How Printed Circuit Board Is Made
The finished photomask was photolithographically reproduced onto a photoresist coating on the blank copper-clad boards. The first PCBs used through-hole know-how, mounting digital parts by leads inserted through holes on one facet of the board and soldered onto copper traces on the opposite aspect.
Starting from the schematic diagram the element pin pads were laid out on the mylar and then traces had been routed to connect the pads. Rub-on dry transfers of frequent component footprints increased efficiency.
Tin-lead or solder plating additionally grows whiskers, solely lowered by reducing the proportion of tin. Reflow to soften solder or tin plate to relieve floor stress lowers whisker incidence.
Another coating problem is tin pest, the transformation of tin to a powdery allotrope at low temperature. After PCBs are etched after which rinsed with water, the solder masks is applied, after which any uncovered copper is coated with solder, nickel/gold, or some other anti-corrosion coating. Initially PCBs have been designed manually by making a photomask on a transparent mylar sheet, often at two or four occasions the true dimension.
Silver, zinc, and aluminum are identified to develop whiskers under the influence of an electric subject. Silver also grows conducting floor paths within the presence of halide and other ions, making it a poor choice for electronics use.