At 08:16 AM 5/17/2002 -0700, [EMAIL PROTECTED] wrote:
>4. It is good practice to provide fabricators with your whole design or at 
>least the .pcb file because often slight corrections are necessary to be 
>made just to adapt the pcb to their technologies, and let them generate 
>files and reports they need.

 From an individual fabricator's point of view, this may be true, though 
the fabricator increases the risk of error by modifying design files, that 
is, the risk of error for which the fabricator will be held responsible, 
which could be very, very expensive. We have seen in recent threads where 
fabricators made incorrect assumptions about design intent.

The raw PCB file, in my opinion, should not, under normal circumstances, be 
provided to the fabricator. It belongs with design engineering. Yes, 
fabricators may wish to make certain modifications in the files in order to 
end up with the nominal track sizes, for example, after the etch process 
behavior is considered. But this is a geometric modification and can 
readily be done in a CAM program, so the fabricator only needs the gerbers.

The supplied gerbers should show nominal dimensions indicating the desired 
finished result, not necessarily what will actually be plotted. For 
example, I may specify an 0.028 hole and that is what will be in the PCB 
file, but the fabricator will probably drill the hole at 0.031 to allow for 
the thickness of the plating. So the fabricator will modify the files 
according to the needs of the fabricator's process, which may vary from 
fabricator to fabricator.

If the relationship between the fabricator and the designer is close, 
possibly the fabricator might have the PCB file, but would not have the 
authority to modify it in a permanent way. Anomalies found by the 
fabricator should always be queried to the designer even if they seem easy 
to fix. One person's anomaly can be another person's special design condition.

Abd ul-Rahman Lomax
PCB design, consulting, and training
Protel EDA license resales
Easthampton, Massachusetts, USA
(413) 527-3881, efax (419) 730-4777

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