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62 The PCB Design Magazine • July 2015 • Create the production panel (a.k.a. working panel). • Compensate for deviations in the manufacturing processes (e.g., scaling to compensate for distortions during lamination). At this stage CAM has a model of the production panel as it will be manufactured. • Send fabrication data to ERP. • Output dedicated digital pro- duction tools to drive the NC fabrication equipment (photoplotting, direct imaging, legend printing, drilling, routing, scoring, AOI, electrical test files, AVI). The content of these production tools is very specific to the fa - bricator's and customer's setups, as the informa- tion needed and format used are often proprietary and specific to the equipment brand and model, while the equipment must "know" customer-specific details such as the location of the fiducials for re- gistration on the direct imager and "don't care" zones for the AOI system. To generate the production tools, the CAM operator needs a perfect physical model of the bare PCB. As we have said, the client's incoming fabrication data is used as digital data to recon- struct that physical model. It may at first sight seem strange that machi- ne files such Gerber, Excellon and IPC-D-56A are used for this, but it actually makes perfect sense. The Gerber format has evolved way beyond its origins as a photoplotter driver to become the perfect vehicle with which to transfer digital image and drill information from CAD to CAM. And it is precisely because of those origins that Gerber files are so perfectly suited to their current role of accurately representing where copper and other materials are. Similarly, Excellon drill files correctly specify where the drill holes are. Which makes these formats capable of describing a PCB. So the incoming Gerber, Excellon and other data is always read into the fabricator's CAM system which analyses, reworks and transfor- ms the image and drill information into pro- duction tools. This is a very different proposi- tion from using the designer's datasets directly as production tools in two aspects: • The data files are not treated as standalone items, but must be viewed as an interconnected dataset that, together, describes a PCB. • PCB CAM needs to "know" more about a PCB than just the image. CAM needs to know, for example, which pads are edge connectors, because these need to be gold plated; CAM needs to know which drill holes are vias, because the solder mask is treated dif - ferently for via pads than for component pads, and so on. These two points affect the way in which the designer's PCB fa- brication data should be structured. The designer need not worry whether his or her files will be able to drive NC production machines; it is the job of the fabricator's CAM to manipulate the de- sign data so that it will run on real production equipment. The designer's focus should be on specifying the end product accurately, comple- tely and unequivocally. Remember: CAD output is CAM input and not machine input. When creating fabrication data from CAD, do not ask, "What can I do to create better production tools?" Instead, ask, "What can I do to create better CAM input?" Part 2 of this series will be published next month. PCBDESIGN article karel Tavernier is managing director of ucamco. Karel has 30 years' experience in software and imaging equipment for the PCB and electronic printed packaging industry, including sales, service and R&D at Barco, Belgium. to generate the production tools, the CaM operator needs a perfect physical model of the bare PCB. " " THE GERBER GUIDE continues

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