If wiring doesn’t require an extra layer, why use it? Wouldn’t reducing layers make a circuit board thinner? If the circuit board is less than a layer, is not the cost lower? However, in some cases, adding a layer may actually lower the cost.
PCB board has two different structures: core structure and foil structure.
In the core structure, all conductive layers in the PCB board are applied on the core material. However, in foil coating structure, only the internal conductive layer of PCB board is applied on core material, and the external conductive layer is applied on foil dielectric board. All conductive layers are bonded together through a medium using a multilayer compression process.
Nuclear materials are the double-sided foils used in factories. Because each core has two sides, when fully utilized, the number of conductive layers of PCB board is even. Why not use foil on one side and nuclear structure on the other? The main reasons are: PCB board cost and PCB board bending.
Cost advantage of even layer PCB board
Due to the lack of a layer of medium and foil, the raw material cost of odd PCB board is slightly lower than that of even PCB. However, the processing cost of odd layer PCB board is significantly higher than that of even layer PCB board. The processing cost of the inner layer is the same; However, foil/core structure significantly increases the processing cost of the outer layer.
The odd layer PCB board needs to add the non-standard laminated layer bonding process on the basis of the nuclear structure process. Compared with the nuclear structure, the production efficiency of the plant with foil coating outside the nuclear structure will decrease. Before laminating, the outer core requires additional processing, which increases the risk of scratches and etching errors on the outer layer.
Balance structure to avoid bending
The best reason to design PCB boards without odd layers is that odd layers are easy to bend. When PCB board is cooled after multilayer circuit bonding process, the different lamination tension between the core structure and foil structure will cause the bending of PCB board. As the thickness of PCB increases, the risk of bending composite PCB with two different structures will increase. The key to eliminating PCB bending is to use balanced lamination. Although PCB boards bent to a certain extent meet the requirements of the specification, the subsequent processing efficiency will be reduced, resulting in increased costs. Because special equipment and technology are needed for assembly, the accuracy of component placement is reduced, which will damage the quality.
When an odd layer PCB board appears in the design, the following methods can be used to balance the lamination, reduce the production cost of the PCB board and avoid the bending of the PCB board. The following methods are arranged by priority level.
1, a layer of signal layer and use. This method can be used if the power layer of PCB board 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 quality of PCB board.
2. Add an additional power layer. This method can be used if the power layer of PCB board 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. Firstly, the odd layer PCB board is wired, and then the strata are duplicated in the middle to mark the remaining layers. This is the same as the electrical properties of foils applied to thickened layers.
3. Add a blank signal layer near the center of the PCB stack. This method minimizes the stacked unbalance and improves the quality of PCB boards. First, the odd layer wiring, and then add a blank signal layer, marking the rest of the layer. Used in microwave circuits and mixed media (media with different dielectric constants) circuits.