Cut through a SDRAM-module, a multi-layer PCB. Note the via, visible as a bright copper-colored band running between the top and bottom layers of the board.
Multi-layer printed circuit boards have trace layers inside the board. This is achieved by laminating a stack of materials in a press by applying pressure and heat for a period of time. This results in an inseparable one piece product. For example, a four-layer PCB can be fabricated by starting from a two-sided copper-clad laminate, etch the circuitry on both sides, then laminate to the top and bottom pre-preg and copper foil. It is then drilled, plated, and etched again to get traces on top and bottom layers.
The inner layers are given a complete machine inspection before lamination because afterwards mistakes cannot be corrected. The automatic optical inspection system compares an image of the board with the digital image generated from the original design data.