Relatively speaking, even-numbered layer PCBs have more advantages than odd-numbered layer PCBs.
Due to the lack of a layer of dielectric and foil, the raw material cost of 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, but the foil/core structure increases the processing cost of the outer layer significantly.
1. The odd-numbered layer PCB needs to add a non-standard laminated core layer bonding process on the basis of the core structure process. Factories that add foil outside the core structure will experience reduced 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.
2. The balance structure avoids bending. The best reason to design a PCB without odd layers is that odd layers of the board tend to bend. After the multi-layer circuit bonding process is completed, when the PCB cools, different lamination tensions can cause the PCB to bend when the core structure and the foil structure cool. Composite PCBs with two different structures have a greater risk of bending as the board thickness increases. The key to eliminating board bow is to employ balanced lamination. Although the curved PCB meets the specification requirements to a certain extent, the subsequent processing efficiency will be reduced, resulting in increased costs. Since special equipment and processes are required in the assembly process, the placement accuracy of parts will be reduced, thereby affecting the quality.
In other words, it is easier to understand: in the PCB process, a four-layer board is easier to control than a 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 size of the three-layer board is larger, the warpage will exceed this standard, which will affect SMT placement and The reliability of the entire product, so the chief designer does not design odd-numbered laminates. Even if the odd-numbered layer realizes the function, it will be designed as a false even-numbered layer, that is, 5 layers are designed as 6 layers, and 7 layers are designed as 8 layers.
Even layer PCB
Based on the above reasons, most PCB multilayer boards are designed with even layers and fewer odd layers.
