PCB Design is used to mechanically support and electrically connect electronic components using conductive pathways or traces, etched from copper sheets laminated onto a non-conductive substrate. PCBs have between one and twenty conductive layers laminated together in a sandwich with insulating plastic. Layers may be connected together through drilled holes called vias. Either the holes are electroplated or small rivets are inserted. High-density PCBs have blind vias, which are visible only on one surface, or buried vias, which are visible on neither. Some designs also have quality control patterns to measure soldering and etching processes.