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From: Chris Taylor[SMTP:[log in to unmask]]
Sent: Monday, June 02, 1997 2:53 PM
To: [log in to unmask]
Subject: RE: Modifying Gerber Data
The Gerber data is almost always "compensated", Scott. Typically, etch is enlarged by one mil or so to compensate from under-etch. The software has an under/ over size edit feature which simply changes the flash geometry of the area you window select. If you have one of the CAM350 products (which I highly recommend) such as PC Gerber or ECAM or know an associate who does, try bringing in an artwork and select Utilities, Over/ Under Size... and try it out on an artwork file. When you send Gerber data to a Rigid-Flex house, they also scale the layers that are flex a fraction of a percent larger to allow for the differential in growth between polyimide and FR-4. The FR-4 tends to move more, polyimide being the more dimensionally stable substrate. This allows the pads to be better aligned after the lamination process for the drill process, especially at the edges.
More and more fab houses are asking designers to provide 1/1 Soldermask screens in reference to the SMT pattern. This allows the fabricator to "micro-modify" the artwork as necessary to achieve the highest yield. The variance changes with layer stackup and panel size and is typically greater at the edges. Each fabricator has their own recipe for modifying Gerber data which is why they prefer a 1/1 Soldermask. With designs utilizing a 1/1 Soldermask artwork you should provide a note on your drill drawing specifying the acceptable constraints of your Soldermask in relation to the SMT pattern. Having gone this direction myself, I have been delighted to find that my fabricator can now lay Soldermask between some fine pitch lands which could never have been done had I simply oversized the pad myself.
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From: [log in to unmask][SMTP:[log in to unmask]]
Sent: Monday, June 02, 1997 2:30 PM
To: [log in to unmask]
Subject: Modifying Gerber Data
Does the PCB fab shop usually modify trace and pad sizes from original
to
match their processes? If so is it usually done photographically or
digitally in a gerber editor?
Thanks in advance.
__________________________________
Scott Buscomb, Senior CAD Designer
[log in to unmask]
voice 770 338-6170
MTCC/Panasonic, Pager Development
1225 Northbrook Pkwy, Ste 2-358
Suwanee, GA USA 30024
__________________________________
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