4 thou across a large board would give you a 2 thou error for print with Fiducial correction. Placement will usually stretch/shrink compensate plus x y & Theta compensate using Fiducials, so even less error.
That doesn't sound too bad.
PCB fabricators compensate for differing X & Y stretch/shrink on etch. Your pcb fabricator should confirm their capability for your size of pcb and materials used.

I'd determine what is really needed and talk to the supplier on how to achieve that.

Rex

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ashok Dhawan
Sent: 26 June 2012 13:02
To: [log in to unmask]
Subject: [TN] board larger than gerber

I have come across a Fab where print is smaller than board. The board is being printed on first side and this is observed prior to any reflow (no shrinkage). No visible warpage on board. 

CMM measurement of boards verified board scaled larger than gerber by 0.004" which is enough to have misregistration on fine pitch parts. 

What is tolerance spec for board size compared to gerber size ? 

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