At 10:06 PM 2/9/01 +1100, Les Grant wrote:

>I have just noticed (after my client told me!) that Protel gerbers of
>octagonal pads end up as rounded pads. Is this just a configuration
>problem or does Protel not like octagonal pads or is there some
>other good reason for this behaviour? The actual problem is that the
>PCB manufacturer reproduced the octagonal pads somehow but
>the paste screen made from Protel gerbers has rounded holes
>which, apparently, causes some minor problems...

Octagonal apertures are a nice idea whose time has not come. The RS-274X 
standard recognizes regular polygons and Protel, if you choose RS-274X 
output, indeed defines a polygon, but last time I looked (SP5) the polygon 
was rotated 22.5 degrees. Someone at Protel misread the specification.

Assuming that this has not been corrected in SP6:

Because octagonal pads are not correctly implemented in RS-274X, one should 
not use RS-274X if you must use octagonal pads. Instead generate aperture 
tables, the old method. Or edit the definitions in the plot files, a 
somewhat hazardous task. It could be automated.

However, user error at the photoplotting stage can easily result in a 
misassignment of pad shapes.

Now, what about SP6? I just created a test board with a 100x200 mil 
octagonal pad and photoplotted it. I looked at the gerber with CAMtastic 
and found that an obround (oval) pad had been plotted. Somebody fixed the 
octagon problem but it looks like they didn't fix it all the way.... I 
loaded the gerber with Protel and even Protel itself recognised the pad as 
obround. I looked at the gerber, and, sure enough, the aperture command was

%ADD15O,0.100X0.200*%

(By the way, someone who does customization please tell me how to get a 
standard Windows rt-click menu for copy, cut, and paste in the new text 
editor.)

The "O" stands for "Obround," not "Octagon."

Then I changed the pad to 100x100. This time the aperture command was:

%ADD15P,0.100X8X0*%

Yes, Virginia, "P" stands for "Polygon," and it must be a regular polygon. 
Unfortunately, the "0" at the end, before the asterisk, is the number of 
degrees of rotation, and this is defined by one of the points of the 
polygon, not by the center of a side, as Protel seems to have assumed.

Protel octagon plot, as we have noted before, is rather thoroughly broken. 
This has been known for a long time, and it is close to mission-critical. 
Why hasn't it been fixed? Either take octagons out of the options or fix 
the gerber. The latter is greatly preferred, since octagons would be very, 
very useful. They are superior even to rounded rectangles, which is not an 
oxymoron.

Frankly, the best solution is to implement RS-274X special apertures, which 
are basically gerber macros that can be used to build any kind of shape. 
These could be used to make true non-regular octagons, which would be super 
for SOIC pads.


And by another way, Protel, many, many thanks for providing us with 
CAMtastic. It should be *routine* for users to look at the Gerbers with 
CAMtastic before sending out the files. It is almost one-button once you 
know what buttons to push.


Abd ul-Rahman Lomax
LOMAX DESIGN ASSOCIATES
PCB design, consulting, and training
Protel EDA brokering (resale) services
Sonoma, California, USA
(707) 939-7021, efax (419) 730-4777
[EMAIL PROTECTED]
[EMAIL PROTECTED]




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