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12 The PCB Design Magazine • January 2015 used. Orthochromatic film is blind to red, so it recorded only the blue traces. Similarly, pan- chromatic film is less sensitive to blue and with filtration it only records the red traces. Thus, the red and blue tapes allowed two negatives to be produced, back and front, while ensuring exact alignment of pads and vias. There was another popular method of de- signing PCBs at the time, although not nearly as popular as tape and Mylar, using a material called Rubylith. This was a polyester base cov- ered with a red plastic laminate. The designer used an X-Acto knife to remove the red where traces and pads would be. Then the "negative" PAST AND FuTuRE TRENDS IN PCB DESIgN continues feature Figure 1: yes, vacuum tubes and PCB technology overlapped. This is from a 1971 television (Mark Forbes' photo and collection).

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