thanks for repost, i will consider this method

yes the shop should have called, FYI it was Imagineering
(not the first problem i have had with them)
though in fairness their quality is adequate when considering the very
fast turn and price

Dennis Saputelli


Abd ulRahman Lomax wrote:
> 
> At 02:41 PM 5/14/2002 -0700, Dennis Saputelli wrote:
> >could you repost your alternative?
> 
> possible implementation: enable an unused mech layer and label it "shorts."
> Add track on this layer to a jumper footprint so that it will short the
> jumper if added to the plot for the copper layer. Use the "shorted" jumper
> on schematics to isolate a net section (i.e., to control connection path).
> Create a separate gerber definition file under the CAM manager for the
> copper layer used which adds the shorts mech layer to the plot; disable
> plot for that copper layer in the regular CAM gerber definition file. If
> both definitions are enabled for plot in the CAM manager, both will be plotted.
> 
> (Copy the main CAM def file to make the special one, then disable all
> layers except the one you want to plot and change the mech layer to plot
> simultaneously in on the mech layer tab.)
> 
> I have not actually used this procedure, only the microgap virtual short,
> but it seems quite straightforward in theory. If anyone has used it, please
> confirm that it works or tell us how it didn't, if known.
> 
> I would expect that the most likely failure would be that the designer
> forgets to set the special CAM definition, which would be relatively
> harmless at the prototype level since the shorting jumper could manually be
> shorted. (This is also the case with the microgap short).
> 
> This, by the way, is a good argument for requiring the fabricator to
> provide its customer the actual CAM files used for fabrication. Fabricators
> routinely make small changes without informing customers in order to
> improve manufacturability.... But I've never seen a fabricator provide the
> actual files.
> 
> In any case, the fabricator who unshorted the pads should very clearly have
> queried the customer as to whether or not the pads should be shorted (as
> they would if the board had been fabricated as-is) or opened to a
> fabricatable gap. So those missing shorts were due to fabricator error...
> 
> >i missed it
> >i did find that origin pin 1 seemed to nail the issue and make no gap
> >even in gerber 2.4
> >my gap was i think .005 mil
> >
> >in the (bad) case of using center origin and using gerber 2.3 i think
> >the
> >result was a 2 mil gap as the edges of the pads were pulled back toward
> >the center to the nearest mil
> >
> >this notwithstanding it has proven to be a useful and clever tool
> >
> >Dennis Saputelli
> >
> >
> >Abd ulRahman Lomax wrote:
> > >
> > > If gerber plots were not rounded off, there would be no problem with the
> > > "virtual shorts," and, in fact, if fab houses fabbed the boards as-is
> > > without modifying the gerber, there would also be no problem.
> > >
> > > But Protel does some rounding and it is not easy to exactly control
> > > aperture assignments while using the much easier RS-274X, though it can be
> > > done; properly implemented, aperture match would cause the gap to actually
> > > disappear as long as the pad distance is such as to leave the pads on a 1
> > > mil grid. But as Mr. Saputelli discovered, it is fairly easy to set up and
> > > place a virtual short footprint in such a way as to leave a tiny gap,
> > > enough to puzzle the fab house inspector.
> > >
> > > Complexities like this have led me to recommend the alternate method I
> > > indicated in another post in this thread. It is a little easier to document
> > > and no fab house will be tempted to modify the gerber.
> > >
> >

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