Gary, 

Typically fabrication shops swell all outer layer features to compensate for
their etch process tolerance. This has been going on since the beginning of
PCB fabrication.

I've never heard the board house performing micro-modification, but my guess
is that might be possible to gain space between conductive etch. A space is
more difficult to manufacturer than a trace or pad. i.e.: if the space is
tight to begin with and then the fabrication shop swells the features for
etch compensation this makes the space between features tighter. So they can
swell and then micro-modify. 

I'm just guessing here because I don't have all the facts, but PCB designs
often go through the CAM process on the 2nd or 3rd shift. The more notes you
have regarding "Not permitted without prior approval" will surely postpone
delivery if fabrication can't contact you at midnight. 

I don't know of any IPC standard that addresses this issue. However, I'm not
sure if a fabrication CAM operator will be familiar with the term
"Micro-modififications" and they might just throw all modifications into
that category. So it's best to discuss the terminology with your fabricator
to eliminate any misinterpretations. 

Tom


-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Gary
Koven
Sent: Tuesday, December 18, 2012 4:06 PM
To: [log in to unmask]
Subject: [DC] Micro-modification fab drawing note

We just had a case of a CM taking it upon themselves to micro-modify
footprints on source Gerber's and then complain to us about the yield,
blaming the areas which they micro-modified.

This has motivated us to consider adding a micro-modification note to our
boilerplate drill drawing layer.  This is my first cut at such a note:


"MICRO-MODIFICATIONS OR ANY OTHER CHANGES TO CONDUCTIVE FEATURE PATTERN ON
SOURCE GERBERS ARE NOT 
PERMITTED WITHOUT PRIOR APPROVAL OF (COMPANY NAME)".

There isn't much to go by on this topic in any of the IPC specs or on the
Web.  Please correct me if there's an IPC spec out there that addresses this
issue.


Anyone else who has experienced this, and can improve upon this wording,
please share.



TIA and Best Regards,
 
Gary M. Koven, C.I.D.
InVue Security Products

Charlotte, NC, USA
Board Of Directors, RTP Chapter

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