Separations in Inner Layers
Let us first understand basic multilayer PCB construction. There are two basic elements: (a) copper foil of varying thicknesses, and (b) FR-4 (“fire-resistant 4” category, as per IEC). The FR-4 may be in the form of prepreg (preimpregnated) material, or in the form of a core. The core material is cured fiberglass-epoxy resin (i.e., hardened to start with). Prepreg however consists of thin sheets of fiberglass impregnated with uncured epoxy resin. Several layers of prepreg may be combined to achieve a desired final prepreg thickness. Finally, the entire assembly is pressed together, and under heat, the uncured epoxy also gets cured (it hardens).
We can have many variations of thicknesses, and finally, the distance ...
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