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70 The PCB Design Magazine • November 2015 article • Poor implementation of NC output in some CAD systems Some NC files are really terrible. They can, for example, have binary junk in the middle of the file, or non-standard commands. As images are considered to be more critical than drill fi- les, the quality of Gerber files is typically much better. • Needlessly using two formats where one will do Copper layers must be in Gerber as NC files cannot describe a copper image, so Gerber is a given. As Gerber can also describe the simpler drill data, it makes sense to keep things simple by using this format for both copper and drill/ rout data. By using an NC format for drill, a se- cond format is needlessly introduced, and the risk of problems is doubled. There is not a single disadvantage in using Gerber rather than an NC format to express CAD-to-CAM drill data. On the contrary, it brings many advantages in terms of complete- ness, accuracy, consistency and simplicity. One may object that we cannot send Gerber files to a drilling machine. True enough, but as outlined above, this is a non-issue. The CAM sy- stem will generate the drill files for fabrication in Excellon, Hitachi, Sieb & Meyer, or whatever format best suits the drilling machine, and will take care of the offset, rotation, feeds, speeds, tool diameter, resolution, optimisation, size compensation and whatever else is needed for optimal performance on the fabricator's equi- pment. This means that Gerber is hands-down the best format for input in CAM. One may object that Gerber is an image description format—true again—and that drill data is not image data. This is actually not true. A Gerber copper file describes where there is copper and where there is no copper. Just as a legend file describes where there is legend ink and where there is no legend ink. These files use images to show the presence and absence of material. So do drill and rout files. In this case they describe where material must be removed: a hole or slot is the absence of material. So a drill file, just like a copper layer, is an image file. Yes, drill holes and copper patterns are produ- ced by different processes, but at design stage, they are images. Copper layers and legends are also produced by different processes for that matter, but no one claims they must be described by different image formats. The only difference between drill and copper layers is that the drill file ap- plies to a range of layers, the span, and not to a single layer, and of course, the span must be cle- arly specified, in both Gerber and NC formats. Here too, Gerber is better: NC formats have no inherent mechanism for describing spans, whi- le Gerber's attributes allow span to be specified in a standard, machine-readable manner. Conclusion: Gerber is far superior to the NC formats for transferring drill and rout data from design to fabrication. Output your drill and rout files in Gerber. Next month we'll move on to Chapter 5. See you then. PCBDESIGN This column has been excerpted from the Gui- de to PCB Fabrication Data: Design to Fabrication Data Transfer. karel Tavernier is managing director of ucamco. " There is not a single disadvantage in using Gerber rather than an Nc format to express caD-to-caM drill data. on the contrary, it brings many advantages in terms of completeness, accuracy, consistency and simplicity. " THE GERBER GuIDE, CHAPTER 4