Are they still needed in the age of machine assembly?
If you can’t have reference designators, what should you do?
The first answer will probably be along the lines of, “Put them on the board.”
But, sometimes you can’t have reference designators on the board. Maybe it’s too densely populated and there isn’t room. Maybe, for aesthetic reasons, you chose to leave them off. With some products, like development boards, it’s sometimes necessary to use the space for instruction or functional identification, and reference designators would confuse customers.
It’s always best to put reference designators as close to the part as possible, and on the same side as the part, but if that’s not possible, you can still create an assembly drawing. When laying out the board, put the reference designators in a different layer than the text you want in silkscreen. Then, create a PDF that has all the component outlines in their place with reference designators. Make one for the top and one for the bottom. Call this document “assembly drawing” and include it in the files sent in to be manufactured.
FIGURE 1 shows a good assembly drawing format. It has reference designators and polarity marks.
One might ask why reference designators are needed when all the surface-mount parts are machine-assembled. First, any through-hole parts are hand-assembled. Their locations and board side need to be clear for the people stuffing them.
Second, CAD systems don’t always have 100% accurate information. If the center point of the footprint is off, surface mount machines (ours and anyone else’s) will center the part where the file says to put it, which, in this case, would be the wrong spot.
The reference designators are also part of quality control. It’s another opportunity to remove ambiguity. Ambiguity bad. Certainty good.