What is a multilayer PCB?

A multilayer PCB (Printed Circuit Board) is a type of circuit board that has multiple layers of conductive material and insulating material sandwiched together. 

In a multilayer PCB, the layers of conductive material are typically made of copper and are separated by layers of insulating material, such as fiberglass, ceramic, or resin. The conductive layers are connected to each other through plated through-holes, which are small holes drilled through the board and coated with a thin layer of conductive material.

Multilayer PCBs are commonly used in electronic devices where space is at a premium, and there is a need for complex and dense circuitry. They are used in applications such as smartphones, computers, servers, medical devices, and aerospace equipment. 

The number of layers in a multilayer PCB can vary, depending on the complexity of the circuitry and the space available. Multilayer PCBs can have anywhere from four to dozens of layers, with 6-10 layers being most common.