I think I can answer that in a round-about way, but I have to say something
to set the foundation...

Gerber data formats were developed as a way to control laser photoplotters,
instructing the machine to expose film (instead of drawing ink on paper).
So, you can think of them as a type of plot file.

Well, just as a camera lets light in through an aperture, a laser
photoplotter
can control feature sizes by using different sizes of apertures.

Some apertures are called flashes, which moved the light source to a
specific
location and "flashed" the shape onto the film, like a rectangle or circle.

Other apertures were called "draws" which opens the light source and THEN
moves it to specific locations, effectively "drawing" a light beam of a
specific
width.

Thinking about a circuit board film layer, you can imagine that usually all
of
the pads are created with flashes, and traces are created with draws.

In the past the designer had to think of these things and supply a custom
"aperture list" with every order.
These days, gerber formats (274X) can embed the apertures directly into
the file, and most modern CAD systems can assign the apertures
automatically.

Unfortunately, since most of this has been automated, many designers don't
have a clue what is going on when they create the data files. On the upside,

if the designer hasn't made flashes available, or no flash is the correct
size when
the data file is being made, the CAD system is smart enough to DRAW the
feature (which far better than ignoring the feature, right?)

Using draws makes it more difficult for DRC checking in CAM, but usually the
front-end engineering will convert them to flashes as a normal part of the
process, so their checks can run more optimally.

The answer to your question really depends on the CAD system and the
designer,
so I'm sorry I can't be more specific, but usually YES, the designer can use
flashes
instead of draws for pad sizes if they know how to use their system.

Anyway, I think I answered ONE of your questions, the technical names are
DRAWs and FLASHes

hope that helps,
Jack

On Thu, Jan 29, 2009 at 9:41 PM, Ioan Tempea <[log in to unmask]> wrote:

>  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
>
> *P* *N'imprimer que si nécessaire - Print only if you must*
>
>
>

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