A multilayer board is a type of printed circuit board with three or more layers of printed circuits on an insulated substrate. It is a combination of several thin single-sided or double-sided boards, with a thickness typically ranging from 0.4 to 6.0mm. To connect the circuits sandwiched between the insulating substrates, the holes where components are mounted on the multilayer board need to be metallized, meaning a metal layer is applied to the inner surface of the small holes to connect them to the printed circuits between the insulating substrates.