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PCBD-Apr2016

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78 The PCB Design Magazine • April 2016 which needs CNC data that has been generated by CAM. If the fabricator only has a drill map, how do you expect him to generate the drill data? Visually, by measuring the drill map on an XY table and typing in the coordinates? Another example is the excellent profile drawing in Figure1. This drawing defines and il- lustrates the profile perfectly. The profile draw- ing may still be useful as a check, but it does not replace digital data specifying the profile, which can be read in automatically (see Chap- ter 3 in this series). Note that while the draw- ing does not specify the exact position of the profile with regard to the copper layout, profile digital data does. Remember, drawings are no substitute for digital data. Chapter 10: Use only the Gerber Format for your Image Data Keep data formats to a minimum, using only those that are truly needed. Every extra format used adds output and input processing require- ments and increases the risk of bugs and version problems. Mixing formats also increases the risk of misalignment between files (see Chapter 2 in this series). Copper layers must be expressed in Gerber, so Gerber is a given. Consequently, what can reasonably be expressed in Gerber must be ex- pressed in Gerber. Drawings are images, so they can—and must—be transferred in Gerber. Other formats are often used for drawings: PDF, HPGL, DXF, DWG etc. These may be fine formats, and DPF is definitely a first-rate data exchange format, but for PCB fabrication data drawings, Gerber is the better choice: Your fabricator needs to read your drawings into his Gerber-capable CAM sys- tem to relate it to, and shed light on, the image data—which is, after all, the very reason why you sent the drawings at all. His CAM system is definitely proficient in handling Gerber data but it was not designed to handle other formats. So, although PDF may be a better choice for other workflows, Gerber is the format for draw- ings in the PCB workflow. Avoid complicated formats such as DXF and DWG like the plague. You cannot expect your professional CAM operator, who is highly skilled in the Gerber format, to have access to, and be familiar with, the high-end profession- al software that would be necessary to handle such formats. Simpler software is often of mixed quality, and not safe or reliable enough to faith- fully transfer your professional work. Definitely do not use DXF or DWG for data files such as copper layers. These formats were neither designed for, nor suitable for, PCB data. Indeed, such files are loathed by CAM operators. In fact, the number of formats needed is very limited. Copper, drill, rout, solder mask, legend must all be expressed in Gerber, and so must the drawings. The netlist cannot be expressed in Gerber, IPC-D-356A must be used. The infor- mal data intended for human eyes—delivery the gerBer guiDe, chapters 9 & 10 Figure 1: A profile drawing.

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