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18 PCB007 MAGAZINE I MAY 2024 Subtractive etching has been the technol- ogy of choice since the era of PCBs began in the mid-1950s. I can remember TV ads for one manufacturer that claimed their TV sets were of much higher quality claiming, "We use wires to make the connections for our television sets, not cheap printed circuit boards." Imagine that. Many alternative technologies have been tried—e.g., conductive inks applied with an ink jet printer, direct metallization, and semi- and fully-additive manufacturing—but none have been able to match the low cost and high productivity of the subtractive process. However, the subtractive process has its limits, and those limits are being challenged by the calls for the higher and higher circuit densities demanded today. e problem, of course, is that the subtrac- tive etching process etches sideways as well as downward in a ratio of one unit sideways to every three or four units down, depend- ing on which etchant is used and the etchant parameters. is means there is a limit to how deeply one can etch before completely under- cutting the etch resist at the top of the circuit. What are those limits? A reasonably good cir- cuit board manufacturer today should be able to achieve 75-µm (0.003") lines and spaces in 35-µm (1-ounce) foil, 50-µm (0.002") lines and spaces in 17-µm (half-ounce) foil, and (maybe, Can the Limits of Subtractive Etching Be Extended? The Chemical Connection Feature Column by Don Ball, CHEMCUT

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