Why are PCB multilayer boards mostly even-numbered layers? There are few odd-numbered layers. There are single-sided, double-sided and multi-layer PCB boards for these reasons. The number of multi-layer boards is not limited. There are already more than 100 layers of PCBs, while the common multi-layer PCBs are four. Layer and six-layer boards. Then why do people have the question "Why are PCB multilayer boards all even-numbered layers?"? Relatively speaking, even-numbered PCBs do have more than odd-numbered PCBs, and they have more advantages.

Because of the lack of a layer of dielectric and foil, the cost of raw materials for odd-numbered PCBs is slightly lower than that of even-numbered PCBs. However, 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, but the foil/core structure obviously increases the processing cost of the outer layer.

The odd-numbered PCB needs to add a non-standard laminated core layer bonding process on the basis of the core structure process. Compared with the nuclear structure, the production efficiency of factories that add foil to the nuclear structure will decrease. Before lamination and bonding, the outer core requires additional processing, which increases the risk of scratches and etch errors on the outer layer.

Balanced structure to avoid bending The best reason to design PCB without odd-numbered layers is that the odd-numbered layer circuit boards are easy to bend. When the PCB is cooled after the multi-layer circuit bonding process, the different lamination tensions of the core structure and the foil-clad structure will cause the PCB to bend. As the thickness of the circuit board increases, the risk of bending of a composite PCB with two different structures becomes greater. The key to eliminating circuit board bending is to use a balanced stack. Although the PCB with a certain degree of bending meets the specification requirements, the subsequent processing efficiency will be reduced, resulting in an increase in cost. Because special equipment and craftsmanship are required during assembly, the accuracy of component placement is reduced, which will damage the quality.

To put it more easily, 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 size of the three-layer board is large, the warpage will exceed this standard, which 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 the odd-numbered layer realizes the function, it will It is designed as a fake even-numbered layer, that is, 5 layers are designed into 6 layers, and 7 layers are designed into 8-layer boards.

Based on the above reasons, most PCB multi-layer boards are designed with even-numbered layers and fewer odd-numbered layers.