When circuits are too complex for single layer wiring or copper foil is needed for grounding shielding, double-layer or even multilayer boards are needed. The most typical difference between a laminate and a monolayer is the addition of a through-hole structure to connect the layers of copper foil. General substrate + transparent adhesive + copper foil the first processing process is the production of holes. First in the substrate and copper foil drilling, cleaning and plating a certain thickness of copper, and then through the hole is finished. And the next process is almost identical to that of a single board.
HDI Green Soldermask flex-rigid Board
2layer Cem-3 Stiffener Black Flexible Printed Circuit Board | YMSPCB