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PCBD-Nov2015

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68 The PCB Design Magazine • November 2015 which are essential for automated CAD to CAM data transfer. Attributes specify the span and plating of a file, the function of a drill hole— that it is, say, a via hole—and the drill toleran- ces. Gerber is the only format that supports at- tributes. NC formats do not. In addition, CAM departments often face the following problems with NC files: • Drill files and copper layers are not registered as they use a different datum point This occurs in nearly 50% of all job data sets. Where this happens, the CAM engineer has to work the data, shifting and rotating each drill file to register it with the copper layers. This is a medieval way of working. The only secure so- lution is for the CAD professional to generate the drill and copper layers in perfect register, preferably using the coordinates of the CAD sy- stem on all layers, to facilitate communication between designer and fabricator. The under- lying reason for misregistration is that copper layers are output in Gerber and the drill data in an NC format. The NC output in CAD can pos- sibly be set up to register with the Gerber data, but the simplest way to guarantee registration is to output both in Gerber. • Drill holes are not centered on the copper pads When this occurs, part of the drill toleran- ce is gone before drilling even starts, increasing the risk of breakout. The root cause is different rounding and resolution in the Gerber and NC drill files. To overcome this issue, the CAM en- gineer may "snap" the copper pads to the drill holes. This manual operation takes time and carries risk, forcing the CAM engineer to mani- pulate the design, which is something the desi- gner does not want. The simplest solution is to output the drill files in Gerber at the same re- solution as the copper layers. Then everything matches perfectly. • Incomplete NC file All too often—in a whopping 75% of PCB data sets, in fact—the NC files are incomplete and require manual input of scale, unit and tool diameters. Such incomplete files relate to a complete NC file as the (obsolete) Standard Gerber relates to a proper Extended Gerber file. If you use Extended Gerber for image layers, it is not terribly consistent to stick to incomplete drill files. (See Chapter 21.) You must use com- plete drill files; the question is how to do this. Although it may be possible to set up the NC output of your CAM system to generate com- plete NC files, but the simplest solution is to output in Extended Gerber, which is guaranteed to be complete. • Limited resolution of Excellon file The Excellon format suffers from severe li- mitations on resolutions, so while it is quite adequate for drilling, it is not recommended for design transfer. There is no such limitation in Gerber. article THE GERBER GuIDE, CHAPTER 4 Figure 1: Drill and copper both in the same format, centered. Figure 2: Drill and copper in different formats, not centered.

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