3D Printed Circuit Board
- Method 1 - 3D print serves as resist on copper clad board, which is then etched in standard fashion.
- Method 2 - Solderless version uses conductive paint. 3D print conduits for conductive paint, put components into small holes on other side, bending leads for good contact. Resistivity of the commercial conductive paint is 1 ohm per inch - workable for logic circuits but not power circuits above 1A. Instructable - . Compare to resistance of copper, perhaps 1000x less? 
- Method 3 - 3D printer just uses a scribe, where a black paint mask has been applied to a circuit board. Doesn't sound like a scribe would cut through the copper clad? Doubt that this works. 
- Method 4 - Conductive Composite FDM Filament
- Melting of low temperature metals offers good conductivity for large power circuits.
- Paste extruders can yield satisfactory results for logic circuits, but not for power circuits.
- Assuming logic is up to 50mA signals - 12mOhm/square (Voltera PCB Printer) is fine.
- Take the case of a 1 mm lead 10 cm long - carrying 50 mA. This would be a typical logic case. 12 mOhm/square gets us 1 Ohm resistance over 10 cm. For 50mA, that is a voltage drop V=IR of 0.05*1= 0.05V - which is fine in 5 volt systems. See Ohms Per Square.