It’s not always possible to have all of the information needed for a successful PCB assembly printed on the blank PC board. When this is the case, we ask for an assembly drawing – like I suggest here. But what about things that are important at the PCB fabrication stage rather than at assembly.
That’s where the fab drawing comes in.
One of the problems with this system is that the “standards” for fab and assembly drawings are only loosely adhered to, if you can call them standards at all. If in doubt, label the fab drawing “Fab drawing” and the assembly drawing, “Assembly drawing.” That may seem obvious, but in the wide world of technology, obvious too often is anything but.
(image from xkcd.com)
Clearly label anything that the fab shop needs that isn’t obvious from the Gerber files, make a PDF, and label it “Fab drawing.pdf.” Do the same for any assembly information and instructions and label it “Assembly drawing.pdf.” If information is needed by both the fab shop and us, the assembler, put it in both drawings.
We recently had a case where a component polarity wasn’t marked on the board or in the assembly drawing, but was in the fab drawing. We do our best to catch such things, but it ads a bit of ambiguity to the process. If you’ve been reading this blog before, you’ve likely picked up that I do not like ambiguity. I do not like it one bit.
Duane Benson
Vote for clarity! Kick ambiguity out to the street