Hi,

  Let me try to answer your questions:

First your 100 x 200 pad example.

  A 100 x 200 rectangular pad is almost always drawn. The early gerber
formats as drawn by ACD software would only draw or flash rounds and
squares. Rectangles of 100 x 200 would have been created by selecting a 100
wide square aperture and then drawing for a distance of 100.

 In RS-274-X you can define such a rectangle but it needs to be created for
each angle at which you intend to use it. In software it is easier to simply
draw.

Questions:
o        How is each of them generated?

CAD software that generates gerber files uses whatever schemes or ideas the
programmer could think of which would result in the required drawn image.

This "Freedom to Choose" can create a lot of problems as the RS-274
specifications leave many issues undefined. As an example can a programmer
use arcs as segments of a -X polygon fill? Some programmers think so but
some gerber readers do not render this correctly.

The programmers who write gerber readers (like me) have to "Guess" at what
is legal AND possible. Doing a good job of reading gerbers requires finding
all the quirks that CAD software programmers can think of.


o        Are there CAD systems that can only generate the latter?

I would suggest NO.

I have seen CAD systems that use only G36 G37 polygons. I have seen CAD
systems draw every shape using lines. Most CAD systems use a mix of many
techniques.

 Using a single mode of drawing makes drawing of all the required shapes
very difficult.

o        Any precautions that can be taken by the designer so that only the
first one be generated?

 Some CAD systems allow some control over the gerber output. But, if you
place a part with square pads at 45 degrees you need a draw. In RS-274-X you
can use "Aperture Macros" which can be rotated.  

 
o        If I want to call each method by its technical name, what would
that be?

Bob Kondner


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Thursday, January 29, 2009 10:42 PM
To: [log in to unmask]
Subject: [TN] Aperture generation in gerber files

Dear Colleagues,
 
The many gerber files I have seen can be divided into two categories,
according to how the geometrical figures (apertures) of the land patterns
are generated:
o        Apertures declared as stand alone geometrical figures (e.g. a
rectangular 100x200 pad is declared as a 100x200 rectangle)
o        Apertures generated by successive positions of a smaller element
(e.g. 100x200 rectangle generated by adjoining twenty 100x10 lines)
 
The second version does not come handy at all when trying to measure the
center of an aperture with a gerber viewer.
 
Questions:
o        How is each of them generated?
o        Are there CAD systems that can only generate the latter?
o        Any precautions that can be taken by the designer so that only the
first one be generated?
o        If I want to call each method by its technical name, what would
that be? 
 
Thank you,
 
Ioan Tempea, ing.
Ingénieur Principal Fabrication / Sr. Manufacturing Engineer 
 
30 ans déjà! - Already 30 years!
950 rue Bergar, Laval, Québec, H7L 5A1
t : 450-967-7100 ext : 244
Mtl : 514-990-5762
f : 450-967-7444
[log in to unmask]
www.digico.cc <http://www.digico.cc/> 
P N'imprimer que si nécessaire - Print only if you must
 

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