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60 The PCB Design Magazine • July 2015 What does a PCB fabricator do with CAD fabrication data? PCBs are typically fabricated in about 22 steps, many of which are digitally controlled and require dedicated data modules called pro- duction tools. Some designers believe that their PCB fabri- cation data will drive the fabricator's production machines directly; that the Gerber files will be used directly on the PCB fabricator's photoplot- ter; that Excellon drill files will go straight onto the fabricator's drilling machines; and that IPC- D-356A netlist will go right into electrical test machines. Not so. Fabricators never use the Gerber or Excellon files directly on their equipment. There are many reasons for this, the sim- plest of which is panelization. Even though the designer's data describes a single PCB or maybe an array, the job is never manufactu- red as such. It is always put on a production panel, which will typically have multiple jobs on it, as well as a border for plating, test coupons, etc. This is illustrated in Figures 1–3. It follows that the fabricator can do nothing with production tools for a single job: he ne- eds films and drill files for panel production. Another reason is that deviations are inevitably introduced by the fabrication processes, such as layer distortion during lamination and line width reduction during etching. These must be compensated for prior to manufacture. A third reason is that the production tools driving the fabricator's equipment must fit the fabricator's specific requirements so must often be conver- ted to a proprietary format associated with the machine. For all these reasons, the production tools that will drive the fabricator's equipment are generated by the fabricator's CAM department. PCBs cannot be professionally fabricated wi- thout this step: no CAM, no fabrication. It's as simple as that. THE GERBER GUIDE continues Figure 1: a single PCB. Figure 2: a panel with multiple PCBs. article

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