I’m curious about the prevalence of Foil Construction vs. Core Construction of pcb’s. #1, I didn’t know Foil Construction was a thing (the copper foil on the prepreg itself, basically a one layer which is then attached to the core with top and bottom copper), and I’m wondering how common this is?
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Foil construction is by far the preferred method as it eliminates a number of issues with Core/Cap construction and reduces manufacturing costs. Basically if you take a 6 layer foil construction stack-up layers 2&3 and layers 4&5 are printed and etched cores with the foil becoming layers 1&6. Both sides of the cores are processed at the same time and provide good layer to layer registration. With Core/Cap construction layer 2 and layer 5 are printed and etched on one side of the cores while the other side remains full copper so there are material stress issues. Layers 3&4 are a printed and etched core. While still necessary in some cases, especially for sub-assemblies, this method takes more time and makes layer to layer registration more difficult.