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64 The PCB Design Magazine • February 2016 plated. The diameter is easily specified by the aperture (tool) diameter in the Gerber drill file, or the NC drill file for that matter. Span and plating are transferred in Gerber with the '.FileFunction' attribute. To quote from the Gerber format specification (see Table 1). As can be seen from this excerpt, the Gerber format provides unequivocal language to describe drill span and plating. It should be clear from this that any one file will describe one drill span and one plating instruction, so different spans and plating instructions must be put into separate files. A typical PCB fabrication data set will therefore contain several drill files: one for PTH holes, another for NPTH holes, and others for the different blind and buried spans. By creating data sets in this way, we can ensure that the whole drill file structure is standard and can be read automatically. If you are not able to add attributes to the file in this way, the CAM operator must manually determine the file function on his CAM system. In this case, you will be providing the information informally, but it should still be as simple and unequivocal as possible. The best way to achieve this is to make the function clear in the file name (e.g., NPTH.GBR). A more indirect method is to list the files and their functions in a text file. Sometimes both plated and non-plated holes are lumped together in a single file. Some argue that this is OK because the drill map indicates which holes are plated. The drill map does indeed indicate this (usually). But it is not OK. Drill data must be standardized and machine- readable. Drill maps are neither standardized nor machine readable; they must be read offline by CAM operators, who must work out the drill coordinates visually and then indicate manually which holes are plated and which are not. This terrible practice is bad enough if the plated and non-plated holes are specified using different tool numbers. But where plated and non-plated holes have the same diameter and the file is "optimized" by using the same tool for both, it becomes really excruciating work for the CAM engineer, and risky for the successful outcome of CAM and production. This is why plated and non-plated holes should always be put in separate files. If this is not possible, at least use separate tools for them. One more thing. In some cases, fabrication data comes in with just a drill map, and no digital drill data at all. This is simply unacceptable. Drill machines cannot read a drill map: drill machines have been using CNC data for decades. Without digital drill data, the CAM operator must pore over the drill map, Table 1. the gerBer guide, chapter 6