PCB board has one layer, two layers and multiple layers, among which there is no limit on the number of layers of multilayer board. Currently, there are more than 100 layers of PCB, and the common multilayer PCB is four layers and six layers. So why do people say, “why are PCB multilayers mostly even?” The question? Even layers do have more advantages than odd layers.
1. Low cost
Because of one layer of media and foil, the cost of raw materials for odd-numbered PCB boards is slightly lower than that for even-numbered PCB boards. However, processing cost of odd-layer PCB is significantly higher than that of even-layer PCB.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 needs to add non-standard laminated core bonding process on the basis of nuclear structure process.Compared with the nuclear structure, the production efficiency of the plant with foil coating outside the nuclear structure will decrease.The outer core requires additional processing prior to laminating, which increases the risk of scratches and etch errors on the outer layer.
2. Balance structure to avoid bending
The best reason to design PCBS without odd-numbered layers is that odd-numbered layers are easy to bend.When PCB is cooled after multi-layer circuit bonding process, different laminating tension between core structure and foil-coated structure will cause PCB bending.As the thickness of the board increases, the risk of bending a composite PCB with two different structures increases.The key to eliminating circuit board bending is to use balanced layering.Although a certain degree of bending PCB meets the specification requirements, the subsequent processing efficiency will be reduced, resulting in an increase in cost.Because the assembly requires special equipment and process, components placement accuracy is reduced, so it will damage the quality.
Change more easy to understand: in the process of PCB technology, four layer board is better than three layer board control, mainly in terms of symmetry, the warp degree of four layer board can be controlled under 0.7% (IPC600 standard), but the three layer board size, warp degrees will exceed the standard, this will affect the SMT and the reliability of the entire product, so the general designer, is not an odd number of layer board design, even if is an odd layer functions, will be designed to fake an even layer, the 5 designs 6 layers, layer 7 8 layer board.
For the above reasons, most PCB multilayers are designed as even layers, and odd layers are less.