How printed circuit boards are made

A circuit board is glass cloth soaked in epoxy and clad in copper, with wiring left behind after acid eats the rest.

stations: 8
things upstream: 48
raw sources: 18
steps from the ground: 8

The short version

A printed circuit board is a sandwich of glass-epoxy sheets and copper foil. Light exposes a pattern on a resist, and acid eats away the unprotected copper to leave traces. Layers are pressed together, holes are drilled and plated, solder paste is printed, parts are placed by machine and a reflow oven melts the solder.

Bond copper to glass-epoxy

Copper foil is laminated onto sheets of FR-4, a woven glass cloth soaked in epoxy. This is the most common base material for boards.

Goes in
Copper Fiberglass Epoxy
Expose the wiring pattern

Photoresist is coated on the copper and exposed to light projected in the pattern of the artwork. Resist that remains protects the copper that will become wires.

Goes in
Photoresist
Etch away the extra copper

The etching solution dissolves every unprotected part of the copper layer. The board is then cleaned, leaving traces, pads and solid planes.
Press the layers together

Multilayer boards stack several etched cores with insulating layers between them, so inner traces do not take up surface space.

Goes in
Epoxy
Drill and plate the vias

Holes are drilled through the stack and lined with metal, linking one copper layer to another. Since 1995, microvias have let makers build denser boards.

Goes in
Copper
Cover with solder mask

A protective coating goes over the traces except the pads, which reduces the chance of solder shorts.
Print solder paste

Solder paste, a sticky mix of powdered solder and flux, is printed onto the pads.

Goes in
Solder
Place parts and reflow

Machines drop components onto the paste, then the board rides through a reflow oven. Each zone is set to a temperature profile until the paste melts into permanent joints, for anywhere from one to thousands of parts.

End of the linePrinted circuit board Next it goes into hearing aid, pacemaker, digital thermometer and 38 more.

Worth repeating

Albert Hanson described flat foil conductors laminated to an insulating board in layers in 1903, long before anything we would call electronics was cheap.
A 4-layer board with microvias is considered equal in quality to an 8-layer through-hole board.
Thomas Edison experimented with plating conductors onto linen paper in 1904.