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PCBD-Nov2015

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66 The PCB Design Magazine • November 2015 The Gerber Guide chapter 4 by karel Tavernier uCAMCo It is possible to fabricate PCBs from the fa- brication data sets currently being used; it's being done innumerable times every day all over the glo- be. But is it being done in an efficient, reliable, automated and standardized manner? At this mo- ment in time, the honest answer is no, because the- re is plenty of room for improvement in the way in which PCB fabrication data is currently transfer- red from design to fabrication. This is not about the Gerber format, which is used for more than 90% of the world's PCB pro- duction. There are very rarely problems with Ger- ber files themselves; they allow images to be tran- sferred without a hitch. In fact, the Gerber format is part of the solution, given that it is the most reliable option in this field. The problems actually lie in which images are transferred, how the format is used and, more often, in how it is not used. Each month we look at a different aspect of the design to fabrication data transfer process. In this monthly column, Karel Tavernier explains in detail how to use the newly revised Gerber data format to communicate with your fabrication par- tners clearly and simply, using an unequivocal yet versatile language that enables you and them to get the very best out of your design data. Chapter 4: Output drill files in Gerber rather than in an NC format Incoming drill files in particular are never used directly on the fabricator's drilling machi- nes. There are plenty of reasons for this, and we've already seen some of these in earlier chap- ters. But the specific issue with drill files is that incoming CAD files specify the end-diameter of the hole on the finished bare board, while in re- ality a bigger hole must be drilled to make room for plating. Furthermore, the PCB designer is unlikely to know the origin and orientation of the fabricator's drilling machine, its working parameters and all the rest, which will also af- fect the way in which the production panels are drilled and routed. Therefore, the CAD designer need not fret about whether the drill machine works in me- tric or imperial, or can handle the resolution, feeds and speeds: the fabricator's CAM system takes care of all that. The designer needs only to concern himself with how to transfer the design data to CAM, optimally, accurately, completely and safely with a minimum of manual work for all parties. It may come as a surprise, but the best file format for CAD to CAM drill information tran- sfer is a Gerber file, not an NC or Excellon file. The most important reason for this is attributes, article

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