Hi Steve,
Can't quite tell if you're concerned with the film image size or the etched
circuit image size.
I'll bet that IPC-D-310, Guidelines for phototool generation and measurement
techniques, contains some definitive answers. Unfortunately, I don't have a
copy.
But:
In the old taped artwork days (pre-surface mount), typical artwork (film)
tolerance was +/- 0.005".
Towards the end of the taped artwork era, tolerances were often tightened to
+/- 0.002".
Occassionally, PCB Fab drawings include an image size tolerance (often +/-
0.002").
Currently our PCB Imaging Dept. uses a +/- 0.0025" (inhouse) tolerance for
photoplotted artwork (panel lengths up to 22"). There's some give & take
based on PCB complexity & technology.
We chose the 2 1/2 mil tolerance to ensure that, when added to the other
manufacturing tolerances (imaging registration, drilling, material stretch,
etc.), the resulting PCB complies to IPC-A-600 & 6012 tolerances (annular
ring, trace to hole, hole to edge, etc.).
On boards with large pads & small holes (or no holes), we add panel
registration holes & targets to ensure that the image size and registration
are accurate. (I can't see where this is required by IPC-A-600, 6012, etc).
Good Luck,
Mark Hargreaves
-----Original Message-----
From: Steve Vargas [SMTP:[log in to unmask]]
Sent: Friday, January 17, 2003 10:52 AM
To: [log in to unmask]
Subject: [TN] PWB artwork
Technetters:
Can someone tell me what the requirements are for PWB artwork
stretch? I
have a 12" long board where the artwork appears to be about .004"
longer
than the data from the gerber files. This is causing us much
heartache
during our screenprint process. As you would expect, it is also
causing
problems with 0402 tombstoning and fine pitch soldering as well. I
could
not find this in IPC-A-600. Thanks.
Regards,
Stephen M. Vargas
Polaris Contract Mfg. Inc.
[log in to unmask]
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