At 03:07 PM 6/27/01 -0500, Jon Elson wrote:
>I tried this on a recent board, and had mixed results. I had a row of
>12 3-pin jumpers, and 4 of the 12 were successfully shorted, the other
>8 had a visible gap in the copper, about .004", I'd guess. It seems there
>may have been roundoff either in Protel or at the photoplotter that caused
>some of these .00005" gaps to become much larger. I'm going to have to
>come up with a better scheme for this that ensures the jumpers are truly
>shorted.
Well, I don't know for sure what happened here, but I have my suspicion,
and it is not about Protel or the photoplotter, that is the machine the
does the photoplotting. It is about the fab house, which may have decided
to alter the apertures. Why this would affect only some of the jumpers and
not others remains a mystery, though.
Frankly, I'd be *very* concerned about variations of 4 mils between design
and fab. Definitely, talk to the fabricator.
Fabricators commonly alter films to tweak them for their process
conditions. However, the unmodified film is, in fact, a specification, and
if there were 4 mil opens when the film had no open or only 50 microinches
open (I don't make the gaps that large, but that is the figure Mr. Elson
wrote), the board is defective and the fabricator should fix it. In this
case that should be little more than touching the pads with a soldering iron.
The procedure does require a little more thought and investigation. It
could be, for example, that the films were plotted with apertures of, say,
49.999 mils. If the plot routines in Protel truncated that to 49, instead
of rounding it to 50, this could create a gap of 2 mils. Then a fabricator
might have looked at that and said, "I can't make that," so he enlarged it.
Reducing the database to three or four or five place plots should be
through round-off, not through truncation, but sometimes programmers take a
shortcut: truncation is a tad faster. Since feature to feature positioning
error is the same both ways one might think that truncation could be used,
but the combination of feature position and size produces double the
potential error with truncation than it does with rounding.
Another place where truncation might be taking place would be in the
photoplotter, particularly when the apertures are read in. If an aperture
table is being used, any potential problem would be avoided, but I'd prefer
to stick with RS-274X. What is worrisome about this is that, if the error
is being introduced in the photoplotter, gerber software might not show it.
I've used virtual shorts without any reported problems, but it would be
worth looking into how Protel rounds off apertures and positions, as well
as how apertures are assigned under various conditions. If there is a
single pad of 49.999 mils, and match is set to .005 mils, what aperture
will Protel assign? Does aperture assignment vary with plot resolution?
Perhaps I'll find some time to look at this tonight.
[EMAIL PROTECTED]
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433
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