About the automated printing machines:
They are all a little different, but the alignment part of the process is basically the same.
I don't know if you have ever seen an automatic printer, so I'll do my best to explain it.
The PCB rides into the machine on two edge rails. It is positioned (either by hard stop or sensor and software).
Next the PCB is clamped either by edge clamping or by a vacuum plate from below.
Next a camera with upward and downward looking lenses finds the pre-programmed alignment features. Then these (two at least) features are aligned with each other.
Next, the camera moves out of the way and the board and or stencil is moved to print height. After that, the Squeegee does it's thing.
About the Alignment features, They are typically round dots in opposite corners of the PCB. We call them fiducials or fids for short. There are often fids close to fine pitch pads as well for local alignment. If the PCB was designed without fids, most printers can be programmed to use pads for alignment.
This Youtube link shows the alignment process on an automatic printer: http://www.youtube.com/watch?v=BfAYF8jsEWM
reply »