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Date: | Sat, 21 Oct 95 19:52:15 EST |
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Jerry:
You are correct in saying that most board houses don't
do a "true" net list test based on your supplied netlist;
rather a test fixture is created from the Gerber in most
cases. At least this is the case at HADCO. HADCO takes the
Gerber data provided and "pre-netlists" it before any
modifications. These modifications (teardropping, adjusting
pad & line widths, thieving, etc.) are then performed and a
"post-netlist" is generated. We then do a pre/post netlist
compare to ensure that we did not chnage the intent of the
design in any way.
We have embraced IPC-D-356 netlist formatting as the
vehicle for those of our customers who prefer to supply us
with a netlist from which to build the fixture. We viewed
-356 as the most universally accepted format out there.
Unfortunately, there are usually discrepancies between
supplied netlist and Gerber (on one job there were over 700
discrepancies!). For this reason, we have chosen to use a
certification program with our customers who desire us to
use their -356 data. Three successive designs must be sent
in with no discrepancies to become certified. It's a great
learning curve for both HADCO and our customer. At present,
I believe we only have two "certified" customers out of a
customer base of over 500. Let me share with you some of
the common problems encountered:
- Non-plated holes called out as a net
- Ground areas which are physically disconnected but
part of the same net, which are not listed in the
same net
- Naming convention errors (ex: "Signal A" later listed
as "Sig A" and then not tied together)
- Manual changes to Gerber not reflected in CAE data
(ex: split power/ground)
- CAE netlist not in IPC-D-356 format. Much of the
data that we receive seems to have a personal touch
mixed in with the IPC format. We need data that has
been built under the guidelines of IPC-D-356 from the
first column to the last (70th).
We will gladly work with any customer who wants us to
generate fixtures from their supplied IPC-D-356 netlist
data. We need the name, telephone #, and fax # of a person
designated to work resolutions on the CAD vs. Gerber
conflicts.
In summary, IPC-D-356 testing is a computer-generated
file in a specific format which describes which points are
interconnected, or part of a common net. Gerber-extract
testing is taking customer Gerber image files, processing
this information on a CAM workstation, and then
computer-generating a netlist program for electrical test.
Regards,
Tom Coyle
Field Services Engineer
HADCO Corporation
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