PCB design engineers may design odd-numbered printed circuit boards (PCBs). If wiring doesn’t need an extra layer, why use it? Won’t reducing layers make the circuit board thinner? If the circuit board is one less layer, isn’t the cost lower? However, in some cases, adding one floor will 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 are coated on the core material; In the foil-coated structure, only the inner conductive layer of the circuit board is coated on the core material, and the outer conductive layer is a foil-coated dielectric plate. All the conductive layers are bonded together by a multilayer lamination process through a medium.
The material is the double-sided foil in the factory. Because each core has two sides, when fully utilized, the number of conductive layers of PCB is even. Why not use foil on one side and nuclear structure on the rest? The main reasons are: the cost of PCB and the curvature of PCB.
Cost advantage of even layer circuit board
Because there is one layer of dielectric and foil less, the raw material cost of odd PCB is slightly lower than that of even PCB. But the processing cost of odd-numbered PCB is obviously higher than that of even-numbered PCB. The processing cost of the inner layer is the same; However, the foil/core structure obviously increases the processing cost of the outer layer.
Odd layer PCB needs to add non-standard laminated core layer bonding process on the basis of core structure process. Compared with the nuclear structure, the factory production efficiency of adding foil outside the nuclear structure will decrease. Before laminating and bonding, the outer core needs additional processing, which increases the risk of scratching and etching errors of the outer layer.
Balance structure to avoid bending
The best reason to design PCB without odd-numbered layers is that odd-numbered layers are easy to bend. When the PCB is cooled after the multilayer circuit bonding process, the different lamination tension between the core structure and the foil structure will cause the PCB to buckle. With the increase of circuit board thickness, the risk of bending of composite PCB with two different structures is greater. The key to eliminate circuit board bending is to adopt balanced lamination. Although the PCB bent to a certain extent meets the requirements of the specification, the subsequent processing efficiency will decrease, resulting in an increase in cost. Because special equipment and technology are needed during assembly, the accuracy of component placement will be reduced, which will damage the quality.
Use even-numbered PCB
When odd layers of PCB appear in PCB design, the following methods can be used to achieve balanced lamination, reduce PCB manufacturing cost 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 PCB is even and the signal layer is odd. The additional layer does not increase the cost, but it can shorten the delivery time and improve the quality of PCB.
2. Add an additional power layer. This method can be used if the power layer of PCB is designed to be odd and the signal layer is even. A simple method is to add a stratum in the middle of the stack without changing other settings. First, wire according to the PCB type of odd layers, then copy the stratum in the middle and mark the remaining layers. This is the same as the electrical characteristics of the foil for thickening the stratum.
3. Add a blank signal layer near the center of PCB stack. This method minimizes the unbalance of lamination and improves the quality of PCB. First, lay the wires in odd numbers, then add a blank signal layer, and mark the remaining layers. It is used in microwave circuit and mixed dielectric (dielectric with different dielectric constants) circuit.
Advantages of laminated PCB: low cost, not easy to bend, shortened delivery time and guaranteed quality.
