Two thoughts on this: 1. Have you seen CNC PCB milling machines? A lot of similarities: but they are subtractive, rather than additive. 2. Conductive Epoxy is starting to make inroads into commercial PCB technology instead of solder. This might be a better medium to print with. To be frank, you've got no chance of 3d printing solder and getting it to stay put: solder is sociable: i.e. it prefby MaherGBR - Let's design something! (I've got an idea ...)