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60 The PCB Design Magazine • October 2015 one ends. Thus the order in which the segments of a contour are defined is significant. Non-con- secutive segments that meet or intersect fortui- tously are not considered to connect. A contour is closed: The end point of the last segment must connect to the start point of the first segment." The Gerber format regions are defined by contours using the G36/G37 commands. This is precise and unequivocal, and is the recommen- ded way to specify the profile. The filled contour covers the PCB exactly. If this is not possible, the profile can be spe- cified by drawing the contour with a zero size or very small size aperture. If the aperture is not zero size, the profile is the center line of the stroked line; in other words, do not compensa- te for aperture size. You are transferring an ima- ge, not a production tool for a drill machine. The profile layer is not copper. It is more akin to a drill or rout file as it affects all layers. The outline should therefore be put into a separate file, and not shoved into a copper layer. It is helpful to provide a mechanical drawing with the profile. However, this is not a substitute for digital data. Corner marks are sometimes used to indicate the profile. Again, corner marks are meant for vi- sual interpretation and do not constitute digital data. Therefore, they are not a valid specification of the profile. What is definitely unacceptable is to take a copper layer and add a crude manually-drawn line that, to make matters worse, is drawn with the same aperture as the copper tracks. Next month we'll move on to Chapter 4. See you then. PCBDESIGN This column has been excerpted from the Guide to PCB Fabrication Data: Design to Fabrica- tion Data Transfer. article THE GERBER GuIDE, CHAPTER 3 Figure 1: Profile defined by g35/g37 region. Figure 2: Profile stroked with a thin aperture. Karel Tavernier is managing director of ucamco.

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