PCBs are divided into single-sided circuit boards, double-sided PCB boards and multi-layer circuit boards. Common multilayer PCBs are four-layer circuit boards, six-layer circuit boards and 8-layer circuit boards. The simple four-layer board adds a power layer and a ground layer on the basis of the Top Layer and Bottom Layer, which greatly solves the problem of electromagnetic interference, improves the reliability of the system, and can also improve the routing rate of wires and reduce the size of The area of the PCB circuit board.
The reason why the number of layers of a PCB multilayer board is even and not odd
1. The cost is low. Due to the lack of one layer of dielectric and foil, the cost of raw materials for odd-layer PCBs is slightly lower than that of even-layer PCBs, but the processing cost of odd-layer PCBs is significantly higher than that of even-layer PCBs. When the processing cost of the inner layer is the same, the foil/core structure of the odd-numbered layers significantly increases the processing cost of the outer layer.
Odd-layer PCBs require a non-standard lamination core layer bonding process on the basis of the core structure process. Factories 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 scratching and etching errors on the outer layer.
2. Do not use odd-numbered layers to design PCBs because odd-numbered layers are easy to bend. When the PCB is cooled after the multilayer circuit bonding process, the different lamination tensions of the core structure and the foil-clad structure as they cool 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 flex is a balanced stackup. Although a PCB with a certain degree of curvature meets the specification requirements, the subsequent processing efficiency will be reduced, resulting in increased costs. Because special equipment and processes are required for assembly, the accuracy of component placement is reduced, so quality will be compromised.
In short, 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% (the standard of IPC600), but the size of the three-layer board is larger. When the warpage exceeds this standard, this will affect the reliability of the SMT patch and the entire product. Therefore, the general designer does not design an odd-numbered layer board. Even if an odd-numbered layer is required to realize the function, it will be designed as a false even-numbered layer. The five layers are designed to be six layers, and the seven layers are designed to be eight layers. For the above reasons, most of the PCB multilayer boards are designed with even layers, and there are fewer odd layers.
