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72 The PCB Design Magazine • December 2015 The Gerber Guide chapter 5 karel Tavernier uCAMCo It is possible to fabricate PCBs from the fa- brication data sets currently being used; it's being done innumerable times every day all over the glo- be. But is it being done in an efficient, reliable, automated and standardized manner? At this mo- ment in time, the honest answer is no, because the- re is plenty of room for improvement in the way in which PCB fabrication data is currently transfer- red from design to fabrication. This is not about the Gerber format, which is used for more than 90% of the world's PCB pro- duction. There are very rarely problems with Ger- ber files themselves; they allow images to be tran- sferred without a hitch. In fact, the Gerber format is part of the solution, given that it is the most reliable option in this field. The problems actually lie in which images are transferred, how the format is used and, more often, in how it is not used. Each month we look at a different aspect of the design to fabrication data transfer process. In this monthly column, Karel Tavernier explains in detail how to use the newly revised Gerber data format to communicate with your fabrication par- tners clearly and simply, using an unequivocal yet versatile language that enables you and them to get the very best out of your design data. Chapter 5: The Layer Structure When a PCB fabricator receives a data set, the first thing he does is load it into his CAM system to recreate a model of the PCB. This is the model that will drive manufacturing. To create this model, all image files must be con- verted and the function of each file in the stack must be clearly defined so it is clear which is the top copper layer, which is the solder mask, etc. The layer structure—the position of each layer—must be defined. After all, a PCB is not a pile of unrelated images, but a strictly ordered set of layers that are laminated together. The goal is to define this layer structure in an automatic process following a defined stan- dard to eliminate manual work and subjective interpretation. This is precisely what the Ger- ber specification's .FileFunction attribute does. To quote from the Gerber specification: 5.4.1.1 .FileFunction The .FileFunction file attribute identifies the function of the file in the PCB. The attribute must be set in the file header. Of all the attributes it is the most important. When this attribute is present in the data set, the fabricator's CAM software can use it to assign the file to its correct position in the layer structure, automatically and without any need for manual input. Job done, with no risk of ope- rator error. To transfer the layer structure according to the standard, you must choose Gerber "X2" output on your layout software, where X2 is shorthand for "Gerber with attributes." article