GBRUnion allows sizing up and down while simultaneously unionizing the data. Sizing is most often used to compensate for over or under etching during the board manufacturing process. However sizing can also be used to create contours that are offset by a tool diameter - very useful for laser or mechanical routing.
Sizing is always measured from the edge of the shape. If you size a 0.010 inch line up by 0.001 inch, then the final width of the line is 0.012.
Sizing is controlled from the GBRUnion dialog box. The default value = 0.000 which turns off the sizing operation.
Below is an example of a PCB top layer (pads, traces and ground plane) that has been sized up:
The magenta layer is the original size (the oblong pads are 0.024 inches wide.) The green is sized up 0.002 inches, the blue is sized up 0.004 inches and the red is sized up 0.006 inches.
The sizing value should normally be significantly smaller than the feature size of the pads and traces; otherwise there is a possibility, particularily where the shape has sharp turns or indentations, of distortion in the final results. There is also the chance that if the sizing value is larger than the gaps between conductors that independent nets become shorted together.
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