FastCAD V7 Gerber / NC drilling file export
PHOTO PLOTTING FILES
FastCAD V7 can be used for printed circuit board design. Using EXP
Computing's Gerber and NC file export tools, the CAD drawing is
converted
to printed circuit board artwork by saving
the drawing as several different Gerber format files corresponding to
the photo tooling films needed to fabricate the PCB.
The drawing should
be organised in layers or sets of layers corresponding to the different
films. For a two-sided PCB this might involve separate layers for the
following:-
Top
- showing circuit traces on the component side of the
board.
- Bottom - showing circuit traces on the solder side of the board.
- Pads - showing all the pads that require soldering. Used for producing the solder mask.
- Silkscreen - showing component outlines, part numbers and identifying information.
- Outline - showing the mechanical features of the design and the board outline dimensions.
- Drill Pattern - showing all the holes.
For boards with more layers or complexity, further drawing
layers will be required.
PREPARING THE DRAWING
The circuit traces connecting the pads are usually drawn using the PATH
command with the linewidth property set to the required trace width. If
a path linewidth is zero it will be converted to the minimum width
setting
when saved to Gerber format. Paths have square ends in the drawing but
are
converted to Gerber strokes of the same width using a circular aperture
of the same diameter. This produces radiused ends and allowance must
therefore be made when preparing the drawing to avoid any overlaps at
the ends of traces - especially where wide traces are used. If square
ends are need on some traces a filled polygon or box can be used to
draw the end pads or traces.
Lines and arcs can as be used in a similar way to paths.
Polygons and circles with a non zero fill style and a linewidth of zero
are converted to solid filled areas.
Polygons and circles with fill style = 0 and linewidth = 0 are
converted to closed paths with the minimum linewidth.
Polygons and circles with a non zero fill style and a linewidth
> 0 are converted to closed paths of that linewidth.
SAVING GERBER FILES
The organised information is converted to Gerber files one file at a
time by showing the required layer(s), using the Layer Management tool
to hide all the other layers except the layer(s) for the film you want
to print e.g. the top of the board.
Only visible line, path, polygon, circle, arc and exploded text
entities are exported. Groups are ungrouped into their components
entities. All other entity types and all entities on hidden layers are
ignored in the export process. It is important, therefore, that the
drawing should be checked for other types of visible entities before
exporting - the COUNT command can be used to list selected entities.
Symbol references and dimensions should be exploded before exporting.
Only vector fonts (.FNT) should be used in the drawing and the PTKEXPL
command used to explode these into line vectors before exporting.
Use the File menu Save As... command to save the visible layer(s) to a
Gerber file using the type RS-274X file (GBR) option. Repeat the
process for each PCB film changing the visible layer(s) each time. Use
separate 8-character maximum file names for each layer.
The units setting for this file is determined automatically from the
drawing file units setting which must be inches or millimeters. The
number of digits is determined automatically from the extents of the
drawing, and the number of decimal places is determined from the
drawing setting - normally this is preset in your drawing template. The
minimum linewidth setting for entities which have a zero linewidth
setting is preset but can set temporarily using the GEROPT command.
NC DRILL FILE
In addition to the set of gerber files a NC drill file is also
required. This is produced by first making the Drill layer visible,
then using the File menu Save As... command with the type Excellon NC
Format drilling file (DRL) option, to save the visible layer(s). Only
circle entity centre co-ordinates and diameters are saved.
The units setting for this file is determined automatically from the
drawing file units setting which must be inches or millimeters. For
inches units the output format is six digits 00.0000 and for
millimeters 0000.00, with leading zeros an absolute units.
README FILE
To complete the set of artwork files a "Readme" text file should be
included containing a list of all the file names with a brief
description of each ones contents. Usually all the files are placed in
a zip file for transmission to the fabricator.
MORE DETAILS
For more information
about Gerber and NC file export tools contact