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April 1997

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Subject:
From:
"Matt Mahlau" <[log in to unmask]>
Date:
Sun, 20 Apr 1997 13:20:13 -0400
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> Can anyone elaborate on the differences between testing
> PWBs to gerber data versus testing to customer-supplied
> netlists?
> 
For a PCB manufacturer, the most common problem with testing to customer
supplied netlist is that a netlist to a designer/layout person is a
different animal than a netlist for testing.  A testing netlist needs X and
Y coordinates of the pads/holes on the board.  The layout netlist does not
have that info.  Some CAD systems can output a sort of testing netlist with
X and Y coordinates that, with a little massaging in a text editor, can be
used to test the board.

Gerber data is a standard that all CAM systems recognize and this
eliminates a lot of the incompatibility problems.

> What are the advantages for the customer if he/she provides
> a netlist?
> 
As long as the netlist can be used by the tester, a more accurate test will
result.  It is possible, and I've seen it happen, where a board tested to
the gerber data will test GOOD when the board is actually BAD.  How?  A
change in the netlist can be made without a new set of gerber files being
generated.  Of course, this won't be discovered until after the board is
manufactured, but at least it will save the customer from populating a
board and then finding out it is no good. 

> Is there a standard (or even commonly used) netlist format?
A standard has been proposed (IPC-D-350) that all parties follow in the
lifecycle of a PCB.  From CAD, CAM, testing and assembly.  This way
everyone will be working from the exact same data without each process
having to convert from one type of file to another.

The newest CAM systems and bare board testers do recognize this format.  A
lot of older software is still out there though that do not.  Maybe specify
that the IPC format file should be used, if possible.  If not, fall back on
the gerber data.  That is pretty much universal.

> 
> Thanks in advance,
> 
> Andy Wolf
> RMS Information Systems
> [log in to unmask]

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