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60 DESIGN007 MAGAZINE I MAY 2024 for all physical layers and features of the PCB, such as copper traces, drill holes, solder mask application, and silkscreen (legend) place- ments. e CAM tool makes minor adjustments to your design to make manufacturing as reliable as possible. It will not make design-related changes that could change the product's func- tionality. e designer must clearly designate in the fabrication notes any areas of the design that absolutely cannot be modi- fied. ese CAM outputs com- municate the designer's intent to the machinery, operators, and technicians who will bring the PCB to life, ensuring every layer, hole, and label is precisely where it belongs. e clarity, accuracy, and complete- ness of these outputs dictate how smoothly the transition from design to product goes. Understanding the Materials When we discuss the level of difficulty that CAM outputs are meant to address, we are usually concerned with multilayer designs. A multilayer design is more complex than just a double-sided board. First, the internal layers must be created. ese layers are manufac- tured in pairs, two to a board. e boards are copper-clad laminate, which is a rigid epoxy layer clad in copper on both sides. A polymer film—called photoresist—is applied over the copper on both sides, then the board goes to imaging. An imaging process (described below) transfers the electronic cir- cuit design to the photoresist. Portions of the resist are exposed to light and remain on the panel while the unexposed resist is removed (developed) from the panel. On interior layers, the remaining photoresist protects the desired retained copper, while the rest of the copper is etched away. Of course, other conductive materials can also be used in the production of circuit boards. Some of them use similar techniques, while others require that a new approach be taken. Tin, silver, and gold can all be used, for instance. Each comes with its own challenges. For example, electrolytic silver can form den- drites over time, which end up creating unexpected shorts in a circuit. The Imaging Process Imaging refers to the process of applying the electronic design to the physical manu- facturing panel. is process is central to creating the PCB, as it defines the board's copper layout, such as circuit traces. During imaging, a photosensi- tive polymer—referred to as a photoresist—is applied to the copper-clad panel. e image is usually transferred to the photoresist in one of two ways: digitally using direct imaging or by plotting a film and exposing the film to UV light. Direct imaging is a newer process with less tol- erance variation built into it than plotting. e image is transferred to the photoresist using a laser, which offers much greater precision. e imaging machine can also calculate stretch and skew that is created during the process, allow- ing it to make corrections to the data so that fin- ished images can more accurately line up with slight variations in manufacturing panels. Plotting film on clear Mylar is an older tech- nique that relies heavily on the skill and expe- rience of the operator. For each copper layer, a corresponding film is plotted and used in the imaging process. e films are placed on either side of the panel, and a vacuum is used to ensure tight contact before exposing it to high-intensity UV light. is exposure process alters the photoresist (cross-linking the poly- mer) through the clear parts of the film, allow- The designer must clearly designate in the fabrication notes any areas of the design that absolutely cannot be modified.