Why are PCB multilayer boards all even-numbered layers?RenhotecIC feel that even-numbered PCBs do have more than odd-numbered PCBs, which is more advantageous.

　　PCB board has one side, two sides and multi-layer, in which the number of layers of multilayer board is unlimited, there are more than 100 layers PCB, and the common multi-layer PCB and six layers. And why do we have "PCB multilayer why are all even ?" What about this question? Relatively speaking, even layer PCB is indeed more than odd layer PCB, also has the advantage.


　　1.Relatively low cost

　　The cost of raw materials for odd-numbered PCB plates is slightly lower than that for even-numbered PCB. because of the absence of one layer of medium and foil The processing cost PCB odd layer is much higher than that PCB. even layer The processing cost of the inner layer is the same, but the foil / core structure obviously increases the processing cost of the outer layer.

　　Odd layer PCB need to add non-standard laminated core layer bonding process based on nuclear structure process. Compared with the nuclear structure, the factory production efficiency of adding foil to the nuclear structure will decrease. Before lamination bonding, the outer core needs additional process treatment, which increases the risk of the outer layer being scratched and etched incorrectly.


　　2.Balance structure avoid bending

　　Best reason to design PCB without odd layer is: odd layer circuit board is easy to bend. When the PCB is cooled after the multilayer circuit bonding process, the different lamination tension of the core structure and the foil coating structure will cause the PCB bending when they are cooled. As the thickness of the circuit board increases, the greater the risk of composite PCB bending with two different structures. The key to eliminating circuit board bending is to adopt balanced stacking. Although some degree of bending PCB meet the specification requirements, the efficiency of subsequent processing will be reduced, resulting in increased costs. Because special equipment and process are needed in assembly, the accuracy of component placement is reduced, so the quality will be damaged.

　　 What is easier to understand is that in the PCB process, the four-layer board is better controlled than the three-layer board, mainly in terms of symmetry. The warpage of the four-layer board can be controlled below 0.7%(IPC600 standard), but when the three-layer board is large, the warpage will exceed this standard, which will affect the reliability of the SMT patch and the whole product.

　　 Based on the above reasons, most PCB multilayer boards are designed as even layers, and odd layers are less.


　　3.How do odd layer PCB balance stacking and reduce cost?

　　If there is an odd layer PCB in the design, what should be done? Use the following methods can achieve balanced stacking, reduce the cost of PCB fabrication, and avoid PCB bending.

　　​1）A layer of signal layer and utilization. If the power layer of the design PCB is even and the signal layer is odd, this method can be used. Additional layers do not increase costs, but can shorten delivery time and improve PCB quality.?

　　2）Add an additional power layer. If the power layer of the design PCB is odd and the signal layer is even, this method can be used. A simple method is to add a layer in the middle of the stack without changing other settings. First according to the odd layer PCB kinds of wiring, then in the middle copy of the formation, marking the remaining layer. This is the same as the electrical characteristics of the thickened formation.

　　3）Add a blank signal layer in the center of the close PCB stack. this method minimizes stacking imbalance and improves PCB quality. Wiring by odd layer, then adding a blank signal layer to mark the rest. In microwave circuit and mixed medium (medium has different dielectric constant) circuit.


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