Inner layers are used inside multilayer PCBs. Later the inner layers are hardly recognizable from the outside of the finished PCB multi-layer.
Inner Layer Automated Optical Inspection (AOI)
The inner layers are given a complete machine inspection before lamination because afterwards mistakes cannot be corrected. The automatic optical inspection system scans the board and compares it with the digital image generated from the original design data.
Multi-layer printed circuit boards have trace layers inside the board. One way to make a 4-layer PCB is to use a two-sided copper-clad laminate, etch the circuitry on both sides, then laminate to the top and bottom prepreg and copper foil. Lamination is done by placing the stack of materials in a press and applying pressure and heat for a period of time. This results in an inseparable one piece product. It is then drilled, plated, and etched again to get traces on top and bottom layers. Finally the PCB is covered with solder mask, marking legend, and a surface finish may be applied. Multi-layer PCBs allow for much higher component density.