Don't ask what the designers prefer, ask what the customers (users) prefer.
The most common size/shape is a 1mm (0.040") circle. It is prefered to have
a flat finish, so that the vision cameras do not get confused by variations
in reflections and glare. That is also why solder mask is not put up to the
edge of the fiducial. Any error in putting down the solder mask will cover
part of the fiducial.

Have fun,
Ahne. 
Fine Line Stencil, Inc.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas McCall
Sent: Wednesday, 04 June, 2008 03:14
To: [log in to unmask]
Subject: [TN] Fiducial solder resist clearance

Hi,
When desiging the PWB for fiducials, what is the experience of having the
solder resist up to the edge of the fiducial pad.
 
If the recommended clearance window round (2-3 x fiducial radius) the
fiducial is not maintained does this in general cause excessive vision
system problems, - looking at the solder resist compared to looking at the
laminate material.
 
I understand that this clearance should be left with no resist and leaving
the bare PWB laminate and etched fidicial feature.
 
What is the prefered fiducial shape used by most designers?
 
Thanks in advance for any information.
D. McCall.
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