason for designing PCB boards without odd layers is that odd-numbered PCBs are easy to bend. When the PCB board in the multi-layer circuit bonding process after cooling, the nuclear structure and foil structure cooling when the different laminated tension will cause the PCB board bending. As the thickness of the circuit board increases, the risk of bending a composite PCB with two different structures is greater. The key to eliminating PCB bending is to use a balanced stack. Although a certain degree of bending of the PCB board to meet the regulatory requirements, but the follow-up efficiency will be reduced, resulting in increased costs. Because the assembly requires special equipment and processes, components, reduce the accuracy of placement, it will damage the quality.

Use even-numbered PCBs
When the design of the odd number of PCB board, the following methods can achieve a balanced stack, reduce the PCB board production costs, to avoid PCB board bending. The following methods are arranged according to the preferred level.

1, a layer of signal layer and use. This method can be used if the power plane of the PCB board is even and the signal layer is odd. The increased 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 plane of the PCB board is odd and the signal layer is even. A simple way is to add a formation in the middle of the stack without changing the other settings. First by odd number of PCB board wiring, and then copy the formation in the middle, marking the remaining layers. This is the same as the electrical properties of foil deposited in thickened formations.

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. First by odd number of wiring, and then add a layer of blank signal layer, marking the remaining layers. In the microwave circuit and mixed media (dielectric have different dielectric constant) circuit used.

Balanced stack PCB advantages: low cost, easy to bend, shorten the delivery time to ensure quality.