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82 The PCB Design Magazine • August 2016 production tolerances, as tolerances on pads are often tighter than those on conductive copper. Thus CAM needs to know not only where and what the pads are, but must also be able to edit them efficiently. Consequently, pads must be differentiated from other copper features, but they must also be classified separately by type, size and function so that they can be modified separately. The way to do this is in Gerber is simple: All pads are represented using the flash of an aperture, with different apertures representing different types of pads, even if they are similar in size and shape. So a component pad and a via pad that are the same size will have diffe- rent aperture numbers, making them easy to se- lect and treat according to their different needs. Pads should only be made using flashes, and fla- shes should only be used to make pads. Painted (stroked) pads are evil because they are so hard to edit. If a Gerber file served merely as an image, painted pads would be fine. Howe- ver, a Gerber file must also be editable, which, as we said, is not possible for painted pads. Remember: All pads must be flashes, and all flashes must be pads. Further Notes on Copper Pads Always flash embedded pads Embedded pads are pads that are fully wi- thin a bigger copper area (e.g., a via pad in an SMD pad, or an SMD pad on a copper pour). Image-wise these pads have no effect; whether they are present or absent, the image remains the same. However, CAM must know where and what the pads are, and a valid Gerber file will convey that information. The embedded pads must be present in your Gerber file; don't "opti- mize" your output by removing them! Do not use donuts! Sometimes pads on a plated hole are repre- sented by a donut rather than a solid pad (Figu- re1). Presumably, this is to make room for the drill hole or to indicate that copper will be re- moved. And it is true, copper will be removed. However the donut is very impractical because it does not fit how the copper is removed. The copper is not removed by imaging. First a solid pad is created by imaging, and then the pad is drilled. Consequently, CAM must laboriously re- place all the donuts with solid pads to prepare for imaging. Furthermore with non-plated pads which typically truly are donuts, plated donuts are confusing. Drill plated pads must be solid (Figure 2). Whatever you want to express by using a donut is better expressed in another way. Chapter 18: The contents of the fabrication data set All files in a fabrication data set are stored in an industry standard archive format (e.g., rar, zip, 7z). The archive should only contain data THE GERBER GUIDE, CHAPTERS 17 & 18 Figure 2: Proper pads—everything is clear. Figure 1: All donut pads—very confusing.

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