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PCBD-June2016

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82 The PCB Design Magazine • June 2016 system. The file names in MS-DOS were restrict- ed to a measly eight characters. Abusing the file extension to gain a few extra characters may have made sense in those days, but it makes no sense today. File extensions such as .ger, .pho, etc. are also sometimes used. They do indeed express the format, but in a proprietary, non-standard way. Always use the standard extension .gbr or .GBR. One may object that the supply chain is used to a file name convention such as in pn674847. top and it is not possible to simply throw it overboard. Fine: give this file its full name, in this case, pn674847.top.gbr. This is valid and the old file name is still prominent. Remember: Always use the standard file ex- tension ".gbr" for Gerber files. Chapter 14: Negative Copper Layers Negative layers are a relic of the 1960s and 1970s and the age of the vector photoplotter, which are now as obsolete as the mechanical typewriter. The vector photoplotter was similar to a pen plotter, but instead of using ink and paper, it wrote onto photosensitive film using a stationary light "pen." The film was held firm on a flat table that moved in the X-Y plane under the pen's light beam which was switched on and off as the image dictated. Every movement was governed by commands in input Gerber files. This was fine for drawing tracks. The prob- lem started with planes, or anything with large copper pours such as that shown in Figure 2. This is because vector plotters created these large copper pours using a technique called "vector-fill," "painting," or "stroking." This in- volved repeatedly moving the table back and forth under the pen, just as a child moves a crayon back and forth over an area until it is completely filled, as in Figure 3. In principle, this worked. But in practice, the input Gerber file, containing the zillions of draws that were needed for the vector fill, was huge. More importantly, it took ages—easily an entire shift—to plot a plane, so it was highly impractical. The solution was to plot in negative, in oth- er words, creating the clearances rather than the copper pours, as shown in Figure 4. This eliminated the need for painting, and the Gerber file size and plotting times were kept to a minimum. The negative film thus created was then used in the photolab as a phototool, to generate the positive film that was necessary for downstream production processes. Althou- gh this added a step and involved manual work, it was infinitely better than blocking the expen- sive photoplotter for a whole shift with impa- tient customers breathing down one's neck and demanding their plots. Negative films made a lot of sense in the days of vector photoplotters. But time has moved on since then, and so THE GERBER GUIDE, CHAPTERS 13 & 14 Figure 1: Vector photo plotter. Figure 2: A plane layer.

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