Designers may design odd-numbered-layer printed circuit boards (PCBs). If the routing doesn't require the extra layer, why use it? Wouldn't reducing the layers make the board thinner? Wouldn't the cost be lower if the circuit board had one less layer? However, in some cases, adding a tier can actually lower the cost.
Circuit boards come in two different constructions: core construction and foil clad construction.
In the core structure, all the conductive layers in the circuit board are deposited on the core material; while in the foil structure, only the internal conductive layer of the circuit board is deposited on the core material, and the outer conductive layer uses a foil-coated dielectric board. All conductive layers are bonded together through the dielectric using a multi-layer lamination process.
The nuclear material is double-sided foil-clad board in the factory. Since each core has two sides, the PCB has an even number of conductive layers when fully utilized. Why not use foil on one side and core construction on the rest? The main reasons are: the cost of the PCB and the curvature of the PCB.
Cost advantage of even-numbered layer boards
Because there is one less 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-numbered layer PCBs is significantly higher than that of even-numbered layer PCBs. The processing cost of the inner layer is the same; however, the foil/core structure significantly increases the processing cost of the outer layer.
Odd-numbered layer PCBs need to add a non-standard stacked core layer bonding process on the basis of the core structure process. Plants that add foil outside the core structure will lose productivity compared to the core structure. The outer core requires additional processing prior to lamination bonding, which increases the risk of scratches and etching errors on the outer layers.
Balanced structure avoids bending
The best reason not to design a PCB with an odd number of layers is that odd layer boards tend to bend. When the PCB is cooled after the multilayer circuit bonding process, different lamination tensions when the core structure and the foil-clad structure are cooled can cause the PCB to bend. As the thickness of the board increases, the risk of bending a composite PCB with two different structures increases. The key to eliminating board bow is a balanced stackup. Although the 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 processes are required during assembly, the accuracy of component placement is reduced, and quality will be compromised.
Use an even layer PCB
When there are odd-numbered PCB layers 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 listed in order of preference.
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. Additional layers do not increase cost, but can reduce lead time and improve PCB quality.
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 route according to the odd-numbered PCB, then copy the ground layer in the middle, and mark the remaining layers. This is the same as the electrical characteristics of the thickened formation foil.
Add a blank signal layer near the center of the PCB stackup. This approach minimizes stack-up imbalance and improves PCB quality. Route the odd-numbered layers first, then add a blank signal layer, and mark the remaining layers. It is used in microwave circuits and mixed media (dielectrics have different dielectric constants) circuits.
Advantages of balanced laminated PCB: low cost, not easy to bend, shorten delivery time, and ensure quality.