People get an impression that the PCBs are always in even number layers, so why there isn't any odd number layer PCB? Except single layer of course.
To answer this question, we will need to ask ourselves:
What are the "layers" mean in PCB?
Layers doesn't mean how many cores or PP used, it only reflects to the sheets number of copper clad.
So when we use are core inside and we attached its two sides with copper clad, it is a double layer PCB. And if we use two core and attached both them two sides with copper clads, it will be a four layers PCB.
Why electronic engineers do not design one copper clad in the core?
in terms of multilayers PCB manufacturing process, the pressing procedure is a must, to press a three copper clads layer PCB or four copper clads layer PCB doesn't have any difference for PCB manufacturers
The difference for a 3 layers PCB and a 4 layers PCB is a copper clad and adhensive sheet, there is no big difference in cost, so PCB manufacturer normally quote 3/4 layers PCB in the same price level, and so do 5/6 layers and 7/8 layers.
in the real PCB manufacturing process, a even number PCB is easier to control then odd layers PCB, especially the twist parameter.
So that's why we rarely see odd number layers PCB!
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