The layer count of PCB (Printed Circuit Board) determines its manufacturing technology difficulty and PCB manufacturing cost. PCB boards can be briefly classified into two categories: single-layer (single-sided) PCB and double-layer (double-sided) PCB. When it comes to high-end electronic products, a couple of signal layers can be added to the inside of PCB apart from surface routing due to some limitations in terms of board real estate and board design. During the manufacturing procedure, after routing for each layer has been finished with positioning and lamination completed, multiple layers of signals will be pressed into a single board that is called multilayer PCB. Therefore, a multilayer PCB refers to any piece of circuit board containing more than two layers of signals. Multilayer PCBs can be multilayer rigid PCB, multilayer flex PCB and flex-rigid multilayer PCB.