The Gerber data file comes in a well defined format if the originator
followed the Gerber specifications!  But even here some programmers have
found the need to "improve" on their output format and the result is that
those improvements can lead to errors.
The aperture list that is needed to go with the Gerber (274-D) data file is
not specified as far as the format is concerned. The result is that reader
of that list has to "guess" how to interpret it. Most programs that need to
input Gerber data have a number of templates that match various known list
formats from different board design programs. Some of these vary by only
having the X-column and the Y-column switched. The resulting error is easy
to detect. Other differences may result in much harder to detect errors.
In your case you essentially provided two different aperture lists and the
wrong one apparently matched an aperture list template and did not result in
easy to detect errors. You had a 50% chance to get it done correctly and in
this case you got the wrong 50.

When you use the Gerber 274-X format, the aperture list is embedded in the
data file and should follow the specified format. The result is a more
predictable interpretation of the data, easier to transfer the data (fewer
different files) and you will more likely be sending the right files. The
most recent specification document
(http://www.maniabarco.com/transdown/rs274xrevd_e.pdf) for the Gerber format
emphasizes 274-X. So would I.

Good luck in the future.

Best regards,
Ahne Oosterhof, GM

503-641-9428


-----Original Message-----
From:   TechNet [mailto:[log in to unmask]] On Behalf Of Andre Leclair
Sent:   Tuesday, March 11, 2003 04:55
To:     [log in to unmask]
Subject:        [TN] Aperture Data

Morning all

I have a question regarding Aperture List for Gerber Data.  This is an area
I know little about so bare with me.
We had an incident with a PCB house which resulted in the boards being
manufactured with all traces/pads 0.020" bigger then they were supposed to
be.  The shop said the problem arouse do to the way the aperture list was
set up.
The list has a set of 24 station #s with dimensions (which I understand get
translated to d-codes).  These Station#s are then repeated further down
plus have extra #s to 999.  We noticed that some of the dimensions in the
2nd set of station #s are differant from the 1st set. The 2nd set are the
correct #s.
The shop tells us their software (Infinite Graphics) will import the 1st
set then ignore the 2nd set of repeated #s then will continue to import any
station #s that are differant.
When we look at the Data with GC-Place and look at the used Apertures we
get the correct values.
Could the error be due to a differance in the way the software interprets
the list?..
The customer's data was created using P-Cad at least 10 years ago.  The
format the Lists are in are the same for all the customers product.  A
previous PCB shop had no reported difficulty.

Any ideas would be helpful
Thanks
Andre

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