Good morning Douglas (OK, good afternoon in the UK).

We (as builders) insist on 2x the fiducial radius to ensure a good
vision check.  We prefer a round fiducial, .0050" diameter.

Not having a good fiducial usually isn't a show stopper, but will
aggravate the assembly people as it means manual intervention to align
the machine to the board.  Obviously the finer the pitch the more
problematic this can be as a manual alignment can be "close" but usually
isn't quite as good as if the machine finds it.

Worst fiducials I've worked with - a designer had the brilliant idea
that the fiducial should double as a ground point, so all fiducials were
drilled and plated through to connect to the ground plane.  Essentially
then the fiducial was a big via.  Great for troubleshooting, but a royal
pain in the butt for us - after first reflow the HASL solder in the via
would flow down, so each fiducial would have it's own unique dome shape
- whereas we were looking for a nice flat surface.  Aaaggg!!! 


regards,
 - Graham

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas McCall
Sent: Wednesday, June 04, 2008 7:14 AM
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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