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Configuring the Gerber Translator for Photoplotters
Types of Photoplotters
There are two types of photoplotters: vector and raster.
Vector Photoplotters
Vector photoplotters process each draw and flash command directly from the Gerber database. These are normally mechanical plotters with an X-Y table, a light head, and an aperture wheel. Examples of vector plotters include the Gerber 3200 and 4000 series flatbed plotters.
Raster Photoplotters
Raster photoplotters rasterize the input Gerber data using a computer, and then use the resulting bitmap to modulate a laser that is scanned across the film. What is interesting about raster plotters is that many of them can accept polygons in addition to draws and flashes. The ability of a photoplotter to fill a polygon is extremely useful to the microwave and RF designer. Examples of powerful raster plotters that support polygons include the Gerber Crescent family of plotters and the Cymbolic Sciences family of FIRE 9000 plotters.
Vector Plotter Configuration
The mask file input to the Gerber translator is essentially a collection of polygons that need to be filled. Therefore, when you run the translator you should:
- Set the Translation Settings option Outline/Fill to FILL.
- Have a reasonable selection of round apertures available to fill. The smallest aperture diameter should be at least 2-3 times less than your smallest line width, otherwise you may not get a good representation of your design.
- Typical D-code diameters range from 0.001 inch up to 0.200 inch. Most mechanical photoplotters support up to 24 D-codes.
The Gerber translator uses an advanced multi-aperture fill instead of what we call a pen plotter fill. The multi-aperture fill generates Gerber files with the same resolution as a pen plotter fill, but creates data files 5-10 times smaller.
Polygon Filling Rules
Each polygon is filled independently of any other polygon in the mask file.
Any arcs that are part of a polygon boundary are broken into segments using the ArcRes parameter. This parameter is the number of degrees per segment. The default of ArcRes is 9 degrees. If you need smoother arcs in your film, reduce the number to 6 or even 4 degrees.
The routines start filling at the inner edge of the polygon with the smallest aperture (the one you specify as the Start Aperture in the translation configuration menu). This aperture is normally used twice and is offset from the edge of the polygon by 1/2 diameter (see A below). The first two strokes overlap by 1/2 diameter (see B below).
The routines then select a larger aperture (but normally no larger than 2 times the starting aperture) and repeat this process.
The interior of most large polygons is scan-filled with a fat aperture. There is no overlap between strokes once the routines jump into scan-fill mode.
Empty Polygons
Because the Gerber translator fills each polygon as it encounters it in the data stream, when it encounters an empty polygon it cannot clear away areas already filled. Therefore, for vector photoplotters, avoid using empty polygons in your layout. (This limitation is not in effect for some raster plotters.)
The translator issues a warning in the log file when it does encounter an empty polygon so that you do not accidentally plot over it.
If you have used empty polygons in your design, you may select the Advanced Design System Gerber export option Auto Merge, enabling you to merge filled and empty polygons to form a single filled polygon. This creates what we call a re-entrant polygon and is supported by the Gerber translator. However, while we recommend this function for relatively simple structures, we do not recommend you use such a function where hundreds of drill holes must appear in a power plane.
Compensation
Compensation works by swelling or shrinking each polygon prior to filling it. Again, because the translator views polygons independently, it cannot take into account spatial relationships between polygons. If you attempt to use a shrink compensation with butting polygons, a narrow gap will form between them.
FIRE 9000 Photoplotter Configuration
The Cymbolic Sciences FIRE 9000 photoplotter is a raster laser plotter that is ideal for creating microwave and RF artwork. Not only does this photoplotter have a very high resolution (typically 1/8 mil), but its RIP front end supports two very important extensions to standard Gerber (RS274D) data:
Because of these high level commands, the Gerber translator can translate a mask file with empty polygons directly into a stream of POEX and POIN commands. Configuring the translator for MDA output is the only mode that supports empty figures in the mask file.
Not only does configuring the translator for MDA output eliminate the need to stroke out the interior of each polygon, but the resulting artwork is limited only by the precision of the photoplotter. The FIRE 9000 autoplot format also embeds all Gerber format, unit, and data mode information into its header so that a separate aperture and information list is not needed.
We highly recommend (if possible) that you send your data to a photoplot or board shop equipped with a raster photoplotter such as the FIRE 9000.
The Gerber Viewer can properly view both POEX and POIN data so that you can verify the correctness of the output.
Recommended Settings for FIRE 9000 Output
The proper settings for FIRE 9000 output are summarized in the table below. When Outline/Fill is set to OUTLINE, each mask polygon is outlined. If a polygon is filled, then it is sent to either D20 or D21; these are both assigned as POEX. If it is an empty polygon or a hole, it is assigned to D22 or D23 which correspond to a POIN. Any open mask entities are sent to D10, which is a standard round aperture.
RS274X Output Configuration
Gerber Scientific's laser photoplotters read the extended RS274X specification. These photoplotters also support a polygon definition. Unfortunately, however, "empty" polygons are not supported. If you use empty figures in the mask file they will be covered up.
Other photoplotters may also support the RS274X specification, but before using them you should verify that they properly support the G36/G37 command used to switch into polygon mode.