See, the figure during the check black line keep track of is coloured with mild; therefore the black marker is useful for the dark monitor, as seen in the picture. First, use a printer to print the PCB drawing on movie or sulfuric acid paper to make a “detrimental movie�? https://pcb-board72183.gynoblog.com/34141029/helping-the-others-realize-the-advantages-of-electronic-components