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Out of several layers that make up a PCB, one of them is a layer of copper foil (see image below), What purpose does this layer serve to the PCB?

enter image description here

Also, some PCBs have just one copper foil layer on either sides of the substrate and some other PCBs have two layers on on each sides of the substrate. What difference does having a layer on either sides or on both sides of the substrate make?

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    \$\begingroup\$ It's the layer that the traces are made out of... The "non-trace" areas are etched off the copper layer to leave just the traces/pads. The substrate is just a structural component (mostly). \$\endgroup\$ – Ron Beyer Jan 21 at 18:25
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    \$\begingroup\$ It's the traces, standard size is 0.127mm and a pcb is 1.8mm. So yeah, traces are thin. \$\endgroup\$ – sidA30 Jan 21 at 18:28
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    \$\begingroup\$ "What difference does having a layer on either sides or on both sides of the substrate make?" Basically so you can cross connections without shorting them. I think you would benefit by looking at a real PCB instead of a diagram. \$\endgroup\$ – Oldfart Jan 21 at 18:43
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    \$\begingroup\$ These days, many PC boards have two or more copper layers inside the board, as well as the two on the surfaces. The copper layers provide the "wires" that connect the components. \$\endgroup\$ – Peter Bennett Jan 21 at 18:51
  • \$\begingroup\$ This video will make it clearer m.youtube.com/watch?v=Z2LgmIGE2nI&feature=youtu.be \$\endgroup\$ – Martin Jan 21 at 19:25
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The copper is etched into conductive paths between components to connect them electrically. Components are soldered from their terminals to the copper, to make firm electrical connection, and also to keep components mechanically attached to the PCB. The more copper layers there are, the more complex wiring is possible.

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Sometimes it helps to have a little history of the name. PCB => Printed Circuit Board. A few years back they used to be called PWB => Printed Wiring Board.

This last acronym, PWB, helps a lot with the purpose of the copper. It is a method of placing wires between components. The fiberglass is to assist in mechanical stability and the solder mask keeps solder from sticking to other areas of copper that don't have components connecting to them.

Having the copper foil on top and on bottom allows the traces (wires) to crisscross around the board without shorting (unintentional connections), and thus allows for more complicated circuits to be created.

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