What is Circuit Design

3.5. Lamination

To create a 4-layer, 6-layer, or 32-layer PCB, fabricators separate cores with a material referred to as prepreg.  Prepreg is short for pre-impregnated, and it refers to a dielectric material that is saturated with a thermally-activated epoxy.

When the cores and prepregs are placed in a hot-oil press for several hours, the epoxy permanently bonds the cores together.

This hot oil press permanently bonds the various layers of a PCB together.

If your board has blind or buried vias, this will increase the number of press operations required to fabricate your PCB.  Each additional press operation adds additional time and increases the risk of something going wrong during fabrication.  So to keep the cost of your PCB down, try to limit the number of press operations required to make your PCB.