Initially PCBs were designed manually by creating a <answer>photomask</answer> on a clear mylar sheet, usually at two or four times the true size. Starting from the schematic diagram the component pin pads were laid out on the mylar and then traces were routed to connect the pads. Rub-on dry transfers of common component footprints increased efficiency. Traces were made with self-adhesive tape. Pre-printed non-reproducing grids on the mylar assisted in layout. To fabricate the board, the finished <answer>photomask</answer> was photolithographically reproduced onto a photoresist coating on the blank copper-clad boards.
What did PCB designers used to employ on a mylar sheet?
photomask