Designers may design odd-layer printed circuit boards (PCBs). If the wiring doesn’t need extra layers, why use it? Wouldn’t fewer layers make the board thinner? Wouldn’t it cost less if the board had one less layer? However, in some cases, adding a layer will instead reduce the cost.
There are two different structures of circuit boards: core structure and foil structure.
In the core structure, all the conductive layers in the circuit board on the core material; and in the foil structure, only the internal conductive layers of the board on the core material, the outer conductive layer with the foil dielectric board. All the conductive layers are bonded together using a multilayer lamination process through the dielectric.
The core material is the double-sided foil board in the factory. Because each core has two sides, when fully utilized, the PCB has an even number of conductive layers. Why not use the foil on one side and the rest of the core structure? The main reason is: the cost of PCB and PCB curvature.
The cost advantage of even-layer circuit boards
Because there is one less layer of media and foil, the cost of raw materials for odd-layer PCBs is slightly lower than for even-layer PCBs, but 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; however, the foil/core structure significantly increases the processing cost of the outer layer.
Odd-layer PCBs require a non-standard laminated core layer bonding process on top of the core structure process. Compared to the core structure, the factory productivity will be reduced by adding the foil outside the core structure. Before lamination bonding, the outside core requires additional process processing, which increases the risk of scratches and etching errors on the outer layer.
Balanced structure to avoid bending
The best reason for not using odd-layer design PCBs is that odd-layer boards tend to bend. When the PCB is cooled after the multilayer circuit bonding process, the different lamination tensions between the core structure and the foil structure as they cool can cause the PCB to bend. As the board thickness increases, the greater the risk of bending of composite PCBs with two different structures. The key to eliminating circuit board bending is to use a balanced lamination. Although a certain degree of bending of the PCB to meet the specification requirements, but the subsequent processing efficiency will be reduced, resulting in increased costs. Because special equipment and processes are required for assembly, component placement accuracy is reduced, so quality will be compromised.
Use of even-layer PCBs
When an odd-layer PCB is present in the design, the following methods can be used to achieve balanced lamination, reduce PCB production costs, and avoid PCB bending. The following methods are arranged in order of preference level.
1. A layer of signal layers and use. If the design of the PCB power supply layer for an even number of signal layers can be used for this method. The additional layers do not increase the cost, but can shorten the delivery time and improve the quality of the PCB.
2. Add an additional power supply layer. If the design of the PCB power supply layer for an odd number of signal layers can be used for this method. A simple method is to add a ground layer in the middle of the stack without changing the other settings. First by the odd number of layers PCB kind of wiring, and then copy the ground layer in the middle, marking the remaining layers. This is the same as the electrical characteristics of the layup foil with a thicker ground layer.
3. Add a blank signal layer near the center of the PCB stack. This method minimizes lamination imbalance and improves the quality of the PCB. Wiring by odd number of layers first, then add a blank signal layer to mark the remaining layers. Used in microwave circuits and mixed dielectric (dielectric with different dielectric constants) circuits.
Balanced lamination PCB advantages: low cost, less bending, shorten delivery time, quality assurance.